<div>The shear-phase compound TiNb₂O₇ has recently emerged as a safe and high-volumetric density replacement for graphite anodes in lithium ion batteries. An appealing feature of TiNb₂O₇ is that it retains capacity even at high cycling rates. Here we demonstrate that phase pure and crystalline TiNb₂O₇ 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.</div>

The Wadsley-Roth compound PNb 9 O 25 is a promising fast charging lithium ion battery anode material with high operating voltage to prevent solid electrolyte interface formation.Here, we present potentiometric entropy measurements featuring signatures of semiconductorto-metal transition and intralayer ordering upon lithiation in the anode material PNb 9 O 25 that could not be observed with in situ X-ray diffraction. In addition, the instantaneous heat generation rates at the PNb 9 O 25 working electrode and at the lithium metal counter electrode during galvanostatic cycling were measured individually for the first time by operando isothermal calorimetry. The heat generation rate decreased at the PNb 9 O 25 electrode upon lithiation due to the decrease in electrical resitivity caused by the semiconductor-to-metal transition observed in potentiometric entropy measurements. Furthermore, the heat generation rate at the lithium metal electrode was positive during delithiation due to the exothermic plating of Li + ions on the lithium metal counter electrode associated with dendrite formation. Furthermore, calorimetric measurements established that the entropy change dominated the reversible heat generation rate at each electrode. Finally, the contribution of enthalpy of mixing was relatively small even at high C-rates thanks to the high Li + ion mobility in Pnb 9 O 25 confirming its promises as a fast charging anode material.

<div>The shear-phase compound TiNb₂O₇ has recently emerged as a safe and high-volumetric density replacement for graphite anodes in lithium ion batteries. An appealing feature of TiNb₂O₇ is that it retains capacity even at high cycling rates. Here we demonstrate that phase pure and crystalline TiNb₂O₇ 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.</div>