Introduction to the characteristics of lithium ion batteries

Lithium battery is a new type of battery successfully developed in the 20th century, which can be understood as a battery containing lithium elements (including lithium metal, lithium alloy, lithium ion, lithium polymer). And it can be divided into lithium metal battery (very little production and use) and lithium ion battery (nowadays used in large quantities).

Because of its high specific energy, high battery voltage, wide operating temperature range, long storage life and other advantages, it has been widely used in military and civilian small appliances, such as cell phones, portable computers, video cameras, cameras, etc., partially replacing the traditional battery.

Structure and principle of lithium ion battery

Main components

● Cathode

The active material mainly refers to lithium cobaltate, lithium manganate, lithium iron phosphate, lithium nickelate, lithium nickel cobalt manganate, etc. The conductive collector fluid is generally used to achieve a thickness of 10 – 20 microns of aluminum foil.

● Separator

A special plastic film that allows lithium ions to pass through, but is an insulator of electrons. There is also a class of solid inorganic diaphragm, such as aluminum oxide diaphragm coating, which is a kind of solid inorganic diaphragm.

● Anode

Lithium ion battery anode mainly refers to graphite, lithium titanate, or carbon materials that resemble the structure of graphite, and the conductive collector generally uses copper foil with a thickness of 7-15 microns.

● Electrolyte

Generally organic systems, such as lithium hexafluorophosphate dissolved in carbonate solvents, and some polymer batteries use gel electrolyte.

● Battery shell

Mainly divided into hard shell (steel shell, aluminum shell, nickel-plated iron shell, etc.) and soft pack (aluminum-plastic film).

When the battery is charged, lithium ions are removed from the cathode and embedded in the anode, and vice versa when discharged. This requires an electrode to be in the lithium-embedded state before assembly.

Various carbon materials including natural graphite, synthetic graphite, carbon fiber and metal oxides are selected for the cathode material.

● The electrolyte is a mixed solvent system with alkyl carbonate such as ethylene carbonate (EC), propylene carbonate (PC) and low viscosity diethyl carbonate (DEC) of LiPF6.

● Diaphragm using polyene micro porous membrane such as PE, PP or their composite film, especially PP/PE/PP triple diaphragm not only has a lower melting point, but also has a high strength of puncture resistance, which plays a thermal insurance role.

● The housing is made of steel or aluminum, and the cover body assembly has an explosion-proof power failure function.

Basic working principle

When charging the battery, the lithium-containing compound at the cathode has lithium ions released, and lithium ions move through the electrolyte to the anode. The carbon material of the anode has a layered structure, which has many micropores. Lithium ions reaching the anode are embedded in the microholes in the carbon layer, and the more lithium ions are embedded, the higher the charging capacity.

When the battery is discharged, lithium ions embedded in the carbon layer of the anode are released and moved back to the cathode. The more lithium ions that return to the cathode, the higher the discharge capacity. Battery capacity is usually referred to as discharge capacity.

Charging and discharging mechanism

The charging process of Li-ion batteries is divided into two stages: the constant current charging stage and the constant voltage current charging stage.

Overcharging and discharging lithium-ion batteries can cause permanent damage to the cathode and anode terminals. Overcharge causes lithium ions to be released during the charging process. Overcharging causes some of these lithium ions to no longer be released.

The charging and discharging method that maintains the best performance is shallow charging and discharging. Generally 60% DOD is 2-4 times the cycle life under 100% DOD.

Main performance indicators

● Battery capacity

There is a difference between the rated capacity and the actual battery capacity.
The rated capacity of the battery is the power that should be provided when the battery is discharged to the termination voltage at an ambient temperature of 15℃~25℃ at a rate of 5h, expressed as C5.

The actual capacity of the battery is the actual power discharged from the battery under certain discharge conditions, which is mainly affected by the discharge multiplier and temperature. Therefore, strictly speaking, the battery capacity should specify the charging and discharging conditions.

● Battery internal resistance

The internal resistance of the battery is the resistance to the flow of current through the interior of the battery when it is in operation. A large value of the internal resistance of the battery will lead to a lower working voltage and a shorter discharge time of the battery. The size of the internal resistance is mainly influenced by the material of the battery, the manufacturing process, the structure of the battery and other factors. The internal resistance of the battery is an important parameter to measure the performance of the battery.

● Voltage

When the voltage of the lithium ion battery voltage is in a non-operating state, or when there is no current in the circuit, it is called an open circuit voltage.

Working voltage, also known as terminal voltage, is the potential difference between battery terminals when the battery is in working condition, or when there is current flowing through the circuit.

In the working state of the battery discharge, when the current flows through the battery, it does not need to overcome the resistance caused by the internal resistance of the battery, so the working voltage is always lower than the open circuit voltage, and the opposite when charging.

● Discharge platform time

The discharge platform time is the discharge time to a certain voltage with the battery fully charged.

Because the working voltage of certain appliances using lithium-ion batteries has voltage requirements, if they are lower than the required value, they will not work. Therefore, the discharge platform is one of the important criteria to measure the performance of the battery.

● Charge/discharge rate

The charge/discharge multiplier is the current value required when the battery discharges its rated capacity within the specified time. 1C is numerically equal to the rated capacity of the battery, usually expressed by the letter C. If the nominal rated capacity of the battery is 10Ah, then 10A is 1C and 100A is 10C.

● Self-discharge rate

Self-discharge rate, also known as charge retention capacity, refers to the ability of the battery to retain the charge stored in the battery under certain conditions in the open circuit state. It is mainly influenced by the manufacturing process, materials and storage conditions of the battery. It is an important parameter for measuring battery performance.

● Efficiency

Charging efficiency is a measure of the degree to which the electrical energy consumed by the battery during charging is converted into the chemical energy that the battery can store. It is mainly affected by the battery process, formulation and working temperature of the battery. Generally, the higher the ambient temperature, the lower the charging efficiency.

Discharge efficiency is the ratio of the actual power discharged to the terminal voltage to the rated capacity of the battery under certain discharge conditions, which is mainly affected by the discharge rate, ambient temperature, internal resistance and other factors. In general, the higher the discharge rate, the lower the discharge efficiency. The lower the temperature, the lower the discharge efficiency.

● Cycle life

Lithium ion battery life cycle is the number of times the battery has been charged and discharged under a certain charge/discharge regime when the battery capacity drops to a specified value.

The main classification

● Electrolyte materials: liquid lithium batteries and polymer lithium batteries.

● Charging method: non-rechargeable and rechargeable types.

● Appearance: there are square lithium batteries (such as commonly used cell phone batteries) and columnar batteries (such as 18650, 18500).

● Outer pack materials: aluminum shell lithium battery, steel shell lithium battery, soft pack battery.

● Anode and cathode materials: lithium cobaltate, lithium manganate, lithium iron phosphate, disposable lithium manganese dioxide

In the process of continuous research and development, batteries are used in many fields. With the rapid development of science and technology, the market has become more demanding. Battery technology is also constantly evolving to adapt to the times.