First-Principles Calculation Study on Degradation-Related Phenomena around Cathode/Solid Electrolyte Interfaces in Solid-State Battery

Control of battery degradation has been a critical issue for the EV application, and a variety of technologies have been developed so far. However, understanding of the microscopic behavior of ions and electrons is still very lacking, and thus the degradation control has room to be improved. In this respect, we have investigated the microscopic interfacial and mechanical phenomena around cathode / solid electrolyte interfaces via density-functional-theory (DFT) based first-principles calculations, and drawn general theories effective for larger-scale consideration.<br/>In this talk, I introduce our recent works related to degradation; (1) stress/strain effects on Li⁺ diffusion in Li_xCoO₂ [1], (2) oxygen evolutions and cation migrations around the surfaces of Li_xCoO₂ [2], (3) Li⁺ transfer around cathode/coating layer/solid electrolyte interfaces [3,4].<br/>(1) We found that the barrier in the single vacancy mechanism decreases with biaxial expansion, contrasting with an increase in the double vacancy mechanism. This phenomenon is attributed to the c-axis position in the Li⁺ diffusion pathway. Our DFT-MD simulations revealed that compressive biaxial strain enhances Li⁺ diffusivity in Li-deficient Li_xCoO₂ (decreased to 0.22 and 0.21 eV), while tensile biaxial strain and hydrostatic pressure hinder it. Surprisingly, "Co layer distances" play a pivotal role in Li⁺ diffusion, and our study uncovers intricate interactions between Li-Li Coulomb interactions, state-of-charge (SOC), and Li⁺ diffusion in Li_xCoO₂. Besides, we reported the activation volume of LiCoO₂ under hydrostatic pressure and show that compressive biaxial strain weakens Li-O bond interactions, reducing the Li intercalation potential.<br/>(2) We examined oxygen release and Co-ion migration both in the bulk and on the surfaces (001), (104), and (110) of Li_xCoO₂, taking into account the process of spinel-like Co₃O₄ formation. The calculated oxygen vacancy formation energies are 2.38, 2.42, 1.46 and 1.10 eV for bulk, the (001), (104) and (110) surfaces, respectively. These trends are reasonably consistent with the experiments. We also clarified the significant reduction in activation energies of Co-ion migration in the presence of oxygen vacancies, indicating the correlation among the oxygen vacancy formation and the Co-ion migration.<br/>(3) Focusing on LiCoO₂ / LiNbO₃ / Li₃PS₄ interfaces in typical solid-state battery, we calculated ionic PES across the interfaces by first-principles calculations, and demonstrated how such energy profiles look like around the interfaces. The results correspond to the discharging situation, and indicate how to understand the charging situation by separating ion and electron movements. Our interface analyses also suggested a way of understanding interfacial ion and electron transfer by DFT-based standard electrochemical potentials of Li, Li⁺ and e-. The picture well accounts for the mitigation of ionic interface resistance by insertion of oxide coating layer between cathode and sulfide electrolytes. This DFT-based framework gives a crucial insight into ion as well as electron transfer across general interfaces.<br/><br/>These works were done in collaboration with Mr. Z. Zhou, Dr. H. D. Luong, Dr. R. Jalem in NIMS, Prof. B. Gao in Jilin Univ., and Prof. H.-K. Tian in National Cheng-Kung Univ. The works were partly supported by MEXT as “Program for Promoting Researches on the Supercomputer Fugaku / Fugaku Materials Physics & Chemistry Project" (JPMXP1020230325) and JSPS KAKENHI “Interface IONICS” (JP19H05815).<br/><br/>References: [1] Z. Zhou, Y. Tateyama <i>et al.</i>, under review. [2] H. D. Luong, Y. Tateyama <i>et al.</i>, in preparation, [3] B. Gao, R. Jalem, Y. Tateyama ACS Appl. Mater. Interfaces, 13, 11765 (2021). [4] H.-K. Tian, Y. Tateyama <i>et al.</i>, ACS Appl. Mater. Interfaces, 12, 54752 (2020).