Batteries with flammable organic electrolytes have an inherent fire risk and create a level of insecurity in applications where energy density is not the priority. Frank Lev examines the state of play with aqueous electrolytes as a replacement technology.
Common electrolytes used in rechargeable batteries are organic (carbon-based) or aqueous (water-based). Both are solutions of solvents and salts. For example, lead-acid batteries use sulfuric acid as an electrolyte, whereas nickel-based batteries use an alkaline KOH solution.
Aqueous electrolytes are highly conductive, inexpensive and do not burn. Organic electrolytes use carbon-based solvents that can withstand higher charging voltages than water-based electrolytes. Most lithium-ion batteries (LIB) and sodium-ion batteries (SIB) use organic electrolytes. The vapours produced by organic electrolyte batteries experiencing thermal runaway are flammable, explosive and toxic, which is the Achilles heel of such battery technologies.
Extensive development work worldwide is underway to produce energy-dense batteries with non-flammable electrolytes. A solid-state inorganic electrolyte is an example of such technologies. It provides the ultimate solution to the flammability issue, simultaneously enabling a dramatic increase in energy density. However, the solid-state batteries are complex, fraught with challenges and expensive.
In contrast, advanced aqueous electrolytes offer a promising alternative for certain applications where safety, longevity, and low cost take precedence over energy density. While an electric drone or a specialised EV must have a battery with the highest possible energy density, some stationary energy storage systems or low-speed vehicles may successfully use lower-energy-density batteries, enjoying lower safety risks.
General Motors EV1
The first American purpose-built electric car, EV1, used lead-acid batteries (Fig 1 above). Supplied by GM’s Delco Remy division, these were AGM valve-regulated lead-acid batteries (VRLA), weighing 533kg and storing 16.5kWh. Their energy density was approximately 31Wh/kg, enabling a range of up to 160km.
The second generation of the EV1, introduced in 1999, was equipped with 31.7Wh/kg Panasonic VRLAs. Later on, EV1 was refitted with NiMH batteries, whose 55Wh/kg energy density enabled a range of 240km.
GM ended production of the EV1 in 1999 and completely terminated the program in 2003. In 2008, only five years later, Tesla Motors unveiled its first electric car that achieved 394km on a single charge. Just like GM, Tesla used Panasonic batteries in its EVs, except they were not aqueous electrolyte-based. Tesla used lithium-ion batteries (LIB) with an organic electrolyte, the technology that launched the era of automotive electrification, smartphones and military drones.
How does cell voltage affect energy density?
The higher cell voltage is the reason batteries switched from aqueous to organic electrolytes. If a battery consists of a single cell, its energy can be expressed by the following equation.
Ecell = Ccell x Vcell, where:
- Ecell – battery cell energy, in [Wh]
- Ccell – battery cell capacity, in [Ah]
- Vcell – battery cell voltage, in [V]
For example, the energy content of a NiMH battery with a cell voltage of 1.2 V and a capacity of 3.2 Ah is Ecell = 3.2 x 1.2 = 3.84Wh. The comparable capacity Panasonic NCR18650B has a cell voltage of 3.6V and capacity of 3.35Ah, with Ecell = 3.35 x 3.6 = 12.06Wh. Owing to its higher cell voltage, the Panasonic cell has energy almost three times that of a NiMH cell. To reach Panasonic‘s voltage, the NiMH would require a string of three cells connected in series. Such a battery will inevitably be heavier and occupy more space.
Typically, aqueous electrolytes must not exceed 1.23V at charging due to electrolysis that depletes their water content. The only practical exception is lead-acid batteries, since they use a lead-based cathode with a high hydrogen overpotential, enabling a nominal cell voltage of 2.0V, making this technology extremely successful for over a century across various applications. Yet, compared to Tesla-type LIBs, the lead-acid cell voltage is almost 1.8x lower.
Reinventing electrolytes
As mentioned, the crucial caveat of organic electrolytes is their high flammability due to the inherent volatility of the organic solvents. In search of new solutions to the flammability of organic electrolytes and confirming that “all that is new is the forgotten old,” the battery developers turned their attention to aqueous electrolytes again. Recent developments have shown that aqueous electrolytes, combined with new electrode materials, may offer a safe alternative to organic electrolytes. However, the utility of aqueous electrolytes is limited by their relatively low cell voltages, impacting their energy output.
The ongoing R&D is pursuing aqueous electrolytes with voltages comparable to those of organic electrolytes. For instance, Suo et al. developed an aqueous electrolyte with a voltage window of 3.0V. Suo’s electrolyte is based on lithium bis(trifluoromethanesulfonyl)imide commonly known as LiTFSI –a highly conductive and water-soluble salt, often used as a safer, more stable alternative to conventional LiPF6.
High-concentration LiTFSI solutions can suppress hydrogen evolution and electrode oxidation. An experimental LIB with this aqueous electrolyte underwent 1,000 cycles, demonstrating nearly 100% coulombic efficiency at various C-rates. Thus, aqueous electrolytes may become a feasible alternative to organic electrolytes (Table 1).
Cosmos Lab and SoftBank will commercialise aqueous batteries
The Nikkei Business Daily reported recently that SoftBank of Japan and Korean Cosmos Lab plan to jointly develop a “zinc-halogen battery” and begin mass production in Japan during fiscal 2027. Cosmos Lab has been working on its aqueous electrolyte battery technology since 2021.
The new battery uses zinc and bromine – which are readily available in Japan – instead of lithium and cobalt. The fact that the electrolyte uses water rather than an organic solvent makes the risk of ignition “almost nonexistent in principle,” said Nikkei.
Cosmos Lab’s battery technology
The biggest challenge of any battery technology is to achieve inflammability and high energy density in the same package. As mentioned, the cell voltage is a crucial factor because it depends on the electrolyte’s electrolysis threshold.
Organic solvent-based electrolytes enabled threefold increases in cell voltage compared to aqueous electrolytes, and, in combination with higher specific-capacity electrode materials, resulted in commercial batteries with gravimetric energy densities of 260Wh/kg and above, which are 6x as energy-dense as
VRLA batteries.
In contrast to conventional water-based batteries, Cosmos Lab’s batteries – the company claims – are “breaking through the limits of energy density.” However, the cell voltage achieved by Cosmos Lab remains relatively low – in the same ballpark as standard aqueous electrolyte batteries with potassium hydroxide electrolytes (Table 2).
One of the Cosmos Lab’s presentations in 2024 shows the following energy density milestones, which, in tune with the company’s “out of this world” name, reflect “a new universe of energy solutions” (Table 3).
There is no timeframe showing the current state of Cosmos Lab’s energy density milestones, but, according to Nikkei, SoftBank/Cosmos aims to begin mass production in 2027, so it is likely that the energy density of the batteries will achieve the EARTH status of 160Wh/kg. It suffices to note that the most commercial LFP batteries used for BESS and some EVs in China have an energy density of 160Wh/kg, but, unlike the EARTH (Table 3), they are flammable and toxic.
The Cosmos Lab battery architecture
Inspired by the assurance that “only batteries designed with aqueous electrolytes can guarantee safety”, Cosmos Lab developed a battery technology with a unique architecture that leverages the synergetic effects of Faradaic and double-layer capacitor electrochemistries. Cosmos Lab also uses:
- Anode-less zinc anode – a technology of forming an anode active material in-situ during the first charge
- Dry electrode technology. An environmentally friendlier and less expensive method of preparing electrodes, compared to conventional wet slurry methods
- Separator optimised for aqueous electrolyte. In contrast with organic solvents, water requires special hydrophilic materials for adequate wetting.
The Cosmos Lab’s cell comprises two electrodes with metal current collectors electronically insulated from each other by a fibrous separator. An aqueous electrolyte contained within the separator pores provides the ionic conductivity. The cathode active material consists of an activated carbon matrix saturated with bromine. In combination with the zinc-based anode, such electrochemical coupling functions as a supercapattery.
The latter is a generic term for hybrid electrochemical devices that combine the merits of supercapacitors and batteries. The Journal of Energy Storage, Volume 46, describes supercapattery as an energy storage device with energy densities rivalling those of batteries, high power, and fast charging/discharging capabilities. Also, with an extended cyclic lifespan (Fig 2).
Inherent safety
In contrast to LIBs that use highly flammable, explosive, and toxic organic solvents as electrolytes, water remains non-flammable and maintains consistent discharge energy even with wide temperature fluctuations, provided it does not undergo phase transformations, such as high-pressure steam or hydrogen evolution. As a result, Cosmos Lab batteries can operate in various environments without risk of thermal runaway.
Ecological friendliness
Cosmos Lab uses wood waste as a precursor and a low-carbon footprint process to produce its electrodes. In contrast to Tesla-type LIBs, no lithium, cobalt, nickel, or manganese is used in the Cosmos Lab batteries.
Low cost
Cosmos Lab can produce its battery cells at less than $50/kWh owing to the low cost of zinc and bromine. Such raw materials can be locally sourced and independently supplied in many countries, making domestic battery production more feasible.
The Asia Business Daily noted that Cosmos Lab has demonstrated, for the first time in the world, the feasibility of an aqueous electrolyte technology with an energy density comparable to that of LFP and sodium-ion batteries while setting an inflammability target beyond the reach of batteries with organic electrolytes.
Addressing technological challenges
Zinc-based batteries, however, typically have a shorter lifespan than LIBs. During charging and discharging, dendrites can form on the electrode surface, accelerating degradation. According to Nikkei, Cosmos Lab mitigated dendrite formation by optimising the anode’s porosity. Since 2021, the company has independently developed all key battery components, including separators, electrodes, and electrolytes, achieving prices comparable to LFP.
Furthermore, unlike existing redox flow batteries, Cosmos Lab uses the same prismatic form factor as LIBs, ensuring high compatibility and enabling direct application to existing infrastructure.
While most experimental aqueous batteries have struggled to exceed 1Ah capacity and have remained at the initial small-scale stage, Cosmos Lab has achieved a structural innovation suitable for large-scale commercialisation. By combining the combustion-suppressing properties of bromine-based halogens with a water-based electrolyte, the company has eliminated the risk of fire.
Cosmos Lab greatly reduced requirements for spacing, cooling, and fire suppression systems, doubling the space utilisation. SoftBank plans to validate the Cosmos Lab batteries in a large data centre it is building in Sakai, Japan. A Cosmos Lab representative stated, “We are currently operating a pilot line, producing prototypes and verifying performance. Our goal is to begin full-scale mass production in 2026, using industry-standard prismatic cells to ensure high compatibility with existing BESS infrastructure. We also plan to start global sales from 2026 and have already begun preparations for certification.”
Alsym Energy’s aqueous electrolyte technology
Alsym Energy of Massachusetts, USA, developed aqueous electrolyte sodium-ion batteries (SIBs), which, similarly to the Cosmos Lab batteries, are non-flammable.
Such improvement might be a game-changer for conventional SIBs, which are rapidly maturing into viable alternatives to LFPs but remain flammable and toxic.
A global leader in SIB development, CATL is commercialising its Naxtra cell, rated at 175Wh/kg and operating over a -40°C to 70°C temperature range. According to Future Market Insights, the SIB market is currently valued at $2.9 billion and is projected to reach $6.2 billion in 2031. Alsym aims to become an indispensable part of the global SIB market.
Alsym is also a participant in the recently announced American Battery Leadership Coalition (ABLC), dedicated to establishing SIBs as an essential battery technology for US energy storage, manufacturing competitiveness, and national security strategy.
Safe sodium-ion batteries
Alsym claims its revolutionary aqueous electrolyte technology makes SIBs safe (Fig 3) and dramatically simplifies fire suppression and safety controls of BESS.
A key advantage of SIB technology, regardless of its electrolyte chemistry, is that it enables the storage and transportation of SIBs in a fully discharged state, thereby enhancing their safety. Also, SIBs can be made with aluminium current collectors instead of copper – which is used in LIBs – making them lighter and less expensive.
“Our batteries are exactly what is needed by the market right now, and can deliver peace of mind to homeowners, businesses, communities, and grid operators who are eagerly seeking safe and reliable energy storage solutions,” said CEO and co-founder of Alsym, Mukesh Chatter.
Diving into Alsym technology
Alsym has published technical specifications for its SIB cells, showing cell-level energy densities of 135Wh/kg and 250Wh/L. These figures are in line with the lower-end LFP cells. The most advanced LFP cells from BYD can deliver 200Wh/kg. Alsym does not publicly state its nominal cell voltage.
Based on the known cell’s electrochemistry, the likely cell voltage is almost certainly lower than that of LFP and SIB with organic electrolytes, which explains Alsym’s relatively low energy density of 135Wh/kg.
Cathode. Alsym referenced Sodium Iron Phosphate Pyrophosphate (Na2FeP2O7) as a material of choice for its cathodes. In contrast to layered oxides and Prussian Blue, Na2FeP2O7 is known for its excellent thermal stability, strong cycle life, and low oxygen release.
Anodes. Alsym states that its anodes use “robust” hard carbon (HC) material. HC is a typical anode material used by many SIB manufacturers, such as CATL, Alstrom, and others. Graphite, widely used in LIB anodes, is not suitable for sodium ions, which are larger than lithium ions. In contrast, HC has larger pore structures, turbostratic graphene layers, defect sites and nanopores that allow sodium insertion and storage. To be suitable for SIB anodes, HC must have the following properties (Table 4).
Typically, HCs are synthesised from biomass, coconut shells, lignin, cellulose, and other carbonaceous, non-graphitisable materials. Even synthetic graphite precursors, such as petrol-based pitches and coal, can be used.
Alsym cell architecture
Table 5 reflects the basic architecture of the Alsym SIB cell.
Alsym overcame numerous challenges, focusing on its principal component – a stable water-based electrolyte that resists hydrogen evolution and enables long cycle life. The rest of the components are pretty typical for a SIB technology, except perhaps the separator, which must be well wetted with a water-based solution.
Alsym noted that its technology is not yet in mass production, but that it is “advancing its commercialisation roadmap” and collaborates with strategic partners. Recently, Alsym announced a strategic partnership with Juniper Energy, a California-based renewable energy developer, to deploy 500MWh of non-flammable BESS across California. This strategic agreement aims to maximise domestic content and eliminate active cooling in extreme environments such as the Mojave Desert. Given the history of BESS-related fires, the need for safe, enabling battery technology is long overdue. By integrating Alsym’s SIB technology, Juniper Energy will eliminate the fire and explosion risks associated with organic electrolyte batteries and significantly reduce BESS costs.
Crucial advantages of the water-based batteries
Unlike LIBs, which require energy-intensive active cooling to prevent thermal runaway and fire, Cosmos Lab’s and Alsym’s water-based technologies can operate efficiently without active cooling, dramatically simplifying BESS safety equipment, lowering upfront capital expenditures, and greatly reducing ongoing operational and maintenance costs.
By utilising Alsym’s domestically manufactured battery cells, Juniper Energy may benefit from the recently unveiled in the USA “One Big Beautiful Bill” tax credits and Department of Energy (DOE) incentives. “Our sodium-ion technology was built for exactly this: providing high-performance, fast-charging storage that doesn’t require complex cooling or risk community safety. By manufacturing here in the US, we are ensuring that partners like Juniper can deploy projects faster and more profitably,” said Mukesh Chatter, CEO of Alsym.
In the contest where human lives are involved, safety outweighs energy considerations
While organic electrolyte-based LFP batteries are inherently flammable and can release toxic hydrogen fluoride and hydrogen cyanide gases, Alsym and Cosmos Lab batteries offer safe operation. Their lower energy densities are offset by lower costs and simpler safety measures typically required of LFP batteries. Also, storage systems that use these water-based batteries can enjoy much lower insurance rates.
As mentioned, many stationary applications are not severely handicapped by the higher weight and larger footprint of the Alsym and Cosmos Lab batteries. Lower-cost materials, such as sodium and aluminium rather than lithium and copper, are also a decisive cost-reducing factor with the potential for wide applications not only in AI centres but also across construction, mining and agriculture.
Even the mobile applications are not out of the picture. Some low-speed and assured-safety vehicles may use aqueous electrolyte batteries instead
of LFPs.
Imagine a petrol or propane hauling truck. Which batteries would you choose for it?
Owing to life-endangering thermal runaway events involving EVs with lithium batteries, public perception in many instances favours lead-acid batteries, despite their lower energy density and cycle life. It appears that not every application needs LFP. For low-speed, short-range applications, LFP is not always the optimal commercial solution. For instance, the following considerations may justify choosing lead-acid batteries:
- Weight reduction is less critical for a low-speed vehicle
- Fast charging is not required if the EV is recharged overnight
- Peace of mind that your EV will not involuntarily burst into flames.
The continued use of lead-acid batteries in low-speed EVs, UPS and other applications is a reminder that safety issues and the high initial cost of lithium batteries remain serious barriers.
Advanced aqueous electrolyte batteries like Cosmos Lab and Alsym may provide “a sustainable future” by meeting rising global energy demand from the exponentially growing AI data centres.
Conclusion
As mentioned, the currently widely used LIBs pose an inherent fire risk due to the high flammability of their organic electrolytes. The emerging SIBs are only marginally safer. Although safe, solid-state batteries are still expensive. Only advanced aqueous electrolytes can ensure total non-flammability, an acceptable energy density and manageable cost. Cosmos Lab and Alsym aqueous electrolyte technologies are harbingers of the safe, efficient, and economically viable batteries that will soon enter our lives.
