Introduction To Basic Knowledge Of Lithium Battery Cells

1, Composition of lithium battery cells

1. Positive electrode material: Lithium compounds such as lithium cobalt oxide (LiCoO ₂), lithium manganese oxide (LiMn ₂ O ₄), lithium iron phosphate (LiFePO ₄), and ternary materials (LiNixCoyMnzO ₂) are commonly used. Different positive electrode materials have different performance characteristics. For example, lithium cobalt oxide has a high energy density but relatively low safety, while lithium iron phosphate has high safety and long lifespan but relatively low energy density.

2. Negative electrode material: Generally made of carbon materials such as graphite, its function is to store lithium ions during charging and release lithium ions during discharging. Negative electrode materials also include intermediate phase carbon microspheres, lithium titanate, etc.

3. Diaphragm: A porous polymer film that serves to isolate positive and negative electrodes, prevent short circuits, and allow lithium ions to pass through. The material of the diaphragm is usually a polyolefin porous membrane.

4. Electrolyte: mainly composed of lithium salts (such as lithium hexafluorophosphate LiPF ₆) and organic solvents, responsible for conducting lithium ions between the positive and negative electrodes. The electrolyte can be liquid or gel.

5. Shell: The shell of the battery can be made of steel, aluminum, nickel plated iron, or aluminum-plastic film. The shell also includes a cap for the battery, which serves as the outlet for the positive and negative electrodes.

 

2, The working principle of lithium battery cells

During charging, lithium ions are released from the positive electrode material, pass through the electrolyte through the separator, and embedded into the negative electrode material; During discharge, lithium ions are released from the negative electrode material, pass through the electrolyte through the separator, and return to the positive electrode material, generating current in this process.

 

3, Classification of lithium battery cells

 

1. Classified by appearance: square lithium battery, cylindrical lithium battery, soft pack lithium battery;

2. Classified by outsourcing materials: aluminum shell lithium batteries, steel shell lithium batteries, and soft pack batteries;

3. Classified by positive electrode material: lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), ternary lithium (LiNixCoyMnzO2), lithium iron phosphate (LiFePO4);

4. Classified by electrolyte state: lithium-ion batteries (LIB) and polymer batteries (PLB);

5. Classified by purpose: ordinary batteries and power batteries.

6. Classified by performance characteristics: high-capacity batteries, high rate batteries, high-temperature batteries, low-temperature batteries, etc

 

4, Characteristics of lithium battery cells

1. High energy density: Lithium battery cells can store more energy, allowing lithium batteries to have higher energy output than other types of batteries at the same volume or weight.

2. Long cycle life: After multiple charge and discharge cycles, it can still maintain good performance, generally reaching hundreds or even thousands of cycles.

3. Low self discharge rate: When not in use, the self discharge speed is slow and can maintain power for a long time.

4. Environmental protection: It does not contain heavy metals such as mercury and cadmium, and is relatively environmentally friendly.

 

5, Explanation of Common Terms

1. Capacity

Refers to the amount of electricity that can be obtained from lithium in a battery under certain discharge conditions. The formula for battery capacity is Q=I * t, measured in coulombs. The capacity unit of a battery is specified as Ah (ampere hour) or mAh (milliampere hour), which means that a 1Ah battery can be discharged for 1 hour with a current of 1A when fully charged.

Previously, the battery of Nokia's old phones (such as BL-5C) was usually 500mAh. Nowadays, the battery of smartphones is 800-1900mAh, electric bicycles are usually 10-20Ah, and electric cars are usually 20-200Ah.

2. Charge Rate/Discharge Rate

It represents the amount of current used for charging and discharging, usually calculated as a multiple of the nominal capacity of the battery, commonly referred to as several degrees Celsius. For a battery with a capacity of 1500mAh, 1C is specified as 1500mAh. If discharged at 2C, it is discharged with a current of 3000mA, and charged and discharged at 0.1C, it is charged and discharged with a current of 150mA.

3. Voltage (OCV: Open Circuit Voltage)

The voltage of a battery generally refers to the nominal voltage (also known as the rated voltage) of a lithium battery. The nominal voltage of a regular lithium battery is generally 3.7V, and we also refer to its voltage plateau as 3.7V.

When the battery has a capacity of 20~80%, the voltage is concentrated at around 3.7V (3.6~3.9V), and if the capacity is too high or too low, the voltage changes greatly.

4. Energy/Power

The energy (E) that a battery can release when discharged to a certain standard is measured in Wh (watt hours) or KWh (kilowatt hours), with 1KWh=1 kWh.

E=U*I*t, It is also equal to multiplying the battery voltage by the battery capacity

The formula for power is, P=U*I=E/t, Indicates the amount of energy that can be released per unit time. The unit is W (watts) or KW (kilowatts). A battery with a capacity of 1500mAh typically has a nominal voltage of 3.7V, corresponding to an energy of 5.55Wh.

5. Resistance

Due to the fact that charging and discharging cannot be equivalent to an ideal power source, there is a certain internal resistance. Internal resistance consumes energy, of course, the smaller the internal resistance, the better.

The unit of internal resistance of the battery is milliohms (m Ω).

The internal resistance of a typical battery is composed of ohmic resistance and polarization resistance, and the magnitude of the internal resistance is influenced by the material, manufacturing process, and structure of the battery.

6. Cycle Life

Charging and discharging a battery once is called a cycle, and the cycle life is an important indicator to measure the performance of battery life.

The IEC standard stipulates that mobile phone lithium batteries should be discharged from 0.2C to 3.0V and charged from 1C to 4.2V. After 500 cycles, the battery capacity should be maintained at 60% or more of the initial capacity. That is to say, the cycle life of lithium batteries is 500 times.

According to the national standard, after a cycle life of 300 times, the capacity should be maintained at 70% of the initial capacity. If the battery capacity is less than 60% of the initial capacity, it is generally considered to be scrapped.

7. Depth of Discharge (DOD)

Defined as the percentage of the capacity released by the battery to the rated capacity.

The deeper the discharge depth of lithium batteries, the shorter the battery life.

8. Cut Off Voltage

The termination voltage is divided into charging termination voltage and discharging termination voltage, which means the voltage at which the battery cannot continue to charge or discharge. Continuing to charge or discharge at the termination voltage has a significant impact on the battery's lifespan.

The charging termination voltage of lithium batteries is generally 4.2V, and the discharging termination voltage is 3.0V.

Deep charging or discharging of lithium batteries beyond the termination voltage is strictly prohibited.

9. Self Discharge Rate

The rate at which the capacity of a battery decreases during storage, expressed as a percentage of the decrease in capacity per unit of time.

The self discharge rate of a typical lithium battery is 2% to 9% per month.

10. SOC(State of Charge)

Refers to the percentage of remaining battery power to the total amount of power that can be discharged, ranging from 0 to 100%. Reflect the remaining battery level.

 

6, Naming convention for batteries

Different manufacturers have different naming conventions, but universal batteries all follow a unified standard, and the size of the battery can be determined by its name.

According to IEC 61960, the rules for cylindrical and square batteries are as follows:

1. Cylindrical battery

3 letters followed by 5 numbers.

Three letters, the first letter represents the negative electrode material, I represents the presence of built-in lithium ions, and L represents the lithium metal or lithium alloy electrode; The second letter represents the positive electrode material, C represents cobalt, N represents nickel, M represents manganese, and V represents vanadium; The third letter, R, represents cylindrical shape.

5 digits, the first 2 digits represent diameter and the last 3 digits represent height, all in millimeters.

2. Square battery

3 letters followed by 6 numbers.

Three letters, the first letter represents the negative electrode material, I represents the presence of built-in lithium ions, and L represents the lithium metal or lithium alloy electrode; The second letter represents the positive electrode material, C represents cobalt, N represents nickel, M represents manganese, and V represents vanadium; The third letter P represents a square.

6 digits, the first 2 digits represent thickness, the middle 2 digits represent width, and the last 2 digits represent height (length), all in millimeters.

For example, ICR 18650 is a universal cylindrical battery with a diameter of 18mm and a height of 65mm; ICP 053353 is a square battery with a thickness of 5mm, a width of 33mm, and a height (length) of 53mm.