U.S. patent application number 12/286898 was filed with the patent office on 2009-04-23 for high elastic modulus polymer electrolytes suitable for preventing thermal runaway in lithium batteries.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Nitash Pervez Balsara, Hany Basam Eitouni, Enrique Daniel Gomez, Scott Mullin, Ashoutosh Panday, Mohit Singh.
Application Number | 20090104523 12/286898 |
Document ID | / |
Family ID | 38801947 |
Filed Date | 2009-04-23 |
United States Patent
Application |
20090104523 |
Kind Code |
A1 |
Mullin; Scott ; et
al. |
April 23, 2009 |
High elastic modulus polymer electrolytes suitable for preventing
thermal runaway in lithium batteries
Abstract
A polymer that combines high ionic conductivity with the
structural properties required for Li electrode stability is useful
as a solid phase electrolyte for high energy density, high cycle
life batteries that do not suffer from failures due to side
reactions and dendrite growth on the Li electrodes, and other
potential applications. The polymer electrolyte includes a linear
block copolymer having a conductive linear polymer block with a
molecular weight of at least 5000 Daltons, a structural linear
polymer block with an elastic modulus in excess of 1.times.10.sup.7
Pa and an ionic conductivity of at least 1.times.10.sup.-5
Scm.sup.-1. The electrolyte is made under dry conditions to achieve
the noted characteristics. In another aspect, the electrolyte
exhibits a conductivity drop when the temperature of electrolyte
increases over a threshold temperature, thereby providing a shutoff
mechanism for preventing thermal runaway in lithium battery
cells.
Inventors: |
Mullin; Scott; (Berkeley,
CA) ; Panday; Ashoutosh; (Berkeley, CA) ;
Balsara; Nitash Pervez; (El Cerrito, CA) ; Singh;
Mohit; (Berkeley, CA) ; Eitouni; Hany Basam;
(Berkeley, CA) ; Gomez; Enrique Daniel;
(Princeton, NJ) |
Correspondence
Address: |
Weaver Austin Villeneuve & Sampson LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
38801947 |
Appl. No.: |
12/286898 |
Filed: |
October 1, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2007/008435 |
Apr 3, 2007 |
|
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12286898 |
|
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60744243 |
Apr 4, 2006 |
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60820331 |
Jul 25, 2006 |
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Current U.S.
Class: |
429/188 ;
320/134; 429/309; 525/187 |
Current CPC
Class: |
C08L 53/00 20130101;
H01B 1/122 20130101; G01N 27/4166 20130101; H01M 10/052 20130101;
H01M 10/443 20130101; Y02E 60/10 20130101; H01M 2300/0082 20130101;
H01M 10/0565 20130101; C08F 297/02 20130101; C08L 53/00 20130101;
C08L 2666/02 20130101 |
Class at
Publication: |
429/188 ;
525/187; 429/309; 320/134 |
International
Class: |
H01M 6/18 20060101
H01M006/18; C08L 53/00 20060101 C08L053/00; H02J 7/00 20060101
H02J007/00 |
Goverment Interests
STATEMENT OF GOVERNMENTAL SUPPORT
[0002] This invention was made with government support under
Contract DE-AC02-05CH11231 awarded by the United States Department
of Energy to The Regents of the University of California for the
management and operation of the Lawrence Berkeley National
Laboratory. The government has certain rights in this invention.
Claims
1. A polymer electrolyte, comprising: a linear block copolymer
having, a Li-ion conductive non-crosslinked linear polymer block
with a molecular weight of at least 5000 Daltons; a structural
linear polymer block with an elastic modulus in excess of
1.times.10.sup.7 Pa; and an ionic conductivity of at least
1.times.10.sup.-5 Scm.sup.-1.
2-20. (canceled)
21. The electrolyte of claim 1, wherein the copolymer is a triblock
copolymer of polystyrene-block-polyisoprene-block poly(ethylene
oxide) (S--I-EO) triblock copolymer.
22. The electrolyte of claim 1, wherein the block copolymer is a
polystyrene-block-poly(ethylene oxide) (SEO) diblock copolymer.
23. The electrolyte of claim 22, wherein the poly(ethylene oxide)
block has a molecular weight of at least 50,000 Daltons.
24. The electrolyte of claim 23, wherein the polystyrene block has
a molecular weight of at least 50,000 Daltons.
25. (canceled)
26. The electrolyte of claim 1, wherein the volume fraction
poly(ethylene oxide) block is between 0.25-0.35.
27. The electrolyte of claim 1, wherein the electrolyte is
configured to dissolve a lithium salt at a first temperature and to
precipitate the lithium salt at a higher temperature.
28. (canceled)
29. The electrolyte of claim 27, wherein the precipitation occurs
at the threshold temperature between 100.degree. C. and 120.degree.
C.
30. (canceled)
31. The electrolyte of claim 27, wherein the precipitation is
accompanied by at least 5-fold drop of conductivity of the
electrolyte comprising a lithium salt.
32. (canceled)
33. A method of making a polymer electrolyte, comprising: in an
oxygen and moisture free environment, forming a linear block
copolymer having a Li-ion conductive linear polymer block with a
molecular weight of at least 5000 Daltons and a structural linear
polymer block with an elastic modulus of at least 1.times.10.sup.7
Pa; and incorporating a Li salt into the linear block copolymer;
wherein the resulting polymer electrolyte has a ionic conductivity
of at least 1.times.10.sup.-5 Scm.sup.-1.
34. (canceled)
35. The method of claim 33, further comprising: after incorporating
the Li salt into the block copolymer, exposing the block copolymer
to a temperature above 25.degree. C. in an annealing and/or casting
process, wherein the annealing and/or casting process is performed
below a threshold precipitation temperature without causing Li salt
precipitation.
36. The method of claim 35 comprising performing annealing and/or
casting at a temperature below 150.degree. C.
37. (canceled)
38. The method of claim 35, wherein the block copolymer is a
polystyrene-block-poly(ethylene oxide) (SEO) diblock copolymer, and
wherein the annealing and/or casting is performed at a temperature
range of between 90-110.degree. C.
39. The method of claim 33, wherein the method comprises:
synthesizing the structural polymer block by living anionic
polymerization; adding a monomer of the conductive polymer block
and a cryptand catalyst to the structural polymer block living
anionic polymerization mixture; allowing a diblock copolymerization
reaction to proceed; terminating the reaction; precipitating and
freeze-drying the resulting diblock copolymer product; blending the
freeze-dried diblock copolymer with the Li salt in a dry solution
such that the Li salt is dissolved into the conductive polymer
block; and freeze-drying the polymer/salt solution.
40. The method of claim 39, further comprising subjecting a portion
of the freeze-dried polymer/salt solution to heat and pressure to
form a freestanding polymer electrolyte film.
41-44. (canceled)
45. The method of claim 33, wherein incorporating a Li salt into
the linear block copolymer comprises blending solutions of the
lithium salt and the linear block copolymer in one or more liquid
solvents, and subjecting the formed solution to heat to evaporate
the one or more liquid solvents and to cast a copolymer film of
desired dimensions, wherein said subjecting to heat dose not expose
the formed solution to the threshold temperature above which
lithium salt precipitation occurs.
46. A battery cell, comprising: a Li anode; a cathode; and a solid
phase polymer electrolyte as claimed in claim 1 disposed between
the anode and cathode.
47. (canceled)
48. The cell of claim 46, wherein the cell is configured for
thermal run-away shutoff by providing a conductivity drop in the
linear block copolymer responsive to a temperature increase of the
block copolymer beyond a threshold temperature.
49. The cell of claim 48, wherein the conductivity drop is at least
5-fold.
50-59. (canceled)
60. A method of operating a battery cell equipped with a thermal
run-away shutoff, the method comprising: (a) providing a battery
cell comprising a Li anode, a cathode, and a solid block copolymer
electrolyte having a lithium salt dissolved therein, wherein the
block copolymer electrolyte is configured for precipitating lithium
salt above a threshold temperature; (b) charging the battery cell;
and (c) discharging the battery cell, wherein the battery cell is
configured for cooling or for reduced heating once the electrolyte
reaches the threshold temperature during battery cell operation,
wherein said cooling or reduced heating is due to conductivity drop
in the electrolyte associated with lithium salt precipitation.
61. (canceled)
62. The method of claim 60, wherein the threshold temperature is
between 100.degree. C. and 120.degree. C.
63. (canceled)
64. The method of claim 60, wherein the block copolymer electrolyte
comprises polystyrene-block-poly(ethylene oxide) (SEO) di-block
copolymer wherein the volume fraction of poly(ethylene oxide) block
is between 0.2 and 0.6.
65. (canceled)
66. A method of screening block copolymer electrolytes for a
block-copolymer electrolyte suitable for preventing thermal runaway
in a cell, the method comprising: (a) providing a plurality of
block-copolymers having a lithium salt dissolved therein; (b)
measuring dependence of conductivity, SAXS, or light scattering
profiles versus temperature for at least some of the provided block
copolymers; (c) based on the measurements obtained in (b),
identifying the block copolymers which exhibit a drop in
conductivity or exhibit evidence of salt precipitation in the SAXS
or light scattering profile with increase of temperature above a
threshold temperature.
67. The method of claim 66, wherein the block copolymers provided
in (a) have a conductivity of at least 1.times.10.sup.-5 Scm.sup.-1
at least at some temperatures of the measured temperature
range.
68. The method of claim 66, further comprising identifying block
copolymers exhibiting at least 5-fold drop in conductivity at a
temperature range of about 90.degree. C.-150.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of the
international application PCT/US2007/008435 filed on Apr. 3, 2007,
titled HIGH ELASTIC MODULUS POLYMER ELECTROLYTES; which claims
priority to U.S. Provisional Patent Application No. 60/744,243
filed Apr. 4, 2006, titled HIGH ELASTIC MODULUS POLYMER
ELECTROLYTES; and to U.S. Provisional Patent Application No.
60/820,331 filed Jul. 25, 2006, titled SYNTHESIS OF DRY POLYMER
ELECTROLYTES. The disclosures of applications listed above are
incorporated herein by reference in their entirety and for all
purposes.
FIELD OF THE INVENTION
[0003] The invention relates to polymer electrolytes. More
particularly, the invention relates to high elastic modulus, high
ionic conductivity polymer electrolytes comprising linear block
copolymers, and methods of making them. The invention further
relates to methods for preventing thermal runaway in lithium
batteries.
BACKGROUND OF THE INVENTION
[0004] Polymer membranes with high ionic conductivity are important
for applications such as solid-state batteries and fuel cells. The
performance of these materials depends not only on their electrical
properties but also on other properties such as shear modulus,
permeability, and the like. The mechanical properties of polymer
electrolytes are particularly important in secondary solid-state
lithium (Li) batteries. One of the challenges in the field of
rechargeable Li ion batteries is to combine high energy density
with good cyclability and electrode stability. Batteries that
employ Li metal anodes for high energy density applications suffer
from failures due to side reactions and dendrite growth on the Li
electrodes. Repeated cycling of the batteries causes roughening of
the Li surface and eventually to dendrite formation, which reduces
battery life and compromises safety.
[0005] Recent theoretical work indicates that dendrite growth can
be stopped if the shear modulus of current polymer electrolytes can
be increased by three orders of magnitude without a significant
decrease in ionic conductivity. Other studies have shown that
cation transport is intimately coupled to segmental motion of the
polymer chains. These studies indicate that dendrite growth on the
electrode surface can be prevented by introducing a highly rigid
electrolyte (elastic modulus of about 1 GPa) between the two
electrodes. This high modulus requirement essentially renders most
rubbery polymer electrolytes incompatible with the electrode
material, as the elastic moduli of typical rubbery polymers are
about 1 MPa. For example, poly(ethylene oxide) (PEO) melt, one of
the most studied polymer electrolytes, has an elastic modulus of
less than 1 MPa. High ionic conductivity is obtained in soft
polymers such as PEO because rapid segmental motion needed for ion
transport also decreases the rigidity of the polymer. Glassy
polymers such as polystyrene offer very high moduli (about 3 GPa)
but are poor ion conductors. Thus, conductivity and high modulus
have appeared to be almost mutually exclusive goals in polymer
electrolytes.
[0006] There is, therefore, a need to develop a new methodology for
decoupling the electrical and mechanical properties of polymer
electrolyte materials. Such a material would be useful as a solid
phase electrolyte for high energy density, high cycle life
batteries that do not suffer from failures due to side reactions
and dendrite growth on the Li electrodes.
[0007] A separate problem encountered during operation of
rechargeable lithium batteries is thermal runaway characterized by
uncontrolled heating of the battery cell during operation (e.g.,
during charging). Such uncontrolled heating occurs because of a
positive feedback which generally exists between cell temperature
and conductivity of the electrolyte in a battery. As the
temperature of the cell rises, the conductivity of electrolyte
increases, leading to additional increase in cell temperature,
leading, in turn, to an increase in electrolyte conductivity, and
so on. When uncontrolled, this cycle can lead to overheating
(thermal runaway) of the cell, which can cause melting of lithium
metal and violent chemical reactions. Thermal runaway is a serious
safety problem in lithium battery design, which is currently
addressed by complex engineering solutions. Development of
additional methods for preventing thermal runaway in rechargeable
lithium batteries is desirable.
SUMMARY OF THE INVENTION
[0008] The present invention provides a polymer that combines high
ionic conductivity with the structural properties required for Li
electrode stability. The polymer is useful as a solid phase
electrolyte for high energy density, high cycle life batteries that
do not suffer from failures due to side reactions and dendrite
growth on the Li electrodes, and other potential applications. In
one aspect, the polymer electrolyte includes a linear block
copolymer having a conductive linear polymer block with a molecular
weight of at least 5000 Daltons, a structural linear polymer block
with an elastic modulus in excess of 1.times.10.sup.7 Pa and an
ionic conductivity of at least 1.times.10.sup.-5 Scm.sup.-1. The
electrolyte is made under dry conditions to achieve the noted
characteristics. The electrolyte includes a lithium salt dissolved
within the conductive linear block of the block copolymer.
[0009] In specific embodiments, the linear block copolymer is a
diblock copolymer characterized by bicontinuous lamellar phases of
the polymer block constituents. In some embodiments the structural
polymer block can form a continuous rigid framework through which
the conductive polymer block forms continuous nanostructured
ionically conductive channels. In one embodiment, the structural
polymer block occupies at least 50%, e.g., at least 60% of block
copolymer by volume, while the conductive block forms cylindrical
nanostructure embedded within the rigid framework of the structural
block. Notably, in some embodiments the structural block is a
glassy polymer in a large temperature range (e.g., up to 90.degree.
C., or even up to 100.degree. C.). In some embodiments, the entire
block copolymer is effectively glassy at least to a temperature of
up to 70.degree. C.
[0010] In one aspect, the invention relates to a polymer
electrolyte. The electrolyte includes a linear block copolymer
having a conductive polymer block with a molecular weight of at
least 5000 Daltons, a structural polymer block with an elastic
modulus in excess of 1.times.10.sup.7 Pa, and an ionic conductivity
of at least 1.times.10.sup.-5 Scm.sup.-1.
[0011] In another aspect, the invention relates to a polymer
electrolyte which is suitable for preventing thermal runaway in a
lithium battery cell. It was unexpectedly discovered that
conductivity of certain polymer electrolytes doped with lithium
salts (specifically, block copolymers) decreases once the
temperature increases above a threshold value. These polymer
electrolytes may be suitable for preventing thermal runaway in
lithium battery cells. Specifically, in some embodiments, the
threshold temperature at which the conductivity drop occurs is
between 90.degree. C. and 150.degree. C., e.g., between 100.degree.
C. and 120.degree. C. In some embodiments the conductivity drop is
at least 5-fold, preferably at least 10-fold. This conductivity
drop is due to precipitation of the lithium salt in the block
copolymer, which, unexpectedly occurs with temperature increase.
Accordingly, in some aspects, a block-copolymer configured for
dissolving a lithium salt at a first temperature and precipitating
this salt at a higher temperature, wherein the precipitation is
accompanied with a conductivity drop, is provided.
[0012] In another aspect, the invention relates to a method of
making a polymer electrolyte. The method involves, in an oxygen and
moisture free environment, forming a linear block copolymer having
a conductive polymer block with a molecular weight of at least 5000
Daltons and a structural polymer block with an elastic modulus of
at least 1.times.10.sup.7 Pa and incorporating a Li salt into the
linear block copolymer such that the resulting polymer electrolyte
has an ionic conductivity of at least 1.times.10.sup.-5
Scm.sup.-1.
[0013] In some embodiments, the method of making the polymer
electrolyte further involves heating the block copolymer having the
lithium salt dissolved therein in a casting and/or annealing
process, to form the polymer electrolyte film. In some embodiments
the casting and/or annealing is performed at a temperature, at
which lithium salt does not precipitate. For example, for those
block copolymers which exhibit salt precipitation with increase of
temperature, the film may be heated at a temperature that is below
the threshold temperature from the range of 90.degree. C. to
150.degree. C. The threshold temperature at which the salt
precipitation occurs therefore, is an important parameter, which is
considered for determining the optimal temperature range for
copolymer film processing. In some embodiments, casting and/or
annealing is performed at a temperature above the glass transition
temperature of the structural block of the copolymer but below the
threshold temperature at which salt precipitation occurs.
[0014] In a further aspect, the invention relates to battery cells
comprising a Li anode, a cathode and linear block copolymer
electrolyte in accordance with the invention disposed between the
anode and cathode. The cell can be cycled without detrimental
dendrite growth on the anode. In one embodiment, the battery cell
includes a linear block copolymer electrolyte which exhibits a drop
in conductivity once the temperature exceeds a threshold
temperature. In some embodiments, the threshold temperature is in a
range from 90.degree. C. to 150.degree. C., and the conductivity
drop is at least 2-fold, or at least 5-fold, and, preferably, at
least 10-fold. Such polymer electrolytes can be used to prevent
thermal runaway in a lithium metal battery.
[0015] In a further aspect, the invention relates to a method of
operating a rechargeable lithium cell. The method involves
providing a battery cell having a lithium anode, a cathode, and a
solid block copolymer electrolyte having a lithium salt dissolved
therein, wherein the block copolymer electrolyte is configured for
precipitating the dissolved lithium salt with increase of
temperature above a threshold temperature. The method includes
charging and discharging the battery cell, with the cell being
configured for cooling or reduced heating once the electrolyte
reaches the threshold temperature. The cooling or reduced heating
is due to conductivity drop in the electrolyte which is caused by
lithium salt precipitation.
[0016] In a further aspect, the invention provides a method for
screening block copolymer electrolytes for those which are suitable
for preventing thermal runaway in a battery cell. The screening
method involves providing a plurality of block copolymers, having a
lithium salt dissolved therein; measuring dependence of
conductivity versus temperature for at least some of the provided
block copolymers; and, based on the obtained measurements,
identifying the block copolymers which exhibit a drop in
conductivity with increase of temperature above a threshold
temperature. For example, block copolymers exhibiting a 5-fold,
preferably 10-fold decrease in conductivity, after the temperature
is increased above the threshold temperature from the range of
between 90.degree. C. and 150.degree. C., may be selected for use
in battery cells configured for thermal runaway shutoff.
[0017] These and other aspects and advantages of the present
invention are described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The accompanying drawings, which are incorporated into and
constitute a part of this specification, illustrate one or more
embodiments and, together with the detailed description, serve to
explain principles and implementations of the invention.
[0019] In the drawings:
[0020] FIG. 1A is an illustration of a block copolymer in
accordance with the present invention in isolation, showing the
linearity of the polymer block chains.
[0021] FIG. 1B depicts the chemical structure of a
polystyrene-b-poly(ethylene oxide) (PS-b-PEO, also referred to as
SEO) diblock in accordance with one embodiment of the present
invention.
[0022] FIGS. 2A-C illustrate embodiments of a PS-b-PEO diblock.
[0023] FIG. 3 is a graph illustrating Small Angle X-ray Scattering
(SAXS) profiles obtained from SEO/salt and SIEO/salt mixtures in
accordance with the present invention.
[0024] FIG. 4A is a graph illustrating the dependence of ionic
conductivity on r (Li salt per EO unit) and temperature in SEO
(36-25), a block copolymer electrolyte in accordance with the
present invention.
[0025] FIG. 4B is a graph illustrating the dependence of ionic
conductivity on r (Li salt per EO unit) in SEO (74-98), a block
copolymer electrolyte in accordance with the present invention.
[0026] FIGS. 5A-5C are plots illustrating the dependence of ionic
conductivity of block copolymer electrolytes in accordance with the
present invention on the molecular weight of their respective PEO
blocks.
[0027] FIG. 6 is a graph illustrating the frequency dependence of
the storage and loss shear moduli of SEO(36-25), a block copolymer
electrolyte in accordance with the present invention, with and
without salt, and PEO.
[0028] FIG. 7 is a graph illustrating the frequency dependence of
elastic moduli for the series of copolymers referenced in FIG.
3.
[0029] FIG. 8 is a plot of the results of DC cycling of a Li/SEO/Li
cell with an electrolyte in accordance with the present
invention.
[0030] FIG. 9A is a bright field electron micrograph of a block
copolymer in accordance with the present invention.
[0031] FIG. 9B is a lithium energy filtered electron micrograph of
the same block copolymer region as FIG. 9A.
[0032] FIG. 10 presents data showing lamellae thicknesses for block
copolymers in accordance with the present invention obtained from
various measurements.
[0033] FIG. 11 is a plot of Li lamellae thickness normalized by the
PEO lamellae thickness as a function of molecular weight.
[0034] FIG. 12 is a plot comparing the normalized Li lamellae
thickness and normalized conductivity versus molecular weight for
various block copolymers in accordance with the present
invention.
[0035] FIG. 13 is a simple schematic diagram of a battery cell in
accordance with the present invention.
[0036] FIG. 14 is a plot illustrating the dependence of
conductivity on inverse temperature for a SEO (54-23) block
copolymer. A drop in conductivity above a threshold temperature is
illustrated.
[0037] FIG. 15 is a SAXS plot illustrating precipitation of lithium
salt in a SEO (54-23) block copolymer with increase in
temperature.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0038] Reference will now be made in detail to specific embodiments
of the invention. Examples of the specific embodiments are
illustrated in the accompanying drawings. While the invention will
be described in conjunction with these specific embodiments, it
will be understood that it is not intended to limit the invention
to such specific embodiments. On the contrary, it is intended to
cover alternatives, modifications, and equivalents as may be
included within the spirit and scope of the invention as defined by
the appended claims. In the following description, numerous
specific details are set forth in order to provide a thorough
understanding of the present invention. The present invention may
be practiced without some or all of these specific details. In
other instances, well known process operations have not been
described in detail in order to not unnecessarily obscure the
present invention.
[0039] It will, of course, be appreciated that in the development
of any actual implementation of the invention, numerous
implementation-specific decisions must be made in order to achieve
the developer's specific goals, such as compliance with
application- and business-related constraints, and that these
specific goals will vary from one implementation to another and
from one developer to another. Moreover, it will be appreciated
that such a development effort might be complex and time-consuming,
but would nevertheless be a routine undertaking of engineering for
those of ordinary skill in the art having the benefit of this
disclosure.
[0040] Introduction
[0041] As noted above, in one aspect, the present invention
provides a polymer that combines high ionic conductivity with the
structural properties required for Li electrode stability. The
polymer is useful as a solid phase electrolyte for high energy
density, high cycle life batteries that do not suffer from failures
due to side reactions and dendrite growth on the Li electrodes, and
other potential applications. The polymer electrolyte includes a
linear block copolymer having a conductive linear polymer block
with a molecular weight of at least 5000 Daltons, a structural
linear polymer block with an elastic modulus in excess of
1.times.10.sup.7 Pa and an ionic conductivity of at least
1.times.10.sup.-5 Scm.sup.-1. In some embodiments, the block
copolymer has an elastic modulus of greater than 100 MPa, e.g.,
300-700 MPa. The electrolyte is made under dry conditions to
achieve the noted characteristics.
[0042] In specific embodiments, the linear block copolymer is a
diblock copolymer characterized by bicontinuous lamellar phases of
the polymer block constituents, and the structural polymer block
can form a rigid framework in which the conductive polymer block
forms nanostructured ionically conductive channels. In other
embodiments, the conductive block forms a cylindrical nanostructure
within the rigid framework formed by the structural block.
[0043] Linear Block Copolymers
[0044] A linear block copolymer can be used to produce a polymer
electrolyte that can combine high ionic conductivity with the
structural properties required for Li metal electrode stability.
The electrolyte is useful as a solid phase electrolyte for high
energy density, high cycle life batteries that do not suffer from
failures due to side reactions and dendrite growth on the Li
electrodes. The linear block copolymer may be a composite polymer
electrolyte having relatively soft (e.g., having an elastic modulus
on the order of 1 MPa or less) nanoscale conducting channels
embedded in a relatively hard (e.g., having an elastic modulus on
the order of 1.times.10.sup.7 Pa or more) polymer matrix that need
not be ionically conductive. The resulting block copolymer
preferably has an elastic modulus of at least 100 MPa, e.g.,
300-700 MPa. Ionic conductivity in the channels is conferred by a
Li salt incorporated with the soft polymer block. It has been found
that particular configurations and fabrications of such linear
block copolymers enable the combination of high ionic conductivity
and high elastic modulus in a polymer electrolyte.
[0045] While prior work in the field has suggested block copolymers
as suitable materials for electrolytes, these prior block
copolymers were composed of relatively soft (non-glassy) polymer
blocks that were modified with various side chains.
[0046] The electrolytes of the present invention, however, comprise
block copolymers with linear polymer block constituents, one of
which is soft and conductive to Li ions; another of which is hard
and need not be ionically conductive. FIG. 1A provides an
illustration of such a copolymer in isolation, showing the
linearity of the polymer block chains.
[0047] The inventive polymer electrolytes, in one aspect, include a
linear block copolymer having a conductive linear polymer block
with a molecular weight of at least 5000 Daltons, a structural
linear polymer block with an elastic modulus in excess of
1.times.10.sup.7 Pa and an ionic conductivity of at least
1.times.10.sup.-5 Scm.sup.-1. The polymer, in some embodiments, is
non-crosslinked. Remarkably, even without crosslinking high elastic
modulus values of at least 100 MPa are achieved, due to rigidity of
the structural block. In other embodiments, the structural block of
the copolymer is cross-linked, while the conductive block remains
non-crosslinked.
[0048] In a specific embodiment, the copolymer is a linear diblock
copolymer of poly(ethylene oxide) (a rubbery, linear polymer,
highly ionically conductive when embedded with an appropriate Li
salt) with polystyrene (a non-conductive, glassy, linear polymer);
a polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock, the
chemical structure of which, is illustrated in FIG. 1B. Notably,
both polystyrene (PS) and polyethyleneoxide (PEO) blocks are linear
and are covalently connected with one another without forming
branches. This combination has been found to provide both the high
ionic conductivity and high rigidity sought after for high
performance solid state polymer electrolytes. It is understood that
the SEO polymer can have a variety of terminating groups instead of
illustrated sec-Butyl group at the polystyrene terminus and OH at
the PEO terminus. For example, a variety of alkyl groups can be
introduced at the polystyrene terminus (such as methyl, ethyl,
propyl, i-propyl, n-butyl, etc.). The PEO terminus, in some
embodiments, contains an alkoxy terminal group (such as methoxy,
ethoxy, propoxy group, etc.). In some embodiments alkoxy-terminated
polymers are preferable over hydroxy-terminated polymers, due to
the lower reactivity of the former.
[0049] FIGS. 2A-C illustrate embodiments of a PS-b-PEO diblock in
accordance with the invention. The PS-b-PEO diblock, generally
numbered 10, exhibits an extensive phase behavior, which allows for
bicontinuous phases. In these embodiments, in the bicontinuous
PS-b-PEO phases, the major (e.g., greater than 50%) phase (PS) 12
provides a rigid framework for the nanostructured ionically
conducting channels (PEO) 14.
[0050] FIGS. 2A-C illustrate the shape and placement of polymer
blocks comprising conceptual diblocks in accordance with the
present invention, and are not meant to be limiting. It will be
known and understood that the microstructure of the diblock may be
in any shape that is necessary and the placement of the polymers
within the diblock may vary as needed. As schematically illustrated
in the FIG. 2A, the cylindrical PEO channels 14 may be positioned
in an orderly placement within the PS matrix 12. The nanostructure,
shown in FIG. 2A is referred to as cylindrical nanostructure.
Alternatively, as illustrated in FIG. 2B, the PEO channels 14 may
be branched, such as in a gyroid phase, within the PS matrix 12.
FIG. 2C illustrates another specific embodiment in which the PEO
channels 18 are layered in a lamellar arrangement with the PS
matrix 16. While it has previously been believed that network
morphologies are essential for ion transport, it was surprisingly
found that diblock copolymers in accordance with the present
invention with such non-network lamellar morphologies are very
effective ion transporters.
[0051] In many instances, the diblock copolymer has a
non-conducting phase that is continuous, and, in most cases, the
major component, which differs from previously proposed block
copolymer electrolytes. The mechanical properties of such block
copolymer electrolytes are dominated by those of the non-conducting
component. This enables independent control over the electrical and
mechanical properties of the electrolyte. The structural linear
polymer block major phase forms a rigid framework, with the
conductive polymer block forming nanostructured ionically
conductive channels through the rigid framework.
[0052] In specific embodiments, the linear block copolymer is a
diblock copolymer and may be characterized by bicontinuous phases
of the polymer block constituents such that there are continuous
ionically-conductive channels (or pathways) through a continuous
rigid framework. In this way, the beneficial properties of both
polymer block materials (structural and conductive) exist
throughout the material. Block copolymers are known in the art to
self-assemble into a variety of structures or arrangements. As
noted above, a particular arrangement for diblock copolymers in
accordance with the present invention with advantageous properties
is lamellar, although other arrangements are possible.
[0053] An important feature of the block copolymers is the
combination of soft and hard polymer blocks to form a rigid
material that is also highly ionically conductive. Thus, the
structural polymer block has an elastic modulus of at least
1.times.10.sup.7 Pa, and generally from about 1.times.10.sup.7 Pa
to 3 GPa. A suitable polymer for the structural block is
polystyrene, a glassy polymer with an elastic modulus of about 3
GPa. Other mechanisms for obtaining suitable rigid frameworks
include cross-linking and crystallization of structural blocks. The
ionically conductive polymer, on the other hand, typically has an
elastic modulus of no more than 1 MPa; poly(ethylene oxide), a
specific material for which advantageous properties have been found
in block polymers of the present invention, has an elastic modulus
of less than 1 MPa. The resulting block copolymer in some
embodiments has an elastic modulus of at least 100 MPa. Further,
the resulting block-copolymer, in some embodiments, is glassy at a
relatively large temperature range (e.g., up to 70.degree. C., and
even up to 90.degree. C.).
[0054] Other examples of polymers suitable as conductive or
structural polymers for these block copolymer electrolytes include
poly vinyl pyrrolidine (conductive), poly acrylates (conductive),
polyvinylcyclohexane (structural), epoxies (structural), and
polypropylene (structural).
[0055] As noted above, the conductive polymer block chains of the
block copolymer have a molecular weight of at least 5000 Daltons.
This is notable since prior work in the field has indicated an
inverse relationship between the molecular weight of conductive
polymers and their ionic conductivity. To the contrary, it has been
found that the ionic conductivity of the block copolymers of the
present invention increases with increasing molecular weight of the
constituent polymer blocks. Accordingly, in various embodiments,
the structural polymer has a molecular weight of at least 5000
Daltons. Further, in various embodiments, the ionic conductivity of
the block copolymer increases with increasing molecular weight of
the conductive polymer block, so the conductive and structural
polymer block chains have molecular weights of at least 15,000
Daltons; or at least 20,000 Daltons; or at least 50,000 Daltons; or
up to about 100,000 Daltons or more. In some embodiments, the
molecular weight of the structural block is at least 200,000, e.g.,
or at least 300,000. Ionic conductivity of greater than
1.times.10.sup.-4 Scm.sup.-1 and up to about 1.times.10.sup.-3
Scm.sup.-1 has been achieved with these relatively high molecular
weight block copolymers in accordance with the present
invention.
[0056] As noted above, ionic conductivity is achieved for the
conductive polymer block by incorporating an appropriate Li salt
into the conductive polymer block. It is important that
incorporation of the Li salt be done in a controlled environment
and under conditions selected to achieve the desired performance in
the resulting polymer electrolyte product. This is discussed
further below. Suitable Li salts include lithium
bis(pentafluoroethane sulfonyl) imide, lithium
bis[1,2-oxalato(2-)--O,O']borate (LiBOB), and lithium
bis(trifluoromethane sulfonyl) imide (LiN[SO.sub.2CF.sub.3].sub.2,
also known as LiTFSI) LiClO.sub.4, LiPF.sub.6, LiAsF.sub.6,
LiSO.sub.2CF.sub.3 (also known as lithium triflate), and others
known in the art. One specific salt is LiN[SO.sub.2CF.sub.3].sub.2
(also known as LiTFSI). Self-assembly of the conductive polymer
block results in ionically conductive channels in the rigid
structural polymer block framework. The salts and the polymers are
selected such that the lithium salt is capable of dissolving within
the conductive portion of the block copolymer. For example LiTFSI
has a solubility limit of 56 wt % in PEO. The concentration of
lithium salt within the block copolymer is also given by a ratio of
molar concentration of lithium ion per molar concentration of
conductive block monomer unit. For example, for SEO block
copolymers, the concentration of lithium salt is given by
r=[Li]/[EO], where EO is ethylene oxide monomer unit. In some
embodiments r ranges from 0.02 to 0.12, e.g., from 0.05 to 0.1.
[0057] Further, as also described below, it has been found that the
enhanced ionic conductivity obtained with increasing molecular
weight of the conductive polymer block is correlated with a
concentration of Li salt in a central portion of the ionically
conductive channels in the block copolymer electrolytes.
[0058] While many specific embodiments of the invention exhibit a
diblock copolymer structure, other structures are also possible.
For example, an additional polymer block may be added to adjust the
properties of the resulting copolymer, in this case a triblock
copolymer. One suitable triblock copolymer is a
polystyrene-block-polyisoprene-block poly(ethylene oxide) (S--I-EO)
triblock copolymer. For example, a S--I-EO triblock copolymer with
molecular weights of 11,000 Daltons for the polystyrene block, 6000
Daltons for the polyisoprene block, and 9000 Daltons for the
poly(ethylene oxide) block was found to have a measured
conductivity range from 4.times.10.sup.-4 Scm.sup.-1 to
9.times.10.sup.-4 Scm.sup.-1 in the 90-120.degree. C. temperature
range, for a molar ratio of Li ions to ethylene oxide, r=0.02.
[0059] In a separate aspect, solid polymer electrolytes which
exhibit a conductivity drop upon increase of temperature above a
threshold temperature value are provided. This intrinsic property
of certain polymer electrolytes can be used to prevent thermal
runaway in lithium battery cells. The term "prevent", as used
herein, means "to decrease the probability of thermal runaway
failure in a battery cell", and is not intended to mean that the
described polymers entirely eliminate the possibility of thermal
runaway under all conditions.
[0060] It was unexpectedly discovered that certain Li salt doped
block copolymers, such as linear block copolymers described above,
exhibit a drop in conductivity when the temperature of polymer
electrolyte increases above a threshold value. The conductivity
drop was found to be caused by precipitation of dissolved lithium
salt, which occurs above a threshold temperature. This behavior is
entirely unexpected, because it is typical for the conductivity of
salt-doped polymers to increase with the temperature increase. For
example, conductivity of Li salt doped PEO polymer increases with
increasing temperature, without exhibiting a drop, and without any
indication of lithium salt precipitation with temperature increase.
Remarkably, in block copolymers described herein, the conductivity
increases until a threshold temperature value which lies in a range
of between 90.degree. C. and 150.degree. C., and starts to drop
after the threshold temperature value has been reached. In some
embodiments conductivity drop is at least 2-fold, or at least
5-fold, or even at least 10-fold. For example, the conductivity can
increase until a threshold temperature of 110.degree. C. is
reached. Above this temperature, the conductivity can drop by an
order of magnitude because of salt precipitation. The conductivity
drop typically occurs at least within 30 minutes of exceeding the
threshold temperature.
[0061] A number of block copolymers described above were found to
exhibit this interesting and valuable property. While it is
preferable to use copolymers having improved structural properties
as described in the previous sections, the conductivity drop is
considered to be an intrinsic property of certain block copolymer
systems, and is not limited to block copolymers having high elastic
modulus.
[0062] While many of the copolymers described above exhibit the
conductivity drop, it is particularly pronounced in copolymers
having cylindrical nanostructure morphology (such as illustrated in
FIG. 2A). At least some lamellar samples also exhibit the drop, but
sometimes to a lesser degree. The drop is also particularly
pronounced in block copolymers having a volume fraction of
conductive block in the range of 0.2-0.6, more preferably, of
0.2-0.5, such as 0.25-0.35. Further, the conductivity drop, in some
embodiments, was found to be pronounced in relatively high
molecular weight block copolymers, e.g., in block copolymers having
molecular weight of greater than 200 kg/mol, and even greater than
300 kg/mol. For example, SEO samples SEO (54-23), SEO (216-102),
and SEO (360-165) having cylindrical ordered nanostructure and
having volume fraction of conductive EO phase 0.28, 0.3, and 0.3
respectively, all exhibited a conductivity drop of at least 2-fold
(e.g., a 10-fold drop) upon reaching a threshold temperature in a
range of 90-120.degree. C. In SEO (54-23), the first value (54)
refers to a molecular weight of polystyrene structural block, while
the second value (23) refers to molecular weight of conductive PEO
block, where both values are given in kg/mol. Other polymer samples
are labeled using an analogous scheme.
[0063] In one aspect, a method of screening block copolymers for a
block copolymer electrolyte suitable for preventing thermal runaway
in a cell, is provided. The method involves providing a plurality
of block copolymers doped with a lithium salt (such as LiTFSI or
other salt mentioned above), and measuring the conductivity
dependence of the formed solid electrolyte versus temperature. The
measurements will preferably, include at least a portion of the
90.degree. C.-150.degree. C. temperature range. For example
conductivity dependence can be measured in a range from 25.degree.
C. to 150.degree. C. The plurality of block copolymers can include
any block copolymer without limitation, but, in some embodiments,
includes only linear block copolymers having linear conductive and
structural phases, such as polymers described above. Further,
preferably, but not necessarily, all block copolymers used for
screening should have good structural properties, such as high
elastic modulus, e.g., a modulus of greater than 100 MPa. The
measurement of conductivity vs. temperature dependence is performed
individually for each polymer and can be performed either serially
or in parallel. Such measurements can be performed, for example,
using ac impedance spectroscopy on a Solartron 1260 frequency
response analyzer available from Solartron Analytical of
Farnborough, UK.
[0064] Based on the results of this measurement, copolymers
exhibiting a conductivity drop at a threshold temperature are
identified, and are selected as candidates for solid polymer
electrolytes suitable for thermal shutoff of a Li battery cell. In
some embodiments, copolymers exhibiting at least a 2-fold
conductivity drop, or at least a 5-fold conductivity drop, or at
least a 10-fold conductivity drop are selected. Further,
preferably, polymers having high conductivity (e.g., at least
1.times.10.sup.-5 Scm.sup.-1 prior to the conductivity drop) and
good structural properties (as previously described) are selected.
In some embodiments only polymers having high conductivity and good
structural properties are screened.
[0065] Table 1 illustrates linear SEO block copolymers which were
screened for conductivity drop by measuring the dependence of
conductivity on temperature. The conductivities were measured
during heating from 25 to 120.degree. C. Of the 13 samples that
were screened, five samples exhibited conductivity drops greater
than about 2-fold. "About" as used herein refers to a range which
includes .+-.0.5 interval around the recited value. The molecular
weights for PS and PEO fraction, a volume fraction of EO in the
polymer, and the conductivity drop data are shown in Table 1.
TABLE-US-00001 TABLE 1 Block Copolymers screened for conductivity
drop Mn Mn Volume SAMPLE (kg/mol) (kg/mol) Fraction Conductivity
Threshold ID PS PEO of EO drop (fold) Temperature SEO-1 36.3 24.6
0.38 SEO-2 39.7 31.3 0.42 SEO-3 18.8 29.1 0.58 SEO-4 39.6 53.6 0.55
SEO-5 74.0 98.0 0.55 SEO-6 16.2 16.3 0.48 about 4 100.degree. C.
SEO-7 52.9 67.6 0.54 SEO-8 37.0 25.4 0.385 SEO-9 53.5 22.9 0.281
about 10 100.degree. C. SEO-11 6.3 7.2 0.519 about 3 90.degree. C.
SEO-15 246.8 116.1 0.3 about 2-3 120.degree. C. SEO-16 352.1 165.6
0.3 about 3-4 110.degree. C. SEO-17 178.6 199.9 0.5
[0066] Of the polymers listed in Table 1, SEO9, SEO15, and SEO16
had a cylindrical morphology. The remaining polymers which showed a
conductivity drop had a lamellar morphology.
[0067] An alternative method for identifying polymers that are
suitable for preventing thermal runaway in batteries involves
screening salt-doped polymers for evidence of salt precipitation
occurring with temperature increase. In one embodiment, a plurality
of salt-doped polymers are screened using SAXS. A SAXS profile for
each polymer is measured at several temperatures, starting at lower
temperature, and up to e.g., 120-150.degree. C. Precipitation of
salt (which correlates with conductivity drop) manifests itself in
a SAXS profile by increase of intensity in low-q scattering. The
samples which exhibit increase in low-q scattering with increase of
temperature are identified as candidate polymers for preventing
thermal runaway in lithium batteries.
[0068] In another embodiment screening for salt precipitation is
performed by measuring light scattering of a plurality of samples.
Salt precipitation manifests itself by an increase in light
scattering. Similarly to the SAXS-based screening method described
above, a plurality of salt-doped polymers are subjected to light
scattering measurement. Light scattering for each polymer sample is
recorded at several temperatures, typically starting at lower
temperature, and increasing the temperature to, e.g.,
120-150.degree. C. Those polymers which exhibit increase in light
scattering after the temperature reaches a threshold value, are
then selected.
[0069] Electrolyte Fabrication
[0070] In one aspect, a method of making a polymer electrolyte in
accordance with the present invention comprises, in an oxygen and
moisture free environment, forming a linear block copolymer having
a Li ion conductive linear polymer block with a molecular weight of
at least 5000 Daltons and a structural linear polymer block with an
elastic modulus of at least 1.times.10.sup.7 Pa; and incorporating
a Li salt into the linear block copolymer, such that the resulting
polymer electrolyte has an ionic conductivity of at least
1.times.10.sup.-5 Scm.sup.-1. Care must be taken in the manufacture
of the linear block copolymer electrolytes to prevent adverse
reaction of the Li salt and to obtain the polymer product with the
advantageous performance noted herein.
[0071] A block copolymer electrolyte in accordance with the present
invention can be made by synthesizing the structural polymer block
by living anionic polymerization. Then, a monomer of the conductive
polymer block and a crypt and catalyst are added to the structural
polymer block living anionic polymerization mixture. The
polymerization of the conductive block is then allowed to proceed.
The reaction may proceed to completion and, after it is terminated
with a suitable reagent, the resulting diblock copolymer product is
precipitated and freeze-dried. Other living polymerization methods
like cationic and radical polymerization techniques can also be
used to synthesize the block copolymers.
[0072] To render the copolymer conductive to Li ions, a Li salt is
blended with the freeze-dried diblock copolymer using a
moisture-free solvent such that the Li salt is dissolved with the
conductive polymer block. The resulting polymer/salt solution is
then freeze dried to remove the solvent. The freeze-dried dry
polymer/salt mixture can then be subjected to heat and pressure
(compression molded) to form a freestanding polymer electrolyte
film.
[0073] A detailed description of a method of making a linear
diblock copolymer as applied to a specific embodiment follows. The
following example will be described using SEO; however, it will now
be realized that other materials and preparation methods may be
used. As such, the following example is discussed for exemplary
purposes only and is not intended to be limiting.
[0074] Synthesis of PS-b-PEO
[0075] In an exemplary embodiment of the invention, the diblock
copolymer, polystyrene-b-poly(ethylene oxide) (SEO) is synthesized
using living anionic polymerization to attain good control of the
molecular weight and achieve low polydispersity. All of the
synthetic steps are performed on a high-vacuum line and inside an
argon glove box to ensure an oxygen and moisture free
environment.
[0076] The first step is to purify the solvent, benzene, over
calcium hydride for at least 12 hours at room temperature. Benzene
is then distilled onto a sec-butyllithium stage for further
purification.
[0077] The next step is purification of the styrene monomer, and
synthesis of polystyrene. The styrene monomer is purified on
calcium hydride (CaH.sub.2) by stirring for a few hours to remove
trace amounts of water. The monomer is then distilled into a flask
(e.g., short-neck round-bottom flask) containing dibutyl magnesium
(DBM). The monomer is stirred over DBM overnight. A clean reactor,
in which polymerization will take place, is placed on the
high-vacuum line and flamed with a torch to remove adsorbed water
and solvent. After flaming, about 20 mL of the purified solvent is
distilled from the sec-butyllithium stage into the clean reactor.
This is done to prevent freezing and degassing a large amount of
solvent. After the addition of 20 mL of benzene, the reactor is
taken off the vacuum line, and into the glove box at which point
the calculated amount of sec-butyllithium initiator required for
reaction is pipetted into the reactor.
[0078] A very good seal is maintained during transfer back onto the
vacuum line. After the reactor is placed back on the line, the
mixture is frozen with liquid nitrogen and degassed 1, 2, 3, 4, or
5 times to remove trapped argon from the vessel and promote fast
distillation. The remainder of the solvent can then be distilled
into the reactor. In one arrangement, enough solvent is added to
yield a mixture of approximately 8-10 wt % polymer in solution.
[0079] The first block, polystyrene (seen as 12 in FIGS. 2A-B, and
16 in FIG. 2C) is synthesized by distilling a calculated amount of
styrene from the DBM stage into a graduated ampoule and then into
the reacting vessel, which contains benzene and the
sec-butyllithium initiator. The reaction is carried out at room
temperature for approximately 12 hours and a characteristic yellow
"living" polymer mixture is obtained upon completion. The mixture
is stirred continuously during the reaction to ensure uniform
mixing of the monomer. The reactor is then taken into the glove box
and an aliquot is taken out and terminated with degassed methanol
or isopropanol for characterization using gel permeation
chromatography to determine its molecular weight.
[0080] This completes the synthesis of the first block, ready for
the addition of the ethylene oxide (EO) block. A flask (e.g.,
short-neck round-bottom flask) with freshly powdered calcium
hydride is placed on the vacuum line and opened up to vacuum to
remove all air. The flask is maintained at 0.degree. C. Ethylene
oxide monomer, a gas at room temperature (b.p. 10.degree. C.) is
condensed onto the calcium hydride at 0.degree. C. and left to stir
overnight to remove residual moisture. The monomer-containing flask
is maintained at a dry ice/IPA mixture temperature. The ethylene
oxide is kept cold at all times during purification. The ethylene
oxide/calcium hydride mixture is frozen and degassed twice before
the purification steps.
[0081] The monomer is purified with two stages of n-butyllithium by
stirring for 30 minutes at 0.degree. C. at each stage. 10 mL of
n-butyllithium in cyclohexane is used for each stage. The purifying
agent, n-butyllithium, is completely dried by pulling vacuum to get
rid of the cyclohexane solvent in which the n-butyllithium is
dissolved. This can prevent contamination and ensures an accurate
measurement of the amount of ethylene oxide (EO) monomer. The
ethylene oxide monomer is distilled into a graduated ampoule
maintained at 0.degree. C. after purifying at both stages of
n-butyllithium.
[0082] Following purification and distillation into a graduated
ampoule, 1 ml of the EO monomer is distilled into the living
polystyrene reactor for addition of a single EO unit. Only one EO
unit is added to the living polystyrene chain because of the strong
complexation of the lithium cation to the oxygen in the ethylene
oxide unit. The Li cation shows a strong association with the
ethylene oxide unit and, thus, no propagation reaction can occur.
To polymerize the second block, a cryptand catalyst (e.g.,
tert-butyl phosphazene base
1-tert-Butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosph-
oranylidenamino]-2.LAMBDA..sup.5,
4.LAMBDA..sup.5-catenadi(phosphazene) (tBu-P.sub.4), available from
Aldrich. Milwaukee, Wis.) is added to the reaction mixture
(containing living EO capped polystyrene chains) to complex the
lithium ions and facilitate the polymerization of ethylene oxide.
To prepare the catalyst base, the solvent in which it is dissolved
is removed and a known amount of benzene distilled into the flask.
This solution can then be pipetted into the reactor in the
glovebox. The reactor is then reattached to the high-vacuum line
and degassed 1, 2, 3, 4, or 5 times to remove trapped argon from
the vessel and promote fast distillation. The remainder of the EO
is then distilled from the graduated ampoule into the reactor.
[0083] The reaction is allowed to proceed for three to four days at
45.degree. C. during which time the color changes to a dark blue.
The diblock copolymer (SEO) is then terminated with methanol in the
glove box and purified by precipitation in cold hexane. In those
instances, when alkoxy-terminated polymer is desired, the reaction
is quenched with an alkylating agent, such as alkyl iodide (e.g.,
methyl iodide). The precipitated polymer is dissolved in benzene
and filtered through a 0.2 .mu.m filter. The filtered polymer is
then freeze dried to remove all solvent. The freeze-dried polymer
is characterized by gel permeation chromatography (GPC) to
determine the molecular weight and by nuclear magnetic resonance
(NMR) to calculate the volume fraction of each block.
[0084] Electrolyte Preparation
[0085] Polymer electrolytes are prepared by blending the
freeze-dried SEO copolymers with the lithium salt,
LiN[SO.sub.2CF.sub.3].sub.2 (LiTFSI, available from Aldrich) in a
moisture free environment. This is achieved in a few steps. A
LiTFSI/THF solution (about 10% w/w) is prepared in the glove box,
and stored as a stock solution. All the copolymer samples are
weighed and vacuum dried in a heated antechamber and brought into
the glove box. The dry copolymer samples are then dissolved in dry
benzene. To this copolymer/benzene solution, the necessary amount
of the LiTFSI stock solution is added in the argon glove box. All
solvents used in the electrolyte preparation are doubly-distilled
to remove trace amounts of moisture. The polymer/salt solution is
then freeze-dried in a glove box compatible desiccator for one week
to remove all solvent. The freeze-dried polymer/salt mixture is
loosely packed into a pellet in a die that has a diameter slightly
less than the spacer inner diameter. The pellet is placed in a
Teflon.TM. bag along with a spacer, such that the pellet is in the
center of the spacer's slot. The spacer is made of a non-conducting
reinforced resin, and can withstand high pressure and temperature
without deforming. The pellet is then subjected to a high pressure
(about 1 kpsi) at about 100.degree. C. This yields a clear,
freestanding polymer electrolyte film with the spacer around it.
The spacer defines the edge and the thickness (and hence the area,
and the volume) of the freestanding polymer electrolyte film, and
provides a means for easy transfer in and out of the
membrane-electrode assembly. All the steps are carried out in the
glove box.
[0086] In a separate aspect, a method of making block copolymers is
adjusted such that during preparation, the block copolymer with an
embedded salt is not exposed to a temperature that is higher than
the threshold temperature for lithium salt precipitation. This
adjustment is made for those polymers which exhibit conductivity
drop above a threshold temperature due to salt precipitation.
Therefore, in these embodiments after the polymer/salt mixture is
made and is freeze-dried, the freeze-dried pellet is subjected to
high temperature and pressure to form a freestanding polymer
electrolyte film, where the temperature of the anneal is adjusted
to be below the threshold temperature at which lithium salt
precipitation occurs. Depending on a particular polymer and the
threshold temperature, the anneal is performed below 150.degree.
C., or below 110.degree. C. In some embodiments, due to these
considerations, a relatively narrow temperature range for anneal is
found to be preferable, such as 80-120.degree. C., or
100-110.degree. C. When annealing is performed above the threshold
temperature for lithium salt precipitation, due to precipitation of
the salt, the conductivity of polymer electrolyte substantially
drops. When annealing is performed at too low a temperature, the
film cannot always properly form to have the desired nanostructure.
In some embodiments the temperature for annealing and/or casting is
selected such that it is greater than the glass transition
temperature of the structural block material of the copolymer but
is lower than the threshold salt precipitation temperature. One of
skill in the art will understand how to select a temperature range
for annealing a polymer electrolyte sample, given these
guidelines.
[0087] In a separate, but related aspect, the method of making the
polymer electrolyte film involves casting the polymer at high
temperature. In this case, instead of freeze-drying, evaporation of
the solvent from the solution of the polymer and the salt dissolved
in a solvent, is performed. Upon evaporation, the polymer is heated
and is casted such that it adopts a shape needed for the
electrolyte film. Similarly to the previous example, in those
embodiments where the polymer electrolyte precipitates the lithium
salt above a threshold temperature, the process is adjusted such
that casting and/or evaporation of the solvent is performed below
the threshold temperature of salt precipitation. In some
embodiments, these considerations provide a relatively narrow
temperature window for the casting and/or evaporation process.
Specifically, many copolymers described herein can be cast only
above 90-100.degree. C. (or their corresponding Tg temperature), as
they are too hard and glassy at lower temperatures. However, above
the threshold temperature for lithium salt precipitation, the
polymer loses a lot of its conductivity. Therefore, casting is
performed at a temperature range at which the polymer is
sufficiently soft for casting and at which salt precipitation does
not occur. In some embodiments, this temperature range is between
90-110.degree. C.
[0088] Structure and Performance
[0089] While the invention is not limited by any particular theory,
it is believed that the high elastic modulus, high ionic
conductivity block copolymers of the present invention are
correlated with a particular morphology. It has been found that for
at least some specific embodiments of the invention with
advantageous properties, the bicontinuous phases of the polymer
block constituents of the diblock copolymer have a lamellar
arrangement and that the Li salt segregates itself to the ionically
conductive block channels. In addition, with increasing molecular
weight of the polymer block constituents, the Li salt increasingly
concentrates itself to a central portion of the ionically
conductive block channels and ionic conductivity increases.
[0090] Further, block copolymers having cylindrical morphology were
also found to be suitable in terms of conductivity and structural
properties. Further, the conductivity drop is often more pronounced
in samples having cylindrical morphology, which makes them
attractive as polymer electrolytes having an intrinsic property of
preventing thermal runaway in lithium battery cells.
[0091] As described further below with reference to FIG. 8, the
linear block copolymer electrolytes of the present invention
exhibit good cycling performance. No dendrite formation was
observed for 80 cycles in DC Li/polymer/Li test cells.
EXAMPLES
[0092] The following examples provide details relating to
composition, fabrication and performance characteristics of block
copolymer electrolytes in accordance with the present invention. It
should be understood the following is representative only, and that
the invention is not limited by the detail set forth in these
examples.
[0093] To characterize the electrolyte, experimental data results
were obtained by methods such as transmission electron microscopy
(TEM), small angle X-ray scattering (SAXS), AC impedance
spectroscopy, and rheology. AC impedance spectroscopy measurements
were made using a test cell on thermo-stated pressed samples in the
glove box, and a Solartron 1260 Frequency Response Analyzer machine
connected to a Solartron 1296 Dielectric Interface. The polymer
samples for TEM and SAXS were annealed using the same thermal
history as that used for conductivity measurements. The electrolyte
samples were kept at 120.degree. C. for 2 hrs, and cooled to room
temperature. Thin sections (about 50 nm) were prepared using an RMC
Boeckeler PT XL Cryo-Ultramicrotome operating at -100.degree. C.
Imaging was done on a Zeiss LIBRA 200FE microscope operating at 200
kV. SAXS measurements were made on hermetically sealed samples. The
data were collected for various temperatures between 140.degree. C.
and room temperature during cooling. Rheological measurements were
performed using an ARES rheometer from Rheometric Scientific Inc.
with a parallel-plate geometry. Approximately 1 mm thick samples
were placed between 8 mm plates for SEO, and 50 mm plates for PEO,
in a closed oven with a Nitrogen (N.sub.2) atmosphere. The ratio of
Li ions to ethylene oxide moieties, r, in the electrolyte was
varied from 0.02 to 0.10. Unless otherwise noted, due to the lack
of qualitative changes in properties with salt concentration, data
was obtained at r=0.02. Alternating current (AC) impedance and
rheological data, obtained from mixtures of a 20 kg/mol PEO
homopolymer and Li[N(SO.sub.2CF.sub.3).sub.2], serve as the
baseline for evaluating the properties of the composite
electrolyte.
[0094] Table 2 lists the characteristics of the different kinds of
copolymers that will be discussed:
TABLE-US-00002 TABLE 2 Mn (PS) Mn (PEO) d spacing Copolymer g/mol
g/mol .phi..sub.EO Morphology (nm) SEO(36-25) 36400 24800 0.38
Perforated Lamellae 47.9 .+-. 0.8 SEO(74-98) 74000 98100 0.55
Lamellar 101.2 .+-. 3.4 SEO(40-54) 39700 53700 0.55 Lamellar 66.4
.+-. 1.4 SEO(40-31) 39700 31300 0.42 Perforated Lamellae 44.4 .+-.
0.6 SEO(16-16) 16200 16300 0.48 Lamellar 30.4 .+-. 0.3 SIEO(11-6-9)
11200 9000 0.31 Lamellar 33.0 .+-. 0.3
[0095] Two different block polymer types were utilized to obtain
the experimental data. The first is polystyrene-block-poly(ethylene
oxide) diblock copolymers (SEO) doped with lithium
bis(trifluoromethylsulfonyl)imide, Li[N(SO.sub.2CF.sub.3).sub.2].
The PS-rich phase provides mechanical rigidity, while the PEO phase
provides ionic conductivity. The second is a
polystyrene-block-polyisoprene-block-poly(ethylene oxide) triblock
copolymer (SIEO). The presence of the polyisoprene rubbery domains
is known to increase the impact strength of polystyrene.
[0096] FIG. 3 is a graph illustrating SAXS profiles obtained from
SEO/salt and SIEO/salt mixtures. The SAXS data were obtained from
SEO/salt and SIEO/salt mixtures with r=0.02 and at a temperature of
90.degree. C. For clarification, SIEO (11-6-9) (marked A) has a
ratio of polystyrene-polyisoprene-poly(ethylene oxide) of 11-6-9.
SAXS data obtained from pure SEO and SIEO samples were
indistinguishable from the data shown in FIG. 3. The long range
order, as gauged by the sharpness of the primary and higher order
peaks, is better in low molecular weight samples than in the high
molecular weight samples. This is expected due to the slow
diffusion in strongly segregated block copolymers.
[0097] The SAXS profiles of SEO(36-25) (marked C) are similar to
perforated lamellar phases, while those from the other samples show
the presence of a lamellar phase. TEM images of SEO(36-25) (not
shown for brevity) indicate that the PEO lamellae appear dark and
perforated, while the PS lamellae show no evidence of perforations,
in agreement with the SAXS data shown in FIG. 3. Based on the
similarity in the SAXS profiles obtained with and without salt, the
addition of salt at low concentrations to the SEO copolymer may not
affect the morphology. The TEM and SAXS data indicate the absence
of an interpenetrating network phase in the block copolymer
electrolytes.
[0098] FIG. 4A is a graph illustrating the dependence of ionic
conductivity on r and temperature, respectively, in SEO(36-25).
FIG. 4B illustrates the dependence of ionic conductivity on r at
100.degree. C. in SEO(74-98). FIG. 4B illustrates that the
conductivity has a maximum at r.apprxeq.0.06-0.09. This trend is
similar to that observed in PEO-salt homopolymer-based systems. At
low salt concentrations, ionic conductivity increases with salt
concentration due to the increase in the number of charge carriers.
At high salt concentrations, transient cross linking of the polymer
chains and neutral ion pairs result in reduced conductivity. FIG.
4A illustrates the conductivity at various temperature ranges. It
illustrates that SEO (36-25) was conductive at 90.degree. C. and
higher.
[0099] FIGS. 5A-5C are graphs comparing ionic conductivity of the
composite electrolytes as a function of the molecular weights of
the PEO blocks. FIG. 5A illustrates a systematic increase in
conductivity with the molecular weight of the PEO block
(M.sub.PEO). The observed trend is opposite to that obtained in
pure PEO/salt mixtures where the conductivity decreases with
increasing molecular weight and eventually reaches a plateau. The
conductivity, .sigma..sub.PEO, of the PEO homopolymer with r=0.02
at 90.degree. C. and 120.degree. C. was measured to be
3.7.times.10.sup.-4 S/cm and 5.6.times.10.sup.-4 S/cm,
respectively. The conductivity obtained in the highest molecular
weight sample, SEO(74-98) (marked F in FIG. 3), is comparable to
that of pure PEO. Data obtained (not shown) from SEO(74-98) at
100.degree. C. show C peaks at a value of about 10.sup.-3 S/cm at
r=0.085. These values of conductivity are adequate for some battery
applications. FIG. 5C illustrates dependence of conductivity on the
molecular weight of the PEO block separately for lamellar and
cylindrical samples. It can be seen that in lamellar samples the
conductivity increases with increase of molecular weight of PEO and
then plateaus above 60 kg/mol. In samples having cylindrical
morphology, increase of conductivity with increase in PEO block
weight is observed, for all molecular weights screened.
[0100] The PEO volume fraction in the composite electrolyte,
.phi..sub.PEO, varied from 0.38 to 0.55. If all of the PEO channels
in the nanostructured electrolyte provided conducting pathways for
the ions, and if the conductivity of the PEO channels were
identical to that of PEO homopolymers, then the value of the
conductivity of a doped SEO sample, .sigma..sub.max would be the
product of .phi..sub.PEO.sigma..sub.PEO. The ratio
.sigma./.sigma..sub.max thus normalizes the measured conductivity
data for differences in .phi..sub.PEO. FIG. 5B is a plot of
.sigma./.sigma..sub.max versus M.sub.PEO. FIG. 5B shows that the
ionic conductivity of the electrolytes may be mainly affected by
M.sub.PEO and not by block copolymer composition.
[0101] FIGS. 5A and 5B illustrate that: (1) it is possible to make
self-assembled conducting nanostructured electrolytes with
non-conducting matrix phases, and (2) the magnitude of the
conductivity of such electrolytes is in the range of the
theoretical upper limit that can be expected from such systems. The
samples were not subjected to any special processing steps to
ensure that the PEO lamellae were connected or aligned.
Connectivity of the PEO phase occurs reproducibly by quiescent
annealing at 120.degree. C. In other embodiments, annealing is
performed at lower temperatures to avoid salt precipitation within
the polymer. Although it has been widely understood that highly
connected network phases are essential for high conductivity, the
above data indicates that that is not the case.
[0102] FIG. 5B illustrates the fact that .sigma./.sigma..sub.max
for the highest molecular weight sample is close to the theoretical
maximum of 0.67 expected for a structure comprised of randomly
oriented grains. It is believed that dissociated Li ions are
tightly coordinated with the ether linkages in PEO, and thus
disruption of this coordination could lead to faster ion transport.
The disruption of coordination may be due to two reasons: (1) the
contact between the ions and the polystyrene/poly(ethylene oxide)
interfaces, and (2) the deformation of PEO chains due to
self-assembly. The polystyrene/poly(ethylene oxide) interfacial
area (per unit volume) decreases with increasing molecular weight.
As noted above with reference to FIG. 3, potential reason (1)
appears incorrect. As such, ion transport within microphases may be
faster than in bulk.
[0103] In contrast, it is known that block copolymer chains stretch
when they form ordered phases due to the traditional balance of
energy and entropy. On average, PEO chains are stretched in the
lamellae, compared to the homopolymer. The extent of stretching
depends on copolymer chain length, N, and the Flory-Huggins
interaction parameter between the blocks, .chi.. The range of the
product .chi.N in the SEO diblock copolymers ranged from 25 to 130
at 90.degree. C., indicating that these copolymers in accordance
with the invention are far from the weak segregation limit. Another
indication of the stretched nature of the PEO chains is the scaling
of the PEO lamellae thickness, d.sub.PEO, calculated from the
periodic length scale, d, given in Table 1 with N.sub.EO, the
number of EO units in PEO block. A least squares power law fit
through the d versus N data yields d.sub.PEO=0.18*N.sub.EO.sup.0.69
(both the prefactor and exponent were free parameters in the fit),
which is the expected result in the strongly segregated limit.
Thus, the stretched PEO chains in the high molecular weight block
copolymers are not as tightly coordinated with Li ions compared to
PEO random coils, which leads to enhanced conductivity.
[0104] FIG. 6 is a graph illustrating the frequency dependence of
the storage and loss shear moduli. The graph shows the frequency
(.omega.) dependence of the storage and loss shear moduli, G' and
G'', respectively, of pure SEO(36-25), SEO(36-25) with r=0.020, and
pure PEO homopolymer. The data obtained from the pure SEO(36-25)
and SEO(36-25) with r=0.020 are indistinguishable. The
frequency-independence of the moduli and the fact that G' is an
order of magnitude larger than G'' indicate that the block
copolymer electrolytes are elastic solids. The data also indicates
that the addition of small amounts of salt has no detrimental
effect on the mechanical properties of the materials. The value of
G' obtained from the SEO(36-25) electrolyte is 100 times larger
than the plateau modulus of pure PEO and 6 orders of magnitude
larger than G' of the PEO homopolymer. The molecular weight of the
PEO homopolymer is similar to that of the PEO block in the
SEO(36-25) copolymer. Thus, nanostructured electrolytes have
roughly half the conductivity of homopolymer PEO but larger shear
moduli by several orders of magnitude.
[0105] FIG. 7 is a graph illustrating the frequency dependence of
elastic moduli for the diblock copolymers referenced in FIG. 3. The
higher molecular weight copolymers that exhibit high ionic
conductivity yield elastic moduli of the order of 10.sup.8 Pa. By
adding the PS block to the PEO chain, the shear modulus increases
by several orders of magnitude while keeping the ionic conductivity
near the level of pure PEO.
[0106] FIG. 8 plots the results of cycling a Li/SEO(74-98)
r=0.02/Li cell with a 250 .mu.m thick electrolyte at 90.degree. C.
The cell was formed with a freestanding polymer electrolyte film
formed as described above herein. A spacer with the freestanding
polymer electrolyte film was then sandwiched between two aluminum
masks that defined the exposed electrolyte area available for
lithium (Li) deposition. This assembly was then secured to a
slotted disk. The slots in the disk ensure that the electrolyte
surface was exposed for Li deposition. The slotted-disk with 30
electrolyte/mask assemblies was then secured to a spindle in the Li
deposition chamber. The spindle was connected to a motor that can
rotate the spindle, along with the slotted plate with
electrolyte/mask assemblies. Once the slotted plate was secured to
the spindle, Li pellets were placed in the crucible of the
vapor-deposition chamber. The deposition chamber was then closed
and left overnight under a very high vacuum (<10.sup.-3 mbar).
The high vacuum ensures that the electrolyte and Li pellets are
free of any adsorbed solvent molecules that may react with Li
vapors. When the pressure in the deposition chamber was below
10.sup.-3 mbar, the crucible was heated to .about.200.degree. C.,
and Li was deposited from Li vapors onto the exposed surface of the
electrolyte. The thickness of the deposited Li layer/film was
monitored, and vapor deposition was stopped once a desired
(.about.1 .mu.m) Li-layer thickness was achieved. Then the
deposition chamber was opened and the slotted plate was flipped and
secured back to the spindle, exposing the other face of the
freestanding electrolyte film that had not yet received any
deposited Li. The deposition chamber was closed and the process of
Li-deposition was repeated. This yields symmetric Li/SEO/Li cells
for performing DC cycling measurements. In each cycle, a 50
.mu.A/cm.sup.2 current density was applied to the cell for 2 h,
followed by a rest period of 1 h, followed by a 50 .mu.A/cm.sup.2
current density applied in the opposite direction. The voltage
required to achieve this current density was independent of time
over 80 cycles, indicating the lack of dendrite growth during these
experiments.
[0107] The structure of the block copolymer electrolytes of the
present invention was investigated to determine if a structural
correlation for the observed properties could be determined.
Poly(styrene-block-ethylene oxide) (SEO) copolymer electrolytes
were prepared as described above. Samples were pressed at
120.degree. C. into 0.1-1 mm thick disks using a mechanical press.
All sample preparation was done without exposing the materials to
atmospheric water or oxygen.
[0108] Electron microscopy images were analyzed to determine the
width of the lithium and poly(ethylene oxide) channels. Micrographs
were segmented into boxes that were approximately two times the
periodicity of the structures. The minimum periodicity, d.sub.min,
was taken to be the true spacing of the nanostructured electrolyte.
Regions which show a periodicity within 10% of d.sub.min were
selected for further quantification. The intensity distribution of
these images was normalized to maximize the dynamic range of
contrast. A high-pass filter which removes low-frequency data below
a radius of 30 pixels was used to remove some of the noise from the
images. The image was thresholded, such that pixels are either
black or white, to measure systematically the lamellae thicknesses.
The channel thicknesses were measured by hand every 004d.sub.min
along the lamellae normal.
[0109] FIG. 9A is a bright field image of SEO(16-16)/salt mixture
(r=0.085). The amorphous nature of the polymer is clearly visible.
FIG. 9B is the lithium energy filtered micrograph of the same
region as FIG. 9A. The light regions correspond to the presence of
lithium, and the dark regions to the absence of lithium. Since the
salt segregates itself to the PEO channels, the microstructure is
now clearly visible and it is highly suggestive of a lamellar
morphology.
[0110] Measurements of the thickness of the lithium salt and
poly(ethylene oxide) lamellae are shown in FIG. 10 for SEO(74-98)
at a salt concentration of r=0.085. The first column corresponds to
the product of the periodicity observed in our images and the
volume fraction of ethylene oxide in the copolymer. The second
column is the thicknesses of PEO lamellae measured from bright
field images of neat SEO copolymers stained with RuO.sub.4. The
last three columns correspond to lamellae measurements taken on
energy filtered TEM images of O, Li and F. The first three columns
are a measure of the PEO lamellae thickness, while the last two are
a measure of the Li channels. It is clear from FIG. 10 that the
salt lamellae are thinner than the PEO channels for SEO(74-98) at a
salt concentration of r=0.085. The size of the Li domains were
normalized by the ethylene oxide domains to determine the extent of
this concentration effect as a function of the molecular weight of
PEO, M.sub.PEO, and these results are plotted in FIG. 11. It is
clear that the thinning of the lithium lamellae is more dramatic
for polymers with higher molecular weight.
[0111] FIG. 12 is a comparison of the lithium salt distribution and
the ionic conductivity of the copolymer electrolytes. There is a
clear inverse relationship between the extent to which the Li
segregates away from the interface and the ionic conductivity. It
is concluded that by constructing SEO copolymer/salt mixtures where
M.sub.PEO is high, a novel lithium salt structure within the
nanostructured copolymer, with Li salt concentrated in a central
portion of the ionically conductive polymer block, is produced,
which results in highly conductive materials.
[0112] FIG. 14 illustrates dependence of conductivity on inverse
temperature (1000/T) in a SEO (54-23) sample, having lithium salt
concentration r=0.05. Heating and cooling plots are shown with
arrows. It can be seen that during heating the conductivity
increases with the increase in temperature until a threshold of
100-110.degree. C. is reached. Above this temperature range, the
conductivity stops rising and eventually drops by almost an order
of magnitude. Interestingly, during cooling the conductivity does
not rise to the initial high value. Therefore, it appears that at
least in this sample, and at the time scale used during measurement
(about 30 minutes for each data point), the salt precipitation is
irreversible.
[0113] FIG. 15 provides evidence that the observed conductivity
drop is due to lithium salt precipitation. FIG. 15 provides SAXS
plots for the same SEO polymer. SAXS profiles at 53.degree. C.,
88.degree. C., and 134.degree. C. in a heating regime. The
134.degree. C. temperature value is above the threshold temperature
for conductivity drop in this polymer. It can be observed that the
profile at this temperature exhibits increased low-Q scattering,
which is a sign of macrophase separation (salt precipitation). The
profiles at lower temperatures do not show signs of salt
precipitation.
[0114] Nanostructured polymer electrolytes with soft conducting
channels embedded in a hard insulating matrix are beneficial for
applications that require independent control over mechanical and
electrical properties. The data presented above indicate that
network phases are not necessary to obtain ionic conductivity,
which allows for the possibility of using a wide variety of
morphologies for designing ionically conducting polymers. No
special processing is needed to create percolating conducting
pathways in these materials; they are formed entirely by quiescent
self-assembly. This suppresses dendrite growth in Li-polymer
batteries and provides a cost-effective solution for combining high
energy density with good cyclability.
[0115] Applications
[0116] The electrolytes of the present invention are useful as
solid phase electrolytes for high energy density, high cycle life
batteries that do not suffer from failures due to side reactions
and dendrite growth on the Li electrodes. In one aspect, the
invention relates to a battery cell, a simple schematic diagram of
which is provided in FIG. 13, comprising a lithium anode, a
cathode, and a solid phase block copolymer electrolyte in
accordance with the present invention disposed between the anode
and cathode. The cell can be cycled without detrimental dendrite
growth on the anode.
[0117] In one aspect the battery cell includes a polymer
electrolyte which is configured for preventing thermal runaway in
the battery cell. The polymer electrolyte has a lithium salt
dissolved therein, and is configured for precipitating this salt
once the temperature of electrolyte exceeds a threshold value. In
some embodiments, this threshold value lies in the range between
90.degree. C. and 150.degree. C., e.g., between 100.degree. C. and
110.degree. C. The precipitation of lithium salt leads to a drop or
at least a plateau in conductivity of the polymer above the
threshold temperature, which, in turn, leads to cooling, or at
least reduced heating of the cell.
[0118] Advantageously, the precipitation of lithium salts in the
block copolymers described herein is an intrinsic unexpected
property of the copolymers, related to the thermodynamics of the
copolymer/salt system. Therefore, in some embodiments the
electrolyte itself provides a thermal shutoff mechanism for the
lithium battery cell, without the need for complicated mechanical
shutoff designs. In other embodiments, both shutoff designs may
complement each other and be used in concert.
[0119] Preferably, the polymer electrolyte is configured to have a
conductivity drop and a threshold temperature which are sufficient
to prevent melting of lithium metal in the battery cell. For
example, polymer electrolytes having a threshold temperature at
which conductivity drops or plateaus (or lithium salt precipitates)
well below the melting point of Li metal (180.degree. C.) are
preferred.
[0120] Further, in another aspect, a method of using a lithium
battery cell is provided. The cell includes a lithium anode, a
cathode, and a solid block copolymer electrolyte having a lithium
salt dissolved therein. The block copolymer electrolyte is
configured for precipitating the lithium salt above a threshold
temperature (e.g., above a temperature from the range of 90.degree.
C.-150.degree. C.). The method of operating a cell involves
charging and discharging the rechargeable cell. The cell is
configured for cooling or for reduced heating once the electrolyte
reaches the threshold temperature during battery cell operation. As
it was described, the reduced heating or cooling is due to
conductivity drop in the electrolyte which occurs above the
threshold temperature upon salt precipitation.
[0121] The invention may also be used, for example, as a power
source for electronic media such as cell phones, media players
(MP3, DVD, and the like), laptops and computer peripherals, digital
cameras, camcorders, and the like. It may also be beneficial for
applications in electric vehicles and hybrid electric vehicles.
[0122] In addition, as it was previously mentioned, it has been
discovered that careful processing of polymer films needs to be
performed in order to prevent salt precipitation during fabrication
of the polymer films. The choice of processing temperatures used in
annealing and/or casting operations will depend on the polymer
system, specific polymer chain architecture, the type of salt, the
salt concentration and other parameters. One of skill in the art
will understand how to select optimal temperature window for
polymer film fabrication, based on provided considerations and
guidelines.
CONCLUSION
[0123] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims. It should be noted that
there are many alternative ways of implementing both the process
and compositions of the present invention. Accordingly, the present
embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein, but may be modified within the scope and equivalents
of the appended claims.
* * * * *