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

Abstract

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 LixCu3–xP 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 LixCu3–xP phases, Cu3P, Li0.2Cu2.8P, 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-Li2–xCu1+xP phase. This process might be responsible for the poor capacity retention in Cu3P lithium batteries when cycled to a low voltage.