Mo6+ bifunctional substitution of P2-type manganese oxide for high performance sodium-ion batteries

Due to its cost-effectiveness and high specific capacity, P2-type manganese (Mn) oxide is considered a highly promising cathode material for sodium ion batteries (SIBs). However, the practical application of this material is hindered by poor cyclic stability caused by the phase transition of P2-P'2 due to the Jahn-Teller effect of Mn3+ at low voltage and significant volume changes at high voltage. In this study, we utilized Mo6+ dual-function P2-Na0.67Mn0.99Mo0.01O1.997(PO4)0.0008 (NMMPO-1) to achieve exceptional magnification performance and cycle stability. Advanced electron microscopy and X-ray diffraction analysis revealed that Mo6+ acts as a pillar in replacing interlayer Na+ sites within the P2 phase, resulting in extended layer spacing and a stable crystal structure. Additionally, Mo6+ substitution for Mn3+ weakens the Jahn-Teller distortion effect of Mn3+. Ex-situ X-ray diffraction experiments demonstrated that NMMPO-1 effectively inhibits the P2-P'2 phase transition at low voltage while minimizing volume changes during charge–discharge cycles. Overall, our modified NMMPO-1 exhibits excellent cycle stability with 144.55 mA h g−1 after 100 cycles at 0.5C, retaining 83 % capacity retention rate, along with impressive rate performance achieving capacity of 100.57 mA h g−1 at 5C. Furthermore, NMMPO-1 demonstrates superior air stability compared to unmodified samples when exposed to air over multiple cycles. Moreover, through detailed analysis using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic intermittent titration technique (GITT) curves; it is observed that NMMPO-1 possesses enhanced Na+ diffusion capacity and kinetic properties compared to other materials. This research provides novel insights into the role of substituting high valence cations in layered Mn oxide materials which can contribute towards designing improved performance for future sodium ion battery applications.