U.S. patent application number 17/149660 was filed with the patent office on 2021-05-13 for fast formation cycling for rechargeable batteries.
The applicant listed for this patent is UT-BATTELLE, LLC. Invention is credited to Seong Jin An, Jianlin Li, David L. Wood, III.
Application Number | 20210143391 17/149660 |
Document ID | / |
Family ID | 1000005348548 |
Filed Date | 2021-05-13 |
![](/patent/app/20210143391/US20210143391A1-20210513\US20210143391A1-2021051)
United States Patent
Application |
20210143391 |
Kind Code |
A1 |
Wood, III; David L. ; et
al. |
May 13, 2021 |
FAST FORMATION CYCLING FOR RECHARGEABLE BATTERIES
Abstract
A method for fast formation cycling for rechargeable batteries
comprising the steps of: step 1 (First Partial Charge)--charge cell
from open-circuit voltage (OVC) up to 80-90% of an upper cutoff
voltage (UCV) of from 4-5 V at a C rate not less than 0.5 and not
more than 1.5; step 2 (First Shallow Charge)--charge cell from
80-90% of UCV to 97-100% of UCV at a C rate of not less than 0.2
and not more than 0.5; step 3 (First Shallow Discharge)--discharge
cell from 97-100% of UCV to 80-90% of UCV at a C rate of not less
than 0.2 and not more than 0.5; and step 4 (Subsequent
Charge/Discharge Cycles)--repeat steps 2-3 up to 2-10 times where
the charging and discharging rates are progressively increased by
25-75%. A battery made according to the method of the invention is
also disclosed.
Inventors: |
Wood, III; David L.;
(Knoxville, TN) ; Li; Jianlin; (Knoxville, TN)
; Jin An; Seong; (Suwon-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UT-BATTELLE, LLC |
Oak Ridge |
TN |
US |
|
|
Family ID: |
1000005348548 |
Appl. No.: |
17/149660 |
Filed: |
January 14, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
16225889 |
Dec 19, 2018 |
10910628 |
|
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17149660 |
|
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|
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62609376 |
Dec 22, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/0445 20130101;
H01M 4/525 20130101; H02J 7/0068 20130101; H01M 4/0447 20130101;
H01M 10/446 20130101; H01M 10/0525 20130101; H01M 4/505 20130101;
H01M 10/44 20130101; H01M 4/587 20130101 |
International
Class: |
H01M 4/04 20060101
H01M004/04; H02J 7/00 20060101 H02J007/00; H01M 10/0525 20060101
H01M010/0525; H01M 4/587 20060101 H01M004/587; H01M 4/505 20060101
H01M004/505; H01M 4/525 20060101 H01M004/525; H01M 10/44 20060101
H01M010/44 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND
DEVELOPMENT
[0002] This invention was made with government support under
Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of
Energy. The government has certain rights in this invention.
Claims
1. A method for fast formation ion cycling for rechargeable
batteries comprising the steps of: 1) charging the cell from
open-circuit voltage (OVC) up to 80-90% of an upper cutoff voltage
(UCV) of from 4-5 V at a C rate not less than 0.5 and not more than
1.5; 2) charging the cell from 80-90% of UCV to 97-100% of UCV at a
C rate of not less than 0.2 and not more than 0.5; 3) discharging
the cell from 97-100% of UCV to 80-90% of UCV at a C rate of not
less than 0.2 and not more than 0.5; and, repeating steps 2) and 3)
up to 2-10 times where the charging and discharging rates are
progressively increased by 25-75%.
2. The method of claim 1, wherein the cell comprises
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, x.ltoreq.0.5
and the method comprises the steps of: 1) charging the cell from
open-circuit voltage (OVC) of .about.3 V up to 3.7-3.9 V at a C
rate not less than 0.5 (80 mA/g-NMC) and not more than 1.5 (240
mA/g-NMC); 2) charging the cell from 3.7-3.9 V to maximum cell
voltage of 4.2-4.3 V at a C rate of not less than 0.2 (32 mA/g-NMC)
and not more than 0.5 (80 mA/g-NMC); and 3) discharging the cell
from 4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.2 (32
mA/g-NMC) and not more than 0.5 (80 mA/g-NMC).
3. The method of claim 2, further comprising the steps of: 4)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.5 (80 mA/g-NMC) and not more than
0.75 (120 mA/g-NMC). 5) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 0.5 (80 mA/g-NMC) and not
more than 0.75 (120 mA/g-NMC).
4. The method of claim 3, further comprising the steps of: 6)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.75 (120 mA/g-NMC) and not more
than 1.2 (192 mA/g-NMC); and 7) discharging the cell from 4.2-4.3 V
to 3.7-3.9 V at a C rate of not less than 0.75 (120 mA/g-NMC) and
not more than 1.2 (192 mA/g-NMC).
5. The method of claim 4, further comprising the steps of: 8)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.2 (192 mA/g-NMC) and not more than
1.5 (240 mA/g-NMC); and 9) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 1.2 (192 mA/g-NMC) and not
more than 1.5 (240 mA/g-NMC).
6. The method of claim 5, further comprising the steps of: 10)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.5 (240 mA/g-NMC) and not more than
2.0 (320 mA/g-NMC); and 11) discharging the cell from 4.2-4.3 V to
2.5 V at a C rate not less than 0.5 (80 mA/g-NMC) and not more than
1.5 (240 mA/g-NMC).
7. The method of claim 1, wherein the cell comprises
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite,
x<0.5.ltoreq.0.8 and the method comprises the steps of: 1)
charging the cell from open-circuit voltage (OVC) of .about.3 V up
to 3.7-3.9 V at a C rate not less than 0.5 (95 mA/g-NMC) and not
more than 1.5 (285 mA/g-NMC); 2) charging the cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.4 V at a C rate of not less than
0.2 (38 mA/g-NMC) and not more than 0.5 (95 mA/g-NMC); and 3)
discharging the cell from 4.2-4.4 V to 3.7-3.9 V at a C rate of not
less than 0.2 (38 mA/g-NMC) and not more than 0.5 (95
mA/g-NMC).
8. The method of claim 7, further comprising the steps of: 4)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.4
V at a C rate of not less than 0.5 (95 mA/g-NMC) and not more than
0.75 (142.5 mA/g-NMC); and 5) discharging the cell from 4.2-4.4 V
to 3.7-3.9 V at a C rate of not less than 0.5 (95 mA/g-NMC) and not
more than 0.75 (142.5 mA/g-NMC).
9. The method of claim 8, further comprising the steps of: 6)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.4
V at a C rate of not less than 0.75 (142.5 mA/g-NMC) and not more
than 1.2 (228 mA/g-NMC); and 7) discharging the cell from 4.2-4.4 V
to 3.7-3.9 V at a C rate of not less than 0.75 (142.5 mA/g-NMC) and
not more than 1.2 (228 mA/g-NMC).
10. The method of claim 9, further comprising the steps of: 8)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.4
V at a C rate of not less than 1.2 (228 mA/g-NMC) and not more than
1.5 (285 mA/g-NMC); and 9) discharging the cell from 4.2-4.4 V to
3.7-3.9 V at a C rate of not less than 1.2 (228 mA/g-NMC) and not
more than 1.5 (285 mA/g-NMC).
11. The method of claim 10, further comprising the steps of: 10)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.4
V at a C rate of not less than 1.5 (285 mA/g-NMC) and not more than
2.0 (380 mA/g-NMC); and 11) discharging the cell from 4.2-4.4 V to
2.5 V at a C rate not less than 0.5 (95 mA/g-NMC) and not more than
1.5 (285 mA/g-NMC).
12. The method of claim 1, wherein the cell comprises LiFePO.sub.4
(LFP)/graphite and the method comprises the steps of: 1) charging
the cell from open-circuit voltage (OVC) of .about.2.5 V up to
3.2-3.4 V at a C rate not less than 0.5 (75 mA/g-LFP) and not more
than 1.5 (225 mA/g-LFP); 2) charging the cell from 3.2-3.4 V to
maximum cell voltage of 3.6-3.7 V at a C rate of not less than 0.2
(30 mA/g-LFP) and not more than 0.5 (75 mA/g-LFP); and 3)
discharging the cell from 3.6-3.7 V to 3.2-3.4 V at a C rate of not
less than 0.2 (30 mA/g-LFP) and not more than 0.5 (75
mA/g-LFP).
13. The method of claim 12, further comprising the steps of: 4)
charging the cell from 3.2-3.4 V to maximum cell voltage of 3.6-3.7
V at a C rate of not less than 0.5 (75 mA/g-LFP) and not more than
0.75 (112.5 mA/g-LFP); and 5) discharging the cell from 3.6-3.7 V
to 3.2-3.4 V at a C rate of not less than 0.5 (75 mA/g-LFP) and not
more than 0.75 (112.5 mA/g-LFP).
14. The method of claim 13, further comprising the steps of: 6)
charging the cell from 3.2-3.4 V to maximum cell voltage of 3.6-3.7
V at a C rate of not less than 0.75 (112.5 mA/g-LFP) and not more
than 1.2 (180 mA/g-LFP); and 7) discharging the cell from 3.6-3.7 V
to 3.2-3.4 V at a C rate of not less than 0.75 (112.5 mA/g-LFP) and
not more than 1.2 (180 mA/g-LFP).
15. The method of claim 14, further comprising the steps of: 8)
charging the cell from 3.2-3.4 V to maximum cell voltage of 3.6-3.7
V at a C rate of not less than 1.2 (180 mA/g-LFP) and not more than
1.5 (225 mA/g-LFP); and 9) discharging the cell from 3.6-3.7 V to
3.2-3.4 V at a C rate of not less than 1.2 (180 mA/g-LFP) and not
more than 1.5 (225 mA/g-LFP).
16. The method of claim 15, further comprising the steps of: 10)
charging the cell from 3.2-3.4 V to maximum cell voltage of 3.6-3.7
V at a C rate of not less than 1.5 (225 mA/g-LFP) and not more than
2.0 (300 mA/g-LFP); and 11) discharging the cell from 3.6-3.7 V to
2.5 V at a C rate not less than 0.5 (75 mA/g-LFP) and not more than
1.5 (225 mA/g-LFP).
17. The method of claim 1, wherein the cell comprises
LiNi.sub.xCo.sub.yAl.sub.1-x-yO.sub.2 (NCA)/Graphite, y.ltoreq.0.3
and the method comprises the steps of: 1) charging the cell from
open-circuit voltage (OVC) of .about.3 V up to 3.7-3.9 V at a C
rate not less than 0.5 (100 mA/g-NCA) and not more than 1.5 (300
mA/g-NCA); 2) charging the cell from 3.7-3.9 V to maximum cell
voltage of 4.2-4.3 V at a C rate of not less than 0.2 (40 mA/g-NCA)
and not more than 0.5 (100 mA/g-NCA); and 3) discharging the cell
from 4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.2 (40
mA/g-NCA) and not more than 0.5 (100 mA/g-NCA).
18. The method of claim 17, further comprising the steps of: 4)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.5 (100 mA/g-NCA) and not more than
0.75 (150 mA/g-NCA); and 5) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 0.5 (100 mA/g-NCA) and not
more than 0.75 (150 mA/g-NCA).
19. The method of claim 18, further comprising the steps of: 6)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.75 (150 mA/g-NCA) and not more
than 1.2 (240 mA/g-NCA); and 7) discharging the cell from 4.2-4.3 V
to 3.7-3.9 V at a C rate of not less than 0.75 (150 mA/g-NCA) and
not more than 1.2 (240 mA/g-NCA).
20. The method of claim 19, further comprising the steps of: 8)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.2 (240 mA/g-NCA) and not more than
1.5 (300 mA/g-NCA); and 9) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 1.2 (240 mA/g-NCA) and not
more than 1.5 (300 mA/g-NCA).
21. The method of claim 20, further comprising the steps of: 10)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.5 (300 mA/g-NCA) and not more than
2.0 (400 mA/g-NCA); and 11) discharging the cell from 4.2-4.3 V to
2.5 V at a C rate not less than 0.5 (100 mA/g-NCA) and not more
than 1.5 (300 mA/g-NCA).
22. The method of claim 1, wherein the cell comprises LiCoO.sub.2
(LCO)/graphite and the method comprises the steps of: 1) charging
the cell from open-circuit voltage (OVC) of .about.3 V up to
3.7-3.9 V at a C rate not less than 0.5 (70 mA/g-LCO) and not more
than 1.5 (210 mA/g-LCO); 2) charging the cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.2
(28 mA/g-LCO) and not more than 0.5 (70 mA/g-LCO); and 3)
discharging the cell from 4.2-4.3 V to 3.7-3.9 V at a C rate of not
less than 0.2 (28 mA/g-LCO) and not more than 0.5 (70
mA/g-LCO).
23. The method of claim 22, further comprising the steps of: 4)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.5 (70 mA/g-LCO) and not more than
0.75 (105 mA/g-LCO); and 5) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 0.5 (70 mA/g-LCO) and not
more than 0.75 (105 mA/g-LCO).
24. The method of claim 23, further comprising the steps of: 6)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.75 (105 mA/g-LCO) and not more
than 1.2 (168 mA/g-LCO); and 7) discharging the cell from 4.2-4.3 V
to 3.7-3.9 V at a C rate of not less than 0.75 (105 mA/g-LCO) and
not more than 1.2 (168 mA/g-LCO).
25. The method of claim 24, further comprising the steps of: 8)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.2 (168 mA/g-LCO) and not more than
1.5 (210 mA/g-LCO); and 9) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 1.2 (168 mA/g-LCO) and not
more than 1.5 (210 mA/g-LCO).
26. The method of claim 25, further comprising the steps of: 10)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.5 (210 mA/g-LCO) and not more than
2.0 (280 mA/g-LCO); and 11) discharging the cell from 4.2-4.3 V to
2.5 V at a C rate not less than 0.5 (70 mA/g-LCO) and not more than
1.5 (210 mA/g-LCO).
27. The method of claim 1, wherein the cell comprises
Li.sub.1+xNi.sub.yMn.sub.zCo.sub.1-x-y-zO.sub.2 (NMC)/graphite,
0<x.ltoreq.0.2. z.gtoreq.0.5 and the method comprises the steps
of: 1) charging the cell from open-circuit voltage (OVC) of
.about.3 V up to 3.9-4.1 V at a C rate not less than 0.5 (115
mA/g-NMC) and not more than 1.5 (345 mA/g-NMC); 2) charging the
cell from 3.9-4.1 V to maximum cell voltage of 4.3-4.5 V at a C
rate of not less than 0.2 (46 mA/g-NMC) and not more than 0.5 (115
mA/g-NMC); and 3) discharging the cell from 4.3-4.5 V to 3.9-4.1 V
at a C rate of not less than 0.2 (46 mA/g-NMC) and not more than
0.5 (115 mA/g-NMC).
28. The method of claim 27, further comprising the steps of: 4)
charging the cell from 3.9-4.1 V to maximum cell voltage of 4.3-4.5
V at a C rate of not less than 0.5 (115 mA/g-NMC) and not more than
0.75 (172.5 mA/g-NMC); and 5) discharging the cell from 4.3-4.5 V
to 3.9-4.1 V at a C rate of not less than 0.5 (115 mA/g-NMC) and
not more than 0.75 (172.5 mA/g-NMC).
29. The method of claim 28, further comprising the steps of: 6)
charging the cell from 3.9-4.1 V to maximum cell voltage of 4.3-4.5
V at a C rate of not less than 0.75 (172.5 mA/g-NMC) and not more
than 1.2 (276 mA/g-NMC); and 7) discharging the cell from 4.3-4.5 V
to 3.9-4.1 V at a C rate of not less than 0.75 (172.5 mA/g-NMC) and
not more than 1.2 (276 mA/g-NMC).
30. The method of claim 29, further comprising the steps of: 8)
charging the cell from 3.9-4.1 V to maximum cell voltage of 4.3-4.5
V at a C rate of not less than 1.2 (276 mA/g-NMC) and not more than
1.5 (345 mA/g-NMC); and 9) discharging the cell from 4.3-4.5 V to
3.9-4.1 V at a C rate of not less than 1.2 (276 mA/g-NMC) and not
more than 1.5 (345 mA/g-NMC).
31. The method of claim 30, further comprising the steps of: 10)
charging the cell from 3.9-4.1 V to maximum cell voltage of 4.3-4.5
Vat a C rate of not less than 1.5 (345 mA/g-NMC) and not more than
2.0 (460 mA/g-NMC); and 11) discharging the cell from 4.3-4.5 V to
2.5 V at a C rate not less than 0.5 (115 mA/g-NMC) and not more
than 1.5 (345 mA/g-NMC).
32. The method of claim 1, wherein the cell comprises
LiNi.sub.xFe.sub.yAl.sub.1-x-yO.sub.2 (NFA)/graphite,
0.25.ltoreq.x.ltoreq.0.85 and the method comprises the steps of: 1)
charging the cell from open-circuit voltage (OVC) of .about.3 V up
to 3.7-3.9 V at a C rate not less than 0.5 (100 mA/g-NFA) and not
more than 1.5 (300 mA/g-NFA); 2) charging the cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
0.2 (40 mA/g-NFA) and not more than 0.5 (100 mA/g-NFA); and 3)
discharging the cell from 4.2-4.3 V to 3.7-3.9 V at a C rate of not
less than 0.2 (40 mA/g-NFA) and not more than 0.5 (100
mA/g-NFA).
33. The method of claim 32, further comprising the steps of: 4)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.5 (100 mA/g-NFA) and not more than
0.75 (150 mA/g-NFA); and 5) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 0.5 (100 mA/g-NFA) and not
more than 0.75 (150 mA/g-NFA).
34. The method of claim 33, further comprising the steps of: 6)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 0.75 (150 mA/g-NFA) and not more
than 1.2 (240 mA/g-NFA); and 7) discharging the cell from 4.2-4.3 V
to 3.7-3.9 V at a C rate of not less than 0.75 (150 mA/g-NFA) and
not more than 1.2 (240 mA/g-NFA).
35. The method of claim 34, further comprising the steps of: 8)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.2 (240 mA/g-NFA) and not more than
1.5 (300 mA/g-NFA); and 9) discharging the cell from 4.2-4.3 V to
3.7-3.9 V at a C rate of not less than 1.2 (240 mA/g-NFA) and not
more than 1.5 (300 mA/g-NFA).
36. The method of claim 35, further comprising the steps of: 10)
charging the cell from 3.7-3.9 V to maximum cell voltage of 4.2-4.3
V at a C rate of not less than 1.5 (300 mA/g-NFA) and not more than
2.0 (400 mA/g-NFA); and 11) discharging the cell from 4.2-4.3 V to
2.5 V at a C rate not less than 0.5 (100 mA/g-NFA) and not more
than 1.5 (300 mA/g-NFA).
37. The method of claim 1 wherein the total time is less than 48
hours.
38. A battery produced by the method of claim 1, wherein the
battery has no less than 95% rated capacity retention after 100
0.33 C/-0.33 C cycles.
39. The battery of claim 38, wherein the battery has no less than
80% rated capacity retention after 1000 0.33 C/-0.33 C cycles.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No. 16/225,889 filed Dec. 19, 2018, which
claims priority to U.S. Provisional Patent Application No.
62/609,376 filed on Dec. 22, 2017, entitled "Fast Formation Cycling
for Rechargeable Batteries", the entire disclosures of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention is directed to the manufacture of
rechargeable batteries, and more particularly to protocols for the
formation of a stable solid electrolyte interphase (SEI).
BACKGROUND OF THE INVENTION
[0004] Lithium-ion batteries (LIBs) are common power sources for
portable electric devices and attractive for electric vehicle
applications. Increasing energy density of LIBs has been a major
focus of recent research, with many scientists developing and
improving cathode materials (e.g. higher nickel contents) and anode
materials (e.g. silicon or tin composites) for high voltage and
high energy LIBs. Concurrently, reducing LIB production cost
without sacrificing cell performance is another focus especially
for electrical vehicle applications. One of the largest
contributors to processing cost during LIB production is the
electrolyte interphase formation step.
[0005] The solid electrolyte interphase (SEI) acts as a protective
layer to impede continuous electrolyte decomposition and solvent
co-intercalation into graphitic layers during subsequent cycles.
This passivation layer is electrically resistive but ionically
conductive. Imperfect SEI layers could expose fresh graphite to the
electrolyte, cause continuous electrolyte decomposition, and lead
to graphite exfoliation. The cell formation protocol is, therefore,
essential to create a stable SEI layer and minimize active lithium
loss, electrolyte depletion, and capacity fade over the lifetime of
the battery.
[0006] The anode solid electrolyte interphase (SEI) and cathode
electrolyte interphase (CEI) form when the electrolyte is
accessible to electrons at the electrode and, simultaneously the
electrolyte experiences an unstable voltage range. During a
charging cycle, the electrolyte decomposes and precipitates at low
potentials at the anode via reduction reactions and at high
potential on cathode via oxidation reactions. Irreversible capacity
loss indicating electrolyte interphase formation is the highest
after the first charge/discharge cycle (ca. 10% in the case of
graphite anode), significantly lower after the second cycle, and
even lower after the third cycle and so on (less than 0.05%). The
irreversible capacity loss varies depending on such factors as the
negative-to-positive capacity ratio, electrode materials, surface
area of particles, electrolyte, and operation conditions.
[0007] Most solid electrolyte interphase (SEI) forms during the
first charge/discharge cycle because the pristine anode and cathode
do not have previously formed passivation layers that
electronically insulate the electrode from the electrolyte. If
after the first cycle, the anode graphite was not significantly
exfoliated, further cycling results in significantly lower
electrolyte interphase formation because the preformed interphase
layer (from the first charging cycle) impedes solvent molecule
diffusion towards the electrode surface and electron transfer
between the electrode and electrolyte.
[0008] The manufacturing cost of cell formation scales with the
required length of time. Besides material cost, the electrolyte
wetting and SEI formation steps are the most expensive processes
because of the slow wetting and slow charge/discharge rates, for
example 3-5 cycles at C-rate of C/20 and 3-5 cycles at higher
C-rate at a higher temperature. This process may take up to 1.5-3
weeks, depending on the cell manufacturer and cell chemistry,
requiring a tremendous number of charge/discharge cycles for mass
production of LIBs, large floor space, and intense energy for the
cyclers and environmental chambers. These processes are a major
production bottleneck; therefore, it is important to reduce wetting
and formation time for cost and production rate benefits.
[0009] Complete wetting of the active material and separator with
electrolyte is typically a slow process (hours to days). After
wetting, the first charge/discharge cycle(s) is at low rate to
ensure formation of a robust SEI and avoid lithium plating. Typical
formation protocols found in the academic literature include 3 to 5
cycles at a C-rate of C/10 to C/20. In an industrial setting, the
formation protocol may be faster but remains a bottleneck for
production. The formation step in battery manufacturing requires a
tremendous number of battery cyclers, which occupy a sizeable
footprint and consume considerable energy. Therefore, it is
important to reduce formation time to increase production rate and
lower cost.
[0010] There have been several electrolyte interphase formation
studies that attempted to reduce the required time. For example,
skipping the high state-of-charge (SOC) region reduced formation
time, but it also resulted in a decrease in capacity retention.
Increasing C-rates also reduced formation time. However, it
generally caused negative effects on electrolyte interphase
formation such as non-uniform thickness and discontinuity of the
layer on the anode. Formation at high voltage (4.2 V) has rarely
been reported, although high-voltage operation is beneficial for
high-energy batteries.
SUMMARY OF THE INVENTION
[0011] A method for fast formation cycling for rechargeable
batteries comprising the steps of: step 1 (First Partial
Charge)--charge cell from open-circuit voltage (OCV) up to 80-90%
of an upper cutoff voltage (UCV) of from 4-5 V at a C rate not less
than 0.5 C and not more than 1.5 C; step 2 (First Shallow
Charge)--charge cell from 80-90% of UCV to 97-100% of UCV at a C
rate of not less than 0.2 C and not more than 0.5 C; step 3 (First
Shallow Discharge)--discharge cell from 97-100% of UCV to 80-90% of
UCV at a C rate of not less than 0.2 C and not more than 0.5 C; and
step 4 (Subsequent Charge/Discharge Cycles)--repeat steps 2-3 up to
2-10 times where the charging and discharging rates are
progressively increased by 25-75%.
[0012] The invention is suitable for a number of lithium battery
cathode chemistries combined with graphite. These include
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, where
x.ltoreq.1.0 and y.ltoreq.1.0,
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, where
x.ltoreq.0.5 and y.ltoreq.0.4,
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, where
0.5.ltoreq.x.ltoreq.0.8 and 0.1.ltoreq.y.ltoreq.0.4, LiFePO.sub.4,
LiNi.sub.xCo.sub.yAl.sub.1-x-yO.sub.2 where 0.5<x.ltoreq.0.85
and 0.1.ltoreq.y<0.25, LiCoO.sub.2,
Li.sub.1+xNi.sub.yMn.sub.zCo.sub.1-x-y-zO.sub.2, where
0.01.ltoreq.x.ltoreq.0.2, 0.1.ltoreq.y<0.3 and
0.4<z.ltoreq.0.65, and LiNi.sub.xFe.sub.yAl.sub.1-x-yO.sub.2
where 0.5<x.ltoreq.0.8 and 0.1.ltoreq.y<0.4. Other battery
chemistries are possible, especially at the anode where the
graphite may be mixed with up to 80 wt % Si.
[0013] A battery made according to the method of the invention is
also disclosed. The performance of cells should preferably meet
acceptable standards. These include no less than 95% rated capacity
retention after 100 0.33 C/-0.33 C cycles, and no less than 80%
rated capacity retention after 1000 0.33 C/-0.33 C cycles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] There are shown in the drawings embodiments that are
presently preferred it being understood that the invention is not
limited to the arrangements and instrumentalities shown,
wherein:
[0015] FIG. 1A and FIG. 1B present a typical cathode potential
(.mu..sub.C), anode potential (.mu..sub.A), and voltage between
anode and cathode (V.sub.OC) from a three-electrode pouch cell
(graphite anode/Li
reference/Li.sub.1.02Ni.sub.0.50Mn.sub.0.29Co.sub.0.19O.sub.2
cathode) with potential ranges where the electrolyte is not stable.
FIG. 1A is cell voltage profiles from a baseline and FIG. 1B is SEI
formation protocol according to the invention.
[0016] FIG. 2A is voltage profiles for three baseline formation
protocols (extended times in dark gray) and three alternative
protocols (C-rates denoted with "a" and shortened times in light
gray). FIG. 2B is corresponding formation times.
[0017] FIG. 3A is average discharge capacities with 90% confidence
intervals using different formation protocols during formation
cycling (several alternative protocols denoted with "a" compared to
C/20 baseline). FIG. 3B is post-formation rate capability
testing.
[0018] FIG. 4A and FIG. 4C are discharge capacity and FIG. 4B and
FIG. 4D are discharge capacity retention during aging for each 1 C
and -1 C cycle (FIG. 4A, FIG. 4B) and each C/5 and -C/5 loop (FIG.
4C and FIG. 4D) where each loop is 50 cycles.
[0019] FIG. 5 is the equivalent circuit model used in this
impedance analysis.
[0020] FIGS. 6A-6F are electrochemical impedance spectroscopy (EIS)
Nyquist plots from cells with different formation protocols (FIG.
6A, FIG. 6C, and FIG. 6E) near 3.9 V.+-.0.05 V during discharge;
average areal specific resistances (ASR) of F@C/20 and F@C/5a (FIG.
6B, FIG. 6D, and FIG. 6F) at different voltages before aging cycles
(FIG. 6B), after 300 cycles (FIG. 6D), and after 1300 cycles (FIG.
6F).
[0021] FIG. 7 (A-E) shows voltage profiles of NMC 811/Graphite
cells with different formation protocols for FIG. 7A F_86h cell;
FIG. 7B F_30h cell; FIG. 7C F_26 h cell; FIG. 7D F_10h cell and
FIG. 7E F_10 h@ 40 cell. FIG. 7F shows the coulombic efficiency of
each cell during the first formation cycle (light gray) and their
total efficiency during the formation steps (dark gray).
[0022] FIG. 8A is a plot of rate performance of NMC811/Graphite
cells with different formation protocols. FIG. 8B is a plot of
capacity retention of NMC811/Graphite cells with different
formation protocols. FIG. 8C is a plot of discharge capacities of
NMC811/Graphite cells with different formation protocols. Voltage
window: 3.0-4.2V
[0023] FIG. 9A is Nyquist plots of NMC811/Graphite cells after
formation cycles and FIG. 9B is after aging cycles. FIG. 9C is the
area specific resistance (ASI) of F_86h and F_30h cells during
aging cycles. FIG. 9D is the discharge capacity of cells after
aging cycles at 0.1 C rate.
[0024] FIG. 10 is an equivalent circuit used to model the impedance
spectra, where R is a resistor, Q is a constant phase element and W
is the Warburg element.
[0025] FIG. 11 A-C are XPS sputter depth profiles of cycled anodes.
FIG. 11A is lithium composition from light spots (where lithium
plated, dashed lines) and dark spots (where lithium did not plate,
solid lines) on F_10h and F_10 h@40 anodes. Surface C, O, Li, and P
compositions are shown in FIG. 11B for F_86h anodes; in FIG. 11C
for F_30h anodes; in FIG. 11D for F_26h anode; in FIG. 11E for
F_10h anodes, dark spots; and in FIG. 11F for F_10 h@40 anodes,
dark spots.
[0026] FIG. 12A is core level spectra of carbon from light spots
(where lithium plated) and dark spots (where lithium did not plate)
on F_10h and F_10 h@40 anodes. First derivative of XPS sputter
depth profiles for surface C, O, Li, and P atomic compositions are
provided for FIG. 12B for F_86h anodes; FIG. 12C for F_30h anodes;
FIG. 12D for F_26h anode; FIG. 12E for F_10h anodes, dark spots;
and FIG. 12F for F_10h@40 anodes, dark spots.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The formation process for lithium ion batteries typically
takes several days or more, and it is necessary for providing a
stable solid electrolyte interphase on the anode (at low potentials
vs. Li/Li.sup.+) for preventing irreversible consumption of
electrolyte and lithium ions. An analogous layer known as the
cathode electrolyte interphase layer forms at the cathode at high
potentials vs. Li/Li.sup.+. However, several days, or even up to a
week or more, of these processes result in either lower lithium-ion
battery (LIB) production rates or a prohibitively large size of
charging-discharging equipment and space (i.e. excessive capital
cost).
[0028] The protocol of the invention can be described generally to
comprise the following steps:
[0029] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) up to 80-90% of an upper cutoff voltage (UCV) of from
4-5 V at a C rate not less than 0.5 C and not more than 1.5 C.
[0030] Step 2 (First Shallow Charge)--charge cell from 80-90% of
UCV to 97-100% of UCV at a C rate of not less than 0.2 C and not
more than 0.5 C.
[0031] Step 3 (First Shallow Discharge)--discharge cell from
97-100% of UCV to 80-90% of UCV at a C rate of not less than 0.2 C
and not more than 0.5 C.
[0032] Step 4 (Subsequent Charge/Discharge Cycles)--repeat steps
2-3 up to 2-10 times where the charging and discharging rates are
progressively increased by 25-75%.
[0033] The invention is suitable for a number of lithium battery
cathode chemistries combined with graphite. These include
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, where
x.ltoreq.1.0 and y.ltoreq.1.0,
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, where
x.ltoreq.0.5 and y.ltoreq.0.4,
LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite, where
0.5<x.ltoreq.0.8 and 0.1.ltoreq.y<0.4, LiFePO.sub.4,
LiNi.sub.xCo.sub.yAl.sub.1-x-yO.sub.2 where 0.5<x.ltoreq.0.85
and 0.1.ltoreq.y<0.25, LiCoO.sub.2,
Li.sub.1+xNi.sub.yMn.sub.zCo.sub.1-x-y-zO.sub.2, where
0.01.ltoreq.x.ltoreq.0.2, 0.1.ltoreq.y<0.3 and
0.4<z.ltoreq.0.65, and LiNi.sub.xFe.sub.yAl.sub.1-x-yO.sub.2
where 0.5<x.ltoreq.0.8 and 0.1.ltoreq.y<0.4. Other battery
chemistries are possible, especially at the anode where the
graphite may be mixed with up to 80 wt % Si.
[0034] The performance of cells should preferably meet acceptable
standards. These include no less than 95% rated capacity retention
after 100 0.33 C/-0.33 C cycles, and no less than 80% rated
capacity retention after 1000 0.33 C/-0.33 C cycles.
[0035] The invention reduces the cycling time substantially.
Cycling times in a range of 150-250 hours can be reduced to 10-50
hours. Cycling times for the invention can be 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
49, and 50 hours, or within a range of any high value and low value
selected from these values.
[0036] The above general protocol can be further refined for
specific battery chemistry. Additional steps can also be added.
There are presented below some examples for different battery
chemistries.
[0037] LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite,
x.ltoreq.0.5, y.ltoreq.0.4.
[0038] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.3 V up to 3.7-3.9 V at a C rate not less
than 0.5 C (80 mA/g-NMC) and not more than 1.5 C (240
mA/g-NMC).
[0039] Step 2 (First Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.2
C (32 mA/g-NMC) and not more than 0.5 C (80 mA/g-NMC).
[0040] Step 3 (First Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.2 C (32
mA/g-NMC) and not more than 0.5 C (80 mA/g-NMC).
[0041] Step 4 (Second Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
0.5 C (80 mA/g-NMC) and not more than 0.75 C (120 mA/g-NMC).
[0042] Step 5 (Second Shallow Discharge--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.5 C (80
mA/g-NMC) and not more than 0.75 C (120 mA/g-NMC).
[0043] Step 6 (Third Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.75
C (120 mA/g-NMC) and not more than 1.2 C (192 mA/g-NMC).
[0044] Step 7 (Third Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.75 C (120
mA/g-NMC) and not more than 1.2 C (192 mA/g-NMC).
[0045] Step 8 (Fourth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.2 C (192 mA/g-NMC) and not more than 1.5 C (240 mA/g-NMC).
[0046] Step 9 (Fourth Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 1.2 C (192
mA/g-NMC) and not more than 1.5 C (240 mA/g-NMC).
[0047] Step 10 (Fifth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.5 C (240 mA/g-NMC) and not more than 2.0 C (320 mA/g-NMC).
[0048] Step 11 (Fifth Full Discharge)--discharge cell from 4.2-4.3
V to 2.5 V at a C rate not less than 0.5 C (80 mA/g-NMC) and not
more than 1.5 C (240 mA/g-NMC).
[0049] LiNi.sub.xMn.sub.yCo.sub.1-x-yO.sub.2 (NMC)/Graphite,
0.5<x.ltoreq.0.8, 0.1.ltoreq.y<0.4.
[0050] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.3 V up to 3.7-3.9 V at a C rate not less
than 0.5 C (95 mA/g-NMC) and not more than 1.5 C (285
mA/g-NMC).
[0051] Step 2 (First Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.4 V at a C rate of not less than 0.2
C (38 mA/g-NMC) and not more than 0.5 C (95 mA/g-NMC).
[0052] Step 3 (First Shallow Discharge)--discharge cell from
4.2-4.4 V to 3.7-3.9 V at a C rate of not less than 0.2 C (38
mA/g-NMC) and not more than 0.5 C (95 mA/g-NMC).
[0053] Step 4 (Second Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.4 V at a C rate of not less than
0.5 C (95 mA/g-NMC) and not more than 0.75 C (142.5 mA/g-NMC).
[0054] Step 5 (Second Shallow Discharge)--discharge cell from
4.2-4.4 V to 3.7-3.9 V at a C rate of not less than 0.5 C (95
mA/g-NMC) and not more than 0.75 C (142.5 mA/g-NMC).
[0055] Step 6 (Third Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.4 V at a C rate of not less than 0.75
C (142.5 mA/g-NMC) and not more than 1.2 C (228 mA/g-NMC).
[0056] Step 7 (Third Shallow Discharge)--discharge cell from
4.2-4.4 V to 3.7-3.9 V at a C rate of not less than 0.75 C (142.5
mA/g-NMC) and not more than 1.2 C (228 mA/g-NMC).
[0057] Step 8 (Fourth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.4 V at a C rate of not less than
1.2 C (228 mA/g-NMC) and not more than 1.5 C (285 mA/g-NMC).
[0058] Step 9 (Fourth Shallow Discharge)--discharge cell from
4.2-4.4 V to 3.7-3.9 V at a C rate of not less than 1.2 C (228
mA/g-NMC) and not more than 1.5 C (285 mA/g-NMC).
[0059] Step 10 (Fifth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.4 V at a C rate of not less than
1.5 C (285 mA/g-NMC) and not more than 2.0 C (380 mA/g-NMC).
[0060] Step 11 (Fifth Full Discharge)--discharge cell from 4.2-4.4
V to 2.5 V at a C rate not less than 0.5 C (95 mA/g-NMC) and not
more than 1.5 C (285 mA/g-NMC).
[0061] LiFePO.sub.4 (LFP)/Graphite
[0062] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.2.5 V up to 3.2-3.4 V at a C rate not less
than 0.5 C (75 mA/g-LFP) and not more than 1.5 C (225
mA/g-LFP).
[0063] Step 2 (First Shallow Charge)--charge cell from 3.2-3.4 V to
maximum cell voltage of 3.6-3.7 V at a C rate of not less than 0.2
C (30 mA/g-LFP) and not more than 0.5 C (75 mA/g-LFP).
[0064] Step 3 (First Shallow Discharge)--discharge cell from
3.6-3.7 V to 3.2-3.4 V at a C rate of not less than 0.2 C (30
mA/g-LFP) and not more than 0.5 C (75 mA/g-LFP).
[0065] Step 4 (Second Shallow Charge)--charge cell from 3.2-3.4 V
to maximum cell voltage of 3.6-3.7 V at a C rate of not less than
0.5 C (75 mA/g-LFP) and not more than 0.75 C (112.5 mA/g-LFP).
[0066] Step 5 (Second Shallow Discharge)--discharge cell from
3.6-3.7 V to 3.2-3.4 V at a C rate of not less than 0.5 C (75
mA/g-LFP) and not more than 0.75 C (112.5 mA/g-LFP).
[0067] Step 6 (Third Shallow Charge)--charge cell from 3.2-3.4 V to
maximum cell voltage of 3.6-3.7 Vat a C rate of not less than 0.75
C (112.5 mA/g-LFP) and not more than 1.2 C (180 mA/g-LFP).
[0068] Step 7 (Third Shallow Discharge)--discharge cell from
3.6-3.7 V to 3.2-3.4 V at a C rate of not less than 0.75 C (112.5
mA/g-LFP) and not more than 1.2 C (180 mA/g-LFP).
[0069] Step 8 (Fourth Shallow Charge)--charge cell from 3.2-3.4 V
to maximum cell voltage of 3.6-3.7 V at a C rate of not less than
1.2 C (180 mA/g-LFP) and not more than 1.5 C (225 mA/g-LFP).
[0070] Step 9 (Fourth Shallow Discharge)--discharge cell from
3.6-3.7 V to 3.2-3.4 V at a C rate of not less than 1.2 C (180
mA/g-LFP) and not more than 1.5 C (225 mA/g-LFP).
[0071] Step 10 (Fifth Shallow Charge)--charge cell from 3.2-3.4 V
to maximum cell voltage of 3.6-3.7 V at a C rate of not less than
1.5 C (225 mA/g-LFP) and not more than 2.0 C (300 mA/g-LFP).
[0072] Step 11 (Fifth Full Discharge)--discharge cell from 3.6-3.7
V to 2.5 V at a C rate not less than 0.5 C (75 mA/g-LFP) and not
more than 1.5 C (225 mA/g-LFP).
[0073] LiNi.sub.xCo.sub.yAl.sub.1-x-yO.sub.2 (NCA)/Graphite,
0.5<x.ltoreq.0.85, 0.1.ltoreq.y<0.25
[0074] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.3 V up to 3.7-3.9 Vat a C rate not less
than 0.5 C (100 mA/g-NCA) and not more than 1.5 C (300
mA/g-NCA).
[0075] Step 2 (First Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.2
C (40 mA/g-NCA) and not more than 0.5 C (100 mA/g-NCA).
[0076] Step 3 (First Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.2 C (40
mA/g-NCA) and not more than 0.5 C (100 mA/g-NCA).
[0077] Step 4 (Second Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
0.5 C (100 mA/g-NCA) and not more than 0.75 C (150 mA/g-NCA).
[0078] Step 5 (Second Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.5 C (100
mA/g-NCA) and not more than 0.75 C (150 mA/g-NCA).
[0079] Step 6 (Third Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.75
C (150 mA/g-NCA) and not more than 1.2 C (240 mA/g-NCA).
[0080] Step 7 (Third Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.75 C (150
mA/g-NCA) and not more than 1.2 C (240 mA/g-NCA).
[0081] Step 8 (Fourth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.2 C (240 mA/g-NCA) and not more than 1.5 C (300 mA/g-NCA).
[0082] Step 9 (Fourth Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 1.2 C (240
mA/g-NCA) and not more than 1.5 C (300 mA/g-NCA).
[0083] Step 10 (Fifth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.5 C (300 mA/g-NCA) and not more than 2.0 C (400 mA/g-NCA).
[0084] Step 11 (Fifth Full Discharge)--discharge cell from 4.2-4.3
V to 2.5 V at a C rate not less than 0.5 C (100 mA/g-NCA) and not
more than 1.5 C (300 mA/g-NCA).
[0085] LiCoO.sub.2 (LCO)/Graphite
[0086] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.3 V up to 3.7-3.9 V at a C rate not less
than 0.5 C (70 mA/g-LCO) and not more than 1.5 C (210
mA/g-LCO).
[0087] Step 2 (First Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.2
C (28 mA/g-LCO) and not more than 0.5 C (70 mA/g-LCO).
[0088] Step 3 (First Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.2 C (28
mA/g-LCO) and not more than 0.5 C (70 mA/g-LCO).
[0089] Step 4 (Second Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
0.5 C (70 mA/g-LCO) and not more than 0.75 C (105 mA/g-LCO).
[0090] Step 5 (Second Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.5 C (70
mA/g-LCO) and not more than 0.75 C (105 mA/g-LCO).
[0091] Step 6 (Third Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.75
C (105 mA/g-LCO) and not more than 1.2 C (168 mA/g-LCO).
[0092] Step 7 (Third Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.75 C (105
mA/g-LCO) and not more than 1.2 C (168 mA/g-LCO).
[0093] Step 8 (Fourth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.2 C (168 mA/g-LCO) and not more than 1.5 C (210 mA/g-LCO).
[0094] Step 9 (Fourth Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 1.2 C (168
mA/g-LCO) and not more than 1.5 C (210 mA/g-LCO).
[0095] Step 10 (Fifth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.5 C (210 mA/g-LCO) and not more than 2.0 C (280 mA/g-LCO).
[0096] Step 11 (Fifth Full Discharge)--discharge cell from 4.2-4.3
V to 2.5 V at a C rate not less than 0.5 C (70 mA/g-LCO) and not
more than 1.5 C (210 mA/g-LCO).
[0097] Li.sub.1+xNi.sub.yMn.sub.zCo.sub.1-x-yO.sub.2
(NMC)/Graphite, 0.01.ltoreq.x.ltoreq.0.2, 0.1.ltoreq.y<0.3,
0.4<z.ltoreq.0.65.
[0098] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.3 V up to 3.9-4.1 V at a C rate not less
than 0.5 C (115 mA/g-NMC) and not more than 1.5 C (345
mA/g-NMC).
[0099] Step 2 (First Shallow Charge)--charge cell from 3.9-4.1 V to
maximum cell voltage of 4.3-4.5 V at a C rate of not less than 0.2
C (46 mA/g-NMC) and not more than 0.5 C (115 mA/g-NMC).
[0100] Step 3 (First Shallow Discharge)--discharge cell from
4.3-4.5 V to 3.9-4.1 V at a C rate of not less than 0.2 C (46
mA/g-NMC) and not more than 0.5 C (115 mA/g-NMC).
[0101] Step 4 (Second Shallow Charge)--charge cell from 3.9-4.1 V
to maximum cell voltage of 4.3-4.5 V at a C rate of not less than
0.5 C (115 mA/g-NMC) and not more than 0.75 C (172.5 mA/g-NMC).
[0102] Step 5 (Second Shallow Discharge--discharge cell from
4.3-4.5 V to 3.9-4.1 Vat a C rate of not less than 0.5 C (115
mA/g-NMC) and not more than 0.75 C (172.5 mA/g-NMC).
[0103] Step 6 (Third Shallow Charge)--charge cell from 3.9-4.1 V to
maximum cell voltage of 4.3-4.5 V at a C rate of not less than 0.75
C (172.5 mA/g-NMC) and not more than 1.2 C (276 mA/g-NMC).
[0104] Step 7 (Third Shallow Discharge)--discharge cell from
4.3-4.5 V to 3.9-4.1 V at a C rate of not less than 0.75 C (172.5
mA/g-NMC) and not more than 1.2 C (276 mA/g-NMC).
[0105] Step 8 (Fourth Shallow Charge)--charge cell from 3.9-4.1 V
to maximum cell voltage of 4.3-4.5 V at a C rate of not less than
1.2 C (276 mA/g-NMC) and not more than 1.5 C (345 mA/g-NMC).
[0106] Step 9 (Fourth Shallow Discharge)--discharge cell from
4.3-4.5 V to 3.9-4.1 V at a C rate of not less than 1.2 C (276
mA/g-NMC) and not more than 1.5 C (345 mA/g-NMC).
[0107] Step 10 (Fifth Shallow Charge)--charge cell from 3.9-4.1 V
to maximum cell voltage of 4.3-4.5 V at a C rate of not less than
1.5 C (345 mA/g-NMC) and not more than 2.0 C (460 mA/g-NMC).
[0108] Step 11 (Fifth Full Discharge)--discharge cell from 4.3-4.5
V to 2.5 V at a C rate not less than 0.5 C (115 mA/g-NMC) and not
more than 1.5 C (345 mA/g-NMC).
[0109] LiNi.sub.xFe.sub.yAl.sub.1-x-yO.sub.2 (NFA)/Graphite,
0.5<x.ltoreq.0.8, 0.1.ltoreq.y<0.4.
[0110] Step 1 (First Partial Charge)--charge cell from open-circuit
voltage (OCV) of .about.3 V up to 3.7-3.9 Vat a C rate not less
than 0.5 C (100 mA/g-NFA) and not more than 1.5 C (300
mA/g-NFA).
[0111] Step 2 (First Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.2
C (40 mA/g-NFA) and not more than 0.5 C (100 mA/g-NFA).
[0112] Step 3 (First Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.2 C (40
mA/g-NFA) and not more than 0.5 C (100 mA/g-NFA).
[0113] Step 4 (Second Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
0.5 C (100 mA/g-NFA) and not more than 0.75 C (150 mA/g-NFA).
[0114] Step 5 (Second Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.5 C (100
mA/g-NFA) and not more than 0.75 C (150 mA/g-NFA).
[0115] Step 6 (Third Shallow Charge)--charge cell from 3.7-3.9 V to
maximum cell voltage of 4.2-4.3 V at a C rate of not less than 0.75
C (150 mA/g-NFA) and not more than 1.2 C (240 mA/g-NFA).
[0116] Step 7 (Third Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 0.75 C (150
mA/g-NFA) and not more than 1.2 C (240 mA/g-NFA).
[0117] Step 8 (Fourth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.2 C (240 mA/g-NFA) and not more than 1.5 C (300 mA/g-NFA).
[0118] Step 9 (Fourth Shallow Discharge)--discharge cell from
4.2-4.3 V to 3.7-3.9 V at a C rate of not less than 1.2 C (240
mA/g-NFA) and not more than 1.5 C (300 mA/g-NFA).
[0119] Step 10 (Fifth Shallow Charge)--charge cell from 3.7-3.9 V
to maximum cell voltage of 4.2-4.3 V at a C rate of not less than
1.5 C (300 mA/g-NFA) and not more than 2.0 C (400 mA/g-NFA).
[0120] Step 11 (Fifth Full Discharge)--discharge cell from 4.2-4.3
V to 2.5 V at a C rate not less than 0.5 C (100 mA/g-NFA) and not
more than 1.5 C (300 mA/g-NFA).
[0121] Testing was performed where graphite, NMC 532, and 1.2 M
LiPF.sub.6 in ethylene carbonate: diethyl carbonate were used as
anodes, cathodes, and electrolytes, respectively. Results from
electrochemical impedance spectroscopy show the new protocol
reduced surface film (electrolyte interphase) resistances, and 1300
aging cycles show an improvement in capacity retention.
[0122] Different C-rates were evaluated with high-voltage cells
(graphite as anodes and layered oxides, NMC 532, as cathodes) and
compared with the protocols of the invention, which not only
reduced formation time, but also increased cell capacity retention.
A simple wetting process was applied in this study. C-rate tests,
aging tests, and performance checks during aging were conducted for
six different formation protocols, three baseline protocols and
three alternative protocols according to the invention.
Electrochemical impedance spectroscopy (EIS) was also measured to
investigate total resistance and resistance components.
[0123] Most SEI and CEI form at a high SOC because electrolytes
undergo more reduction reactions at anode and more oxidation
reactions at cathode. An anode SEI layer at high SOC is more
compact and stable than that at low SOC because the potentials at
high SOC result in more electrolyte instability and more lithium is
available at the anode for reduction with bulk compounds. The SOC
should remain high for a longer period of time and low for a
shorter period of time in order to have a compact and stable
electrolyte interphase layer, but the SOC should not simply be held
at a higher cut-off voltage that results in the current
(electron-flow) dropping down to nearly zero.
[0124] Typical potential profiles (cathode denoted as .mu..sub.C,
anode denoted as .mu..sub.A, and potential difference between anode
and cathode denoted as V.sub.OC) from a three-electrode pouch cell
(graphite/Li/Li.sub.1.02Ni.sub.0.50Mn.sub.0.29Co.sub.0.19O) are
illustrated in FIG. 1A showing the unstable potential ranges of the
cathode and anode. The intensity indicates the relative degree of
instability of the electrolyte. According to the invention a
protocol for electrolyte interphase formation shown in FIG. 1B is
compared with a baseline protocol, the latter of which consists of
a series of charge and discharge cycles at a constant C-rate
without any interruption between the lower and upper cut-off
voltages. The protocol of the invention, however, involves repeated
cycling within a high SOC region (after the first charge) until the
last cycle where a full discharge takes place.
[0125] FIG. 1A and FIG. 1B present a typical cathode potential
(.mu..sub.C), anode potential (.mu..sub.A), and voltage between
anode and cathode (V.sub.OC) from a three-electrode pouch cell
(graphite anode/Li
reference/Li.sub.1.02Ni.sub.0.50Mn.sub.0.29Co.sub.0.19O.sub.2
cathode) with potential ranges where the electrolyte is not stable.
FIG. 1A is cell voltage profiles from a baseline and FIG. 1B is SEI
formation protocol according to the invention.
[0126] The baseline formation protocol was evaluated with three
different equal charge and discharge C-rates: C/20, C/10, and C/5.
Rates of C/20 or C/10 are generally used for at least the first
formation cycle in standard cell manufacturing. The baseline
formation protocols were compared with the invention protocols
using the same three equal charging and discharging C-rates: C/20,
C/10, and C/5. Abbreviations are listed with their respective
descriptions in Table 1. Prior to beginning all formation cycling,
each cell was exposed to a 3-h electrolyte wetting process.
Eighteen .about.1.5 Ah pouch cells were assembled for testing using
the six different formation protocols (three pouch cells were used
or each protocol). The cell chemistry and dimensions are listed in
Table 2.
TABLE-US-00001 TABLE 1 Abbreviations used and associated formation
conditions SEI formation condition Intermediate voltage Number of
Higher turning to Lower charge/ Test group cut-off charge cut-off
discharge abbreviation C-rate voltage mode voltage cycles Baseline
F@C/20 C/20 4.2 v None 2.5 V 5 formation F@C/10 C/10 F@C/5 C/5
Alternative F@C/20a C/20 4.0 V formation F@C/10a C/10 3.9 V F@C/5a
C/5 Test group SEI formation condition abbreviation
TABLE-US-00002 TABLE 2 Cell information Size Composition (loading)
(porosity) Anode Electrode: 92 wt % A12 graphite (ConocoPhillips),
Electrode only 2 wt % C-65 carbon black (Timcal), 6 wt % 84.4 mm
.times. 56 mm .times. polyvinylidene fluoride (PVDF, Kureha 9300)
65 .mu.m (6.36 mg/cm.sup.2) Current collector: Copper foil Tab:
nickel [55%] Cathode Electrode: 90 wt %
Li.sub.1.02Ni.sub.0.50Mn.sub.0.29Co.sub.0.19O.sub.2 (NMC Electrode
only 532 or MCM 532, TODA America Inc.). 5 wt % 84.4 mm .times. 56
mm .times. power grade carbon black (Denka), 5 wt % PVDF 64 .mu.m
(12.02 mg/cm.sup.2) (Solvay Solef 5130) Current collector: Aluminum
[55%] foil Tab: Aluminum Separator
Polypropylene-polyethylene-polypropylene 89 mm .times. 61 mm
.times. (Celgard 2325) 25 .mu.m [39%] Electrolyte 1.2M LiPF.sub.6
in EC:DEC (3:7 by weight, BASF) --
[0127] Electrodes were coated and dried using a slot-die coater
(Frontier Industrial Technology) in the DOE Battery Manufacturing
R&D Facility at ORNL, but they were not calendered. Cell
assembly was completed in a dry room where the relative humidity
was held between 0.1 and 0.2% at a room temperature of 21.degree.
C. Secondary drying of the electrodes was completed overnight at 80
C under vacuum prior to assembly to minimize moisture content. The
electrolyte volume ratio used in each cell was 2.5 (ratio of
electrolyte volume to total cell pore volume) to minimize the
effect of insufficient electrolyte, and the cells were sealed under
vacuum at 700 mm Hg.
[0128] After assembly, all cells were rested for 2 h at 21.degree.
C. for the first electrolyte wetting, then placed in an
environmental chamber (ESPEC Corp.) at 30.degree. C., and connected
to a battery tester (Series 4600, Maccor Inc.). Next the cells were
charged at C/3 until the tap voltage reached 1.5 V to avoid
corrosion of the copper current collector and rested again for 1 h
for the second electrolyte wetting. The pouch cells went through
their respective series of formation cycles using the protocols
shown in Table 1 and FIG. 1B, and were subsequently evaluated at
C/5, C/2, 1 C, and 3 C for initial rate performance. Capacity fade
was measured over 1300 cycles at 1 C charge/discharge rates where 1
C was based on 160 mAh/g (normalized by the NMC 532 wt). Upper and
lower cut-off voltages were 4.4 V and 2.5 V, respectively, for all
charge discharge cycles.
[0129] EIS was measured before the aging cycles, after 300 cycles,
and after 1300 cycles to analyze resistance increases using VSP
potentiostat systems (EC-Lab, Bio-Logic Science Instruments SAS).
These measurements were performed at 25% discharge intervals and
frequencies from 400 kHz to 10 mHz with 5 mV oscillation
amplitudes. Nyquist plots were fitted using EC-Lab software
(Bio-Logic Science Instruments SAS) to analyze ohmic resistance
(R.sub.ohmic), surface film resistance (R.sub.sf), and charge
transfer resistance (R.sub.ct). All other data processing and
calculations were performed using Matlab R2016 (MathWorks,
Inc).
[0130] Most formation processes utilize three cycles or more at
C/5, C/10 or even C/20 charge and discharge rates. Five formation
cycles were conducted to confirm capacity convergence. FIG. 2A
shows experimental results of voltage profiles vs. time for the
baseline and invention protocols at different C-rates. Five
formation cycles with the baseline C/20, C/10, and C/5 charging and
discharging rates resulted in 220, 107 and 55 h, respectively,
while those with the invention C/20 (C/20a), C/10 (C/10a), and C/5
(C/5a) rates resulted in 68, 42, and 21 h as shown in FIG. 2B.
Compared to the baseline protocol, the alternatives reduced
formation time by 60% or more at each C-rate. When the invention
C/5 (C/5a) protocol is compared to the baseline C/20 protocol, a
90% reduction in formation time is realized. For the case of three
C/20 baseline cycles, the formation time with C/5a is still 6 times
faster.
[0131] FIG. 3A is average discharge capacities with 90% confidence
intervals using different formation protocols during formation
cycling. FIG. 3B is post-formation rate capability testing.
Capacities from alternative protocols in FIG. 3A show only one
value at the first cycle for each formation C-rate because the
alternative protocol contains only one full discharge.
[0132] During formation cycling, discharge capacities with the
invention formation protocol exhibited 10-20 mAh g.sup.-1 lower
than those with the baseline protocol (FIG. 3A). The capacity data
sets for the invention formation protocols are all located on cycle
#1 because the protocols contain only one full discharge step.
However, cells cycled with both the baseline and invention
protocols had similar discharge capacities during rate capability
testing as shown in FIG. 3B where error bars correspond to 90%
confidence intervals.
[0133] Initial and final capacities of cells cycled with different
formation protocols were also similar during aging (FIG. 4). FIG.
4A and FIG. 4C are discharge capacity and FIG. 4B and FIG. 4D are
discharge capacity retention during aging for each 1 C charge and
1C discharge cycle (FIG. 4A, FIG. 4B) and each C/5 and .C/5 loop
(FIG. 4C and FIG. 4D) where each loop is 50 cycles. The three
different C-rates (C/20, C/10, and C/5) for both baseline and
invention protocols did not significantly affect capacities at
C-rate and aging tests. A capacity increase occurred for each cell
after EIS measurements at 300th cycle. Capacity retention shown in
FIG. 4B after 1000 cycles at 1 C charge and discharge rates was
about 80% for cells using the baseline protocols and about 82% for
cells using the alternative protocol ones. The cyclability at C/5
charge and discharge rates is also similar with 86% capacity
retention after 1000 cycles (20th loop) as shown in FIG. 4C and
FIG. 4D. Although these results are similar when considering the
error bars of each data, the indication is that the invention
formation protocol had a positive impact on the cell performance
rather than any negative one.
[0134] The equivalent circuit model used for the EIS fitting is
shown in FIG. 5. R.sub.ohmic, R.sub.sf, and R.sub.ct represent the
ohmic resistance, surface film (electrolyte interphase) resistance,
and charge transfer resistance, respectively. On the left in FIG.
6, the EIS intercepts with the real axis in the high-frequency
region are generally considered as R.sub.ohmic, which involves
resistances from lithium ion transport through the electrolyte and
from electron transport through the electrodes, current collectors,
cables, and lead clips between the cell and potentiostat. The first
semicircles at the high-to-medium frequency region (ca. 80 k-200
Hz) are related to R.sub.sf and attributed to impedances from
lithium ion migration through the surface films. The second
semicircles at the medium-to-low frequency region (ca. 200-0.4 Hz)
are related to R.sub.ct and are impedances from charge transfer
between the liquid electrolyte and solid surface. The linear
Warburg-type elements at the low frequency region (ca. 0.4-0.01 Hz)
correspond to lithium-ion diffusion in the active material
particles, which were not included in the data fitting in this
study. In parallel to R.sub.sf and R.sub.ct in the equivalent
circuit model, CPE.sub.sf and CPE.sub.ct represent the capacitance
of the surface film and charge transfer, respectively. A constant
phase element (CPE) was applied instead of an ideal capacitor
element to account for imperfect capacitor behavior in a large,
porous electrode.
[0135] Representative impedances near 3.9 V during discharge from
F@C20, F@C/20a, F@C/5 and F@C/5a are shown in Nyquist plots on the
left side of FIG. 6. FIG. 6 is EIS Nyquist plots from cells with
different formation protocols (FIG. 6A, FIG. 6C, and FIG. 6E) near
3.9 V+0.05 V during discharge; average areal specific resistances
(ASR) of F@C/20 and F@C/5a (FIG. 6B, FIG. 6D, and FIG. 6F) at
different voltages before aging cycles (FIG. 6B), after 300 cycles
(FIG. 6D), and after 1300 cycles (FIG. 6F).
[0136] On the right side of FIG. 6, average areal specific
resistances from the EIS of two extreme cases (F@C/20 and F@C/5a)
were compared at different voltages. Resistances from F@C/5a
(fastest alternative formation protocol) before aging cycles were
slightly lower than those from F@C/20 (slowest baseline formation
protocol). As the cells were cycled, the resistances from F@C/5a
were significantly smaller than those from F@C/20. All ohmic
resistances increased slightly (by ca. 10%) after 1300 cycles while
surface film resistances and charge transfer resistances
significantly increased (by 80% or more). The increase in charge
transfer resistance was larger than that of the surface film
resistance for both F@C/20 and F@C/5a. However, F@C/5a showed
25-30% smaller surface film resistance than F@C/20 both before and
after aging cycles, implying the cells with alternative formation
cycles had more robust electrolyte interphase layers than those
with the baseline formation cycles. Regarding baseline protocols,
SEI analysis using X-ray photoelectron spectroscopy and separation
of anode and cathode resistances using three-electrode cells were
elaborated elsewhere.
[0137] The invention demonstrates the effectiveness of a formation
protocol having more (shallow) charge-discharge cycles between 3.9
V and 4.2 V and fewer (full depth of discharge) cycles below 3.9 V.
The invention formation protocol shortened formation time by 6
times or more without compromising cell performance; rather, it
improved capacity retention, which will have a significant impact
on the operating and capital cost of manufacturing LIBs. For both
protocols, the different C-rates, at least up to C/5 during
formation, did not significantly affect capacities and capacity
fades. Analysis via EIS showed substantially lower surface film
(electrolyte interphase) resistance for the cells that underwent
the fastest alternative formation protocol than those that
underwent the slowest baseline formation protocol, indicating that
the alternative protocol provided a more robust and chemically
stable electrolyte interphase layer.
[0138] Five different formation protocols were tested with
nickel-rich LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2
(NMC811)/graphite cells where the total formation time varied from
10 to 86h. Electrochemical characterization and analysis show that
very long formation times do not necessarily improve long-term
performance while very short formation protocols result in lithium
plating and poorer electrochemical performance. The formation
cycling protocol of the invention is intermediate in length to
minimize impedance growth, improve capacity retention, and avoid
lithium plating.
[0139] Five different formation protocols for large format pouch
cells, where the total formation time varied from 10 to 86h.
Materials and cell design are representative of lithium-ion
batteries with relatively high energy density and low cost. For
example, the cathode is nickel-rich
LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2 (NMC811), which delivers a
higher capacity (>185 mAh/g) over the same operating voltage
window compared to other widely used layered cathode materials such
as LiCoO.sub.2 (140 mAh/g), LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2
(160 mAh/g), and LiNi.sub.0.5Mn.sub.0.3Co.sub.0.2O.sub.2 (175
mAh/g). Moreover, replacing expensive Co with relatively
inexpensive Ni lowers the cost of raw materials.
[0140] A graphite anode was used to match the NMC 811 cathode since
graphite remains the standard active material for negative
electrodes despite considerable progress towards higher-capacity
materials such as metal oxides, graphene, Si and Li metal. Graphite
is inexpensive and non-toxic with desirable electrochemical
properties including a low average voltage, minimal hysteresis,
flat voltage profile, and an adequate specific capacity from 350 up
to 372 mAh/g.
[0141] The electrodes were coated with high areal loadings (>2
mAh/cm.sup.2), which are typical for high energy cells. While thick
electrodes are important to lower the overall cell cost, they
present new challenges for decreasing cell formation time. Ion
transport is limited through thick electrodes, which leads to
lithium plating at high charge rates. Analysis assessed the degree
of lithium plating in cells that underwent the different formation
protocols. These results show that very long formation times do not
necessarily improve long-term performance. The optimum formation
cycling protocol is intermediate in length to minimize impedance
growth, improve capacity retention, and avoids lithium plating.
[0142] Electrode fabrication and cell build were completed at the
U.S. Department of Energy (DOE) Battery Manufacturing R&D
facility at Oak Ridge National Laboratory. All chemicals were
provided by suppliers and used as received. Electrodes were
prepared by coating slurries onto metal foil current collectors (Al
for the cathode and Cu for the anode) using a pilot-scale slot-die
coater (Frontier Industrial Technology). The cathode slurry
contained 90 wt. % LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2 powder
(Targray), 5 wt. % acetylene carbon black (Denka Black), and 5 wt.
% polyvinylidene difluoride (Solvay 5130) in N-methyl-2-pyrrolidone
(NMP). The anode slurry contained 92 wt. % Superior SLC 1520T
graphite, 2 wt. % carbon black (Timcal Super C65), and 6 wt. %
polyvinylidene difluoride (Kureha 9300) in NMP. The areal capacity
of the NMC 811 cathode was 2.3 mAh/cm.sup.2 (11.5 mg/cm.sup.2 mass
loading), and the areal capacity of the graphite anode was 2.6
mAh/cm.sup.2 (8.3 mg/cm.sup.2 mass loading), yielding a negative to
positive capacity ratio (N/P ratio) of around 1.15 in the full
cells. All electrodes were calendared to 35% porosity (2.8
g/cm.sup.3 electrode density for the cathode and 1.4 g/cm.sup.3 for
the anode) after primary drying and underwent secondary drying
under vacuum at 120.degree. C. prior to cell assembly. 1.2 M
LiPF.sub.6 dissolved in ethylene carbonate: ethylmethyl carbonate
(EC:EMC=3:7 by weight, SoulBrain) was used as the electrolyte and
the electrolyte fill factor remained consistent through all cells.
Polyolefin-based separators (Celgard) with 25 .mu.m thickness and
39% porosity were used to build the cells. Single layer pouch cells
(.apprxeq.100 mAh) were assembled in a dry room (dew point of less
than -50.degree. C. and relative humidity (RH) of 0.1%).
[0143] All cells were first charged to 1.5 V to avoid corrosion of
the Cu current collector and rested for 6 h at 30.degree. C. or
40.degree. C. in an environmental chamber. Next, they went through
their respective formation cycles using the protocols summarized in
Table 3.
TABLE-US-00003 TABLE 3 Formation protocols Formation Total Protocol
Wetting Conditions Cycling Conditions Formation Time F_86 h Tap
charge to 1.5 V after C/10 CCCV Charge to 4.2 V till 86 h vacuum
seal, then rest for 6 h Current < C/20 C/10 Discharge to at
30.degree. C. 3.0 V, 4 Cycles Cycling at 30.degree. C. F_30 h Tap
charge to 1.5 V after C/2 CCCV Charge to 4.2 V till 30 h vacuum
seal, then rest for 6 h Current < C/20 C/2 Discharge to at
30.degree. C. 3.0 V, 1 Cycle C/2 CCCV Charge to 4.2 V till Current
< C/20 C/2 Discharge to 3.0 V, 1 Cycle Cycling at 30.degree. C.
F_26 h Tap charge to 1.5 V after C/10 CCCV Charge to 4.2 V till 26
h vacuum seal, then rest for 6 h Current < C/20 at 30.degree. C.
C/10 Discharge to 3.0 V, 1 Cycle Cycling at 30.degree. C. F_10 h
Tap charge to 1.5 V after C/2 CCCV Charge to 4.2 V till 10 h vacuum
seal, then rest for 6 h Current < C/20 C/10 Discharge to at
30.degree. C. 3.0 V, 1 Cycle cycling at 30.degree. C.
[0144] Following formation cycling, the cells were degassed and
resealed under vacuum. All cells were tested on a Maccor battery
cycler under 5 psi stack pressure in an Espec environmental chamber
at 30.degree. C. The cells were cycled between 3.0 and 4.2 V at C/3
charge/discharge rates with a 3-h long voltage hold at the top of
each charge to accelerate cell degradation. Tests for F_86h and
F_30h cells were also interrupted periodically by hybrid pulse
power characterization (HPPC) to obtain areal specific resistance
(ASR) information. Rate performance was also tested within the
voltage window of 3.0-4.2 V using C/5, C/3, C/2, 1 C, 2 C, 3 C, 5
C, and 10 C discharge rates with a constant charging rate of C/5. 1
C corresponds to 195 mAh/gNMc based on the discharge voltage
profiles from half coin cells with lithium metal counter
electrodes. The results presented here represent the average from
two or three cells. Bio-Logic potentiostats/galvanostats (VSP) and
EC-Lab software were used to obtain and analyze electrochemical
impedance spectra (EIS). For EIS the frequency of the excitation
signal was scanned from 500 kHz to 10 mHz with a sinus amplitude of
5 mV.
[0145] Duplicate sets of pouch cells were characterized after
formation cycling and after long-term cycling (300 cycles). The
cells were discharged to 3.0 V at C/10 and disassembled in an
argon-filled glovebox for post-mortem analysis. Optical images of
all harvested electrodes were taken through the glovebox window.
X-ray powder diffraction (XRD) was collected on a PANalytical
X'Pert system with a Cu source (.lamda.=1.54 .ANG.) operated at 45
kV and 40 mA with automatic divergence and anti-scatter slits.
X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha)
was used to analyze the electrode surface chemistry after formation
and after long-term cycling. The X-ray source was monochromatic Al
K.alpha. with 1486.6 eV photon energy and a spot size of 400 .mu.m.
The system used low energy Ar-ions and a low energy electron flood
gun for charge compensation. The harvested electrodes were lightly
rinsed with DMC solvent, dried in the glovebox, and loaded in a
vacuum transfer module to avoid air and moisture exposure. The
transfer module was directly inserted into the XPS chamber with a
base pressure of 10-9 Torr. Scanning electron microscopy (SEM,
Merlin VP, Zeiss) was used to image the electrodes.
[0146] The impact of the formation cycling protocol on SEI
properties and cell performance was explored by varying the number
of formation cycles, the cycling rates, and the electrolyte wetting
temperature. Table 1 summarizes the different formation protocols
adopted in this study. The longest formation protocol consisted of
four C/10-C/10 charge/discharge cycles from 3.0 to 4.2 V. This
protocol is similar to formation processes commonly found in the
literature and serves as a baseline. A total of approximately 86h,
including the wetting step, were required to complete the entire
baseline formation process.
[0147] Three-electrode cells with a Li metal reference electrode
were designed to monitor the potential change on both the cathode
and anode during the baseline formation process. The anode
potential stays below 240 mV after the first formation cycle. The
reduction of electrolyte and formation of the anode SEI take place
at a higher potential (0.5-1.0 V vs. Li.sup.+/Li). Since most of
the SEI is formed during the first charge, the total formation time
could not be reduced by optimizing the first one or two cycles and
eliminating the following formation cycles.
[0148] As summarized in Table 3, faster formation protocols like
F_30h and F_26h have fewer cycles and faster charge/discharge
rates, leading to >65% reduction in total formation time.
Protocol F_10h reduces the formation time drastically to 10 h with
only one high rate C/2-C/2 cycle. Protocol F_10 h@40 was identical
to F_10 h except the wetting step was performed at a higher
temperature of 40.degree. C. The cells were maintained at
30.degree. C. during wetting for all other protocols.
[0149] FIG. 7 shows the voltage profiles of NMC 811/Graphite cells
for the different formation protocols. FIG. 7 (A-E) presents
voltage profiles of NMC 811/Graphite cells with different formation
protocols for FIG. 7A F_86h cell; FIG. 7B F_30h cell; FIG. 7C F_26h
cell; FIG. 7D F_10h cell and FIG. 7E F_10 h@ 40 cell. FIG. 7F
presents the coulombic efficiency of each cell during the first
formation cycle and their total efficiency during formation steps.
Total efficiency is calculated by multiplying the coulombic
efficiency of each cycle during formation.
[0150] All cells delivered a high charge capacity around 224
mAh/gNMc during the first charge. The discharge capacities were all
below 185 mAh/gNMc in the first discharge, yielding a first cycle
Coulombic efficiency (CE) around 85%. The low CE during the first
formation cycle is expected from electrolyte reduction on the anode
and the corresponding formation of the SEI. FIG. 7F shows that a
slower C/10 rate in the first formation cycle resulted in higher CE
compared to C/2 rate. F_30h and F_26h cells have the least
cumulative irreversible capacity loss (ICL) over all the formation
steps, indicating less loss of active lithium during the critical
formation steps. The longest formation protocol (F_86h) did not
have the highest coulombic efficiency. In contrast to cells with
shorter formation protocols, the F_86h anode spent much more time
at low potentials, which potentially resulted in more parasitic
reactions, electrolyte decomposition, and SEI growth. The increase
in side-reactions is reflected in the lower total coulombic
efficiency compared to F_30h and F_26h cells.
[0151] Optical images of anodes that were harvested from cells that
were dissembled immediately after formation cycling revealed no
cracks or delamination were observed in any of the anodes. No
lithium plating was observed on the anodes if the formation cycles
were done at a low rate of C/10 (F_26h and F_86h). However, lithium
plating was observed on all anodes when the formation protocol
included high rate C/2-C/2 cycling (F_30 h, F_10 h, and F_10 h@40).
The F_30h protocol included a slow C/10-C/10 cycle after the
initial C/2-C/2 cycle, which reduced the amount of lithium plating
compared to cells that were cycled at high rate without the slower
recovery step (F_10 h and F_10h@40). This indicates that some of
the plated lithium is still electrochemically active at this early
stage and can be recovered. We note that the sequence of the
cycling conditions could also influence CE and lithium plating. For
example, if the C/10-C/10 cycle was started first in the F_30h
protocol, the CE of the first cycle is expected to be the same as
those from F_86h and F_26 h, and the lithium plating could also be
less significant. Increasing the temperature for the wetting step
did not reduce the extent of lithium plating on the surface, which
suggests that electrode wetting at 30.degree. C. for 6 h is
probably sufficient for these single layer pouch cells. Extended
wetting periods may be required for multiple layer cells especially
with low porosity. Analysis of the anodes immediately after
formation clearly shows the importance of including a relatively
slow charge/discharge cycle in the formation protocol to prevent
undesirable lithium plating, which could deplete the cell inventory
of active lithium and raise safety concerns during long-term
cycling if the plated lithium can't be re-intercalated.
[0152] FIG. 8A is a plot of rate performance of NMC811/Graphite
cells with different formation protocols. FIG. 8B is a plot of
capacity retention of NMC811/Graphite cells with different
formation protocols. FIG. 8C is a plot of discharge capacities of
NMC811/Graphite cells with different formation protocols. The
voltage window was 3.0-4.2V. Despite the significant differences in
total formation time and lithium plating behavior, all cells formed
with the various protocols exhibited similar rate performance, as
demonstrated in FIG. 8A. Each data point is the average of three
cycles from two identical pouch cells. All cells delivered a high
discharge capacity of 194 mAh/gNMc at a low discharge rate of C/5
independent of the formation protocol, which indicates the plated
lithium likely re-intercalated into active materials at such a low
rate. All cells had excellent rate capability delivering 163
mAh/gNMc at 3 C and 140 mAh/gNMc at 5 C. Thus, even at very high
discharge rates of 5 C the cells retained 72% of their full
capacity. However, further increasing the discharge rate to 10 C
caused a steep drop in capacity that was ascribed to mass transport
limitations in active materials and porous electrodes. The drop in
capacity occurred at 10 C independently of the formation protocol
and could be due to Li-ion saturation of the surface layer in the
solid phase and/or Li.sup.+ depletion in the electrolyte phase.
[0153] FIG. 9A is Nyquist plots of NMC811/Graphite cells after
formation cycles and FIG. 9B is after aging cycles. FIG. 9C is the
area specific resistance of F_86h and F_30h cells during aging
cycles. FIG. 9D is the discharge capacity of cells after aging
cycles at 0.1 C rate. FIG. 9A displays EIS results from cells after
formation cycling, which were charged to 3.8 V cell voltage for EIS
measurements. All cells exhibited similar cell impedance after
formation cycling independent of the formation protocol, consistent
with their similar rate performance.
[0154] The impact of different formation protocols on the long-term
cycling performance at C/3 rate is shown in FIG. 8B and FIG. 8C.
F_30h cells demonstrated the highest capacity (173.5 mAh/g) and
capacity retention (91.2%) after 300 aging cycles, even though the
first formation cycle was at a high rate of C/2-C/2. The slowest
formation protocol (F_86h) did not result in higher capacity
retention compared to formation cycles that were significantly
shorter (F_30h and F_26 h). F_10h and F_10h@40 had slightly lower
initial discharge capacities (3-5 mAh/g lower) compared to the
other cells due to lithium plating and active lithium loss during
the formation steps. This discharge capacity recovered slightly
during the first 10 cycles, which indicates that some of the plated
lithium was electrochemically active and stripped during the slower
aging cycles. However, F_10h and F_10 h@40 faded faster than the
cells with longer formation protocols, finishing with 89.6% and
88.1% capacity retention, respectively. Nonetheless, at the end of
300 cycles the capacity retention of all cells were within 3%
compared to the baseline F_86h cells. This is significant given
that the length of the formation protocols varied by over a factor
of 8. The choice of formation protocol had a larger impact on the
total cell capacity, which varied by 6% from 173.5 mAh/g for F_30 h
to 166.1 mAh/g for F_10 h@40 after 300 cycles.
[0155] Capacity fade is driven primarily by loss of active lithium
and impedance rise. More detailed electrochemical characterization
was carried out to distinguish contributions from the different
causes. While all the cells had similar resistance at the beginning
of life, the resistance grew at different rates depending on the
formation protocols. Hybrid pulse power characterization (HPPC)
measured the area specific resistance (ASR) of F_86h and F_30h
cells at about 50% state-of-discharge (SOD) after every 50 cycles
(FIG. 9C). The resistance of both cells started around 20 .OMEGA.
cm.sup.2 after the formation cycles, consistent with the EIS
results shown in FIG. 9A. However, F_86h cells exhibited a much
faster resistance growth than F_30h cells as cycling went on,
consistent with their faster capacity fade. The F_86h cells also
had lower coulombic efficiency during the aging cycles, which
suggests that the SEI layer formed during the long formation steps
did not passivate the anode surface as effectively, leading to
continuous parasitic reactions, SEI growth, and impedance rise.
[0156] EIS further confirmed that the formation protocol impacted
cell impedance rise. EIS was obtained on cells after 300 aging
cycles at 3.8 V cell voltage (FIG. 9B). The full cell EIS measured
here is the convoluted impedance of both the cathode and anode.
However, full cell impedance is typically dominated by the cathode
for similar cell chemistries. The high frequency arc in the Nyquist
plot arises mainly from contributions from the electronic contact
resistance between carbon additives and active material. The
mid-frequency arc corresponds to interfacial phenomenon. The cell
resistance was calculated by fitting the EIS spectra using the
equivalent circuit shown in FIG. 10, and the results are summarized
in Table 4.
TABLE-US-00004 TABLE 4 Cell resistance calculated from EIS using
the equivalent circuit provided in FIG. 10. Cell Resistance
Formation Protocol (R1 + R2 + R3/.OMEGA. cm.sup.2) F_86 h 40 F_30 h
31 F_26 h 34 F_10 h 44 F_10 H@40 42
[0157] The trends in impedance rise matched the trends in capacity
retention (FIG. 8B). F_10h and F_10 h@40 cells exhibited the
greatest impedance rise, while F_30h and F_26h cells showed the
least. F_86h cells had intermediate impedance rise, consistent with
their intermediate capacity retention. The results of both HPPC
tests and EIS measurements show that longer formation protocols
such as F_86h are not necessarily optimal for long term
performance.
[0158] Measurements at low rate minimize the impact of cell
resistance and isolate capacity fade due to loss of lithium
inventory. FIG. 9D shows the discharge voltage profile at a rate of
C/10 for all cells after aging cycles. While F_86h, F_30 h, and
F_26h cells all had a similar discharge capacity around 176 mAh/g,
F_10h and F_10h@40 cells had lower capacity (170 and 166 mAh/g,
respectively), indicating more active lithium was lost in these
cells during cycling.
[0159] After the aging cycles, anodes were recovered from each cell
in the glovebox. No obvious lithium plating was observed in F_86h
and F_26h cells, confirming that low-rate cycling during formation
effectively prevents lithium plating. More significantly, F_30h
cells showed no evidence of lithium plating after long-term
cycling, even though some lithium plated after formation. This
further confirms that some lithium plating is reversible. F_10h and
F_10 h@40 cells still showed obvious lithium plating after
long-term cycling, although it was somewhat less pronounced than
after formation. In addition, F_10 h@40 exhibited more plated
lithium than the F_10 h cell, consistent with the greater capacity
fade from loss of active lithium (FIG. 9D). The slow C/10 cycle in
the F_30h protocol was necessary to recover most plated lithium and
prevent irreversible lithium plating during extensive long-term
cycling.
[0160] The SEM images of uncycled anodes showed graphite particles
with clean surfaces. After aging cycles, all graphite surfaces were
covered by electrolyte decomposition products. The SEM images
showed that the SEI layers were thicker on F_86h graphite anodes
compared to F_30h and F_26h anodes. This difference matched well
with the impedance results, which indicated the F_86h baseline
cells had faster impedance growth, more active lithium loss, and
ended with a higher resistance. Moreover, lithium deposits were
clearly identified on cycled F_10h and F_10 h@40 graphite anodes.
The microscopy images reinforce the importance of optimizing
formation cycling to avoid thick SEI build-up and prevent lithium
plating.
[0161] To further reveal the surface chemistry differences and
estimate the thickness of SEI layers, XPS depth profiles were
employed to investigate cycled anodes. F_10h and F_10 h@40 anodes
had visually heterogenous surfaces with light spots where lithium
plating had occurred and dark spots where no obvious lithium
plating had taken place. Therefore, XPS measurements were taken on
both types of surfaces. Note that the depth was calculated based on
the assumption that the SEI has the same etching rate as a
SiO.sub.2 standard. Therefore, the reported values do not reflect
the absolute thickness of SEI layers but provide a basis for
comparison across the different anode surfaces. FIG. 11A shows the
atomic percentage of Li as a function of etching depth on both
spots of F_10h and F_10 h@40 anodes. Each electrode demonstrated a
higher Li concentration at the locations where Li metal had plated.
Lithium metal is highly reactive with the electrolyte, which leads
to more electrolyte decomposition and SEI formation at these
locations. Moreover, the C 1s signals from spots where lithium
plated (light spots) showed a larger amount of C--O and C.dbd.O
bonds due to the thicker SEI. Lithium plating also reduced the
relative intensity of the C--F.sub.x peaks from the polyvinylidene
fluoride binder (FIG. 12A). Attenuation of the signals from the
binder is additional evidence that a thicker reaction layer forms
after plating.
[0162] FIG. 11 A-C are XPS sputter depth profiles of cycled anodes.
FIG. 11A is lithium composition from light spots (where lithium
plated, dashed lines) and dark spots (where lithium did not plate,
solid lines) on F_10h and F_10 h@40 anodes. Surface C, O, Li, and P
compositions are shown in FIG. 11B for F_86h anodes; in FIG. 11C
for F_30h anodes; in FIG. 11D for F_26h anode; in FIG. 11E for
F_10h anodes, dark spots; and in FIG. 11F for F_10 h@40 anodes,
dark spots.
[0163] Elemental profiles from all five cycled anodes are displayed
in FIG. 11B-F. These depth profiles are taken from locations where
no visible Li plating occurred. Atomic compositions of four
elements (surface C, O, Li, and P) were selected since they are
directly associated with SEI components and could be used to
estimate the SEI thickness.
[0164] To define the SEI thickness, the first derivative of the
atomic percentage of the four surface elements was taken as a
function of depth, as shown in FIG. 12B-F. FIG. 12A is core level
spectra of carbon from light spots (where lithium plated) and dark
spots (where lithium did not plate) on F_10h and F_10 h@40 anodes.
First derivative of XPS sputter depth profiles for surface C, O,
Li, and P atomic compositions are provided for FIG. 12B for F_86h
anodes; FIG. 12C for F_30h anodes; FIG. 12D for F_26h anode; FIG.
12E for F_10h anodes, dark spots; and FIG. 12F for F_10 h@40
anodes, dark spots.
[0165] It was also assumed that the atomic percentage would remain
constant (first derivative .apprxeq.0) in the bulk. Based on these
assumptions, F_10 h@40h and F_10 h had the thickest SEI layers,
which were over 80 nm. This result is consistent with greater
electrolyte decomposition, higher impedance growth, and lower
capacity retention. F_86h, F_30 h, and F_26h formation protocols
all yielded much thinner SEI layers overall. However, the F_86h
anodes seem to have a thicker SEI layer compared to F_30h and F_26
h, which further indicates that an extended formation time may not
be optimal for stable SEI formation.
[0166] Possible structural changes on the cathode side were also
investigated. The XRD patterns of three cycled cathodes harvested
from F_86h, F_30 h, and F_10 h cells were compared. Although the
full cells exhibited different capacity retention, all cathodes
retained good crystallinity with no other phases (spinel or
rock-salt) identified. The high intensity ratio between (003) and
(104) peaks for all cycled cathodes, together with the well-defined
splitting of (006)/(102) and (108)/(110) doublets, confirms the
well-preserved R3m layered structure. SEM images of cycled cathodes
from F_10h cells, which exhibited a low capacity retention after
aging cycles, showed no cracks or delamination, and the primary
particles were still well agglomerated with each other, indicating
minimal active materials loss. The characterization of cycled
cathodes shows minimal degradation of the bulk active material or
electrode structure on the cathode side. Therefore, it is believed
that most of the differences in capacity retention for cells that
underwent different formation protocols arise from differences in
the passivation of the anode. The best formation protocols
passivate the anode effectively, resulting in higher coulombic
efficiency and lower impedance rise.
[0167] Faster formation protocols for NMC811/Graphite full cells
using a standard electrolyte without additives were proposed and
investigated. The shortest formation protocols (10 h) resulted in
lithium plating, faster impedance growth, and the poorest long-term
capacity and capacity retention. Formation protocols that were
intermediate in length (26-30 h) yielded the best long-term
performance with minimal impedance rise. Further increasing the
time for the formation protocol did not offer any improvements.
Instead, the conventional slow formation cycling protocol led to
slightly greater impedance rise and lower capacity retention.
Lithium plating occurred in all cells that were subjected to a fast
C/2 charge during formation. However, the lithium metal was still
electrochemically active and could be stripped provided the
following cycle was at sufficiently low rate. The best
electrochemical performance was obtained in cells where the
formation protocol began with one quick C/2 cycle followed by a
slow C/10 recovery cycle.
[0168] This invention can be embodied in other forms without
departing from the spirit or essential attributes thereof.
Reference should therefore be made to the following claims to
determine the scope of the invention.
* * * * *