Days numbered for 'risky' lithium-ion batteries, scientists say, after fast-charging breakthrough in sodium-ion alternative

Newly developed sodium-ion (Na-ion) batteries could offer much faster charging speeds, higher energy density and improvements in safety compared with conventional lithium-ion (Li-ion) batteries, scientists say.

Using Na-ion batteries, an alternative to the Li-ion batteries found in the majority of today's devices, researchers at the Tokyo University of Science used a new carbon-based electrolyte to improve Na-ion energy density and charge speeds.

Scientists have been investigating Na-ion batteries as an alternative to Li-ion batteries because of their improved stability and low cost, but several bottlenecks and limitations have blocked the technology's advancement.

All batteries contain an anode and a cathode, the two electrodes that determine how current flows into and out of the device. In Li-ion batteries, the cathode is made primarily of graphite, as it's an excellent material for storing lithium ions to be discharged later.

But Na-ion batteries use hard carbon (HC) — a porous combination of thousands of "turbostratic basic structural units," essentially a complex crystalline structure, that excels at storing sodium ions. This is, in theory, a very fast-charging material.

Previous research into HC found it difficult to prove that this theoretical charging rate is practically possible, however, because ions entering the dense electrolyte at high speed experience a slowdown similar to a traffic jam. But in a new study published Dec. 15, 2025, in the journal Chemical Science, the scientists set out to overcome this hurdle.

Limiting the risks of Li-ion batteries

The researchers combined small concentrations of HC with aluminum oxide, a chemically inactive material, into a combined electrode. This allowed ions to flow freely into the HC particles with no "traffic'" issues.

With the problem overcome, the researchers then proved that sodium ions could enter HC at similar rates to lithium ions entering graphite in a Li-ion battery.

The researchers also found that the bottleneck for the entire process is the rate at which ions fill the "pores" within HC, where "pores" describe the process in which ions form pseudo-metallic clusters inside the nanoscopic pores across the surface of HC.

Through careful analysis, the researchers found that sodium ions require less energy to form these clusters. The finding indicates that, under the right conditions, Na-ion batteries — also called SIBs — can achieve faster charge rates than Li-ion batteries can.

"A key point of focus for developing improved HC materials for fast-chargeable SIBs is to attain faster kinetics of the pore-filling process so that they can be accessed at high charging rates," lead study author Shinichi Komaba, a professor in the Department of Applied Chemistry at the Tokyo University of Science, explained in a statement. "Also, our results suggest that sodium insertion is less sensitive to temperature, based on the consideration of smaller activation energy than lithiation."

In the real world, the results could help Na-ion batteries become more widely adopted for uses that require incredibly fast charging or discharging rates. For example, grid-scale battery energy storage systems would benefit from the capability to rapidly discharge energy on demand. It's also of paramount importance for batteries to remain stable when they're used at scale for storing energy produced by renewable sources.

Na-ion batteries are safer than Li-ion batteries, as noted in a 2025 study by researchers at the Islamic University of Technology, Idaho State University, and University of Waterloo. This is because the stable sodium ions they contain are less prone to the chain reaction that causes Li-ion batteries to burn, or even explode, when damaged.

The U.K. National Fire Chiefs Council has stated that battery energy storage systems that use Li-ion batteries pose a "significant fire risk," particularly because once they're on fire, these batteries cannot be easily extinguished.

Thermal runaway, the self-sustaining process that causes Li-ion batteries to ignite, can even sustain itself without oxygen. The British Safety Council has noted that after they ignite, Li-ion batteries in some electric vehicles may burn for hours or even days.

If produced at scale, Na-ion batteries like those tested in the study could avoid these risks altogether.

"Our results quantitatively demonstrate that the charging speed of an SIB using an HC anode can attain faster rates than that of an LIB [lithium-ion battery]," Komaba said in the statement.