Cation-Disordered Rock-Salt Lithium Titanium Oxyfluoride Anode Enabling High-Rate Li-Ion Storage Through a 3D...

Pseudocapacitive materials employ rapid, non-diffusion-limited faradaic processes to store charge, demonstrating significant potential for fast-charging batteries and supercapacitors. However, the high redox potentials of existing pseudocapacitive anodes substantially lower the overall cell voltage and energy density. Here, a cation-disordered rock-salt lithium titanium oxyfluoride (DRX-Li_xTiOF₂, 0 < x < 2) is reported to reversibly accommodate approximately 1.19 mol of Li⁺ (~ 310 mAh g^-1) and delivers high-rate performance (exceeding 64.4 C) via pseudocapacitive Li⁺ storage within a low potential window extending down to 0.1 V vs. Li⁺/Li. This pseudocapacitive behavior is characterized by several structural and electrochemical features: the absence of phase transformation during Li⁺ intercalation, quasi-rectangular cyclic voltammetry curves, sloping charge/discharge profiles and a surface-controlled current response. We further reveal that the pseudocapacitive characteristics originate from a three-dimensional percolation network that facilitates fast Li⁺ migration with low energy barriers, enabled by a cation/anion-disordered structure arising from the mixed occupancy of Li/Ti cations and O/F anions. Owing to its low operating potential and high-rate capability, DRX-Li_xTiOF₂ allows a lithium-ion capacitor to attain 4.0 V cell voltage and achieve energy and power densities several times higher than those obtained with conventional anodes, such as battery-type Li₄Ti₅O₁₂ or pseudocapacitive materials like Nb₂O₅ and TiO₂.