Changes in local Ni/Mn environment in layered LiMg x Ni 0.5−x Mn 0.5 O ₂ (0 ≤ x ≤ 0.10) after electrochemical extraction and reinsertion of lithium

Electron paramagnetic resonance spectroscopy was used to study the changes in local Ni,Mn environment in layered LiMgxNi0.5−xMn0.5O2 (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 LiMgxNi0.5−xMn0.5O2. The Mn4+ environment in the transition metal layer of LiNi0.5Mn0.5O2 displays reversible changes in the potential range of 5.0–2.5 V. The loss of capacity of LiNi0.5Mn0.5O2 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.