Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni_0.2Li_0.2Mn_0.6]O₂

TLDR

In conventional Li‑ion cathodes, Li⁺ extraction is accompanied by transition‑metal oxidation, but in Mn⁴⁺‑based materials Li⁺ can be removed without Mn oxidation, often invoking simultaneous Li and O loss. The study aims to directly observe O₂ evolution during charging of Li[Ni₀.₂Li₀.₂Mn₀.₆]O₂ and to link surface oxygen loss with transition‑metal migration. The authors employed in situ differential electrochemical mass spectrometry to detect O₂ evolution and powder neutron diffraction to track transition‑metal migration from the surface into the bulk. Oxygen loss drives the composition toward MO₂, enabling the cathode to store 200 mAh g⁻¹ of charge compared with 140 mAh g⁻¹ for LiCoO₂.

Abstract

The cathode in rechargeable lithium-ion batteries operates by conventional intercalation; Li+ is extracted from LiCoO2 on charging accompanied by oxidation of Co3+ to Co4+; the process is reversed on discharge. In contrast, Li+ may be extracted from Mn4+-based solids, e.g., Li2MnO3, without oxidation of Mn4+. A mechanism involving simultaneous Li and O removal is often proposed. Here, we demonstrate directly, by in situ differential electrochemical mass spectrometry (DEMS), that O2 is evolved from such Mn4+ -containing compounds, Li[Ni(0.2)Li(0.2)Mn(0.6)]O2, on charging and using powder neutron diffraction show that O loss from the surface is accompanied by diffusion of transition metal ions from surface to bulk where they occupy vacancies created by Li removal. The composition of the compound moves toward MO(2). Understanding such unconventional Li extraction is important because Li-Mn-Ni-O compounds, irrespective of whether they contain Co, can, after O loss, store 200 mAhg(-1) of charge compared with 140 mAhg(-1) for LiCoO(2).