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

“…When normalizing the Si concentration gradient, it was the maximum amount of Si lithiation that was achieved during cycling that was used. Experimental evidence of partial Si lithiation was observed previously using differential capacity analysis for these cells …”

“…Experimental evidence of partial Si lithiation was observed previously using differential capacity analysis for these cells. 38 The Si strain was considerably greater when the higher diffusion coefficient was implemented (Figure 2e) due to increased Si lithiation, which resulted in greater strains but also increased cell capacity. In addition, the Si strain was more homogeneous through the electrode than with the slower experimental diffusion coefficient (Figure 2e), which can be directly linked to the Li concentration gradients throughout the electrode due to diffusion-related transport limitations.…”

“…The 2D mesh consisted of approximately 4000 quadratic elements with 94,000 degrees of freedom, while the solutions were found to be mesh-independent. The Parallel Direct Sparse Solver (PARDISO) was used to solve the discretized transport, electrochemistry, and solid mechanics equations, using the numerical procedure previously outlined …”

Investigating the Influence of the Effective Ionic Transport on the Electrochemical Performance of Si/C‐Argyrodite Solid‐State Composites

“…[8][9][10] However, the main drawback of employing silicon as an anode are the high volume changes during the lithiation/delithiation process (300 %). [11][12][13] Since silicon itself is not sufficiently electronically or ionically conductive and needs conductive additives to perform well, the electrodes in Si-based are often composites of the active material, a SE and an electronically conductive additive such as carbon. [14][15][16] Si-based composites with carbon as a conductive additive have been widely explored as potential candidates for use in lithium-ion batteries as well as solid-state-batteries.…”

“…Hence other materials are researched as possible high capacity anodes, such as silicon which is considered to be one of the most promising materials to be employed as an anode active material, it shows a high specific capacity (3590 mAh g −1 ) and low lithiation potential (0.4 V vs. Li + /Li) compared to other anode materials [8–10] . However, the main drawback of employing silicon as an anode are the high volume changes during the lithiation/delithiation process (300 %) [11–13] . Since silicon itself is not sufficiently electronically or ionically conductive and needs conductive additives to perform well, the electrodes in Si‐based are often composites of the active material, a SE and an electronically conductive additive such as carbon [14–16] …”