View source for NASICON

{{Short description|Class of solid materials}}
[[File:Na3Zr2(SiO4)2(PO4).png|260px|thumb|2×2 unit cell of Na<sub>3</sub>Zr<sub>2</sub>(SiO<sub>4</sub>)<sub>2</sub>(PO<sub>4</sub>) (x = 2), which is the most common NASICON material;<ref name=fergus/> red: O, purple: Na, light green: Zr, dark green: sites shared by Si and P]]
[[File:Na2Zr2(SiO4)(PO4)2.png|260px|thumb|One unit cell of Na<sub>2</sub>Zr<sub>2</sub>(SiO<sub>4</sub>)(PO<sub>4</sub>)<sub>2</sub> (x = 1); red: O, purple: Na, light green: Zr, dark green: sites shared by Si and P]]

'''NASICON''' is an [[acronym]] for sodium  (Na) [[superionic conductor|super ionic conductor]], which usually refers to a family of solids with the chemical formula Na<sub>1+x</sub>Zr<sub>2</sub>Si<sub>x</sub>P<sub>3−x</sub>O<sub>12</sub>, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements. NASICON compounds have high [[Ionic conductivity (solid state)|ionic conductivities]], on the order of 10<sup>−3</sup> S/cm, which rival those of liquid [[electrolyte]]s. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice.<ref name=vithal/>

==Properties==
The crystal structure of NASICON compounds was characterized in 1968. It is a covalent network consisting of ZrO<sub>6</sub> [[octahedra]] and PO<sub>4</sub>/SiO<sub>4</sub> [[tetrahedra]] that share common corners. Sodium ions are located at two types of interstitial positions. They move among those sites through bottlenecks, whose size, and thus the NASICON [[electrical conductivity]], depends on the NASICON composition, on the site occupancy,<ref name=knauth/> and on the oxygen content in the surrounding atmosphere. The conductivity decreases for x < 2 or when all Si is substituted for P in the crystal lattice (and vice versa); it can be increased by adding a rare-earth compound to NASICON, such as [[yttria]].<ref name=fergus/>

NASICON materials can be prepared as single crystals, polycrystalline ceramic compacts, thin films or as a bulk glass called '''NASIGLAS'''. Most of them, except NASIGLAS and phosphorus-free Na<sub>4</sub>Zr<sub>2</sub>Si<sub>3</sub>O<sub>12</sub>, react with molten sodium at 300&nbsp;°C, and therefore are unsuitable for electric batteries that use sodium as an electrode.<ref name=vithal/> However, a NASICON membrane is being considered for a [[Sodium–sulfur battery|sodium-sulfur battery]] where the sodium stays solid.

==Development and potential applications ==
The main application envisaged for NASICON materials is as the solid electrolyte in a [[sodium-ion battery]]. Some NASICONs exhibit a low [[Thermal expansion#Coefficients|thermal expansion coefficient]] (< 10<sup>−6</sup> K<sup>−1</sup>), which is useful for precision instruments and household ovenware. NASICONs can be doped with [[Rare-earth element|rare-earth elements]], such as Eu, and used as [[phosphor]]s. Their electrical conductivity is sensitive to molecules in the ambient atmosphere, a phenomenon that can be used to detect CO<sub>2</sub>, SO<sub>2</sub>, NO, NO<sub>2</sub>, NH<sub>3</sub> and H<sub>2</sub>S gases. Other NASICON applications include [[catalysis]], immobilization of radioactive waste, and sodium removal from water.<ref name=vithal/>

The development of sodium-ion batteries is important since it makes use of an earth-abundant material and can serve as an alternative to [[Lithium-ion battery|lithium-ion batteries]] which are experiencing ever-increasing demand despite the limited availability of [[lithium]]. Developing high-performance sodium-ion batteries is a challenge because it is necessary to develop electrodes that meet the requirements of high-energy density and high cycling stability while also being cost-efficient. NASICON-based electrode materials are known for their wide range of electrochemical potentials, high ionic conductivity, and most importantly their structural and thermal stabilities.<ref>{{Cite journal |last1=Singh |first1=Baltej |last2=Wang |first2=Ziliang |last3=Park |first3=Sunkyu |last4=Gautam |first4=Gopalakrishnan Sai |last5=Chotard |first5=Jean-Noël |last6=Croguennec |first6=Laurence |last7=Carlier |first7=Dany |last8=Cheetham |first8=Anthony K. |last9=Masquelier |first9=Christian |last10=Canepa |first10=Pieremanuele |date=2021-01-05 |title=A chemical map of NaSICON electrode materials for sodium-ion batteries |url=https://pubs.rsc.org/en/content/articlelanding/2021/ta/d0ta10688g |journal=Journal of Materials Chemistry A |language=en |volume=9 |issue=1 |pages=281–292 |doi=10.1039/D0TA10688G |issn=2050-7496}}</ref> NASICON-type cathode materials for sodium-ion batteries have a mechanically robust three-dimensional (3D) framework with open channels that endow it with the capability for fast ionic diffusion.<ref>{{Cite journal |last1=Liu |first1=Yukun |last2=Li |first2=Jie |last3=Shen |first3=Qiuyu |last4=Zhang |first4=Jian |last5=He |first5=Pingge |last6=Qu |first6=Xuanhui |last7=Liu |first7=Yongchang |date=2022-01-01 |title=Advanced characterizations and measurements for sodium-ion batteries with NASICON-type cathode materials |journal=eScience |language=en |volume=2 |issue=1 |pages=10–31 |doi=10.1016/j.esci.2021.12.008 |s2cid=245575050 |issn=2667-1417|doi-access=free }}</ref> A strong and lasting structural framework allows for repeated {{Chem|Na|+}} ion de-/insertions with relatively high operating potentials. Its high safety, high potential, and low volume change make NASICON a promising candidate for sodium-ion battery cathodes.<ref>{{Cite journal |last1=Jiang |first1=Yu |last2=Zhou |first2=Xuefeng |last3=Li |first3=Dongjun |last4=Cheng |first4=Xiaolong |last5=Liu |first5=Fanfan |last6=Yu |first6=Yan |date=June 2018 |title=Highly Reversible Na Storage in Na 3 V 2 (PO 4 ) 3 by Optimizing Nanostructure and Rational Surface Engineering |url=https://onlinelibrary.wiley.com/doi/10.1002/aenm.201800068 |journal=Advanced Energy Materials |language=en |volume=8 |issue=16 |pages=1800068 |doi=10.1002/aenm.201800068|s2cid=104243685 }}</ref>

NASICON cathodes typically suffer from poor electrical conductivity and low specific capacity which severely limits their practical applications. Efforts to enhance the movement of electrons, or electrical conductivity, include particle downsizing<ref>{{Cite journal |last1=Guin |first1=M. |last2=Tietz |first2=F. |last3=Guillon |first3=O. |date=2016-10-01 |title=New promising NASICON material as solid electrolyte for sodium-ion batteries: Correlation between composition, crystal structure and ionic conductivity of Na3+xSc2SixP3−xO12 |url=https://www.sciencedirect.com/science/article/pii/S0167273816301394 |journal=Solid State Ionics |language=en |volume=293 |pages=18–26 |doi=10.1016/j.ssi.2016.06.005 |issn=0167-2738}}</ref> and carbon-coating<ref>{{Cite journal |last1=Hu |first1=Qiao |last2=Yu |first2=Mufan |last3=Liao |first3=Jiaying |last4=Wen |first4=Zhaoyin |last5=Chen |first5=Chunhua |date=2018-01-30 |title=Porous carbon-coated NaTi2(PO4)3 with superior rate and low-temperature properties |url=https://pubs.rsc.org/en/content/articlelanding/2018/ta/c7ta10207k |journal=Journal of Materials Chemistry A |language=en |volume=6 |issue=5 |pages=2365–2370 |doi=10.1039/C7TA10207K |issn=2050-7496}}</ref> which have both been reported to improve the electrochemical performance.

It is important to consider the relationship between lattice parameters and [[activation energy]] as the change in lattice size has a direct influence on the size of the pathway for {{Chem|Na|+}} conduction as well as the hopping distance of the {{Chem|Na|+}} ions to the next vacancy. A large hopping distance requires a high activation energy.<ref>{{Cite journal |last1=Wang |first1=Qianchen |last2=Gao |first2=Hongcai |last3=Li |first3=Jingbo |last4=Liu |first4=Gui-Bin |last5=Jin |first5=Haibo |date=2021-03-31 |title=Importance of Crystallographic Sites on Sodium-Ion Extraction from NASICON-Structured Cathodes for Sodium-Ion Batteries |url=https://pubs.acs.org/doi/10.1021/acsami.1c01663 |journal=ACS Applied Materials & Interfaces |language=en |volume=13 |issue=12 |pages=14312–14320 |doi=10.1021/acsami.1c01663 |pmid=33749228 |s2cid=232309319 |issn=1944-8244}}</ref>

NASICON-phosphate {{Chem|Na|3|V|2|(|PO|4|)|3}} compounds are considered promising cathodes with a theoretical [[specific energy]] of 400 W h kg<sup>−1</sup>. [[Vanadium]]-based compounds exhibit satisfactory high energy densities that are comparable to those of lithium-ion batteries as they operate through multi-electron redox reactions (V3+/V4+ and V4+/V5+) and a high operating voltage.<ref>{{Cite journal |last1=Chen |first1=Mingzhe |last2=Hua |first2=Weibo |last3=Xiao |first3=Jin |last4=Cortie |first4=David |last5=Chen |first5=Weihua |last6=Wang |first6=Enhui |last7=Hu |first7=Zhe |last8=Gu |first8=Qinfen |last9=Wang |first9=Xiaolin |last10=Indris |first10=Sylvio |last11=Chou |first11=Shu-Lei |last12=Dou |first12=Shi-Xue |date=2019-04-01 |title=NASICON-type air-stable and all-climate cathode for sodium-ion batteries with low cost and high-power density |journal=Nature Communications |language=en |volume=10 |issue=1 |pages=1480 |doi=10.1038/s41467-019-09170-5 |issn=2041-1723 |pmc=6443767 |pmid=30931938|bibcode=2019NatCo..10.1480C }}</ref> The use of vanadium is toxic and expensive which introduces a critical issue in real applications. This concern holds true for other electrodes based on costly 3d transition metal elements such as Ni- or Co-based electrodes. The most abundant and non-toxic 3d element, iron, is the favored choice as the redox center in the polyanionic or mixed-polyanion system.<ref>{{Cite journal |last1=Kim |first1=Hyungsub |last2=Kim |first2=Haegyeom |last3=Ding |first3=Zhang |last4=Lee |first4=Myeong Hwan |last5=Lim |first5=Kyungmi |last6=Yoon |first6=Gabin |last7=Kang |first7=Kisuk |date=October 2016 |title=Recent Progress in Electrode Materials for Sodium-Ion Batteries |url=https://onlinelibrary.wiley.com/doi/10.1002/aenm.201600943 |journal=Advanced Energy Materials |language=en |volume=6 |issue=19 |pages=1600943 |doi=10.1002/aenm.201600943|s2cid=100280194 }}</ref>

== Lithium analogues ==
Some [[lithium]] phosphates also possess the NASICON structure and can be considered as the direct analogues of the sodium-based NASICONs.<ref name=":4">{{Cite journal|last1=Gao|first1=Zhonghui|last2=Sun|first2=Huabin|last3=Fu|first3=Lin|last4=Ye|first4=Fangliang|last5=Zhang|first5=Yi|last6=Luo|first6=Wei|last7=Huang|first7=Yunhui|date=2018|title=Promises, Challenges, and Recent Progress of Inorganic Solid-State Electrolytes for All-Solid-State Lithium Batteries|url=http://doi.wiley.com/10.1002/adma.201705702|journal=Advanced Materials|language=en|volume=30|issue=17|pages=1705702|doi=10.1002/adma.201705702|pmid=29468745 |bibcode=2018AdM....3005702G |s2cid=5011915 }}</ref> The general formula of such compounds is {{Chem|Li|3=M|4=2|5=(|6=P|8=O|9=4|10=)|11=3}}, where M identifies an element like [[titanium]], [[germanium]], [[zirconium]], [[hafnium]], or [[tin]].<ref name="vithal" /><ref name=":5">{{Cite journal|last1=Pershina|first1=S. V.|last2=Pankratov|first2=A. A.|last3=Vovkotrub|first3=E. G.|last4=Antonov|first4=B. D.|date=2019|title=Promising high-conductivity Li1.5Al0.5Ge1.5(PO4)3 solid electrolytes: the effect of crystallization temperature on the microstructure and transport properties|url=https://doi.org/10.1007/s11581-019-03021-5|journal=Ionics|language=en|volume=25|issue=10|pages=4713–4725|doi=10.1007/s11581-019-03021-5|s2cid=182714008 |issn=1862-0760}}</ref> Similarly to sodium-based NASICONs, lithium-based NASICONs consist of a network of MO<sub>6</sub> octahedra connected by [[Phosphate|PO<sub>4</sub>]] tetrahedra, with lithium ions occupying the interstitial sites among them.<ref name=":0">{{Cite journal|last1=Francisco|first1=Brian E.|last2=Stoldt|first2=Conrad R.|last3=M’Peko|first3=Jean-Claude|date=2014|title=Lithium-Ion Trapping from Local Structural Distortions in Sodium Super Ionic Conductor (NASICON) Electrolytes|url=https://pubs.acs.org/doi/10.1021/cm5013872|journal=Chemistry of Materials|language=en|volume=26|issue=16|pages=4741–4749|doi=10.1021/cm5013872|issn=0897-4756}}</ref> Ionic conduction is ensured by lithium hopping among adjacent interstitial sites.<ref name=":0" />

Lithium NASICONs are promising materials to be used as [[Solid-state electrolyte|solid electrolytes]] in all-solid-state [[Lithium-ion battery|lithium-ion batteries]].<ref name=":6">{{Cite journal|last1=Campanella|first1=Daniele|last2=Belanger|first2=Daniel|last3=Paolella|first3=Andrea|date=2021|title=Beyond garnets, phosphates and phosphosulfides solid electrolytes: New ceramic perspectives for all solid lithium metal batteries|url=https://linkinghub.elsevier.com/retrieve/pii/S0378775320312489|journal=Journal of Power Sources|language=en|volume=482|pages=228949|doi=10.1016/j.jpowsour.2020.228949|bibcode=2021JPS...48228949C |s2cid=224953720 }}</ref>

=== Relevant examples ===
The most investigated lithium-based NASICON materials are {{Chem|Li|3=Zr|4=2|5=(|6=P|8=O|9=4|10=)|11=3}}, {{Chem|Li|3=Ti|4=2|5=(|6=P|8=O|9=4|10=)|11=3}},<ref name="vithal" /> and {{Chem|Li|3=Ge|4=2|5=(|6=P|8=O|9=4|10=)|11=3}}.<ref name=":2">{{Cite journal|last1=DeWees|first1=Rachel|last2=Wang|first2=Hui|date=2019|title=Synthesis and Properties of NaSICON‐type LATP and LAGP Solid Electrolytes|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.201900725|journal=ChemSusChem|language=en|volume=12|issue=16|pages=3713–3725|doi=10.1002/cssc.201900725|pmid=31132230 |s2cid=167209150 |issn=1864-5631}}</ref>

==== Lithium zirconium phosphate ====
[[Lithium zirconium phosphate]], identified by the formula {{Chem|Li|3=Zr|4=2|5=(|6=P|8=O|9=4|10=)|11=3}} (LZP), has been extensively studied because of its [[Polymorphism (materials science)|polymorphism]] and interesting conduction properties.<ref name="vithal" /><ref name=":3">{{Cite journal|last1=Brummel|first1=Ian A.|last2=Drury|first2=Daniel E.|last3=Kitahara|first3=Andrew R.|last4=El Gabaly|first4=Farid|last5=Ihlefeld|first5=Jon F.|date=2021|title=Temperature and processing effects on lithium ion conductivity of solution‐deposited lithium zirconium phosphate (LiZr 2 P 3 O 12 ) thin films|journal=Journal of the American Ceramic Society|language=en|volume=104|issue=2|pages=711–721|doi=10.1111/jace.17483|issn=0002-7820|doi-access=free}}</ref> At room temperature, LZP has a [[Triclinic crystal system|triclinic crystal structure]] (''C''<span style="text-decoration:overline">1</span>) and undergoes a [[phase transition]] to [[Rhombohedral crystal system|rhombohedral crystal structure]] (R<span style="text-decoration:overline">3</span>c) between 25 and 60&nbsp;°C.<ref name=":3" /> The rhombohedral phase is characterized by higher values of ionic conductivity (8×10<sup>−6</sup> S/cm at 150&nbsp;°C) compared to the triclinic phase (≈ 8×10<sup>−9</sup> S/cm at room temperature):<ref name=":3" /> such difference may be ascribed to the peculiar distorted tetrahedral coordination of lithium ions in the rhombohedral phase, along with the large number of available empty sites.<ref name="vithal" />

The ionic conductivity of LZP can be enhanced by elemental doping, for example replacing some of the zirconium cations with [[lanthanum]],<ref name=":3" /> [[titanium]],<ref name="vithal" /> or [[aluminium]]<ref name=":9">{{Cite journal|last1=Zhang|first1=Yibo|last2=Chen|first2=Kai|last3=Shen|first3=Yang|last4=Lin|first4=Yuanhua|last5=Nan|first5=Ce-Wen|date=2017|title=Enhanced lithium-ion conductivity in a LiZr 2 (PO 4 ) 3 solid electrolyte by Al doping|url=https://linkinghub.elsevier.com/retrieve/pii/S0272884217310131|journal=Ceramics International|language=en|volume=43|pages=S598–S602|doi=10.1016/j.ceramint.2017.05.198}}</ref><ref>{{cite journal |last1=Reddy |first1=I. Neelakanta |last2=Akkinepally |first2=Bhargav |last3=Reddy |first3=Ch. Venkata |last4=Sreedhar |first4=Adem |last5=Ko |first5=Tae Jo |last6=Shim |first6=Jaesool |title=A systematic study of annealing environment and Al dopant effect on NASICON-type LiZr2(PO4)3 solid electrolyte |journal=Ionics |date=September 2020 |volume=26 |issue=9 |pages=4287–4298 |doi=10.1007/s11581-020-03622-5|s2cid=219183976 }}</ref> atoms. In case of lanthanum doping, the room-temperature ionic conductivity of the material approaches 7.2×10<sup>−5</sup> S/cm.<ref name=":3" />

==== Lithium titanium phosphate ====
[[Lithium titanium phosphate]], with general formula {{Chem|Li|3=Ti|4=2|5=(|6=P|8=O|9=4|10=)|11=3}} (LTP or LTPO), is another lithium-containing NASICON material in which TiO<sub>6</sub> octahedra and [[Phosphate|PO<sub>4</sub>]] tetrahedra are arranged in a rhombohedral unit cell.<ref name=":2" /> The LTP crystal structure is stable down to 100 K and is characterized by a small coefficient of thermal expansion.<ref name=":2" /> LTP shows low ionic conductivity at room temperature, around 10<sup>−6</sup> S/cm;<ref name=":4" /> however, it can be effectively increased by elemental substitution with isovalent or aliovalent elements ([[Aluminium|Al]], [[Chromium|Cr]], [[Gallium|Ga]], [[Iron|Fe]], [[Scandium|Sc]], [[Indium|In]], [[Lutetium|Lu]], [[Yttrium|Y]], [[Lanthanum|La]]).<ref name=":4" /><ref name=":2" /><ref name=":7">{{Cite journal|last1=Zhang|first1=Bingkai|last2=Tan|first2=Rui|last3=Yang|first3=Luyi|last4=Zheng|first4=Jiaxin|last5=Zhang|first5=Kecheng|last6=Mo|first6=Sijia|last7=Lin|first7=Zhan|last8=Pan|first8=Feng|date=2018|title=Mechanisms and properties of ion-transport in inorganic solid electrolytes|url=https://linkinghub.elsevier.com/retrieve/pii/S2405829717300685|journal=Energy Storage Materials|language=en|volume=10|pages=139–159|doi=10.1016/j.ensm.2017.08.015}}</ref> The most common derivative of LTP is [[lithium aluminium titanium phosphate]] (LATP), whose general formula is {{chem|Li|1+x|Al|x|Ti|2-x|(|9=P|11=O|12=4|13=)|14=3}}.<ref name=":2" /> Ionic conductivity values as high as 1.9×10<sup>−3</sup> S/cm can be achieved when the microstructure and the aluminium content (''x'' = 0.3 - 0.5) are optimized.<ref name=":4" /><ref name=":2" /> The increase of conductivity is attributed to the larger number of mobile lithium ions necessary to balance the extra electrical charge after Ti{{sup|4+}} replacement by Al{{sup|3+}}, together with a contraction of the ''c'' axis of the LATP unit cell.<ref name=":2" /><ref name=":7" />

In spite of attractive conduction properties, LATP is highly unstable in contact with lithium metal,<ref name=":2" /> with formation of a lithium-rich phase at the interface and with [[Reduction reaction|reduction]] of Ti{{sup|4+}} to Ti{{sup|3+}}.<ref name=":6" /> Reduction of tetravalent titanium ions proceeds along a single-electron transfer reaction:<ref name=":8">{{Cite journal|last1=Safanama|first1=Dorsasadat|last2=Adams|first2=Stefan|date=2017|title=High efficiency aqueous and hybrid lithium-air batteries enabled by Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 ceramic anode-protecting membranes|url=https://linkinghub.elsevier.com/retrieve/pii/S0378775316316275|journal=Journal of Power Sources|language=en|volume=340|pages=294–301|doi=10.1016/j.jpowsour.2016.11.076|bibcode=2017JPS...340..294S }}</ref>

<chem>LiTi2(PO4)3 + Li -> Li2Ti2(PO4)3</chem>

Both phenomena are responsible for a significant increase of the [[Electrical conductivity|electronic conductivity]] of the LATP material (from 3×10<sup>−9</sup> S/cm to 2.9×10<sup>−6</sup> S/cm), leading to the degradation of the material and to the ultimate cell failure if LATP is used as a solid electrolyte in a lithium-ion battery with [[Lithium|metallic lithium]] as the anode.<ref name=":6" />

==== Lithium germanium phosphate ====
[[File:LGP crystal structure.png|thumb|LGP crystal structure.<ref>{{Cite journal|last1=Weiss|first1=Manuel|last2=Weber|first2=Dominik A.|last3=Senyshyn|first3=Anatoliy|last4=Janek|first4=Jürgen|last5=Zeier|first5=Wolfgang G.|date=2018|title=Correlating Transport and Structural Properties in Li 1+ x Al x Ge 2– x (PO 4 ) 3 (LAGP) Prepared from Aqueous Solution|url=https://pubs.acs.org/doi/10.1021/acsami.8b00842|journal=ACS Applied Materials & Interfaces|language=en|volume=10|issue=13|pages=10935–10944|doi=10.1021/acsami.8b00842|pmid=29516733 |osti=1434719 |issn=1944-8244}}</ref>]]
[[Lithium germanium phosphate]], {{Chem|Li|3=Ge|4=2|5=(|6=P|8=O|9=4|10=)|11=3}} (LGP), is closely similar to LTP, except for the presence of GeO<sub>6</sub> octahedra instead of TiO<sub>6</sub> octahedra in the rhombohedral unit cell.<ref name=":2" /> Similarly to LTP, the ionic conductivity of pure LGP is low and can be improved by doping the material with aliovalent elements like [[aluminium]], resulting in [[lithium aluminium germanium phosphate]] (LAGP), {{chem|Li|1+x|Al|x|Ge|2-x|(|9=P|11=O|12=4|13=)|14=3}}.<ref name=":2" /> Contrary to LGP, the room-temperature ionic conductivity of LAGP spans from 10<sup>−5</sup> S/cm up to 10<sup>−3</sup> S/cm,<ref name=":7" /> depending on the microstructure and on the aluminium content, with an optimal composition for ''x'' ≈ 0.5.<ref name=":5" /> In both LATP and LAGP, non-conductive secondary phases are expected for larger aluminium content (''x'' > 0.5 - 0.6).<ref name=":2" />

LAGP is more stable than LATP against lithium metal anode, since the reduction reaction of Ge{{sup|4+}} cations is a 4-electron reaction and has a high [[Chemical kinetics|kinetic]] barrier:<ref name=":8" />

<chem>2LiGe2(PO4)3 + 4Li -> 3GeO2 + 6LiPO3 + Ge</chem>

However, the stability of the lithium anode-LAGP interface is still not fully clarified and the formation of detrimental interlayers with subsequent battery failure has been reported.<ref name=":10">{{Cite journal|last1=Liu|first1=Yijie|last2=Li|first2=Chao|last3=Li|first3=Bojie|last4=Song|first4=Hucheng|last5=Cheng|first5=Zhu|last6=Chen|first6=Minrui|last7=He|first7=Ping|last8=Zhou|first8=Haoshen|date=2018|title=Germanium Thin Film Protected Lithium Aluminum Germanium Phosphate for Solid-State Li Batteries|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.201702374|journal=Advanced Energy Materials|language=en|volume=8|issue=16|pages=1702374|doi=10.1002/aenm.201702374|s2cid=103584410 |issn=1614-6840}}</ref>

=== Application in lithium-ion batteries ===
Phosphate-based materials with a NASICON crystal structure, especially LATP and LAGP, are good candidates as solid-state electrolytes in lithium-ion batteries,<ref name=":2" /> even if their average ionic conductivity (≈10<sup>−5</sup> - 10<sup>−4</sup> S/cm) is lower compared to other classes of solid electrolytes like [[garnet]]s and [[Sulfide|sulfides]].<ref name=":6" /> However, the use of LATP and LAGP provides some advantages:

* Excellent stability in humid air and against [[Carbon dioxide|CO<sub>2</sub>]], with no release of harmful gases or formation of Li<sub>2</sub>CO<sub>3</sub> [[Passivation (chemistry)|passivating]] layer;<ref name=":6" />
* High stability against water;<ref name=":2" />
* Wide [[Electrochemical window|electrochemical stability window]] and high voltage stability, up to 6 V in the case of LAGP, enabling the use of high-voltage cathodes;<ref name=":10" />
* Low toxicity compared to sulfide-based solid electrolytes;<ref name=":2" />
* Low cost and easy preparation.<ref name=":2" />

A high-capacity lithium metal anode could not be coupled with a LATP solid electrolyte, because of Ti{{sup|4+}} reduction and fast electrolyte decomposition;<ref name=":6" /> on the other hand, the reactivity of LAGP in contact with lithium at very negative potentials is still debated,<ref name=":8" /> but protective interlayers could be added to improve the interfacial stability.<ref name=":10" />

Considering LZP, it is predicted to be electrochemically stable in contact with metallic lithium; the main limitation arises from the low ionic conductivity of the room-temperature triclinic phase.<ref name=":9" /> Proper elemental doping is an effective route to both stabilize the rhombohedral phase below 50&nbsp;°C and improve the ionic conductivity.<ref name=":9" />

== See also ==

* [[Lithium aluminium germanium phosphate]]
* [[LISICON]]
* [[Solid-state electrolyte]]
* [[Sodium-ion battery]]
* [[Lithium-ion battery]]

==References==
{{reflist|refs=
<ref name=fergus>{{Cite journal | doi = 10.1016/j.ssi.2012.09.019| title = Ion transport in sodium ion conducting solid electrolytes| journal = Solid State Ionics| volume = 227| pages = 102–112| year = 2012| last1 = Fergus | first1 = J. W. }}</ref>
<ref name=knauth>{{Cite journal | doi = 10.1016/j.ssi.2009.03.022| title = Inorganic solid Li ion conductors: An overview| journal = Solid State Ionics| volume = 180| issue = 14–16| pages = 911–916| year = 2009| last1 = Knauth | first1 = P. }}</ref>
<ref name=vithal>{{Cite journal | doi = 10.1007/s10853-011-5302-5| title = A wide-ranging review on Nasicon type materials| journal = Journal of Materials Science| volume = 46| issue = 9| pages = 2821| year = 2011| last1 = Anantharamulu | first1 = N.| last2 = Koteswara Rao | first2 = K.| last3 = Rambabu | first3 = G.| last4 = Vijaya Kumar | first4 = B.| last5 = Radha | first5 = V. | last6 = Vithal | first6 = M.| bibcode = 2011JMatS..46.2821A| s2cid = 136385448}}</ref>
}}{{Subject bar|portal1=Chemistry|portal2=Energy|portal3=Science}}

{{Lithium compounds}}

[[Category:Electrolytes]]
[[Category:Sodium compounds]]
[[Category:Lithium compounds]]
[[Category:Phosphates]]