Determination of Electrochemically Active Surface Area in All-Solid-State Batteries

Abstract

All-solid-state batteries (ASSBs) are currently receiving much attention as one of the most promising next-generation rechargeable battery systems due to their promise of high energy density and high safety. Despite many efforts to commercialize ASSBs, however, there are still many unresolved technical challenges. Unlike conventional lithium-ion batteries (LIBs), the interface between the cathode and electrolyte in ASSBs exhibits a complex degradation pattern that combines chemical and mechanical degradations. In particular, the high Ni content cathode materials in ASSBs are commonly known to suffer from significant additional degradation mechanism, e.g., volume expansion/contraction due to H2-H3 phase transitions at high voltages, resulting in particle cracking. This degradation of the integrity of the active material leads to a reduction in electrochemically active surface area (active area afterward) due to loss of contact between the active material and solid electrolyte, slow diffusion rate of lithium in the active material, and isolation of the active material. These issues are recognized as major contributors to cell performance degradation. Among these issues, the changes in the active area have a decisive impact on the rate of redox reactions that are key to battery charging and discharging kinetics, so accurate determination of the active area is crucial for the diagnosis and improvement of batteries. In the electrochemical analysis of ASSBs, the poor contact between the solid electrolyte and the active material, or the change of the contact properties during the battery operation, should be a very important consideration. Especially when determining kinetic factors such as charge transfer resistance and chemical diffusion coefficient of cathode material by electrochemical methods, the models we usually use include the electrode surface area value as an important variable, so the accurate estimation of the active area is indispensable for obtaining reliable electrochemical properties. Unlike typical LIBs, in ASSBs the active material/electrolyte interface contact properties change relatively significantly during repeated cycling, so devising a reliable technique to analyze the active area of active materials in ASSBs is very crucial for the research and development of ASSBs. However, to the best of our knowledge, there are still very few studies on this.In this study, we propose promising methods to measure the contact characteristics between the cathode material and the solid electrolyte in ASSBs, namely the electrochemically active surface area, and discuss their reliability based on intercomparison and literature reports. For this purpose, first, the method for determining the active area of active materials based on galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) reported in conventional LIBs was reviewed. Then, its applicability to ASSBs was explored, and a way to improve the reliability of the analysis was proposed. Specifically, a three-electrode cathode half-cell experiment was conducted to analyze the active area of the Ni-rich cathode. The active area was calculated by comparing the diffusion coefficient measured by EIS with that obtained by GITT. When comparing the active area determined at different voltages, it was observed that the active area varies significantly with voltage most likely due to the volume change caused by the H2-H3 phase transition. The second electrochemical method performed for the determination of the active area of the active material is based on the quantification of the lithium intercalation sites on the cathode surface based on the analysis of the frequency factor in Arrhenius expression. For this purpose, battery at different charge states were kept at high temperatures for long periods of time to induce interfacial degradation. The temperature-dependent charge transfer resistance was then determined by EIS analysis, which was used to interpret the redox (or lithium absorption/desorption) reactions kinetically. The experimental results showed that the redox reaction rate followed exactly the Arrhenius relationship for all the charge states used, but the activation energies and frequency factors changed differently depending on the charge state. In this study, we discuss these results in relation to various phenomena that may occur during high-temperature aging, such as the formation of rock salt structures and cracks on the nickel-rich cathode surface, and try to estimate the change in active area. In this study, the electrochemical analysis methods of the contact characteristics between the cathode and the solid electrolyte in ASSBs will be examined in depth and their reliability will be assessed. In addition, further improvements in active area analysis techniques for active materials in ASSBs will be discussed.