Observing Framework Expansion of Ordered Mesoporous Hard Carbon Anodes with Ionic Liquid Electrolytes via in Situ Small-Angle Neutron Scattering

The reversible capacity of materials for energy storage, such as battery electrodes, is deeply connected with their microstructure. Here, we address the fundamental mechanism by which hard mesoporous carbons, which exhibit high capacities versus Li, achieve stable cycling during the initial "break-in" cycles with ionic liquid electrolytes. Using in situ small-angle neutron scattering we show that hard carbon anodes that exhibit reversible Li⁺ cycling typically expand in volume up to 15% during the first discharge cycle, with only relatively minor expansion and contraction in subsequent cycles after a suitable solid electrolyte interphase (SEI) has formed. While a largely irreversible framework expansion is observed in the first cycle for the 1-methyl-1-propypyrrolidinium bis(trifluoromethanesulfonyl)imide (MPPY.TFSI) electrolyte, reversible expansion is observed in the electrolyte lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI)/1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulf-onyl)imide (EMIM.TFSI) related to EMIM⁺ intercalation and deintercalation before a stable SEI is formed. We find that irreversible framework expansion in conjunction with SEI formation is essential for the stable cycling of hard carbon electrodes.