Electrochemical Properties of Nanocrystalline Li4Mn5O12 for Hybrid Capacitor

Abstract

The hybrid capacitor or asymmetric capacitor is the hybridization of electrochemical capacitor and rechargeable batteries. The device is developed by hybridization on the electrode level; therefore, it is based on a battery anode (charged by faradic reaction) and electrochemical capacitor electrode (charge stored in the electrochemical double layer) [1]. Sometimes, the name “lithium-ion” capacitor was introduce to described a device using a graphite negative electrode and activated carbon (AC) based positive electrode in combination with a Li+ containing electrolyte [2]. In such device, different types of materials have been proposed for faradic component. Lithium insertion compounds used in lithium-ion batteries have attracted a lot of interest due to high specific charge. They have been typically used in organic electrolyte but the aqueous solutions are not missing [3,4]. In this work, nanocrystalline Li4Mn5O12 was low-temperature synthesized by sol-gel method with citric acid as a chelating agent. Various initial synthesis conditions were studied such as: pH of solution, the ratio of citric acid to total metal ions, the precursor of lithium salts. The structure, morphology and electrochemical properties of materials are characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), cyclic voltammetry (CV) and galvanostatic cycling with potential limit (GCPL). The stoichiometric of formula was exactly determined by reduction-oxidation titration and flame atomic absorption spectrophotoscopy (FAAS). The resulting materials exhibited the particles size of 50 – 100 nm. The electrochemical results showed that Li4Mn5O12 synthesized at pH = 9 and the ratio of citric acid to total metal ions 1:1 exhibited the best electrochemical capacitive performance within potential range of 0 – 1.4 V in 1M Li2SO4. A maximum specific capacitance of 20 F.g-1 during 200 cycles was obtained for hybrid capacitor for AC/Li4Mn5O12 at high discharge rate of 5 A.g-1. Acknowledgements The authors would like to thank Office of Naval Research Global (ONRG) for the grant N62909-13-1-N235. The authors acknowledge funding from VNU-HCM under the grants HS2013-76-01.