A lithium-ion battery is a rechargeable electrochemical energy-storage device in which lithium ions move between a negative electrode and a positive electrode through an electrolyte during charging and discharging. It is called lithium-ion because lithium ions are the principal charge-carrying species moving within the cell. Unlike a primary battery, which is normally designed for one discharge cycle, a lithium-ion cell can undergo repeated charge and discharge cycles, although its capacity and performance gradually decline with use and age. Lithium-ion technology is widely used in smartphones, laptops, power tools, electric vehicles, grid storage systems and portable electronics because it combines relatively high energy density, useful power capability, rechargeable operation and comparatively low self-discharge with a wide range of possible chemistries and cell designs.
A lithium-ion cell contains several essential components with different functions. The negative electrode is commonly made from graphite in conventional commercial cells, although other materials can be used. The positive electrode contains a lithium-containing compound, with examples including lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminium oxide and lithium iron phosphate.
An electrolyte provides an ionically conductive medium through which lithium ions can move, while a separator electrically isolates the electrodes and helps prevent an internal short circuit while allowing ionic transport. Current collectors provide conductive pathways between the electrode materials and the external circuit. These components are assembled into cylindrical, prismatic or pouch cells, and multiple cells can be combined into modules and battery packs.
Lithium-ion batteries encompass a family of chemistries rather than one universal chemical formulation. Lithium iron phosphate, commonly abbreviated LFP, generally offers strong thermal stability, long cycle life and lower reliance on nickel and cobalt, while having lower gravimetric energy density than some nickel-rich chemistries. Nickel-manganese-cobalt and nickel-cobalt-aluminium systems can provide high energy density, which is valuable where weight and volume are constrained, but they require careful thermal and charging management. Lithium titanate is another specialised chemistry that can offer very high cycle life and fast charging characteristics at the expense of lower energy density. The electrode materials strongly influence voltage, energy density, power capability, cost, lifespan and thermal behaviour.
The term battery technically refers to an assembly of one or more electrochemical cells, although everyday usage often calls a single cell a battery. In an electric vehicle, many individual cells are electrically connected and packaged into a high-voltage battery pack. The pack includes electrical connections, sensors, contactors, fuses, cooling or heating systems and a battery-management system. The battery-management system monitors cell voltages, temperatures, current and other parameters and can control charging and discharging within safe operating limits.
Cell balancing is used to reduce differences between cells so that individual cells do not become excessively charged or discharged relative to their neighbours. These supporting systems are essential because a high-energy battery is an engineered system rather than simply a collection of chemical cells.
Lithium-ion batteries store energy electrochemically rather than by storing electricity in the same way as a conventional capacitor. During discharge, chemical reactions at the electrodes allow lithium ions to move through the electrolyte while electrons travel through the external electrical circuit, providing useful electrical current. During charging, an external power source drives the electrochemical processes in the opposite direction, restoring energy to the cell. The electrodes are designed to reversibly host lithium ions, allowing repeated cycling. The exact reaction pathways differ between chemistries, but the general architecture separates ionic movement inside the cell from electron movement through the external circuit.
Important characteristics of lithium-ion batteries include nominal voltage, energy capacity, power capability, charge acceptance, cycle life, calendar life, energy density and safety limits. Energy density describes how much energy can be stored for a given mass or volume, whereas power capability describes how rapidly energy can be delivered or accepted. These properties can involve trade-offs. A chemistry optimised for maximum energy density may not have exactly the same characteristics as one optimised for long cycle life or very high charging power. Temperature also has a substantial effect: low temperatures can reduce available power and charging capability, while high temperatures can accelerate degradation and increase safety risks.
Lithium-ion batteries require protection against electrical, thermal and mechanical abuse. Excessive charging, severe over-discharge, physical damage or internal defects can create conditions capable of producing dangerous heating. A phenomenon known as thermal runaway can occur when heat generation exceeds the system's ability to dissipate it and self-reinforcing reactions accelerate temperature rise. Modern battery packs therefore use protective electronics, temperature monitoring, current limits, thermal management and mechanical containment. Safety also depends on cell manufacturing quality, pack design and charging equipment. A properly engineered battery system can be operated safely, but safety is not an intrinsic guarantee provided solely by the phrase lithium-ion.
Lithium-ion technology became commercially significant through the development of rechargeable cells using lithium intercalation materials, culminating in widespread commercial products during the 1990s. Its development transformed portable electronics and subsequently enabled large-scale electric mobility and stationary energy storage. Continued research focuses on improving energy density, charging speed, longevity, safety, cost and sustainability, including developments involving silicon-rich anodes, solid-state electrolytes, sodium-ion alternatives and improved recycling processes. Lithium-ion batteries remain a broad technological family whose properties depend strongly on chemistry, cell construction and battery-management engineering rather than on lithium alone.
