# Cathode
A **cathode** is the electrode at which reduction takes place, that is, the electrode at which electrons are taken up by atoms, molecules, or ions.[1](https://goldbook.iupac.org/terms/view/C00905) The name is defined by the reaction occurring at the electrode, not by its electrical sign: in a discharging battery the cathode is the positive terminal, while in an electrolytic cell or a battery being recharged it is the negative terminal.[2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497) • [3](https://www.chemeurope.com/en/encyclopedia/Cathode.html)
| Key fact | Detail |
|---|---|
| Definition | IUPAC: the electrode at which reduction takes place[1](https://goldbook.iupac.org/terms/view/C00905) |
| Polarity | Positive in a discharging battery, negative in an electrolytic cell or recharging battery[2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497) • [3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) |
| Origin | Coined by Michael Faraday in the 1830s, in consultation with William Whewell and Whitlock Nicholl[4](https://royalsocietypublishing.org/rsnr/article/16/2/187/55220/Faraday-consults-the-scholars-the-origins-of-the) |
| Electron flow | During discharge, electrons flow through the external circuit from anode to cathode[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) |
| Li-ion cathodes | Cathode materials such as LiCoO2, LiMn2O4, and LiFePO4, into which lithium ions intercalate during discharge[5](https://technav.ieee.org/topic/cathodes/) |
| Energy density | LFP cells reached 160 Wh/kg in 2022; LMFP about 210–240 Wh/kg; NMC cells 300 Wh/kg and above[6](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf) |
| Market share | LFP took 42% of the global battery market in 2023[6](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf) |
## Definition and terminology
IUPAC pairs the two names by reaction type: the cathode is the electrode at which reduction takes place, and the anode is the electrode at which oxidation takes place.[1](https://goldbook.iupac.org/terms/view/C00905) • [7](https://goldbook.iupac.org/terms/view/A00370) According to IUPAC recommendations, anode and cathode are defined solely by which way electrons move, and have nothing to do with the electrodes' polarity.[8](https://www.degruyterbrill.com/document/doi/10.1515/pac-2018-0109/pdf) The 1974 IUPAC electrode-kinetics convention expresses the same idea in current terms: a current of positive electricity passing from the electrode into the electrolyte is taken as positive, and an electrode at which a net positive current flows is called an anode.[9](https://media.iupac.org/publications/pac/1974/pdf/3704x0499.pdf)
**Origin of the word.** [Michael Faraday](https://www.edgepedia.org/michael-faraday) introduced the electrochemical vocabulary in the 1830s, and his fifteenth series of *Experimental Researches in Electricity*, published in the *Philosophical Transactions* in 1839, is the primary publication carrying the terminology.[10](https://royalsocietypublishing.org/rstl/article-pdf/doi/10.1098/rstl.1839.0002/1453872/rstl.1839.0002.pdf) He did not coin the Greek-derived words alone: the Royal Society's historical account identifies his consultants as Whitlock Nicholl and [William Whewell](https://www.edgepedia.org/william-whewell), Master of Trinity College, Cambridge, whose role was disclosed in 1868 by John Tyndall.[4](https://royalsocietypublishing.org/rsnr/article/16/2/187/55220/Faraday-consults-the-scholars-the-origins-of-the) In Faraday's own 1834 formulation, the cathode is the surface at which the current leaves the decomposing body, in contact with the negative electrode, where metals, alkalies, and bases are evolved; he called bodies passing to the anode anions and those passing to the cathode cations, with ion as the collective term.[11](https://www.chemteam.info/Chem-History/Faraday-electrochem.html)
## Physical mechanism
Reduction is the defining process. In either a galvanic or an electrolytic cell, the cathode is the electrode at which electrons are being taken up by atoms, molecules, or ions.[12](http://encyclopedia-loadbalancer-1-1782916326.us-west-2.elb.amazonaws.com/science-and-technology/computers-and-electrical-engineering/electrical-engineering/cathode) In a discharging battery, electrons arrive from the external circuit and cations migrate from the electrolyte toward the positive cathode.[2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497) During discharge, electrons flow through the external circuit from the anode to the cathode.[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html)
The energy bookkeeping is captured by the relation \( -\Delta G = nFE \), where \( \Delta G \) is the change in [Gibbs free energy](https://www.edgepedia.org/gibbs-free-energy) during ion and electron insertion or extraction, with the dominant effect arising from the ions, \( n \) is the number of electrons stored per formula unit, \( F \) the [Faraday constant](https://www.edgepedia.org/faraday-constant), and \( E \) the cell voltage.[13](https://www.sciencedirect.com/science/article/pii/S1369702115003181) In practice the cathode is a microstructure, not a flat surface: cathode active material consists of primary particles 10–500 nm in size fused into secondary particles of 3–20 µm, and during discharge lithium ions must intercalate into these secondary particles.[14](https://iopscience.iop.org/article/10.1088/2515-7655/acc139/meta)
## Cathodes in electrochemistry
When a battery or [galvanic cell](https://www.edgepedia.org/galvanic-cell) discharges, its cathode serves as the positive terminal through which conventional current exits; when a battery is being recharged or an electrolytic cell is operating, the cathode instead acts as the negative terminal, returning current to the outside generator. A common mnemonic is CCD, Cathode Current Departs.[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) During charging of a secondary battery the processes reverse, yet the positive electrode is still commonly, if somewhat inaccurately, referred to as the cathode in lithium-ion literature.[2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497)
**Electroplating.** The object to be plated is made the cathode. When metal ions are reduced from ionic solution onto the cathode, they form a pure metal surface; in the classic example, an iron spoon made the cathode in a cell with a silver anode and silver nitrate solution acquires a shiny, rust-resistant silver coating.[12](http://encyclopedia-loadbalancer-1-1782916326.us-west-2.elb.amazonaws.com/science-and-technology/computers-and-electrical-engineering/electrical-engineering/cathode) • [3](https://www.chemeurope.com/en/encyclopedia/Cathode.html)
**Dual intercalation.** Li-ion cells operate by dual intercalation: their cathode and anode each possess frameworks permitting lithium cations to be inserted and removed reversibly.[2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497)
## Cathodes in vacuum and electronic devices
In a vacuum tube the cathode is the negative electrode, with more electrons on its surface than the anode; heating it releases free electrons that travel to the anode as cathode rays, the operating principle of cathode-ray tubes used in oscilloscopes, televisions, and computer monitors.[12](http://encyclopedia-loadbalancer-1-1782916326.us-west-2.elb.amazonaws.com/science-and-technology/computers-and-electrical-engineering/electrical-engineering/cathode) Historically, Johann Hittorf showed that these rays could be deflected by a magnet, and in 1876 they were dubbed Kathodenstrahlen (cathode rays) by [Eugen Goldstein](https://www.edgepedia.org/eugen-goldstein).[15](http://scholar.uoa.gr/sites/default/files/tarabatz/files/2009a.pdf)
**Hot and cold cathodes.** Cathodes used for field emission in vacuum tubes are called cold cathodes, while heated electrodes, or hot cathodes, frequently called filaments, are much more common; most radios and televisions before the 1970s used filament-heated-cathode electron tubes, and hot cathodes remain the electron-beam source in CRTs, fluorescent lamps, and X-ray tubes.[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html)
**Semiconductors.** In a semiconductor diode the cathode is the N-doped layer of the PN junction, where electrons combine with holes from the P-doped layer. A [Zener diode](https://www.edgepedia.org/zener-diode) has fixed anode and cathode but conducts in reverse above its breakdown voltage.[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html)
## Battery cathode materials
The principal lithium-ion cathode families are LCO (LiCoO2), LMO (LiMn2O4), LFP (LiFePO4), NCA, and NMC, with LCO, LMO, and LFP among the most widely employed.[16](https://journals.co.za/doi/pdf/10.1016/j.sajce.2024.09.008?download=true) In lithium-ion cells the cathode is typically a layered transition-metal oxide into which lithium ions intercalate during discharge.[5](https://technav.ieee.org/topic/cathodes/)
Material-level capacities and average potentials, as tabulated in one comparative review, are: LiCoO2, 272 mAh/g theoretical with about 140 mAh/g used in practice, at 4.2 V versus Li/Li+; NMC111 (LiNi1/3Mn1/3Co1/3O2), 272 mAh/g theoretical and 200 mAh/g practical at 4.0 V; LiMn2O4 spinel, 148 mAh/g theoretical and 120 practical at 4.1 V; and LiFePO4 olivine, 170 mAh/g theoretical and 160 practical at 3.45 V.[17](https://www.mdpi.com/2304-6740/2/1/132) A second review gives somewhat different operating figures for the same materials, for example LCO at 160 Ah/kg and 3.9 V and NCM at 160 Ah/kg and 3.7 V; the two tabulations disagree on practical capacity and average potential, and no single set of values is settled across the literature.[18](https://www.uni-ulm.de/fileadmin/website_uni_ulm/nawi.inst.250/publications/Review_EES_preprint.pdf)
Cathode chemistry largely determines a cell's specific capacity, voltage plateau, cycle life, and thermal stability.[5](https://technav.ieee.org/topic/cathodes/) LFP is cobalt-free and offers longer cycle life and better thermal stability than NMC, while NMC811 has poor thermal stability and moderate cycle life.[6](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf) Cathode identity also reaches the whole cell: in anode-free batteries, a 2024 study found that NMC811 cathodes deliver lower Coulombic efficiencies than LFP because of tortuous Li deposits, acidic species in the electrolyte, and accumulation of [dead lithium](https://www.edgepedia.org/dead-lithium).[19](https://www.cell.com/chem/fulltext/S2451-9294(24)00255-9)
## By the numbers
Cell-level energy density has moved substantially. State-of-the-art LFP cells rose from 90 Wh/kg in 2010 to 160 Wh/kg in 2022.[6](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf) LMFP (lithium manganese iron phosphate) offers about 210–240 Wh/kg at cell level, roughly 40% more than standard LFP; CATL's Qilin NMC pack offers 250 Wh/kg, and NMC-type cells can typically reach 300 Wh/kg and above at cell level.[6](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf) On market share, LFP took 42% of the global battery market in 2023, driven by high penetration in China; under the Faraday Institution's baseline scenario, by 2040 NMC declines to around 38% of the European EV market with LFP at 32%.[6](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf)
## How it compares with the anode
| Property | Cathode | Anode |
|---|---|---|
| Reaction | Reduction (electrons taken up)[1](https://goldbook.iupac.org/terms/view/C00905) | Oxidation (electrons given up)[7](https://goldbook.iupac.org/terms/view/A00370) |
| Discharging battery | Positive terminal[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) | Negative terminal[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) |
| Electrolytic cell / recharging | Negative terminal[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) | Positive terminal[3](https://www.chemeurope.com/en/encyclopedia/Cathode.html) |
| Ion migration | Cations move toward it[2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497) | Anions move toward it[11](https://www.chemteam.info/Chem-History/Faraday-electrochem.html) |
The confusion is structural, not careless: the names track reaction type and electron-flow direction, so the same physical electrode changes name when a rechargeable cell switches from discharge to charge, while battery engineers keep calling the positive electrode the cathode in both modes.[8](https://www.degruyterbrill.com/document/doi/10.1515/pac-2018-0109/pdf) • [2](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497)
## Recent developments
**Lattice-oxygen redox.** High-energy-density oxide cathodes increasingly rely on hybrid anion- and cation-redox (HACR), invoking oxidized oxygen ions at high voltages; extended cycling under these conditions triggers chemical, electrochemical, mechanical, and microstructural degradation that shortens battery life and raises safety issues.[20](https://pubs.acs.org/doi/full/10.1021/acs.chemrev.2c00251) A 2025 review in *Chemical Society Reviews* notes that achieving high specific capacity in high-voltage LCO, Ni-rich oxides, and Li-rich cathodes is always accompanied by the oxygen redox process, which shows unsatisfactory reversibility and significantly affects structural stability, and that a unified understanding of high-voltage LCO and Ni-rich oxide behavior has not yet been established.[21](https://pubs.rsc.org/en/content/articlelanding/2025/cs/d4cs00797b) A *Nature Energy* review frames oxide cathode performance as governed by three interrelated chemical factors, electronic configuration, chemical bonding, and chemical reactivity, with improvement strategies including compositional tuning, surface doping, and electrolyte optimization.[22](https://preview-www.nature.com/articles/s41560-025-01963-x)
**Sodium-ion cathodes.** An iron-mediated strategy in a Na2/3Mn7/12Mg1/4Fe1/6O2 layered cathode improved lattice-oxygen redox reversibility from 75% to 99%, enabling a sodium-ion pouch cell with 206 Wh/kg energy density, stable for 100 cycles at 50 mA/g with 87.8% capacity retention.[23](https://www.nature.com/articles/s41560-026-02112-8) Composition screening of doped [sodium manganese oxides](https://www.edgepedia.org/sodium-manganese-oxide) produced 28 compositions, with Na0.5Mn0.66Fe0.33O2 delivering the highest specific capacity of 178 mAh/g, small Ti substitution improving high-rate capacity by 77% at 1000 mA/g, and an equimolar VFT111 composition retaining 74% of capacity after 100 cycles versus 55% undoped.[24](https://pubs.rsc.org/ta/article/doi/10.1039/d6ta04588j/1360221/Probing-the-composition-space-of-V-Fe-and-Ti) Air stability, a practical obstacle for [sodium layered oxides](https://www.edgepedia.org/sodium-layered-oxide), has been addressed by proton-exchange surface engineering: a treated O3-type Na0.82Cu0.19Fe0.39Mn0.42O2 delivered 110.2 mAh/g at 0.2 C with 92.3% retention after 100 cycles, against 48.6% for the pristine sample, and kept its performance after one month of air exposure.[25](https://pubs.acs.org/doi/10.1021/acsami.6c16266) The underlying constraint is ionic: the larger radius of the sodium ion, 1.02 Å versus 0.76 Å for lithium, leads to sluggish diffusion kinetics and significant volume expansion during cycling.[26](https://link.springer.com/article/10.1007/s44508-026-00022-x)
Degradation under high-voltage operation remains a central problem for oxide cathodes; high-voltage charging causes unusual anion and cation mobilities and extensive room-temperature lattice reconstructions, in both useful senses (plasticity and self-healing phenomena such as grain boundary sliding and lattice microcrack healing) and damaging ones (phase change, corrosion, and damage).[20](https://pubs.acs.org/doi/full/10.1021/acs.chemrev.2c00251)
## References
1. [IUPAC Gold Book: cathode (C00905)](https://goldbook.iupac.org/terms/view/C00905)
2. [Battery Cathodes, Springer encyclopedia entry](https://link.springer.com/rwe/10.1007/978-1-4419-0851-3_497)
3. [Cathode, Chemeurope Encyclopedia](https://www.chemeurope.com/en/encyclopedia/Cathode.html)
4. [Faraday consults the scholars: the origins of the terms of electrochemistry, Notes and Records of the Royal Society](https://royalsocietypublishing.org/rsnr/article/16/2/187/55220/Faraday-consults-the-scholars-the-origins-of-the)
5. [Cathodes, IEEE Technology Navigator](https://technav.ieee.org/topic/cathodes/)
6. [Developments in lithium-ion battery cathodes, Faraday Institution (Faraday Insights 18)](https://www.faraday.ac.uk/wp-content/uploads/2023/09/Faraday_Insights_18_FINAL.pdf)
7. [IUPAC Gold Book: anode (A00370)](https://goldbook.iupac.org/terms/view/A00370)
8. [IUPAC Recommendations, Pure and Applied Chemistry (PAC-2018-0109)](https://www.degruyterbrill.com/document/doi/10.1515/pac-2018-0109/pdf)
9. [IUPAC Pure and Applied Chemistry 37(4), 1974, Electrode kinetics conventions](https://media.iupac.org/publications/pac/1974/pdf/3704x0499.pdf)
10. [Michael Faraday, Experimental researches in electricity, fifteenth series, Philosophical Transactions (1839)](https://royalsocietypublishing.org/rstl/article-pdf/doi/10.1098/rstl.1839.0002/1453872/rstl.1839.0002.pdf)
11. [Faraday and electrochemistry from 1834, ChemTeam](https://www.chemteam.info/Chem-History/Faraday-electrochem.html)
12. [Cathode, Encyclopedia.com](http://encyclopedia-loadbalancer-1-1782916326.us-west-2.elb.amazonaws.com/science-and-technology/computers-and-electrical-engineering/electrical-engineering/cathode)
13. [Understanding electrochemical potentials of cathode materials in rechargeable batteries, Materials Today](https://www.sciencedirect.com/science/article/pii/S1369702115003181)
14. [Status and outlook for lithium-ion battery cathode material synthesis, IOPscience](https://iopscience.iop.org/article/10.1088/2515-7655/acc139/meta)
15. [Cathode Rays, historical lecture notes, University of Athens](http://scholar.uoa.gr/sites/default/files/tarabatz/files/2009a.pdf)
16. [Lithium-ion battery fundamentals and exploration of cathode materials: a review, S. Afr. J. Chem. Eng.](https://journals.co.za/doi/pdf/10.1016/j.sajce.2024.09.008?download=true)
17. [Comparative issues of cathode materials for Li-ion batteries, Inorganics (MDPI)](https://www.mdpi.com/2304-6740/2/1/132)
18. [Fundamentals of electrochemical energy storage: theory and practice, Universität Ulm review](https://www.uni-ulm.de/fileadmin/website_uni_ulm/nawi.inst.250/publications/Review_EES_preprint.pdf)
19. [Elucidating the role of cathode identity: voltage-dependent reversibility of anode-free batteries, Chem (2024)](https://www.cell.com/chem/fulltext/S2451-9294(24)00255-9)
20. [Oxide cathodes: functions, instabilities, self healing, and degradation mitigations, Chemical Reviews](https://pubs.acs.org/doi/full/10.1021/acs.chemrev.2c00251)
21. [A comprehensive understanding on the anionic redox chemistry of high-voltage cathode materials, Chemical Society Reviews (2025)](https://pubs.rsc.org/en/content/articlelanding/2025/cs/d4cs00797b)
22. [Chemical factors controlling the behaviour of oxide cathodes in batteries, Nature Energy (2025)](https://preview-www.nature.com/articles/s41560-025-01963-x)
23. [Iron-mediated reversible lattice-oxygen redox enables stable 200 Wh/kg sodium-ion batteries, Nature Energy](https://www.nature.com/articles/s41560-026-02112-8)
24. [Probing the composition space of V, Fe, and Ti substituted sodium manganese oxides, Journal of Materials Chemistry A](https://pubs.rsc.org/ta/article/doi/10.1039/d6ta04588j/1360221/Probing-the-composition-space-of-V-Fe-and-Ti)
25. [Proton-exchange surface engineering enables air-stable sodium layered oxide cathodes, ACS Applied Materials & Interfaces](https://pubs.acs.org/doi/10.1021/acsami.6c16266)
26. [Review of AI-driven discovery of cathode, anode, and electrolyte materials for sodium-ion batteries, Springer](https://link.springer.com/article/10.1007/s44508-026-00022-x)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Electromagnetism › Electric and magnetic fields › Electrostatics › Electric charge*
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