Lithium batteries are the most common type of rechargeable battery used in electronics today.

They are known for high energy density and good cycle life and etc. These qualities make them ideal for use in portable electronics like phones, laptops, and tablets.

To achieve these characteristics, lithium batteries have to contain a special kind of electrolyte which is able to withstand repeated charges without degrading or leaking out. Since this electrolyte is not visible to the naked eye, it’s important to understand how it works so that you can prevent any problems from occurring!

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

1. The ion conductivity is high over a wide temperature range and the number of lithium ion migrations is large to reduce the concentration polarization of the battery during charge and discharge.
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 undergo significant side reactions at the poles, and to meet the singularity of electrode reactions in the electrochemical process.
4. When used instead of the diaphragm, it must also have good mechanical properties and processability.
5. Low price and cost.
6. Good safety, high flash point or no burning.
7. It is non-toxic and will not cause harm to the environment.

Electrolytes are generally formulated from

1.high-purity organic solutions, C3H4O3, C4H6O3,C3H6O3, PF5.etc.

2.electrolyte lithium salts:

3.necessary additives

4.other raw materials

Above all under certain conditions and in certain proportions.

Different types of batteries rely on different types of chemical reactions and different electrolytes.

For example,

Lithium hexafluorophosphate (LiPF6) is a lithium salt solution which is the most common electrolyte in lithium batteries. Potassium hydroxide is the electrolyte in common household alkaline batteries. A lead-acid battery usually uses sulfuric acid to create the intended reaction. Zinc-air batteries rely on oxidizing zinc with oxygen for the reaction.

There are three kinds of lithium ion battery electrolyte. According to the presence state of electrolytes, 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

solid-liquid composite electrolytes are gel electrolytes composed of solid polymer and liquid electrolytes.

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The following are detailed explanation among those five kinds of  lithium ion battery electrolytes.

Organic liquid electrolyte: The electrolyte obtained by dissolving the lithium salt electrolyte in a polar aprotic organic solvent

advantages:
1.Organic liquid electrolyte has good electrochemical stability,
2.Organic liquid electrolyte has low freezing point and high boiling point
3.Organic liquid electrolyte can be used in a wide temperature range.

disadvantages:
1.Organic solvents  of organic liquid electrolyte have a small dielectric constant
2.Organic solvents  of organic liquid electrolyte have a high viscosity
3.Organic liquid electrolyte has poor ability to dissolve inorganic salt electrolytes
4.Organic liquid electrolyte has low electrical conductivity
5.Organic liquid electrolyte are particularly sensitive to trace amounts of water.
6.Organic liquid lithium batteries are prone to leakage.
7.The product must use a sturdy metal shell, the shell model and size are fixed which lacks of flexibility.
8.Poor safety because of the flammablity of organic solvents. Thus the protection measures for the battery must be very perfect.

Room temperature ionic liquid electrolyte: A functional material or medium composed of specific cations and anions that are liquid at or near room temperature.

advantages:

1.Room temperature ionic liquid electrolyte has outstanding advantages such as high conductivity
2.Room temperature ionic liquid electrolyte has low vapor pressure
3.Room temperature ionic liquid electrolyte has wide liquid range
4.Room temperature ionic liquid electrolyte has good chemical and electrochemical stability
5.Room temperature ionic liquid electrolyte has no pollution
6.Room temperature ionic liquid electrolyte  is easy to recovery
7.Perfect safety:
Room temperature molten salt is used as an electrolyte for lithium-ion batteries to improve the safety of batteries at high power densities and completely eliminate the safety hazards of batteries.

Gel electrolyte: Liquid plasticizers such as PC, EC, etc. are attracted to the polymer matrix to obtain a solid-liquid composite gel electrolyte.

advantages: this ternary electrolyte composed of polymer compounds, lithium salts and polar organic solvents has the properties of both solid electrolytes and liquid electrolytes.

Inorganic solid electrolytes: Solid materials with high ion conductivity. The inorganic solid electrolytes used in all-solid lithium-ion batteries are divided into glass electrolytes and ceramic electrolytes.

Advantages:

1.Solid electrolytes can play the role of electrolytes and it can replace the diaphragm in the battery.
2.No leakage problem
3.The battery can be miniaturized and miniaturized although the number of lithium ion migrations in this type of material is large

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The influence of lithium-ion battery electrolytes on the macroscopic electrochemical properties of batteries includes the following aspects：

• 1.Impact on battery capacity

• 2.Impact on battery internal resistance and magnification charge and discharge performance

• 3.Impact on battery operating temperature range

• 4.Impact on battery storage and cycle life

• 5.Impact on battery safety

• 6.Impacton battery self-discharge performance

• 7.Impact on battery overcharging and over-discharging

Let’s break them down one by one

1.Impact on battery capacity

Firstly, 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 large extent.

That is,

Because the embedding, de-lithium process and circulation process of the electrode material are always the process of interaction with the electrolyte. This interaction has an important impact on the interface condition of the electrode material and the changes in the internal structure.

Secondly, in 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 embedded and removed, there are also a large number of side reactions.

Such as

1. the oxidation and reduction decomposition of electrolytes on the surface of the positive and negative electrodes,
2. the surface passivation of electrode active substances,
3. the high interface impedance between the electrode and the electrolyte interface….

These factors affect the embedding and de-lithium capacity of the electrode material.

That is why some electrolyte systems can make the electrode material exhibit excellent embedding and de-lithium capacity, while some electrolyte systems are very destructive to the electrode material.

2.Impact on battery internal resistance and magnification charge and discharge performance

What is the internal resistance of lithium ion batteries?

The internal resistance refers to the resistance of current when it passes through the battery. It includes the ohmic internal resistance and the polarization resistance of the electrode during the electrochemical process. For lithium-ion batteries, it should include the interface resistance between the electrode and electrolyte.

Thus, the sum of ohmic internal resistance, electrode/electrolyte interface resistance and polarization internal resistance is the total internal resistance of lithium-ion batteries.

It is an important indicator to measure the performance of chemical power supplies and directly affects the battery’s operating voltage, operating current, output energy and power.

Why do electrolyte have impact on battery internal resistance?

The ohmic internal resistance of the battery is mainly due to the conductivity of the electrolyte, and it should also include the resistance of the electrode material and the diaphragm.

The conductive mechanism of the electrolyte part is ion conductivity, and the resistance during the conductive process is usually much greater than that of the electronic conductive part.

The resistance of the interface between the electrode and the electrolyte is of great significance in lithium-ion batteries.

The greater the resistance of lithium ions when passing through the interface, the higher the internal resistance of the battery. Under normal circumstances, the interface resistance is significantly higher than the ohm internal resistance.

Why do electrolyte have impact on magnification charge and discharge performance?

The definition of  discharge and charge rate:

Magnification charge and discharge performance is an important indicator to measure the capacity retention capacity of lithium-ion batteries under fast charge and discharge conditions.

Reasons of electrolyte have impact on magnification charge and discharge performance:

The magnification charge and discharge performance of the battery are determined by

1.the mobility of lithium ions in the electrode material,

2.the conductivity of the electrolyte,

3.the mobility of lithium ions at the electrode or electrolyte phase 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 phase, among all environmental factors, temperature has the most obvious impact on battery performance.

1. Under low temperature conditions, the rate of electrode reaction decreases, and even the reaction terminates, and the performance of the battery decreases significantly, or even cannot be used normally.
2. When the temperature is raised, the electrode reaction intensifies, but the side reactions at the electrode or electrolyte phase interface are also intensified at the same time. 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 electrode reaction without obvious side reactions.

For example

1. the operating temperature range of liquid lithium-ion batteries is usually -10-45℃;
2. the minimum operating temperature is generally not lower than -20℃,
3. the maximum operating temperature generally does not exceed 60℃.

For liquid electrolytes, the main way to broaden their operating temperature range is

1.to expand the liquid range of the electrolyte

2.improve the conductivity of the electrolyte under low temperature conditions

3.improve the stability of the electrolyte under high temperature conditions.

For solid electrolytes, to broaden their operating temperature range,

1. to try to increase the conductivity of the electrolyte at room temperature or even low temperature,
2. reduce the interface impedance between it and the electrode material.

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4.Impact on battery storage and cycle life

For storage life:

The aging of lithium-ion batteries during long-term storage is the key to affecting the storage performance of batteries. 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.

the main reasons:

The corrosion of the electrode collector fluid

The loss of electrochemical activity of the electrode active substance from the collector fluid

The nature of the electrolyte is closely related to the corrosion of the collector fluid and the stability of the electrode material in it. Therefore, the electrolyte to a large extent affects and even determines the battery storage life.

For cycle life:

The cycle life is an important indicator for evaluating the merits of secondary batteries. It is generally measured by the number of cycles when the capacity of the battery is reduced to a certain value.

There are many factors that affect the cycle life of lithium-ion batteries, including

1.the stability of the electrode material,

2.the stability of the electrolyte,

3.the charge and discharge rate,

4.the charge and discharge depth

5.temperature. For lithium-ion batteries

6.correct use and maintenance,

the main further reasons for the short cycle life of the battery are the following：

1. The active specific surface of the electrode active substance during the charging and discharging process continues to decrease, the true current density of the battery during operation increases, and the internal resistance of the battery gradually increases.
2. The active substance of the electrode collector falls off or transfers, losing its due electrochemical activity
3. During the operation of the battery, certain materials age or corrode in the electrolyte
4. The diaphragm is damaged or partially closed
5. Due to the oxidation or reduction reaction of the electrolyte at the electrode interface, impurities in the electrolyte increase

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 nature of the electrolyte.

5. Impact on battery safety

Lithium-ion batteries replace the dissolution and deposition of lithium metal in traditional lithium secondary batteries with a lithium storage mechanism inside the lattice, eliminating the growth of dendrite lithium on the negative electrode surface and reducing the chance of short circuit of the battery, but this does not fundamentally eliminate the safety risks of the battery.

For example, liquid lithium-ion batteries will also deposit lithium metal on the negative electrode surface under overcharging conditions, while the positive electrode surface will oxidize and decompose the electrolyte under high potential conditions, and a series of unsafe side reactions will occur inside the battery.

In addition, the large amount of heat generated by the battery under the condition of high current charge and discharge cannot be lost in time, resulting in a rapid increase in the temperature of the battery, which will also bring significant safety problems to the battery.

We all know that the main factors affecting the safety of lithium-ion batteries are as following:

1. the stability of the electrode material,
2. the composition of the electrolyte,
3. the manufacturing process
4. operating conditions of the battery itself

However,

the root cause of the safety problem of liquid lithium-ion batteries is the volatility and high flammability of the organic liquid electrolyte itself.

Therefore,  to fundamentally eliminate the safety risks of batteries, the flammability of organic solvents must be eliminated.

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6. Impact on battery self-discharge performance

The self-discharge rate of lithium-ion batteries is determined by:

1. the type and structure of the electrode material,
2. the interface properties of the electrode/electrolyte,
3. the composition of the electrolyte
4. the production process of the battery.

The main reasons for the self-discharge of lithium-ion batteries:

1.  Self-discharge of the negative electrode.

The self-discharge of the negative electrode is mainly due to the lithium from the negative electrode prolapse or entering the electrolyte in the form of Li+.

The rate of which depends on the surface condition and surface catalytic activity of the negative electrode. The surface condition of the negative electrode is obviously affected by the electrolyte, so optimizing the composition of the electrolyte can reduce the self-discharge density of the battery.

1.  Self-discharge of the positive electrode:

It refers to the lithium ions in the electrolyte embedded in the lattice of the positive electrode material, which causes the positive electrode to self-discharge. Its rate depends on the kinetic factors in the Li+embedded positive electrode, mainly the interface properties of the positive electrode or the electrolyte.

In addition, the appearance of impurities in the electrolyte is also an important reason for the self-discharge of the battery.

This is because the oxidation potential of impurities is generally lower than the positive electrode potential of lithium-ion batteries, which is easy to oxidize on the surface of the positive electrode, and the oxide will be reduced at the negative electrode, thereby continuously consuming the active substances of the positive and negative electrode materials, causing self-discharge.

Therefore, lithium-ion batteries have high requirements for the composition and purity of electrolytes.

7.Effect on battery overcharge and over discharge

Since the lithium-ion battery electrolyte cannot provide anti-overcharge or over-discharge protection when the battery is working 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 a higher voltage, there is often a significant capacity mismatch.

When the battery pack is charged, there will always be individual batteries overcharged,

When discharging, there will also be individual batteries over-discharged.

This aspect causes irreversible damage to battery performance and affects the life of the battery pack; at the same time, it also brings obvious safety risks to the battery.

Why lithium battery electrolyte has impact on overcharging and over-discharging?

The modification and modification of electrolytes is an important way to prevent battery overcharge and discharge.

There is an inherent overcharge and discharge protection mechanism inside the organic liquid electrolyte.

For example, some addiction are added to the electrolyte. Under overcharge conditions, the substance oxidizes at the positive electrode, and the oxidant is reduced to the surface of the negative electrode

Thereby avoiding the continuous increase in battery voltage.

(1) It should be a good ionic conductor and electronic insulator, so that ion (Li+) transport can be facile and self-discharge can be kept to a minimum;

(2) It should have a wide electrochemical window, so that electrolyte degradation would not occur within the range of the working potentials of both the cathode and the anode;

(3) It should also be inert to other cell components such as cell separators, electrode substrates, and cell packaging materials;

(4) It should be thermally stable, for liquid electrolytes both the melting and boiling points should be well outside the operation temperatures;

(5) It must have low toxicity and successfully meet also other measures of limited environmental hazard;

(6) It must be based on sustainable chemistries, meaning that the elements are abundant and the synthesis processes are as low impact as possible, and

(7) it must carry as low total cost, materials and production, as possible

What Is the Battery Electrolyte Made Of?

The electrolyte inside most consumer batteries (and many industrial ones) is made up of two parts: water and potassium hydroxide (KOH). When these two components are combined with each other, they form a solution that allows electricity to flow freely between electrodes within a battery cell—this is known as “electrolysis” or “electrolytic action.”

Is Lithium Battery Electrolyte Safe?

Yes.

Lithium batteries are safer than most other types of battery because they contain no heavy metals or toxic chemicals. They also have a longer lifespan than other batteries, so you won’t need to replace them as often.Lithium batteries are considered safe, but they do have a few challenges.Lithium batteries are very stable and have a high energy density. They can be used in small devices like phones or large ones like cars, and they come in different shapes and sizes. Lithium batteries also have an impressive shelf life of up to 20 years when stored at room temperature. The most common lithium battery uses a lithium-ion (Li-ion) chemistry, which means that lithium ions move from the negative electrode to the positive electrode through an electrolyte solution within the battery cell.

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