P2-type layered oxide cathode with honeycomb-ordered superstructure for sodium-ion batteries

Sodium-ion batteries (SIBs) have been proved as one of the most alternative to Lithium-ion batteries (LIBs) by the virtue of the abundant resources of Na. Among the cathodes, layered oxides stand out owing to their high energy density. The Jahn–Teller distortion, structural phase transition and oxygen release are main drawbacks of Mn/Ni based layered oxides. Hence, Li-Ni-Cu co-substituted Na0.67Li0.1(Mn0.7Ni0.2Cu0.1)0.9O2 (NLMNC) sample was prepared. The NLMNC sample exhibits the honeycomb-ordered superstructure at about 22° which is ascribed to cation ordering. By virtue of the Li-Ni-Cu co-substitute, the Jahn–Teller distortion of Mn is suppressed. Attributed to Li+ migrating from the metal oxide layer to the interlayer and the Li+ can play the part of pillar in the interlayer, NLMNC cathode displays the capacity retention rate of 82.5% at 200 mA g−1 after 100 cycles. The anionic redox reaction of NLMNC is related to the addition of Li ions and O 2p non-bonding which is different from the anionic redox reaction of electrons removing from O 2p in the eg* (Ni–O) states of Na2/3Ni1/3Mn2/3O2. During charge process, peroxo O− emerges with the oxidation of lattice oxygen. During discharge process, the intensity of peroxo O− gradually decreases. DFT calculations were also carried out to reveal the reaction mechanism of NLMNC. The DFT results showed that the anionic redox reaction occurs near Mn ions, while the Ni ions are stable which is corresponding with our results. It is proven that such Li-Ni-Cu co-substituted NLMNC presents a promising cathode material for SIBs.