Deciphering the Origin of High Electrochemical Performance in a Novel Ti-Substituted P2/O3 Biphasic Cathode for Sodium-Ion Batteries.

The layered Mn-based oxides (NaxMnO2), which is one of the most promising cathode families for rechargeable sodium-ion batteries, have received considerable attention because of their tunable electrochemical performances and low costs. Herein, a novel P2/O3 intergrown Li-containing Na0.8Li0.27Mn0.68Ti0.05O2 cathode material prepared by Ti-substitution into Mn-site is reported. Benefiting from the synergistic effects of the biphasic composite structure and inactive d0 element substitution, this P2/O3 electrode exhibits high initial charge/discharge capacity and excellent cycling performance. The combination of different characterization techniques including solid-state NMR, electron paramagnetic resonance, X-ray adsorption spectroscopy, and high-resolution transmission electron microscopy gives insights into the local electronic environment, the redox chemistry, and also the microstructure rigidity of these cathode materials upon cycling. On the basis of comprehensive comparison with the Ti-free P2/O3-Na0.8Li0.27Mn0.73O2, the observed improvement on the electrochemical performance is primarily attributed to the mitigation of notorious Mn3+/Mn4+ redox and the enhanced stability of the oxygen charge compensation behavior. From the viewpoint of structure evolution, Ti-substitution restrains the Li+ loss and irreversible structural degradation during cycling. This study provides an in-depth understanding of the electronic and crystal structure evolutions after inactive d0 element substitution and may shed light on the rational design of high-performance P2/O3 biphasic Mn-based layered cathodes.