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Lithium-ion batteries are the most popular battery storage option today, controlling more than 90% of the global grid battery storage market, according to some estimates. However, the lithium-ion supply chain is becoming constrained. Zinc-ion batteries may offer a safer, and ultimately cheaper, energy storage option.

Lithium-ion batteries have emerged as an important technology in the fight against climate change. They are the key enabling technology for continued improvements in electric vehicles (EVs), and for renewable energy storage installations.

However, lithium-ion raw materials are not produced in sufficient quantities to meet the imminent demand from both of those markets, and a quick comparison between projections for adoption of these technologies and investments made by miners show that lithium-ion’s supply chain will soon be very constrained. This shortage will become even more severe as governments around the world pass legislation that accelerates the transition to EVs and renewables.

New battery technologies are sorely needed to address this shortage. The need for light batteries means that lithium is unlikely to be replaced for EVs; lithium’s position on the periodic table all but guarantees that it will remain the king of energy density.

Renewable energy storage, on the other hand, really only requires a low lifetime cost. Here lithium’s advantage is primarily due to its position as the incumbent. In fact, lithium-ion’s safety risks make it a poor fit for a market that seeks to place massive battery packs in people’s homes and businesses. Non-lithium batteries are far more likely to succeed in energy storage for renewables. The question then becomes, what technologies can beat lithium-ion for energy storage, while being able to scale at the rate demanded by climate change?

Many companies have tried to build new energy storage batteries over the past decades. None have really succeeded. Even for technologies that made it out of the lab, the rapidly decreasing manufacturing costs for lithium-ion eroded their competitive position before they could scale up.

It has become increasingly clear that any alternative to lithium-ion batteries needs to adopt standard manufacturing processes to allow for a rapid and low-cost scale-up. So far, the zinc-ion battery (Figure 1) is the only non-lithium technology that can adopt lithium-ion’s manufacturing process to make an attractive solution for renewable energy storage, particularly for its compatibility along with other advantages.

1. Salient Energy’s zinc-ion battery cell has various components, as shown here. The zinc-ion battery, like a lithium-ion battery, functions using intercalation. Zinc ions react at both electrodes and travel between them through a water-based electrolyte. During discharge, zinc metal at the anode is dissolved into the electrolyte as zinc ions. At the same time, zinc ions are absorbed into the cathode from the electrolyte. This process is reversed during charge. Courtesy: Salient Energy

How Lithium-ion Batteries Are Made

To appreciate lithium-ion batteries, one has to start with the science. Lithium-ion batteries are what is referred to as an intercalation battery. This means that the same ion (lithium) reacts at both the anode and the cathode, traveling between the two through a liquid electrolyte. When the battery is discharged, the graphite anode releases a lithium ion into the electrolyte at the same time that the cathode absorbs one. During charge, the process is reversed.

Importantly, the electrolyte does not need to store large amounts of ions, it only needs to serve as a conduit between the electrodes. Most other battery chemistries do not use intercalation, depending instead on each electrode reacting with the electrolyte. This means they typically require a large quantity of electrolyte to store reactants. By needing a minimal amount of electrolyte, lithium-ion batteries can be very compact.

Another key feature of lithium-ion batteries is their ability to store a large amount of energy in a small amount of material. This means that lithium-ion electrodes can be built with relatively thin coatings of active material (that is, the materials at each electrode that react), with total electrode thickness of less than 0.1 millimeters. This is in contrast to lead-acid batteries, whose electrodes are multiple millimeters thick. The use of thin coatings allows for higher energy efficiency and better performance in high-power applications.

The combination of these two traits, low electrolyte volume and thin electrodes, drives the lithium-ion manufacturing process. Electrodes are made by applying thin coatings to thin metal substrates. These thin coatings allow for fairly rapid application and in-line drying in a continuous, roll-to-roll production process.

A separator, which can be quite thin since it does not need to store excess electrolyte, is placed between the electrodes before they are (typically) wound together and placed in a container. Electrolyte is injected into the cell before it is sealed and sent off for initial cycling. This carefully controlled cycling, called formation cycling, causes reactions to happen within the cell that protect its longevity.

Conditions for Lithium-ion Manufacturing Compatibility

This understanding of lithium-ion manufacturing reveals the requirements for a novel chemistry to be adapted to it. A novel chemistry must have the ability to store a large amount of energy in a small amount of active material at both electrodes. Without this, the thin lithium-ion style electrodes will yield a small amount of energy compared to the amount of metal substrate and separator required to support it. The cost of these non-active components is significant, representing about one-third of a lithium-ion cell’s material costs. Unless a battery chemistry can store similarly large amounts of energy in a small amount of material, the cost of non-active components will make the use of thin, lithium-ion style electrodes infeasible.

The second requirement is the ability to use a small amount of electrolyte. The need for excess electrolyte requires the use of thicker separators and limits the amount of energy that can be stored in a cell container of a given size. While the effect of this on cost is not as pronounced as active material energy density, it remains an important consideration.

There are very few battery chemistries that meet the above requirements, and even fewer meet them while also meeting the cost and performance requirements demanded by the market. This is why the zinc-ion battery, which meets all these requirements, has such strong potential to replace lithium-ion in stationary energy storage.

The Zinc-ion Battery

Like lithium-ion, the zinc-ion battery functions using intercalation. Zinc ions react at both electrodes and travel between them through a water-based electrolyte. During discharge, zinc metal at the anode is dissolved into the electrolyte as zinc ions. At the same time, zinc ions are absorbed into the cathode from the electrolyte. This process is reversed during charge.

Zinc-ion batteries meet the conditions for lithium-ion compatibility. The use of intercalation means that the electrolyte’s only function is as a conduit for ions, enabling a small amount to be used. Also, the active materials used in zinc-ion batteries are very energy dense, allowing for sufficiently high energy to be stored even in thin electrodes.

In fact, zinc-ion batteries (Figure 2) can improve on lithium-ion manufacturing processes. Lithium’s violent reactivity with water requires many of its production steps to take place in a highly controlled atmosphere that makes the process more costly, and more complicated. As a water-based battery, zinc-ion does not have this constraint.

2. Salient Energy workers assemble the company’s zinc-ion battery cells. The zinc-ion battery is considered safer than its lithium-ion counterpart, because it uses water as the electrolyte. It also could take better advantage of domestic supply chains within the U.S. Courtesy: Salient Energy

Additionally, zinc-ion batteries do not require formation cycling at the end of life. This means they can more quickly move from the manufacturing line to the customers. This ability to use lithium-ion manufacturing means that the production of zinc-ion batteries can be rapidly and inexpensively scaled-up.

Zinc-ion’s Competitive Advantages

In the short term, zinc-ion’s key differentiators from lithium-ion are safety and supply chain security. Zinc-ion’s intrinsic safety, due to its use of water as the electrolyte, means it will be able to gain traction in markets where lithium-ion adoption has been limited due to safety concerns.

An example would be dense urban centers where fire regulations prevent lithium-ion adoption. Zinc-ion’s ability to be built with materials that are produced in abundance in North America is another key differentiator. As energy storage plays an increasingly important role in critical infrastructure, customers will seek to develop domestic supply chains. The need for domestic supply chains will become even more acute if U.S.-China relations worsen, or if government subsidies enforce strong “Buy American” requirements.

As zinc-ion production ramps up and takes advantage of economies of scale, zinc-ion batteries will become a lower-cost alternative to lithium-ion. Paired with their long service life, this will allow zinc-ion batteries to offer a far lower cost of storage than can be achieved with lithium-ion today.

Future of the Energy Storage Industry

As the energy storage sector continues to expand on innovative solutions, zinc-ion batteries provide an alternate solution that will greatly challenge lithium-ion as the leader in the category. As progress continues to be made, it is important that investment in new resources and innovation continues to enhance the success of the industry.

Although it is relatively new, zinc-ion has demonstrated that it offers substantial improvements in supply chain security and safety. Pairing these advantages with a scalable manufacturing process will ensure that zinc-ion batteries have an important role in the future of clean technology.

As the world continues to fight to reduce carbon emissions, the need for better batteries and easier-to-source materials will become more acute. Zinc-ion’s unique properties are well-positioned to meet this growing need. Through research and increasing credibility within the industry, zinc-ion batteries are poised to become the default choice for stationary energy storage. ■

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