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

Solid-state batteries (SSBs) are promising alternatives to the incumbent lithium-ion technology; however, they face a unique set of challenges that must be overcome to enable their widespread adoption. These challenges include solid-solid interfaces that are highly resistive, with slow kinetics, and a tendency to form interfacial voids causing diminished cycle life due to fracture and delamination. This modeling study probes the evolution of stresses at the solid electrolyte (SE) solid-solid interfaces, by linking the chemical and mechanical material properties to their electrochemical response, which can be used as a guide to optimize the design and manufacture of silicon (Si) based SSBs. A thin-film solid-state battery consisting of an amorphous Si negative electrode (NE) is studied, which exerts compressive stress on the SE, caused by the lithiation-induced expansion of the Si. By using a 2D chemo-mechanical model, continuum scale simulations are used to probe the effect of applied pressure and C-rate on the stress-strain response of the cell and their impacts on the overall cell capacity. A complex concentration gradient is generated within the Si electrode due to slow diffusion of Li through Si, which leads to localized strains. To reduce the interfacial stress and strain at 100% SOC, operation at moderate C-rates with low applied pressure is desirable. Alternatively, the mechanical properties of the SE could be tailored to optimize cell performance. To reduce Si stress, a SE with a moderate Young's modulus similar to that of lithium phosphorous oxynitride (∼77 GPa) with a low yield strength comparable to sulfides (∼0.67 GPa) should be selected. However, if the reduction in SE stress is of greater concern, then a compliant Young's modulus (∼29 GPa) with a moderate yield strength (1-3 GPa) should be targeted. This study emphasizes the need for SE material selection and the consideration of other cell components in order to optimize the performance of thin film solid-state batteries

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

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 can cause significant mechanical stresses and strains that affect cell performance and durability, however their role and nature in SSBs 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 taken as a representative cross-section of a non-porous, thin-film solid-state battery with an amorphous Si (a-Si) negative electrode, lithium phosphorous oxynitride (LiPON) solid electrolyte and LiCoO2 (LCO) positive electrode. A viscoplastic model is used to predict the build-up of strains and plastic deformation of a-Si as a result of (de)lithiation during cycling. A suite of electrochemical tests, including 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 (a-Si∣LiPON) 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

Solid State Batteries: Materials and Interfaces

In recent years, solid-state batteries (SSBs) have garnered not only academic research attention but also of that of the electric vehicle (EV) and consumer electronics industry. The use of a solid electrolyte (SE) in a SSB, in place of the flammable liquid electrolyte (LE) used in conventional lithium-ion batteries (LIBs) could result in improved safety. Additionally, SSBs are promising alternatives to the incumbent LIB which contains a LE, due to their higher energy densities by pairing the SE with lithium (Li) or silicon (Si) negative electrodes (NEs). These offer a ≈10x theoretical energy density compared to the graphite NE used in LIBs. However, SSBs are plagued by a unique set of challenges which must be overcome to enable the widespread adoption of this technology: solid-solid interfaces which are highly resistive, slow kinetics, and the formation of interfacial voids leading to Li penetration, which ultimately results in capacity loss and low cycle life. In this work, electrochemical testing, physical characterisation and an electrochemo-mechanical (echem-mech) model is used to investigate and analyse the solid-solid interface, predominantly at the NE|SE interface. First, experimental testing of a commercial thin film Si based SSB is used to parameterise and validate an echem-mech model at the continuum level. Then, the validated model is used to explore the interplay between electrochemistry and mechanics by probing the relationship and between C-rate, applied pressure and capacity on the stress-strain response of the cell. Complex evolution of concentration gradients at high C-rates is found to influence the internal stresses and could point towards sites for fracture propagation within the electrode. Finally, this thesis addresses a key shortcoming of lithium phosphorus oxynitride (LiPON) SE - the requirement for vacuum deposition techniques, to a new non-crystalline (NC) material albeit synthesised using scalable processing methods. Electrochemical testing and physical characterisation of this material Li2.8AlP1.25Ox (LAPO) and its stability against Limetal for use in an “anode-free” battery is explored. The methods and characterisation tools presented in this thesis are designed so that they can be implemented for future testing across different SE battery systems

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

Solid-state batteries are promising alternatives to the incumbent lithium-ion technology however, they face a unique set of challenges that must be overcome to enable their widespread adoption. These challenges include solid-solid interfaces that are highly resistive, with slow kinetics, and a tendency to form interfacial voids leading to delamination which results in diminished cycle life. This modelling study probes the evolution of stresses at the solid electrolyte (SE) solid-solid interfaces, by linking the chemical and mechanical material properties to their electrochemical response, which can be used as a guide to optimise the design and manufacture of silicon (Si) based SSBs. A thin-film solid-state battery consisting of an amorphous Si negative electrode (NE) is studied, which exerts compressive stress on the SE, caused by the lithiation-induced expansion of the Si. By using a 2D chemo-mechanical model, continuum scale simulations are used to probe the effect of applied pressure and C-rate on the stress-strain response of the cell and their impacts on the overall cell capacity. A complex concentration gradient is generated within the Si electrode due to slow diffusion of Li through Si which leads to localised strains. To reduce the interfacial stress and strain at 100% SOC, operation at moderate C-rates with low applied pressure are desirable. Alternatively, the mechanical properties of the SE could be tailored to optimise cell performance. To reduce Si stress, a SE with a moderate Young’s modulus similar to that of lithium phosphorous oxynitride (~ 77 GPa) with a low yield strength comparable to sulfides (~ 0.67 GPa) should be selected. However, if the reduction in SE stress is of greater concern, then a compliant Young’s modulus (~ 29 GPa) with a moderate yield strength (1-3 GPa) should be targeted. This study emphasises the need for SE material selection and to consider other cell components in order to optimise the performance of thin film solid-state batteries

Engineering Solution-Processed Non-Crystalline Solid Electrolytes for Li Metal Batteries

Non-crystalline Li+-ion solid electrolytes (SEs), such as lithium phosphorous oxynitride, can uniquely enable high-rate solid-state battery operation over thousands of cycles in thin film form. However, they are typically produced by expensive and low throughput vacuum deposition, limiting their wide application and study. Here, we report a non- crystalline SE of composition Li-Al-P-O (LAPO) with an ionic conductivity >10-7 S cm-1 at room temperature by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesised with sub- micron thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic con- ductivity, with increased Li and decreased P content being optimal, while higher annealing temperatures result in decreased ionic conductivity. Activation energy analysis reveals a Li+-ion hopping barrier of 0.42(1) eV. Additionally, these SEs exhibit low room temperature electronic conductivity (<10-11 S cm-1) and moderate Young’s modulus of ≈54 GPa, which may be beneficial in preventing Li dendrite formation. In contact with Li metal, LAPO is found to form a stable, but high impedance passivation layer comprised of Al metal, Li-P and Li-O species. These findings should be of value when engineering non- crystalline SEs for Li-metal batteries with high energy and power densities