Glossary of Electrochemistry Terms
Battery: a device that stores chemical energy and converts it into electrical energy. It typically consists of one or more electrochemical cells.
Primary battery: single use, not rechargeable
Secondary battery: rechargeable
Cathode: the electrode where reduction reactions occur during the electrochemical process. In a discharging battery, the cathode gains electrons and is typically the positive electrode.
Anode: the electrode where oxidation reactions occur during the electrochemical process. In a discharging battery, the anode is typically the negative electrode, releasing electrons to the external circuit.
Electrolyte: A substance that conducts electricity through the movement of ions, but does not conduct electrons. Liquid electrolytes consist of a polar solvent, a salt, and additives to improve battery performance.
Separator: a thin, porous membrane placed between the anode and the cathode to physically separate them while allowing ions to flow between them. It prevents direct contact between the electrodes, which would cause a short circuit.
Capacity: the amount of electrical charge a battery can store and deliver at a time. Capacity is measured in ampere-hour [Ah] or miliampere-hour [mAh] units and indicates how long a battery can provide a certain current before it is fully discharged. Capacity can also be expressed in watt-hours (Wh) when considering voltage.
State of Charge (SOC): the percentage of energy stored in a battery relative to its maximum capacity. 0% SOC is fully discharged, 100% SOC is fully charged. Equal to 1 - DOD (depth of discharge).
State of Health (SOH) is a metric used to describe the overall condition of a battery relative to its ideal or pristine state. It is typically expressed as a percentage, where 100% represents a brand-new battery operating at peak capacity, and lower percentages indicate aging or degradation. SOH evaluates a battery's performance and predicts its remaining useful life.'
Depth of discharge (DOD) measures the energy or capacity consumed (as a percentage) while the battery discharges. Equal to 1- SOC (state of charge). DOD is more commonly used when referring to primary batteries, whereas SOC is used for secondary batteries.
C-rate is a measure of the rate at which a battery is (dis)charged relative to its maximum capacity. A C/2 rate means the battery can be fully charged or discharged in two hours, while a 2C rate means it can be done in 30 minutes.
Dendrites are needle-like growths that appear on the surface of the anode. They induce unwanted side reactions that reduce energy density and may cause shorting of the electrodes that can lead to thermal runaway.
Coulombic efficiency (CE) describes the efficiency with which charge (electrons) is transferred in a system facilitating an electrochemical reaction.
Solid Electrolyte Interphase (SEI): a thin, passivating layer formed at the anode/electrolyte interface. Ideally stable during cycling, electronically insulating, and permits fast Li transport. Creation of the SEI consumes Li, resulting in capacity fade and decreasing coulombic efficiency. An unstable SEI can continuously grow thicker, consuming more electrolyte and Li, leading to reduced capacity and increased internal resistance. Electrolyte additives are used to create a stable SEI.
graphite - the most commonly used anode material in lithium ion batteries. The layered structure allows it to host lithium ions.
Lithium Cobalt Oxide (LCO - LiCoO₂) - one of the earliest and most widely used lithium-ion cathode materials, especially in consumer electronics.
Lithium Nickel Manganese Cobalt Oxide (NMC- Li(NixMnyCoz)O₂) - A versatile lithium-ion cathode material balancing energy density, power, and thermal stability. Various formulations (e.g., NMC111, NMC532, NMC622, NMC811) optimize trade-offs between cost (nickel content), capacity, and stability.
Lithium Iron Phosphate (LFP - LiFePO₄) - A phosphate-based lithium-ion cathode material known for excellent safety and long cycle life.
intercalation- insertion between layers in a crystal lattice, ions move in and out of a layered structure. The chemistry has the term “ion” in it i.e. li-ion or zinc ion.
conversion- A different type of battery/electrochemistry where material is electrochemically converted to another material. The most commonly known conversion batteries are household AA and AAA batteries - sometimes called alkaline batteries due to the basic electrolyte (KOH) used.
BOL: beginning of life
EOL: end of life
CC: constant current cycling (charge or discharge)
CCCV: constant current charge followed by a constant voltage charge
OCV: open circuit voltage. The voltage of a cell at rest
IR: internal resistance in ohms. Could be DCIR (direct current), or ACIR (alternating current)
Coin cell - a small, cylindrical button-shaped battery commonly used for testing lithium-ion battery materials or as a power source for low-power devices. Coin cells typically have a stainless steel metallic casing, and have very low capacity due to their small size. The size typically ranges from 3 mm to 25 mm in diameter and 1 mm to 6 mm in height. Common types include CR2032, CR2025, and CR2016 (where "CR" stands for lithium-manganese dioxide chemistry, and the numbers indicate dimensions, 2032 = 20mm diameter and 3.2mm height).
Single-layer pouch (SLP) - type of lithium-ion battery (LIB) cell packaging that consists of a single flexible pouch made from a laminated polymer-aluminum composite. This provides a lightweight and compact form factor compared to rigid cylindrical or prismatic cells. SLPs are the industry standard for testing a cell larger than a coin cell.
Hybrid Pulse Power Characterization (HPPC): Involves subjecting a battery to a series of rapid charge and discharge pulses at different SOCs to evaluate dynamic power capabilities and performance. Essentially, mimics real-world usage in an electric vehicle to assess the battery's ability to handle sudden power demands (accelerating and braking) effectively.
N/P ratio - calculated by dividing the negative electrode's capacity by the positive electrode's capacity
Current density- the amount of electric current flowing per unit cross-sectional area of a material [mA/cm2]