The Mechanical and Electrochemical Behaviour of Red Phosphorus in Lithium, Sodium and Potassium-Ion Batteries

Red phosphorus (RP) is a promising anode material for alkali-ion batteries due to its ability to alloy at low potentials with Li, Na and K to form the phases Li3P, Na3P and KP, delivering a high theoretical capacity of 2596, 2596 and 865 mAh g-1, respectively. However, its main disadvantage is its very large volumetric expansion during the alloying reaction (168% with Li, 331% with Na and 161% with K) that can cause, loss of electrical contact, low Coulombic efficiency, and ultimately poor cycle life. Other alloying materials such as silicon have shown to suffer from the fracturing of the particles during the reaction with lithium. In this work, we discuss if this is the case for phosphorus too [1]. Through comprehensive electrochemo-mechanical characterisation and modelling of the cycling stresses, we show that RP can be cycled at high current densities without fracture. A model that can simulate the volume expansion of RP during the reaction with the alkali metals Li, Na and K, as well as the generation and evolution of stresses on the particles is developed. Its use required the implementation of the elastic, plastic and fracture properties of RP that are measured by novel application of in-situ nanoindentation and powder compression. The validity of the model is also tested through in-situ TEM observation with extreme conditions (anisotropic ion diffusion and high current density) where no catastrophic failure was observed. The mechanical and the electrochemical characterisation, together with the model developed in this work allows for predictions to be made for the application of RP in alkali-ion batteries.REFERENCE[1] Capone et al., Matter 2020 3 (6), 2012-2028