Electrical conductivity in lithium orthophosphates K. Rissouli a , K. Benkhouja a , J.R. Ramos-Barrado b , C. Julien c, * a LMCM, De´partement de Chimie, Faculte´ des Sciences, Universite´ Chouaib Doukkali, BP 20, El Jadida 24000, Morocco b Departamento de Fisica Aplicada, Universidad de Malaga, 299071 Malaga, Spain c LMDH, UMR7603, Universite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France Received 20 March 2002; accepted 24 October 2002 Abstract The electrical conductivity of lithium orthophosphates was investigated as a function of the nature of the transition-metal cation. LiMPO 4 (M/Mn, Co, Ni) compounds and their derivatives were investigated. The enhancement in ionic conductivity appeared for compounds containing manganese cations. A relationship between crystal structure and ionic conductivity was established and discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Olivine-type compounds; Lithium orthophosphates; Electrical conductivity 1. Introduction Recently, solid-state ionics gains intensive develop- ment, especially in the energy storage related field, due to the potential applications of solid electrolyte and electrode materials as well. Extensive works have been carried out for the search of superior fast lithium-ion conductors as membrane in advanced batteries has been carried out [1,2]. It has been shown by Goodenough et al. that frameworks built up from PO 4 or from P 2 O 7 polyanions are good candidates for the use of solid electrolytes [3] and electrode materials [4] in recharge- able lithium cells. The low atomic weight of lithium and the high negative Gibbs energies of formation of many lithium compounds, which may be formed in the course of discharge, are very promising in this respect. Previous studies of the crystal chemistry and ion transport properties of alkali-based phosphates A x B 2 (PO 4 ) 3 , with A /Li, Na and B /transition-metal, have shown that the sodium members have a Nasicon- type structure [5], while lithium compounds have been shown to exhibit various structures depending on the substitution at the B site [6]. Lithium orthophosphates LiMPO 4 (M /Mn, Co, Ni) [7,8] adopt an olivine- related structure, which consists of a hexagonal closed- packing (HCP) of oxygen atoms with Li  and M 2 cations located in half of the octahedral sites and P 5 cations in 1/8 of tetrahedral sites. This structure may be described as chains (along the c direction) of edge- sharing MO 6 octahedra which are cross-linked by the PO 4 groups forming a three-dimensional network (Fig. 1). Tunnels perpendicular to the [0 1 0] and [0 0 1] directions contain octahedrally coordinated Li  cations (along the b axis), which are mobile in these cavities. The ionic conductivity of Li 3 Sc 2 (PO 4 ) 3 ceramics was optimized to 2 /10 9 S cm 1 at room temperature [9]. The increase of the conductivity was also obtained in Li 3 Sc 2 (PO 4 ) 3 by the substitution of the scandium with aluminium and yttrium. Recently, orthorhombic phos- pho-olivine were investigated as high-voltage positive electrode active materials for lithium secondary bat- teries. LiCoPO 4 exhibited the highest 4.8 V discharge plateau of 100 mA h g 1 vs. Li/Li  [10,11]. In contrast, LiNiPO 4 and LiMnPO 4 had no voltage plateaus in their discharge profiles even after initial charging to 5.2 V but the observed irreversible capacity is attributed to the poor electronic conductivity [10]. Thus, it is an impor- tant issue to collect data on electrical transport in these compounds. * Corresponding author. Tel.: /33-144-274561; fax: /33-144- 273854 E-mail addresses: krissouli@caramail.com (K. Rissouli), cjul@ccr.jussieu.fr (C. Julien). Materials Science and Engineering B98 (2003) 185 /189 www.elsevier.com/locate/mseb 0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5107(02)00574-3