Synergistic effect of Mg and Ti ions by dual-site modulation strategy induces enhanced ordering and electrochemical performance of layered cathode for sodium‐ion batteries

Abstract

Herein, high-performance P2-Na2/3Mg1/18[Ni1/4Ti5/36Mn11/18]O2 (NMNTM) cathode material is designed via a dual-site modulation strategy of Mg/Ti ions in different crystallographic sites. Unlike the P63/mmc space group identified by X-ray diffraction, neutron diffraction confirms the distributions of Mg-ion in Na sites and Ti in transition-metal sites, and a larger super cell structure with the P63 space group, indicating the existence of superlattice ordering in NMNTM. Electrochemically inert Mg/Ti ions do not smooth the charge/discharge profiles, but lead to the staircase-like voltage profiles upon electrochemical cycling, which is due to enhanced superlattice ordering confirmed by neutron diffraction. However, Mg/Ti ions effectively inhibits the P2-O2 phase transition at high voltage ranges, indicating the phase-transition-free solid-solution reaction. NMNTM delivers a reversible capacity of 113 mAh g-1 with largely improved rate capability, corresponding to 87% of theoretical capacity, and a great capacity retention of 80.2% after 150 cycles. Dual-site modulation of Mg and Ti ions in different crystallographic sites is beneficial for achieving the synergistic effect, which effectively tunes the Mn3+/Mn4+ ratio to avoid the Jahn-Teller distortion by eliminating Mn3+ ions and resulting structure degradation benefiting from Mg ions, leads to pillar effect of Mg ions in Na sites, enhances structure integrity by strong Ti-O bond in contrast to Mn-O bond, suppresses the P2-O2 transition and promotes the Na-ion movement, thereby improving the electrochemical performance of NMNTM.