Lithium-ion batteries, as portable energy storage devices, are widely used in mobile phones, laptops, cameras, electric bicycles, electric vehicles, and other fields. Among these, the electrolyte is a crucial aspect. After all, the electrolyte, which accounts for 15% of the battery cost, plays a vital role in battery energy density, power density, wide operating temperature range, cycle life, and safety performance.
The electrolyte is one of the four key materials in lithium-ion batteries: the positive electrode, the negative electrode, the separator, and the electrolyte itself. Often referred to as the "blood" of a lithium-ion battery, it conducts electrons between the positive and negative electrodes. It is essential for achieving high voltage and high efficiency in lithium-ion batteries, ensuring advantages such as high energy density.
As is well known, the main components of a lithium-ion battery include four aspects: positive electrode material, negative electrode material, electrolyte, and separator. As a crucial component of lithium-ion batteries, the electrolyte plays an irreplaceable role in improving the cycle performance and energy density, thereby further increasing the driving range of electric vehicles. The energy density of a lithium-ion battery depends on its voltage and capacity. To increase the battery's energy density, besides increasing the capacity of the positive and negative electrode materials, another method is to increase the battery's operating voltage. This means the battery will be affected by the electrolyte at higher operating voltages. High-voltage performance also presents new technical requirements.
The electrolyte plays a crucial role in conducting electrons between the positive and negative electrodes of a lithium-ion battery, ensuring its high voltage and high specific energy. Electrolytes are generally formulated under specific conditions and in specific proportions using high-purity organic solvents, lithium salts, and necessary additives.
High-energy-density electrolyte: 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 of batteries.
Organic liquid electrolytes: Carbonate organic liquids are good solvents for lithium salts, with an oxidation potential of 4.7V and a reduction potential of approximately 1.0V (all voltage values in this article are relative to the potential of lithium). Furthermore, carbonates have relatively low viscosity and low activation energy for lithium-ion migration. Therefore, the most commonly used electrolytes are carbonates and their mixtures, including PC, EC, DEC, DMC, EMC, etc.
Liquid electrolyte: The choice of electrolyte has a significant impact on the performance of lithium-ion batteries. It must be chemically stable, especially resistant to decomposition at higher potentials and temperatures, and possess high ionic conductivity (>10⁻³ S/cm). Furthermore, it must be inert to the anode and cathode materials and must not corrode them. Due to the high charge and discharge potentials of lithium-ion batteries and the chemically active lithium embedded in the negative electrode material, organic compounds must be used instead of water as the electrolyte.
Ionic liquids: In recent years, due to their high oxidation potential (approximately 5.3), room-temperature ionic liquids (such as 1M LiTFSI/EMI-TFSI, EMIBF4, BMIBF4, etc.) have been considered as potential alternatives to lithium-ion battery electrolytes. They also possess advantages such as low vapor pressure, better thermal stability, non-toxicity, high boiling point, and high lithium salt solubility. However, the high viscosity of ionic liquids reduces the mobility of lithium ions.
Safe electrolytes: The safety of lithium-ion batteries is crucial in the event of combustion or even explosion. Firstly, the battery itself is flammable. Therefore, overcharging, over-discharging, short circuits, and excessively high external temperatures can all lead to safety accidents. Thus, flame retardants are an important area of research in safe electrolytes.
Solid electrolytes: Directly using metallic lithium as the anode material offers high reversible capacity, with a theoretical capacity as high as 3862 mAh·g⁻¹, more than ten times that of graphite, and at a lower cost. It is considered the most attractive anode material for next-generation lithium-ion batteries, but it produces dendritic lithium. Using a solid electrolyte as the ion conductor can suppress the growth of dendritic lithium, allowing metallic lithium to be used as the anode material.
