Meaning of lithium-ion battery electrolyte？

◆What are aqueous electrolytes? 

◆Introduction to Solid Electrolytes

The electrolyte, an indispensable component of lithium-ion batteries, plays a crucial role in the battery's charge-discharge cycles.

It is not only responsible for the efficient transport of lithium ions and the conduction of current, but also possesses electronic insulation properties to effectively prevent direct electron flow between the positive and negative electrodes. Figuratively speaking, the electrolyte is like the "blood" inside a lithium-ion battery, ensuring the connectivity between the positive and negative electrode materials, thereby guaranteeing the smooth progress of the entire charge-discharge process.

An ideal electrolyte for a lithium-ion battery should meet the following five requirements：

With the ever-increasing demands for battery energy density and power density, battery technology is developing rapidly, and electrode materials have made tremendous progress. In contrast, the development of electrolyte systems has lagged behind. Currently, the development of lithium-ion battery electrolytes can be broadly classified into three types: non-aqueous solvent electrolytes, aqueous electrolytes, and solid-state electrolytes.

Non-aqueous solvent electrolyte

 

Non-aqueous solvent electrolytes in lithium-ion batteries refer to electrolyte systems that do not contain water, mainly composed of solvents, solutes (usually lithium salts), and additives. These non-aqueous solvents are typically organic solvents, rather than aqueous solvents, to avoid electrolysis of water or adverse reactions with electrode materials. Lithium salts are the primary carriers for lithium-ion transport, solvents serve as the dissolution, dispersion, and support for lithium salts, and additives primarily function to improve the electrochemical performance or safety of lithium-ion batteries.

Commercially available electrolytes (i.e., liquid electrolytes) used in lithium-ion batteries are primarily composed of one or more lithium salts dissolved in two or more organic solvents; electrolytes composed of a single solvent are very rare. The reason for using multiple solvents is that real-world batteries have different, even contradictory, requirements that are difficult to meet using a single solvent. For example, electrolytes may require high fluidity while also having a high dielectric constant; therefore, solvents with different physicochemical properties are often used in combination, exhibiting different characteristics simultaneously. Furthermore, lithium salts are generally not used simultaneously because the selection of lithium salts is limited, and their advantages are not easily apparent.

Ideal organic solvents should possess the following key properties: First, they need a high dielectric constant to ensure good dissolution of lithium salts; second, they should have a low melting point and a high boiling point to broaden the operating temperature range of the electrolyte; third, low viscosity helps promote efficient migration of lithium ions in the medium; and finally, these solvents should be inexpensive and have low toxicity (ideally non-toxic). Carbonate compounds, as one of the earliest and most widely used organic solvents in the lithium-ion battery industry, occupy a crucial position in the field of battery electrolytes.

Currently, this type of solvent mainly includes two structural forms: cyclic and chain. The table below summarizes the relevant physical parameters of several commonly used non-aqueous solvents, electrolytes, and organic solvents.

Currently, alkyl carbonate solvents are widely used in electrolytes. These solvents possess good oxidation resistance and exhibit excellent stability under high voltage conditions. Cyclic carbonates, such as ethylene carbonate and propylene carbonate, are known for their high dielectric constants, meaning they can dissolve lithium salts more effectively; however, due to strong intermolecular forces, these solvents have high viscosity, which slows down the movement of lithium ions within them. In contrast, chain carbonates, such as dimethyl carbonate and diethyl carbonate, while having lower viscosity, also have relatively low dielectric constants, resulting in relatively poor dissolution efficiency for lithium salts. Therefore, to prepare solution systems with superior ionic conductivity, different types of solvents are often mixed, such as PC+DEC or EC+DMC combinations. Lithium salts, as the source of lithium ions in the electrolyte, play a major role in lithium-ion ion transport during the charging and discharging process of lithium-ion batteries. Their performance directly affects many aspects of lithium-ion batteries, including energy density, power density, operating voltage range, cycle life, and safety. Currently, in laboratory research and industrial practice, lithium salts with large anionic radii and high redox stability are typically selected. Based on their chemical composition, lithium salts can be broadly classified into two categories: inorganic lithium salts and organic lithium salts. Several inorganic lithium salts have been developed, including LiPF6, LiClO4, LIBF, and LIASF. In contrast, commonly used organic lithium salts in lithium-ion batteries are formulated by adding electron-withdrawing groups to the anions of these inorganic lithium salts, such as lithium dioxalato-borate (LiBOB), lithium difluorooxalato-borate ([iODFB]), lithium difluorosulfonylimide (LiFSI), and lithium ditrifluoromethylsulfonylimide (LTFSI).The table below shows the relevant physicochemical properties of several commonly used lithium salts in lithium-ion batteries.

Additives are substances added to the electrolyte in low concentrations (usually no more than 10% by mass) that have specific functions and can significantly improve the electrochemical characteristics of the battery. Based on their functions, these additives can be broadly classified into several categories: film-forming additives, flame retardants, and additives to prevent overcharging. In addition, there are additives used to enhance conductivity, optimize performance under low-temperature conditions, or control trace amounts and HF concentrations in the electrolyte solution.