Cation Co-Intercalation with Anions: The Origin of Low Capacities of Graphite Cathodes in Multivalent Electrolytes

Dual-ion batteries involving anion intercalation intographitecathodes represent promising battery technologies for low-cost andhigh-power energy storage. However, the fundamental origins regardingmuch lower capacities of graphite cathodes in earth abundant and inexpensivemultivalent electrolytes than in Li-ion electrolytes remain elusive.Herein, we reveal that the limited anion-storage capacity of a graphitecathode in multivalent electrolytes is rooted in the abnormal multivalent-cationco-intercalation with anions in the form of large-sized anionic complexes.This cation co-intercalation behavior persists throughout the stageevolution of graphite intercalation compounds and leads to a significantdecrease of sites practically viable for capacity contribution insidegraphite galleries. Further systematic studies illustrate that thephenomenon of cation co-intercalation into graphite is closely relatedto the high energy penalty of interfacial anion desolvation due tothe strong cation-anion association prevalent in multivalentelectrolytes. Leveraging this understanding, we verify that promotingionic dissociation in multivalent electrolytes by employing high-permittivityand oxidation-tolerant co-solvents is effective in suppressing multivalent-cationco-intercalation and thus achieving increased capacity of graphitecathodes. For instance, introducing adiponitrile as a co-solvent toa Mg2+-based carbonate electrolyte leads to 83% less Mg2+ co-intercalation and a similar to 29.5% increase in deliveredcapacity of the graphite cathode.