lithium-ion battery

A lithium-ion battery is a rechargeable electrochemical cell in which lithium ions move between a negative electrode (anode) and positive electrode (cathode) through an electrolyte, releasing or absorbing electrical energy as they travel. Unlike primary batteries, these cells can be charged and discharged hundreds to thousands of times before degradation becomes severe. The anode is typically graphite, the cathode is a lithium metal oxide (commonly lithium iron phosphate, LiCoO2, or NCA variants), and the electrolyte is an organic solvent carrying dissolved lithium salts.

Industrial applications depend heavily on cell chemistry and format. Cylindrical cells (18650, 21700 sizes) pack into tool batteries and backup power systems. Prismatic cells with flat form factors suit laptops and stationary storage. Pouch cells offer weight savings for electric vehicles and aerospace. Energy density ranges from roughly 150 Wh/kg for iron phosphate chemistry to 250 Wh/kg for nickel-cobalt blends. Temperature performance, cycle life, and cost vary dramatically: LFP cells cost less and tolerate cold and charge abuse, while NCA and NCM chemistries deliver higher energy but demand thermal management and careful charging profiles.

Failure modes and field reality

Thermal runaway, uncontrolled exothermic reaction, occurs when internal resistance causes heat, the separator (a microporous film) melts, electrodes touch internally, and temperature accelerates past 200 degrees Celsius. This happens through external short circuits, overcharging beyond the cell voltage limit (typically 4.2 to 4.3 volts), mechanical puncture, or manufacturing defects. Once started, thermal runaway cannot be stopped; the cell vents flammable electrolyte gas and heat. Battery management systems (BMS) monitor cell voltage and temperature to prevent these states, but no system catches every fault.

Cycle fade and calendar fade both reduce capacity over time. Cycle fade results from structural changes in the electrode materials and growth of resistive films (solid electrolyte interphase, or SEI) on the anode each time lithium ions are extracted and reinserted. Calendar fade occurs even on the shelf: parasitic chemical reactions continue slowly, especially at elevated temperature or high state of charge. Industrial equipment deployed in hot climates or kept charged 24/7 suffers accelerated loss of usable capacity.

The term "lithium-ion" distinguishes these cells from older lithium metal primaries (non-rechargeable, containing actual metallic lithium) and from other rechargeable chemistries like nickel-metal hydride or lead-acid. The emphasis on ions reflects the electrochemistry: no bulk lithium metal is present during normal operation; instead, Li+ cations in solution and solid phases carry the charge. This design eliminates dendrite growth that plagued early lithium metal rechargeable attempts and makes the cells safer to handle, though not risk-free. Cost, energy density, and the ability to be manufactured in many formats have made lithium-ion the dominant rechargeable battery type in industrial and consumer equipment since the early 2000s.