Improved reversibility and kinetics of oxygen redox reaction in Na2Mn3O7 by dual-site regulation strategy

Abstract

Enhancing structural stability under high voltage is essential for design of sodium-ion batteries with high energy density and cycling stability. Layered oxides possesses non-bonding O 2p orbitals contribute to extra capacity from oxygen redox reaction (ORR). Nevertheless, the irreversibility of ORR and structural evolution result in capacity decline, voltage hysteresis, and inferior kinetics. Here, dual-site regulation strategy is applied in Na₂Mn₃O₇. A strategic incorporation of Li⁺ and Ti⁴⁺ into alkali and transition-metal layers, respectively, to activate higher capacity and enhance structural stability. Combination of X-ray diffraction (XRD) and pair distribution function refinement results confirms the crystal structure. In-situ XRD shows that Na_1.65Li_0.35 [Mn_2.5Ti_0.5]O_7-δ (NLMT35) undergoes less phase transitions, indicating better structural stability. In addition, hard X-ray absorption, soft X-ray absorption and X-ray photoelectron spectra analysis reveal the reductive coupling mechanism of electron transfer from oxygen to manganese ions, boosting the reversibility and kinetics of anionic redox reactions. Consequently, the NLMT35 delivers an initial capacity of 171 mAh g⁻¹, among which the reversible oxygen redox capacity increased by 49 % and the polarization decreased by 36 %, ensuring a capacity retention of 93 % after 200 cycles in full cells. This finding facilitates the development of reliable ORR-based cathode materials with high energy efficiency.