A lithium-ion battery works by reversibly moving lithium ions between two electrodes while electrons travel through an external electrical circuit. During discharge, lithium contained within the negative electrode is oxidised, releasing lithium ions and electrons. The lithium ions move through the electrolyte towards the positive electrode, while the electrons cannot normally pass through the electrolyte and therefore travel through the external circuit, where they can perform useful electrical work.
During charging, an external electrical source forces the process in the opposite direction: lithium ions move back towards the negative electrode and electrons are driven through the external electrical circuit towards that electrode. The battery therefore separates ionic transport inside the cell from electronic transport through the external circuit, creating a controllable flow of electrical energy.
In a conventional lithium-ion cell, the negative electrode is often graphite and the positive electrode is a lithium-containing transition-metal compound. During discharge, lithium atoms associated with the negative electrode release electrons and become lithium ions. The ions migrate through the electrolyte and separator into the positive electrode, where they are incorporated into its structure. The electrons instead travel through the external circuit because the separator and electrolyte prevent ordinary electronic conduction between the electrodes. This separation is fundamental to battery operation. During charging, the applied voltage reverses the electrochemical driving forces and causes lithium ions to leave the positive electrode and return to the negative electrode, while the charging circuit supplies the corresponding electrons.
The term intercalation describes an important feature of many lithium-ion cells. Lithium ions can be inserted into or removed from the crystal or layered structure of electrode materials without completely destroying the host structure. Graphite, for example, can accommodate lithium between its carbon layers during charging. Many positive-electrode materials similarly accommodate and release lithium ions through changes in their chemical state and structure. This reversibility allows a lithium-ion cell to cycle repeatedly. The precise electrochemical reactions depend on the materials used, so it is inaccurate to assume that every lithium-ion battery has exactly the same reactions. Different cathode and anode chemistries change voltage, capacity, rate capability, stability and degradation behaviour.
A battery's voltage arises from the difference in electrochemical potential between the two electrodes. When the cell is connected to a load, this potential difference drives electrons through the external circuit. The amount of energy available depends on both voltage and the quantity of charge that can be transferred. Electrical energy is commonly expressed in watt-hours, while electrical charge capacity is expressed in ampere-hours.
The two are related through voltage, although the relationship is not simply constant throughout a real discharge because cell voltage changes with state of charge, temperature and load. Internal resistance also causes voltage to drop when current flows, so the voltage measured under load can be lower than the cell's open-circuit voltage.
Charging a lithium-ion battery requires controlled electrical conditions. A common charging strategy uses constant-current charging followed by constant-voltage charging. Initially, the charger supplies approximately constant current while the cell voltage rises. Once a specified voltage limit is reached, the charger holds approximately constant voltage while current gradually decreases. The precise limits depend on cell chemistry and manufacturer specifications. Charging outside appropriate voltage, temperature and current ranges can accelerate degradation or create hazardous conditions. Electric-vehicle charging systems therefore coordinate the charger, vehicle electronics and battery-management system rather than simply applying an arbitrary voltage directly to the cells.
The battery-management system is particularly important in multi-cell batteries. Individual cells do not behave identically, and small differences in manufacturing, temperature, state of charge and ageing can cause their voltages to diverge. The management system monitors individual cell or cell-group voltages, pack current and temperatures and can control contactors and charging limits. Cell balancing attempts to reduce state-of-charge differences, especially near the upper end of charging. The system can also restrict charging or power output when temperatures or voltages approach defined limits. In a vehicle, thermal management can heat the battery in cold conditions or remove heat during high-power operation and charging.
Battery performance changes with operating conditions. Low temperatures can increase internal resistance and reduce power capability, while charging a cold lithium-ion cell under inappropriate conditions can cause undesirable lithium deposition on the negative electrode. High temperatures generally accelerate chemical degradation and can increase safety risks. High current operation can produce heat through internal resistance and other losses. Repeated operation at very high or very low states of charge can also influence long-term degradation depending on chemistry and design. Consequently, manufacturers establish recommended operating windows that deliberately avoid the extreme electrochemical conditions that would maximise stress.
Lithium-ion battery operation is therefore an interaction between electrochemistry, electrical engineering, thermal management and control software. Charging stores energy by driving lithium ions back into the negative electrode, while discharging allows them to return towards the positive electrode and produces electron flow through an external circuit. The process is reversible but not perfectly lossless, which is why batteries heat up and gradually degrade. Real-world performance depends on electrode chemistry, cell construction, temperature, current, state of charge, charging strategy and age. Understanding these dependencies explains why battery capacity, charging speed, range and longevity cannot be represented by a single fixed number independent of operating conditions.
