Highly Reversible Na-Intercalation into Graphite Recovered from Spent Li-Ion Batteries for High-Energy Na-Ion Capacitor.

High-performance Na-ion capacitor (NIC) was constructed with graphite recovered from spent Li-ion batteries (LIBs) as battery-type negative electrode and high-surface-area activated carbon as a supercapacitor component. Unlike Li-insertion into graphite, Na-insertion into graphite is extremely limited; hence, a "solvent-co-intercalation" mechanism was proposed for high reversibility using ether family solvents. First, the Na-insertion properties were assessed in the half-cell assembly with 0.5 m NaPF6 in tetraethylene glycol dimethyl ether as an electrolyte solution and compared with the commercial graphite. The NIC comprised pre-sodiated graphite as a negative electrode and commercial activated carbon as a cathode. This fascinating NIC configuration displayed the maximum energy density of 59.93 Wh kg-1 with exceptional cyclability of 5000 cycles at ambient temperature with approximately 98 % retention. Interestingly, the electrode aging process in the presence of electrolyte resulted in approximately 19 % higher energy density than the routine electrode heat treatment. Further, the electrochemical activity of the NIC at various temperatures was studied, and it was found that the graphite recovered from spent LIBs could be effectively reused towards the construction of high-performance charge storage devices with exceptional performance.