U.S. patent application number 14/364326 was filed with the patent office on 2014-10-23 for positively charged silicon for lithium-ion batteries.
The applicant listed for this patent is Jean-Sebastien Bridel, Kris Driesen, Jan Gilleir, Dan V. Goia, Delphine Longrie, Nicolas Marx, John I. Njagi, Stijn Put. Invention is credited to Jean-Sebastien Bridel, Kris Driesen, Jan Gilleir, Dan V. Goia, Delphine Longrie, Nicolas Marx, John I. Njagi, Stijn Put.
Application Number | 20140315086 14/364326 |
Document ID | / |
Family ID | 48611875 |
Filed Date | 2014-10-23 |
United States Patent
Application |
20140315086 |
Kind Code |
A1 |
Put; Stijn ; et al. |
October 23, 2014 |
Positively Charged Silicon for Lithium-Ion Batteries
Abstract
This invention relates to a negative electrode material for
lithium-ion batteries comprising silicon and having a chemically
treated or coated surface influencing the zeta potential of the
surface. The active material consists of particles or particles and
wires comprising a core (11) comprising silicon, wherein the
particles have a positive zeta potential in an interval between pH
3.5 and 9.5, and preferably between pH 4 and 9.5. The core is
either chemically treated with an amino-functional metal oxide, or
the core is at least partly covered with O.sub.ySiH.sub.x groups,
with 1<x<3, 1<y<3, and x>y, or is covered by
adsorbed inorganic nanoparticles or cationic multivalent metal ions
or oxides.
Inventors: |
Put; Stijn; (Olmen, BE)
; Gilleir; Jan; (Mortsel, BE) ; Driesen; Kris;
(Hasselt, BE) ; Bridel; Jean-Sebastien; (Geel,
BE) ; Marx; Nicolas; (Geel, BE) ; Longrie;
Delphine; (Gent, BE) ; Goia; Dan V.; (Potsdam,
NY) ; Njagi; John I.; (Potsdam, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Put; Stijn
Gilleir; Jan
Driesen; Kris
Bridel; Jean-Sebastien
Marx; Nicolas
Longrie; Delphine
Goia; Dan V.
Njagi; John I. |
Olmen
Mortsel
Hasselt
Geel
Geel
Gent
Potsdam
Potsdam |
NY
NY |
BE
BE
BE
BE
BE
BE
US
US |
|
|
Family ID: |
48611875 |
Appl. No.: |
14/364326 |
Filed: |
December 13, 2012 |
PCT Filed: |
December 13, 2012 |
PCT NO: |
PCT/EP2012/075409 |
371 Date: |
June 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61570375 |
Dec 14, 2011 |
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61630873 |
Dec 21, 2011 |
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Current U.S.
Class: |
429/220 ;
427/122; 427/215; 429/218.1; 429/221; 429/223; 429/228; 429/229;
429/231; 429/231.5; 429/231.6; 429/231.8; 429/231.95 |
Current CPC
Class: |
H01M 4/0428 20130101;
H01M 4/134 20130101; H01M 4/366 20130101; H01M 4/483 20130101; H01M
4/0416 20130101; H01M 4/0404 20130101; H01M 4/38 20130101; H01M
4/62 20130101; H01M 4/622 20130101; H01M 10/0525 20130101; H01M
4/0492 20130101; H01M 4/386 20130101; H01M 4/1395 20130101; Y02E
60/10 20130101; H01M 2004/027 20130101; H01M 4/0471 20130101 |
Class at
Publication: |
429/220 ;
429/218.1; 429/229; 429/231; 429/231.8; 429/221; 429/231.5;
429/231.6; 429/228; 429/231.95; 429/223; 427/215; 427/122 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/04 20060101 H01M004/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2012 |
EP |
12164773.9 |
May 11, 2012 |
EP |
12167592.0 |
Claims
1-38. (canceled)
39. A negative electrode material for a lithium rechargeable
battery, the material comprising a core comprising silicon, wherein
the surface of the core carries O.sub.ySiH.sub.x groups, with
1<x<3, 1.ltoreq.y.ltoreq.3, and x>y, and wherein the
material has a positive zeta potential in an interval between pH
3.5 and 9.5.
40. A negative electrode material for a lithium rechargeable
battery, the material comprising a core comprising silicon, wherein
the surface of the core is at least partly covered by a coating
comprising inorganic nanoparticles, and wherein the material has a
positive zeta potential in an interval between pH 3.5 and 9.5.
41. The negative electrode material of claim 40, wherein the
inorganic nanoparticles comprise an aluminum compound, a zinc
compound or an antimony compound.
42. The negative electrode material of claim 41, wherein the
aluminum compound is either aluminum or Al.sub.2O.sub.3, the zinc
compound is either zinc or zinc oxide, and the antimony compound is
either antimony or antimony oxide.
43. The negative electrode material of claim 40, wherein the
nanoparticles form a first coating layer on the core, the first
coating layer having a thickness of less than 10 nm.
44. The negative electrode material of claim 43, wherein the
particles further comprise a second coating layer located between
the core and the nanoparticles, the second coating layer comprising
either carbon or aluminum.
45. The negative electrode material of claim 43, wherein the first
coating layer has a thickness between 1 and 5 nm.
46. The negative electrode material of claim 43, wherein the first
coating layer is either conformal or porous.
47. The negative electrode material of claim 44, wherein either one
or both of the first and second coating layer is electrochemically
active.
48. The negative electrode material of claim 40, wherein the
nanoparticles comprise a precursor material susceptible of being
converted to aluminum, zinc or antimony by reduction.
49. A negative electrode material for a lithium rechargeable
battery, the material comprising a core comprising silicon, wherein
the surface of the core is at least partly covered by adsorbed
cationic multivalent metal ions, and wherein the material has a
positive zeta potential in an interval between pH 3.5 and 9.5.
50. The negative electrode material of claim 49, wherein the metal
ions are selected from the group consisting of Al-, Sb-, Fe-, Ti-
and Zn-ions and combinations thereof.
51. A negative electrode material for a lithium rechargeable
battery, the material comprising a core comprising silicon, wherein
the surface of the core is at least partly covered by silanol
groups covalently bound to amino-functional metal compounds,
wherein the metal compound is selected from the group consisting of
Si, Al and Ti and combinations thereof, and wherein the material
has a positive zeta potential in an interval between pH 3.5 and
9.5.
52. A negative electrode material for a lithium rechargeable
battery, the material comprising a core comprising silicon, wherein
the surface of the core is at least partly covered by adsorbed
nanoparticles of cationic multivalent metal oxides, and wherein the
material has a positive zeta potential in an interval between pH
3.5 and 9.5.
53. The negative electrode material of claim 52, wherein the metal
oxides are selected from the group consisting of Al-oxide,
Ca-oxide, Mg-oxide, Pb-oxide, Sb-oxide, Fe-oxide, Ti-oxide,
Zn-oxide and In-hydroxide and combinations thereof.
54. The negative electrode material of claim 40, wherein the
material has a positive zeta potential in an interval between pH 4
and 9.5.
55. The negative electrode material of claim 40, wherein the
material has a point of zero-charge at pH 4 or higher.
56. The negative electrode material of claim 40, comprising either
particles or a mixture of particles and wires.
57. The negative electrode material of claim 56, wherein both the
particles and the wires are nano-sized, and wherein the average
particle size of the particles is at least 5 times the average
width of the wires.
58. The negative electrode material of claim 40, wherein the core
has an average particle size between 20 nm and 200 nm and comprises
either pure silicon; or a silicon monoxide powder, which comprises
a mixture at nanometric scale of Si and SiO.sub.2; or silicon
having a SiO.sub.x surface layer, with 0<x<2, the surface
layer having an average thickness between 0.5 nm and 10 nm; or a
homogeneous mixture of silicon- and metal-oxides, having the
formula SiO.sub.x (M.sub.aO.sub.b).sub.y, with 0<x<1 and
0.ltoreq.y<1, wherein a and b are selected to provide
electroneutrality, and wherein M is selected from the group
consisting of Ca, Mg, Li, Al, and Zr; or an alloy Si--X, wherein X
is either one or more metals selected from the group consisting of
Sn, Ti, Fe, Ni, Cu, Co and Al.
59. The negative electrode material of claim 40, wherein the
material has a BET value between 1 and 60 m.sup.2/g.
60. A process for preparing the negative electrode material
according to claim 49, comprising: providing a nanosized silicon
material; dispersing the silicon material in water; providing a
quantity of cationic multivalent metal ions in the dispersion,
adjusting the pH of the dispersion to a value between 2 and 3.5,
and thereafter adjusting the pH of the dispersion to a value
between pH 3.5 and 4, determining the zeta potential of the
dispersion, and, if the zeta potential is negative, further
adjusting the pH of the dispersion to a value 0.5 above the
previous pH value, and determining the zeta potential of the
solution, and repeating the adjustment step until a positive zeta
potential is measured.
61. The process according to claim 60, wherein the step of
adjusting the pH of the dispersion to a value between 2 and 3.5 is
performed by addition of HCl, and wherein both the step of
adjusting the pH of the dispersion to a value between 3.5 and 4,
and, if applicable, the steps of further adjusting the pH of the
dispersion to a value 0.5 above the previous pH value, are
performed by addition of NaOH.
62. A process for preparing the negative electrode material
according to claim 40, comprising: providing a nanosized silicon
material, and subjecting the silicon material to an atomic layer
deposition process in a reaction chamber under a vacuum of at least
1 mbar and at a temperature between 50 and 500.degree. C., making
use of a gaseous organo-aluminum, organo-zinc or organo-antimony
stream and water vapour, until a layer with a thickness between 2
and 10 nm is formed on the surface of the silicon material.
63. The process according to claim 62, wherein the organo-aluminum
compound is trimethyl aluminum.
64. A process for preparing the negative electrode material
according to claim 49, comprising: providing a nanosized silicon
material, and dispersing the silicon material in water, providing a
quantity of cationic multivalent metal ions in the dispersion,
mixing the dispersion whereby the silicon material is at least
partly covered by adsorbed cationic multivalent metal ions, and
drying the metal ion-silicon mixture.
65. The process according to claim 64, further comprising:
redispersing the dry metal ion-silicon mixture in water, and
acidifying the dispersion to a pH between 2 and 6.
66. The process according to claim 64, wherein the metal ions are
selected from the group consisting of Al-, Sb-, Fe-, Ti-, Zn-ions
and combinations thereof.
67. A process for preparing the negative electrode material
according to claim 51, comprising: providing a nanosized silicon
material, dispersing the silicon material in water comprising
ammonium ions, whereby the surface of the silicon material is at
least partially covered with silanol groups, providing an
amino-functional metal oxide compound to the dispersion mixture,
agitating the mixture, whereby the silanol groups are covalently
bound to the amino-functional metal compounds, and drying the
mixture.
68. The process according to claim 67, wherein the metal oxide
compound is selected from the group consisting of Si, Al and Ti and
combinations thereof.
69. A process for preparing the negative electrode material of
claim 52, comprising: providing a nanosized silicon material,
dispersing the silicon material in water, adding a quantity of
nanoparticles of cationic multivalent metal oxides to the
dispersion, agitating the dispersion whereby the surface of the
silicon material is at least partly covered by adsorbed
nanoparticles of cationic multivalent metal oxides, and drying the
metal oxide-silicon mixture.
70. The process according to claim 69, wherein the metal oxides are
selected from the group consisting of Al-oxide, Ca-oxide, Mg-oxide,
Pb-oxide, Sb-oxide, Fe-oxide, Ti-oxide, Zn-oxide, In-hydroxide and
combinations thereof.
71. The process according to claim 60, wherein the silicon material
comprises either particles or a mixture of particles and wires,
wherein both the particles and the wires are nano-sized, and
wherein the average particle size of the particles is at least 5
times the average width of the wires.
72. The process according to claim 60, wherein the silicon material
comprises either pure silicon; or a silicon monoxide powder, which
comprises a mixture at nanometric scale of Si and SiO.sub.2; or
silicon having a SiO.sub.x surface layer, with 0<x<2, the
surface layer having an average thickness between 0.5 nm and 10 nm;
or a homogeneous mixture of silicon- and metal-oxides, having the
formula SiO.sub.x.(M.sub.aO.sub.b).sub.y, with 0<x<1 and
0.ltoreq.y<1, wherein a and b are selected to provide
electroneutrality, and wherein M is selected from the group
consisting of Ca, Mg, Li, Al, Zn and combinations thereof; or an
alloy Si--X, wherein X is one or more metals selected from the
group consisting of Sn, Ti, Fe, Ni, Cu, Co and Al.
73. A process for preparing an electrode assembly for a
rechargeable Li-ion battery comprising the negative electrode
material according to claim 40, comprising: dispersing the negative
electrode material according to claim 40 in an aqueous solution,
thereby obtaining a first slurry, adjusting the pH of the first
slurry to a value in the interval where the zeta potential of the
negative electrode material is positive, dissolving a CMC salt in
water so as to obtain an aqueous solution of binder material,
adjusting the pH of the aqueous solution of binder material to a
value in the interval where the zeta potential of the negative
electrode material is positive, mixing the first slurry and the
aqueous solution of binder material to obtain a second slurry,
dispersing conductive carbon in the second slurry, spreading the
second slurry on a current collector, and curing the electrode
assembly comprising the second slurry at a temperature between 105
and 175.degree. C.
74. The process for preparing an electrode assembly according to
claim 73, wherein the aqueous solution of binder material has a
concentration of 1-4 wt % of Na-CMC.
Description
TECHNICAL FIELD AND BACKGROUND
[0001] This invention relates to a negative electrode material for
lithium-ion batteries comprising silicon and having a chemically
treated or coated surface influencing the zeta potential of the
surface.
[0002] The Li-ion technology has dominated the portable battery
market and stands as a serious candidate for EVs and HEVs
applications. Therefore for such large volume applications, besides
the need for higher energy density and power rate electrodes,
safety and cost issues must be overcome. To address this challenge,
a wide variety of research directions enlisting different Li
reactivity mechanisms (conversion, displacement and alloying
reactions) as compared to the classical Li insertion mechanism have
been explored with more or less success. Recent findings have
definitively put nano-materials on the stage with fast
implementations in commercial cells, both at the positive side with
intercalation compounds (olivine LiFePO.sub.4) and at the anode
side with alloying reactions.
[0003] Experimental studies on the electrochemical alloying of
elements with lithium started in the early 70's when Dey described
the creation of Li alloys at room temperature with Sn, Pb, Al, Au .
. . and pointed out they were similar to those prepared by
metallurgical ways. In 1976, Li was found to electrochemically
react with Si at high temperatures through the consecutive
formation of Li.sub.12Si.sub.7, Li.sub.14Si.sub.6,
Li.sub.13Si.sub.4 and Li.sub.22Si.sub.5 phases. Even if we are now
aware that this electrochemical reaction is limited at room
temperature to the Li.sub.15Si.sub.4 end-member both the
corresponding gravimetric (3579 mAh/g) and volumetric (8330
mAh/cm.sup.3) capacities are far ahead all the other Li-uptake
reactions so far identified, regardless of their nature
(intercalation, alloying, conversion), and self-justify the past,
present and surely future focus on this system.
[0004] Therefore, an inherent drawback to electrodes based on
alloys lies in their poor cycling life (e.g. rapid capacity fading)
caused by the large volume swings upon subsequent
charges/discharges, which results in an electrochemical grinding of
the electrode, and hence its electric percolation loss, and in the
mean time a huge electrolyte degradation on the particles surface.
To avoid this issue the first optional solution was to use
electrodes made of nanoparticles as the usual mechanisms of
deformation and dislocation are not the same as the micro scale,
with namely the small particles being capable of releasing strains
without fracturing. Unfortunately, these nanoscale silicon powders
rapidly oxidize when exposed to air and the surface of commercially
available Si or Si made in-situ in a plasma process, as disclosed
in WO2012-000858, is covered by (protonated) silanol groups SiOH.
These surface silanol groups cause a non-optimal behavior of the
silicon particles in the anode electrode during the life of the
battery.
[0005] Other approaches to keep the electrode integrity are
either
[0006] 1) the preparation of metal thin-films, which provide the
best electrical contact via their strong adherence to the
substrate, hence enabling high quality electrical contact during
cycling, or 2) the fabrication of (metal-carbon-binder) composite
electrodes with the proper binder so that
Si/C/Carboxy-methyl-cellulose (CMC) electrodes having attractive
cycling properties are achieved, or 3) the elaboration via the
pyrolysis of organic precursors of metal-carbon (Si/C, Sn/C)
composites with the carbon acting as a volume buffer matrix.
[0007] Huge progress has been made since 10 years to improve the
behaviour of the silicon based electrode by an improvement of the
slurry preparation and the choice of a good binder. Nevertheless,
the active material particles need to be further tuned to realize
the next step of improvement, especially to provide a better
capacity retention during cycling. If today, the anode capacity can
be maintained with a lithium counter-electrode, the silicon based
metal-based electrodes present generally a weak capacity retention
with a cathode that contains a limited quantity of lithium. To
avoid this issue, the silicon surface needs to be modified to limit
the electrolyte degradation.
[0008] Different strategies can be used to modify and protect the
silicon surface. But the new surface needs also to be chosen
according to the reactivity of the electrolyte, since the material
needs to increase the potential window of stability of the
electrolyte, and hence decrease the electrolyte decomposition. In
US2011/0292570 Si nanoparticles having a positive surface charge
are coating with graphene. The positive charge is caused by
modification of the surface of the nanoparticles by functional
groups, that are preferably selected from amino groups and ammonium
groups, such as NR.sub.2 and NR.sub.3.sup.+, where R is selected
from H, C.sub.1-C.sub.6-alkyl or -hydroxyalkyl.
[0009] The invention aims at disclosing new Si based particles used
in the negative electrode of a rechargeable battery that are
capable of providing a better capacity retention during
cycling.
SUMMARY
[0010] Viewed from a first aspect, the invention can provide a
negative electrode material for use in a lithium rechargeable
battery, the material comprising a core comprising silicon, wherein
the material has a positive zeta potential in an interval between
pH 3.5 and 9.5, and preferably between pH 4 and 9.5. In certain
embodiments the zeta potential is at least +10 mV, preferably at
least +20 mV, in the pH interval 3.5-9.5 and even in the interval
4-9.5. The advantage of having a higher zeta potential is to
improve the dispersion between carbon, active material and binder
during the paste preparation in aqueous media. By zeta potential is
naturally meant the potential measured in demineralized water. The
core material may be nanometric (thus having at least one dimension
below 100 nm), of submicron size, or micron-sized, and may comprise
a mixture of nano-sized Si particles and nano-sized Si wires,
wherein the average particle size of the Si particles is at least 5
times the average width of the Si wires, and preferably at least 10
times the average width of the Si wires, such as described in
WO2012-000854. This negative electrode material for Li-ion
secondary batteries offers considerable advantages in limiting the
loss of capacity during cycling. In one embodiment, the material
has a point of zero-charge at pH 4 or higher, preferably at pH 7 or
higher.
[0011] In another embodiment, the core has an average particle size
between 20 nm and 200 nm (the average particle diameter was
calculated assuming nonporous spherical particles and a theoretical
density of the individual materials. Without going through the
entire derivation, the equation for calculating the average
particle diameter in nanometers is 6000/(BET surface area in
m.sup.2/g).times.(density in g/cm.sup.3)) and consists either of
[0012] pure silicon (at least metallurgical grade), or of [0013] a
silicon monoxide powder, which consists of a mixture at nanometric
scale of Si and SiO.sub.2, the powder may be micrometric, or of
[0014] silicon having a SiO.sub.x surface layer, with 0<x<2,
the surface layer having an average thickness between 0.5 nm and 10
nm, or of [0015] a homogeneous mixture of silicon and metal oxides,
having the formula SiO.sub.x.(M.sub.aO.sub.b).sub.y, with
0<x<1 and 0.ltoreq.y<1, wherein a and b are selected to
provide electroneutrality, and wherein M is either one or more of
Ca, Mg, Li, Al, and Zr, or of [0016] an alloy Si--X, wherein X is
either one or more metals of the group consisting of Sn, Ti, Fe,
Ni, Cu, Co and Al. The pure silicon may be nanometric. In some
embodiments the silicon may be coated with carbon. The silicon
material may also be a Si powder having an average primary particle
size between 20 nm and 200 nm, wherein the powder has a SiO.sub.x
surface layer, with 0<x<2, the surface layer having an
average thickness between 0.5 nm and 10 nm, such as disclosed in
WO2012-000858. The alloys mentioned above may comprise, besides Si,
metals such as Sn, Ti, Ni, Fe, Cu, Co and Al, as are disclosed in
US2010-0270497, WO2007-120347 or co-pending application
PCT/EP2011/068828.
[0017] In one particular embodiment, the surface of the core
carries O.sub.ySiH.sub.x groups, with 1<x<3,
1.ltoreq.y.ltoreq.3, and x>y. The O.sub.ySiH.sub.x groups could
also be considered to be a mixture of silanol (SiOH) and SiH.sub.x,
with 1<x<3. The presence of silanol groups and SiH.sub.x can
be detected by [0018] .sup.1H MAS NMR spectrometry which shows two
distinct regions at around 1.7 ppm and around 4 ppm. The first
corresponds to the Si--OH while the second is assigned to adsorbed
water or SiH.sub.x groups. [0019] .sup.29Si MAS/CPMG NMR
spectrometry which shows a first band at -100 ppm, attributable to
silicon atoms carrying single hydroxyl groups, being either
isolated silanols or vicinal silanols for both silicon. Another
band at -82 ppm is assigned to silicon which carry at least one
hydrogen (SiH.sub.x). [0020] DRIFT Infra-red spectrometry which
reveals the presence of various crystal modes as well as a
Si--O--Si mode at around 1100 cm.sup.-1. The peaks around 3500
cm.sup.-1 are attributed to hydroxylated silanol group. Between
2260 and 2110 cm.sup.-1, different peaks can be assigned to OySiHx
deformation modes.
[0021] In another embodiment, the surface of the core is at least
partly covered by a coating consisting of inorganic nanoparticles.
The surface being at least partly covered means that here too the
surface of the core may further carry O.sub.ySiH.sub.x groups, with
1<x<3, 1.ltoreq.y.ltoreq.3, and x>y. These inorganic
nanoparticles may be either one of an aluminium compound (such as
Al.sub.2O.sub.3), a zinc compound (such as zinc oxide) and an
antimony compound (such as antimony oxide). In one embodiment, the
nanoparticles in the coating consist of a precursor material
susceptible of being converted to either one of aluminium, zinc and
antimony by reduction. The nanoparticles may form a first coating
layer on the core having a thickness of less than 10 nm. There may
be provided a second coating layer located between the core and the
first coating of nanoparticles, the second coating layer comprising
either carbon or aluminium. Either one or both of the first and
second coating layer may be electrochemically active. The
embodiment where the surface of the core is at least partly covered
by a coating consisting of inorganic nanoparticles can be combined
with the embodiment where the surface of the core carries
O.sub.ySiH.sub.x groups, with 1<x<3, 1.ltoreq.y.ltoreq.3, and
x>y.
[0022] In another particular embodiment, the surface of the core is
at least partly covered by adsorbed cationic polymers having either
one or more of primary, secondary and tertiary amine functional
groups. The surface being at least partly covered means that here
too the surface of the core may further carry O.sub.ySiH.sub.x
groups, with 1<x<3, 1.ltoreq.y.ltoreq.3, and x>y. In still
another particular embodiment, the surface of the core is at least
partly covered by adsorbed cationic multivalent metal ions. The
surface being at least partly covered means that here too the
surface of the core may further carry O.sub.ySiH.sub.x groups, with
1<x<3, 1.ltoreq.y.ltoreq.3, and x>y. In one embodiment,
the metal ions may be either one or more of the group consisting of
Al-, Sb-, Fe-, Ti- and Zn-ions. This embodiment can be combined
with the embodiment where the surface of the core carries
O.sub.ySiH.sub.x groups, with 1<x<3, 1.ltoreq.y.ltoreq.3, and
x>y. In a further particular embodiment, the surface of the core
is at least partly covered by adsorbed nanoparticles of cationic
multivalent metal oxides. The surface being at least partly covered
means that here too the surface of the core may further carry
O.sub.ySiH.sub.x groups, with 1<x<3, 1.ltoreq.y.ltoreq.3, and
x>y. In one embodiment, the metal oxides are either one or more
of the group consisting of Al-oxide, Mg-oxide, Pb-oxide, Sb-oxide,
Fe-oxide, Ti-oxide, Zn-oxide and In-hydroxide. In another further
particular embodiment, the surface of the core is at least partly
covered by silanol groups (--Si--O.sup.- groups) covalently bound
to amino-functional metal compounds, wherein the metal is either
one or more of the group consisting of Si, Al and Ti. The surface
being at least partly covered means that here too the surface of
the core may further carry O.sub.ySiH.sub.x groups, with
1<x<3, 1.ltoreq.y.ltoreq.3, and x>y. In one embodiment,
the amino-functional metal compound used to chemically treat the
core surface may be an alkoxide.
[0023] Viewed from a second aspect, the invention can provide the
use of the active material described before in a negative electrode
further comprising either a water-soluble, or a
N-methylpyrrolidone-soluble binder material. The negative electrode
may further comprising graphite.
[0024] Viewed from a third aspect, the invention can provide a
process for preparing a negative electrode material, comprising the
steps of: [0025] providing a nanosized silicon material, [0026]
dispersing the silicon material in water, [0027] providing a
quantity of cationic multivalent metal ions in the dispersion,
[0028] adjusting the pH of the dispersion to a value between 2 and
3.5, and preferably between 2 and 2.5, and thereafter [0029]
adjusting the pH of the dispersion to a value between pH 3.5 and 4,
[0030] determining the zeta potential of the dispersion, and, if
the zeta potential is negative, [0031] further adjusting the pH of
the dispersion to a value 0.5 above the previous pH value, and
determining the zeta potential of the solution, and [0032]
repeating the adjustment step until a positive zeta potential is
measured. When the cationic multivalent ions are mixed into the
dispersion, the silicon material is partly covered by adsorbed
cationic multivalent metal ions, that are converted into a coating
consisting of inorganic nanoparticles during the subsequent steps
of pH adjustment. The nanosized silicon material may be dispersed
in a demineralised water at neutral pH. The step of adjusting the
pH of the dispersion to a value between 2 and 3.5 may be performed
by addition of HCl. If the pH is adjusted to a value lower than 2,
there is a risk of dissolving Si in the acid solution. Both the
step of adjusting the pH of the dispersion to a value between 3.5
and 4, and, if applicable, the steps of further adjusting the pH of
the dispersion to a value 0.5 above the previous pH value, may be
performed by addition of NaOH. The cationic multivalent metal ions
may be present in the nanosized silicon material as (accidental)
dopant, or can be added in the water used to make the
dispersion.
[0033] The invention can also provide a process for preparing a
negative electrode material, comprising the steps of: [0034]
providing a nanosized silicon material, [0035] subjecting the
silicon material to an atomic layer deposition process in a
reaction chamber under a vacuum of at least 1 mbar and at a
temperature between 50 and 500.degree. C., making use of gaseous
organo-aluminium, organo-zinc or organo-antimony stream and water
vapour, until a layer with a thickness between 2 and 10 nm is
formed. The vacuum may be at last 10.sup.-1 mbar, and the
temperature may be between 150 and 250.degree. C. to make the
process more economical.
[0036] The invention can also provide a process embodiment for
preparing a negative electrode material, comprising the steps of:
[0037] providing a nanosized silicon material, and dispersing the
silicon material in water, [0038] providing a quantity of cationic
multivalent metal ions in the dispersion, [0039] mixing the
dispersion whereby the silicon material is at least partly covered
by adsorbed cationic multivalent metal ions, and [0040] drying the
metal ion-silicon mixture. The nanosized silicon material may be
dispersed in a demineralised water at neutral pH. The metal ions
may be for example either one or more of the group consisting of
Al-, Sb-, Fe-, Ti- and Zn-ions. The drying step may comprise a
heating step under vacuum at a temperature between 70 and
100.degree. C. for at least 1 hr. This process may comprise the
additional steps of redispersing the dry metal ion-silicon mixture
in water, and acidifying the dispersion to a pH between 2 and 6.
This way a higher value for the zeta potential is achieved. Both
additional steps may be combined by redispersing in an acid
solution at a pH between 2 and 6. This method embodiment can also
be combined with the first named process in the third aspect of the
invention.
[0041] The invention can also provide a process for preparing a
negative electrode material, comprising the steps of: [0042]
providing a nanosized silicon material, [0043] dispersing the
silicon material in water comprising ammonium ions, the surface of
the silicon material being covered with silanol groups, [0044]
providing an amino-functional metal oxide compound to the mixture,
[0045] agitating the mixture, whereby the silanol groups are
covalently bound to the amino-functional metal compounds, and
[0046] drying the mixture. Ammonium ions are added to control the
pH of the solution. The solution may comprise ethanol to render the
dissolution of the metal oxide compound easier. The drying step may
comprise a heating step under vacuum at a temperature between 70
and 100.degree. C. for at least 1 hr. It may be preceded by a
washing step with ethanol. The metal in the metal oxide compound
may be for example either one or more of the group consisting of
Si, Al and Ti.
[0047] The invention can also provide a process for preparing a
negative electrode material, comprising the steps of: [0048]
providing a nanosized silicon material, [0049] dispersing the
silicon material in water, [0050] adding a quantity of
nanoparticles of cationic multivalent metal oxides to the
dispersion, [0051] agitating the dispersion whereby the surface of
the silicon material is at least partly covered by adsorbed
nanoparticles of cationic multivalent metal oxides, and [0052]
drying the metal oxide-silicon mixture. The nanosized silicon
material may be dispersed in a demineralised water at neutral pH.
The metal oxides may be for example either one or more of the group
consisting of Al-oxide, Mg-oxide, Pb-oxide, Sb-oxide, Fe-oxide,
Ti-oxide, Zn-oxide and In-hydroxide. The drying step may comprise a
heating step under vacuum at a temperature between 70 and
100.degree. C. for at least 1 hr.
[0053] In the different process embodiments, the final product may
have an open porous volume lower than 0.01 cc/g. In one process
embodiment, the nanosized material in each of the processes above
may consist of either particles or a mixture of particles and
wires, wherein both the particles and the wires are nano-sized, and
wherein the average particle size of the particles is at least 5
times the average width of the wires, and preferably at least 10
times the average width of the wires. In another process
embodiment, the silicon material may consist of either of [0054]
pure silicon, or of [0055] a silicon monoxide powder, which
consists of a mixture at nanometric scale of Si and SiO.sub.2, the
powder may be micrometric, or of [0056] silicon having a SiO.sub.x
surface layer, with 0<x<2, the surface layer having an
average thickness between 0.5 nm and 10 nm, or of [0057] a
homogeneous mixture of silicon- and metal-oxides, having the
formula SiO.sub.x.(M.sub.aO.sub.b).sub.y, with 0<x<1 and
0.ltoreq.y.ltoreq.1, wherein a and b are selected to provide
electroneutrality, and wherein M is either one or more of Ca, Mg,
Li, Al, and Zr, or of [0058] an alloy Si--X, wherein X is either
one or more metals of the group consisting of Sn, Ti, Fe, Ni, Cu,
Co and Al.
[0059] Viewed from a fourth aspect, the invention can provide a
process for preparing an electrode assembly for a rechargeable
Li-ion battery comprising the negative electrode material described
before, comprising the steps of: [0060] dispersing the negative
electrode material in an aqueous solution, thereby obtaining a
first slurry, [0061] adjusting the pH of the first slurry to a
value in the interval where the zeta potential of the material is
positive, [0062] dissolving a CMC salt in water so as to obtain an
aqueous solution of binder material, [0063] adjusting the pH of the
aqueous solution of binder material to a value in the interval
where the zeta potential of the material is positive, preferably
the pH value of the first slurry, [0064] mixing the first slurry
and the aqueous solution of binder material to obtain a second
slurry, [0065] dispersing conductive carbon in the second slurry,
[0066] spreading the second slurry on a current collector,
preferably a copper foil, and [0067] curing the electrode assembly
comprising the second slurry at a temperature between 105 and
175.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] FIG. 1: schematic representation of a silicon particle (21)
with a positively charged layer (22) surrounded by a negatively
charged polymer (23)
[0069] FIG. 2: scheme of the silicon particles (or primary
particles) (11) with A: coating with inorganic layer (12); B:
physically adsorbed organic molecules (13); C: chemically attached
organic molecules (14) and D: adsorbed nanoparticles (secondary
particles) (15) at the surface of the primary particles
[0070] FIG. 3: scheme of the ALD reactor with P for pump, N-p for
nanopowder and Ar for the Argon gas source.
[0071] FIG. 4: TEM picture of silicon particles (61) with alumina
coating (62)
[0072] FIG. 5: Zeta potential (mV) versus pH of coated silicon (1)
and comparison with pristine silicon (2).
[0073] FIG. 6: Viscosity evolution according to time for silicon
and coated silicon suspension in CMC solution, .quadrature.=for CMC
alone (blank test), =for pristine silicon (reference), .DELTA.=for
alumina coated silicon.
[0074] FIG. 7: Reversible delithiation capacity of a battery using
coated silicon as anode material (1) and comparison with
delithiation capacity of a battery using pristine silicon as anode
material (2).
[0075] FIG. 8: Reversible delithiation capacity of a battery using
alumina coated silicon (25 cycles of ALD) as anode material.
[0076] FIG. 9: TEM picture of silicon nanowires with alumina
coating (a). EFTEM map of the samples with alumina contrast (b),
silicon contrast (c) and oxygen contrast (d).
[0077] FIG. 10: Zeta potential (mV) according to pH of nanowires:
coated nanowires (1) and comparison with pristine nanowires
(2).
[0078] FIG. 11: Zeta potential (mV) according to pH of acid-base
treated silicon
[0079] FIG. 12: Reversible delithiation capacity of a battery using
treated silicon as anode material (1) and comparison with
delithiation capacity of a battery using pristine silicon as anode
material (2).
[0080] FIG. 13: Zeta potential (mV) versus pH of APTS (Si:APTS=4/5)
coated silicon (1) and comparison with pristine silicon (2).
[0081] FIG. 14: Zeta potential (mV) versus pH of silicon carrying
adsorbed alumina nanoparticles (1) and comparison with pristine
silicon (2).
[0082] FIG. 15: Reversible delithiation capacity of a battery
(mAh/g vs. cycle number) using alumina absorbed silicon as anode
material (1) and reversible delithiation capacity of a battery
using alumina treated silica absorbed silicon as anode material
(2).
[0083] FIG. 16: Zeta potential (mV) versus pH of silicon carrying a
physically adsorbed cationic polymer (1) and comparison with
pristine silicon (2).
[0084] FIG. 17: Zeta potential (mV) versus pH of cation coated
silicon (1) and comparison with pristine silicon (2).
[0085] FIG. 18: Reversible delithiation capacity of a battery
(mAh/g vs. cycle number) using cation coated silicon as anode
material
[0086] FIG. 19: Zeta potential (mV) versus pH of alumina coated
silicon (different BET) (1) and comparison with pristine silicon
(same initial BET) (2).
[0087] FIG. 20: Zeta potential (mV) versus pH of alumina coated
silicon monoxide (1) and comparison with pristine silicon monoxide
(2).
DETAILED DESCRIPTION
[0088] The goal of this invention is to propose a modified particle
surface for a Si comprising particle, hereby maintaining the
capacity throughout cycles by a good interaction with the binder,
and decreasing the reactivity of the electrolyte with the surface.
The proposed approach in the invention is to modify the particle
surface and to create a positively charged surface that is a
surface having a positive zeta potential, in parts of the range of
pH 3.5 to 9.5.
[0089] Positively charged particles are characterised by dispersing
20 g/l of powder in distilled water. Zeta potentials `Z` of the
dispersion in the aqueous medium are measured by using for example
a Zetaprobe Analyser.TM. from Colloidal Dynamics. Finally, a
particle is defined as being a positively charged particle at a
certain pH, if Z is higher than 0, and a negatively charged
particle is defined as a particle having Z<0. This is
illustrated in FIG. 1.
[0090] A silicon surface is natively negatively charged at neutral
pH and can become positive between pH 2 and 3 (the isoelectric
point or IEP of silica). The chemical surface groups are mainly
deprotonated silanol groups. This charge is not in favour of a good
interaction with the negatively charged binder. Indeed, the
preferred binder is a water soluble polymer, like
carboxylmethylcellulose binder (CMC) or polyacrylates PM, which are
negatively charged at neutral or even at lower pH.
[0091] We propose 4 different ways to modify the particle surfaces
(see FIG. 2): [0092] (A) with a at least partial coating of an
inorganic layer (as for example coating of alumina, zinc oxide, or
antimony oxide); by a deposition method of the layer; or whereby
the particles are composed of a core comprising silicon, and an
inorganic layer is attached to the core; where the coating is
obtained with a chemical treatment consisting of a sequential
acid-base treatment of the powder containing the elements of the
future coating; [0093] (B) with physical adsorption of cations (as
for example cationic multivalent metal ions) or organic molecules
to the core of the particles (as for example cationic molecules);
[0094] (C) with a chemical adsorption (creation of covalent bond)
of organic molecules (as for example grafting of silane type
molecule--or in general any amino-functional metal oxide--on the
surface silanols); [0095] (D) and with adsorption of nanoparticles
(secondary particles) to the core of the particles (for example
adsorption of nano particles of alumina at the surface of the
silicon surface).
[0096] Although FIG. 2 represents spherical particles, the
particles may also consist of wires comprising silicon.
[0097] In case a coating is applied, this coating may be
insulating, since the lithium can diffuse through the thin layer
(organic or inorganic layer). Also, when applying a thin metal
oxide coating, there is the additional advantage that the oxide can
be converted in a metal and lithium oxide during the first cycle in
the lithium-ion battery. This irreversible conversion will create a
metallic surface which can, as the silicon, create alloys with the
lithium and generate extra capacity and allowing a volumic
expansion; the proposed alumina/aluminium, zinc or antimony coating
can grow with the volumic expansion of the silicon and protect
continuously the electrolyte/silicon contact.
[0098] This positively charged silicon or material comprising
silicon is tested as an anode material in a lithium-ion battery.
The powder is mixed with a binder and a carbon conductor to create
a slurry that is coated on copper foil. The invention allows to
improve the electrochemical behaviour of the silicon comprising
particles by improving the dispersion of the silicon, carbon and
polymer in the coating on the copper foil, improving the capacity
retention and lowering the irreversibility.
Example 1
Alumina Coated Silicon
[0099] In this example, an alumina coating is applied by Atomic
layer deposition (ALD), which is a deposition method to prepare
nano-sized coatings. In the ALD process two (or more) alternating
surface self-limiting chemical vapour deposition reactions are
performed. The technology is also used to coat nano-powders. Small
quantities of powder can be coated using a stationary system, but
for larger quantities a fluidized particle bed reactor or a
rotating reactor can be used, as illustrated in US 2011/0200822. A
coating of Al.sub.2O.sub.3 can be deposited by thermal ALD using
Tri-Methyl Aluminium (TMA) and H.sub.2O as reactants. The reaction
temperature is around 200.degree. C. The saturation of the reactive
surface can be monitored using mass spectroscopy on the
decomposition products of the precursors.
[0100] In the Example, 5 g of a nanosilicon powder, made according
to WO2012-000858, and having a BET of 21 m.sup.2/g with a open
porous volume lower than 0.001 cc/g (measurement done by ASAP
equipment by isotherm adsorption-desorption of N.sub.2 at 77K after
preheating the sample for 1 h under a flow of argon at 150.degree.
C.), an oxygen content<4 wt %, a primary particle size defined
as 80 nm<D80<200 nm and an initial negative zetapotential
(defined at pH7 in water) is weighed and put in a glass reactor
(see FIG. 3). The reactor exit 3 is connected to a vacuum oil pump
at 10.sup.-2 mbar. The reactor gas inlet 1 is connected to a
trimethyl aluminium (TMA) supply (97%, Sigma-Aldrich) using
Swagelok.RTM. tubes and an automated valve. The reactor gas inlet 2
is connected to a glass bottle of H.sub.2O (de-ionized) using
Swagelok.RTM. tubes and an automated valve. During one "cycle" a
gas flow of TMA and water is used. The valve connecting the reactor
to TMA is opened until the surface is saturated with TMA (5
minutes) and closed. This is followed by opening the valve
connecting the reactor to the H.sub.2O for 5 minutes. 6 cycles (TMA
followed by water) are used for the preparation of the coating. An
equal quantity of pristine nano-silicon powder is used as reference
example.
[0101] The microscopic pictures show conformal homogeneous and thin
(3 nm) coatings of alumina on the surface of silicon particles (see
FIG. 4). The BET surface of this powder is determined at 20
m.sup.2/g (compared to 21 m.sup.2/g for the pristine silicon) and
with a porous volume lower than 0.001 cc/g from the results of
isotherm adsorption-desorption of N.sub.2 at 77K after preheated
for 1 h under a flow of argon at 150.degree. C. The aluminium
quantity is measured by ICP and a value of 2 wt % is
calculated.
[0102] The ALD process thus comprises the steps of: [0103]
providing a nanosized silicon material in a reaction chamber under
a vacuum of at least 10.sup.-1 mbar and at a temperature between
150 and 250.degree. C., [0104] injecting a gaseous
organo-aluminium, organo-zinc or organo-antimony compound in the
reaction chamber, [0105] saturating the surface of the silicon
material with the organo-aluminium, organo-zinc or organo-antimony
compound, and subsequently [0106] injection water vapour in the
reaction chamber, thereby [0107] providing the surface of the
silicon material with a coating of aluminium-, zinc- or antimony
oxide, [0108] repeating the steps of injecting a gaseous
organo-aluminium, organo-zinc or organo-antimony compound in the
reaction chamber, [0109] saturating the surface of the silicon
material with the organo-aluminium, organo-zinc or organo-antimony
compound, and subsequently [0110] injecting water vapour in the
reaction chamber, until a coating with a thickness between 2 and 10
nm is formed.
[0111] The measure of the zeta potential of the resulting material
is made according to the following procedure: 150 ml of both a
reference 2 wt % nano-silicon powder and the alumina coated silicon
suspension in demineralised water is prepared by ultrasonification
(120 s at 225 W). The zeta potential of this suspension in the
aqueous medium is measured with the Zetaprobe Analyser.TM. from
Colloidal Dynamics. The samples are automatically titrated from
neutral pH to acid pH with 0.5 M HCl and to more basic pH with 0.5
M NaOH.
[0112] The high negative surface charge on nano-silicon powder can
be clearly measured (see FIG. 5, line 2). The zeta potential is
negative from pH 5 to 2. In the case of the alumina coated silicon,
the powder has a positive zeta potential from pH 9.5 to at least pH
2.5. (FIG. 5, line 1)
[0113] A slurry is prepared using 50 wt % of this powder (based on
the dry residue), 25 wt % of a Na-CMC binder (Molecular weight
<200 000) and 25 wt % of a conductive additive (Super C65,
Timcal). In a first step, a 2.4% Na-CMC solution is prepared and
dissolved overnight. Then, the conductive carbon is added to this
solution and stirred for 20 minutes using a high-shear mixer. Once
a good dispersion of the conductive carbon is obtained, the active
material is added and the slurry is stirred again using a
high-shear mixer during 30 minutes. One of the proofs that the
coating prevents contact between the electrolyte and the silicon
surface is the evolution of the viscosity of the CMC solution. Flow
properties are measured under shear rate controlled conditions with
the Fysica MCR300 rheometer with cone-plate measuring geometry at
23.degree. C. When the polymer (CMC in this example) is in contact
with the pristine silicon surface, we observe a drop of the
viscosity of a CMC/Si solution/suspension. As shown in FIG. 6, the
viscosity is maintained when the silicon is coated with alumina
versus the blank test with CMC alone (.quadrature.=for CMC alone,
=for pristine silicon (reference), .DELTA.=for alumina coated
silicon), which proves that the silicon is fully covered with
another material (here Al.sub.2O.sub.3).
[0114] The electrodes are prepared by coating the resulting slurry
on a copper foil (17 .mu.m) using 125 .mu.m wet thickness and then
dried at 70.degree. C. for 2 hours. Round electrodes are punched
and dried at 150.degree. C. during 3 hours in a small vacuum oven.
The electrodes are electrochemically tested versus metallic lithium
using coin cells prepared in a glovebox (dry Argon atmosphere). The
electrolyte used is LiPF.sub.6 1M in a mix of EC/DEC (50/50 wt
%)+10% FEC+2% VC (Semichem). The coin cells are tested in a CC mode
between 10 mV and 1.5 Vat a C-rate of C/5 (meaning a full charge of
discharge of 3570 mAh/g of active material in 5 hours). The result
is shown in FIG. 7.
[0115] We clearly see that the behaviour of the electrode is
improved with the coating of alumina (line 1): after 100 cycles,
the delivered capacity remains around 2400 mAh/g, versus 1000 mAh/g
for the pristine silicon (line 2). It was found also that a coating
that is thinner than 1 nm does not have the desired effect.
Counter-Example 1
Alumina Coated Silicon with High Thickness of Alumina
[0116] Alumina coated silicon is prepared with the ALD process as
in the Example 1. 25 cycles (TMA followed by water) are used for
the preparation of this powder (compared to 6 cycles in the Example
1). The alumina layer has a thickness of 12 nm. The BET of the
surface decreases to 16 m.sup.2/g and the quantity of alumina is
measured at 8 wt % of the powder. Slurries and batteries are
prepared as in the Example 1, and the result is shown in the FIG.
8. The capacity is lower than 500 mAh/g from the first cycle, and
this capacity drops in the following cycles. This result clearly
shows the importance to have a thin layer at the surface of the
silicon to allow the electrochemical reaction.
Example 2
Alumina Coated Silicon Particles and Nanowires
[0117] In this example, an alumina coating is applied by Atomic
layer deposition (ALD) on Si particles and nanowires made according
to WO2012-000854, and having an oxygen content <4 wt %. The
synthesis procedure is described in the Example 1.
[0118] The microscopic pictures (see FIG. 9) show conformal
homogeneous and thin coatings of alumina on the surface of the
silicon particles. The coated powder is dispersed in ethanol, after
which it is placed on a carbon grid mounted on the Cu support. The
crushing step in the sample preparation was forgone, to avoid
damaging the powder. EFTEM maps showing silicon, alumina and oxygen
contrast are acquired at 300 kV using the Philips CM30-FEG
microscope (see FIG. 9). The aluminium quantity is measured by ICP
and a value of 2 wt % is calculated. As in the example 1, it is
possible to prove that the silicon is fully covered with another
material (here Al.sub.2O.sub.3) by measuring the viscosity
evolution of a CMC-powder solution according to time.
[0119] The measure of the zeta potential is made according to the
following procedure: 150 ml of both a reference 2 wt % powder and
the alumina coated silicon nanowires suspension in demineralised
water is prepared by ultrasonification (120 s at 225 W). The zeta
potential of this suspension in the aqueous medium is measured with
the Zetaprobe Analyser.TM. from Colloidal Dynamics. The samples are
automatically titrated from neutral pH to acid pH with 0.5 M HCl
and to more basic pH with 0.5 M NaOH.
[0120] The high negative surface charge on nanowires can be clearly
measured (see FIG. 10, line 2). The zeta potential is negative from
pH 8 to 2. In the case of the alumina coated silicon nanowires, the
powder has a huge positive zeta potential from pH 8 to at least pH
2. (FIG. 10, line 1)
Example 3
Acid-Base Treatment of Silicon Surface
[0121] 150 ml of a 2 wt % nano-silicon suspension in demineralised
water is prepared by ultrasonification (120 s at 225 W). The
nano-silicon is made according to WO2012-000858 and has a BET of 25
m.sup.2/g, an oxygen content <4 wt %, a particle size defined as
80 nm<D80<200 nm, an aluminium contamination of at least 0.1
wt % (typical for a plasma generated silicon powder), the
contamination being concentrated at the particles' surface, and has
an initial negative zetapotential (defined at pH7 in water. To the
suspension, a known quantity of 0.5 M HCl is added to lower the pH
to 2. Later, an addition of 0.5 M NaOH allows to bring back the pH
of the suspension to pH 4. The zeta potential of this suspension in
the aqueous medium is measured with the Zetaprobe Analyser.TM. from
Colloidal Dynamics. The measured charge on these particles is
positive as a zetapotential of +12 mV is measured.
[0122] FIG. 11 describes the charge variation of the suspension
during the acid-base treatment. The measure has been done with the
Zetaprobe Analyser.TM. from Colloidal Dynamics and with solutions
of 0.5 M HCl and 0.5 M NaOH. During the acid treatment, below pH 5,
a steep decrease in surface charge is observed (translated by a
decrease of the absolute value of zetapotential), most probably due
to the protonation of silanol groups on the oxidized Si-surface and
due to a dissolution of the aluminium compound contaminant which
liberates aluminium ions. During the back titration (=the base
treatment), the zeta potential becomes positive between pH 3.4 and
pH 4.9. During the addition of base, a precipitation of alumina
occurs at the surface of the electrode and thus creates the
positive charge. The coverage is not total as the quantity of
alumina is low, and this explain the behaviour of the surface which
potential becomes negative for a pH higher than 5.
[0123] To prepare an electrode, the first step is the preparation
of 2.4% Na-CMC solution by dissolving overnight, and then adjusting
its pH to the pH of the silicon suspension prepared previously. The
conductive carbon is added and the mixture is stirred for 20
minutes using a high-shear mixer. Once a good dispersion of the
conductive carbon is obtained, the active material suspension
(treated silicon) is added and the resulting slurry is stirred
again using a high-shear mixer during 30 minutes. The slurry is
prepared with a final composition of 50 wt % of this powder, 25 wt
% of a Na-CMC binder (Molecular weight<200 000) and 25 wt % of a
conductive additive (Super C65, Timcal).
[0124] Electrodes are prepared by coating the resulting slurry on a
copper foil (17 .mu.m) using 125 .mu.m wet thickness and are then
dried at 70.degree. C. for 2 hours. Round electrodes are punched
and dried at 150.degree. C. during 3 hours in a small vacuum oven.
The electrodes are electrochemically tested versus metallic lithium
using coin cells prepared in a glovebox (under dry Argon
atmosphere). The electrolyte used is LiPF.sub.6 1M in a mix of
EC/DEC (50/50 wt %)+10% FEC+2% VC (Semichem). The coin cells are
tested in a CC mode between 10 mV and 1.5 Vat a C-rate of C/5
(meaning a full charge of discharge of 3570 mAh/g of active
material in 5 hours). The result is shown in FIG. 12, where the
evolution of the capacity during cycling of the pristine
nano-silicon powder (2) is compared to the evolution for alumina
coated silicon (1).
[0125] We clearly see that the behaviour of the electrode is
improved with the acid/base treatment in the presence of aluminium
ions; after 100 cycles, the delivered capacity remains around 3000
mAh/g versus 1000 mAh/g for the pristine silicon.
Example 4
Silane Coated Silicon
[0126] 12 g of nano silicon particles (made according to
WO2012-000858) is dispersed in a 1000 cm.sup.3 solution containing
NH.sub.4OH, H.sub.2O and C.sub.2H.sub.5OH (pure ethanol) in the
ratio 1:10:14 respectively. Ammonium hydroxide (NH.sub.4OH, 30%)
was supplied by J. T Baker. The suspension is sonicated for 20
minutes and further stirred overnight. Next, 6 g of APTS (a silicon
alkoxide: 3-aminopropyltriethoxysilane) is added to the suspension,
sonicated for 10 minutes and further stirred overnight. The APTS
was purchased from Sigma Aldrich. The particles are allowed to
settle, and mother liquid is removed. Next, the amine modified Si
particles are washed three times with ethanol and then dried in a
vacuum oven overnight at 60.degree. C.
[0127] The effectiveness of surface modification is evaluated by
the zeta potential (ZP), which was determined from the
electrophoretic mobility measurements over a broad range of pH
values. The surface zeta potential of this powder is investigated
using ZetaPALS from Brookhaven Instruments. A dispersion containing
0.25 mg/cm.sup.3 of modified silicon is prepared by dissolving the
powder in 0.001 mol/L of KCl. To keep the ionic strength constant,
the pH is adjusted using 0.1 mol/L of KOH and 0.1 mol/L HCl
solutions. Different weight ratios of APTS:Si were tested to
concluded that a ratio of at least 1:2 is recommended to obtain a
positive charge in the particles surface.
[0128] FIG. 13 shows the zetapotential of the amine modified- (with
weight ratios of APTS:Si=5:4) (line 1) and pristine silicon (line
2) as a function of pH at a constant ionic strength. The success in
surface modification is attested by the change in surface charge of
the particles from negative to positive at pH values lower than 7,
the amine treated particles having an iso-electric point at neutral
pH. On the other hand, the pristine Si particles are negatively
charged over the pH range of 3-11. All the silicon particles
traited with APTS have a similar zetapotential profile if the
weight ratios of APTS:Si is higher than 1:2).
[0129] APTS is one of the examples of cationic silane which can be
used to create a positive charge. The same effect can be obtain
with derivates of APTS (such as aminomethyltriethoxysilane,
2-aminoethyltriethoxysilane, aminotriethoxysilane,
3-aminopropyltrimethoxysilane), derivates of
triethoxy(3-isocyanatopropyl)silane, or derivates of
N-[3-(Trimethoxysilyl)propyl]aniline.
Example 5
Adsorption of Nano-Alumina Particles on Silicon Particles
[0130] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, a known
quantity of 2 wt % of Al.sub.2O.sub.3 nanoparticles (commercially
available: DISPARAL P2, Sasol; particles size<30 nm) dispersed
in water is added in order to have a Al.sub.2O.sub.3/Si ratio of at
least 2. In this example, 5 wt % of Al.sub.2O.sub.3 nanoparticles
have been added. This combined dispersion is placed on a rollerbank
for 30 minutes. Below a weight ratio alumina/silicon of 2/1, the
agglomeration (silicon particles with alumina adsorbed particles)
charge stays negative; indeed a minimum quantity of alumina
particles is recommended to cover enough silicon surface and have a
positive average charge of the agglomerations. The analysis of the
porosity shows that some micro-porosity appear after the treatment
(<0.01 cc/g). This is probably created between the nanoparticles
of alumina.
[0131] After this mixing the pH of the solution is equal to 6 and
the zeta potential of this suspension in the aqueous medium is
measured with the Zetaprobe Analyser.TM. from Colloidal Dynamics.
The measured charge on these particles is positive as a
zetapotential of +45 mV is measured. This value shows that the
alumina colloids were absorbed on the surface of silicon particles.
The combined dispersion is then dried via a rotavap, heated to
80.degree. C. under vacuum during 10 hours.
[0132] The measure of the zeta potential is made according to the
following procedure: 100 ml of both a reference 1 wt % nano-silicon
powder and the treated silicon powder suspension in demineralised
water is prepared by ultrasonification (120 s at 225 W). The zeta
potentials of these suspensions in the aqueous medium are measured
with the Zetaprobe Analyser.TM. from Colloidal Dynamics. The
samples are automatically titrated from neutral pH to acid pH with
0.5 M HCl and to more basic pH with 0.5 M NaOH. The initial charge
of the treaded silicon is positive as the zeta potential is
measured at 53 mV. Thus, this value shows that the adsorption of
nanoparticles of alumina is kept during the drying process. The
high negative surface charge on nano-silicon powder can be clearly
measured (see FIG. 14, line 2). While the zeta potential is
negative from pH 2 to at least pH 5 in the case of the pristine
silicon, the treated silicon powder has a positive zeta potential
from pH 9.5 to at least pH 2.5. (FIG. 14, line 1)
[0133] Slurries and batteries are prepared as in the Example 1, and
the result of coin cell tests is shown in the FIG. 15. We clearly
see that the behaviour of the electrode is improved with the
adsorption of nano-alumina particles (line 1): the initial
delivered capacity is similar to the theoretical capacity of the
material and after 100 cycles, the delivered capacity remains
around 2200 mAh/g, versus 1000 mAh/g for the pristine silicon (see
FIG. 7 line 2).
Example 6
Adsorption of Cationic Polymer
[0134] For cationic polymer adsorption, a known dispersing agent is
used: Poly ethylene-imine (PEI) whose chemical formula is
(C.sub.2H.sub.4NH)n or
##STR00001##
[0135] This polymer is a branched polymer containing primary,
secondary and tertiary amine functional groups. The nitrogen can be
protonated to make the polymer highly positively charged. It also
presents the advantages to be soluble in water.
[0136] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, a quantity
of 1 wt % PEI dispersed in water (PEI 10 kmol/g, pH adjusted to 6)
is added. This combined dispersion is placed on a rollerbank for 30
minutes. After this mixing the zeta potential of the suspension in
the aqueous medium is measured with the Zetaprobe Analyser.TM. from
Colloidal Dynamics. The measured charge on the particles is
positive as a zetapotential of +30 mV is measured. This value shows
that the polymer chains are absorbed on the surface of silicon
particles. To obtain the positive effect, a PEI/Si weight ratio of
at least 0.35/1 is recommended or a mass of PEI of at least 14 mg
per m.sup.2 of silicon surface. This value shows that considerably
less mass of PEI is necessary to obtain a positive charge than for
nano-alumina in Example 5. The combined dispersion is then dried
via a rotavap, heated to 80.degree. C. under vacuum during 10
hours.
[0137] The measure of the zeta potential is made according to the
following procedure: 100 ml of both a reference 1 wt % nano-silicon
powder and the treated silicon powder suspension in demineralised
water is prepared by ultrasonification (120 s at 225 W). The zeta
potentials of these suspensions in the aqueous medium are measured
with the Zetaprobe Analyser.TM. from Colloidal Dynamics. The
samples are automatically titrated from neutral pH to acid pH with
0.5 M HCl and to more basic pH with 0.5 M NaOH. The initial charge
of the treaded silicon is positive as the zeta potential is
measured at +35 mV. Thus, this value shows that the adsorption of
PEI is kept during the drying process.
[0138] The high negative surface charge on nano-silicon powder can
be clearly measured (see FIG. 16, line 2). Where the zeta potential
is negative from pH 12 to 2 in the case of the pristine silicon,
the treated silicon powder has a positive zeta potential from pH 6
to at least pH 2.5. (FIG. 16, line 1).
[0139] This effect can be also obtain by adsorption of others
cationic surfactant and polymer based on pH-dependent primary,
secondary, or tertiary amines as for example Octenidine
dihydrochloride, Poly(4-vinylpyridine), Poly(2-vinylpyridine
N-oxide), Poly(N-vinylpyrrolidone), . . . .
[0140] Slurries and batteries are prepared as in the Example 1. The
adsorption of cationic polymer improve the capacity retention of
the electrode: after 100 cycles, the delivered capacity remains
above 1500 mAh/g, versus 1000 mAh/g for the pristine silicon (FIG.
7 line 2).
Example 7
Adsorption of Cation on Silicon Particles
[0141] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, at least 26
mg of Al.sup.3+ (in a form of AlCl.sub.3 salt) is added. This
result of a Al/Si ratio of at least 0.026 or at least 1 mg of Al
per m.sup.2 of silicon. This combined dispersion is placed on a
rollerbank for 30 minutes. The combined dispersion is then dried
via a rotavap, heated to 80.degree. C. under vacuum during 10
hours.
[0142] The measure of the zeta potential is made according to the
following procedure: 100 ml of both a reference 1 wt % nano-silicon
powder and the treated silicon powder suspension in demineralised
water is prepared by ultrasonification (120 s at 225 W). The zeta
potentials of these suspensions in the aqueous medium are measured
with the Zetaprobe Analyser.TM. from Colloidal Dynamics. The
initial pH of this solution is 5.3 and the zetapotential is +40 mV.
The sample is then automatically titrated from neutral pH to acid
pH with 0.5 M HCl and to more basic pH with 0.5 M NaOH. The
redispersed treated powder has a positive zetapotential from 2 to 7
(FIG. 17, line 1) with a stable value of +65 mV from 2 to 5. The
zeta potential is negative from at least pH 5 to 2 in the case of
the pristine silicon (FIG. 17, line 2).
[0143] Slurries and batteries are prepared as in the Example 1, and
the result is shown in the FIG. 18 (capacity in mAh/g versus cycle
number). We clearly see that the behaviour of the electrode is
improved with the adsorption of cation (line 1): after 100 cycles,
the delivered capacity remains around 1950 mAh/g, versus 1000 mAh/g
for the pristine silicon (FIG. 7 line 2).
[0144] Besides AlCl.sub.3, other water soluble aluminium salts or
Sb, Ti and Zn (all cationic multivalent metals) can be used to
obtain a similar result.
Example 8a
Alumina Coated Silicon: Variation of Specific Surface
[0145] In this Example, 5 g of a nanosilicon powder, made according
to WO2012-000858, and having a BET of 40 m.sup.2/g with a open
porous volume lower than 0.001 cc/g (measurement done by ASAP
equipment by isotherm adsorption-desorption of N.sub.2 at 77K after
preheated for 1 h under a flow of argon at 150.degree. C.), an
oxygen content<4 wt %, and an initial negative zetapotential
(defined at pH7 in water) are use and treated as in Example 1.
[0146] Microscopic pictures show conformal homogeneous and thin (3
nm) coatings of alumina on the surface of the silicon particles.
The BET surface of this powder is determined at 40 m.sup.2/g (no
modification of the BET after the ALD treatment) and with a porous
volume lower than 0.001 cc/g from the results of isotherm
adsorption-desorption of N.sub.2 at 77K after preheated for 1 h
under a flow of argon at 150.degree. C. The aluminium quantity is
measured by ICP and a value of 3.4 wt % is calculated. The measure
of the zeta potential of the resulting material is made according
to the following procedure: 150 ml of both a reference 2 wt %
nano-silicon powder and the alumina coated silicon suspension in
demineralised water is prepared by ultrasonification (120 s at 225
W). The zeta potential of this suspension in the aqueous medium is
measured with the Zetaprobe Analyser.TM. from Colloidal Dynamics.
The samples are automatically titrated from neutral pH to acid pH
with 0.5 M HCl and to more basic pH with 0.5 M NaOH. The high
negative surface charge on nano-silicon powder can be clearly
measured (see FIG. 19, line 2). The zeta potential is negative from
pH 6 to 2. In the case of the alumina coated silicon, the powder
has a positive zeta potential from at least pH 9 to at least pH
2.5. (FIG. 19, line 1)
[0147] The electrodes are prepared and tested as described in the
Example 1. The result shows that the behaviour of the electrode is
improved with the coating of alumina: after 100 cycles, the
delivered capacity remains around 2500 mAh/g, versus 1000 mAh/g for
the pristine silicon (see FIG. 7, line 2). It was found also that a
coating that is thinner than 1 nm does not have the desired
effect.
Example 8b
Alumina Coated Silicon: Variation of Specific Surface
[0148] In this Example 1, 5 g of a commercial micrometric powder
(Aldrich), having a BET of 1 m.sup.2/g with a open porous volume
lower than 0.001 cc/g (measurement done by ASAP equipment by
isotherm adsorption-desorption of N.sub.2 at 77K after preheated
for 1 h under a flow of argon at 150.degree. C.), an oxygen
content<4 wt %, and an initial negative zetapotential (defined
at pH7 in water) are use and treated as in Example 1.
[0149] Microscopic pictures show conformal homogeneous and thin (3
nm) coatings of alumina on the surface of the silicon particles.
The measure of the zeta potential of the resulting material is made
as in Example 8a. The negative surface charge on silicon powder can
be clearly measured. The zeta potential is negative from pH 6 to 2.
In the case of the alumina coated silicon, the powder has a
positive zeta potential from at least pH 7 to at least pH 2.5.
Example 9
Alumina Coated Silicon Monoxide
[0150] In this Example, 5 g of a micrometric silicon monoxide
powder, which consists of a mixture at nanometric scale of Si and
SiO.sub.2, and having a BET of 2 m.sup.2/g, an oxygen content
around 32 wt %, and an initial negative zetapotential (defined at
pH7 in water) are used and treated as in Example 1.
[0151] Microscopic pictures show conformal homogeneous and thin (3
nm) coatings of alumina on the surface of the silicon particles.
The BET surface of this powder and the oxygen content did not
change during the ALD treatment. The measure of the zeta potential
of the resulting material is made according to the following
procedure: 150 ml of both a reference 2 wt % nano-silicon powder
and the alumina coated silicon suspension in demineralised water is
prepared by ultrasonification (120 s at 225 W). The zeta potential
of this suspension in the aqueous medium is measured with the
Zetaprobe Analyser.TM. from Colloidal Dynamics. The samples are
automatically titrated from neutral pH to acid pH with 0.5 M HCl
and to more basic pH with 0.5 M NaOH. The high negative surface
charge on micrometric silicon monoxide powder can be clearly
measured (see FIG. 20, line 2). The zeta potential is negative from
pH 7 to at least 4. In the case of the alumina coated silicon
monoxide, the powder has a positive zeta potential from at least pH
3 to pH 6.8. (FIG. 20, line 1)
Example 10
Alumina and Carbon Coated Silicon
[0152] In this Example, 5 g of a carbon coated nanometric silicon
powder, which consists of a silicon core made according to
WO2012-000858 with a carbon coating made by CVD (chemical vapour
deposition of toluene) technic, and having a BET of 20 m.sup.2/g,
an oxygen content around 4 wt %, and an initial zetapotential
(defined at pH7 in water) near to zero are used and treated as in
Example 1. After the ALD treatment, the particles characteristics
(alumina layer thickness and BET and oxygen content) are similar as
the previous example. The increase of positive charge can be
measured by zetapotential measurement (performed as in the previous
Examples).
Example 11
Adsorption of Nanoparticles on Silicon Surface: Nanoparticles of
In(OH).sub.3
[0153] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, a known
quantity of 1 wt % of In(OH).sub.3 nanoparticles (commercially
available: particles size<30 nm) dispersed in water is added in
order to have a In(OH).sub.3/Si ratio of at least 0.02. This
combined dispersion is placed on a rollerbank for 30 minutes. Below
this weight ratio indium hydroxide/silicon of 0.02, the
agglomeration (silicon particles with adsorbed particles) charge
stays negative; indeed a minimum quantity of particles is
recommended to cover enough silicon surface and have a positive
average charge of the agglomerations.
Example 12
Adsorption of Nanoparticles on Silicon Surface: Nanoparticles of
Alumina Treated Silica
[0154] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, a known
quantity of 2 wt % of alumina treated silica (Levasil 200s)
nano-particles (dispersed in water is added in order to have a
treated SiO.sub.2/Si ratio of at least 1.5. This combined
dispersion is placed on a rollerbank for 30 minutes. Above this
weight ratio treated SiO.sub.2/silicon of 1.5, the agglomeration
(silicon particles with adsorbed particles) charge stays negative;
indeed a minimum quantity of particles is recommended to cover
enough silicon surface and have a positive average charge of the
agglomerations.
[0155] Slurries and batteries are prepared as in the Example 1, and
the result is shown in the FIG. 15 (line2). We clearly see that the
behaviour of the electrode is improved with the adsorption of this
type of nano-particles: after 100 cycles, the delivered capacity
remains around 1600 mAh/g, versus 1000 mAh/g for the pristine
silicon (FIG. 7 line 2).
Example 13
Adsorption of Nanoparticles on Silicon Surface: Nanoparticles of
Iron Oxide
[0156] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, a known
quantity of at least 2 wt % of iron oxide (commercially available,
particles size 20.25 nm) nanoparticles dispersed in water is added
in order to have a Fe.sub.2O.sub.3/Si ratio of at least 2. This
combined dispersion is placed on a rollerbank for 30 minutes. Below
this weight ratio iron oxide/silicon of 2, the agglomeration
(silicon particles with adsorbed particles) charge stays negative;
indeed a minimum quantity of particles is recommended to cover
enough silicon surface and have a positive average charge of the
agglomerations. The negative surface charge on nanometric silicon
powder can be clearly measured. The zeta potential is negative from
pH 4.5 to at least 9. In the case of the iron oxide coated silicon,
the powder has a positive zeta potential from at least pH 2 to pH
8.
[0157] Slurries and batteries are prepared as in the Example 1. We
clearly see that the behaviour of the electrode is improved with
the adsorption of nanoparticles of iron oxide: after 100 cycles,
the delivered capacity remains around 2000 mAh/g, versus 1000 mAh/g
for the pristine silicon (FIG. 7 line 2). In parallel, we can also
observe that the absorbed particles participle to the energy
storage. Indeed, it know that the iron oxide nanoparticles can be
reversibly reduce according to the conversion process.
Example 14
Adsorption of Nanoparticles on Silicon Surface: Nanoparticles of
Magnesium Oxide
[0158] 100 ml of a 1 wt % nano-silicon (made according to
WO2012-000858) suspension in demineralised water is prepared by
ultrasonification (120 s at 225 W). To this suspension, a known
quantity of at least 2 wt % of magnesium oxide (commercially
available) nano-particles (dispersed in water is added in order to
have a MgO/Si ratio of at least 1. This combined dispersion is
placed on a rollerbank for 30 minutes. Below this weight ratio
MgO/silicon, the agglomeration (silicon particles with adsorbed
particles) charge stays negative; indeed a minimum quantity of
particles is recommended to cover enough silicon surface and have a
positive average charge of the agglomerations. The negative surface
charge on nanometric silicon powder can be clearly measured. The
zeta potential is negative from at least pH 3.5 to at least 9. In
the case of the iron oxide coated silicon, the powder has a
positive zeta potential from at least pH 2 to at least pH 9.
* * * * *