Zinc-ion hybrid supercapacitor-batteries with a leaf-like ZIF-L/MgNiO2 micro-sphere composite and a Zn2+/sulfonated poly(ether ether ketone) gel

Abstract

A Zn-ion hybrid supercapacitor-battery (ZHSB) type storage device combines the advantages of the high power density of a supercapacitor and the high energy density of a battery. This work reports the fabrication and study of a unique ZHSB architecture with a composite of a Zn-metal–organic framework (MOF: ZIF-L) and MgNiO₂ as the cathode, a composite of Zn microparticles and activated carbon derived from orange peels (ZnμPs-AC) as the anode and a dual cationic Zn²⁺/sulfonated poly(ether ether ketone) (SPEEK) gel as the electrolyte. The 2D-layered leaf-like structure of the ZIF-L MOF affords a large number of zero-dimensional pores and multiple Zn²⁺ metal nodes as well as many nitrogens (from imidazolium moieties) enabling maximum ion-adsorption and faradaic reactions. Simultaneously, MgNiO₂ exists in the form of porous micro-spheres, again allowing a good amount of ion-insertion/extraction during charge–discharge. The ZnμPs-AC anode is characterized by a high DC conductivity and permits facile plating/stripping of Zn supporting long term cyclability. The synergy between the two materials comes to the fore in the ZIF-L/MgNiO₂//Zn²⁺/SPEEK//ZnμPs-AC based ZHSB, which achieves a specific capacity of ∼197 mA h g⁻¹ at 1 A g⁻¹, an energy density of ∼157 W h kg⁻¹ at a power density of 0.8 kW kg⁻¹, a slow self-discharge rate and an ultra-long cycling lifetime of 10 000 cycles, with a capacity retention of ∼98%. The Zn²⁺/H⁺ conducting Zn²⁺/SPEEK gel also contributes to this exemplary performance via its superior ionic conductivity of 0.065 S cm⁻¹ and enhanced redox activity compared to the SPEEK gel. Mechanistically, while the diffusion process for charge storage is dominant at low scan rates, the microstructure of the cathode and electrolyte conductivity enable fast intercalation kinetics, thus leading to the observed deliverance of high specific power and energy. The ZHSB is thus an efficient, cost effective, highly scalable and easily implementable next generation energy storage system.