Factors Affecting Long-Term Cycle Stability of Cobalt-Free Li 2 MnO 3 -Based Materials

A layered-type solid solution material between Li 2 MnO 3 and LiNi 0.5 Mn 0.5 O 2 , Li 1.2 Ni 0.2 Mn 0.6 O 2 , delivers a large reversible capacity over 220 mA h g –1 , which exceeds the theoretical capacity calculated based on cationic redox of transition metal ions, through reversible anionic redox of oxide ions. However, a practical problem of this material is found in the poor electrode reversibility associated with partial oxygen loss during charging process. In this study, Li 1.2 Al 0.04 Ni 0.18 Mn 0.58 O 2 , where Ni and Mn ions are partially substituted with Al ions, is synthesized via a solid-state reaction under different synthesis conditions, and its electrochemical performance as a positive electrode material is examined. Li 1.2 Al 0.04 Ni 0.18 Mn 0.58 O 2 samples are synthesized at various sintering temperatures ranging from 850 to 1100 o C, and single-phase samples are obtained except for 850 o C. Al 3+ substitution effectively suppresses the capacity fading and voltage decay of Li 1.2 Ni 0.2 Mn 0.6 O 2 . In addition, Li 1.2 Al 0.04 Ni 0.18 Mn 0.58 O 2 delivers a large reversible capacity of over 200 mA h g –1 through optimizing the specific surface area. Furthermore, the use of highly concentrated electrolytes with high oxidative stability and low solubility of Mn ions effectively improves electrode reversibility of Li 1.2 Al 0.04 Ni 0.18 Mn 0.58 O 2 . From these results, factors affecting the electrode properties of cobalt-free Li-rich layered oxides will be discussed in detail.