Experimental evidence suggests sodium-ion batteries can outpace lithium-ion charging
As the transport and energy sectors accelerate toward electrification, demand is increasing not only for higher energy density batteries, but also for systems that can charge rapidly, operate reliably, and rely on materials with secure global supply chains
While lithium-ion batteries (LIBs) remain the dominant technology today, growing attention is being paid to sodium-ion batteries (SIBs) as a complementary or alternative solution.
New experimental research published in Chemical Science by Tokyo University of Science, provides quantitative evidence that SIBs can achieve faster charging rates than LIBs under the right conditions, offering potential advantages for high-power transport and infrastructure applications.
ADVANTAGES OF SODIUM-ION
Sodium-ion batteries have long been attractive due to the abundance and low cost of sodium compared with lithium. However, questions have remained over whether SIBs can match or exceed the electrochemical performance of LIBs, particularly in demanding use cases. A research team led by Professor Shinichi Komaba at Tokyo University of Science has now addressed a critical part of this debate by directly comparing the intrinsic charging kinetics of sodium and lithium ions at hard carbon (HC) negative electrodes.
Hard carbon (HC) is a low-crystallinity, porous carbon material widely regarded as the most promising anode material for SIBs. It can store large quantities of sodium and enables energy densities comparable to commercial LIBs. Although HC has been considered a fast-charging material, conventional battery testing methods often obscure its true rate capability. In composite electrodes, dense packing of active material can lead to concentration overvoltage during rapid charging, where ion transport through the electrolyte becomes the limiting factor. This creates what the researchers describe as effective “ion traffic jams,” preventing accurate measurement of the material’s intrinsic kinetics.
AN EXPERIMENTAL APPROACH
To overcome this limitation, the research team employed an innovative experimental approach known as the diluted electrode method. In this technique, HC particles are mixed with an electrochemically inactive material, aluminium oxide (Al2O3), in carefully controlled ratios. By sufficiently diluting the active material, each HC particle is surrounded by an ample supply of ions, minimising electrolyte transport limitations and allowing the fundamental reaction rates to be measured directly.
Using this method, the team compared sodiation (sodium insertion), lithiation (lithium insertion), and lithium intercalation behaviour in hard carbon and graphite electrodes. The results showed that sodiation in diluted HC electrodes exhibited rate capability comparable to lithium intercalation in diluted graphite electrodes, already a benchmark for fast-charging anodes in LIBs. More significantly, detailed electrochemical analysis demonstrated that sodium insertion into hard carbon is intrinsically faster than lithium insertion in the same material.
The researchers applied cyclic voltammetry, electrochemical impedance spectroscopy, and potential-step chronoamperometry to quantify ion transport behaviour. From these measurements, they calculated apparent diffusion coefficients, which describe how quickly ions move within the electrode material. Across a wide range of conditions, the apparent diffusion coefficients for sodium were generally higher than those for lithium. As Komaba states: “Our results quantitatively demonstrate that the charging speed of an SIB using an HC anode can attain faster rates than that of an LIB.”
KEY FINDINGS
Beyond confirming faster sodium kinetics, the study also identified the key mechanism that ultimately limits charging speed. Hard carbon stores ions through a multi-step process. Initial adsorption and intercalation at defect sites and between graphene layers occur rapidly for both sodium and lithium. However, the final stage involves a pore-filling mechanism, where ions aggregate to form pseudo-metallic clusters inside nanoscale pores within the hard carbon structure. The researchers found that this pore-filling step is the rate-determining process for overall charging.
Through detailed kinetic and thermodynamic analysis, the team showed that sodium requires less activation energy than lithium to form these clusters. This lower energy barrier explains why sodium insertion proceeds more rapidly during the pore-filling stage. Komaba highlighted the design implications of this finding: “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. Also, our results suggest that sodium insertion is less sensitive to temperature, based on the consideration of smaller activation energy than lithiation.”
This reduced temperature sensitivity is particularly relevant for transport applications, where batteries must operate reliably across a wide range of climates and duty cycles. Faster intrinsic kinetics and lower activation energy could enable more consistent performance during cold starts or high-power operation, areas where LIBs can face limitations.
PROMISING PROGRESS
The findings suggest that sodium-ion batteries should not be viewed solely as a lower-cost or resource-secure alternative to lithium-ion technology. Instead, they may offer genuine performance advantages in charging speed, especially for applications requiring frequent fast charging or high-power throughput. For transport sectors such as buses, commercial vehicles, rail auxiliaries, and stationary systems supporting charging infrastructure, these characteristics could prove highly valuable.
While further development is required to optimise HC structures and full-cell designs, this work provides a clear experimental foundation. By isolating and quantifying the fundamental kinetic processes at the negative electrode, the study offers actionable guidance for materials engineering and cell optimisation. As electrified transport expands, such advances indicate that sodium-ion batteries could play a significant role alongside lithium-ion systems in building more resilient and sustainable energy storage solutions.