A battery stores energy in a chemical form, which it can later convert and release as electrical energy. This process of energy delivery, known as discharge, is governed by controlled chemical reactions within the battery cell. When a battery is connected to an external power source, the entire chemical process is forcibly reversed. This article explains the physical and chemical transformations that occur internally when external electrical power is applied to reverse the battery’s natural energy flow, restoring its stored chemical potential.
How Batteries Store and Release Energy
A rechargeable battery consists of three main components: a positive electrode, a negative electrode, and an electrolyte. The electrolyte is a medium that facilitates the movement of ions but prevents electrons from passing directly between the electrodes. Chemical potential energy is stored when lithium ions are held within the structure of the negative electrode.
When the battery is connected to a device, it begins the discharge process, converting its stored chemical energy into electrical energy. Lithium atoms within the negative electrode oxidize, releasing lithium ions and electrons. The electrons flow out through the external circuit to power the device, while the lithium ions move through the electrolyte toward the positive electrode to maintain charge balance. This movement continues until the chemical potential difference between the two electrodes is equalized, at which point the battery is considered discharged.
The Electrochemical Reversal of Charging
Charging uses an external power source to force ions and electrons to move opposite to their natural flow. The charger applies a voltage higher than the battery’s own voltage, overriding the chemical drive toward equilibrium. This external electrical current provides the necessary energy to reverse the chemical reactions that occurred during discharge.
During this reversal, lithium ions are compelled to move out of the positive electrode. These ions travel across the electrolyte back toward the negative electrode. Simultaneously, electrons are forced through the external circuit and into the negative electrode, where they recombine with the incoming lithium ions.
The lithium ions then re-enter the porous structure of the negative electrode material through a process called intercalation. This forced insertion stores the ions within the electrode’s lattice, restoring the battery to a high-energy, chemically unstable state. The charging process is complete when the maximum amount of lithium ions has been successfully re-intercalated into the negative electrode, restoring the battery’s chemical potential.
Energy Conversion and Heat Production
The charging process is not entirely efficient; not all electrical energy supplied is converted into stored chemical energy. A portion of the input energy is wasted as heat due to the internal resistance of the battery components. This resistance, known as ohmic resistance, is present in the electrodes, the electrolyte, and the connections, converting electrical current into heat.
Heat is also generated by the electrochemical reactions themselves. Reversible heat, called entropic heat, is associated with the entropy changes of the materials as ions are inserted and removed during charging. There is also irreversible heat from kinetic losses and structural changes within the materials, collectively known as polarization.
The accumulation of this heat affects the battery’s performance and lifespan. A Battery Management System (BMS) continuously monitors the temperature and voltage of the cells. The BMS regulates the rate of charge to keep the temperature within safe limits, preventing excessive heat build-up that could lead to thermal runaway.
Causes of Capacity Degradation
The chemical processes of charging and discharging are not perfectly reversible, leading to a reduction in the battery’s ability to hold a charge over time. A primary cause is the slow, continuous growth of the Solid Electrolyte Interphase (SEI) layer on the surface of the negative electrode.
The SEI layer forms when components of the electrolyte decompose and react on the electrode surface. While a thin, stable SEI layer is necessary to protect the electrode, its continuous thickening consumes lithium ions that would otherwise be available to shuttle between the electrodes. This loss of mobile lithium ions reduces the battery’s energy storage capacity.
Another form of degradation is lithium plating, which occurs when lithium ions are unable to intercalate into the negative electrode quickly enough, forming metallic lithium deposits on the electrode surface instead. This is more likely during fast charging or when charging in cold temperatures, which slow down the intercalation process. This metallic lithium is chemically inactive and permanently removes those lithium atoms from the energy storage cycle, reducing the battery’s maximum capacity.