Towards Optimised Cell Design of Thin Film Silicon-Based Solid-State Batteries via Modelling and Experimental Characterisation

Abstract

To realise the promise of solid-state batteries, negative electrode materials exhibiting large volumetric expansions, such as Li and Si, must be used. These volume changes cause significant mechanical stresses and strains that affect cell performance and durability, however their role and nature are poorly understood. Here, a 2D electro-chemo-mechanical model is constructed and experimentally validated using steady-state, transient and pulsed electrochemical methods. The model geometry is a representative cross-section of a non- porous, thin-film solid-state battery with an amorphous Si negative electrode, LiPON solid electrolyte and LiCoO2 positive electrode. A viscoplastic model is used to predict the build-up of strains and plastic deformation of Si as a result of (de)lithiation during cycling. Electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique and hybrid pulse power characterisation are carried out to establish key parameters for model validation. The validated model is used to explore the peak interfacial stress and strain as a function of the relative electrode thickness (up to a factor of 4), revealing a peak volumetric expansion from 69% to 104% during cycling at 1C. The validation of this electro-chemo- mechanical model under load and pulsed operating conditions will aid in the cell design and optimisation of solid-state battery technologies.

Silicon-based Solid-State Batteries: Electrochemistry and Mechanics to Guide Design and Operation

“…LPSCl and LLZTO were also modelled as elastic-perfectly plastic solids. LCO was assumed to be an isotropic linear-elastic solid, whereas a-Si was treated as an isotropic elastic-viscoplastic solid with its Young's modulus, yield strength and Poisson's ratio varying with the state of lithiation (further details can be found in Vadhva et al, 39 and Leo et al, 43 ). The CCs were assumed to be electronically conductive, linear elastic solids with Young's moduli of ≈100 GPa.…”

“…The Parallel Direct Sparse Solver (PARDISO) was used to solve the discretised transport, electrochemistry, and solid mechanics equations, using the numerical procedure previously outlined. 39 3. RESULTS AND DISCUSSION…”

“…Incrementing the rate to 1C (Figure 2b) while modelling Si as an elastic material resulted in greater first cycle capacity reduction when compared to C/5 cycling, albeit with a lower capacity reduction than when Si plasticity was considered. To understand the cause of this phenomenon, the diffusion of Li in Si, as taken from experimentally extracted diffusion coefficient in literature (Dexp ~ 10 -16 m 2 s -1 ), 39 was increased by ~ 4 orders of magnitude to a hypothetical diffusion coefficient (Dhyp = 10 -12 m 2 s -1 ). As a result, the difference between first and second cycle capacity was significantly reduced for both the Si viscoelastic and viscoplastic cases (Figure 2c) showing the Li-ion diffusion in Si is a limiting factor and that cycling rate greatly influences the electrochemical response.…”

“…To date, comprehensive, validated, thin film SSB continuum-level models are limited. 39 Our previous work, 39 demonstrated an experimentally validated electro-chemo-mechanical model of a Si thin film SSB. However, the electrochemical and mechanical interplay was not explored, nor was the dependency of externally applied pressure or C-rate.…”

“…This work aims to understand the influence of these parameters to guide cell material design. 39 The thin film SSB model consists of an amorphous Si (a-Si) NE, lithium phosphorous oxynitride (LiPON) SE and LiCoO2 (LCO) PE. First, the effect of SE mechanical properties and kinetics on first cycle efficiency are explored.…”