Li-doped P3-type Mn-Ni-Based Cathodes with Improved Electrochemical Performance for Na-ion Batteries

The Mn-Ni-based oxides used as cathodes for Na-ion batteries (NIBs) are inexpensive and exhibit high capacities, but the Jahn-Teller distortions of Mn3+ rapidly decrease the capacity. Here, Li-doped P3-type Na2/3Ni1/3Mn2/3O2 (named Li-NaNMO) was prepared with a microwave-assisted sol-gel method. After structural analyses, we found that Li doping decreased the content of Mn3+ and replaced it with Mn4+, the content of Mn3+ was reduced from 66.2% for NaNMO to 32.4% for Li-NaNMO, the content of Mn4+ was increased from 33.8% to 67.6%, and the content of Ni2+ varied to balance the charge, which was verified with XRD and XPS results. As the cathode of a Na-ion battery (NIB), NaNMO showed a capacity retention rate of 60% after the 100th cycle over the voltage range 2.0-4.2 V, while Li-NaNMO exhibited good cycling stability and 90% retention under the same testing conditions, which resulted from suppression of the Jahn-Teller effect and the improved conductivity. It also displayed a good rate performance (73 mAh/g at 200 mA/g), and the increased conductivity was confirmed by an EIS analysis.