Insight into the capacity decay of layered sodium nickel manganese oxide cathodes in sodium ion batteries

P2-type NaxNi1/3Mn2/3O2 is considered as one of the promising cathodes for sodium-ion batteries due to its low cost, low toxicity and high operating voltage. However, this material suffers the serious capacity decay and poor stability during cycling. Here, we performed the in-situ X-ray diffraction measurement to investigate the relationship among the voltage window, crystal structure evolution and capacity decay. Based on the finding that the capacity decay is mainly caused by the formation of oxygen anion group [O2]x−(0＜x＜4), we controlled the voltage in 2.6–3.8 V to avoid the oxygen reaction and the capacity retention of the material has been improved from ∼14% (for 100 cycles) to 94.5% (for 500 cycles). [Display omitted] •The inferior capacity retention (only ∼14% for 100 cycles) was improved to 94.5% (for 500 cycles) by optimizing the voltage window.•The formation of oxygen anion group under high voltage is the main cause to the capacity decay.