Reliable Diagnostics for All-Solid-State Battery Composite Cathodes by Decoupling Electrode and Interfacial Processes
J.H Kim, Soyoung Joo, Heon‐Cheol Shin
Rapid electric vehicle adoption has elevated battery safety and high energy density from desirable attributes to core requirements, bringing all-solid-state batteries (ASSBs) employing sulfide solid electrolytes (SEs) to the forefront as credible next-generation candidates. Yet, in practice, the composite cathode remains the principal bottleneck. In Ni-rich high-capacity cathode active materials (CAMs) paired with sulfide SEs, parasitic interfacial reactions and contact degradation during charge–discharge are recognized sources of performance loss. In addition, the spatial distribution and percolation characteristics of CAM and SE complicate the ionic and electronic pathways, hindering uniform electrochemical reactions throughout the electrode. These issues are exacerbated in thick electrodes, where ionic transport resistance, electronic transport resistance, and contact resistance—collectively termed “electrode resistance”—become markedly larger than in conventional lithium-ion batteries. In impedance measurements, the electrode resistance contribution rarely manifests as an isolated feature; instead, it overlaps with signals arising from interfacial processes, including charge-transfer resistance (R ct ) and film resistance (R film ), thereby complicating mechanistic interpretation. The thicker the electrode, the stronger this overlap becomes, and the more uncertain the assignment of spectral features to purely interfacial or purely transport origins. Despite this reality, prior studies have largely emphasized half-cell-based impedance analyses focused on interfacial reaction resistances. Under conditions where electrode resistance and interfacial resistance coexist and co-evolve, such a narrow focus limits the reliability of impedance interpretation and can lead to ambiguous or even misleading conclusions. To enable trustworthy assessment of interfacial behavior in ASSBs and to guide high-energy designs, it is therefore essential to systematically separate and quantify electrode resistance components from genuine interfacial contributions, and to clarify their relative weights under practically relevant electrode conditions. In this study, we investigate the impedance of composite ASSB cathodes with the goal of improving the reliability of interpretation by explicitly decoupling electrode and interface contributions. Using electrochemical impedance spectroscopy (EIS) coupled with Distribution of Relaxation Times (DRT) analysis on composite-cathode half cells, we examine how the impedance response varies with state of charge (SoC) and areal loading—the latter determining electrode thickness. By scanning SoC and areal loading in a systematic manner, we construct a frequency-resolved picture in which processes can be separated by their SoC sensitivity and their evolution with thickness. This approach enables identification of distinct features associated with electrode resistance, charge transfer at the CAM/SE interface, and the formation of interfacial films that arise during cycling. Across SoC and areal-loading conditions, three dominant DRT peaks emerge, labeled P1, P2, and P3. P1 remains essentially invariant with SoC and occupies the high-frequency region of the spectrum, indicating an origin in electrode resistance that reflects through-plane ionic/electronic pathways and contacts within the composite network. P2 exhibits a U-shaped dependence on SoC, with larger values at Li-poor and Li-rich extremes and a minimum at intermediate states, a hallmark of R ct governed by composition-dependent kinetics at the CAM/SE interface. P3 gradually stabilizes as cycling proceeds, consistent with the growth and subsequent stabilization of a cathode–electrolyte interphase (CEI) that contributes a film-resistance component. The concurrent presence of these features explains why interfacial analysis alone can be unreliable in thick electrodes: as electrode resistance rises, its high-frequency signature can extend into the mid-frequency range and partially obscure or distort interfacial signals. Areal-loading-dependent measurements further reveal a critical loading that separates two regimes of behavior. Below this threshold, increasing electrode thickness leads to a pronounced growth in P1, reflecting the amplification of electrode resistance with extended transport pathways. In the same regime, P2 and P3 decrease with increasing areal loading, a trend attributed to the effective increase in electrochemically active area and improvements in local CAM–SE contact as the composite volume increases. Above the critical loading, however, spectral separation deteriorates: signals associated with electrode resistance and R ct begin to overlap. Taken together, these results quantitatively map how electrode and interfacial resistances contribute to the overall impedance as a function of areal loading and SoC. They show that electrode resistance can significantly bias the mid- and low-frequency response and thereby distort conclusions about interfacial reactions if it is not first isolated and properly accounted for in the analysis. On this basis, we advocate an analysis sequence in which high-frequency features associated with electrode resistance are identified and considered before interpreting the mid- and low-frequency regime associated with R ct and CEI-related film resistance. The study strengthens the foundation for accurate interfacial analysis in thick ASSB electrodes and supports rational strategies for energy-density enhancement and performance optimization.
https://doi.org/10.1149/ma2025-0283546mtgabs
Cathode
Electrode
Contact resistance
Electrolyte
Electrical impedance
Decoupling (probability)
Ionic bonding
Dielectric spectroscopy
Battery (electricity)
