NASICON

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2×2 unit cell of Na3Zr2(SiO4)2(PO4) (x = 2), which is the most common NASICON material;[1] red: O, purple: Na, light green: Zr, dark green: sites shared by Si and P
One unit cell of Na2Zr2(SiO4)(PO4)2 (x = 1); red: O, purple: Na, light green: Zr, dark green: sites shared by Si and P

NASICON is an acronym for sodium (Na) Super Ionic CONductor, which usually refers to a family of solids with the chemical formula Na1+xZr2SixP3−xO12, 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 conductivities, on the order of 10−3 S/cm, which rival those of liquid electrolytes. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice.[2]

Properties[]

The crystal structure of NASICON compounds was characterized in 1968. It is a covalent network consisting of ZrO6 octahedra and PO4/SiO4 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,[3] 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.[1]

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 Na4Zr2Si3O12, react with molten sodium at 300 °C, and therefore are unsuitable for electric batteries that use sodium as an electrode.[2] However, a NASICON membrane is being considered for a sodium-sulfur battery where the sodium stays solid.

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 coefficient (< 10−6 K−1), which is useful for precision instruments and household ovenware. NASICONs can be doped with rare-earth elements, such as Eu, and used as phosphors. Their electrical conductivity is sensitive to molecules in the ambient atmosphere, a phenomenon that can be used to detect CO2, SO2, NO, NO2, NH3 and H2S gases. Other NASICON applications include catalysis, immobilization of radioactive waste, and sodium removal from water.[2]

Lithium analogues[]

Some lithium phosphates also possess the NASICON structure and can be considered as the direct analogues of the sodium-based NASICONs.[4] The general formula of such compounds is LiM
2(PO
4)
3, where M identifies an element like titanium, germanium, zirconium, hafnium, or tin.[2][5] Similarly to sodium-based NASICONs, lithium-based NASICONs consist of a network of MO6 octahedra connected by PO4 tetrahedra, with lithium ions occupying the interstitial sites among them.[6] Ionic conduction is ensured by lithium hopping among adjacent interstitial sites.[6]

Lithium NASICONs are promising materials to be used as solid electrolytes in all-solid-state lithium-ion batteries.[7]

Relevant examples[]

The most investigated lithium-based NASICON materials are LiZr
2(PO
4)
3, LiTi
2(PO
4)
3,[2] and LiGe
2(PO
4)
3.[8]

Lithium zirconium phosphate[]

, identified by the formula LiZr
2(PO
4)
3 (LZP), has been extensively studied because of its polymorphism and interesting conduction properties.[2][9] At room temperature, LZP has a triclinic crystal structure (C1) and undergoes a phase transition to rhombohedral crystal structure (R3c) between 25 and 60 °C.[9] The rhombohedral phase is characterized by higher values of ionic conductivity (8×10−6 S/cm at 150 °C) compared to the triclinic phase (≈ 8×10−9 S/cm at room temperature):[9] 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.[2]

The ionic conductivity of LZP can be enhanced by elemental doping, for example replacing some of the zirconium cations with lanthanum,[9] titanium,[2] or aluminium[10][11] atoms. In case of lanthanum doping, the room-temperature ionic conductivity of the material approaches 7.2×10−5 S/cm.[9]

Lithium titanium phosphate[]

, with general formula LiTi
2(PO
4)
3 (LTP or LTPO), is another lithium-containing NASICON material in which TiO6 octahedra and PO4 tetrahedra are arranged in a rhombohedral unit cell.[8] The LTP crystal structure is stable down to 100 K and is characterized by a small coefficient of thermal expansion.[8] LTP shows low ionic conductivity at room temperature, around 10−6 S/cm;[4] however, it can be effectively increased by elemental substitution with isovalent or aliovalent elements (Al, Cr, Ga, Fe, Sc, In, Lu, Y, La).[4][8][12] The most common derivative of LTP is (LATP), whose general formula is Li
1+xAl
xTi
2-x(PO
4)
3.[8] Ionic conductivity values as high as 1.9×10−3 S/cm can be achieved when the microstructure and the aluminium content (x = 0.3 - 0.5) are optimized.[4][8] The increase of conductivity is attributed to the larger number of mobile lithium ions necessary to balance the extra electrical charge after Ti4+ replacement by Al3+, together with a contraction of the c axis of the LATP unit cell.[8][12]

In spite of attractive conduction properties, LATP is highly unstable in contact with lithium metal,[8] with formation of a lithium-rich phase at the interface and with reduction of Ti4+ to Ti3+.[7] Reduction of tetravalent titanium ions proceeds along a single-electron transfer reaction:[13]

Both phenomena are responsible for a significant increase of the electronic conductivity of the LATP material (from 3×10−9 S/cm to 2.9×10−6 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 metallic lithium as the anode.[7]

Lithium germanium phosphate[]

LGP crystal structure.[14]

, LiGe
2(PO
4)
3 (LGP), is closely similar to LTP, except for the presence of GeO6 octahedra instead of TiO6 octahedra in the rhombohedral unit cell.[8] 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), Li
1+xAl
xGe
2-x(PO
4)
3.[8] Contrary to LGP, the room-temperature ionic conductivity of LAGP spans from 10−5 S/cm up to 10−3 S/cm,[12] depending on the microstructure and on the aluminium content, with an optimal composition for x ≈ 0.5.[5] In both LATP and LAGP, non-conductive secondary phases are expected for larger aluminium content (x > 0.5 - 0.6).[8]

LAGP is more stable than LATP against lithium metal anode, since the reduction reaction of Ge4+ cations is a 4-electron reaction and has a high kinetic barrier:[13]

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.[15]

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,[8] even if their average ionic conductivity (≈10−5 - 10−4 S/cm) is lower compared to other classes of solid electrolytes like garnets and sulfides.[7] However, the use of LATP and LAGP provides some advantages:

  • Excellent stability in humid air and against CO2, with no release of harmful gases or formation of Li2CO3 passivating layer;[7]
  • High stability against water;[8]
  • Wide electrochemical stability window and high voltage stability, up to 6 V in the case of LAGP, enabling the use of high-voltage cathodes;[15]
  • Low toxicity compared to sulfide-based solid electrolytes;[8]
  • Low cost and easy preparation.[8]

A high-capacity lithium metal anode could not be coupled with a LATP solid electrolyte, because of Ti4+ reduction and fast electrolyte decomposition;[7] on the other hand, the reactivity of LAGP in contact with lithium at very negative potentials is still debated,[13] but protective interlayers could be added to improve the interfacial stability.[15]

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.[10] Proper elemental doping is an effective route to both stabilize the rhombohedral phase below 50 °C and improve the ionic conductivity.[10]

See also[]

References[]

  1. ^ a b Fergus, J. W. (2012). "Ion transport in sodium ion conducting solid electrolytes". Solid State Ionics. 227: 102–112. doi:10.1016/j.ssi.2012.09.019.
  2. ^ a b c d e f g h Anantharamulu, N.; Koteswara Rao, K.; Rambabu, G.; Vijaya Kumar, B.; Radha, V.; Vithal, M. (2011). "A wide-ranging review on Nasicon type materials". Journal of Materials Science. 46 (9): 2821. Bibcode:2011JMatS..46.2821A. doi:10.1007/s10853-011-5302-5. S2CID 136385448.
  3. ^ Knauth, P. (2009). "Inorganic solid Li ion conductors: An overview". Solid State Ionics. 180 (14–16): 911–916. doi:10.1016/j.ssi.2009.03.022.
  4. ^ a b c d Gao, Zhonghui; Sun, Huabin; Fu, Lin; Ye, Fangliang; Zhang, Yi; Luo, Wei; Huang, Yunhui (2018). "Promises, Challenges, and Recent Progress of Inorganic Solid-State Electrolytes for All-Solid-State Lithium Batteries". Advanced Materials. 30 (17): 1705702. doi:10.1002/adma.201705702.
  5. ^ a b Pershina, S. V.; Pankratov, A. A.; Vovkotrub, E. G.; Antonov, B. D. (2019). "Promising high-conductivity Li1.5Al0.5Ge1.5(PO4)3 solid electrolytes: the effect of crystallization temperature on the microstructure and transport properties". Ionics. 25 (10): 4713–4725. doi:10.1007/s11581-019-03021-5. ISSN 1862-0760.
  6. ^ a b Francisco, Brian E.; Stoldt, Conrad R.; M’Peko, Jean-Claude (2014). "Lithium-Ion Trapping from Local Structural Distortions in Sodium Super Ionic Conductor (NASICON) Electrolytes". Chemistry of Materials. 26 (16): 4741–4749. doi:10.1021/cm5013872. ISSN 0897-4756.
  7. ^ a b c d e f Campanella, Daniele; Belanger, Daniel; Paolella, Andrea (2021). "Beyond garnets, phosphates and phosphosulfides solid electrolytes: New ceramic perspectives for all solid lithium metal batteries". Journal of Power Sources. 482: 228949. doi:10.1016/j.jpowsour.2020.228949.
  8. ^ a b c d e f g h i j k l m n o DeWees, Rachel; Wang, Hui (2019). "Synthesis and Properties of NaSICON‐type LATP and LAGP Solid Electrolytes". ChemSusChem. 12 (16): 3713–3725. doi:10.1002/cssc.201900725. ISSN 1864-5631.
  9. ^ a b c d e Brummel, Ian A.; Drury, Daniel E.; Kitahara, Andrew R.; El Gabaly, Farid; Ihlefeld, Jon F. (2021). "Temperature and processing effects on lithium ion conductivity of solution‐deposited lithium zirconium phosphate (LiZr 2 P 3 O 12 ) thin films". Journal of the American Ceramic Society. 104 (2): 711–721. doi:10.1111/jace.17483. ISSN 0002-7820.
  10. ^ a b c Zhang, Yibo; Chen, Kai; Shen, Yang; Lin, Yuanhua; Nan, Ce-Wen (2017). "Enhanced lithium-ion conductivity in a LiZr 2 (PO 4 ) 3 solid electrolyte by Al doping". Ceramics International. 43: S598–S602. doi:10.1016/j.ceramint.2017.05.198.
  11. ^ Reddy, I. Neelakanta; Akkinepally, Bhargav; Reddy, Ch. Venkata; Sreedhar, Adem; Ko, Tae Jo; Shim, Jaesool (September 2020). "A systematic study of annealing environment and Al dopant effect on NASICON-type LiZr2(PO4)3 solid electrolyte". Ionics. 26 (9): 4287–4298. doi:10.1007/s11581-020-03622-5.
  12. ^ a b c Zhang, Bingkai; Tan, Rui; Yang, Luyi; Zheng, Jiaxin; Zhang, Kecheng; Mo, Sijia; Lin, Zhan; Pan, Feng (2018). "Mechanisms and properties of ion-transport in inorganic solid electrolytes". Energy Storage Materials. 10: 139–159. doi:10.1016/j.ensm.2017.08.015.
  13. ^ a b c Safanama, Dorsasadat; Adams, Stefan (2017). "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". Journal of Power Sources. 340: 294–301. doi:10.1016/j.jpowsour.2016.11.076.
  14. ^ Weiss, Manuel; Weber, Dominik A.; Senyshyn, Anatoliy; Janek, Jürgen; Zeier, Wolfgang G. (2018). "Correlating Transport and Structural Properties in Li 1+ x Al x Ge 2– x (PO 4 ) 3 (LAGP) Prepared from Aqueous Solution". ACS Applied Materials & Interfaces. 10 (13): 10935–10944. doi:10.1021/acsami.8b00842. ISSN 1944-8244.
  15. ^ a b c Liu, Yijie; Li, Chao; Li, Bojie; Song, Hucheng; Cheng, Zhu; Chen, Minrui; He, Ping; Zhou, Haoshen (2018). "Germanium Thin Film Protected Lithium Aluminum Germanium Phosphate for Solid-State Li Batteries". Advanced Energy Materials. 8 (16): 1702374. doi:10.1002/aenm.201702374. ISSN 1614-6840.
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