Researchers from Tokyo University of Science have shown that sodium ion batteries with hard carbon anodes can charge faster than lithium ion batteries by using a dilute electrode method that reveals that sodium insertion is intrinsically faster than lithium.
Researchers from Tokyo University of Science (TUS) have found that sodium-ion batteries (SIB) using hard carbon anodes (HC) can charge faster than lithium-ion batteries (LIB), challenging long-held assumptions in battery research.
The team sought to address the problem of conventional battery testing where the actual HC charging rate is often underestimated due to concentration overvoltage issues in a composite electrode.
During rapid charging, the dense composite structure of the electrode can cause ‘ion traffic jams’, where ion transport in the electrolyte limits the reaction rate, making the limit on the charging rate of HC, and also how the rate of sodium insertion compares to lithium, unclear.
Published findings in Chemical Science called “Revealing the kinetic limits of sodium and lithiation in hard carbon using the dilute electrode method”, the TUS research team outlines the quantitative comparison of sodium and lithium insertion kinetics without electrolyte transport limitations, using a dilute electrode method (DEM).
The DEM was developed by Professor Kingo Ariyoshi of Osaka Metropolitan University, whose research showed that active HC powder with negative electrode was partially replaced by alumina powder, which is electrochemically inactive.
The sodium process was found to be faster than lithiation for the same hard carbon negative electrode, with the rate-determining step identified as the pore-filling mechanism, where sodium requires less energy than lithium to form pseudometal clusters in hard carbon nanopores.
This low-crystalline, porous type of carbon can store large amounts of sodium, allowing SIBs to achieve energy density comparable to commercial LIBs.
Professor Dr. Shinichi Komaba from the TUS Department of Applied Chemistry said the results quantitatively show that the charging speed of a SIB using an HC anode can reach higher speeds than that of a LIB.
“A key focus in developing improved HC materials for fast-charging SIBs is to achieve faster kinetics of the pore filling process so that they can be accessed at high charging rates,” Komaba said.
Working with third-year doctoral student Yuki Fujii and Assistant Professor Zachary T. Gossage from the Department of Applied Chemistry, the team’s dilute electrode method involved creating an electrode that combines both HC particles and an electrochemically inactive material such as aluminum oxide.
At the correct ratio, it ensures that each HC particle is surrounded by an ample supply of ions, eliminating the typical ion transport problems within the electrolyte and at the negative electrode.
Using this approach, the researchers were able to effectively measure and compare the maximum amounts of sodium (sodium insertion), lithium intercalation, and lithiation (lithium insertion) in HC.
Moreover, the post-treatment in a dilute HC electrode showed similar rate performance as lithium intercalation in dilute graphite electrodes.
“Our results provided clear and quantitative evidence of the high potential of HC. Through detailed testing and analysis using cyclic voltammetry, electrochemical impedance spectrometry and potential step chronamperometry, the team found that the sodium process is intrinsically faster than lithiation for the same negative electrode,” Komaba said.
“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.”
Komaba added that the results quantitatively demonstrate that the charging speed of a SIB using an HC anode can reach higher speeds than that of a LIB.
Rate-determining step
Furthermore, the team accurately determined that the rate-determining step for the entire charging process is the pore-filling mechanism, which occurs when ions come together to form pseudo-metal clusters in the nanopores of HC.
Although the initial charging phase (adsorption/intercalation) was found to be very fast for both ions, the rate of the overall 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 observed speed benefits. By identifying this bottleneck, this research provides a clear direction for faster and more energy-efficient battery designs.
Komaba said a key focus in developing improved HC materials for fast-charging SIBs is to achieve faster kinetics of the pore filling process so that they can be accessed at high charging rates.
“Our results also suggest that sodium insertion is less sensitive to temperature, based on the consideration of smaller activation energy than lithiation.”
The team concludes that their findings suggest that SIBs are a cheaper and safer alternative to LIBs, offering performance benefits in charging speed and can provide more stable operation than LIBs.
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