Sodium-ion batteries could charge faster than lithium-ion batteries, according to an experimental study led by the Department of Applied Chemistry at Tokyo University of Science that directly compares the motion and reactions of sodium and lithium ions in hard carbon anodes.

The findings, published in Chemical Science, provide quantitative evidence that sodium insertion into hard carbon is intrinsically faster than lithium insertion, challenging the assumption that lithium-based systems are always superior in performance.

Lithium-ion batteries currently dominate energy storage for electric vehicles, grid storage, and consumer electronics, but concerns around lithium availability, cost, and long-term sustainability have driven interest in alternatives.

Sodium is far more abundant and cheaper than lithium, making sodium-ion batteries an attractive option for large-scale applications. A key component enabling competitive performance in sodium-ion batteries is the negative electrode material, hard carbon.

Hard carbon is a low-crystallinity, porous form of carbon that can store large amounts of sodium, enabling sodium-ion batteries to approach energy densities comparable to commercial lithium-ion batteries using lithium iron phosphate and graphite as cathodes.

Despite this potential, accurately measuring the true charging speed of hard carbon has been difficult. Conventional battery testing relies on dense composite electrodes, where ion transport through the electrolyte becomes a limiting factor at high charging rates.

This leads to concentration overvoltage, effectively creating “ion traffic jams” that mask the intrinsic reaction speed of the active material. As a result, the real kinetic limits of sodium and lithium insertion into hard carbon have remained unclear.

To overcome this, the researchers used the diluted electrode method. In this approach, part of the hard carbon in the electrode is replaced with an electrochemically inactive material, aluminum oxide.

At an appropriate ratio, this ensures that each hard carbon particle has sufficient access to sodium or lithium ions, eliminating electrolyte transport limitations while preserving the composite electrode structure, including porosity and binder effects. This method allowed the team to evaluate the intrinsic kinetics of ion insertion directly.

The study confirmed that sodium insertion into hard carbon is faster than lithium insertion when both reaction mechanisms are considered. The apparent ion diffusion coefficients, which indicate how quickly ions move through the solid material, were measured to be in the range of 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ for sodium and 10⁻¹⁰ to 10⁻¹² cm² s⁻¹ for lithium.

This shows that sodium generally diffuses faster than lithium within hard carbon. Importantly, the rate capability and diffusion coefficients for sodium insertion into diluted hard carbon were found to be comparable to lithium intercalation into diluted graphite electrodes, which are widely used in commercial lithium-ion batteries.

The researchers also examined temperature dependence and calculated activation energies for the insertion reactions.

The lower activation energy for sodium indicates that sodium insertion is less sensitive to temperature changes and requires less energy to proceed, supporting faster charging, particularly at lower temperatures.

The analysis identified the pore-filling mechanism as the rate-determining step for overall charging. In this process, sodium or lithium ions aggregate within the nanopores of hard carbon to form pseudo-metallic clusters.

While the initial adsorption and intercalation steps were found to be very fast for both ions, the formation of these clusters limits the total reaction rate. Sodium was shown to form these clusters more easily than lithium, contributing to its faster overall kinetics.

At very high charging rates in non-diluted electrodes, lithium sometimes retained higher capacity, which the researchers attributed to lithium’s larger adsorption and intercalation capacity.

However, this advantage does not reflect intrinsic material kinetics and is influenced by electrode design and transport limitations. Overall, the study shows that charge-transfer resistance at the electrolyte–hard carbon interface and solid-state diffusion within hard carbon particles are the main factors limiting insertion rates.

From a technology perspective, the findings suggest that sodium-ion batteries are not only a lower-cost alternative to lithium-ion systems but also offer genuine performance advantages in charging speed.

Faster charging and lower temperature sensitivity make sodium-ion batteries particularly attractive for high-power and grid-scale energy storage applications, where cost, safety, and durability are critical.

The study also provides a clear development direction: improving hard carbon materials to accelerate pore-filling kinetics could further enhance fast-charging performance.

As efforts continue to scale up sodium-ion battery technology, these results strengthen the case for sodium-ion batteries as a competitive and sustainable option in the global energy storage market.

Last December, a team of researchers at Dalhousie University found that a new type of lithium-ion battery featuring single-crystal electrodes could extend the lifespan of electric vehicles and power grid storage systems.

In 2023, in a new study, scientists at the Tokyo Institute of Technology used two lithium-based solid electrolyte chemical compositions to ensure stable ionic movement in millimeter-thick battery electrodes.