Structural and transport properties of lithium-conducting NASICON materials

Lithium-containing NASICON-structured materials are a promising class of solid-state Li-ion conductors for application in electrochemical energy storage devices. Amongst the wide variety of possible compositions the highest conductivities are reported for materials according to the formula (Li1+xMxM2-x(IV))-M-(III)(PO4)(3), in which the substitution of tetravalent with trivalent metal cations leads to incorporation of additional lithium ions and a higher mobility of the charge carriers. For this study, we surveyed more than 300 research articles about Li-NASICON materials. The relations between composition, structure and conductivity are evaluated to give a comprehensive overview of published data on synthesized compositions. A special focus is laid on Li1+xAlxTi2-x (PO4)(3) as the single most conductive and investigated material. The collected conductivities show a wide scattering in a range of 10(-10) S cm(-1) up to 10(-3) S cm(-1). The highest values are obtained for materials with M(III) to M(IV) cation ratios of x = 0.3-0.4. Further characteristics for high conductivity are evaluated and the rhombohedral structure as well as cation sizes of around 50-60 p.m. are identified as crucial prerequisites, favoring titanium-based compositions. Considering the evaluated data, selected compositions are suggested for further investigation to support future research.