Solvent for more powerful batteries

Solvent for more powerful batteries

A new electrolyte could speed up the charging of lithium-ion batteries.© Just_Super/ iStock

Whether in smartphones or electric cars: lithium-ion batteries are common in many areas. What is impractical, however, is that they often take a long time to charge and that they lose large parts of their capacity at low temperatures. Now researchers have developed a new electrolyte solution that could improve both areas. The solvent consists of very small molecules that can improve the transport of ions within the battery and thus speed up charging. In addition, the capacity remained comparatively high even at -70 degrees Celsius.

Anyone who drives an electric car can tell you a thing or two: routes that are not a problem in summer require a stopover to charge in winter. That in turn takes a while. How quickly a battery can be charged is limited by the conductivity of the electrolyte inside it. This determines how well the lithium ions can move within the battery. The following applies to previous batteries: the higher the conductivity, the faster the battery can be charged – but the greater the loss of performance in the cold.

Seemingly contradictory properties

“The ideal electrolyte would require contradictory properties,” explains a team led by Di Lu from Zhejiang University in China. “At the same time, it would have to allow the lithium salts to dissolve to a large extent, would only have a low energy barrier for the transport of the lithium ions and would also have to form a layer on the negative electrode during charging to enable rapid recharging. These properties can never be achieved at the same time with current electrolytes.”

Lu and his team have now found a way to combine fast charging and high performance in cold conditions: While the solvents previously used as electrolytes consist of relatively large molecules, they propose using a solvent called fluoroacetonitrile , whose individual molecules are significantly smaller. The solvent molecules can surround the lithium ions like a shell. While larger solvents only form one shell around each lithium ion, which results in slow ion transport, fluoroacetonitrile forms two shells that work together like a kind of tunnel for the ions. In this way, the lithium ions quickly move from one pole of the battery to the other.

Performance even in sub-zero temperatures

In the research team’s experiments, batteries using fluoroacetonitrile as a solvent achieved an ionic conductivity of 40.3 millisiemens per centimeter at room temperature – around four times more than previous batteries. At the same time, the solvent can also withstand low temperatures, as the tests showed. Even at −70 degrees Celsius, the ionic conductivity was still similar to that of ordinary batteries at room temperature – and around 10,000 times higher than with previous electrolytes at such low temperatures.

“The electrolyte with small solvent molecules enables the lithium-ion batteries to simultaneously achieve high energy density, fast charging and a wide operating temperature range,” write Lu and his colleagues. According to the researchers, the lifespan of the new battery is also long. In their tests, the battery managed more than 3,000 cycles of charging and discharging.

Next generation of lithium-ion batteries

“This research opens new avenues for the development of the next generation of lithium-ion batteries,” write Chong Yan and Jia-Qi Huang of the Beijing Institute of Technology in an accompanying commentary to the study, also published in the journal Nature. “The previously unknown form of structural transport of lithium ions within the battery could inspire innovative approaches to fast charging and the development of low-temperature batteries.”

Source: Di Lu (Zhejiang University, China) et al., Nature, doi: 10.1038/s41586-024-07045-4

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