Binder-free NiCoFe layered double hydroxide nanosheets as electrode materials for flexible hybrid energy storage devices with high-rate and high-retention ability

C-S Ni, S-F Liu, J-F Li, C-W Pao, J-L Chen, H-Y Chen, J-H Huang
National Tsing Hua University,
Taiwan

Keywords: hybrid energy storage devices, NiCoFe hydroxides, Flexible supercapacitors

Summary:

In order to improve the conductivity of the battery-type electrode materials, binder-free NiCoFe layered double hydroxide (LDH) nanosheets with different composition ratios are synthesized via a simple electrodeposition method. Among these samples, the NiCoFe LDH with a composition ratio of 1:2:0.1 shows the best performance with a specific capacity of 727 C/g at 1 A/g and a high rate retention of 77 % at 50 A/g. It also exhibits a high cycling stability of 87 % capacity retention after 10000-cycle operation. The improvement of conductivity is mainly attributed by the addition of Fe ions. The valence changes of each transition metal ions in the LDHs are investigated by the quick in operando X-ray absorption spectra during the oxidation/reduction process. Although the electrochemical activity was contributed by the valence change of Ni, the Fe ions keep at the +3 valence state which helped stabilizing the LDH structure. To demonstrate practical hybrid energy storage applications, a flexible solid-state hybrid energy storage device was fabricated with NiCoFe (1:2:0.1) as the positive electrode, commercial activated carbon as the negative electrode, and PVA-KOH as the solid-state electrolyte. The energy densities of this solid-state device are 8.7 Wh/kg @62.8 W/kg and 6.8 Wh/kg @3139.2 W/kg. The cycling stability was also tested for the device under folded and unfolded conditions. The capacity retention after 2000 cycles were 97 % and 90 % for unfolding and folding conditions, respectively. It indicates that the NiCoFe LDHs can offer good electrochemical performance with high-rate capability and well flexibility.