Electrochemical impedance spectroscopy (EIS) is widely used to probe the physical and chemical processes in lithium (Li)-ion batteries (LiBs). The key parameters include state-of-charge, rate capacity or power fade, degradation and temperature dependence, which are needed to inform battery management systems as well as for quality assurance and monitoring. All-solid-state batteries using a solid-state electrolyte (SE), promise greater energy densities via a Li metal anode as well as enhanced safety, but their development is in its nascent stages and the EIS measurement, cell set-up and modelling approach can be vastly different for various SE chemistries and cell configurations. This review aims to condense the current knowledge of EIS in the context of state-of-the-art solid-state electrolytes and batteries, with a view to advancing their scale-up from the laboratory to commercial deployment. Experimental and modelling best practices are highlighted, as well as emerging impedance methods for conventional LiBs as a guide for opportunities in the solid-state.

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.

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.

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.

Non-crystalline Li-ion solid electrolytes (SEs), such as lithium phosphorus 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 non-crystalline SEs of composition Li−Al− P−O (LAPO) with ionic conductivities > 10 −7 S cm −1 at room temperature made by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesized with submicrometer thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic conductivity, 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 ≈0.4 eV. Additionally, these SEs exhibit low room temperature electronic conductivity (< 10 −11 S cm −1 ) and a 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.

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.