Lithium-ion car batteries are the core components of modern electric vehicles. Understanding their working principles will help us better understand electric vehicle technology.
1. Basic Structure
- Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, electrolytes and separators.
- The positive electrode material is usually a lithium compound, such as lithium cobalt oxide (LiCoO₂), lithium nickel manganese cobalt oxide (LiNMC), etc. These positive electrode materials can provide storage locations for lithium ions. Among them, lithium cobalt oxide is widely used in early lithium-ion batteries and has a high energy density, but the cost is relatively high; while lithium nickel manganese cobalt oxide is a positive electrode material with good comprehensive performance and is used by many modern electric vehicle batteries.
- Graphite is generally used as the negative electrode material. Graphite has a layered structure that can accommodate the insertion and extraction of lithium ions. During the charging process, lithium ions migrate from the positive electrode and embed into the graphite interlayer of the negative electrode; during discharge, the opposite is true, and lithium ions are extracted from the graphite negative electrode and return to the positive electrode.
- The electrolyte is a medium that can conduct lithium ions, usually a mixture of organic solvents and lithium salts. It plays the role of ion transport inside the battery, allowing lithium ions to move freely between the positive and negative electrodes. Common lithium salts include lithium hexafluorophosphate (LiPF₆), and organic solvents include ethylene carbonate (EC), dimethyl carbonate (DMC), etc.
- The separator is located between the positive and negative electrodes and is a thin film with a microporous structure. Its main function is to prevent the positive and negative electrodes from directly contacting each other and causing a short circuit, while allowing lithium ions to shuttle between the positive and negative electrodes through the micropores of the separator.
2. Charging process
1. When the lithium-ion car battery is connected to an external power source for charging:
- The electric energy provided by the external power source drives the lithium ions to escape from the positive electrode material. Under the action of the electric field force, the lithium ions in the positive electrode material migrate to the negative electrode through the electrolyte.
- The lithium ions pass through the separator and enter the graphite interlayer of the negative electrode, where they obtain electrons. This process allows lithium ions to be embedded in the lattice structure of the negative electrode, realizing the conversion of electrical energy to chemical energy.
- As charging proceeds, the lithium ions in the positive electrode material continue to decrease, while the lithium ions in the negative electrode continue to increase until the battery is fully charged, at which time a chemical equilibrium state is established inside the battery.
III. Discharge process
1. During the driving of an electric vehicle, the battery begins to discharge:
- The lithium ions in the negative electrode lose electrons, escape from the graphite layer, and become free lithium ions.
- These lithium ions migrate to the positive electrode through the electrolyte, pass through the diaphragm and are re-embedded in the positive electrode material.
- In the process of lithium ions moving from the negative electrode to the positive electrode, electrons flow from the negative electrode to the positive electrode through the external circuit, thereby providing electrical energy for the motor and other equipment of the electric vehicle, realizing the conversion of chemical energy to electrical energy.
IV. The role of the battery management system
- The operation of lithium-ion automotive batteries is also inseparable from the battery management system (BMS).
- The BMS is responsible for monitoring various parameters of the battery, such as voltage, current, temperature, etc. By monitoring these parameters in real time, BMS can ensure that the battery operates in a safe and efficient state. For example, when the battery temperature is too high, BMS can take measures to dissipate heat to prevent the battery from overheating and damage; when the battery voltage is too low, BMS will limit the discharge of the battery to avoid irreversible damage to the battery caused by overdischarge.
- BMS can also balance the power of each battery cell in the battery pack. Due to manufacturing processes and other reasons, there may be certain differences in the performance of each single cell in the battery pack, which will lead to a decrease in the overall performance of the battery pack. BMS transfers part of the power of the battery cell with high power to the battery cell with low power through the balancing circuit, thereby ensuring the consistency of the power of each single cell in the battery pack and improving the overall performance and service life of the battery pack.
