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

Herein, a high-performance P2-Na_2/3Mg_1/18[Ni_1/4Ti_5/36Mn_11/18]O₂ (NMNTM) cathode material is designed <i>via</i> a dual-site modulation strategy of Mg/Ti ions in different crystallographic sites. Unlike the <i>P</i>6₃/<i>mmc</i> space group identified by X-ray diffraction, neutron diffraction confirms the distribution of Mg ions in the Na sites and Ti ions in the transition metal sites, as well as a larger supercell structure with the <i>P</i>6₃ 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 an enhanced superlattice ordering confirmed by neutron diffraction. However, Mg/Ti ions effectively inhibit 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 to alleviate the structure degradation benefiting from Mg ions leads to the pillar effect of Mg ions in Na sites, enhances structural integrity by strong Ti-O bonds in contrast to Mn-O bonds, suppresses the P2-O2 transition and promotes the diffusion of Na ions, thereby improving the electrochemical performance of NMNTM.