Both nickel-zinc and nickel-metal hydride batteries are relatively environmentally friendly secondary batteries (compared to nickel-cadmium batteries, which contain cadmium). However, there are differences in their environmental performance, primarily in terms of raw material composition, potential environmental risks, recycling difficulties, and resource sustainability. The following is a detailed analysis.
Environmental Differences in Raw Material Composition
Nickel-zinc Batteries
The core materials of nickel-zinc batteries are zinc, nickel, and an alkaline electrolyte (such as potassium hydroxide).
Zinc is an abundant metal in the Earth’s crust (approximately 0.007%). It is chemically stable and non-toxic, and even if leaked, it will not cause heavy metal contamination in soil or water sources.
Although nickel is a heavy metal, the amount used in nickel-zinc batteries is far lower than in nickel-metal hydride batteries (where zinc is the primary active material). Furthermore, nickel poses less environmental risk at low concentrations and can be removed through standard wastewater treatment.
The electrolyte is a strong alkaline solution, and while corrosive, it does not contain heavy metals or toxic organic matter. Leakage can be mitigated through neutralization.
Nickel-metal hydride batteries
The core materials are hydrogen storage alloys (containing rare earth elements, nickel, cobalt, etc.), nickel, and alkaline electrolyte.
Hydrogen storage alloys are a key variable in environmental protection: their composition typically includes rare earth elements (such as lanthanum and cerium), nickel, cobalt, and aluminum. While rare earth elements are non-toxic, they are scarce strategic resources, and over-mining can damage the ecology of mining areas (such as soil desertification and vegetation loss).
Nickel usage is high: nickel can account for 30%-50% of the content in nickel-metal hydride batteries (much higher than in nickel-zinc batteries). If large quantities of discarded batteries are not recycled, nickel can accumulate in organisms after entering the environment, and long-term exposure may affect the survival of aquatic life.
Some low-cost nickel-metal hydride batteries may contain a small amount of cobalt (to enhance alloy stability). Cobalt mining (such as artisanal mining in the Democratic Republic of the Congo) is often associated with child labor and environmental pollution (wastewater contains cobalt ions).
Potential Environmental Risks During Use
Nickel-zinc Batteries: Risks are Controllable
Although nickel-zinc batteries use an alkaline electrolyte, their superior sealing technology prevents leakage. Furthermore, nickel-zinc batteries pose no risk of heavy metal pollution and pose minimal long-term environmental risks.
Zinc itself is chemically stable, and even if the battery casing is damaged, the zinc electrode material is unlikely to form toxic compounds when exposed to the environment.
NiMH Batteries: Excellent sealing performance, but limited material degradation
NiMH batteries utilize a rigorous sealing design (e.g., metal casing + sealing ring), resulting in an extremely low risk of leakage and virtually no release of harmful substances during use.
However, the rare earth elements and nickel in the hydrogen storage alloy are inert metals and difficult to degrade in the natural environment. If left in the soil for a long time, they may slowly release metal ions, which, if accumulated, could affect soil microbial activity.
Differences in Recycling Systems and Resource Circularity
Recycling Difficulty and Cost
Zinc-zinc batteries: The recycling process is simple. Zinc and nickel can be directly separated and purified through pyrometallurgy or hydrometallurgy (zinc has a low melting point and is easily separated from nickel). Recycling costs are low, and market demand for zinc is stable (it can be used for galvanizing steel, alloy manufacturing, etc.), making recycling highly economical. Nickel-metal hydride batteries: Recycling is more challenging. The hydrogen storage alloy has a complex composition (including multiple metals), requiring complex separation processes (such as high-temperature smelting and chemical extraction) to separate rare earth elements, nickel, and cobalt. Consequently, recycling costs are higher than for nickel-zinc batteries.
Recycling System Maturity
Due to the widespread use of nickel-metal hydride batteries (such as in homes and consumer electronics), the recycling system is more mature. For example, automakers such as Toyota and Honda have established closed-loop recycling networks, recycling the nickel and rare earth elements from retired nickel-metal hydride batteries into new battery production, achieving a resource recycling rate of over 90%.
Nickel-zinc batteries currently have limited application scenarios (such as homes, emergency power supplies, and some energy storage devices) and are still in the development stage. The recycling industry chain has yet to achieve scale, and recycling channels are limited.
Differences in Resource Sustainability
Zinc reserves and mining sustainability are superior to rare earths: Global zinc reserves are approximately 250 million tons, with annual mining output of approximately 14 million tons. Furthermore, zinc recycling technology is mature (recycled zinc accounts for over 30% of global zinc production), resulting in a stable resource supply. While the rare earth elements (such as lanthanum and cerium) that nickel-metal hydride batteries rely on are not scarce, their distribution is highly concentrated (China holds 36% of global reserves). Furthermore, the mining process is energy-intensive and highly polluting (for example, rare earth ore extraction requires large amounts of strong acid, which can easily cause water pollution). In the long term, this resource is less sustainable than zinc.
Overall, nickel-zinc batteries offer advantages in terms of environmental friendliness and resource sustainability, while nickel-metal hydride batteries, thanks to their mature recycling system, pose lower environmental risks during actual disposal. Both are significantly superior to cadmium-based batteries, but the higher voltage characteristics of nickel-zinc batteries better accommodate the high energy demands of devices.
