Electrochemical Study on Rate Characteristics of All-Solid-State Batteries
Authors: Joo Soyoung , Kim Seong-Yoon , Min Yu-Jeong , Kim Seong-Yoon 
Published: December 2024

All-solid-state batteries (ASSBs) are next-generation batteries expected to significantly improve safety and energy density by replacing the liquid electrolyte of conventional lithium-ion batteries (LIBs) with a solid electrolyte. Although research and development for the commercialization of ASSBs are actively ongoing, many challenges remain to be addressed. In particular, the rate characteristic is directly related to the duration of discharge, which is a must-have performance for its use as a power source for transportation systems, including electric vehicles. Unlike conventional LIBs, where porous electrodes are fabricated and then filled with liquid electrolytes, ASSBs use a solid electrolyte with low fluidity, so the electrode components such as solid electrolyte, active materials, and binders are mixed at once to fabricate a composite electrode. The size, size distribution, and proportion of the solid electrolyte determine its occupancy within the composite electrode, which critically influences the lithium-ion transport through the electrolyte within the electrode. For example, increasing the proportion of active material within the electrode to achieve high energy density reduces the electrolyte content, making lithium-ion transport more difficult, leading to increased overvoltage and poor rate characteristics in the battery. In addition, during repeated cycling of ASSBs, the volume changes of the active material are not well accommodated by the solid electrolyte, inevitably causing stress at the active material | electrolyte interface. If this stress exceeds the fracture stress, the interfacial contact deteriorates. Particularly, for Ni-rich layered oxides, the most prominent cathode materials currently, rapid volume changes due to H2-H3 phase transitions at high voltage can lead to contact loss at the interface and isolation of the active materials. This increases resistance and the associated overvoltage, significantly degrading the rate characteristics of batteries. To improve the rate characteristics of ASSBs, various theoretical analyses under different design conditions of composite electrode have been attempted. However, the complexity of lithium transport within the composite electrode during high-rate charge and discharge makes it difficult to accurately simulate and understand rate characteristics. To fully understand the rate characteristics, it is necessary to non-destructively track ion transport within the electrode in real-time during battery operation. However, to the best of our knowledge, there is very few studies on this.
In this study, we electrochemically investigate the lithium transport behavior inside the composite electrode during high rate operation to understand the high rate characteristics of ASSBs. For the analysis, an ASSB with a sulfide-based solid electrolyte and a Ni-rich cathode was used, and the rate performance was firstly assessed by galvanostatic rate capability test (GRCT). Incremental capacity (dQ/dV) analysis of the discharge curves at high rate discharges, where the capacity decreases rapidly, showed that the dQ/dV peak intensity significantly decreased and the peak shifted in the cathodic direction. Based on the experimental results, we proposed that the tendency of the dQ/dV curve to change with increasing rate is attributed to the incomplete reaction of the active material due to the high overvoltage during high-rate discharge. To further analyze this, the interfacial resistance was obtained by two methods, stationary electrochemical impedance spectroscopy (SEIS) under open circuit state and dynamic electrochemical impedance spectroscopy (DEIS) under various discharge rates, and the obtained resistance values were comparatively analyzed. The interfacial resistance from SEIS showed an asymmetrical U-shaped curve with a minimum value at approximately mid-level depth of discharge (DoD). On the other hand, for the interfacial resistance from DEIS, the shape of the resistance change with DoD was generally similar to that of the SEIS, but it was found that the DoD with the minimum resistance gradually shifted to higher discharge states with increasing discharge rate. The discrepancies between the results obtained in the SEIS and DEIS were discussed in terms of the uneven degree of lithiation of the active material and the resulting heterogeneity in interfacial resistance. By considering the experimental/analytical results of this study and previous theoretical analysis of high rate operation in the literature, it was proposed that kinetic limitations, such as uneven lithiation of active material and poor supply of lithium ions in the direction of the electrode depth during high discharge rate, limited the utilization of cathode, leading to a sharp drop in cell potential and degradation of rate characteristics. In this work, an electrochemical approach to analyze the rate characteristics of ASSBs will be presented, and the rate-limiting mechanisms of ASSBs will be discussed from the reaction heterogeneity perspective.