α-TiPO₄ as a Negative Electrode Material for Lithium-Ion Batteries

To realize high-power performance, lithium-ion batteries require stable, environmentally benign, and economically viable noncarbonaceous anode materials capable of operating at high rates with low strain during charge-discharge. In this paper, we report the synthesis, crystal structure, and electrochemical properties of a new titanium-based member of the MPO₄ phosphate series adopting the α-CrPO₄ structure type. α-TiPO₄ has been obtained by thermal decomposition of a novel hydrothermally prepared fluoride phosphate, NH₄TiPO₄F, at 600 °C under a hydrogen atmosphere. The crystal structure of α-TiPO₄ is refined from powder X-ray diffraction data using a Rietveld method and verified by electron diffraction and high-resolution scanning transmission electron microscopy, whereas the chemical composition is confirmed by IR, energy-dispersive X-ray, electron paramagnetic resonance, and electron energy loss spectroscopies. Carbon-coated α-TiPO₄/C demonstrates reversible electrochemical activity ascribed to the Ti³⁺/Ti²⁺ redox transition delivering 125 mAh g^-1 specific capacity at C/10 in the 1.0-3.1 V versus Li⁺/Li potential range with an average potential of ∼1.5 V, exhibiting good rate capability and stable cycling with volume variation not exceeding 0.5%. Below 0.8 V, the material undergoes a conversion reaction, further revealing capacitive reversible electrochemical behavior with an average specific capacity of 270 mAh g^-1 at 1C in the 0.7-2.9 V versus Li⁺/Li potential range. This work suggests a new synthesis route to metastable titanium-containing phosphates holding prospective to be used as negative electrode materials for metal-ion batteries.