Unveiling Anode Kinetics and Its Impact on All-Solid-State Battery Performance by Three-Electrode Measurements

The progressive development of solid-state batteries (SSBs) is driven by the need to meet the stringent demands of the electric vehicle market, including high energy density, fast charging, enhanced safety, and a broad operating temperature range. Solid electrolytes (SEs), particularly sulfide-based compounds like Li₆PS₅Cl, Li₁₀GeP₂S₁₂, or Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, exhibit exceptionally high ionic conductivities at room temperature, some even surpassing that of liquid electrolyte counterparts. Note that advanced SEs help enhance the rate capability of SSBs, especially under extreme operating conditions.
Simultaneously, extensive research has focused on the development of high-capacity cathode materials, such as Ni- and/or Li-rich layered oxides, both with polycrystalline and single-crystalline morphologies, as well as anode materials like silicon and lithium metal (incl. anode-less configurations). However, one of the critical challenges in developing robust and efficient SSBs is addressing the unfavorable interfacial kinetics between the SE and the electrodes. This requires the assist of advanced characterization techniques. Furthermore, monitoring of the individual electrodes and their kinetics under extreme conditions, such as high current densities or low temperatures, is crucial for improving battery performance.
To date, most SSB research and development relies on two-electrode (2E) cell configurations, often tested under high external pressure (tens to hundreds of MPa). Standardized testing setups remain scarce, and the use of three-electrode (3E) configurations, common in liquid electrolyte-based cells, has been rarely reported for SSBs.
To address the challenges posed by extreme operating conditions in SSBs and to better differentiate the individual kinetic processes at anode and cathode, we developed a reliable 3E cell setup for SSB testing that is comparable to the 2E counterpart. In our study, after successfully extracting individual impedances, we identified and distinguished the characteristic time constants of typical anode materials through distribution of relaxation times (DRT) analysis. Our results highlight the necessity of 3E measurements for thorough analysis of the kinetic response of individual electrodes, given their overlapping timescales. Moreover, by tracking individual electrode potentials, we are able to study the polarization behavior of the anode and its impact on the electrochemical performance in conjunction with a layered Ni-rich oxide cathode.
Overall, we believe that the 3E setup proposed in our work will significantly reduce the performance gap between 2E and 3E configurations, providing valuable insights into the key challenges facing SSB research, particularly under extreme operating conditions.