Unveiling Coexisting Battery-Type and Pseudocapacitive Intercalation Mechanisms in Lithium Titanate

Conventional lithium-ion (Li-ion) batteries and supercapacitors face inherent trade-offs between power and energy densities, restricting their adaptability in applications requiring dynamic performance across both regimes. Here, a “zero-strain” lithium titanate (Li₄Ti₅O₁₂) as a new class of battery-capacitive material exhibiting dual lithiation mechanisms, combining diffusion-controlled battery-type redox reactions and surface-controlled pseudocapacitive intercalation, depending on the operating potential, is revealed. At ≈1.55 V (vs Li/Li⁺), lithium titanate undergoes a two-phase transition reaction between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂, involving Li migration between 8a and 16c Wyckoff sites. Upon deeper lithiation to potentials near 0 V, Li ions reoccupy the 8a sites, triggering a reversible pseudocapacitive response with fast kinetics. Leveraging these dual lithiation mechanisms, lithium titanate delivers a high reversible capacity of ≈215 mAh g⁻¹ at 20 mA g⁻¹, retaining 148 mAh g⁻¹ at 2000 mA g⁻¹. The high-rate capability and cycling stability are attributed to a unique structure with minimal lattice strain during Li-site occupation. This work presents the first clear demonstration of a unique dual-mode charge storage mechanism in lithium titanate, which can reversibly operate in either battery-type or pseudocapacitive regimes.