NASICON
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.
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.
Other NASICON applications include catalysis, immobilization of radioactive waste, and sodium removal from water.
NaSICON-based electrode materials are known for their wide range of electrochemical potentials, high ionic conductivity, and most importantly their structural and thermal stabilities.
[4] 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.
[6] 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[7] and carbon-coating[8] which have both been reported to improve the electrochemical performance.
The most abundant and non-toxic 3d element, iron, is the favored choice as the redox center in the polyanionic or mixed-polyanion system.
[16] Lithium zirconium phosphate, identified by the formula LiZr2(PO4)3 (LZP), has been extensively studied because of its polymorphism and interesting conduction properties.
[2] The ionic conductivity of LZP can be enhanced by elemental doping, for example replacing some of the zirconium cations with lanthanum,[17] titanium,[2] or aluminium[18][19] atoms.
[17] Lithium titanium phosphate, with general formula LiTi2(PO4)3 (LTP or LTPO), is another lithium-containing NASICON material in which TiO6 octahedra and PO4 tetrahedra are arranged in a rhombohedral unit cell.
[12][16] 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.
[16][20] In spite of attractive conduction properties, LATP is highly unstable in contact with lithium metal,[16] with formation of a lithium-rich phase at the interface and with reduction of Ti4+ to Ti3+.
[16] Contrary to LGP, the room-temperature ionic conductivity of LAGP spans from 10−5 S/cm up to 10−3 S/cm,[20] depending on the microstructure and on the aluminium content, with an optimal composition for x ≈ 0.5.
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] However, the use of LATP and LAGP provides some advantages: A high-capacity lithium metal anode could not be coupled with a LATP solid electrolyte, because of Ti4+ reduction and fast electrolyte decomposition;[15] on the other hand, the reactivity of LAGP in contact with lithium at very negative potentials is still debated,[21] but protective interlayers could be added to improve the interfacial stability.
[23] 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.
[18] Proper elemental doping is an effective route to both stabilize the rhombohedral phase below 50 °C and improve the ionic conductivity.
