What do endurance athletes and lithium-ion batteries have in common? Both need electrolytes. Stemming from the Greek word lytós, meaning “able to be untied or loosened,” electrolytes are electrically conducting solutions. In animals, the positive or negative ions of electrolytes derive from sodium, potassium, calcium, magnesium, chloride, hydrogen phosphate and hydrogen carbonate, helping to control cellular fluid balance, blood pressure and muscle function.
In electrochemistry, positive and negative electrodes (anodes and cathodes) are placed in an electrolyte solution. Cathode materials provide electrons to the electrolyte, and anodes consume electrons from the electrolyte. Connect the two electrodes, we get electricity. This principle is the foundation of an electrochemical cell. Connect two or more cells, and we have an electrical battery.
As we’ve seen in previous entries in Advancing Materials, rechargeable lithium-ion batteries provide high energy density in a lightweight package, making them suitable for zero-emission vehicles. However, to make electric vehicles more affordable with a longer driving range, we need further research into lithium-ion batteries. A critical component in the lithium-ion battery is its electrolyte material, which interacts with electrodes to produce lithium ions used in battery discharge and charging.
Several factors go into evaluating a good electrolyte material for the Li-ion battery, including good ionic conduction, mitigating degradation over usage, reaction (or lack thereof) to other cell components such as separators, substrates and packaging, thermal stability, and low toxicity.
Raman spectroscopy is useful not only for the evaluation of new electrolyte materials but also for studying subtle changes in a material’s structure and chemistry. For example, Raman spectroscopy can be used to study the degree of association of electrolyte ions in solutions and in polymer materials. The association of ions has a direct effect on the ion mobility and ion conductivity, thus affecting battery performance. In the final installation of our series on improving lithium-ion battery components, we review several papers discussing research into electrolytes for Li-ion batteries. Materials studied include solid polymer electrolytes, novel lithium salts, ceramic fillers such as alumina and titania, and additives designed to partially immobilize anions and improve cation charge transfer.