Co-intercalation reaction for graphite in sodium ion batteries: Revival or revolution?

Reversible intercalation of guest ions in graphite laid the foundation of modern battery technology, exemplified by the successful launch of Li-ion batteries (LIBs) 35 years ago. With sodium’s natural abundance and low cost, sodium ion batteries (SIBs) are increasingly regarded an appealing contender for large-scale energy storage. However, the conventional intercalation chemistry that unlocks graphite anodes in LIBs is ineffective in SIBs. This obstacle is recently overcome by a co-intercalation chemistry involving the insertion of solvated ions, which rejuvenated graphite as a viable anode for SIBs by offering ultrafast reaction kinetics, excellent rate capability and long cycle life. Nonetheless, the incorporation of solvent molecules induces substantial challenges, including pronounced volume change, elevated redox potential, and limited Na storage capacities. Despite extensive efforts to accommodate these issues, the practical implementation of graphite anodes in SIBs remain elusive. In this Perspective, we briefly overview the principles and key advances in graphite co-intercalation chemistry, highlight the overlooked challenges, and propose potential directions for achieving regulated co-intercalation for practical graphite-based SIBs.