Synthesis and electrochemical performance of in-situ and ex-situ carbon- coated Na2Ti3O7, as a promising anode for sodium-ion batteries

Insertion-type layered Na₂Ti₃O₇ has attracted the attention of the researchers and is considered to be one of the promising low-voltage anode materiasl for sodium-ion batteries. In spite of its fascinating electrochemical properties, the low electronic conductivity and structural instability of Na₂Ti₃O₇ are major drawbacks that restrict its practical application. Surface modification with pyrolytic carbon is one of the effective ways to reduce irreversible capacity loss caused by electrolytic degradation. In this work, attempts have been made to investigate the effects of different carbon coating approaches on the electrochemical properties of sol-gel-synthesized Na₂Ti₃O₇ microrods. The as-synthesized Na₂Ti₃O₇ rods are coated with a uniform carbon layer both by in-situ and ex-situ methods using citric acid and polyvinyl alcohol as carbon source, respectively. Ex-situ carbon-coated Na₂Ti₃O₇ (Na₂Ti₃O₇@C), due to better coating uniformity and higher graphitized carbon percentage, shows enhanced cyclability and rate performance compared to bare material and in-situ carbon composite (Na₂Ti₃O₇/C). Following the ex-situ carbonization method using PVA as carbon source, it is found that increase of carbon content from 5wt% to 10wt% significantly improves its electrochemical properties. However, further increase in PVA amount has adverse effect on the cycling as well as rate performance of Na₂Ti₃O7@C. Surface modified Na₂Ti₃O₇@C with optimum carbon content (10wt% C) shows improved cycling capacity (capacity retention ∼74.75% after100 cycle) and rate performance (∼67 mAhg-1 at 1.5 Ag-1). Both excess and inadequate carbon content have detrimental effect on the electrochemical properties of Na2Ti3O7 anode.