Operando calorimetry informs the origin of rapid rate performance in microwave-prepared TiNb2O7 electrodes

Abstract The shear-phase compound TiNb 2 O 7 has recently emerged as a safe and high-volumetric density replacement for graphite anodes in lithium ion batteries. An appealing feature of TiNb 2 O 7 is that it retains capacity even at high cycling rates. Here, we demonstrate that phase pure and crystalline TiNb 2 O 7 can be rapidly prepared using a high-temperature microwave synthesis method. Studies of the charging and discharging of this material, including through operando calorimetry, permit key thermodynamic parameters to be revealed. The nature of heat generation is dominated by Joule heating, which sensitively changes as the conductivity of the electrode increases with increasing lithiation. The enthalpy of mixing, obtained from operando calorimetry, is found to be small across the different degrees of lithiation, pointing to the high rate of lithium ion diffusion at the origin of rapid rate performance.