Operando Calorimetry Investigation of Particle Size Effects on Heat Generation in Wadsley–Roth (W_0.2V_0.8)₃O₇-Based Electrodes

The Wadsley–Roth shear phase compound (W0.2V0.8)3O7 is a promising fast-charging electrode material with the ability to engage in multielectron redox reactions and to sustain high specific capacity under large currents. Here, solid-state methods and sol–gel methods combined with freeze-drying were used to synthesize (W0.2V0.8)3O7 microparticles and nanoparticles, respectively. Cyclic voltammetry and galvanostatic cycling demonstrated that electrodes made of (W0.2V0.8)3O7 nanoparticles have superior electrochemical performance compared to those made of microparticles, but the origin of this difference was not well understood. Here, entropic potential measurements performed at slow C-rate indicated that both nanoparticles and microparticles undergo a semiconductor to metal transition. However, the nanoparticles display a two-phase coexistence over a narrower range of compositions than the microparticles. Operando calorimetry measurements at high C-rate established that, regardless of particle size, the heat generation rate at the (W0.2V0.8)3O7 electrode increased with lithiation due to an increase in charge transfer resistance. The time-averaged irreversible heat generation rate was slightly but systematically smaller at the electrode made of nanoparticles. However, the specific dissipated energy and the contribution from enthalpy of mixing caused by the lithium concentration gradient were notably smaller for the (W0.2V0.8)3O7 nanoparticles. These observations were attributed to the fact that nanoparticles were less ionically resistive and able to accommodate more lithium while lithium-ion intercalation therein was more kinetically favorable.