Electrolyte is an ionic conductor that conducts electricity between the positive and negative electrodes of a battery. It is made up of electrolyte lithium salt, high-purity organic solvent and necessary additives in a certain proportion. It plays a crucial role in the energy density, power density, wide temperature range, cycle life and safety performance of a battery.
Lithium-ion batteries consist of a casing, positive electrode, negative electrode, electrolyte, and separator. Among these, electrode materials are undoubtedly the focus of attention and research. However, the electrolyte is also an aspect that cannot be ignored. After all, the electrolyte, which accounts for 15% of the battery cost, plays a crucial role in the battery's energy density, power density, wide-temperature application range, cycle life, and safety performance.
Electrolyte is an ionic conductor that conducts electricity between the positive and negative electrodes of a battery. It is composed of lithium salt electrolyte, high-purity organic solvent, and necessary additives in a specific ratio. As the applications of lithium-ion batteries become increasingly widespread, the requirements for their electrolytes inevitably differ depending on the type of lithium-ion battery.
The pursuit of high specific energy is currently the biggest research direction for lithium-ion batteries, especially as mobile devices occupy an increasingly larger proportion of people's lives, battery life has become the most critical performance characteristic of batteries.
Silicon anodes have attracted attention due to their large specific capacity, but their swelling properties limit their application. In recent years, research has shifted to silicon-carbon anodes, which offer relatively high specific capacity and smaller volume changes. Different film-forming additives result in varying cycling performance on silicon-carbon anodes.
2. High-power electrolyte
Currently, commercially available lithium-ion batteries struggle to achieve high-rate continuous discharge, primarily due to severe overheating of the battery tabs and excessively high overall battery temperature caused by internal resistance, which can easily lead to thermal runaway. Therefore, electrolytes must maintain high conductivity while suppressing rapid battery temperature rise. Furthermore, achieving fast charging is also a crucial direction for electrolyte development in the field of power lithium-ion batteries.
High-power batteries not only place demands on electrode materials such as high solid-phase diffusion, nano-sizing to shorten ion migration paths, and control of electrode thickness and compaction, but also place higher demands on electrolytes: 1. High-dissociation electrolyte salts; 2. Solvent composites - lower viscosity; 3. Interface control - lower membrane impedance.
3. Wide-temperature range electrolyte
At high temperatures, batteries are prone to electrolyte decomposition and increased side reactions between materials and electrolyte components; while at low temperatures, electrolyte salt precipitation and a significant increase in the impedance of the negative electrode SEI film may occur. A wide-temperature electrolyte is designed to provide the battery with a broader operating environment. The following figure compares the boiling points and solidification of various solvents.
4. Safety Electrolyte
Battery safety is primarily concerned with combustion and even explosion. Batteries themselves are flammable; therefore, overcharging, over-discharging, short-circuiting, external irritation such as punctures or pressure, and excessively high temperatures can all trigger safety incidents. Consequently, flame retardancy is a crucial area of research in safe electrolytes.
Flame retardancy is achieved by adding flame retardant additives to conventional electrolytes, typically phosphorus-based or halogen-based. These additives must be reasonably priced and not impair electrolyte performance. Furthermore, the use of room-temperature ionic liquids as electrolytes is under research, which would completely eliminate the use of flammable organic solvents in batteries. Ionic liquids also possess characteristics such as extremely low vapor pressure, good thermal/chemical stability, and non-flammability, significantly improving the safety of lithium-ion batteries.
5. Long-circulation electrolyte
Since the recycling of lithium-ion batteries, especially power lithium batteries, still faces significant technical challenges, improving battery life is one way to alleviate this situation.
There are two important research ideas for long-cycle electrolytes: first, the stability of the electrolyte, including thermal stability, chemical stability, and voltage stability; second, the stability with other materials, requiring stable film formation with the electrodes, no oxidation with the separator, and no corrosion with the current collector.
