Towards Improved All-Solid-State Batteries: A Multi-Scale Simulation of Cation Inter-Diffusion and Mechanical Failures at NASICON-Oxide/LiCoO₂ Interfaces

All-solid-state batteries (ASSBs) hold great promise, yet their commercialization has faced challenges due to interfacial complications, which encompass issues such as solid electrolyte decomposition, mechanical degradation, and Li dendrite growth. Specifically, NASICON-type solid electrolytes, like Li_1+xAl_xTi_2−x(PO₄)₃ (LATP) and Li_1+xAl_xGe_2−x(PO₄)₃ (LAGP), have attracted attention because of their stability against LiCoO₂ (LCO) and remarkable ionic conductivity (ranging from 10^-4 to 10^-3 S/cm). However, a decline in discharge/charge capacity due to mechanical breakdown at their interfaces has been a concern. Leveraging an integrated multi-scale simulation approach that combines Density Functional Theory (DFT) with Finite Element Analysis (FEA), we delved into the issues of cation inter-diffusion and the resulting mechanical challenges at LATP/LCO and LAGP/LCO interfaces. Our DFT studies pinpointed energetically favorable sites for Co to replace Ti atoms, mirroring atomic structures emerging from Co and Li inter-diffusion events. Evaluations of the elastic attributes of Co-LATP and Co-LAGP configurations showed a decline in modulus values (both Young’s and Shear), signaling an interfacial softening. However, the diffusion of Co and Li away from LCO transforms into Co₃O₄, characterized by a greater Young's modulus compared to LCO and both LATP/LAGP. By channeling these insights into a 2D continuum model through FEA, we could visualize the Li-ion concentration gradient adjacent to the LCO particle and the subsequent stress distribution across various discharge phases. Our results identified peak first-principal stresses at the interfaces, reaching approximately 800 MPa for LATP/ LCO and around 1000 MPa for LAGP/ LCO. In comparison, non-interface scenarios registered values between 400-500 MPa. These findings accentuate the importance of inhibiting Co diffusion to safeguard ASSB integrity. Furthermore, simulations suggest that enhancing the interfacial contact area to 75% of optimal contact can effectively counteract peak stress induced by interfaces, underscoring the significance of this parameter in counteracting interface-induced mechanical degradation. In conclusion, this study offers a thorough exploration into the alterations in material properties and the potential mechanical decline triggered by cation inter-diffusion at these interfaces, urging its prevention in subsequent ASSB innovations. The analytical methods adopted here also present a valuable methodological framework for probing other ASSB material pairings, potentially guiding the prediction and alleviation of mechanical setbacks during the discharging phases.