Pseudocapacitance enhanced by N-defects in Na3MnTi(PO4)3/N-doped carbon composite for symmetric full sodium-ion batteries

Abstract The concept of symmetric full battery attracts increasing attentions in recent years. Symmetric battery consists of two identical “bi-functional” electrode materials, which can be used as both the cathode and anode. The NASICON-structured Na3MnTi(PO4)3 is capable to be used as a bi-functional electrode for symmetric sodium-ion full battery due to its multi-redox reaction with suitable voltage gap. However, it suffers from limited capacity and poor rate performance. In this study, a Na3MnTi(PO4)3 particulates embedding in N-doped carbon matrix material (NMTP/C-N) is constructed. Both the experiments and density functional theory (DFT) calculations show that the N-defects in carbon matrix have stronger adsorption energy towards Na+, and the N-vacancy defects have lower diffusion barriers for sodium-ion diffusion, thus enabling higher pseudocapacitance of the NMTP/C-N. By virtue of the enhanced reaction kinetics and pseudocapacitance, the NMTP/C-N demonstrates improved specific capacity and high-rate capability in both high- and low-voltage ranges (2.5-4.2 V vs. Na/Na+; 1.5-2.5 V vs. Na/Na+), where it is operated as the cathode and anode basing on the redox of Mn2+/Mn4+ and Ti4+/Ti3+, respectively. When constructed to a symmetric full battery, it exhibits a moderate reversible capacity of 91.8 mAh g-1 with a high initial Columbic efficiency of 85.2%, and maintains 70.8% of discharge capacity after 400 cycles at 1 C. This work deepens our understanding on materials design for enhanced pseudocapacitance and electrochemical performances.