Revealing the Self-Regulating Phase Transition Mechanism of Low-Volume P2+“Z” in P2-Type Sodium Manganese with High-Entropy Substitution for Sodium-Ion Batteries

Abstract

Layered transition metal oxide P2-Na0.7MnO2 typically faces challenges such as low potential plateau, phase transitions, and poor rate capability. A cost-effective high-entropy substitution strategy is developed to synthesize P2-Na0.7Li0.02K0.02Fe0.2Cu0.2La0.02Ti0.02V0.02Mn0.5O2. The study elucidates that the synergistic influence of substituted elements, specifically Cu2+/Cu3+ and Fe3+/Fe4+, engenders higher redox potentials, substantially enhancing the potential plateau (the discharge midpoint potential increased from 2.36–3.5 V). The addition of La improves structural stability and conductivity. This high-entropy substitution methodology adeptly mitigates volumetric O2 phase transitions, fostering the formation of a moderate self-regulating P2+“Z” phase. The material's volume change in the second cycle at approximately the same potential (≈2.6 V) is about -0.17%. Notably, Hall effect testing indicates that the material primarily conducts electricity through holes. Both bulk and surface carrier concentrations, as well as mobility, have increased. The conductivity increases up to four times (from 8.43E-06 (S cm−1) to 3.49E-05 (S cm−1)). The material delivers a reversible capacity of 151.8 mAh g−1 at 0.1 C within the range of 1.5–4.5 V, retaining 81.3% of capacity at 1 C after 100 cycles. It retains 97.6% capacity at 1 C after 50 cycles within the range of 2.0–4.2 V.