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 Cu₃P electrode toward lithium. Taking advantage of the different nuclear spin characteristics, we have obtained real-time ³¹P and ⁷Li 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 ³¹P facilitates the observation of the chemical evolutions of different lithiated and delithiated Li_<i>x</i>Cu_3–<i>x</i>P phases, whereas the quadrupolar line features in ⁷Li enable identification of asymmetric Li sites. These combined NMR data offer an unambiguous identification of four distinct Li_<i>x</i>Cu_3–<i>x</i>P phases, Cu₃P, Li_0.2Cu_2.8P, Li₂CuP, and Li₃P, 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 Cu⁰ atomic aggregates into the Li₂CuP structure to form a Cu⁰-Li_2–<i>x</i>Cu_1+<i>x</i>P phase. This process might be responsible for the poor capacity retention in Cu₃P lithium batteries when cycled to a low voltage