Effect of Mg/Sn ratio on the structure and electrochemical performance of O3-type high entropy layered oxides for sodium-ion battery cathodes

The design of high-entropy O3-type layered oxides has been considered as an effective method for suppressing multiple phase transitions during cycling and improving stability. Herein, a series of high entropy layered oxides (HEOs) with different Mg:Sn ratios are investigated. It is found that the structural stability and electrochemical performance of these HEOs can be optimized by varying the Mg to Sn ratio. A lower Mg content is conducive to the formation of a single-phase HEO. An optimized HEO composition (namely NaNi0.1Co0.2Mn0.2Fe0.2Cu0.1Mg0.04Sn0.16O2) exhibits high specific capacity (137.3 mAh g−1 at 0.1C) and good rate capability (~100 mAh g−1 at 1C). The cycling performance (capacity retention of ~87 % after 200 cycles at 0.5C) is also comparable to the reported O3-type HEOs. Such good performance can be attributed to its good redox activity, low interfacial impedance, higher Na+ diffusion coefficient (2.26 × 10−11 cm2 s−1) and reversible O3-P3 phase transition.