Sodium-ion batteries can be charged 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 movement and reactions of sodium and lithium ions in hard carbon anodes.
The findings, published in the journal Chemical Science, provide quantitative evidence that sodium insertion into hard carbon is inherently faster than lithium insertion, challenging the assumption that lithium-based systems always outperform.
Lithium-ion batteries currently dominate energy storage in electric vehicles, grid storage, and consumer electronics, but concerns about lithium availability, cost, and long-term sustainability are driving interest in alternative batteries.
Sodium is much more abundant and cheaper than lithium, making sodium-ion batteries an attractive option for large-scale applications. A key element in achieving the competitive performance of sodium-ion batteries is the negative electrode material, hard carbon.
Hard carbon is a porous form of carbon with low crystallinity that can store large amounts of sodium, allowing sodium-ion batteries to approach energy densities comparable to commercial lithium-ion batteries that use lithium iron phosphate and graphite as positive electrodes.
Despite this possibility, it has been difficult to accurately measure the true charging rate of hard carbon. Traditional battery testing relies on dense composite electrodes, where ion transport through the electrolyte becomes the limiting factor at high charging rates.
This causes a concentration overpotential and effectively creates an “ionic traffic jam” that hides the intrinsic reaction kinetics of the active substance. As a result, the actual rate limit for sodium and lithium insertion into hard carbon remains unknown.
To overcome this, the researchers used a dilute electrode method. In this approach, some of the hard carbon in the electrode is replaced with aluminum oxide, an electrochemically inert material.
This ensures that each hard carbon particle has sufficient access to sodium or lithium ions in the appropriate ratio, eliminating electrolyte transport limitations while preserving the composite electrode structure, including porosity and binder effects. This method allowed the team to directly assess the intrinsic kinetics of ion insertion.
This study confirmed that sodium insertion into hard carbon is faster than lithium insertion when both reaction mechanisms are considered. The apparent ion diffusion coefficient, which indicates how fast ions move through a solid material, was measured to be in the range of 10-10 to 10-11 cm² s-1 for sodium and 10-10 to 10-11 cm² s-1 for lithium.
This shows that sodium generally diffuses faster than lithium in hard carbon. Importantly, the rate ability and diffusion coefficient of sodium insertion into dilute hard carbon was found to be comparable to lithium insertion into dilute graphite electrodes, which are widely used in commercial lithium-ion batteries.
The researchers also investigated the temperature dependence and calculated the activation energy of the insertion reaction.
The low activation energy of sodium indicates that sodium insertion is less sensitive to temperature changes and requires less energy for progression, supporting faster charging, especially at low temperatures.
The analysis identified the pore-filling mechanism as the rate-determining step of overall charging. In this process, sodium or lithium ions aggregate within the nanopores of hard carbon to form pseudometallic clusters.
Although the initial adsorption and intercalation steps were found to be very fast for both ions, the formation of these clusters limits the overall reaction rate. Sodium has been shown to form these clusters more readily than lithium, contributing to its faster overall reaction rate.
When charged at very high charging rates with undiluted electrodes, lithium could maintain a higher capacity, which the researchers attributed to lithium’s greater adsorption and intercalation capacity.
However, this advantage does not reflect intrinsic material dynamics and is influenced by electrode design and transport limitations. Overall, this study shows that charge transfer resistance at the electrolyte-hard carbon interface and solid-state diffusion within the hard carbon particles are the main factors limiting the insertion rate.
From a technical perspective, the findings suggest that sodium-ion batteries are not only a lower-cost alternative to lithium-ion systems, but also have real performance advantages in charging speed.
Sodium-ion batteries charge quickly and have low temperature sensitivity, making them particularly attractive for high-power and grid-scale energy storage applications where cost, safety, and durability are important.
This study also provides clear directions for development. This means that fast charging performance could be further improved by modifying the hard carbon material to accelerate the pore filling rate.
As efforts to scale up sodium-ion battery technology continue, these results strengthen the case for sodium-ion batteries as a competitive and sustainable option in the global energy storage market.
Last December, a research team at Dalhousie University discovered that a new type of lithium-ion battery with single-crystal electrodes could extend the lifespan of electric vehicles and grid energy storage systems.
In 2023, in a new study, scientists at the Tokyo Institute of Technology used two different lithium-based solid electrolyte chemistries to ensure stable ion movement in millimeter-thick battery electrodes.
