Different roles of oxygen deficiency in performance of spinel lithium manganese oxides as the cathodes for aqueous and non-aqueous systems

Oxygen deficiencies are well recognized to have a detrimental effect on the performance of spinel LiMn2O4-δ as the cathode for non-aqueous systems, whereas the influences have not been explored for aqueous systems yet. Here, we successfully introduce oxygen deficiencies into LiMn2O4-δ by calcination of oxygen deficiency-free LiMn2O4 at 850 °C for 2 h followed by quenching in air. LiMn2O4-δ primary particles show the octahedral shape, the sizes of about 400 nm, and the larger lattice constant of 8.240 A, in contrast with the irregular shapes, the sizes of about 100 nm, and the lattice constant of 8.231 A. In the non-aqueous system, the performances for LiMn2O4-δ are inferior compared to LiMn2O4, as reported previously. However, both LiMn2O4-δ||polyimide and LiMn2O4||polyimide aqueous full cells exhibit the discharge capacity of about 105 mAh g−1 at 100 mA g−1 and the capacity retentions of 87% and 60% after 100 cycles. The plateau of H2O decomposition observed at about 1.85 V for LiMn2O4 leads to much poorer performance which is absent for LiMn2O4-δ. This study demonstrates different requirements by the cathodes in the non-aqueous and aqueous systems and offers a new idea for improving the performance and stability of the electrodes for aqueous cells.