The Origin of voltage hysteresis in CoP conversion materials
Remi Khatib (Université Montpellier)

This work deals with the study of conversion materials for Li­ion batteries. From a mechanistic point of view, these MX materials (M=transition metal, X=ligand of the main group) react with lithium to form a nano­sized composite electrode made of metallic nano­particles embedded into a LinX matrix. These high­capacity electrodes exhibit a large voltage hysteresis upon cycling, which is dramatic for the battery efficiency. This hysteresis seems to have both a kinetic and a thermodynamic origin. While the former might be partially avoided by appropriate engineering such as particle size control, coating, etc..., the latter seems to be intrinsic to the starting material and directly related to the ionic character of the metal­ligand bonds. It is clear that the kinetic of the electrochemical reactions are augmented by the nano­structuration of the electrode, but the thermodynamic effects induced by the augmentation of specific surfaces are not fully understood.
The electrochemical activity of the CoP conversion electrode was investigated through the combination of bulk, surfaces and interfaces calculations. As a first step, the phase diagram of Li x­ Co­P has been established because the MP electrodes are known to form intermediate phases during the reaction.[1] A multi­step 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.
Then, the analyse of the surface reactivity of this system is studied to identify the mechanisms occurring at the nanometric scale. The various elementary reactions susceptible to occur at the surface of the electrode are investigated by means of surface DFT calculations. This mechanistic study shows that the insertion mechanism is not significantly affected by the electrode nano­sizing, 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.
Finally, this work presents a methodology based on a multi­interface superlattice approach to investigate the interface electrochemistry in conversion reactions.[2] References:
[1] S. Boyanov et al.:Chemistry of Materials 18, 15 (2006) 3531­3538
[2] A.­L. Dalverny et al.: Journal of material chemistry 21 (2011) 10134­10142