Lithium-Sodium Ion Capacitors: A New Type of Hybrid Supercapacitors with High Energy Density

Abstract Reportedly, Li4Ti5O12 could provide the novel characteristics of both Li+ insertion and Na+ insertion. In this work, we firstly take advantage of this unique feature and use lithium/sodium mixed organic solvent as electrolyte for evaluating the electrochemical performance of as-synthesized Li4Ti5O12 nano-microspheres (LTO MS). As a result, the potential window of LTO MS is broadened, leading to full utilization of Li4Ti5O12. In addition, the difference between insertion potentials of Li+ and Na+ inserting into LTO MS is applied to reasonably intercept the potential windows of LTO MS, ensuring its remarkable cycling stability and high rate property. Therefore, LTO MS using lithium/sodium mixed electrolyte (LTO MS-L/SIB system) can provide a broadened potential window of 0.4-2.5 V, leading to a high specific capacity without sacrificing rate performance and cycling stability. We also systemically analyze the charge storage mechanism of LTO MS-L/SIB by ex-situ XRD technologies. For the first time, the lithium-sodium hybrid ion capacitor (LTO MS//PSC L/SIC) is constructed with LTO MS as anode, peanut shell derived carbon (PSC) as cathode, and lithium-sodium mixed organic solvent as electrolyte. Compared with the as-constructed lithium ion capacitors (LTO MS//PSC LIC) and sodium ion capacitors (LTO MS//PSC SIC), LTO MS//PSC L/SIC device provides the highest gravimetric energy density of ∼65.3 Wh kg-1, a remarkable cycling stability, and an ultra-low self-discharge rate.