Improved Performance of Na₃TiMn(PO₄)₃ Using a Non-stoichiometric Synthesis Strategy

Na superionic conductor (NASICON)-structured Na 3 TiMn(PO 4 ) 3 (NTMP) is a promising cathode material for sodium-ion batteries. However, it suffers from a low discharge plateau at about 2.5 V, which leads to not only a substantial capacity decrease in high-voltage discharge plateaus at 4.0 and 3.5 V but also a severe voltage hysteresis. This work demonstrates that NTMP material synthesized using a non-stoichiometric strategy displays a negligible discharge plateau at 2.5 V and substantially suppressed voltage hysteresis. The sample also exhibits the highest cycling stability among all polyanionic Mn-based cathode materials, with capacity retention of 91.7% after 2000 cycles. Experimental and computational data show that the 2.5 V plateau is associated with Na + /Mn 2+ cation mixing, which can be substantially suppressed using the non-stoichiometric strategy.