Researchers in Spain are proposing the use of a dilithium sodium phosphate as an inorganic solid electrolyte. As reported in the ACS journal Inorganic Chemistry, the Li2NaPO4 material showed ionic conductivity values of 6.5 × 10−6 and 1.5 × 10−5 S cm−1 at 25 and 70 °C, respectively. The conductivity value is higher than those of β- and γ-Li3PO4 phases by about 2 orders of magnitude, making Li2NaPO4 an appealing candidate for future solid electrolytes in batteries.
Lithium phosphates are promising candidates as lithium ionic conductor materials for high-energy batteries and other related technologies. Indeed, they are materials permeable to lithium ions and impermeable to electrons, which serve as good candidates for solid electrolytes. In addition, they are lightweight and have a wide potential range of stability versus lithium and lithium-containing electrodes. Nevertheless, lithium phosphate electrolytes have not been used in practical lithium ion batteries because their ionic conductivities are generally lower than those of superionic conductors (10−2 S cm−1) and do not meet the required current densities. Chemical modifications of Li3PO4 might result in interesting ionic conductors.
…While there have been a large number of reports dealing with the study of Li3PO4 polymorphs, Li2NaPO4 has been proportionally considered in less detail. Dilithium sodium phosphate (Li2NaPO4, nalipoite) is new mineral species from Mont Saint-Hilaire, Québec. Its structure was resolved by T. S. Ercit in 1991. To the best of our knowledge, no data reporting the ionic conductivity of this composition are found in the literature.
… These results open new perspectives by finding alternative diffusion paths that allow enhanced lithium/sodium ionic conductors in the Li3−xNaxPO4 family. DFT calculations were used to evaluate the thermodynamic stabilities of several polymorphs for this family. We found that for small x values (ca. x < 1.5), Na ions could be accommodated in the β-Li3PO4 network. Compounds with greater Na contents could be stable within crystal structures possessing octahedral sites.—López et al.
María C.López, Gregorio F. Ortiz, Elena M. Arroyo-de Dompablo, and José L.Tirado (2014) “An Unnoticed Inorganic Solid Electrolyte: Dilithium Sodium Phosphate with the Nalipoite Structure,” Inorganic Chemistry doi: 10.1021/ic4030537