Formation and conductivity studies of lithium argyrodite solid electrolytes using in-situ neutron diffraction R. Prasada Rao a , N. Sharma b , V.K. Peterson b , S. Adams a, ⁎ a Department of Materials Science and Engineering, National University of Singapore, Singapore-117576, Singapore b Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia abstract article info Article history: Received 30 April 2012 Received in revised form 28 August 2012 Accepted 13 September 2012 Available online 7 October 2012 Keywords: Fast ion conducting solids Neutron diffraction Microstructure–conductivity relationships All solid state lithium batteries Lithium-ion conducting argyrodites Li 6 PS 5 X (X = Cl, Br, I) are a promising class of fast-ion conductors for all-solid state Li-ion batteries. To gain a deeper insight into the phase formation of Li 6 PS 5 Cl, in situ neutron diffraction studies are carried out on a stoichiometric ball-milled precursor mixture during thermal treat- ment. The evolution of the S 2- /Cl - anion disorder and its correlation with ionic conductivity are reported here. In contrast to earlier reports, an argyrodite phase is found to form between 80 and 150 °C, but the phase shows only moderate conductivity when crystallized at such low temperatures and further thermal treatment is required to access the highly conducting phase. The maximum room-temperature ionic conduc- tivity of 1.1 × 10 -3 S/cm is observed for samples annealed at intermediate temperatures (250 °C). When ball-milled glass-ceramic precursors for Li 6 PS 5 Cl are crystallized with a constant slow heating rate, the initial- ly formed argyrodite phase is found to be Li 7 PS 6 , which is then gradually converted into Li 6 PS 5 Cl at higher temperatures. The industrial requirements for minimizing cost by using lower annealing temperatures thus need to be balanced with the requirements of obtaining the highest conducting composition of the phase for performance in all-solid state batteries. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Among the fast-ion conducting solids, sulphur-based solid electro- lytes are known for their high ionic conductivity due to the larger size and higher polarizability of the sulphide ions (when compared to O 2- ) that facilitate the migration of mobile cations [1–4]. Argyrodites are a class of chalcogenide structures related to the mineral Ag 8 GeS 6 , which include various fast A + cation conductors A 6 PS 5 X (A=Ag + , Cu + ) [5,6]. Recently, Deiseroth et al. [7] and our group [8] synthesized the analogous cubic Li + argyrodites with formula Li 6 PS 5 X (X = Cl, Br, I). 7 Li-NMR relax- ation studies suggest local Li-ion mobility in the Li-argyrodite crystals to be as high as 10 -2 –10 -3 S/cm at room temperature, close to that of liquid electrolytes used in Li-ion batteries which typically comprises a LiPF 6 salt dissolved in various polycarbonates. With such high Li mobil- ities, these argyrodite-type materials may be ideal for use as solid elec- trolytes in Li-ion or Li batteries [9]. In our previous work, we prepared argyrodite-type Li 6 PS 5 X (X=Cl, Br, I) using mechanical milling followed by annealing. Rietveld refine- ments using joint high-resolution neutron and laboratory X-ray powder diffraction data provided insight into the influence of disorder on the ionic conductivity. Besides the disorder in the Li-distribution, we find that the disorder in the S 2- /Cl - or S 2- /Br - distribution promotes ion mobility. The larger I - cannot be exchanged for S 2- resulting in an anion-ordered Li 6 PS 5 I exhibiting low ionic conductivity [8,10]. Due to the complex Li + transport pathways consisting of interpenetrating cages, the direct current (DC) conductivity of Li 6 PS 5 Cl annealed at 550 °C is slightly lower (0.7×10 -3 S/cm) than to be expected from the NMR-based local mobilities, but still in the technologically interest- ing range [10]. To further understand the influence of preparation conditions on the conductivity of crystalline Li 6 PS 5 Cl, we monitored the process of argyrodite phase formation from ball-milled precursors using in situ neutron diffraction. In parallel, we investigated the conductivity varia- tion by impedance spectroscopy for identical ball-milled precursor mix- tures during their heating to various temperatures, which permitted us to correlate the identified intermediate phases with their respective ionic conductivities. Finally we demonstrate the performance of all- solid state batteries using Li 7-x PS 6-x Cl x as the solid electrolyte. 2. Sample preparation and characterisation Li 6 PS 5 Cl was synthesized by reacting stoichiometric quantities of Li 2 S (Alfa Aesar, 99.8%), P 2 S 5 (Loba Chemie, 99%), and LiCl (Alfa Aesar, 99.8%). The samples were prepared by high-energy mechanical milling using an agate bowl and balls (45 ml bowl and 15 balls of 10 mm diameter) for 20 h. All procedures were conducted under Ar atmosphere. In situ neutron powder diffraction (NPD) data were collected using the Solid State Ionics 230 (2013) 72–76 ⁎ Corresponding author at: National University of Singapore, Department of Materials Science and Engineering, 5 Engineering Drive 2, E2 #05-22, Singapore 17576, Singapore. Tel.: +65 6516 6869; fax: +65 6776 3604. E-mail address: mseasn@nus.edu.sg (S. Adams). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2012.09.014 Contents lists available at SciVerse ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi