In Situ NMR Insights into the Electrochemical Reaction of Cu₃P Electrodes in Lithium Batteries

This study reports a multinuclei in situ (real-time) NMR spectroscopic characterization of the electrochemical reactions of a negative Cu 3 P electrode toward lithium. Taking advantage of the different nuclear spin characteristics, we have obtained real-time 31 P and 7 Li NMR data for a comprehensive understanding of the electrochemical mechanism during the discharge and charge processes of a lithium battery. The large NMR chemical shift span of 31 P facilitates the observation of the chemical evolutions of different lithiated and delithiated Li x Cu 3– x P phases, whereas the quadrupolar line features in 7 Li enable identification of asymmetric Li sites. These combined NMR data offer an unambiguous identification of four distinct Li x Cu 3– x P phases, Cu 3 P, Li 0.2 Cu 2.8 P, Li 2 CuP, and Li 3 P, and the characterization of their involvement in the electrochemical reactions. The NMR data led us to propose a delithiation process involving the intercalation of metallic Cu 0 atomic aggregates into the Li 2 CuP structure to form a Cu 0 -Li 2– x Cu 1+ x P phase. This process might be responsible for the poor capacity retention in Cu 3 P lithium batteries when cycled to a low voltage.