Multi-elemental doping modulated P2-type layered cathodes for high performance sodium-ion batteries

Abstract

The P2-Na_0.67Ni_0.33Mn_0.67O₂ layered oxide cathode manifests remarkable promise for sodium-ion batteries (SIBs) due to its high operating voltage and superior theoretical energy density compared to lithium iron phosphate. However, its performance is greatly hindered by significant phase transitions and the trade-off between stability and capacity. To tackle this dilemma, a multi-elemental doping strategy is proposed in this work. The specially designed Na_0.7Ca_0.01Ni_0.29Li_0.06Mn_0.57Ti_0.07O_1.97F_0.03 (NCNLMTOF) cathode delivers an invertible capacity of 105.2 mAh g^-1 after 100 cycles at 0.2 C, along with a high reversible capacity of 89.8 mAh g^-1 and 85.2% capacity retention over 500 cycles at 5 C in the wide voltage range of 2.0-4.35 V. Especially, the unit cell volume change of only 0.15% for NCNLMTOF as well as a highly reversible phase structural evolution upon cycling is substantiated via the in-situ XRD test. It is noted that Ca²⁺ located within the sodium layers function as stabilizing "pillars", disrupting Na⁺/vacancy ordering during charge/discharge process. Furthermore, the strengthened Ca─O bonds effectively alleviate oxygen release from the lattice. Simultaneously, DFT calculations reveal that the replacement of Li and Ti greatly fosters localized electron distribution around oxygen ions, reducing the Jahn-Teller distortion of Mn and stabilizing lattice oxygen. The F^- dopant could favor Na⁺ transport and buffer phase evolution in high-voltage regions. When coupled with a hard carbon anode, the full cell achieves a commendable energy density of 328.6 Wh kg^-1. This study introduces an innovative protocol for exploiting high-energy SIBs.