New frontiers in alkali metal insertion into carbon electrodes for energy storage

With rising interest in new electrodes for next-generation batteries, carbon materials remain as top competitors with their reliable performance, low-cost, low voltage reactions, and diverse tunability. Depending on carbon's structure, it can attain high cyclability as with Li⁺ at crystalline graphite or exceptional capacities with Na⁺ at amorphous, porous hard carbons. In this review, we discuss key results and research directions using carbon electrodes for alkali ion storage. We start the first section with hard carbon (HC), a leading material of interest for next-generation Na-ion batteries. Methods for tuning the HC structure towards a high capacity pore-filling mechanism are examined. The rate performance of hard carbon electrodes is further discussed. We finish this section with soft carbons that mostly remain as low performing materials compared to other carbons. In the second section, we discuss alkali ion insertion into graphite and graphite-like materials. Though graphite has a long history with Li-ion batteries, it also shows promising characteristics for K-ion batteries. We discuss the significant progress made on improving the electrolyte for high cyclability of graphite with K⁺. Thereafter, we evaluate B/C/N materials that have a similar structure to graphite but can attain higher capacities for both Li⁺ and Na⁺. Finally, we touch on the recent developments using alternative solvents for Na⁺ cointercalation at graphite and deeper knowledge on the intercalant structure. Despite steady progress, carbon electrodes continue to improve as a key group of materials for alkali energy storage.