From a structural perspective, lithium-ion batteries are mainly composed of the following five parts:
1. Cathode material: Transition metal oxides or polyanionic compounds with layered or spinel structures that have high electrode potential and stable structure and lithium intercalation capability, such as lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, ternary materials, etc.
2. Negative electrode materials: Layered graphite, elemental metals and metal oxides with potential close to the lithium potential, stable structure and high lithium storage capacity, such as graphite, centrophase carbon microspheres, lithium titanate, etc.
3. Electrolyte: An organic solvent containing lithium electrolyte salts to supply lithium ions. Lithium electrolyte salts include LiPF6, LiClO4, and LiBF4. The organic solvent is mainly composed of one or more of the following: diethyl carbonate (DEC), propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC).
4. Barrier: A polyolefin microporous membrane placed between the positive and negative electrodes to prevent direct contact between them and to allow Li+ ions to pass through, such as polyethylene (PE), polypropylene (PP), or their composite membranes, such as a PP/PE/PP three-layer barrier.
5. Casing: Battery encapsulation, mainly consisting of aluminum casing, cover plate, tabs, insulating sheets, etc.
The positive terminal cap is located on the raised part at the top of the battery. It is connected to the positive terminal inside and marks the positive terminal of the battery.
The battery casing is made of nickel-plated steel. Its purpose is to prevent the active materials inside the battery from directly contacting the outside world. In addition, it is connected to the negative electrode and serves as a current collector for the negative electrode.
Battery labels are printed plastic sleeves used to identify the type, model, and other information of batteries.
A separator is a material that separates the positive and negative electrode active materials and retains the electrolyte; an electrolyte is a lithium perchlorate organic electrolyte used to transfer ions in a lithium manganese primary battery.
In a lithium-manganese primary battery, the positive electrode material is manganese dioxide.
The batteries in our everyday mobile phones and laptops are actually lithium-ion batteries. Lithium-based batteries are divided into lithium batteries and lithium-ion batteries, but they are commonly referred to as lithium batteries. True lithium batteries are rarely used in everyday electronic products due to their high risk. Today, we'll use images to explain the working principle and structure of lithium batteries in detail, giving you a comprehensive understanding of them.
I. Lithium-ion battery structure
The components of a lithium-ion battery are as follows:
(1) Positive electrode - The active material is generally lithium manganese oxide or lithium cobalt oxide, or lithium nickel cobalt manganese oxide. Electric bicycles generally use lithium nickel cobalt manganese oxide (commonly known as ternary) or ternary + a small amount of lithium manganese oxide. Pure lithium manganese oxide and lithium iron phosphate have gradually faded out due to their large size, poor performance or high cost. Electrolytic aluminum foil with a thickness of 10-20 micrometers is used as the conductive electrode fluid.
(2) Separator - a kind of specially shaped polymer film with a microporous structure that allows lithium ions to pass through freely, while electrons cannot pass through.
(3) Negative electrode - The active material is graphite or carbon with a similar graphite structure, and the conductive current collector uses electrolytic copper foil with a thickness of 7-15 micrometers.
(4) Organic electrolytes are carbonate solvents containing lithium hexafluorophosphate, while gel electrolytes are used for polymers.
(5) Battery casing - divided into steel casing (square type is rarely used), aluminum casing, nickel-plated iron casing (used for cylindrical batteries), aluminum-plastic film (soft packaging), etc., as well as the battery cap, which is also the positive and negative terminals of the battery.
II. Working principle of lithium batteries
The working principle of a lithium battery will be explained in three main parts: the charging process, the discharging process, and the battery protection board.
1. Lithium battery charging process
Working principle of lithium batteries
The positive electrode of the battery is generated by lithium ions. The generated lithium ions "jump" from the positive electrode into the electrolyte, "crawl" through the electrolyte and the winding holes on the separator, and move to the negative electrode, where they combine with the electrons that have already reached the negative electrode through the external circuit.
●The reaction occurring at the positive electrode is: LiCoO2 ==charging== Li1-xCoO2 + Xli+ + Xe (electrons)
●The reaction occurring at the negative electrode is: 6C + XLi + Xe =====LixC6
During charging, Li+ ions are released from the positive electrode LiCoO2 and enter the electrolyte. Under the influence of the external electric field added by the charger, they move towards the negative electrode and enter the negative electrode composed of graphite or coke C in turn, forming LiC compounds at the negative electrode.
2. Lithium battery discharge process
During discharge, electrons and Li+ ions move simultaneously, in the same direction but along different paths. Electrons travel from the negative electrode to the positive electrode through the external circuit; lithium ions (Li+) "jump" from the negative electrode into the electrolyte, "crawl" through the winding holes in the separator, and "swim" to the positive electrode, where they combine with the electrons that have already arrived. The battery capacity we usually refer to is the discharge capacity.
3. Battery protection board
As the name suggests, a battery protection board is an integrated circuit board that primarily protects rechargeable batteries (generally lithium batteries). Lithium batteries (rechargeable) require protection because the materials used in lithium batteries dictate that they cannot be overcharged, over-discharged, over-currented, short-circuited, or charged and discharged at excessively high temperatures. Therefore, lithium batteries always have a protection board and a current fuse.
The diagram below shows the solar panel protection circuit. PTC: Positive Temperature Coefficient Thermistor; NTC: Negative Temperature Coefficient Thermistor, whose resistance decreases when the ambient temperature rises, allowing the electrical or charging equipment to react promptly and control internal interruption to stop charging and discharging; U1 is the circuit protection chip, and U2 are two reverse-connected MOSFET switches.
Under normal conditions, both CO and DO of the solar panel U1 output high voltage, and both MOSFETs are in the open state, allowing the battery to charge and discharge freely.
Simplified principle of charging protection
Overcharge protection: When U1 detects that the battery voltage has reached the overcharge protection threshold, the CO pin outputs a low level, the MOSFET switch 2 changes from on to off, the charging circuit is turned off, and the charger can no longer charge the battery, thus achieving overcharge protection.
Over-discharge protection: During battery discharge, when U1 detects that the battery voltage is lower than the over-discharge protection threshold, the DO pin changes from high level to low level, and MOSFET switch 1 is turned off, preventing the battery from discharging further. Under the over-discharge protection state, the battery voltage cannot be lowered further, requiring the protection circuit to have extremely low current and the control circuit to enter low power consumption mode.
Overcurrent protection: Under normal circumstances, the battery discharges to the load, and the current passes through two series-connected MOSFET switches. The VM pin detects the voltage drop between the two MOSFETs as U. If the load causes an abnormal U for some reason, increasing the circuit current, when U exceeds a certain value, the DO pin changes from high voltage to low voltage, MOSFET switch 1 closes, thereby reducing the discharge circuit current to zero and achieving overcurrent protection.
I. Introduction to Lithium-ion Batteries
Lithium-ion batteries are secondary batteries that use Li+ intercalation compounds as both the positive and negative electrodes.
The positive electrode uses lithium compounds LiCoO2, LiXNiO2, LiXMnO2, LiFePO4, and ternary composite materials.
The negative electrode uses a lithium-carbon interlayer compound - LiXC6
During charging, Li+ ions repeatedly insert and remove themselves between the two electrodes, a process figuratively called a "rocking chair battery."
During charging, Li+ ions are released from the positive electrode, pass through the electrolyte, and are inserted into the negative electrode, which is in a lithium-rich state. The opposite occurs during discharging.
II. Lithium-ion Battery Structure
Positive electrode: active material, conductive agent, solvent, binder, matrix;
Anode: Active material (graphite, MCMB, CMS), solvent, binder, matrix, separator, electrolyte;
Housing hardware: steel housing, aluminum housing, cover plate, electrode lugs, insulating sheet, insulating tape;
1. Lithium-ion battery structure - positive electrode
The electrodes that receive electrons from the external circuit during battery discharge undergo a reduction reaction. These are typically electrodes with high potentials, such as lithium cobalt oxide and lithium manganese oxide electrodes in lithium-ion batteries.
2. Lithium-ion battery structure - negative electrode
The electrodes that transfer electrons from the external circuit during battery discharge undergo oxidation. These are typically electrodes with low potential, such as the graphite electrodes in lithium-ion batteries.
3. Lithium-ion battery structure - separator
A separator is a device placed between two electrodes to act as a barrier between them. It prevents direct contact between the active materials on the electrodes, thus avoiding a short circuit inside the battery. However, the separator must still allow charged ions to pass through, forming a pathway.
Diaphragm requirements:
High ion permeability
Appropriate mechanical strength
It is an insulator
It does not react with electrolytes or battery cells.
