High-Voltage Degradation of Ni-Rich Cathode Materials in All-Solid-State Batteries

Abstract

Lithium-ion battery (LIB) has been widely used as electrochemical energy storage devices in a variety of fields in our daily lives. In particular, recently there are strong market demands in the energy storage systems and electric vehicles, mainly due to its high energy and power densities. However, the customers’ strong needs for the safer and longer-lasting battery than commercial ones have sparked special interest in all-solid-state lithium battery (ASSB), which has a great possibility of replacing the conventional LIB that uses liquid electrolyte solution. Though much effort has been made for its practical applications, there are still a number of critical issues involved in the electrode/cell design, fabrication, scale-up, efficient operating conditions, etc. Upper cut-off voltage is one of the most important parameters, having a determinative effect on the energy density and cycling stability of the ASSB esp. when one uses Ni-rich layered oxide cathodes. Actually, the upper cut-off voltage is treated with high priority also in the LIB systems. In the case of the LIB, the secondary particle cracking is followed by gap-filling with electrolyte during the repetitive high voltage charging, giving additional electrochemical active area. Whereas, the particle cracking in the ASSB usually leads to the isolation of the primary particles. In addition, it is reported that the interface between active materials and solid electrolyte is degraded in a complex manner due to simultaneous chemical side reactions (e.g. cathode electrolyte interphase (CEI) formation) and mechanical breakdown (e.g. contact loss between electrolyte and particle), different from that of the LIB. Therefore, an analysis of cathode degradation in the ASSB needs to be dealt with in a different way from that in the LIB.In this work, the degradation of ASSB, consisting of sulfide-based solid electrolyte and Ni-rich cathode, undergoing the high voltage charge cycling is systematically studied using the electrochemical techniques. For this purpose, first, the deterioration tendency of reversible capacity and cell resistance was analyzed in constant current-constant voltage (CC-CV) charging mode at different upper cut-off voltages (CC charging) and the following charging times (CV charging). In addition, the effect of charging rate on the degradation trend was investigated in terms of the duration time that the cathode potential stays higher than a specific potential value in CC charging mode. It was confirmed that there are particular degradation tendencies of capacity and resistance for upper cut-off voltage and duration time, and the empirical relations were accordingly derived. Notably, the degradation trends between capacity and resistance were observed differently, strongly indicative of different governing factor for each degradation process. In the experiments at various charging rates, the lower charging rate led to larger degradation of the cathode. It is suggested that the application of a driving force for irreversible phase transition (i.e., from H2 to H3) for longer time at lower charging rate is the most probable reason.In order to more clearly understand the governing factors affecting high voltage cathode degradation, the degradation trends of capacity and resistance at the aforementioned operating conditions were thoroughly investigated using incremental capacity-differential voltage (IC-DV) analysis and electrochemical impedance spectroscopy (EIS), respectively. From the IC-DV analysis, it proved that the loss of active materials (LAM) is accelerated at higher cut-off voltage, longer duration time, and lower charging rate, indicating that the LAM might be one of the governing factors for capacity degradation. It is notable, by the way, that the interfacial resistance of the cathode was considerably increased at high voltage charging and implies the loss of lithium inventory (LLI) due to the CEI formation could be another major degradation mode. Accordingly, we tried to estimate the contribution of LLI to total degradation from the comparison of half-cell and full-cell analyses. On the other hand, from the EIS analysis, it was found that the increasing rate of the interfacial resistance was far higher than the decreasing rate of capacity. This tells us that the degradation of the cathode|electrolyte interface is mainly responsible for the resistance or power degradation at high voltage charging. In this presentation, the deterioration modes of ASSB at various high voltage charging conditions are explained in terms of the interface and bulk of the cathode. Moreover, the strategy to minimize the degradation at high voltage charging will be discussed.