Phase transformations and capacity fade mechanism in LixSn nanoparticle electrodes revealed by operando 7Li NMR

Abstract

Operando ⁷Li nuclear magnetic resonance (NMR), ex situ⁷Li magic-angle spinning (MAS) NMR and pair distribution function (PDF) methods are used to investigate the electrochemical lithiation and delithiation of 60 nm particles of tin. While the structural transformation pathways between Li–Sn intermetallics during lithiation and delithiation of Sn nanoparticles are somewhat consistent with the structural evolution of Li–Sn phases expected from the equilibrium binary phase diagram, there are some notable exceptions with the observation of a metastable phase Li₂Sn₃, and two vacancy rich metastable phases, Li_7−ζSn₃, and Li_13−δSn₅ during delithiation. The capacity fade of ∼30% per cycle is not attributed to particle cracking, but rather to particles losing contact with the carbon/PVDF binder. The disconnecting particles are associated with Li_xSn phases undergoing large decreases in diameters on delithiation, i.e., Sn, Li₂Sn₅, LiSn, and Li₇Sn₃. A peculiar behavior of capacity fade in this system is that it only occurs during delithiation. This is explained by a model of disconnecting–reconnecting Li_xSn-containing particles which remain in the electrode and become reconnected every cycle by the volume increase of neighboring connected particles during the lithiation process.