Charge storage performance of lithiated iron phosphate/activated carbon composite as symmetrical electrode for electrochemical capacitor

• We examine LiFePO4-activated carbon (AC) composite as the core electrode materials in Na2SO3 and Li2SO4 aqueous solutions, respectively.
• The maximum specific capacitance of 112.41 F/g is obtained in 40 wt % LiFePO4-AC electrode in comparison to that of pure AC electrode (76.24 F/g) in Na2SO3.
• The capacitive enhancement is attributed to the contribution of synergistic effect of electric double layer capacitance (EDLC) on the surface of AC and pseudocapacitance via intercalation/extraction of H+, OH−, Na+ and SO32− and Li+ ions in the LiFePO4 lattices.

In this study, a symmetric electrochemical capacitor was fabricated by adopting a lithium iron phosphate (LiFePO4)-activated carbon (AC) composite as the core electrode material in 1.0 M Na2SO3 and 1.0 M Li2SO4 aqueous electrolyte solutions. The composite electrodes were prepared via a facile mechanical mixing process. The structural properties of the nanocomposite electrodes were characterised by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis. The electrochemical performances of the prepared composite electrode were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (CD) and electrochemical impedance spectroscopy (EIS). The experimental results reveal that a maximum specific capacitance of 112.41 F/g was obtained a 40 wt% LiFePO4 loading on an AC electrode compared with that of a pure AC electrode (76.24 F/g) in 1 M Na2SO3. The improvement in the capacitive performance of the 40 wt% LiFePO4–AC composite electrode is believed to be attributed to the contribution of the synergistic effect of the electric double layer capacitance (EDLC) of the AC electrode and pseudocapacitance via the intercalation/extraction of H+, OH−, Na+ and SO32− and Li+ ions in LiFePO4 lattices. In contrast, it appears that the incorporation of LiFePO4 into AC electrodes does not increase the charge storage capability when Li2SO4 is used as the electrolyte. This behaviour can be explained by the fact that the electrolyte system containing SO42− only exhibits EDLC in the Fe-based electrodes. Additionally, Li+ ions that have lower conductivity and mobility may lead to poorer charge storage capability compared to Na+ ions. Overall, the results reveal that the AC composite electrodes with 40 wt% LiFePO4 loading on a Na2SO3 neutral electrolyte exhibit high cycling stability and reversibility and thus display great potential for electrochemical capacitor applications.

Schematic diagram explains the charge storage mechanisms proposed for LiFePO4-based electrodes in Na2SO3 solution: (i) adsorption/desorption of proton (H+), hydroxide (OH−), alkali metal cation (Na+) and sulphide anions (SO32−) from electrolyte on LiFePO4/AC composite electrodes forming Helmholtz layer, (ii) intercalation of H+, OH−, Na+ and SO32− ions from the electrolyte into the bulk composite electrodes during reduction while deintercalation of ions upon oxidation.Figure optionsDownload as PowerPoint slide