Origins of rapid solvated-calcium ion co-intercalation in graphite anodes

Co-intercalation reaction unlocks reversible calcium (Ca) ion storage in graphite anodes, a breakthrough unattainable with conventional intercalation chemistry. However, the underlying mechanisms driving the rapid co-intercalation reaction remain unclear. Here we reveal that reversible Ca storage in graphite only occurs in electrolytes that simultaneously meet three critical criteria: high Ca ²⁺ solvation strength in bulky electrolyte, the low intercalation energy at electrolyte/graphite interface, and fast [Ca-solvent] ²⁺ diffusion kinetics within graphite galleries. Among various solvents, only those based on amides enable Ca ²⁺ intercalation in graphite, attributable to their high dielectric constants (ε > 20) and strong binding energies ( E _b > 5 eV) with Ca ²⁺ . Upon intercalation, both thermodynamic (the intercalation energy: E _int < 0) and kinetic (diffusion energy barriers: E _diff = 0.19 eV) factors are equally critical in regulating the Ca ion storage behaviors. Positive intercalation energies and slow [Ca-solvent] ²⁺ migration kinetic result in reduced storage capacities and lattice rearrangements in graphite intercalation compounds. In the optimized dimethylacetamide-based electrolyte, graphite anodes achieve 80% capacity retention over 750 cycles and high-rate capability at 0.2 A g ^− 1 in Ca-ion cells. The intriguing co-intercalation chemistry expands the promise for graphite anodes in non-lithium ion battery systems.