Editing Electrode (section)

== Electrodes in lithium-ion batteries ==
A modern application of electrodes is in [[Lithium-ion battery|lithium-ion batteries]] (Li-ion batteries). A Li-ion battery is a kind of [[flow battery]] which can be seen in the image on the right.

[[File:Redox Flow Battery.jpg|alt=Redox Flow Battery|thumb|upright=1.5|A typical flow battery consists of two tanks of liquids which are pumped past a membrane held between two electrodes.<ref name="Qi 040801">{{cite journal|last1=Qi|first1=Zhaoxiang|last2=Koenig|first2=Gary M.|date=2017-05-12|title=Review Article: Flow battery systems with solid electroactive materials|journal=Journal of Vacuum Science & Technology B|volume=35|issue=4|pages=040801|doi=10.1116/1.4983210|bibcode=2017JVSTB..35d0801Q|issn=2166-2746|doi-access=free}}</ref>]]

Furthermore, a Li-ion battery is an example of a secondary cell since it is rechargeable. It can both act as a [[Galvanic cell|galvanic]] or [[electrolytic cell]]. Li-ion batteries use lithium ions as the solute in the electrolyte which are dissolved in an [[Organic compound|organic]] [[solvent]]. Lithium electrodes were first studied by [[Gilbert N. Lewis]] and [[Frederick G. Keyes]] in 1913.<ref>{{cite journal|first1=Gilbert N.|last1=Lewis|first2=Frederick G.|last2=Keyes|title=The Potential of the Lithium Electrode|journal=Journal of the American Chemical Society|year=1913|volume=35|issue=4|pages=340–344|doi=10.1021/ja02193a004|bibcode=1913JAChS..35..340L |url=https://zenodo.org/record/2233227}}</ref> In the following century these electrodes were used to create and study the first Li-ion batteries. Li-ion batteries are very popular due to their great performance. Applications include mobile phones and electric cars. Due to their popularity, much research is being done to reduce the cost and increase the safety of Li-ion batteries. An integral part of the Li-ion batteries are their anodes and cathodes, therefore much research is being done into increasing the efficiency, safety and reducing the costs of these electrodes specifically.<ref name="sigma">{{citation |last1=Kam |first1=Kinson C. |last2=Doeff |first2=Marca M. |title=Electrode Materials for Lithium Ion Batteries |work=Sigma-Aldrich Technical Documents: Lab & Production Materials |url=https://www.sigmaaldrich.com/AU/en/technical-documents/technical-article/materials-science-and-engineering/batteries-supercapacitors-and-fuel-cells/electrode-materials-for-lithium-ion-batteries}}</ref>

=== Cathodes ===
In Li-ion batteries, the cathode consists of a [[intercalation (chemistry)|intercalated]] lithium compound (a layered material consisting of layers of molecules composed of lithium and other elements). A common element which makes up part of the molecules in the compound is [[cobalt]]. Another frequently used element is [[manganese]]. The best choice of compound usually depends on the application of the battery. Advantages for cobalt-based compounds over manganese-based compounds are their high specific heat capacity, high [[volumetric heat capacity]], low self-discharge rate, high discharge voltage and high cycle durability. There are however also drawbacks in using cobalt-based compounds such as their high cost and their low [[thermostability]]. Manganese has similar advantages and a lower cost, however there are some problems associated with using manganese. The main problem is that manganese tends to dissolve into the electrolyte over time. For this reason, cobalt is still the most common element which is used in the lithium compounds. There is much research being done into finding new materials which can be used to create cheaper and longer lasting Li-ion batteries <ref name="sigma"/> For example, Chinese and American researchers have demonstrated that ultralong single wall [[Carbon nanotube|carbon nanotubes]] significantly enhance lithium iron phosphate cathodes. By creating a highly efficient conductive network that securely binds lithium iron phosphate particles, adding carbon nanotubes as a conductive additive at a dosage of just 0.5% by weight helps cathodes to achieve a remarkable rate capacity of 161.5 mA⋅h⋅g<sup>−1</sup> at 0.5&nbsp;C and 130.2&nbsp;mA⋅h⋅g<sup>−1</sup> at 5&nbsp;C, whole maintaining 87.4% capacity retention after 200 cycles at 2&nbsp;C.<ref>{{cite journal |last1=Guo |first1=Mingyi |last2=Cao |first2=Zengqiang |last3=Liu |first3=Yukang |last4=Ni |first4=Yuxiang |last5=Chen |first5=Xianchun |last6=Terrones |first6=Mauricio |last7=Wang |first7=Yanqing |date=May 2023 |title=Preparation of Tough, Binder-Free, and Self-Supporting LiFePO 4 Cathode by Using Mono-Dispersed Ultra-Long Single-Walled Carbon Nanotubes for High-Rate Performance Li-Ion Battery |journal=Advanced Science |language=en |volume=10 |issue=13 |doi=10.1002/advs.202207355 |issn=2198-3844 |pmc=10161069 |pmid=36905241}}</ref>

=== Anodes ===
The anodes used in mass-produced Li-ion batteries are either carbon based (usually graphite) or made out of spinel lithium titanate (Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>).<ref name="sigma"/> Graphite anodes have been successfully implemented in many modern commercially available batteries due to its cheap price, longevity and high energy density.<ref>{{cite journal |last1=Zhang |first1=Hao |last2=Yang |first2=Yang |last3=Ren |first3=Dongsheng |last4=Wang |first4=Li |last5=He |first5=Xiangming |date=2021-04-01 |title=Graphite as anode materials: Fundamental mechanism, recent progress and advances |url=https://www.sciencedirect.com/science/article/pii/S2405829720304906 |journal=Energy Storage Materials |language=en |volume=36 |pages=147–170 |doi=10.1016/j.ensm.2020.12.027 |bibcode=2021EneSM..36..147Z |s2cid=233072977 |issn=2405-8297}}</ref> However, it presents issues of dendrite growth, with risks of shorting the battery and posing a safety issue.<ref>{{cite journal |last1=Zhao |first1=Qiang |last2=Hao |first2=Xiaoge |last3=Su |first3=Shiming |last4=Ma |first4=Jiabin |last5=Hu |first5=Yi |last6=Liu |first6=Yong |last7=Kang |first7=Feiyu |last8=He |first8=Yan-Bing |date=2019 |title=Expanded-graphite embedded in lithium metal as dendrite-free anode of lithium metal batteries |url=http://xlink.rsc.org/?DOI=C9TA04240G |journal=Journal of Materials Chemistry A |language=en |volume=7 |issue=26 |pages=15871–15879 |doi=10.1039/C9TA04240G |s2cid=195381622 |issn=2050-7488}}</ref> Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> has the second largest market share of anodes, due to its stability and good rate capability, but with challenges such as low capacity.<ref>{{cite journal |last1=Zhang |first1=Hao |last2=Yang |first2=Yang |last3=Xu |first3=Hong |last4=Wang |first4=Li |last5=Lu |first5=Xia |last6=He |first6=Xiangming |date=April 2022 |title=Li 4 Ti 5 O 12 spinel anode: Fundamentals and advances in rechargeable batteries |journal=InfoMat |language=en |volume=4 |issue=4 |doi=10.1002/inf2.12228 |issn=2567-3165|doi-access=free }}</ref> During the early 2000s, silicon anode research began picking up pace, becoming one of the decade's most promising candidates for future lithium-ion battery anodes.<ref name=":0"/> Silicon has one of the highest gravimetric capacities when compared to graphite and  Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> as well as a high volumetric one. Furthermore, Silicon has the advantage of operating under a reasonable open circuit voltage without parasitic lithium reactions.<ref>{{cite journal |last=Zhang |first=Wei-Jun |date=2011-01-01 |title=A review of the electrochemical performance of alloy anodes for lithium-ion batteries |url=https://www.sciencedirect.com/science/article/pii/S0378775310011699 |journal=Journal of Power Sources |language=en |volume=196 |issue=1 |pages=13–24 |doi=10.1016/j.jpowsour.2010.07.020 |bibcode=2011JPS...196...13Z |issn=0378-7753}}</ref><ref>{{cite journal |last1=Liang |first1=Bo |last2=Liu |first2=Yanping |last3=Xu |first3=Yunhua |date=2014-12-01 |title=Silicon-based materials as high capacity anodes for next generation lithium ion batteries |url=https://www.sciencedirect.com/science/article/pii/S0378775314007897 |journal=Journal of Power Sources |language=en |volume=267 |pages=469–490 |doi=10.1016/j.jpowsour.2014.05.096 |bibcode=2014JPS...267..469L |issn=0378-7753}}</ref> However, silicon anodes have a major issue of volumetric expansion during lithiation of around 360%.<ref>{{cite journal |last1=Li |first1=Xiaolin |last2=Gu |first2=Meng |last3=Hu |first3=Shenyang |last4=Kennard |first4=Rhiannon |last5=Yan |first5=Pengfei |last6=Chen |first6=Xilin |last7=Wang |first7=Chongmin |last8=Sailor |first8=Michael J. |last9=Zhang |first9=Ji-Guang |last10=Liu |first10=Jun |date=2014-07-08 |title=Mesoporous silicon sponge as an anti-pulverization structure for high-performance lithium-ion battery anodes |journal=Nature Communications |language=en |volume=5 |issue=1 |pages=4105 |doi=10.1038/ncomms5105 |pmid=25001098 |issn=2041-1723|doi-access=free |bibcode=2014NatCo...5.4105L }}</ref> This expansion may pulverize the anode, resulting in poor performance.<ref>{{cite journal |last1=Zhang |first1=Huigang |last2=Braun |first2=Paul V. |date=2012-06-13 |title=Three-Dimensional Metal Scaffold Supported Bicontinuous Silicon Battery Anodes |url=https://pubs.acs.org/doi/10.1021/nl204551m |journal=Nano Letters |language=en |volume=12 |issue=6 |pages=2778–2783 |doi=10.1021/nl204551m |pmid=22582709 |bibcode=2012NanoL..12.2778Z |issn=1530-6984}}</ref> To fix this problem, scientists looked into varying the dimensionality of the Si.<ref name=":0">{{cite journal |last1=Zuo |first1=Xiuxia |last2=Zhu |first2=Jin |last3=Müller-Buschbaum |first3=Peter |last4=Cheng |first4=Ya-Jun |date=2017-01-01 |title=Silicon based lithium-ion battery anodes: A chronicle perspective review |url=https://www.sciencedirect.com/science/article/pii/S2211285516304931 |journal=Nano Energy |language=en |volume=31 |pages=113–143 |doi=10.1016/j.nanoen.2016.11.013 |bibcode=2017NEne...31..113Z |issn=2211-2855}}</ref> Many studies have been developed in [[Silicon nanowire|Si nanowires]], Si tubes as well as Si sheets.<ref name=":0" /> As a result, composite hierarchical Si anodes have become the major technology for future applications in lithium-ion batteries. In the early 2020s, technology is reaching commercial levels with factories being built for mass production of anodes in the United States.<ref>{{cite web |last=Ohnsman |first=Alan |title=Ex-Tesla Engineer Building Silicon Anode Plant As U.S. Amps Up EV Battery Production |url=https://www.forbes.com/sites/alanohnsman/2022/05/03/ex-tesla-engineer-building-silicon-anode-plant-as-us-amps-up-ev-battery-production/ |access-date=2022-11-19 |website=Forbes |language=en}}</ref>  Furthermore, metallic lithium is another possible candidate for the anode. It boasts a higher specific capacity than silicon, however, does come with the drawback of working with the highly unstable metallic lithium.<ref name="cen.acs.org">{{cite web |url=https://cen.acs.org/materials/energy-storage/battery-materials-world-anodes-time/97/i14 | title=In the battery materials world, the anode's time has come | author=Alex Scott | date=April 7, 2019  |access-date=2022-11-19 |website=cen.acs.org}}</ref> Similarly to graphite anodes, dendrite formation is another major limitation of metallic lithium, with the solid electrolyte interphase being a major design challenge.<ref>{{cite journal |last1=Liu |first1=Bin |last2=Zhang |first2=Ji-Guang |last3=Xu |first3=Wu |date=2018-05-16 |title=Advancing Lithium Metal Batteries |journal=Joule |language=English |volume=2 |issue=5 |pages=833–845 |doi=10.1016/j.joule.2018.03.008 |issn=2542-4785|doi-access=free |bibcode=2018Joule...2..833L }}</ref> In the end, if stabilized, metallic lithium would be able to produce batteries that hold the most charge, while being the lightest.<ref name="cen.acs.org"/> In recent years, researchers have conducted several studies on the use of single wall [[Carbon nanotube|carbon nanotubes]] (SWCNTs) as conductive additives. These SWCNTs help to preserve electron conduction, ensure stable electrochemical reactions, and maintain uniform volume changes during cycling, effectively reducing anode pulverization.<ref>{{cite journal |last1=Park |first1=Gun |last2=Moon |first2=Hyeongyu |last3=Shin |first3=Sunyoung |last4=Lee |first4=Sumin |last5=Lee |first5=Yongju |last6=Choi |first6=Nam-Soon |last7=Hong |first7=Seungbum |date=2023-07-14 |title=Spatially Uniform Lithiation Enabled by Single-Walled Carbon Nanotubes |journal=ACS Energy Letters |language=en |volume=8 |issue=7 |pages=3154–3160 |doi=10.1021/acsenergylett.3c01060 |issn=2380-8195|doi-access=free }}</ref><ref>{{cite journal |last1=Dressler |first1=R. A. |last2=Dahn |first2=J. R. |date=March 2024 |title=Optimization of Si-containing and SiO based Anodes with Single-Walled Carbon Nanotubes for High Energy Density Applications |journal=Journal of the Electrochemical Society |language=en |volume=171 |issue=3 |pages=030520 |doi=10.1149/1945-7111/ad30dc |issn=1945-7111|doi-access=free |bibcode=2024JElS..171c0520D }}</ref>

=== Mechanical properties ===
A common failure mechanism of batteries is mechanical shock, which breaks either the electrode or the system's container, leading to poor conductivity and electrolyte leakage.<ref name="Why do batteries fail">{{cite journal |last1=Palacín |first1=M. R. |last2=de Guibert |first2=A. |date=2016-02-05 |title=Why do batteries fail? |url=https://www.science.org/doi/10.1126/science.1253292 |journal=Science |language=en |volume=351 |issue=6273 |pages=1253292 |doi=10.1126/science.1253292 |pmid=26912708 |hdl=10261/148077 |s2cid=11534630 |issn=0036-8075|hdl-access=free }}</ref> However, the relevance of mechanical properties of electrodes goes beyond the resistance to collisions due to its environment. During standard operation, the incorporation of ions into electrodes leads to a change in volume. This is well exemplified by Si electrodes in lithium-ion batteries expanding around 300% during lithiation.<ref>{{cite journal |last1=Li |first1=Dawei |last2=Wang |first2=Yikai |last3=Hu |first3=Jiazhi |last4=Lu |first4=Bo |last5=Cheng |first5=Yang-Tse |last6=Zhang |first6=Junqian |date=2017-10-31 |title=In situ measurement of mechanical property and stress evolution in a composite silicon electrode |journal=Journal of Power Sources |language=en |volume=366 |pages=80–85 |doi=10.1016/j.jpowsour.2017.09.004 |bibcode=2017JPS...366...80L |issn=0378-7753|doi-access=free }}</ref> Such change may lead to the deformations in the lattice and, therefore stresses in the material. The origin of stresses may be due to geometric constraints in the electrode or inhomogeneous plating of the ion.<ref name="doi.org">{{cite journal |last1=Xu |first1=Rong |last2=Zhao |first2=Kejie |date=2016-12-12 |title=Electrochemomechanics of Electrodes in Li-Ion Batteries: A Review |url=https://doi.org/10.1115/1.4035310 |journal=Journal of Electrochemical Energy Conversion and Storage |volume=13 |issue=3 |doi=10.1115/1.4035310 |issn=2381-6872}}</ref> This phenomenon is very concerning as it may lead to electrode fracture and performance loss. Thus, mechanical properties are crucial to enable the development of new electrodes for long lasting batteries. A possible strategy for measuring the mechanical behavior of electrodes during operation is by using [[nanoindentation]].<ref>{{cite journal |last1=de Vasconcelos |first1=Luize Scalco |last2=Xu |first2=Rong |last3=Zhao |first3=Kejie |date=2017 |title=Operando Nanoindentation: A New Platform to Measure the Mechanical Properties of Electrodes during Electrochemical Reactions |journal=Journal of the Electrochemical Society |language=en |volume=164 |issue=14 |pages=A3840–A3847 |doi=10.1149/2.1411714jes |s2cid=102588028 |issn=0013-4651|doi-access=free }}</ref> The method is able to analyze how the stresses evolve during the electrochemical reactions, being a valuable tool in evaluating possible pathways for coupling mechanical behavior and electrochemistry.

More than just affecting the electrode's morphology, stresses are also able to impact electrochemical reactions.<ref name="doi.org"/><ref>{{cite journal |last1=Zhao |first1=Kejie |last2=Pharr |first2=Matt |last3=Cai |first3=Shengqiang |last4=Vlassak |first4=Joost J. |last5=Suo |first5=Zhigang |date=June 2011 |title=Large Plastic Deformation in High-Capacity Lithium-Ion Batteries Caused by Charge and Discharge: Large Plastic Deformation in Lithium-Ion Batteries |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1551-2916.2011.04432.x |journal=Journal of the American Ceramic Society |language=en |volume=94 |pages=s226–s235 |doi=10.1111/j.1551-2916.2011.04432.x}}</ref> While the chemical driving forces are usually higher in magnitude than the mechanical energies, this is not true for Li-ion batteries.<ref>{{cite journal |last=Spaepen * |first=F. |date=2005-09-11 |title=A survey of energies in materials science |url=https://doi.org/10.1080/14786430500155080 |journal=Philosophical Magazine |volume=85 |issue=26–27 |pages=2979–2987 |doi=10.1080/14786430500155080 |bibcode=2005PMag...85.2979S |s2cid=220330377 |issn=1478-6435}}</ref> A study by Dr. Larché established a direct relation between the applied stress and the chemical potential of the electrode.<ref>{{cite journal |last1=Larché |first1=F |last2=Cahn |first2=J. W |date=1973-08-01 |title=A linear theory of thermochemical equilibrium of solids under stress |url=https://dx.doi.org/10.1016/0001-6160%2873%2990021-7 |journal=Acta Metallurgica |language=en |volume=21 |issue=8 |pages=1051–1063 |doi=10.1016/0001-6160(73)90021-7 |issn=0001-6160}}</ref> Though it neglects multiple variables such as the variation of elastic constraints, it subtracts from the total chemical potential the elastic energy induced by the stress.
<math display="block">\mu = \mu^\text{o} + k\cdot T\cdot\log (\gamma\cdot x)  + \Omega \cdot \sigma</math>

In this equation, ''μ'' represents the chemical potential, with ''μ''<sup>o</sup> being its reference value. ''T'' stands for the temperature and ''k'' the [[Boltzmann constant]]. The term ''γ'' inside the logarithm is the activity and ''x'' is the ratio of the ion to the total composition of the electrode. The novel term Ω is the partial molar volume of the ion in the host and ''σ'' corresponds to the mean stress felt by the system. The result of this equation is that diffusion, which is dependent on chemical potential, gets impacted by the added stress and, therefore changes the battery's performance. Furthermore, mechanical stresses may also impact the electrode's solid-electrolyte-interphase layer.<ref name="Why do batteries fail"/> The interface which regulates the ion and charge transfer and can be degraded by stress. Thus, more ions in the solution will be consumed to reform it, diminishing the overall efficiency of the system.<ref>{{cite journal |last1=Zhao |first1=Kejie |last2=Cui |first2=Yi |date=2016-12-01 |title=Understanding the role of mechanics in energy materials: A perspective |journal=Extreme Mechanics Letters |series=Mechanics of Energy Materials |language=en |volume=9 |pages=347–352 |doi=10.1016/j.eml.2016.10.003 |issn=2352-4316|doi-access=free |bibcode=2016ExML....9..347Z }}</ref>