The increasing demand for solid-state batteries has driven intensive research into fast ion transport in solid-solid interfaces. Optimal performance requires highly ionic conducting electrolytes and low-resistance solid-solid interfaces. Current research focuses on integrating these interfaces, addressing challenges in maintaining chemical and mechanical stability between electrodes and electrolytes during operation. Unlike liquid electrolyte lithium-ion batteries (LELBs) with well-established electrochemical reactions at solid-liquid interfaces, the fundamentals of solid-solid interfacial reactions in all-solid-state lithium batteries (ASSLBs) remain unclear. The initially discontinuous physical contact is traditionally considered a major hurdle for lithium transport, potentially inducing high impedance and degrading performance upon cycling. Unlike liquid electrolytes, solid electrolytes cannot easily penetrate voids and disconnections, impeding lithium transport. This is further complicated in polycrystalline cathode particles, where mesoscale architecture involves ion diffusion through complex inner and outer interfaces. However, existing understanding lacks direct experimental evidence, thus necessitating a deeper investigation into the fundamental interfacial reaction mechanisms in ASSLBs and the impact of physical contact on interfacial ion transport and electrochemistry. This study aims to elucidate these fundamentals and determine the extent to which initial physical contact loss affects solid-state battery performance.

The literature extensively discusses interfacial challenges in solid-state batteries, highlighting the importance of achieving high ionic conductivity in solid electrolytes and minimizing interfacial resistance. Several studies have focused on maximizing ionic conductivity in solid electrolytes (Famprikis et al., 2019; Xiao et al., 2020; Chen et al., 2016). However, the practical implications of physical contact loss on interfacial ion transport and electrochemistry remain largely unproven. Research on interfacial stability in solid-state batteries emphasizes the need for chemical and mechanical stability between electrodes and electrolytes (Xiao et al., 2020). The work by Kasemchainan et al. (2019) explored the relationship between stripping current and dendrite formation in solid electrolyte cells. Other studies have investigated the effects of mesoscopic architecture and grain boundaries on ion diffusion in polycrystalline cathodes (Besli et al., 2019; Xu et al., 2019). These studies provide context for understanding interfacial transport limitations but lack the direct experimental evidence provided in the current work to definitively establish the significance of physical contact loss.

This study used commercial LiNi0.6Co0.2Mn0.2O2 (NCM) polycrystalline particles as a model cathode material due to its hierarchical structure. A flexible solid polymer electrolyte (SPE) based on polyethylene oxide (PEO) complexed with lithium bis(trifluoromethane sulfonimide) (LiTFSI) was prepared via solution casting. ASSLBs (CR2025 coin cells) with a window for X-ray transmission were assembled in an argon glove box, using lithium metal as the anode and the PEO-based SPE as the electrolyte. LELBs were assembled similarly, substituting the SPE with a liquid electrolyte (LiPF6 in EC/DEC/DMC). Electrochemical characterization included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT). In-operando transmission X-ray microscopy (TXM) at the National Synchrotron Light Source II (NSLS-II) was employed to visualize chemical state evolution during charging and cycling. The TXM-XANES data was quantitatively analyzed by fitting with reference spectra. Three-dimensional nanotomography with TXM provided high-resolution 3D morphological information of the cathode particles. Finite element modeling (FEM) was used to simulate stress distribution within polycrystalline NCM particles during cycling, considering the anisotropic mechanical properties of NCM. The FEM simulated the Li-ion concentration and stress distribution during charging. The simulation was based on Fick’s law, where anisotropic diffusion coefficients were considered along different crystal directions. The coupled chemo-mechanical model considered both chemical and elastic strains.

Contrary to expectations, the study found that initial electrochemical performance of ASSLBs was comparable to LELBs, despite significant physical contact loss at the solid-solid interfaces. In-situ EIS showed a decrease in total resistance during initial charging in ASSLBs, with significant reduction in cathode resistance. GITT indicated that lithium diffusion coefficients in the NCM cathode were similar in ASSLBs and LELBs (10⁻¹² cm² s⁻¹ ~ 10⁻¹¹ cm² s⁻¹). In-operando TXM-XANES mapping revealed that despite inhomogeneous initial lithium-ion transport, cathode particles in ASSLBs achieved nearly full delithiation via internal charge propagation driven by ionic-electronic fields. This homogeneity was attributed to the establishment of a local ionic concentration equilibrium driven by concentration gradients and electric fields. In contrast to the homogeneous charge distribution during initial charging, cycled ASSLB cathodes showed significant heterogeneity and SOC separation after 50 cycles, accompanied by capacity fade. This was linked to the formation of microcracks within polycrystalline particles due to uneven stress distribution from uneven interfacial reactions. SEM and 3D nanotomography confirmed the presence of microcracks in cycled ASSLB cathodes but not in LELBs. Synchrotron X-ray analysis revealed a significant increase in nickel valence and distortion of the NiO octahedral coordination in the cycled ASSLB cathodes. FEM simulations showed that heterogeneous stress distribution during charging is responsible for the formation of cracks in polycrystalline NCM particles in ASSLBs. This does not occur in LELBs where uniform Li-ion transport and uniform stress distribution prevent the generation of cracks.

The findings challenge the conventional understanding of solid-solid interfacial limitations in ASSLBs. The initial comparable performance of ASSLBs and LELBs despite physical contact loss suggests that internal charge propagation can compensate for discontinuous interfaces. The observed homogeneity in the initial charging process but heterogeneity after cycling emphasizes the crucial role of maintaining a stable local ionic environment. The microcracks observed in the cycled ASSLB cathodes are directly related to the uneven stress distribution. The finite element model confirms the mechanical origins of the microcracking. These results highlight the importance of considering both the interfacial contact and the interior environment of polycrystalline cathode particles in designing high-performance solid-state batteries.

This study demonstrated that initial electrochemical performance of solid-state batteries is not solely determined by the degree of physical contact between the solid electrolyte and the cathode. Internal charge compensation can overcome initial physical contact deficiencies. However, long-term cycling leads to capacity degradation due to microcrack formation and heterogeneous charge distribution stemming from uneven stress distribution. This necessitates materials and electrode designs that promote stable local ionic concentration equilibrium, addressing both the interfacial challenges and mechanical issues within the cathode particles.

The study focused on a specific cathode material (NCM) and solid-state electrolyte (PEO-LiTFSI). The results might not be generalizable to all cathode and electrolyte combinations. The FEM simulations used idealized models of polycrystalline particles. Real-world particle morphologies and interfacial interactions could be more complex, requiring further refinement of the models. The study primarily focused on the initial charging process and the long-term cycling performance; more detailed studies on intermediate stages of battery life would strengthen the conclusions.