Lithium-ion batteries regularly make the news because of fires started by overheated or damaged cells. Yet, their high energy densities and long runtimes make them the go-to choice for many modern electronics.

A new NUS study published in Advanced Materials is powering up a rising competitor, the zinc metal battery (ZMB). Led by NUS Chemistry Professor Loh Kian Ping, the researchers unlocked a new high-water mark in ZMB energy density while preserving stable electrochemical performance, making ZMBs more attractive as a lithium-ion alternative.

Why zinc?

As a material, zinc has several advantages: it is highly abundant, inexpensive and non-toxic. It is also less reactive than other metals like lithium and compatible with aqueous electrolytes, allowing the development of environmentally benign, intrinsically safer designs.

Zinc-based batteries come in many different forms. Zinc combined with manganese dioxide, for example, make up the ubiquitous alkaline battery – best used for low-drain household devices like remote controls and toys. Unlike the alkaline battery, however, ZMBs are rechargeable; these systems include variations like zinc-vanadium, zinc-manganese, and zinc-iodine batteries.

Why are ZMBs underperforming?

ZMBs have been commercially limited by a critical, underexplored flaw: low energy density. They simply do not store enough power for their weight. So far, a common strategy to advance ZMBs has been to increase the size of the device or multiply the stacks of internal layers.

The paper noted that this strategy increases the battery’s overall capacity but adds bulk and complicates fabrication. Conventional ZMB designs are burdened by low-capacity cathodes and a disproportionately high fraction of inactive materials; in particular, thick separators and excessive electrolyte.

Thus, Loh and his colleagues investigated ZMBs under electrolyte-deficient conditions (also known as lean-electrolyte conditions) to reduce cell weight. The prevailing opinion in the field has been that battery failure in ZMBs under these conditions is caused by the growth of dendrites – uneven zinc deposits that accumulate into branch-like crystal structures that short-circuit the battery. But experiments revealed that water depletion is in fact a more critical factor.

This water depletion is driven by hydrogen evolution reaction (HER), a parasitic side-reaction that breaks down water molecules during the charging and discharging of the battery. In ZMBs, water molecules cluster around zinc ions as they migrate towards the electrode, forming a bulky shell of water. At the electrode surface, these water molecules are broken down into hydrogen and other byproducts, along with nearby free-floating water molecules, leading to water loss.

A playbook for improving ZMB energy density

With this understanding, the researchers shifted away from anode stabilisation, where research efforts have traditionally been focused, to holistic device engineering. First, they developed a thin, structurally stable separator capable of suppressing HER-driven water loss and minimising electrolyte consumption under lean conditions.

The team synthesised a separator with rich hydrophilic groups, anchoring free-floating water molecules to the separator’s nanostructure and disrupting the formation of water shells around zinc ions. This helped to suppress electrolyte loss, while accelerating ion transport. The separator, constructed from covalent organic frameworks grown on the surface of polyacrylonitrile nanofibres (COF@PAN), is over eight times thinner than a conventional glass fibre one.

Next, the researchers evaluated the separator across different battery designs and cathode types under lean-electrolyte conditions. By pairing the separator with representative zinc-battery cathode systems, the researchers demonstrated that their approach to engineering a ZMB “can be applied across different [Zinc] battery configurations”, said Loh.

The study’s best performing ZMB configuration was a pouch cell – a lightweight, flexible battery type commonly used in tablets and smartphones – integrated with a high-capacity zinc-iodine cathode. It achieved the highest energy density and cycle stability for practical ZMBs to date: gravimetric energy density (energy compared to weight) of up to 54.0 Wh kg⁻¹ and volumetric energy density (energy compared to volume) of up to 185.3 Wh L⁻¹ for over 800 stable cycles.

So how do ZMBs compare to the competition?

While this study gives a much-needed energy density boost to ZMBs, the metrics still fall short of lithium-ion standards. But Loh noted that because of ZMBs’ advantages, such as low cost and intrinsic safety, the batteries could be competitive in “specific market niches in applications where safety and affordability are prioritised over maximum energy density, such as e-bikes and some small portable electronics”.

In the same vein, ZMBs also have the potential to replace other traditional rechargeable systems, such as lead-acid and nickel-metal hydride batteries. The design principles uncovered by this study have provided a blueprint for practical, long-life, energy dense ZMBs, making them formidable rivals for today's commercial batteries.