Operando X-Ray Absorption Spectroscopy of Solid Electrolyte Interphase Formation on Silicon Anodes

The development of improved lithium-ion batteries (LiBs) is critical to societies transition towards net-zero. However, more widespread deployment requires improvements in energy density, economic cost and cycle-lifetime. Various cathode and anode materials are under consideration for next-generation LIBs, and the interfacial stability of these materials in contact with the electrolyte is a critical consideration. The formation of a stable solid electrolyte interphase (SEI) is critical to long term LiB performance, however a detailed mechanistic understanding of SEI formation remains elusive, in-part due to the complex electrolyte decomposition reactions that occur. Whilst X-ray spectroscopy, and in particular X-ray absorption spectroscopy, has become a staple characterisation technique to obtain electronic structure and chemical information from battery materials interfaces, it is nearly always performed post-mortem (with cells disassembled in a glovebox, washed, and transferred into the measurement chamber) which can result in unwanted side reactions and removal of labile SEI species.<br/><br/>Here, we demonstrate a novel operando methodology to explore the formation and evolution of the SEI on amorphous silicon (a-Si) anodes during the formation cycle. a-Si is of particular interest in the Li-ion battery community as a result of its extremely high specific capacity¹. A specially designed cell that incorporates a silicon nitride membrane as an X-ray transmissive window allows the electrode surface to be probed in real time whilst performing electrochemical cycling^1,2,3. This synchrotron-based approach uses a modulating beam chopper and lock-in amplifier, to perform total electron yield mode X-ray absorption spectroscopy, providing unique nm-scale sensitivity to buried-interfaces. The methodology allows spectroscopic tracking of the onset potentials for the formation of different SEI species, and for identification of O- and F-containing species in the SEI. Cross-referencing to cycling data, complementary bulk sensitive fluoresecent yield (FY) XAS measurements, and density function theory (DFT) calculated core-hole spectra enables identification of the electrolyte and SEI species, and the dominant mechanisms of SEI formation. Without FEC present, LiF formation is detected at 0.6 V vs. Li/Li⁺ prior to significant lithiation of the a-Si, whilst at lower potentials the SEI grows in thickness with an increased contribution from organic components containing -C(=O)O- species. The observed sequential formation of inorganic and organic components is implicated in layering of the SEI. With FEC as an additive we see the onset of SEI formation at much higher potentials (1.0 V vs. Li/Li⁺), and attribute the improved cycle life seen with this additive to the rapid healing of SEI defects formed during delithiation. This work demonstrates a new X-ray measurement approach that can provide surface sensitive chemical information of buried electrode interfaces, which has the potential to allow us to better understand the complex degradation and formation processes occurring at battery electrode interfaces during cycling. This work was performed in collaboration with two internationally-leading user facilities (Diamond Light Source, UK and ALBA, Spain) and involved collaboration between researchers at the Universities of Oxford and Cambridge.<br/><br/><b>References</b><br/>1. Swallow et al. <i>Nature Commun. </i><b>2022</b>, <i>13, </i>6070.<br/>2. Wu et al. <i>Phys. Chem. Chem. Phys.</i> <b>2015</b>, <i>17</i>, 30229.<br/>3. Weatherup et al. <i>Top. Catal.</i> <b>2018</b>, <i>61</i>, 2085.