Effect of lithium/transition‐metal ratio on the electrochemical properties of lithium‐rich cathode materials with different nickel/manganese ratios for lithium‐ion batteries

A lithium‐rich nickel‐manganese‐based material (Li${}_{1.2}$Ni${}_{0.2}$Mn${}_{0.6}$O${}_2$) exhibited a high discharge capacity at a low C‐rate. However, this material has problems in that it has a poor rate performance, large potential hysteresis, and poor cycle performance. Although increasing the Ni/Mn ratio for Li${}_{1.2}$Ni${}_{0.2}$Mn${}_{0.6}$O${}_2$ ameliorated these problems, it decreased the discharge capacity. In this study, the lithium/transition‐metal (Li/TM) ratios for lithium‐rich layer‐structured cathodes with different Ni/Mn ratios were controlled to attain a high discharge capacity, high rate performance, small potential hysteresis, and high cycle performance. For a low‐Ni/Mn‐ratio (Li${}_{1.2}$Ni${}_{0.2}$Mn${}_{0.6}$O${}_2$) cathode, a decrease in the Li/TM ratio deteriorated the discharge capacity. Conversely, for cathodes with a high Ni/Mn ratio (Li${}_{1.2}$Ni${}_{0.25}$Mn${}_{0.55}$O${}_2$ and Li${}_{1.2}$Ni${}_{0.3}$Mn${}_{0.5}$O${}_2$), the decreases in the Li/TM ratios improved the discharge capacity. In addition, for the high‐Ni/Mn‐ratio cathodes, the decrease in the Li/TM ratio from 1.2/0.8 to 1.18/0.82 increased the discharge capacity at a high C‐rate (3 C) from 157 to 183 Ah kg${}^{-1}$ (Li${}_{1.2}$Ni${}_{0.25}$Mn${}_{0.55}$O${}_2$) and from 139 to 156 Ah kg${}^{-1}$ (Li${}_{1.2}$Ni${}_{0.3}$Mn${}_{0.5}$O${}_2$). Furthermore, the decrease in the Li/TM ratio suppressed potential hysteresis and capacity degradation during cycling for the high‐Ni/Mn‐ratio cathodes. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)