A new electrochemical system developed in Hong Kong delivers 120,000 charge cycles without significant degradation, pointing to water-based batteries that could theoretically last more than 300 years under daily use.
Water-based batteries with 120,000 stable charge cycles
Research into more stable and less toxic water-based batteries is accelerating. A team from City University of Hong Kong and the Southern University of Science and Technology has developed a neutral-electrolyte electrochemical system and published the results in Nature Communications.
The device uses magnesium and calcium salts at pH 7.0, eliminating corrosive acids and alkalis and incorporating an innovative organic polymer-based electrode. In laboratory testing, the battery achieved a specific capacity of 112.8 mAh/g and a theoretical lifespan exceeding 300 years under daily cycling conditions. While challenges remain in volumetric energy density and industrial scalability, the findings offer a credible pathway toward more sustainable storage systems.
Why neutral pH improves water-based battery stability
Conventional water-based batteries face a structural limitation. They rely on acidic or alkaline electrolytes that trigger parasitic reactions, including hydrogen and oxygen evolution at the electrodes. Over time, these reactions degrade internal materials and may pose environmental risks during disposal.
The new system replaces aggressive electrolytes with a neutral solution composed of magnesium and calcium salts. Maintaining a pH of 7.0 prevents the corrosive reactions typical of extreme chemical environments. According to the authors, neutral conditions stabilize the electrode-electrolyte interface, reduce the formation of unwanted byproducts and improve electrochemical reversibility.
How the neutral electrolyte works
The electrolyte consists of MgCl₂ and CaCl₂ dissolved in water. These salts are non-toxic and widely available, commonly used in food processing applications such as tofu brining.
The use of divalent ions such as Mg²⁺ and Ca²⁺ enhances stability compared to monovalent-ion systems. Theoretical simulations and spectroscopic analyses reported in the study show controlled interactions between the ions and the redox-active sites of the organic material. Maintaining a neutral pH prevents proton accumulation and local fluctuations that could otherwise trigger structural degradation.
The Hex-TADD-COP organic electrode innovation
The negative cathode is not metallic but based on a covalent organic polymer known as Hex-TADD-COP. This material features electron-donating bonds that improve conductivity.
Hex-TADD-COP has a hexagonal structure built from molecules capable of reversibly exchanging electrons. In practical terms, it acts as a chemical sponge that stores and releases charge as magnesium or calcium ions move in and out. Its porous architecture facilitates ion diffusion, and electrochemical analyses indicate dominant pseudocapacitive behavior, meaning a significant portion of charge storage occurs through fast surface processes. This contributes to long-term stability.
The positive electrode uses a Prussian blue analogue, CuFe-PBA, a material already recognized for structural stability and reversibility.
How long can these water-based batteries last
Galvanostatic tests show stability up to 120,000 charge and discharge cycles, with minimal capacity decay. By comparison, many conventional aqueous batteries operate for only a few thousand cycles.
Under a daily charging regime, 120,000 cycles correspond to a theoretical lifespan of more than 300 years, a mathematical extrapolation based on laboratory results. The specific capacity reaches 112.8 mAh/g, a high value for an organic aqueous battery. In the full device configuration, specific energy ranges between 40.8 and 48.3 Wh/kg depending on operating conditions.
Environmental advantages and remaining challenges
The absence of strong acids or concentrated bases reduces environmental risk. The materials used do not contain critical heavy metals, and the system meets international environmental standards cited in the study, including GB 18599-2020, ISO 14001 and RCRA.
A neutral electrolyte can be handled with fewer precautions than corrosive solutions, an advantage in the context of circular economy strategies. It simplifies end-of-life management and reduces treatment costs. However, volumetric energy density remains lower than that of lithium-ion batteries. The researchers acknowledge that improving energy storage per unit volume will be essential for large-scale deployment.
