Highly microporous SbPO4/BCx hybrid anodes for sodium-ion batteries

Abstract

The current anode materials greatly restrict the electrochemical performance of sodium-ion batteries. Herein, we propose a highly microporous SbPO₄/BC_x hybrid anode for sodium-ion batteries, exhibiting a high initial reversible capacity of 871 mA h g⁻¹ at 50 mA g⁻¹, a good rate capability of around 300 mA h g⁻¹ even at 5 A g⁻¹ as well as an excellent cycling stability of 500 cycles. The excellent rate capability and cyclability with high capacity are probably due to the novel BC_x structure and stable PO₄³⁻ anions. The abundant micropores serve as reservoirs for storing the sodium ions and shorten the diffusion distance. The high surface area contributes to ample contact area between the electrode and electrolyte, thus achieving a rapid charge-transfer reaction. XPS analysis reveals that the BC_x matrix consists of three B/C structures of BC₃, BC₂O, and BCO₂, and contains around 12.93 at% substitutional boron. Since valence band holes are created by the B/C structures, more sodium ions would be captured easily, which motivates more sodium ions to intercalate electrochemically. Additionally, both the robust BC_x matrix and stable PO₄³⁻ anions as buffers could accommodate the volumetric expansion during the sodium ion insertion, thus optimizing the cycling performance. The strong attachment between SbPO₄ and the BC_x matrix would benefit mutual charge transfer between them and keep the integrity of the electrode during the sodiation/desodiation processes, which are favorable for sodium-ion transport and play a crucial role in enhancing the rate performance. Accordingly, the SbPO₄/BC_x composite is expected to become a promising anode for advanced SIBs.