NMR Study on Silicon Anode Material for Lithium-Ion Batteries

Abstract

This research aims to explore material questions within the field of lithium-ion batteries (LIBs) used for electric vehicles (EVs) through nuclear magnetic resonance (NMR). In EVs, the driving range and fast-charging capabilities need improvement. A promising anode material, silicon, was investigated using both ex situ and operando NMR techniques. Ex situ 7Li NMR was used to identify key lithium silicide phases in the material at various states of charge (SoC). The different phases highlighted throughout this report represent the degree of lithiation per silicon atom. The longevity of the silicon material was tested as a function of cycle count, which provided insight into how the different local environments in silicon were affected. Based on the ex situ 7Li NMR results, mechanisms for capacity fade and degradation were proposed. Operando NMR allowed for the silicon material to be monitored simultaneously with the electrochemical charging cycle. Real-time information regarding structural changes of the LixSi phase under multiple different cycling conditions was achieved and analysed based on deconvolutions and integration. The slower charging rate (C-rate) of C/3 using operando NMR further confirms the formation of phase 1 (LixSi, x < 2.0) and phase 2 (LixSi, 2.0 < x < 3.5) seen via ex situ NMR. More detail was provided with this technique than an ex situ spectrum collected at the top of charge (TOC) and bottom of discharge (BOD) of the post-mortem battery. When the current density of the battery was increased, drastic differences were seen regarding the formation of lithium silicide phases in comparison to slower c-rates. With the faster C-rates, lithium metal was formed during the NMR experiment, and the formation of phase 3, a more highly lithiated crystallin phase, not present under ex situ conditions, was identified. This one-of-a-kind methodology allowed us to probe the lithium environments in the silicon anode material and gain insight into fundamental chemical changes that occur during battery operation.