On the fast-charging capabilities of EV batteries

One of the most common yet complex questions for electric vehicle (EV) owners is, “How long does it take to charge an EV?” While straightforward on the surface, this question requires a nuanced answer due to the many variables at play. Factors such as battery size, maximum charging rates of the EV and charging station, and environmental conditions all influence charging time.

In 2022, the Chinese battery manufacturer CATL introduced its Qilin batteries, supporting 4C fast charging and enabling a 10-minute charge for a 400 km driving range. According to 36kr (1), CATL aims to launch a 6C charging battery later this year, which would allow even faster charging. Other leading battery manufacturers, such as BYD, are also believed to pursue 6C fast-charging technologies. Here, the “C-rate” describes the charging speed of batteries, with 6C referring to a full charge in 1/6 of an hour (10 minutes).

Benefits of fast charging

Fast charging has the potential to significantly alleviate “range anxiety,” a common concern among EV owners. While internal combustion engine (ICE) vehicles typically achieve around 640 km (400 miles) on a full tank, high-performance battery electric vehicles (BEVs) with large battery sizes (100+ kWh) are reaching comparable ranges. Some models, like the Zeeker 001, claim ranges of up to 1000 km (600 miles) per charge. However, range anxiety persists, not due to the total range but the scarcity of charging stations and the time required to charge. While ICE drivers spend about five minutes refueling, EV owners generally need at least 30 minutes for a 2C charge. Increasing the charging rate to 6C would reduce the waiting time to 10 minutes, comparable to a quick refuel.

Physical limits of fast charging

In June 2024, EVE Energy introduced the Omnicell 6C, a cylindrical cell offering 300 km of range after a 5-minute charge (2). But is such rapid charging physically feasible? A simplified calculation using Fick’s Law offers insight into this. The diffusion time t for lithium ions (Li+) is approximated by: t≈x^2/2D, where x represents the diffusion distance, and D is the diffusion coefficient. Assuming a cathode, separator, and anode thickness of 70 μm, 20 μm, and 70 μm, respectively, with an average tortuosity of 4, the diffusion length x totals 0.064 cm. With a Li+ diffusion coefficient D estimated at 5×10−6 cm2/s, the diffusion time is approximately 6.8 minutes—suggesting that 4–6C charging may indeed be feasible for lithium-ion batteries, even if certain rate-limiting steps are not accounted for in this simplified calculation.

Technical challenges and potential solutions

Despite the promise of 6C charging, several limitations exist. From a materials standpoint, lithium-ion diffusion within solid materials is significantly slower than in liquid electrolytes, constraining practical charging rates, especially in graphite anodes. Additionally, at high C-rates, lithium ions may accumulate on the anode surface instead of diffusing into the balk, forming parasitic lithium dendrites. Low temperatures exacerbate this problem, increasing the risk of lithium plating and presenting a major challenge for EVs. Battery makers are developing materials and electrolytes tailored for fast charging, such as CATL’s Freevoy hybrid battery, which combines lithium-ion and sodium-ion cells for enhanced low-temperature performance (3).

Thermal management is another crucial factor, as fast charging can generate substantial heat. According to Joule’s law (P=I^2R), heat production P is proportional to the square of the current I, making effective cooling essential for 6C charging rates. Traditional air cooling may be insufficient due to air’s low thermal conductivity and capacity, necessitating liquid (immersion) cooling systems. Examples include BYD’s direct cooling and direct heating system and CATL’s multifunctional elastic cooling interlayers, designed to maintain optimal battery temperatures under high charge currents (4). Infrastructure is also a bottleneck. As of now, 6C-compatible charging stations are limited, and even 4C infrastructure is not yet widely deployed. Charging a 100 kWh battery at 6C demands around 600 kW power. Ten EVs charging at this rate would impose a 6 MW load, equivalent to the power needs of a small factory. Integrating 6C charging at scale would therefore necessitate substantial upgrades to electrical infrastructure, including transformers, switchgear, and distribution systems (5). Collaboration between battery manufacturers and energy providers will be essential to address these challenges.

Final reflections

The commercialization of lithium-ion batteries has benefited from over thirty years of relentless engineering optimization. By innovating across materials, system design, and infrastructure, the industry continues to push battery performance and charging capabilities to new heights. While 6C fast charging holds the potential to transform the EV landscape and offer new playgrounds for emerging technologies like solid-state batteries and vehicle-to-grid systems, it also introduces complex challenges. As the industry explores solutions, careful evaluation of feasibility and readiness is crucial for successful adoption.

References:

1. https://kr-asia.com/catl-and-byd-lead-the-charge-in-developing-6c-batteries-for-evs
2. https://ev.com/news/eve-energy-unveils-omnicell-6c-battery-with-300-km-range-in-5-minutes-of-charging&blogId=4772
3. https://www.batterydesign.net/catl-freevoy/
4. https://www.sciencedirect.com/science/article/pii/S2590116819300116
5. https://www.sciencedirect.com/science/article/pii/S2352467723001200