Zinc manganese batteries, a typical example of disposable batteries, present a critical challenge in battery materials science: the conflict between the chemical principles of adding cadmium and lead to the zinc anode and environmental requirements. The zinc anode, acting as the electron donor, releases electrons through oxidation during discharge, and its activity directly determines battery capacity and lifespan. However, pure zinc is prone to self-corrosion in electrolytes, especially in chloride-containing environments. Zinc preferentially combines with chloride ions to form soluble complexes, leading to unnecessary consumption of the anode material and the generation of hydrogen gas, causing battery swelling. The addition of cadmium and lead aims to suppress this process by altering the electrochemical behavior of zinc.
Cadmium's chemical effect primarily manifests in increasing the hydrogen overpotential. The hydrogen overpotential is a physical quantity that measures the ease with which hydrogen gas can be evolved from a metal surface. The addition of cadmium significantly increases the hydrogen overpotential on the zinc anode surface, meaning that at the same potential, the hydrogen evolution reaction requires a higher activation energy. From a thermodynamic perspective, this is equivalent to increasing the energy barrier for hydrogen generation, thereby slowing down the rate of hydrogen production during zinc self-corrosion. Furthermore, cadmium can form a solid solution with zinc. Its atomic radius is similar to zinc, allowing it to be uniformly distributed within the zinc lattice. Through lattice distortion, it hinders zinc ion diffusion, further reducing the self-corrosion current density. This dual effect significantly reduces capacity loss in cadmium-containing zinc anodes during storage, extending battery shelf life.
The chemical effect of lead focuses on improving the mechanical properties and corrosion uniformity of the zinc anode. Lead has a low melting point and can form a liquid lead phase during zinc alloy smelting, filling the zinc grain boundaries and acting as a lubricant and plasticizer. This makes the zinc anode less prone to cracking during stamping, improving the yield rate of electrode manufacturing. Simultaneously, lead preferentially oxidizes in the electrolyte to form a lead oxide protective film. This film, though thin, is dense and selectively blocks chloride ion corrosion while allowing zinc ions to pass through, thus transforming the self-corrosion reaction from general corrosion to localized uniform corrosion. This shift in corrosion mode avoids battery failure caused by localized perforation, improving the utilization rate of the zinc anode.
However, the environmental issues of cadmium and lead cannot be ignored. Cadmium is a highly toxic heavy metal. Its compounds are highly soluble and bioaccumulative, and can enter the human body through the food chain, causing irreversible damage to the kidneys and skeletal system. Lead is also neurotoxic, especially affecting children's intellectual development. In traditional zinc-manganese battery production, the use of cadmium and lead additives makes discarded batteries a significant source of pollution. If not properly recycled, cadmium and lead will seep into the soil and groundwater through battery corrosion, causing long-term environmental harm. For example, when cadmium-zinc anodes corrode in acidic electrolytes, cadmium enters the solution in ionic form. If batteries are discarded carelessly, these cadmium ions can leach into water bodies through rainwater, threatening aquatic ecosystems.
To balance chemical performance and environmental protection requirements, modern zinc-manganese battery research focuses on cadmium and lead replacement technologies. On one hand, cadmium- and lead-free zinc anodes are developed through microalloying techniques, such as adding elements like indium and bismuth. These elements can also increase hydrogen overpotential, but their toxicity is significantly lower than that of cadmium and lead. For example, indium has high solid solubility in zinc, which can form a stable surface passivation layer, effectively inhibiting hydrogen evolution. On the other hand, optimizing the electrolyte formulation by using a high-concentration zinc chloride system instead of the traditional ammonium chloride electrolyte reduces the self-corrosion rate of zinc by increasing chloride ion activity, while simultaneously reducing the amount of lead added. Furthermore, surface modification technologies, such as coating the zinc anode with a graphite layer, can create a physical barrier to further prevent direct contact between the electrolyte and zinc.
From a life-cycle perspective, the environmental balance of zinc manganese batteries needs to be integrated into all stages: design, production, use, and recycling. In the production stage, a closed-loop smelting process is used to reduce cadmium and lead volatilization, coupled with a high-efficiency dust removal system to capture heavy metal particles in the exhaust gas. In the use stage, optimized battery structure design, such as the use of leak-proof sealing technology, reduces the risk of electrolyte leakage. In the recycling stage, a comprehensive waste battery recycling system is established, using hydrometallurgical processes to separate and extract metals such as zinc, manganese, cadmium, and lead, achieving resource recycling. For example, through acid leaching-extraction processes, zinc and manganese dioxide in zinc manganese batteries can be efficiently recovered, while cadmium and lead are concentrated into hazardous waste for safe disposal.
In the future, with advancements in materials science and environmental technologies, zinc-manganese batteries are expected to completely eliminate cadmium and lead dependence while maintaining their performance advantages. The application of nanomaterials technology provides new insights into developing high-performance cadmium- and lead-free zinc anodes. For example, zinc nanowire anodes increase reactivity by expanding specific surface area, while their unique crystal structure inhibits dendrite growth and extends battery life. Furthermore, the development of solid-state electrolytes can fundamentally eliminate the risk of electrolyte leakage, providing technological support for the green transformation of zinc-manganese batteries. Through multidisciplinary innovation, zinc-manganese batteries will achieve sustainable development by balancing chemical performance and environmental performance.
