In Situ Characterization of Battery Materials using X-ray Absorption Spectroscopy

X-ray absorption spectroscopy using synchrotron radiation has become an essential tool for operando studies of batteries where nanoparticles and amorphous materials preclude the use diffraction to study structural changes. The ability to take full data sets in only a few minutes is well-matched to typical time scales of battery charge and discharge cycles for in situ and operando experiments. I will briefly discuss the fundamentals of x-ray absorption spectroscopy and present our recent results on a novel, high cycling performance, Sn4P3/graphite composite anode material for lithium ion batteries using the MRCAT beamlines at the Advanced Photon Source.

Building on our previous work in modeling Sn-based anode materials, EXAFS modeling and detailed analysis of local environment changes are correlated to the cell capacity and reveal the mechanism of lithiation/delithiation process. Results show that in the first two lithiation/delithiation cycles crystalline Sn₄P₃ is fully converted to an amorphous SnP_x phase, which in further cycles participates in reversible conversion and alloying reactions. The superior reversibility of this material is attributed to the highly dispersed SnP_x in the graphite matrix, which provides enhanced electrical conductivity and prevents aggregation of Sn clusters during the lithiation/delithiation process. The gradual capacity fading in long-term cycling is attributed to the observed increase in the size and the amount of metallic Sn clusters in the delithiated state, correlated to the reduced recovery of the SnP_x phase. This paper reveals the mechanism responsible for the highly reversible tin phosphides and provides insights for improving the capacity and cycle life of conversion and alloying materials.