Changes in local Ni/Mn environment in layered LiMgxNi0.5−xMn0.5O2 (0 ≤ x ≤ 0.10) after electrochemical extraction and reinsertion of lithium

Abstract

Electron paramagnetic resonance spectroscopy was used to study the changes in local Ni,Mn environment in layered LiMg_xNi_0.5−xMn_0.5O₂ (0 ≤ x ≤ 0.10) after electrochemical extraction and reinsertion of lithium. The stability of the electrode surface in the electrolyte solution was examined by FT-IR and impedance spectroscopy. An EPR response from Mn⁴⁺ ions only has been detected in layered LiMg_xNi_0.5−xMn_0.5O₂. The Mn⁴⁺ environment in the transition metal layer of LiNi_0.5Mn_0.5O₂ displays reversible changes in the potential range of 5.0–2.5 V. The loss of capacity of LiNi_0.5Mn_0.5O₂ in the potential range of 4.6–2.7 V is related to the irreversible formation of complex defects containing Mn⁴⁺ ions. By increasing the voltage limit up to 5.0 V, these defects disappear and an excellent cycling stability is achieved with a Ni²⁺/Ni⁴⁺ redox couple. The Ni²⁺ ions in the LiO₂ layers also participate in the electrochemical reaction. For Mg-substituted compounds, the upper voltage limit is not so critical. The impedance is larger as compared to the Mg-free compound, and the lower voltage limit must be below 2 V to recuperate some capacity.