Comprehensive Electrochemical Studies of Tavorite LiTiPO4F/C Electrode for Rechargeable Lithium Ion Battery

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.

Titanium-based potassium-ion battery positive electrode with extraordinarily high redox potential

The rapid progress in mass-market applications of metal-ion batteries intensifies the development of economically feasible electrode materials based on earth-abundant elements. Here, we report on a record-breaking titanium-based positive electrode material, KTiPO4F, exhibiting a superior electrode potential of 3.6 V in a potassium-ion cell, which is extraordinarily high for titanium redox transitions. We hypothesize that such an unexpectedly major boost of the electrode potential benefits from the synergy of the cumulative inductive effect of two anions and charge/vacancy ordering. Carbon-coated electrode materials display no capacity fading when cycled at 5C rate for 100 cycles, which coupled with extremely low energy barriers for potassium-ion migration of 0.2 eV anticipates high-power applications. Our contribution shows that the titanium redox activity traditionally considered as "reducing" can be upshifted to near-4V electrode potentials thus providing a playground to design sustainable and cost-effective titanium-containing positive electrode materials with promising electrochemical characteristics.