Origin of the Voltage Hysteresis in the CoP Conversion Material for Li-Ion Batteries

The electrochemical activity of the CoP conversion electrode was investigated through the combination of computational and experimental techniques. The carbon-free CoP electrode shows better performances than the carbon-coated electrode, in sharp contrast with the beneficial role of carbon coating reported in many insertion materials. A two-step insertion/conversion process associated with the exchange of 3Li is predicted for this system from the T = 0 K phase stability diagram performed on bulk structures within the DFT framework. The voltage hystereses measured for these two processes through a seven-day relaxation procedure (GITT) are 1 order of magnitude higher for the conversion process (Delta V-conv(exp) = 0.44 V) than for the insertion process (Delta V-ins(exp) = 0.04 V). The various elementary reactions susceptible to occur at the surface of the electrode were investigated by means of surface DFT calculations. This mechanistic study shows that the insertion mechanism is not significantly affected by the electrode nanosizing (Delta V-ins(th) = 0.04 V), while the conversion reaction does. Asymmetric responses are expected upon charge and discharge for this system, due to the growth of different interfaces. This induces different electrochemical equilibriums and then different voltages in charge and discharge. The hysteresis voltage computed for the conversion of LiCoP into Li3P + Co-0 is again in very good agreement with experiments (Delta V-conv(th) = 0.41 V). Such results are very encouraging and open new routes to the rationalization of the microscopic mechanisms acting as limiting reactions in electrode materials for Li-ion batteries.