Investigating self-discharge in a graphite dual-ion cell using in-situ Raman spectroscopy.

Anion intercalation in the graphite positive electrode of a dual-ion battery requires high potential (> 4.3 V vs Li+/Li), which aggravates parasitic reactions involving electrolyte decomposition and Al corrosion, manifesting in poor coulombic efficiency, cycle life, and quick self-discharge. This study aims to investigate the stability of anion-intercalated graphite electrodes in a 4 M solution of lithium bis(fluorosulfonyl)imide (LiFSI) in ethyl methyl carbonate (EMC) using both in-situ and ex-situ Raman spectroscopy. The concentrated electrolyte is essential as it limits parasitic reactions at the cathode-electrolyte interface (CEI) occurring in parallel to anion intercalation. Using electrochemical methods including cyclic voltammetry, and post-mortem electron microscopy it was confirmed that the Al current collector is largely stable at potentials as high as 5.2 V in the electrolyte under consideration; no dissolved Al species were detected using EDX characterization. Results from the cyclic voltammetry study also indicate that parasitic reactions can be mitigated when the cut-off potential is limited to 5.0 V leading to higher coulombic efficiency (CE = 94 %) and more stable discharge capacity (85.17 mAh g-1). However, extending the potential to 5.1 and 5.2 V results in the discharge capacity increasing by almost 20 mAh g-1, though at the expense of the coulombic efficiency, which decreases from 94 to 76 %. Upon raising the cut-off potential to 5.3 V, the CE significantly decreased (20.62 %) as a result of extensive solvent decomposition ultimately leading to much quicker capacity fading. Based on SEM images taken after 50 cycles, graphite particles did not sustain any structural or morphological change during cycling regardless of the cut-off potentials applied. Further tests were conducted on Li-graphite DIBs using galvanostatic methods in the range from 3 to 5 V, and at different specific currents (20, 50, and 100 mA g-1). Though the cells exhibited good