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-NaMg[NiTiMn]O (NMNTM) cathode material is designed a dual-site modulation strategy of Mg/Ti ions in different crystallographic sites. Unlike the 6/ 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 6 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 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 Mn/Mn ratio to avoid the Jahn-Teller distortion by eliminating Mn 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.