Versatile Si/P System as Efficient Anode for Lithium and Sodium Batteries: Understanding of an Original Electrochemical Mechanism by a Full XRD-NMR Study

As illustrated by recent extensive crystallographic studies, the structural richness of the ternary Li/Si/P system offers a unique variety of physical properties. Taking advantage of 3D networks enabling fast ionic conduction properties, new phosphorus silicides based electrodes have recently emerged, exhibiting attractive performance in Li-ion as well as in Na-ion batteries. To date, SiP2 reaction mechanism vs Li or Na has nevertheless not been elucidated. The combination of operando X-ray diffraction and ex situ P-31, Li-7, Na-23, and F-19 MAS NMR spectroscopy allows here probing of both long-range structural information and evolution of local environments. These complementary techniques lead to deeper understanding and depiction of the mechanism behind the lithiation/delithiation and sodiation/desodiation of SiP2. The versatility of connection of tetrahedral SiP4 anions, from threedimensional network to isolated SiP4 units, stabilized by an increasing number of counterions as a function of the state of charge/discharge is the driving force of the new SiP2/Li and SiP2/Na batteries. This richness of SiP4 anions' crystallographic network allows stepwise phase transitions during lithiation/delithiation which limit the detrimental effects of volume expansion.