Ternary-Ion Batteries

The advancement of rechargeable batteries has long been dominated by two principal paradigms: single-ion and dual-ion mechanisms. The single-ion mechanism, operating via the shuttling of monovalent cations, offers high energy density yet suffers from inherent kinetic limitations in multivalent systems. In contrast, the dual-ion mechanism enhances kinetics at the expense of energy density, as it continuously consumes electrolyte components. To overcome this fundamental trade-off, we herein propose a ternary-ion mechanism that orchestrates the cooperative reactions of three distinct ionic species through mutually matched redox reactions and coordinated multivalent-metal ion exchange at the cathode and anode. As a proof-of-concept, we construct an Al/eGr-PTO battery in which, under the same operating voltages, the cathode concurrently accommodates the intercalation of two monovalent AlCl2+ species into pyrene-4,5,9,10-tetraone (PTO) and the compensating deintercalation of one monovalent AlCl4- from expanded graphite (eGr). This ensures that the eGr-PTO composite cathode as a whole intercalates only one aluminum for each complete redox cycle, with the chlorine content within the cathode remaining unchanged, while the anode undergoes the reversible aluminum stripping of an equivalent amount. Throughout cycling, the composition of the ionic liquid electrolyte remains macroscopically invariant. Accordingly, the prototype of Al/eGr-PTO pouch cells with a high capacity of ∼18 mAh and minimal electrolyte uptake achieve fast kinetics, stable long-term cycling, and a high cell-level specific energy of 84 Wh kg-1, outperforming previously reported OEM- and Gr-based dual-ion Al batteries. Our proposed ternary-ion batteries transcend the constraints of conventional single-ion and dual-ion systems, establishing a transformative framework for next-generation electrochemical energy storage.