Researchers find a new way to improve lithium-ion battery performance in extreme cold.
Electric vehicles (EVs) are helping to address the challenges of global warming, but the irony is that they perform better in a warmer world.
The lithium-ion (Li-ion) batteries that power EVs are inefficient at low temperatures, an obstacle for drivers in cold climates in countries like Canada, Sweden and Norway. But new research published by the American Chemical Society (ACS) proposes a way to make Li-ion batteries a little bit cooler.
Freezing Your Batteries Off
When the temperature is below freezing, Li-ion batteries hold less charge and discharge more quickly. For EVs, this results in significantly less range. For example, the Volvo XC40 EV has a specified city driving range of 500 km when the temperature is 23°C (73.4°F), but when the temperature drops to ˗10°C (14°F), so does the range—by nearly a third—to just 340 km.
The new research tackles this problem by focusing on the anode, a common tinkering point for lithium-ion battery research. The most common Li-ion anode material is graphite, which offers high capacity and a light weight. But it is that high capacity that degrades at low temperatures. In their ACS Central Science paper titled “Riemannian Surface on Carbon Anodes Enables Li-Ion Storage at −35°C,” the researchers reveal the precipitous performance decline: at room temperature, graphite has a theoretical capacity of 372 mAh/g, but at ˗20°C, that decays to just 12 mAh/g.
The researchers suspected that the main factor driving down capacity at low temperatures is the anode’s flat surface structure. They replaced the traditional flat graphite anode with a modified carbon-based material that had pentagonal defects embedded into the surface. This novel anode can maintain its rechargeable storage capacity even at temperatures below ˗35°C (˗31°F).
The researchers adjusted the surface electron configurations of the carbon anode to provide better interaction between the solvated lithium ions and adsorption sites for desolvation, aiming to reduce the activation energy of the charge-transfer process. In short, the anode has a rough surface that provides better adsorption of lithium ions.
To test their results, the researchers created an experimental coin-shaped Li-ion battery with their new anode and a standard lithium metal cathode. Their experiments revealed that the battery exhibited good electrical properties all the way down to ˗20°C (˗4°F). The battery had excellent charge transfer capabilities, maintaining 85.9 percent of its room temperature storage capacity just below 0°C (32°F). Perhaps most impressively, the battery is still rechargeable even at ˗35°C (˗31°F).
Out in the Cold
Holding a charge isn’t the only difficulty for Li-ion batteries at low temperatures. Running in the cold is destructive for Li-ion batteries and accelerates their aging. Low temperatures deter the battery’s chemical reactions, leading to lithium plating, which can pose safety concerns.
Adjusting the electrolyte is currently the most common approach for reducing the impact of low temperatures on Li-ion batteries. This involves adjusting proportions of ethylene carbonate solvent, using cosolvents with low melting points and different additives for better low temperature viscosity and increased ionic conductivity. However, this approach is more of a patch than a solution, allowing batteries to operate at low temperatures but not improving their loss in performance.
By focusing on the anode, the new research may help pave the way for better battery performance in cold climates.
