First-Principles Study on the Interplay of Strain and State-of-Charge with Li-Ion Diffusion in the Battery Cathode Material LiCoO2.

ABSTRACT

Cathode degradation of Li-ion batteries (Li+) continues to be a crucial issue for higher energy density. A main cause of this degradation is strain due to stress induced by structural changes according to the state-of-charge (SOC). Moreover, in solid-state batteries, a mismatch between incompatible cathode/electrolyte interfaces also generates a strain effect. In this respect, understanding the effects of the mechanical/elastic phenomena associated with SOC on the cathode performance, such as voltage and Li+ diffusion, is essential. In this work, we focused on LiCoO2 (LCO), a representative LIB cathode material, and investigated the effects of biaxial strain and hydrostatic pressure on its layered structure and Li+ transport properties through first-principles calculations. With the nudged elastic band technique and molecular dynamics, we demonstrated that in Li-deficient LCO, compressive biaxial strain increases the Li+ diffusivity, whereas tensile biaxial strain and hydrostatic pressure tend to suppress it. Structural parameter analysis revealed the key correlation of "Co layer distances" with Li+ diffusion instead of "Li layer distances", as ordinarily expected. Structural analysis further revealed the interplay between the Li-Li Coulomb interaction, SOC, and Li+ diffusion in LCO. The activation volume of LCO under hydrostatic pressure was reported for the first time. Moreover, vacancy formation energy calculations showed that the Li intercalation potential could be decreased under compressive biaxial strain due to the weakening of the Li-O bond interaction. The present findings may serve to improve the control of the energy density performance of layered cathode materials.