NH4+-Modulated Cathodic Interfacial Spatial Charge Redistribution for High-Performance Dual-Ion Capacitors

Abstract

Compared with Zn2+, the current mainly reported charge carrier for zinc hybrid capacitors, small-hydrated-sized and light-weight NH4+ is expected as a better one to mediate cathodic interfacial electrochemical behaviors, yet has not been unraveled. Here we propose an NH4+-modulated cationic solvation strategy to optimize cathodic spatial charge distribution and achieve dynamic Zn2+/NH4+ co-storage for boosting Zinc hybrid capacitors. Owing to the hierarchical cationic solvated structure in hybrid Zn(CF3SO3)2–NH4CF3SO3 electrolyte, high-reactive Zn2+ and small-hydrate-sized NH4(H2O)4+ induce cathodic interfacial Helmholtz plane reconfiguration, thus effectively enhancing the spatial charge density to activate 20% capacity enhancement. Furthermore, cathodic interfacial adsorbed hydrated NH4+ ions afford high-kinetics and ultrastable C‧‧‧H (NH4+) charge storage process due to a much lower desolvation energy barrier compared with heavy and rigid Zn(H2O)6 2+ (5.81 vs. 14.90 eV). Consequently, physical uptake and multielectron redox of Zn2+/NH4+ in carbon cathode enable the zinc capacitor to deliver high capacity (240 mAh g−1 at 0.5 A g−1), large-current tolerance (130 mAh g−1 at 50 A g−1) and ultralong lifespan (400,000 cycles). This study gives new insights into the design of cathode–electrolyte interfaces toward advanced zinc-based energy storage.

1. Introduction

Aqueous zinc-ion hybrid capacitors (ZHCs) have recently emerged as highly competitive power-storage candidates due to their inherited dual superiorities from battery-type Zn anodes and supercapacitor-type carbon-based cathodes [1–5]. The reversible deposition/stripping behavior of Zn anode delivers high theoretical gravimetric capacity (820 mAh g−1) and suitable redox potential (−0.76 V vs. the standard hydrogen electrode), providing sufficient charges for electrochemical energy storage [6–11]. Therefore, significant efforts have been made to develop high-performance cathode materials for propelling ZHCs, mainly focusing on customizing carbon nanostructures [12–17].

However, the developed carbon cathodes still suffer from an energy storage plafond at the electrode–electrolyte interfaces due to intrinsic inadequate zincophilic activity and unsustainable adsorption behavior, which hinder the performance improvement of ZHCs [18–20]. To cope with these dilemmas, the key breakthrough lies in designing highly electroactive and stable cathode–electrolyte interfaces to achieve more efficient charge storage.

Capacitive energy storage of carbon cathodes mainly relies on the electric double-layer (EDL) mechanism, which is largely determined by the specific surface area (SSA) and the distribution of charge carriers in electrolytes [21–24]. A large SSA can effectively expand electrode/electrolyte contact and carbon cathode capacity [25–29]. Nevertheless, the SSA of carbon cathodes is difficult to increase indefinitely, accompanied by negative effects such as inaccessible pores and poor pore-ion compatibility, leading to limited capacity enhancement [30, 31]. Charge carriers play an essential role in regulating the dynamic charge transfer and spatial storage at cathode interfaces, where high-density interfacial charge is beneficial for increasing carbon cathode capacity [12, 32, 33]. In this regard, the size and solvation structure of charge carriers considerably affect interfacial electrochemical behavior and EDL energy storage [34]. Thus, the spatial distribution of charge carriers at the cathode–electrolyte interface is particularly critical for efficient charge storage but has never been unraveled.

Metallic Zn2+ ions usually behave as highly active charge carriers for ZHCs, but their large hydrated structure and high desolvation energy led to a sluggish interfacial charge storage process, especially at high currents [35, 36]. In contrast, non...