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How Lithium-Ion Batteries Work?

Introduction to the working principle of lithium-ion batteries

Lithium-ion batteries use carbon materials as negative electrodes and lithium-containing compounds as positive electrodes. Because there is no metallic lithium in the battery, only with lithium ions, so it is called a lithium-ion battery.

Lithium-ion batteries refer to the general term for batteries with lithium-ion intercalation compounds as positive electrode materials.

The charging and discharging process of lithium-ion batteries is the process of intercalation and deintercalation of lithium ions.

During the intercalation and deintercalation process of lithium ions, it is accompanied by the intercalation and deintercalation of electrons equivalent to lithium ions. The positive electrode is customarily represented by intercalation or deintercalation, while the negative electrode is represented by insertion or deintercalation.

During the charge and discharge process, lithium ions are intercalated/deintercalated and inserted/deintercalated between the positive and negative electrodes.

Main materials of Lithium-ion Batteries

Lithium-ion batteries were first successfully developed by Sony Corporation of Japan in 1990.

It embeds lithium ions in carbon (petroleum coke and graphite) to form a negative electrode (traditional lithium batteries use lithium or lithium alloys as negative electrodes). LixCoO2 is commonly used as the positive electrode material, and LixNiO2 and LixMnO4 are also used. The electrolyte is LiPF6+diethylene carbonate (EC)+dimethyl carbonate (DMC).

Petroleum coke and graphite are non-toxic and abundant resources as negative electrode materials.

Lithium ions are embedded in carbon, which overcomes the high activity of lithium and solves the safety problems of traditional lithium batteries.

The positive electrode LixCo02 can reach a higher level in terms of charge and discharge performance and life, meanwhile reduce the cost.

In short, the comprehensive performance of the lithium-ion battery is improved.

How lithium-ion batteries work

Charging and Discharging Characteristics of Li-ion Batteries

1. Discharge of lithium-ion battery

1.1 Termination discharge voltage of lithium ion battery

The rated voltage of lithium-ion batteries is 3.6V (some products are 3.7V). The termination discharge voltage is 2.5~2.75V. The battery manufacturer gives the working voltage range or the termination discharge voltage, whose parameter is slightly different.

The termination discharge voltage of the battery should not be less than 2.5Vxn (n is the number of batteries connected in series).

Continued discharge after the voltage is lower than the termination discharge voltage is called over-discharge. Over-discharge will shorten the life of the battery. And in severe cases, it will cause the battery to fail.

When the battery is not in use, the battery should be charged to 20% of the capacity, and then stored in a moisture-proof package.

And the voltage should be checked once every 3~6 months, and charged to ensure that the battery voltage is within the range of the safe voltage value (above 3V).

1.2 Discharge current

Li-ion batteries are not suitable for high current discharge. When the excessive current is used to discharge, a higher temperature will be generated inside it, which will consume energy and reduce the discharge time.

If there is no protective element in the battery, the battery will be damaged due to overheating.

Therefore, the battery manufacturer gives the maximum discharge current. Do not exceed the maximum discharge current given in the Product Characteristics table during use.

1.3 Discharge temperature

The discharge curves of lithium-ion batteries are different at different temperatures.

At different temperatures, the discharge voltage and discharge time of lithium-ion batteries are also different.

The battery should be discharged (working) within the temperature range of -20~60°C.

1.4 Precautions for discharging

For batteries, normal use is the process of discharging. Lithium-ion battery discharge needs to pay attention to several points:

(1) The discharge current cannot be too large.

Excessive current can cause the battery to heat up internally, possibly causing permanent damage.

As can be seen from the figure below, the greater the battery discharge current, the smaller the discharge capacity and the faster the voltage drop.

Comparison of discharge current and discharge capacity

(2) Never over-discharge.

Lithium-ion batteries store electrical energy by a reversible electrochemical change. Excessive discharge will cause this electrochemical change to undergo an irreversible reaction, so lithium-ion batteries can’t be over-discharged.

Once the discharge voltage is lower than 2.7V, it may cause the battery to be scrapped.

However, a protection circuit is generally installed inside the battery. Before the voltage is low enough to damage the battery, the protection circuit will work and stop discharging.

2. Charge of the Li-ion battery

When charging a lithium-ion battery, lithium ions are generated on the positive electrode of the battery.

And the generated lithium ions move to the negative electrode through the electrolyte. The carbon material used as the negative electrode has a layered structure with many micropores inside, and the lithium ions that reach the negative electrode are embedded in the micropores of the carbon layer.

The more lithium ions intercalated, the higher the charge capacity.

Similarly, when the battery is discharged (the process in which people use the battery), the lithium ions embedded in the carbon layer of the negative electrode come out and move back to the positive electrode.

The more lithium ions that go back to the positive electrode, the higher the discharge capacity.

Generally, when charging a lithium-ion battery, the greater the current, the faster the charging, and the more the heat generated by the battery.

Moreover, if an excessive current is used to charge, the capacity is not easy to be fully charged. This is because the electrochemical reaction inside the battery takes time, just like people pouring beer, pouring too fast is easy to produce foam, but it is not easy to fill the beer.

When using lithium-ion batteries, it must be noted that the batteries will enter a dormant state after being left for a period of time.

At this time, its capacitance is lower than the normal value, and the use time is also shortened.

However, lithium-ion batteries are easy to activate, as long as 3 to 5 normal charge and discharge cycles can activate the battery and restore normal capacity.

Due to the characteristics of the lithium-ion battery itself, it has almost no memory effect. Therefore, the new lithium-ion battery does not require special methods and equipment during the activation process.

2.1 Charging equipment

A dedicated lithium-ion battery charger should be used to charge the lithium-ion battery. Lithium-ion battery charging adopts the “constant current/constant voltage” method. First, the constant current charge is changed to constant voltage charge when it is close to the termination voltage.

It should be noted that the charger for charging nickel-cadmium batteries (for charging 3 nickel-cadmium batteries) cannot be used to charge lithium-ion batteries (rated voltage is 3.6V). However, due to the different charging methods, it is easy to cause overcharging.

2.2 Charging voltage

The termination charging voltage when fully charged is related to the negative electrode material of the battery.

With the coke of 4.1V, it is generally called 41V lithium-ion battery.

While the 4.2V graphite one is called 42V lithium-ion battery.

When charging, a 4.1V battery cannot be charged with a 4.2V charger, otherwise there will be a risk of overcharging. Because 4.1V and 4.2V chargers use different ICs.

Lithium-ion batteries have high requirements for charging, and it is equipped with a sophisticated charging circuit to ensure the safety of charging. The tolerance of the termination charging voltage accuracy is ±1% of the rated value.

For example, the tolerance for charging a 4.2V lithium-ion battery is ±0.042V. Over-voltage charging will cause permanent damage to the lithium-ion battery.

2.3 Charging current

The charging current of lithium-ion batteries should be determined according to the recommendations of the battery manufacturer, and a current-limiting circuit is required to avoid overcurrent (overheating).

2.4 Charging temperature

When charging lithium-ion batteries, the ambient temperature cannot exceed the temperature range listed in the product characteristics table. The battery should be charged within the temperature range of 0~45°C, away from high temperature (above 60°C) and low temperature (below -20°C) environments.

If a lithium-ion battery is overcharged, overdischarged or overcurrented during charging or discharging, it will damage the battery or reduce its service life.

Therefore, various protection components and protection circuits composed of protection ICs have been developed. It is installed in the battery or battery pack, so that the battery is fully protected.

However, overcharge and overdischarge should be prevented as much as possible during the use of lithium-ion batteries.

For example, the battery used by a small robot should be separated from the charger in time when it is about to be fully charged. When the discharge depth is shallow, the cycle life will be significantly improved. When in use, try to charge the battery before the robot prompts that the battery is low, and stop using it after the prompt signal appears, although there is still a part of the remaining power available for use when this signal appears.

Manufacturing process

Lithium-ion battery manufacturing process generally has the following types.

① Pulping

Use special solvents and binders to mix with powdery positive and negative active materials respectively, and make slurry-like positive and negative materials after stirring evenly.

② Coating film

The positive and negative electrode slurries are evenly coated on the surface of the metal foil by an automatic coating machine. And the positive and negative electrode sheets are automatically cut after automatic drying.

③ Assembly

According to the sequence of positive electrode sheet – separator – negative electrode sheet – separator from top to bottom, the assembly process of the lithium-ion battery is completed through winding and injection of electrolyte, sealing, welding of positive and negative tabs, and finally the finished lithium-ion battery is made.

④ Formation

Place the finished lithium-ion battery in a test cabinet for charge and discharge tests. Screen out qualified finished lithium-ion batteries.

For more information about the production in detail, please check the following for another post:

Lithium-ion Battery Manufacturing Process

Storage of lithium-ion batteries

The storage method of lithium-ion batteries needs to be summarized in combination with its working principle.

Lithium-ion batteries have a very low self-discharge rate and can be stored for more than 3 years. What’s more, most of the capacity can be recovered.

The effect will be better if stored under refrigeration. Therefore, it is a good way to store lithium-ion batteries in low temperature places.

If the voltage of the lithium-ion battery is below 3.6V and needs to be stored for a long time, it will cause the battery to be over-discharged and destroy the internal structure of the battery. What’s worse, it will reduce the service life of the battery. 

Therefore, long-term storage of lithium-ion batteries should be recharged every 3 to 6 months. It is advisable to charge to a voltage of 3.8~3.9V (the best storage voltage is about 3.85V), but it is not suitable to be fully charged.

The application temperature range of lithium-ion batteries is very wide, and it can still be used outdoors in the north in winter, but the capacity will be decreased a lot. 

If it returns to room temperature, the capacity can be restored.

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