Interfaces dominate the performance of electrochemical systems, from interphase formation to charge transfer impedances, and they require careful study to understand. We have undertaken a broadly applicable approach to isolating components of a solid-state battery (SSB) to analyze them using non-destructive electrochemical impedance spectroscopy (EIS), and systematically build up to the full device. We then apply this knowledge to the matter of spontaneous Li transfer from LiPON to V₂O₅ during the sputtering process, which was also seen in other electrode/electrolyte pairs but is poorly studied.

Using sputtering and in vacuo shadow mask exchange, we fabricated an array of 16 batteries (Si/LiPON/LiV₂O₅) in parallel with auxiliary devices with isolated materials and interfaces on a single 3 in Si wafer. The reproducibility of the SSBs was confirmed by electrochemical testing, and the interfaces were evaluated from EIS measurements of the auxiliary devices. Equivalent circuit models for each device were built, ensuring that the components were all physically correlated to a material property or to (non)faradaic reactions of the interface. Interface reaction dynamics were evaluated as the applied voltage was swept over the normal working range of the SSB. The parameters obtained from these auxiliary circuit models were used to build a complete and accurate electric circuit for the SSB.

To evaluate the effect of the sputtering process on the interfaces, two model systems were used offering different conditions. Ultra-thin layers (10-20 nm) of either Li₂O or LiPON were sputtered on top of V₂O₅ using a non-reactive Ar working gas, or reactive N₂, respectively. EIS measurements of the models revealed the presence of an electron blocking LiPON layer, and the absence of a Li₂O layer above the V₂O₅. Furthermore, the properties of the Li₂O/V₂O₅ sample were similar to the lithiated LiV₂O₅ previously measured. An XPS depth profile confirmed that the V₂O₅ was fully lithiated after sputtering Li₂O, while only a fractional lithiation occurred after sputtering LiPON. DC polarization measurements and spectroscopy ellipsometry showed that the interfaces have distinct optical and electronic properties that might indicate interphase formation.

This work presents a materials-agnostic platform for investigating the interfaces in a SSB using non-destructive electrochemical techniques. The auxiliary devices are created in parallel with the SSB, ensuring a 1:1 comparison. Overall, this work presents a comprehensive approach to investigate the interfaces in SSBs, including chemical and physical reactions, and provides insights into improving the interfaces of thin films in SSBs.

Lithium ion batteries (LIBs) employing solid electrolyte offer potential advantages in safety and shelf life over current LIBs made with liquid electrolyte. However, undesired reactions occur between electrode active materials and the solid electrolyte, forming interphase layers that block Li⁺ ion transport and degrade cell performance. To prevent these interphase reactions while maintaining battery operation, protective coatings are of interest which provide both electronic conductivity and Li⁺ ion conductivity. Recent work has demonstrated the use of molecular layer deposition (MLD) to form electrically conducting poly(3,4-ethylenedioxythiophene) (PEDOT), anion-conducting polypyrrole (PPy), and cation conducting lithicone-glycerol (LiGL). However, MLD coatings that conduct both lithium ions and electrons have proven challenging to synthesize. In this work, we report on recent efforts to form MLD films of polyhydroquinone that are cation-conductive and electronically-conductive. We employ in-situ quartz crystal microbalance (QCM) studies during MLD growth to understand the growth mechanisms, as well as ex-situ spectroscopic ellipsometry (SE), electrochemical impedance spectroscopy (EIS), and electrochemical characterization to identify the material properties. We also report early work applying these coatings to nickel rich LiNi_0.6Co_0.2Mn_0.2O₂ (NMC) cathode material to be used in solid electrolyte LIBs with the Li₁₀GeP₂S₁₂ (LGPS) solid electrolyte to understand how these protective coatings affect interphase layer formation in this system.