Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb9O25

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 anode material PNb9O25 that could not be observed with in situ X-ray diffraction. In addition, the instantaneous heat generation rates at the PNb9O25 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 PNb9O25 electrode upon lithiation due to the decrease in electrical resistivity 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 Pnb9O25 confirming its promises as a fast charging anode material.