Defect-Induced Modifications of Ionic and Electronic Properties of LiBO₂: First-Principles Calculations

Lithium metaborate (LiBO₂), a wide-band-gap insulator, has attracted increasing research attention due to its various technological applications, including its use as a surface coating to stabilize the cathode of Li-ion batteries (LIBs) operating at high voltages. While recent experimental studies have demonstrated that LiBO₂ is an effective coating, leading to improved performances of LIBs, its working principles as a surface coating remain unclear. In particular, the impact of lattice vacancies on its coating functionalities-specifically, its ionic and electronic transports-is still an open questions. In this study, we address this question using density functional theory (DFT) calculations to investigate the formation of lattice vacancies and their effects on lithium diffusion, the electronic band structure, and the density of states for both tetragonal (t-LBO) and monoclinic (m-LBO) polymorphs of LiBO₂. Our findings indicate that the formation energy of lattice vacancies increase progressively from lithium to oxygen to boron vacancies. Additionally, lithium vacancies modify slightly the band structures and the value of band gap (E_g) of both the polymorphs. In contrast, boron and oxygen vacancies considerably reduce E_g by introducing defect energy levels within the forbbiden band, transform the material into a degenerate semiconductor. Regarding the lithium-ion transport, our DFT results show that oxygen vacancies decrease the migration energy barrier (E_m) in m-LBO but increase it in m-LBO. In contrast, boron vacancies significantly reduce E_m in both polymorphs. This suggests that generating boron vacancies could be a viable strategy for improving the ionic conductivity of LiBO₂. Generating these boron vacancies simultaneously increases the electronic conductivity of the coating to reduce the ohmic overpotential of LIBs. However, the delocolized electrons in the coating as a result of defect formation could potentially be a source of electrons that facilitate side reactions between the liquid electrolyte and the cathode. These electrons could also contribute to the internal electronic leaking current of LIBs. Optimization strategies are therefore essential to achieve coatings with the desired functional characteristics.