The global shift toward sustainability is increasing the demand for better and more durable battery technologies to support renewable energy systems and everyday electronic devices. While lithium-ion batteries are currently the dominant choice, concerns over lithium availability, cost, and long-term scalability are driving research into alternative technologies. Sodium-ion batteries are emerging as a strong contender because sodium is abundant, widely available, and low-cost, making it suitable for meeting future global energy needs.
A key factor behind the performance of sodium-ion batteries is the material used at the negative electrode, known as hard carbon. This low-crystalline and porous form of carbon can store large amounts of sodium, allowing sodium-ion batteries to reach energy densities comparable to commercial lithium-ion batteries. Scientists have long believed that hard carbon supports fast charging, but confirming this experimentally has been difficult. Conventional battery testing methods often underestimate charging speed because of concentration overvoltage issues in composite electrodes. During rapid charging, the dense structure of these electrodes can restrict ion movement, creating what researchers describe as “ion traffic jams,” which limits reaction speed. As a result, the true charging limits of hard carbon and how sodium insertion compares with lithium insertion have remained unclear.
To overcome this challenge, a research team led by Professor Shinichi Komaba from the Tokyo University of Science, along with PhD candidate Yuki Fujii and Assistant Professor Zachary T. Gossage, adopted an innovative experimental approach. Their study was published in the journal Chemical Science on December 15, 2025. The team used a technique called the diluted electrode method, in which hard carbon particles are mixed with an electrochemically inactive material such as aluminum oxide. This setup ensures that each hard carbon particle has sufficient access to ions, effectively removing ion transport limitations within the electrode and electrolyte.
Using this method, the researchers were able to accurately measure and compare the maximum rates of sodium insertion, lithium insertion, and lithium intercalation into hard carbon. The results showed that sodium insertion into hard carbon has a rate capability comparable to lithium intercalation in graphite electrodes. More importantly, detailed electrochemical analysis revealed that sodium insertion is intrinsically faster than lithium insertion when using the same hard carbon electrode.
The team conducted extensive testing using cyclic voltammetry, electrochemical impedance spectrometry, and potential-step chronoamperometry. By calculating the apparent diffusion coefficients, which indicate how quickly ions move through a material, they found that sodium ions generally move faster than lithium ions within hard carbon. Professor Komaba noted that the results provide clear and quantitative evidence that sodium-ion batteries using hard carbon anodes can achieve faster charging rates than lithium-ion batteries.
The study also identified the key factor limiting the overall charging speed. While the initial adsorption and intercalation stages occur rapidly for both sodium and lithium, the slowest step is the pore-filling process, where ions form pseudo-metallic clusters inside the nanopores of hard carbon. Chemical kinetic analysis showed that sodium requires less energy than lithium to form these clusters, which explains the faster charging behavior. The researchers highlighted that improving the kinetics of this pore-filling process is crucial for developing fast-charging sodium-ion batteries. They also found that sodium insertion is less sensitive to temperature due to its lower activation energy.
These findings suggest that sodium-ion batteries are not just a lower-cost and safer alternative to lithium-ion batteries, but also offer real performance advantages in charging speed and operational stability. Continued research in this area could accelerate the development of next-generation battery technologies and support broader efforts to build more sustainable energy systems.
