All vanadium-based Li-ion hybrid supercapacitor with enhanced electrochemical performance via prelithiation

Highlights

• All vanadium-based Li-ion hybrid supercapacitor is presented.

• Prelithiation is conducted to enhance overall electrochemical performance.

• The fabricated device exhibited stable performance up to 3.2 V.

• Single hybrid supercapacitor can operate LEDs and a strain sensor for body movement.

Abstract

We report on the fabrication of an all-vanadium based Li-ion hybrid supercapacitor whose performance is highly enhanced compared to either batteries or supercapacitors via prelithiation process. Orthorhombic vanadium oxide nanoparticles and amorphous vanadium phosphate nanosheets implemented with carbon nanotubes are applied to battery-type and capacitive electrode, respectively. By utilizing vanadium-based electrodes with different charge-storage mechanisms in one device, advantages of both supercapacitors and batteries can be achieved. After a simple short-circuit prelithiation, the fabricated hybrid supercapacitor exhibits a high electrochemical performance with a capacitance of 111.6 F g⁻¹ at 10 mA g⁻¹, operation voltage window of 3.2 V, energy density of 160.2 Wh kg⁻¹ and power density of 4.484 kW kg⁻¹. Furthermore, it retains 98.3% of its initial capacitance after 2000 charge-discharge cycles. Due to its high operation voltage, a single, fully-charged hybrid supercapacitor successfully lights up variously colored LEDs for longer than one hr. With energy stored in the hybrid supercapacitor, a fragmentized graphene foam based strain sensor is powered to monitor various body-movements. This study demonstrates the high potential of the vanadium-based Li-ion hybrid supercapacitor as a powerful energy storage device obtained via deliberate selection of materials and prelithiation process.

Introduction

With recent developments in electric cars and consumer electronics, demands for advanced energy storage devices are rapidly increasing. Among various energy storage devices, Li-ion Batteries (LIBs) and supercapacitors have been extensively investigated as promising next-generation devices. LIBs exhibit high energy densities and high operating voltage windows but rather low power densities and poor cyclic stability, whereas supercapacitors feature higher power densities and cyclic stability but lower energy densities and operating voltages compared to metal ion batteries. Therefore, a hybrid energy storage device combining the advantages of both LIBs and supercapacitors would be highly desirable. Hybrid supercapacitors (HSC), otherwise known as battery-supercapacitor hybrids, are expected to satisfy such expectations [1]. HSCs are typically comprised of one capacitive electrode and one battery-type electrode, or two electrodes combining both capacitive and battery-type elements [2], [3]. HSCs can be classified into accordion, rocking chair, and hybrid types according to the charge carrier migration mechanism [4]. With two different charge-storage mechanisms based on ion adsorption/desorption and faradaic reactions including diffusion and/or intercalation, HSCs can exhibit higher energy density than supercapacitors, and higher power density and longer cycle life compared to batteries. Though HSCs can offer many advantages, there still exist some short-comings [5]. The cyclic life of HSCs is longer than that of LIBs but much shorter than that of supercapacitors due to the use of battery-type electrodes. The charge-discharge rate is different for each electrode; high at the capacitive electrode but slower at the battery-type electrode [6]. Thus, the energy and power density values of HSCs are in-between those of LIBs and supercapacitors. In addition, battery-type electrodes may lead to a higher chance of forming solid electrolyte interface (SEI), an issue which should be addressed. As a solution, prelithiation, a pre-doping process with Li ions to ensure higher Li ion concentrations in energy storage systems, can alleviate some of those drawbacks. It can compensate for the irreversible capacity loss caused by Li consumption by SEI formation and can raise the overall operation voltage [7], [8]. Up-to now, many reports have suggested that prelithiation significantly enhance the initial coulombic efficiency and operating voltage [9], [10], [11].

Various materials can be utilized as the capacitive or battery-type electrodes in a hybrid energy storage device. Carbon-based materials such as activated carbon, carbon nanotubes (CNTs), or graphene oxide (GO) are often used as the electrodes of electric double layer capacitors (EDLCs) due to their high electrical conductivity, high surface area and mechanical and chemical stability [12]. Conductive polymers and metal oxides are frequently used as the electrodes of pseudo-capacitors owing to their fast surface or near-surface faradaic reaction [13], [14]. Pseudo-capacitive materials exhibit different behavior from bulk battery-type redox reactions in terms of their reaction site and fast kinetics. Furthermore, some of them exhibit different charge storage mechanisms according to nanoparticle size or their crystallinity [15]. For battery electrodes, various metal oxides go through diffusion-controlled charge storage mechanism through intercalation, alloying, or conversion [16], [17]. Among them, vanadium oxides have numerous oxidation states (II-V) and crystalline structures, features allowing their versatile application to energy storage devices, including both LIBs and supercapacitors as both cathode and anode materials [18]. Nanostructured vanadium oxides show extrinsic intercalation pseudo-capacitance with fast intercalation through tunnels and layers with low ion insertion energy, whereas in bulk orthorhombic phase vanadium oxides exhibit typical battery behavior [19]. Many of previous studies have implemented vanadium oxides as battery and supercapacitor electrodes. Zheng et al. synthesized rhombohedral, butterfly-like, and flower-like V₂O₅ as battery-type electrode for hybrid supercapacitors [20], whereas Wang et al. utilized double-wall shelled V₂O₅ hollow microspheres as Li-ion battery electrode [21]. Hu et al. combined polyvinyl alcohol (PVA) with GO with V₂O₅·nH₂O to form electrodes for Li⁺ and Zn²⁺ ion storage [22]. Vanadium phosphates, which generally show faster ion diffusion than vanadium oxides, share advantages similar to those of vanadium oxides, including their capacity to store multiple Li ions, their numerous crystalline forms and relatively high output voltage, and they are also widely utilized in both batteries and supercapacitors [23]. Due to poor conductivity and cyclic stability of vanadium species, carbon-based materials are often composited or applied together to enhance electrostatic and electrochemical performance [24]. Thus the vanadium species, with some reinforcement, can provide a low cost, versatile ways to fabricate energy storage system.

In this work, we suggest an all vanadium-based Li-ion hybrid supercapacitor featuring enhanced performance through prelithiation. The fabricated device consists of a battery-type negative electrode and a prelithiated, pseudo-capacitive positive electrode. Vanadium oxide (V₂O₅), which exhibits a diffusion-controlled charge storage mechanism, was applied as the battery-type electrode, whereas pseudo-capacitive vanadium phosphate (VOPO₄) wrapped on top of CNTs was applied as the capacitive electrode. Although V₂O₅ has often been studied as a cathode material in batteries due to its various oxidation state, numerous crystalline structure, and high operating voltage, its reversible electrochemical activity also allows the application as an anode material [25], [26]. Since VOPO₄ suffers from low electric conductivity and volume expansion during repeated Li-ion intercalation/deintercalation process [27], CNTs are combined to enhance the conductivity and provide structural support for ensuring the uniform distribution of VOPO₄ nanoparticles and suppression of volume expansion, leading to enhanced cycle life [28], [29]. By utilizing vanadium-based electrodes with different charge-storage mechanisms in one device, advantages of both supercapacitors and batteries can be achieved. We also expect that the fabrication of an all vanadium-based hybrid supercapacitor would allow cost reduction of energy storage device with high performance owing to the natural abundance and inexpensive price of vanadium. The coulombic efficiency and cyclic stability of the hybrid supercapacitor were highly enhanced after the prelithiation of capacitive electrode through a facile and reproducible external short-circuit method. It is attributed to the effective expansion of interlayer spacing of VOPO₄ by prelithiation, enhancing the ion migration during charge-discharge process to improve the rate performance [30]. As a result, the fabricated hybrid supercapacitor exhibited a high electrochemical performance, featuring a high operation voltage of 3.2 V; gravimetric capacitance of 111.6 F g⁻¹ at 10 mA g⁻¹; energy density of 160.2 Wh kg⁻¹; and power density of 4.484 kW kg⁻¹. After 2000 charge-discharge cycles, it retained 98.3% of its initial capacitance while maintaining a high coulombic efficiency of 98%. With a single hybrid supercapacitor, LEDs with high operating voltages were successfully lit and a strain sensor was powered to monitor bending motions of human finger and wrist. This work demonstrates a low-cost, facile fabrication route to an alternative energy storage device to conventional batteries and supercapacitors, which can be widely applied to various wearable devices.