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Computational and Experimental investigation of Nalipoite-Li2APO4 (A = Na, K) electrolytes for Li-ion batteries

G. F. Ortiz1, M C. López1, M.E. Arroyo-de Dompablo2 and José L. Tirado1 1 Inorganic Chemistry Laboratory, University of Córdoba, Campus of Rabanales, Marie Curie Building, Cordoba E-14071, Spain 2 Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

ABSTRACT The potential ionic conductors Li2APO4 (A = Na, K) are investigated combining experiments and first principles calculations at the Density Functional Theory level. A high ionic conductivity of 6.5 x10-6 and 1.5 x10-5 S cm-1 at 25 and 70ºC, respectively, is found in Nalipoite-Li2NaPO4. For this mixed phosphate the energy barriers to Li motion are calculated. The lower energy barrier (0.7 eV) implies the inter-chain diffusion of Li in the b-c plane. We predict that ionic mobility is enhanced in the isostructural Li2KPO4, with the lowest calculated energy barrier being 0.4 eV. INTRODUCTION Lithium phosphates materials are permeable to lithium ions and impermeable to electrons, which serve as good candidate for a solid electrolyte in lithium batteries and related technologies. Although Li3PO4 electrolytes fulfill the requirements of an ideal solid state lithium ion conductor [1], they have not been used in practical lithium-ion batteries because of their low ionic conductivities. The measured conductivities at 25 ºC of β and γ-Li3PO4 forms are 8.62 x108 S cm-1 and 4.2 x 10-18 S cm-1, respectively.[2,3]. Chemical or structural modifications of Li3PO4 might result in improved solid electrolytes for Li-ion batteries. To the date little is know about the ionic conductivity of mixed Li3-xAxPO4 materials. Interestingly, for x=1, the mineral Nalipoite exists, having the composition Li2NaPO4.[4,5] The crystal structure of this sodium dilithium phosphate (space group Pmnb) consists on a PO4 and LiO4 tetrahedra corner-linked framework based on a stacking of two-dimensional nets. Na occupies the octahedral cages resulting from the three-dimensional network of PO4 and LiO4 tetrahedra. In this structure chains of LiO4 tetrahedra run parallel to the a axis; these chains are connected by vertex along the c axis. We have recently reported [6] that the nalipoite-Li2NaPO4 displays high ionic conductivities of 6.5 x10-6 and 1.5 x10-5 S cm-1 at 25 and 70ºC, respectively, which has been ascribed to the motion of Li ions. The observed conductivity values of Nalipoite-Li2NaPO4 are higher than that of β- and γLi3PO4 phases by about two orders of magnitude, making very appealing the investigation of mixed monovalent ion phosphates for potential solid electrolytes in Li-ion batteries. In this

sense, other cations could replace Na ions in the octahedral sites of the Nalipoite structure. Following our previous study [6], in this work we combine computations and experiments to investigate the lithium motion in Nalipote-Li2NaPO4 and in the virtual Li2KPO4 phase. METHODOLOGY Nalipoite Li2NaPO4 was synthesized and c