Requirements for lithium-ion battery electrolytes and their impact on battery performance

Basic requirements for lithium-ion battery electrolytes
The electrolyte used in lithium-ion batteries should meet the following basic requirements. These are factors that must be considered to measure the performance of the electrolyte. They are also important prerequisites for achieving high performance, low internal resistance, low price, long life and safety of lithium-ion batteries.

1. The ionic conductivity is high in a wide temperature range and the lithium ion migration number is large to reduce concentration polarization of the battery during the charge and discharge process.
2. Good thermal stability to ensure that the battery operates within a suitable temperature range.

3. The electrochemical window is wide, and it is best to have an electrochemical stability window of 0-5V to ensure that the electrolyte does not have significant side reactions at the two poles and to satisfy the singleness of the electrode reaction in the electrochemical process.
4. When used instead of a diaphragm, it must have good mechanical properties and processability.

5. Low price and cost.
6. Good safety, high flash point or non-flammable.
7. No toxic pollution and no harm to the environment.

2. Classification of lithium-ion battery electrolytes
According to the existence state of the electrolyte, lithium battery electrolytes can be divided into liquid electrolytes, solid electrolytes and solid-liquid composite electrolytes. Liquid electrolytes include organic liquid electrolytes and room temperature ionic liquid electrolytes, solid electrolytes include solid polymer electrolytes and inorganic solid electrolytes, and solid-liquid composite electrolytes are gel electrolytes composed of solid polymers and liquid electrolytes.
Different types of lithium-ion battery electrolytes have their own advantages, as well as their own shortcomings and deficiencies.

1. Organic liquid electrolyte
An electrolyte obtained by dissolving a lithium salt electrolyte in a polar aprotic organic solvent. This type of electrolyte has good electrochemical stability, low freezing point, high boiling point, and can be used in a wide temperature range. However, organic solvents have small dielectric constants, high viscosity, poor ability to dissolve inorganic salt electrolytes, low conductivity, and are particularly sensitive to trace amounts of water. Organic liquid lithium batteries are prone to leakage. The product must use a solid metal casing, with fixed model and size, and lacks flexibility. The flammability of organic solvents results in poor safety, and the battery protection measures must be perfect.

2. Room temperature ionic liquid electrolyte
Functional materials or media composed of specific cations and anions that are liquid at room temperature or near room temperature. They have outstanding characteristics such as high conductivity, low vapor pressure, wide liquid range, good chemical and electrochemical stability, no pollution, and easy recycling. advantage. Room-temperature molten salt is used as an electrolyte for lithium-ion batteries to improve the safety of batteries under high power density and completely eliminate battery safety hazards, making it possible for lithium-ion batteries to be used in large power systems such as electric vehicles or under other special conditions.

3. Solid polymer electrolyte
It has the advantages of non-flammability, low reactivity with electrode materials, and good flexibility. It can overcome the above shortcomings of liquid lithium-ion batteries, allows the volume change of electrode materials during discharge, and is more resistant to impact, vibration, and deformation than liquid electrolytes. , easy to process and shape, and the battery can be made into different shapes according to different needs.

4. Gel electrolyte
Liquid plasticizers such as PC, EC, etc. are introduced into the polymer matrix to obtain a solid-liquid composite gel electrolyte. This ternary electrolyte composed of polymer compounds, lithium salts and polar organic solvents has both solid electrolytes and Properties of liquid electrolytes.

5. Inorganic solid electrolyte
Solid materials with high ionic conductivity. Inorganic solid electrolytes used in all-solid-state lithium-ion batteries are divided into glass electrolytes and ceramic electrolytes. Solid electrolytes not only function as electrolytes, but also can replace the separator in the battery. Therefore, inorganic solid electrolytes are used The prepared all-solid-state lithium battery does not have to worry about leakage, and the battery can be miniaturized and miniaturized. Although the lithium ion migration number is large in this type of material, the conductivity of the electrolyte itself is much smaller than that of the liquid electrolyte. When this type of material is used in lithium-ion batteries, the interface impedance between it and the electrolyte material is high. In addition, inorganic solid electrolytes are highly brittle, and lithium-ion batteries using them as electrolytes have poor earthquake resistance.

3. Effect of electrolyte on battery performance
The influence of lithium-ion battery electrolyte on the macroscopic electrochemical performance of the battery includes the following aspects:

1. Impact on battery capacity
Although the electrode material is a prerequisite for determining the specific capacity of lithium-ion batteries, the electrolyte also affects the reversible capacity of the electrode material to a great extent. This is because the insertion, delithiation and cycling processes of the electrode material always interact with the electrolyte. process, this interaction has an important impact on the interface conditions and internal structure changes of electrode materials.

During the working process of lithium-ion batteries, in addition to the redox reactions that occur at the positive and negative electrodes when lithium ions are inserted and removed, there are also a large number of side reactions, such as the oxidation and reduction decomposition of electrolytes on the surfaces of the positive and negative electrodes, and the electrode active materials. Surface passivation, high interface impedance between the electrode and the electrolyte interface, etc. These factors affect the lithium insertion and removal capacity of the electrode material to varying degrees. Therefore, some electrolyte systems can make the electrode material show excellent lithium insertion and removal capacity. Some electrolyte systems are very destructive to electrode materials.

2. Impact on battery internal resistance and rate charge and discharge performance
Internal resistance refers to the resistance encountered when current passes through the battery. It includes ohmic internal resistance and the polarization resistance exhibited by the electrode during the electrochemical process. For lithium-ion batteries, it should also include the interface resistance between the electrode/electrolyte. For this reason, the sum of ohmic internal resistance, electrode/electrolyte interface resistance and polarization internal resistance is the total internal resistance of the lithium-ion battery. It is an important indicator for measuring the performance of chemical power sources and directly affects the battery's operating voltage, operating current, Output energy and power, etc.

The ohmic internal resistance of a battery is mainly derived from the conductivity of the electrolyte, and should also include the resistance of the electrode materials and separators. The conductive mechanism of the electrolyte part is ionic conduction, and the resistance encountered during the conductive process is usually much greater than the resistance encountered by the electronic conductive part. The interface resistance between electrodes and electrolytes is of great significance in lithium-ion batteries. The greater the resistance when lithium ions cross the interface, the higher the internal resistance of the battery. Typically, the interface resistance is significantly higher than the ohmic internal resistance.

In lithium-ion batteries, the intercalation and delamination of lithium ions are performed at the interface between the electrode and the electrolyte. The ease of the reaction, that is, the degree of electrochemical polarization, is not only related to the nature of the electrode material. , is also related to factors such as the interface condition between the electrolyte and the electrode material, the presence of lithium ions in the electrolyte, and the interaction between lithium ions and the electrolyte. In this sense, the properties of the electrolyte system also affect the polarization resistance of the battery to a certain extent.

Rate charge and discharge performance is an important indicator to measure the capacity retention ability of lithium-ion batteries under rapid charge and discharge conditions. The rate charge and discharge performance of the battery depends on the mobility of lithium ions in the electrode material, the conductivity of the electrolyte, and the mobility of lithium ions at the electrode/electrolyte interface. The latter two are closely related to the composition and properties of the electrolyte.

3. Impact on battery operating temperature range
Due to the large temperature dependence of the electrode reaction that occurs at the interface between the electrode and the electrolyte, among all environmental factors, temperature has the most obvious impact on battery performance. Under low temperature conditions, the rate of electrode reaction decreases or even terminates, and the performance of the battery decreases significantly, or even cannot be used normally. When the temperature is raised, the electrode reaction intensifies, but the side reactions at the electrode/electrolyte interface are also intensified. These side reactions are often very destructive to the battery, and the performance of the battery is affected. Therefore, the optimal temperature for battery operation should be the temperature that is most conducive to the electrode reaction without obvious side reactions. The operating temperature range of liquid lithium-ion batteries is usually -10-45°C; the minimum operating temperature is generally not lower than -20°C. , the maximum operating temperature generally does not exceed 60°C.

For lithium-ion batteries with liquid electrolytes, the main way to widen their operating temperature range is to expand the liquid range of the electrolyte, improve the conductivity of the electrolyte under low temperature conditions, and improve the stability under high temperature conditions. For solid electrolytes, in order to broaden their operating temperature range, we must try to increase the conductivity of the electrolyte at room temperature or even low temperature, and reduce the interface impedance between it and the electrode material.

4. Impact on battery storage and cycle life
The aging of lithium-ion batteries during long-term storage is the key to affecting battery storage performance. Even if a commercial lithium-ion battery is never used, its storage life is only about 3 years. There are many reasons for battery aging. Among them, the corrosion of electrode current collectors and the loss of electrochemical activity of electrode active materials from the current collectors are the main reasons. The properties of the electrolyte are closely related to the corrosion of current collectors and the stability of electrode materials in them. Relevant, therefore, the electrolyte largely affects or even determines battery storage life.

Cycle life is an important indicator for evaluating the quality of secondary batteries. It is generally measured by the number of cycles when the battery capacity is reduced to a specific value. There are many factors that affect the cycle life of lithium-ion batteries, including the stability of electrode materials, the stability of electrolytes, charge and discharge rates, charge and discharge depth and temperature, etc. For lithium-ion batteries, in addition to correct use and maintenance, the main reasons for the short battery cycle life are the following:

A. The active specific surface area of ​​the electrode active material continues to decrease during the charge and discharge process, the actual current density of the battery increases during operation, and the internal resistance of the battery gradually increases.
B. The active material of the electrode current collector falls off or transfers, losing its electrochemical activity. C. During the operation of the battery, some materials age or corrode in the electrolyte. D. The diaphragm is damaged or partially closed.
E. Impurities in the electrolyte due to the oxidation or reduction reaction of the electrolyte at the electrode interface.

Due to the influence of the above factors, the normal service life of lithium-ion batteries is currently about 2-3 years, and most of the above factors have a certain relationship with the properties of the electrolyte.

5. Impact on battery safety
Lithium-ion batteries use an internal lithium storage mechanism in the crystal lattice to replace the dissolution and deposition of metallic lithium in traditional lithium secondary batteries. This eliminates the growth of dendrite lithium on the surface of the negative electrode and reduces the chance of battery short circuit, but this does not fundamentally eliminate it. Battery safety hazards. For example, when a liquid lithium-ion battery is overcharged, metal lithium will also be deposited on the negative electrode surface, and the electrolyte will oxidize and decompose under high potential conditions on the positive electrode surface, leading to a series of unsafe side reactions inside the battery. In addition, the large amount of heat generated by the battery during high current charging and discharging cannot be dissipated in time, causing the temperature of the battery to rise rapidly, which will also bring significant safety issues to the battery.

Although the stability of the electrode material, the composition of the electrolyte, the manufacturing process and usage conditions of the battery itself are all major factors affecting the safety of lithium-ion batteries. However, the root cause of the safety problem of liquid lithium-ion batteries is still the volatility and high flammability of the organic liquid electrolyte itself. Therefore, research on the safety of liquid lithium-ion batteries mainly focuses on the reaction between electrode materials and electrolytes and their thermal effects. These studies have deepened people's understanding of a series of exothermic reactions and combustion mechanisms that occur inside lithium-ion batteries. However, to fundamentally eliminate battery safety hazards, the flammability of organic solvents must be eliminated, and electrolyte systems that are more secure or do not burn at all must be developed, especially for large-scale, high-power-density lithium-ion batteries.

6. Impact on battery self-discharge performance
The self-discharge rate of lithium-ion batteries depends on the type and structure of the electrode material, the interface properties of the electrode/electrolyte, the composition of the electrolyte and the battery production process. The main causes of self-discharge of lithium-ion batteries include the following aspects:

A. Self-discharge of the negative electrode. The self-discharge of the negative electrode mainly originates from the lithium in the negative electrode coming out of or entering the electrolyte in the form of Li+, and its rate depends on the surface condition and surface catalytic activity of the negative electrode. The surface condition of the negative electrode is significantly affected by the electrolyte, so optimizing the composition of the electrolyte can reduce the self-discharge rate of the battery.
B. The self-discharge of the positive electrode means that the lithium ions in the electrolyte are embedded into the crystal lattice of the positive electrode material, thereby triggering the self-discharge of the positive electrode. Its rate depends on the kinetic factors of Li+ embedded in the cathode, and it mainly has the interface properties of the positive electrode.

In addition, the appearance of impurities in the electrolyte is also an important reason for battery self-discharge. This is because the oxidation potential of impurities is generally lower than the positive electrode potential of lithium-ion batteries, and they are easily oxidized on the surface of the positive electrode, and their oxides will be reduced on the negative electrode, thus continuously Consume the active material of the positive and negative electrode materials, causing self-discharge. Therefore, lithium-ion batteries have very high requirements on the composition and purity of the electrolyte.

7. Impact on battery overcharge and overdischarge behavior
Since the lithium-ion battery electrolyte cannot provide protection against overcharge or over-discharge when the battery is operating normally, the battery's ability to resist overcharge and over-discharge is very poor. Under some practical application conditions, when multiple lithium-ion batteries are used in series to obtain higher voltages, there is often an obvious capacity mismatch. When the battery pack is charged, individual cells will always be overcharged, and there will also be overcharging when discharging. The over-discharge phenomenon of individual batteries causes irreversible damage to battery performance and affects the life of the battery pack; at the same time, it also brings obvious safety hazards to the battery.
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