Behavior of LiMn 2 O 4 Single Crystals as Battery Cathodes
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Behavior of LiMn2O4 Single Crystals as Battery Cathodes María Ángeles Monge1, José Manuel Amarilla1, Enrique Gutiérrez-Puebla1, Juan Antonio Campa2, and Isidoro Rasines1 1 Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain. 2 Facultad de Ciencias Geológicas, UCM, 28040 Madrid, Spain. ABSTRACT This paper deals with: i) the growth of LiMn2O4 crystals by electrocrystallisation; ii) the response of these crystals in lithium cells; and iii) a method to follow their structural and morphological changes while working as electrodes. The defects present in LiMn2O4 crystals are determined after refining the occupation at every site of the spinel structure by single-crystal Xray diffraction analyses, which lead to define two possible paths for the process of Li deinsertioninsertion. It is shown how the crystals studied follow one of these paths during cycling of the battery by virtue of a dynamic mechanism consisting in Mn migrations cooperatively induced by Li insertion and extraction.
INTRODUCTION Until recently the electrochemical behavior of LiMn2O4 has been always studied on powdered samples. LiMn2O4 crystals large enough to solve their crystal structure by singlecrystal X-ray diffraction (XRD) were not grown until recently [1]. LiMn2O4, like the mineral spinel, crystallises in the cubic system, space group (S.G.) Fd 3 m, Z = 8, with Li at tetrahedral 8a sites, Mn at the octahedral 16d positions, oxygen at 32e (u,u,u), with a = 8.2483(6) Å and u = 0.26320(18) [1] taking the origin at the inversion center, 3 m. More recently, superlattice cells have also been described [2] for crystals which were mixed with powdered LiMn2O4 and electrochemically delithiated. After growing LiMn2O4 crystals by various techniques, electrocrystallisation led the authors to obtain good-quality black single crystals of various sizes from 0.1 to 1 mm, by systematically varying growth conditions such as flux, temperature, time, voltage and current. All these crystals were examined by single-crystal XRD. EXPERIMENTAL DETAILS LiMn2O4 crystals were grown in a three-electrode cell (Pt electrodes 1.5 mm diameter at a 10 mm distance, rotary Pt crucibles and a Pt wire 0.3 mm diameter as reference electrode) from mixtures of analytical grade reagents which were heated in an open-crucible furnace [3]. Once melted the mixtures were allowed to equilibrate for one hour and a half after reaching the operating crucible temperature. LiBO2 (5 g), Mn2O3 (3 g) and Na2MoO4 (30 g), a temperature of 870 ºC which was maintained for 2 hours, a potential of 0.30 V and currents of 150-175 mA were employed for batch A; and LiBO2 (5 g), Mn2O3 (2 g), and Na2MoO4 (40 g), a temperature of 960 ºC for 12 hours, 0.15 V and 19-26 mA for batch B. Crystals adhered to the electrode were separated from the matrix and washed in warm water. One crystal as grown from each batch was carefully selected: A1 was the most representative crystal of the majority in cell parameter, occupations, and composition; and B1, the most different from A1. After
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