On the Active Components in Crystalline Li-Nb-O and Li-Ta-O Coatings from First Principles

Abstract

Layered-oxide LiNixMnyCo1-x-yO2 (NMC) positive electrodes with high nickel content deliver high voltages and energy densities. However, a high nickel content, e.g., x = 0.8 (NMC811), can lead to high surface reactivity, which can trigger thermal runaway and gas generation. While claimed safer, all-solid-state batteries still suffer from high interfacial resistance. Here, we investigate niobate and tantalate coating materials, which can mitigate the interfacial reactivities in Li-ion and all-solid-state batteries. First-principles calculations reveal the multiphasic nature of Li-Nb-O and Li-Ta-O coatings, containing mixtures of LiNbO3 and Li3NbO4 or of LiTaO3 and Li3TaO4. The concurrence of several phases in Li-Nb-O or Li-Ta-O modulates the type of stable native defects in these coatings. Li-Nb-O and Li-Ta-O coating materials can favorably form lithium vacancies VacLi′ and antisite defects NbLi•••• (TaLi••••) combined into charge-neutral defect complexes. Even in defective crystalline LiNbO3 (or LiTaO3), we reveal poor Li-ion conduction properties. In contrast, Li3NbO4 and Li3TaO4 that are introduced by high-temperature calcinations can provide adequate Li-ion transport in these coatings. Our in-depth investigation of the structure-property relationships in the important Li-Nb-O and Li-Ta-O coating materials helps to develop more suitable calcination protocols to maximize the functional properties of these niobates and tantalates.