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Lithium analogues

Some lithium phosphates also possess the NASICON structure and can be considered as the direct analogues of the sodium-based NASICONs. The general formula of such compounds is LiM
₂(PO
₄)
₃, where M identifies an element like titanium, germanium, zirconium, hafnium, or tin. 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. Ionic conduction is ensured by lithium hopping among adjacent interstitial sites .

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

Relevant examples

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

Lithium zirconium phosphate

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. 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. 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): 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.

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

Lithium titanium phosphate

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. The LTP crystal structure is stable down to 100K and is characterized by a small coefficient of thermal expansion. LTP shows low ionic conductivity at room temperature, around 10^-6 S/cm; however, it can be effectively increased by elemental substitution with isovalent or aliovalent elements (Al, Cr, Ga, Fe, Sc, In, Lu, Y, La). The most common derivative of LTP is lithium aluminium titanium phosphate (LATP), whose general formula is Li
_1+xAl
_xTi
_2-x(PO
₄)
₃. 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. 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 .

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

${\ce {2LiTi2(PO4)3 + Li -> Li2Ti2(PO4)3}}$

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.

Lithium germanium phosphate

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

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:

${\ce {2LiGe2(PO4)3 + 4Li -> 3GeO2 + 6LiPO3 + Ge}}$

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.

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, 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. 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;
• High stability against water;
• Wide electrochemical stability window and high voltage stability, up to 6 V vs. L/Li⁺ in the case of LAGP, enabling the use of high-voltage cathodes;
• Low toxicity compared to sulfide-based solid electrolytes;
• Low cost and easy preparation.

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

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