Material Hardening Caused by Lattice Distortion Enables Good Cycling Stability of Entropy‐Increased Energy‐Storage Materials

Material hardening caused by lattice distortion is demonstrated as the underlying reason for the cycling-stability enhancement in entropy-increased energy-storage materials. The lattice distortion induced by doping/substitution in doped TiNb₂O₇ (Ti_0.95V_0.05Zr_0.05Nb_1.9Ta_0.05O₇) increases its material hardness and its reduced elastic modulus, thus hindering its particle breakage during repeated lithiation–delithiation, although its maximum unit-cell-volume change is larger than that of pristine TiNb₂O₇.

Abstract

Doping/substitution, which leads to an increase in configurational entropy, is a widely used approach to strengthen the electrochemical properties of energy-storage materials. Nevertheless, the key factor behind the enhanced cycling stability after the entropy increase remains elusive. Herein, via selective dopings of TiNb₂O₇ (TNO), an entropy-increased Ti_0.95V_0.05Zr_0.05Nb_1.9Ta_0.05O₇ (TNO-0.05) anode material is exploited, and an intrinsic link between the entropy increase and capacity-retention enhancement is revealed through a systematical study of this new material. As the dopant amount increases, the full-width at half-maximum of the X-ray diffraction (XRD) peaks of doped TNO monotonically widens, and thus the lattice distortion becomes severe. Significant lattice distortion with up to 427% larger lattice strain is introduced in TNO-0.05. Although the unit-cell-volume expansion of TNO-0.05 after lithiation is not reduced, the increased particle hardness, which originates from the increased lattice strain, effectively hinders particle breakage during repeated lithiation–delithiation. Consequently, TNO-0.05 delivers significantly improved cycling stability, showing approximately twice the capacity retention of TNO with a large active-material loading of 4.5 mg cm⁻² after 500 cycles at 1250 mA g⁻¹. Clearly, the material hardening caused by the lattice distortion can solve the structure-degradation problem in intercalation electrodes, undoubtedly promoting the exploitation of long-life energy-storage materials.