Na3mnti(Po4)3/C Composite as an Anode for Na-Ion Batteries with Superior Rate Performance and Long-Term Span

Polyanionic phosphates, one of the Na superionic conductors (NASICON) structured materials, have become the research focus due to their good structural stability, thermal stability, excellent electrochemical performance and long cycle span. Na3MnTi(PO4)3 (NMTP) has been reported as a promising cathode, showing three-electron reaction during cycling with three redox couples of Mn3+/4+ (4.0 V vs. Na+/Na) and Mn2+/3+ (3.5 V) and Ti3+/4+ (2.1 V). Note that the lower redox couple of Ti2+/3+ (0.4 V), due to the further reduction of titanium, endows NMTP the characteristic as an anode for Na-ion batteries. In this work, the Na storage performance and reaction mechanism of NMTP/C in the low potential range of 0.01 – 3.0 V are explored. The NMTP/C composite exhibits a reversible specific capacity of 200 mAh g−1 at 0.1 C, which is superior to the Na3V2(PO4)3/C anode (170 mAh g−1) in the same voltage range. Attractively, NMTP/C composite provides a specific discharge capacity of 71 mAh g−1 at 10.0 C and remains a capacity retention rate of 93.4% after 3000 cycles, showing the superior rate performance and long-term stability. In addition, ex situ XRD and HRTEM are used to reveal the structural evolution of two-phase and single-phase solid solutions during cycling. Our findings confirm that the lower voltage plateau of Ti2+/3+ doubles the sodiation capacity of NMTP and verifies its feasibility as a promising anode for Na-ion batteries.