Reference

Battery science glossary

Terms for reading battery papers, interpreting cell specifications and comparing test results.

Materials and cell components

Active material: The part of an electrode that participates in the intended charge-storage reaction. Capacity per gram of active material excludes binders, conductive additives, current collectors and other cell components.
Cathode: The electrode at which reduction occurs. Battery literature commonly uses “cathode” for the positive electrode according to its role during discharge, even when describing charging. That usage should be distinguished from the reaction-specific definition.
Anode: The electrode at which oxidation occurs. In rechargeable-battery descriptions, “anode” commonly means the negative electrode during discharge. The direction of the reaction reverses during charging.
Electrolyte: The ion-conducting medium connecting the electrodes. It must support the relevant ionic transport while remaining compatible with the electrodes and their operating potentials.
Separator: A layer that prevents direct electronic contact between electrodes while permitting ionic transport. In a liquid-electrolyte cell, its pores normally contain electrolyte; a solid electrolyte may also provide the separating function.
Current collector: An electronic conductor that gathers current from an electrode composite and connects it to the external circuit. Its mass and thickness contribute to complete-cell energy calculations.
LFP: Lithium iron phosphate, LiFePO₄, used as an olivine positive-electrode material. The abbreviation names a chemistry, rather than a complete cell specification.Related reading ↗
LMFP: Lithium manganese iron phosphate, often written LiMnₓFe₁₋ₓPO₄. The manganese fraction must be specified to identify the composition being compared.Related reading ↗
Olivine framework: A crystal-structure family relevant to several transition-metal phosphate electrodes. Framework connectivity and cation occupancy help determine ion pathways; a chemical formula alone does not prove this structure.Related reading ↗
Maricite: A structural form of NaFePO₄ with a cation arrangement different from olivine-derived sodium phosphate. Reports involving activated or transformed maricite need to identify the structure present during charge storage.Related reading ↗

Capacity, energy and operating conditions

Capacity: The charge delivered or accepted under a specified protocol, commonly expressed in ampere-hours. Specific capacity divides that charge by a stated mass. It is distinct from energy because it does not include the voltage profile.
Theoretical capacity: A stoichiometric charge limit calculated from the assumed electron count and molar mass. Complete accessibility and the chosen reaction are assumptions; a measurement may deliver less.Related reading ↗
Energy: Electrical work delivered over a charge or discharge, obtained by integrating voltage over charge. Specific energy uses a mass denominator; energy density uses a volume denominator. The included components and operating window must be stated.
Power: The rate of energy transfer, measured in watts. A high-power discharge may deliver less usable energy than a low-power reference test because polarization reaches the voltage limits sooner.
Electrode potential: An electrode’s electrochemical potential expressed relative to a specified reference electrode. A cathode value versus Li⁺/Li is not the voltage of a full cell with a different negative electrode.
C-rate: A current normalized to a stated capacity. At 1 C, that numerical capacity would be transferred in one hour under an ideal constant-current assumption; cutoffs and actual capacity can change the observed duration. Report which capacity defines the rate.
State of charge (SOC): An estimate of the charge present relative to a defined usable capacity window. Its zero and full-charge references depend on the cell and operating limits; SOC is not a direct measure of health.
State of health (SOH): A health indicator relative to an identified reference, such as retained capacity or increased resistance. Different SOH definitions can give different values for the same cell. The metric and reference conditions must be stated.
Depth of discharge (DOD): The fraction of a defined capacity window removed during discharge. The reference capacity and starting state matter when comparing repeated-cycle protocols.
Coulombic efficiency: Discharge charge divided by charge input for a specified cycle, usually expressed as a percentage. It measures charge balance and should not be substituted for energy efficiency, which also includes voltage differences.
Areal loading: The mass of material per electrode area, with the material basis identified. Combined with utilization and electrode formulation, it affects areal capacity and the transport demands of a thick electrode.
Half-cell and full cell: A half-cell studies an electrode against a reference or counter material, such as lithium metal. A full cell combines intended positive and negative electrodes with a balanced inventory. Results from the two configurations address different constraints.

Transport, interfaces and ageing

Diffusion coefficient: A parameter describing the response of transport to a concentration gradient within a stated model. Crystal migration, chemical diffusion and an apparent coefficient fitted to an electrode are different quantities.Related reading ↗
Antisite defect: A crystal defect in which an atom occupies a site assigned to another species in the ideal structure. In an olivine, a transition-metal ion on a lithium site can affect a lithium pathway; the actual defect population requires evidence.
Polarization: The departure of an operating electrode or cell voltage from its equilibrium value during current flow. Reaction kinetics, concentration gradients and resistive losses can all contribute.
Impedance: The frequency-dependent relationship between a small electrical perturbation and its response. An equivalent-circuit fit is an interpretation of the measurement; assigning a process to a fitted element needs additional justification.
Solid-electrolyte interphase (SEI): A reaction-product layer formed at an electrode–electrolyte interface, commonly discussed on graphite negative electrodes. Its protective behaviour and continued growth can affect cyclable inventory and resistance.Related reading ↗
Lithium plating: Deposition of metallic lithium rather than its intended storage in the negative-electrode host. Its occurrence depends on electrode potential and operating conditions, including charging current and temperature.
Loss of lithium inventory: A reduction in the lithium available to shuttle between the electrodes. Lithium consumed in side reactions can reduce capacity even when much of the active host material remains present.
Loss of active material: A reduction in the active material accessible to the intended reaction, through chemical changes, isolation or structural damage. It is an ageing mode rather than a single universal mechanism.
Calendar ageing: Ageing with elapsed time, including storage without deliberate cycling. Temperature and state of charge remain relevant conditions.
Cycle ageing: Ageing attributed to repeated battery operation. A cycle count needs its depth, current, temperature, time and endpoint conditions to support comparisons.
Interphase: A region with properties or composition different from the two contacting bulk materials. In a solid-state cell, its ionic and electronic transport properties affect whether reaction products passivate the contact or permit further decomposition.Related reading ↗

These definitions are editorial explanations. The sources below provide the material and interface context; the research articles retain the conditions and limitations of specific findings.

References

Wonchan Hwang, Jaehwan Kim, Shin-Yeong Kim, et al.. Unveiling olivine cathodes for high energy-density lithium-ion batteries: a comprehensive review from the atomic level to the electrode scale. Journal of Materials Chemistry A, 2024. DOI: 10.1039/D4TA02338B.
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Yihan Xiao, Yan Wang, Shou-Hang Bo, et al.. Understanding interface stability in solid-state batteries. Nature Reviews Materials, 2020. DOI: 10.1038/s41578-019-0157-5.
Publisher or institutional record ↗
Jacqueline S. Edge, Simon O’Kane, Ryan Prosser, et al.. Lithium ion battery degradation: what you need to know. Physical Chemistry Chemical Physics, 2021. DOI: 10.1039/D1CP00359C.
Publisher or institutional record ↗