The worldwide push for sustainability requires better, more durable batteries to support renewable energy systems and our ubiquitous electronic devices. While lithium-ion batteries (LIBs) are currently the go-to solution, future calls for alternatives built on materials more widely available than lithium. Because sodium is abundant and available at low-cost, sodium-ion batteries (SIBs) are a leading candidate for replacing LIBs while still meeting global energy demands.
The key to SIBs' remarkable performance lies partly in the material used at the negative electrode called hard carbon (HC). This low crystalline, porous type of carbon can store large amounts of sodium, enabling SIBs to reach energy densities comparable to commercial LIBs. Though scientists believe that HC is a fast-charging material, proving this experimentally is challenging. The problem is that conventional battery testing often underestimates the material's true charging rate due to issues with concentration overvoltage in composite electrode. Simply put, during rapid charging, the dense composite structure of the electrode can cause 'ion traffic jams,' where ion transport in the electrolyte limits the reaction speed. Thus, the fundamental charging rate limit of HC, as well as how the rate of sodium insertion compares to lithium, remain unclear.
To address this knowledge gap, a research team led by Professor Shinichi Komaba, alongside a third-year PhD candidate Mr. Yuki Fujii and Assistant Professor Zachary T. Gossage from the Department of Applied Chemistry, Tokyo University of Science, Japan, employed an innovative approach to uncover the kinetic limits of sodium and lithium insertion into HC. Their work was published in the journal Chemical Science on December 17, 2025.
The researchers used a technique known as the 'diluted electrode method.'*1 It involves creating an electrode that combines both HC particles and an electrochemically inactive material like aluminum oxide. At the appropriate ratio, it ensures that each HC particle is surrounded by an ample supply of ions, eliminating the typical ion transport issues within the electrolyte and at the negative electrode. Using this approach, the researchers were able to very effectively measure and compare the maximum rates for sodiation (sodium insertion), lithium intercalation, and lithiation (lithium insertion) into HC. Furthermore, sodiation into diluted HC electrode showed comparable rate capability to lithium intercalation at diluted graphite electrodes.
Our results provided clear and quantitative evidence of HC's high-rate potential. Through detailed testing and analysis using cyclic voltammetry, electrochemical impedance spectrometry, and potential-step chronoamperometry, the team found that the sodiation process is intrinsically faster than lithiation for the same negative electrode. This was confirmed by calculating the apparent diffusion coefficient—a measure of how quickly ions move through the material—which was generally higher for sodium than for lithium. "Our results quantitatively demonstrate that the charging speed of an SIB using an HC anode can attain faster rates than that of an LIB," highlights Prof. Komaba.
Furthermore, the team precisely identified that the rate-determining step for the entire charging process is the pore-filling mechanism, which occurs when ions aggregate to form pseudo-metallic clusters within HC's nanopores. While the initial stage of charging (adsorption/intercalation) was found to be very fast for both ions, the speed of the total reaction is ultimately limited by the efficiency of the pore-filling process. Detailed chemical kinetic analysis revealed that sodium requires less energy than lithium to form these clusters, which helps explain the rate advantages observed. By identifying this bottleneck, this study provides a clear direction for faster and more energy-efficient battery designs. "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, " explains Prof. Komaba.
The findings of this work tell us that SIBs are not simply a cheaper and safer alternative to LIBs, but that they offer genuine performance advantages in charging speed, which are especially relevant in high-power applications. Additionally, SIBs could offer more stable operation than LIBs. Further studies to perfect SIBs will slowly but surely pave the way for new battery technologies, supporting current endeavors to build sustainable societies.
* 1 Diluted electrode method
This unique and effective electrochemical method for evaluating kinetics of insertion materials was originally developed by Associate Professor Kingo Ariyoshi from Osaka Metropolitan University. In this research, the negative electrode active material, i.e. HC powder, was partially replaced by aluminum oxide powder, which is electrochemically inactive.
