A comparison of high voltage outgassing of LiCoO₂, LiNiO₂, and Li₂MnO₃ layered Li-ion cathode materials

One method of increasing the energy density of Li-ion batteries is to access reversible Li intercalation in conventional layered transition metal oxide cathode materials at high potentials (4.3–5 V vs. Li/Li⁺), and thus allow more electrochemical capacity per volume of active material. This comes at the cost of increased interfacial reactivity and often results in capacity fade over many cycles. Tracing gas evolution during electrochemical lithium extraction and insertion provides a useful strategy to understand this high voltage reactivity. In this study, we examine outgassing during Li extraction in well-known layered oxides (LiCoO₂, LCO; LiNiO₂, LNO; and Li₂MnO₃, LMO). We highlight key differences in the outgassing of each material. Whereas negligible O₂ release is found in LCO and LNO, even with voltage holds at 5 V vs. Li/Li⁺, O₂ release is found to account for a large portion of the electrochemical capacity and persists over many cycles in LMO. CO₂ outgassing is observed to varying extents, following the trend of LMO ≈ LNO > LCO, and is due to a combination of residual solid carbonate oxidation and electrolyte degradation. Taken together, these results show the importance of quantitative analysis in understanding the role of transition metal composition on the chemistry of conventional Li-ion battery materials.