Abstract
The development of batteries has become a major challenge and requires new operando techniques for tracking reaction kinetics in battery electrodes during operation. Taking Na 3 V 2 (PO 4 ) 2 F 3 and LiFePO 4 as examples of positive electrode materials, the present work deals with the design of an operando technique to measure the ionic and electronic transport properties of battery electrodes during polarization. In the case of LiFePO 4 , large electronic resistance changes were revealed when crossing the solid-solution domains. Such resistance changes are consistent with thermodynamic models proposing the existence of a diffuse phase boundary between Li-poor and Li-rich domains, as a result of the non-linear variation of the chemical potential of the LFP particles, which in turn leads to restricted lithium diffusion. Concerning Na 3 V 2 (PO 4 ) 2 F 3 , the important variations of electronic resistance measured were correlated with different phase changes and superstructures formed during the insertion-disinsertion of Na + ions, as well as the polarization and entropy heat variations. These results are fully consistent with a substantial correlation of structural changes with transport properties and reaction kinetics, and thus, performances. More generally, this technique shows great promise as a tool to aid in designing battery electrodes with improved ionic and electronic percolations.