Regulating rate performance of P2 layered oxide cathodes by transition metal ordering and phase transition

• Regulating rate performance by TM ordering in 2.0-4.0 V • Regulating rate performance by TM ordering and phase transition in 2.0-4.5 V. • Narrowing the energy difference between Na e and Na f by TM site substitution. • Mg at A site greatly reduces Na + diffusion energy barrier. • Regulating TM ordering by Ti/Mg substitution. The effect of transition metal (TM) ordering and phase transition on rate performance are of great significance for designing P2 layered transition metal oxide (A x TMO 2 , A = alkali metal) cathodes with high-rate performance and energy density. Herein, Ti/Mg/vacancy (◻) were introduced into TM/A site of P2-Na 0.67 [Ni 0.33 Mn 0.67 ]O 2 (NNMO). Ti/Mg and Mg/vacancy at TM site were used to adjust TM ordering and phase transition, respectively. Results show that lower TM ordering in 2.0-4.0 V and P2-OP4 rather than P2-O2 phase transition in 2.0-4.5 V result in better rate performance. Here, P2-Na 0.67 Mg 0.02 [Ni 0.30 Mg 0.03 Mn 0.60 Ti 0.05 ◻ 0.02 ]O 2 (NNMO-MT) shows excellent rate capability with high capacity of 121.4 mAh g -1 at 5C which is 95.4% of its capacity at 1C (127.2 mAh g -1 ) in a wide voltage window of 2.0-4.5 V. Moreover, Mg, Ti and vacancy at TM site can promote rate capability by narrowing the energy difference between Na f and Na e (Na sites right below TM and oxygen, respectively). Mg and vacancy at TM site can promote the energy density and rate capability by activating highly reversible anionic redox reaction (ARR). Mg at A site not only acts as “pillars” that stabilize the structure but also greatly promotes the rate performance by lowering the Na + diffusion energy barrier due to its strong repulsion to sodium ions.