Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e918" altimg="si211.svg"><mml:msub><mml:mrow /><mml:mrow><mml:mn>9</mml:mn></mml:mrow></mml:msub></mml:math>O25

The Wadsley–Roth compound PNb9O25 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 semiconductor-to-metal transition and intralayer ordering upon lithiation in the that could not be observed situ X-ray diffraction. In addition, instantaneous heat generation rates at working electrode metal counter during galvanostatic cycling were measured individually for first time by operando isothermal calorimetry. rate decreased due decrease electrical resistivity caused measurements. Furthermore, was positive delithiation exothermic plating Li+ ions on associated dendrite calorimetric established change dominated reversible each electrode. Finally, contribution enthalpy mixing relatively small even C-rates thanks mobility Pnb9O25 confirming its promises as material.