Rechargeable batteries are crucial for various applications, including electric devices and grid-scale energy storage. Commercial batteries typically rely on electrical grids for charging, limiting their use in harsh environments or remote areas. To address this, energy harvesting technologies (photovoltaic devices, piezoelectric nanogenerators, triboelectric nanogenerators, and thermoelectrics) have been integrated with batteries to create self-charging power systems. However, these systems remain highly dependent on the consistent availability of energy resources and often involve complex configurations with extra components. The need for self-charging systems with simplified designs and adaptability to diverse environments is therefore paramount. Chemical energy stored in molecules presents an alternative energy source, convertible into electrical energy through redox reactions. Oxygen, an abundant resource in air, is particularly attractive for energy conversion and storage devices, as seen in metal-air batteries. These batteries can even charge other energy storage devices. However, these integrated systems often require external power supplies for recharging once both components are exhausted. Thus, a system capable of direct charging through successive chemical energy conversion of oxygen on the cathode is needed. Vanadium-based compounds, with their open-framework crystal structure and vanadium's multiple oxidation states, have shown promise as cathode materials in aqueous zinc-ion batteries (ZIBs). During discharge, Zn²⁺ ion insertion and vanadium reduction occur simultaneously. Importantly, vanadium-based compounds can also be oxidized by oxygen in their low valence state. This characteristic suggests the possibility of self-charging ZIBs where discharged vanadium-based cathodes are oxidized by ambient oxygen, effectively functioning as a charging process. This research aims to leverage this principle to develop a chemically self-charging aqueous ZIB system.

Several studies have explored the integration of energy harvesting technologies with batteries to create self-charging power systems. These systems utilize various energy harvesting mechanisms, including photovoltaic, piezoelectric, triboelectric, and thermoelectric effects, to replenish the battery's charge. However, a common limitation of these systems is their dependence on the continuous availability of the respective energy source, making them unsuitable for applications where reliable energy sources are not guaranteed. The complexity of these integrated systems also presents a challenge. Many systems require additional components, increasing cost and complexity, whereas metal-air batteries, while offering a path to chemical energy conversion, often require external power supplies to fully recharge, falling short of a completely self-sufficient system. The use of vanadium-based compounds as cathode materials in aqueous ZIBs has been explored, demonstrating promising characteristics such as high capacity and fast kinetics due to their open framework and multivalent redox behavior, however, their self-charging capability through oxygen interaction has not been extensively investigated.

CaVO nanoribbons were synthesized via a one-step hydrothermal method. Their morphology and crystal structure were characterized using SEM, TEM, XRD, and TGA. Electrochemical performance was evaluated using coin cells with CaVO nanoribbons as the cathode, zinc foil as the anode, and 4 M Zn(CF₃SO₃)₂ aqueous solution as the electrolyte. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and rate capability tests were performed to assess the electrochemical properties of the Zn/CaVO batteries. The energy storage mechanism was investigated using in situ/ex situ XRD, ex situ TEM, XPS, and XANES. To understand the self-charging mechanism, fully discharged cathodes (CaZn₃.₆VO) were exposed to 4 M Zn(CF₃SO₃)₂ solution containing dissolved oxygen for varying durations, followed by electrochemical characterization. The redox reaction between CaZn₃.₆VO and O₂ was examined in both aqueous and non-aqueous environments to assess the role of water. Finally, "open" coin-type ZIBs were designed to allow for in situ chemical charging via oxygen diffusion. Repeated chemical charging/galvanostatic discharging cycles, and hybrid chemical/galvanostatic charging modes were tested. A timer was used as a visual demonstration of the self-charging functionality of the open cells.

The synthesized CaVO nanoribbons exhibited a layered structure with favorable morphological features for fast Zn²⁺ ion insertion/extraction kinetics. The Zn/CaVO batteries demonstrated a high initial discharge capacity (300 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capability (62 mAh g⁻¹ at 30 A g⁻¹), and outstanding cycling stability (100% capacity retention after 10,000 cycles). The energy storage mechanism involved a two-step redox reaction associated with Zn²⁺ ion insertion/extraction and vanadium valence changes. Importantly, the discharged cathode (CaZn₃.₆VO) could be spontaneously oxidized by ambient oxygen, achieving chemical self-charging. The self-charging process involved the oxidation of vanadium and extraction of Zn²⁺ ions, leading to an open-circuit voltage of ~1.05 V and a discharge capacity of ~239 mAh g⁻¹. The open-cell design allowed for continuous in situ chemical charging, demonstrating reversible chemical charging/galvanostatic discharging cycles and compatibility with hybrid chemical/galvanostatic charging modes. A simple timer successfully demonstrated the practical application of the chemically self-charging batteries.

This research successfully addresses the limitations of existing self-charging power systems by developing a chemically self-charging aqueous ZIB with a simplified design. The high electrochemical performance of the Zn/CaVO batteries is attributed to the favorable nanoribbon morphology, open framework, and expanded interlayer spacing of the CaVO cathode, which enables rapid Zn²⁺ ion transport kinetics. The novel self-charging mechanism, based on the redox reaction between the discharged cathode and ambient oxygen, eliminates the need for external power sources for recharging, making the system particularly suitable for applications in remote or harsh environments. The ability of the system to operate in both chemical and galvanostatic charging modes provides flexibility and enhances its practical applicability. This work represents a significant advancement in self-charging energy storage, offering a promising approach for next-generation self-powered systems. The demonstration of the self-charging battery successfully powering a timer reinforces the practical feasibility and potential of the technology.

This study demonstrates a novel chemically self-charging aqueous zinc-ion battery based on CaV₆O₁₆·3H₂O. This system exhibits high performance, self-rechargeability, and compatibility with hybrid charging modes. The demonstrated ability to power a timer illustrates its practical potential. Future research can focus on optimizing the cathode material to enhance cycling stability, exploring alternative electrolytes, and scaling up the battery for larger-scale applications. This technology offers a promising solution for self-powered systems in various environments where electrical grids are unavailable.

The long-term cycling stability of the chemically self-charging process is limited by the formation of Znₓ₊ᵧ(CF₃SO₃)₂ᵧ(OH)₂ₓ byproduct during chemical charging. Although this byproduct is partially decomposed during galvanostatic charging, some capacity degradation still occurs. Further research is needed to mitigate this limitation and enhance the long-term cycling performance. The current study focuses on a laboratory-scale demonstration. Scaling up the battery for practical applications requires further investigation into material cost-effectiveness and manufacturing processes. The performance of the "open" battery design may also be affected by environmental factors like humidity and dust.