Magnetic and thermoelectric properties of layered Li_xNa_yCoO₂

Abstract

The magnetic, thermoelectric, and structural properties of Li_xNa_yCoO₂, prepared by intercalation and deintercalation chemistry from the thermodynamically stable phase Li_0.41Na_0.31CoO₂, which has an alternating Li/Na sequence along the c-axis, are reported. For the high Li-Na content phases Li_0.41Na_0.31CoO₂ and Li_0.40Na_0.43CoO₂, a sudden increase in susceptibility is seen below 50 K, whereas for Li_0.21Na_0.14CoO₂ an antiferromagnetic-like transition is seen at 10 K, in spite of a change from dominantly antiferromagnetic to dominantly ferromagnetic interactions with decreasing alkali content. The Curie constant decreases linearly with decreasing alkali content, at the same time the temperature-independent contribution to the susceptibility increases, indicating that as the Co becomes more oxidized the electronic states become more delocalized. Consistent with this observation, the low alkali containing phases have metallic-like resistivities. The 300 K thermopowers fall between 30 μV/K (x+y=0.31) and 150 μV/K (x+y=0.83).

Introduction

The study of layered A_xCoO₂ cobaltates (A=Li or Na) is of interest from the viewpoint of both basic science and industrial applications. Li_xCoO₂ is used in Li-ion batteries [1] while Na_xCoO₂ shows a wide variety of electronic states [2], including high thermopower [3], [4] and superconductivity when water is intercalated [5]. The compounds are based on triangular CoO₂ layers built from CoO₆ octahedra sharing edges, alternating with layers of alkali ions. A single thermodynamically stable LiCoO₂ phase exists [1], [6] while for Na_xCoO₂ three thermodynamically stable phases can be obtained [7], [8], [9]. These phases are distinguished by the Na content, number of CoO₂ sheets per unit cell (1, 2 or 3), and by the coordination of the alkali ion (trigonal prismatic or octahedral). By changing the alkali content by chemie douce or electrochemical methods, the electronic properties of the family can be tuned.

Na_xCoO₂ is found in both 2- and 3-layer derived series. Both show similar electronic properties as a function of x in spite of differences in the crystal structures due to layer stacking [8], [9]. This suggests that the physical properties are dominated by charge transport within the CoO₂ layers, with the band filling controlled via the Na content. For Li_xCoO₂, only a 3-layer derived series exists. There are clear similarities between the Na and Li series: an insulator to metal transition is observed on decreasing x, and at x=0.5 an A⁺/vacancy ordering is reported [2], [10]. One notable difference is the impossibility (so far) of making a suitably water intercalated, superconducting Li_0.3CoO₂:yH₂O phase [11]. Here we report an investigation into a related layered cobaltate that contains distinct layers of Li and Na between CoO₂ planes.

The synthesis and structural characterization of a thermodynamically stable Li_xNa_yCoO₂ phase was first reported by Balsys and Davis [12]. Their sample had composition Li_0.43(2)Na_0.36(2)CoO_1.96(6) and was found to crystallize in space group P6₃mc with a=2.839(4) and c=20.36(3) Å. The oxygen packing in this structure is ABCAACBA, and there are four CoO₂ sheets per unit cell. The Li⁺ and Na⁺ ions are not mixed, but are found in separate planes between the CoO₂ sheets: the Li⁺ ions are in octahedral coordination with oxygen, while the Na⁺ ions are in trigonal prisms. The Na⁺ ion has two trigonal prismatic sites available, analogous to the 2-layer Na_xCoO₂ phase [8]. More recently, the thermoelectric properties of this phase at composition Li_0.48Na_0.35CoO₂ were reported [13]. At room temperature, a thermopower of 180 μV/K and resistivity of 20 mΩ cm were observed. The reported thermal conductivity is 2 W m⁻¹ K⁻¹ (cf. Bi₂Te₃ has 3 W m⁻¹ K⁻¹ at 300 K) [14] leading to a thermoelectric figure of merit (ZT) of 0.02 at 300 K. Improvement of ZT will depend on reduction of the resistivity. For example (keeping everything else constant), a resistivity of 1 mΩ cm would lead to a ZT∼0.4 at 300 K. Given the wide variety of electronic states observed in A_xCoO₂ (A=Li, Na) and the potential of Li_0.48Na_0.35CoO₂ as a thermoelectric, it is of much interest to study the structures and physical properties of Li_xNa_yCoO₂, where x and y are varied by intercalation and deintercalation.