Battery terms, in plain language.

The part of an electrode that actually stores lithium — graphite on the negative side, an oxide or phosphate on the positive. It makes up most of the electrode coating; the rest is binder and conductive additive.

A unit of electric charge: one ampere flowing for one hour. Battery capacity is usually rated in ampere-hours, which measure charge, not energy.

The electrode where oxidation happens. On discharge that is the negative electrode; on charge it is the positive one. Battery makers nonetheless call the negative electrode “the anode” at all times, after its role on discharge.

The rule that a rate constant is proportional to exp(−Ea/RT), so a thermally activated reaction speeds up steeply with temperature. Near room temperature many degradation reactions roughly double their rate for every +10 °C.

A polymer that holds electrode particles to each other and to the foil — typically PVDF for positive electrodes and CMC with SBR for graphite. It stores no charge, so cell designers use as little as the electrode needs.

Degradation that happens with time even when a cell is not cycled — in storage, for example. It is driven mainly by side reactions such as SEI growth and accelerates with temperature.

Compressing a dried electrode coating between rollers to set its thickness, density and porosity. It is one of the electrode-making steps in cell production.

The loss of charge a cell can store and deliver, for example through lithium locked into the SEI or active material that has cracked out of service.

The electrode where reduction happens: the positive electrode on discharge, the negative one on charge. In industry usage, “cathode” means the positive electrode and its material, whatever the cell is doing.

Fine carbon, such as carbon black, added in small amounts to an electrode so that electrons can reach every active particle. Many active materials conduct electrons poorly on their own.

Current divided by nominal capacity, in reciprocal hours. 1C on a 20 Ah cell is 20 A; 2C is 40 A. C-rate lets you compare loads on cells of different sizes.

The thin metal foil that carries electrons between an electrode and the external circuit — usually copper on the negative side and aluminium on the positive side.

The terminal-voltage limit that ends a discharge, or caps a charge — in constant-current, constant-voltage (CC-CV) charging the cell is then held at that voltage until the current falls to a cutoff. A real discharge ends at the lower cutoff, which is why actual runtime can differ from the ideal duration.

A cell made by winding the electrodes into a jelly roll and sealing it in a round metal can. Sizes are named by their dimensions: a 21700 cell is 21 mm across and 70 mm long.

Removing the gas a cell releases during formation. A pouch cell’s extra gas pocket is opened in a dry atmosphere, emptied and sealed off; large prismatic cells often vent through their fill opening before final sealing.

A tree-like metal deposit that can grow from the negative electrode through the separator, raising self-discharge or causing an internal short circuit.

How much of a cell’s capacity is used in a cycle. Deep cycles cause more mechanical stress in the active materials than shallow ones.

The proportionality constant D in Fick’s first law, in m²/s. It sets how fast a species spreads for a given concentration gradient.

A way of making electrodes without solvent: the powders are mixed with a binder that draws out into fine fibres under shear, and the resulting film is pressed onto the foil. It removes the slurry, the dryer and solvent recovery.

A production room whose air is kept extremely dry — dew points around −30 °C to −60 °C — so that no moisture reaches the cell while the electrolyte goes in. Water would react with the salt LiPF₆ to form hydrofluoric acid.

The ion-conducting medium between the electrodes. In a liquid-electrolyte cell it fills the pores of both electrodes and the separator, so it is not a separate structural layer.

The final measurements on every finished cell — capacity, internal resistance and open-circuit voltage — used to reject faulty cells and sort good ones into matched grades.

Stored energy per volume, in watt-hours per litre (Wh/L), and per mass, in watt-hours per kilogram (Wh/kg). They are the usual figures for comparing cells and packs.

J = −D dc/dx: diffusive flux is proportional to the concentration gradient and points down it.

The first controlled charge and discharge cycles of a newly assembled cell, during which the SEI forms. It is part of cell production, before aging and grading.

t = 1 / C-rate: how long a constant current would take to transfer the full nominal capacity. A definition, not a prediction of real runtime.

An increase in a cell’s internal resistance as it ages. It costs voltage under load and therefore power, even while the stored charge is still there.

Lithium inserting into, and leaving, the host structure of an electrode material — between graphite layers, for example — without destroying it.

An unintended electronic path between the electrodes inside a cell — through a metal particle, a burr, a damaged separator or lithium deposits. It discharges the cell internally and turns its energy into local heat.

Electrodes and separators wound together into a spiral. It is the inside of a cylindrical cell and of many prismatic cells; large pouch cells usually use stacked sheets instead.

A positive electrode material, LiFePO₄, giving cells a nominal voltage around 3.2 V, long cycle life and good thermal stability, but less energy per kilogram than nickel-rich oxides.

A negative electrode made of lithium metal itself. It stores about ten times more charge per gram than graphite, which is the main attraction of solid-state cells, but it is reactive and prone to uneven deposits.

Lithium metal depositing on the negative-electrode surface instead of intercalating into it. It is promoted by charging at low temperature or high current.

A family of layered-oxide positive electrode materials used in many electric-vehicle cells. Variants are named by their metal ratio: NMC811 has eight parts nickel to one each of manganese and cobalt.

The charge a cell is rated to deliver under specified test conditions — current, temperature, rest and voltage limits — usually in ampere-hours.

A representative average voltage of a cell over a full discharge, such as 3.6–3.7 V for an oxide/graphite cell or about 3.2 V for LFP/graphite. Multiplied by capacity, it gives nominal energy.

The voltage across a cell’s terminals when no current flows and the cell has rested long enough for the voltage to settle. It depends on state of charge and temperature, and a cell whose open-circuit voltage falls unusually fast in storage may have an internal short.

The formation of a surface film that slows further reaction. In lithium-ion cells, the SEI passivates the graphite negative electrode.

The fraction of a layer’s volume that is empty space. In a lithium-ion electrode the pores — roughly 20–35 % of the volume after calendering — are filled with electrolyte, which carries ions to the particles.

An electrode made of active-material particles, conductive additive and binder with electrolyte-filled pores between them. Its large internal surface is where the reaction happens.

A cell sealed in a flexible bag of aluminium laminated with plastic, holding stacked electrode sheets in larger cells or a flattened wound roll in many small ones. It is light and easy to cool, but needs outside support and can visibly swell if gas forms inside.

A cell in a rigid rectangular metal case, holding either a flattened jelly roll or stacked sheets. Large prismatic cells pack tightly into battery packs.

The slow loss of stored charge while a cell is not in use. An unexpected rise can signal a leakage path or a soft internal short.

A porous, electronically insulating film between the electrodes. Electrons cannot cross it; ions move through the electrolyte in its pores.

Heating a powder below its melting point until the particles grow together into a dense solid. Oxide solid electrolytes are sintered at high temperature to give good ionic contact.

The mixture of active material, conductive carbon, binder and solvent that is coated onto a current-collector foil to make an electrode.

A solid ion conductor that replaces the liquid electrolyte in an all-solid-state cell. Oxide, sulfide, halide and polymer families each need a different production route.

A film of electrolyte reaction products on the negative-electrode particles — graphite, and also silicon or lithium metal. It protects the electrode from further reaction but grows slowly for the whole life of the cell, consuming cyclable lithium.

Pressure applied to a cell stack, in production or in use, to keep its layers in contact. Solid-state cells depend on it, because ions can only cross where two solids touch.

How full a cell is, as a fraction of its available capacity. State of charge, like temperature, affects how fast a stored cell ages.

A summary of how far a cell has aged, often expressed through remaining capacity or increased resistance compared with when it was new. By convention a cell has reached end of life at 80 % of its rated capacity.

Self-accelerating heating in a cell: heat speeds up reactions that release more heat, until the cell vents, catches fire or ruptures. An internal short, overcharge or external heating can trigger it.

How winding the path through a porous layer is, compared with a straight line through it. High tortuosity means ions travel much further than the layer is thick, which limits how fast a thick, dense electrode can be charged or discharged.

A unit of energy: one watt for one hour. A cell’s nominal energy in watt-hours is its capacity in ampere-hours times its nominal voltage — 20 Ah at 3.65 V is 73 Wh.

The soaking of electrolyte into every pore of the electrodes and separator after filling. It can take hours or more; a pore left dry is electrode area that never works properly.