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

Sastry – U. Michigan
V.E.2 Thermo-electrochemistry, Capacity Degradation and Mechanics with SEI Layer (U. Michigan)
inside the active particle as well as the SEI layer, in the
latter case by considering a misfit due to the difference in
volume expansion between the two phases. For this
experimental study, the interfacial impedance change as a
function of cycling number and storage at various
conditions is measured via the EIS technique. The
composition change and thickness of the surface layer
were then examined via X-ray Photoelectron
Spectroscopy.
Results
Multiphysics FE model – The microstructure of
percolated particle aggregates has been generated using a
Brownian dynamics simulation as shown in Figure V - 184a.
(blue: active materials, red: inert materials). This
microstructure of particle aggregates has been modeled for
finite element analysis as shown in Figure V - 184b and Figure V
- 184c. At the middle of the cathode, the temporal variation
of reaction current density from the pseudo-2D thermoelectrochemical
model and 3D microscopic model is
compared in Figure V - 185, which reveals a 14% difference.
In Table V - 4, the calculated effective diffusivity and
conductivity are compared with the approach of
Bruggeman’s equation.
(a) (b) (c)
Figure V - 184: (a) aggregated structure (b) voxel mesh (c) tetrahedral mesh
Figure V - 185: reaction current density
Table V - 4: Change in surface atomic concentration of each element
Volume
fraction
Bruggeman 3D
microstructure
D eff active/D active 0.49 0.34 0.346
D eff elyte/D elyte 0.51 0.37 0.378
σ eff solid/σ solid 0.49 0.34 0.300
Capacity fade due to SEI layer formation - The
developed model enabled us to overcome the limitation of
a conventional approach that assumes a steady state (at low
frequencies the system may vary due to the long duration
of measurement). Also, this numerical approach can
facilitate the experimental studies on capacity fade due to
SEI layer formation because capacity fade in Li-ion
batteries comes from complex processes. The simulation
demonstrated that capacity fade was accelerated as the film
resistance increases, as shown in Figure V - 186a. It also
demonstrated that impedance characteristics and frequency
responses were changed depending on film resistance, as
shown in Figure V - 186b.
(a)
Figure V - 186: (a) capacity change (b) frequency response function
Anode particles near the separator underwent high
stress due to (de)intercalation, which resulted in structural
degradation of the active particles as shown in Figure V -
187a. When the SEI layer was formed at the surface of the
(b)
anode particle, this layer changed the stress evolution
inside the particle due to the mismatch of the partial molar
volume as shown in Figure V - 187b. Also, this SEI layer itself
underwent high stress due to the expansion of the active
FY 2011 Annual Progress Report 633 Energy Storage R&D