V. Focused Fundamental Research - EERE - U.S. Department of ...

V.D.1 Polymer Electrolytes for Advanced Lithium Batteries (UCB)
Balsara – UCB
cycling cells as a function of temperature and for making
pouch cells with a solid electrolyte is now available.
Results
Material. We have synthesized three different kinds
of block copolymers: poly(styrene)-b-poly(ethylene oxide)
(SEO) copolymers for stabilizing the lithium metal
electrode, polystyrene-b-polyethylene-b-polystyrene (PS­
PE-PS) copolymers for self-assembled porous separators
and poly(3-hexylthiophene)-b-polyethylene oxide (P3HT­
PEO) copolymers for electronically and ionically
conducting binders.
Block Copolymer Electrolyte Characterization.
The salt diffusion was studied in a series of nanostructured
block copolymer electrolytes using the restricted diffusion
technique. The decay of the open-circuit potential of a
symmetric Li-polymer-Li cell was analyzed by a Laplace
inversion algorithm to give the distribution of relaxation
processes characteristic of the electrolytes. The distribution
function was characterized by two parameters, an average
diffusion coefficient, D avg , and a polydispersity index,
PDI diffusion , which is a measure of the width of the
distribution of diffusion coefficiens. We compare the
parameters obtained from a series of nearly symmetric
SEO block copolymer electrolytes containing lithium
bis(trifluoromethanesulfone)imide salt (LiTFSI) with those
obtained from a homogeneous poly(ethylene oxide)
(PEO)/LiTFSI mixture. D avg of the SEO/LiTFSI mixtures
increases with increasing molecular weight of the PEO
block, M PEO , and reaches a plateau of 2/3 D PEO when M PEO
exceeds 50 kg/mol (D PEO is the average salt diffusion
coefficient in PEO homopolymer). The PDI diffusion values
obtained for SEO copolymers are significantly higher than
those obtained in PEO homopolymer (see Table V - 2).
the cast films are thus converted into pores. This approach
enabled systematic variation of the pore structure at fixed
void fraction by changing the normalized chain length of
the sacrificial PS homopolymer. The efficacy of the
resulting separators was determined by measurement of the
ionic conductivity of separators soaked in a standard
lithium battery electrolyte, 1M lithium
hexafluorophosphate in ethylene carbonate/diethyl
carbonate (1:1 v/v, Novolyte Technologies, Inc.). The
effect of the chain length of the sacrificial homopolymer
on separator morphology and ion transport was then
determined using impedance measurements.
In highly porous separators with a nominal pore
volume fraction of 0.43, conductivity peaked at α = 0.22,
where values as high as 0.39 mS/cm were achieved (α is
the molecular weight of the PS homopolymer normalized
by that of the PS block in the SES copolymer). Nitrogen
adsorption experiments and scanning electron microscopy
were used to determine the underpinnings of this
observation. At α = 0.12, extremely small pores with low
surface area are formed. Increasing α to 0.22 results in a
film with well-connected nanoscale pores. A further
increase in α to 2.02 results in films with micron-sized
pores that are not effective for ion transport. Figure V - 133
shows SEM images of these porous separators.
The main advantage of our approach relative to
conventional separator manufacturing is that the pore
structure is determiend by equilibrium thermodynamics
and thus strict control over processing conditions is not
necessary.
Table V - 2: Average diffusion coefficient, Davg, and PDIdiffusion of the
relaxation distribution functions for the polymers used in this study [see ref. 4
for details].
Polymer D avg (cm 2 /s) Average PDI diffusion
PEO(27) (1.23 ± 0.12) x 10 -7 1.16 ± 0.13
SEO(6 – 7) (2.36 ± 0.49) x 10 -8 1.67 ± 0.32
SEO(16 – 16) (4.62 ± 0.18) x 10 -8 1.59 ± 0.26
SEO(36 – 24) (5.79 ± 1.2) x 10 -8 1.41 ± 0.18
SEO(37 – 25) (6.80 ± 1.5) x 10 -8 1.64 ± 0.35
SEO(40 – 54) (7.44 ± 0.6) x 10 -8 1.31 ± 0.15
SEO(53 – 68) (6.54 ± 1.1) x 10 -8 1.36 ± 0.27
SEO(74 – 98) (8.45 ± 0.8) x 10 -8 1.38 ± 0.21
SEO(240 – 269) (7.83 ± 1.5) x 10 -8 1.35 ± 0.26
Self-assembled Separators. Nanoporous battery
separators were made by blending a poly(styrene-blockethylene-block-polystyrene)
copolymer (SES) and
polystyrene (PS) homopolymers, casting films of the
blend, and selectively dissolving the homopolymer. The
volume previously occupied by the homopolymer chains in
Figure V - 133: Scanning electron micrographs of separators with void
fraction, V = 0.43 at a) α = 0.12 b) α = 0.22 c) α = 0.43 and d) α = 2.02
obtained by cryofracturing washed out films. (Images taken from Wong et
al., paper under review.)
Multi-purpose Binder. A block copolymer that
exhibits simultaneous electronic and ionic conduction was
Energy Storage R &D 594 FY 2011 Annual Progress Report