Achieving battery-level energy density in carbon/metal sulfide asymmetric supercapacitors using organic radicals

Abstract

Asymmetric supercapacitors (ASCs) are promising candidates for high-power output applications; however, their theoretical capacity remains largely unrealized owing to the low specific capacity of carbon negative electrodes. Traditional strategies for enhancing the specific capacity of carbon via structural optimization often compromise the tap density, electrical conductivity, and rate performance of the material. In this study, we address this bottleneck by incorporating 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxyl (4OT) as a redox mediator into the electrolyte to construct ASCs with well-matched capacities and potential windows between the two electrodes. With 50, 100, and 200 mM 4OT added in electrolytes, the activated carbon electrodes achieve specific capacities of 113, 181, and 263 mAh·g−1 at 2 A·g−1. The Ni3S2/CoNi2S4 positive electrode exhibited a specific capacity of 415 mAh·g−1, benefiting from its superior electrical conductivity, abundant active sites, and enhanced electrochemical activity. Notably, introducing 4OT to the electrolyte effectively balances the capacity and potential window of the two electrodes. Consequently, the as-assembled ASCs deliver a maximum energy density of 55 Wh·kg−1, which surpasses previously reported values. Our work demonstrates that the rational selection and application of redox mediators have great potential for balancing electrode capacity and boosting the energy density of high-performance ASCs.

1. Introduction

Given the negative environmental impact of nonrenewable fossil fuels for electricity production, the development of sustainable renewable energy sources is increasingly urgent [1]. Although clean energy sources address environmental issues associated with fossil fuels, they are inherently intermittent, complicating their direct connection to the public electricity grid [2–3]. To address this issue, various electrochemical energy storage systems have been developed to store energy for later distribution, thereby ensuring a more reliable supply [3].

Among these systems, lithium-ion batteries deliver high energy density but suffer from limited power density, which restricts their use in high-power output applications [4–8]. In contrast, asymmetric supercapacitors (ASCs), which pair battery-type electrodes with carbon materials, exhibit high power density and promising theoretical energy density [9–11]. However, achieving the theoretical energy density remains a practical challenge, primarily due to the mismatched capacity and potential windows between the two electrodes [12–13]. The capacity of ASCs is primarily limited by the carbon electrode, which exhibits a lower capacity, whereas the voltage is dictated by the potential windows of the two electrodes [14–15]. During ASC assembly, excess carbon is routinely employed to bridge the capacity gap between the two electrodes [16–18]. For example, in a typical Ni(OH)2/activated carbon ASC, the mass ratio of activated carbon (AC) to Ni(OH)2 can reach 8:1. However, this strategy is inefficient because the specific capacity of the carbon electrode does not scale proportionately with carbon loading [16]. Therefore, developing high-capacity capacitive electrodes and optimizing electrolytes to achieve well-matched capacities and extended stable voltage windows remain key challenges in the development of high energy-density ASCs [10,19].

Carbon electrodes store energy primarily through physical ion adsorption and desorption at the electrode–electrolyte interface [20–21]. A widely adopted strategy for enhancing the capacity of carbon materials involves optimizing their pore structure [22] to increase the effective surface area available for these processes [23–24]. However, pore enlargement often reduces the tap density of carbon materials, thereby compromising the volumetric energy density of ASCs [25–27]. Another strategy involves introducing surface functional groups to the electrode through heteroatom doping to enhance pseudocapacitance [28–29]. However, this approach can diminish the electrical conductivity and rate performance of the carbon materials. Furthermore, these synthesis processes typically require high temperatures, involve corrosive reagents, and generate significant amounts of wastewater and flue gas. Therefore, a mild, efficient, and non-destructive strategy to increase the specific capacitance of the carbon electrode without sacrificing conductivity or rate performance is highly desirable. Unlike conventional methods that focus on electrode materials, introducing redox mediators into the ASC electrolyte presents a facile and effective alternative for enhancing the capacitive performance of carbon electrodes [30–31].