Lithium-sodium ion capacitors: A new type of hybrid supercapacitors with high energy density

Abstract

• New hybrid system adopts lithium/sodium mixed organic solvent as electrolyte. • The potential window of LTO MS L/SIB system is broadened. • A four-step charge storage mechanism is proposed. • New hybrid supercapacitors (LTO MS//PSC L/SIC) show excellent electrochemical properties. Reportedly, Li 4 Ti 5 O 12 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 Li 4 Ti 5 O 12 nano-microspheres (LTO MS). As a result, the potential window of LTO MS is broadened, leading to full utilization of Li 4 Ti 5 O 12 . 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.