International Journal of Inorganic ChemistryVolume 2011 (2011), Article ID 649183, 11 pagesdoi:10.1155/2011/649183

Research Article

Studies on Formation and Decomposition of the Layered Cobaltite LiNa

A. S. Semenova,¹ R. F. Samigullina,¹ E. V. Shalaeva,¹ N. I. Kourov,² and D. G. Kellerman1,3

¹Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Science, 620990 Ekaterinburg, Russia²Institute of Metal Physics, Ural Branch of the Russian Academy of Science, 620041 Ekaterinburg, Russia³Department of Analitical Chemistry, Institute of Natural Sciences, Ural Federal University named after the First Russian President B. N. Yeltsin, 620002 Ekaterinburg, Russia

Received 23 June 2011; Accepted 6 December 2011

Academic Editor: Rabindranath Mukherjee

Copyright © 2011 A. S. Semenova et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Lithium-sodium cobaltite LixNayCoO₂ () with ordered layered structure is promising thermoelectric material. Here we report the peculiarities of this phase formation and decomposition processes studied by TG/DTA, XRD, SEM, and TEM methods. Significant weight loss resulting from oxygen liberation and endothermic effect at °C are the main features of the LixNayCoO₂ formation. Decomposition of the ordered phase is accompanied by exothermic effect at °C and the weight increase. Electron microscopic study shows that the lithium-sodium cobaltite decomposes into LiCoO₂ and γ-Na0.7CoO₂. Lattices of the initial phase and formed ones demonstrate mutual orientation.

1. Introduction

Over the last twenty years, layered metal oxides with the general formula AMeO₂ (A-alkaline and Me-transition metal) have been the most extensively studied materials since the properties they possess make them attractive electrode materials for chemical power sources [1, 2]. Special attention is being paid to the lithium cobaltite LiCoO₂ as a material used in commercial Li-ion batteries. However, the interest in the Li1−x CoO₂ system is due not only to its practical importance but also to the dramatic changes in structural, electronic, and magnetic properties occurring as a result of Li deintercalation/intercalations [3–9].

Regardless of the lithium content, the lithium cobaltites Li1−x CoO₂ have a hexagonal structure with Li–O–Co–O–Li–O⋯ layers stacked in the ABCABC sequence with an octahedral coordination of alkaline and transition elements [10, 11]. A different situation is observed for NaxCoO₂. There are phases, whose structures depend not only on the content of the alkaline element but also on the preparation conditions [12, 13]. Similarly to the lithium cobaltite, the structures of the NaxCoO₂ phases consist of alkaline ion triangular planes positioned between edge-sharing CoO₆ octahedra layers. However, the coordination of sodium ions can be either octahedral or prismatic depending on the three- or two-layer packing [14, 15]. A great interest in the NaxCoO₂ phases was stimulated by the discovery of superconductivity in the two-layer Na0.3CoO₂ intercalated with water [16]. Moreover, the NaxCoO₂ system was found to show a variety of intriguing phenomena [17–21].

By analogy with the lithium cobaltite, NaxCoO₂ was initially proposed as a promising cathode material [22]. However, after it was shown that this compound (specified at first as NaCo₂O₄) exhibits simultaneously high thermoelectric power and low resistivity (S ≈ 100 μV/K and ρ≈ 200 μΩ cm at 300 K) [23], the phases of the Na–Co–O system were considered as potential thermoelectrics [24–26].

The results of sodium and lithium cobaltites studies give us grounds to expect that the phase containing both alkaline metals simultaneously should demonstrate unique physical and functional properties. However, no solid solutions were found to exist in the LiCoO₂-NaxCoO₂ system [27]. At the same time, an ordered phase with a layered structure (Li0.43Na0.36CoO1.96) was reported [28] to form in the region close to the equiatomic alkaline ions concentration. The structure of this phase was attributed to the P6₃mc space group with hexagonal lattice parameters and Å and the ABCAACBA packing of oxygen layers. It is essential that lithium and sodium ions are not mixed but occupy separate planes between CoO₂ layers. Here, Li ions have an octahedral environment, while sodium ions are in the trigonal prisms [28]. This ordered phase is a good thermoelectric material indeed [29]. In addition, effects of magnetic ordering were observed [30] at . However, despite the promising properties of the ordered lithium-sodium cobaltite, there are only few works [27–33] devoted to this phase. The reason is that the double cobaltite is difficult to synthesize because it is thermodynamically stable only at °C [28]. The purpose of the present work is to study the peculiarities of formation and decomposition of the ordered lithium-sodium cobaltite “Li0.5Na0.5CoO₂.” This information should be useful to ascertain the temperature range appropriative for “Li0.5Na0.5CoO₂” practical applications. On the other hand the ordered structure of the “Li0.5Na0.5CoO₂” can be considered as an example of long-period superlattice usually more typical for alloys than for oxides. Peculiarities of such phase decomposition are also of interest.

2. Synthesis of Materials and Experimental Methods

The mixed lithium-sodium cobaltite “Li0.5Na0.5CoO₂” was prepared by a solid-state reaction from a mixture of LiCoO₂ and γ-Na0.7CoO₂ (1 : 1) synthesized in advance as described previously [34]. The unit-cell parameters were Å,Å and Å, Å for LiCoO₂ and Na0.7CoO₂, respectively.

X-ray diffraction experiments were carried out using a SHIMADZU diffractometer MAXima_X XRD-7000. XRD patterns were analyzed with the JCPDS-ICDD database (ICDD, USA, Release 2004), and the lattice parameters were obtained using the WinXpow program. The composition of the mixed lithium-sodium cobaltite was determined by inductively coupled plasma atomic-emission spectrometry ICP-AES measurements (ICAP 6500, Thermo Fisher).

Thermal analysis measurements were carried out with a simultaneous TG-DTA apparatus—SETARAM SETSYS Evolution 1750 both in static air and in dynamic oxygen at the flow rate of 20 mL/min.

The thermopower and electrical resistivity data were collected with system for measuring of the electric properties on base Solartron 7081 Precision Voltmeter (Yokogawa 7651 Programmable DC Source). The experiments were carried out in temperature range 60–300 K.

Transmission electron microscopy was performed in diffraction and imaging modes on a JEM-200CX operating at a voltage of 160 keV. The microdiffraction patterns were obtained from regions of ~200–500 nm in size. The samples suitable for the electron diffraction analysis were prepared through ultrasonic dispersion in ethanol for 5 min of powders precrushed in a mortar. A few droplets of the ethanol suspension were deposited on a holey carbon Cu grid.

The morphology of the samples was observed on a JEOL JSM-6390 LA scanning electron microscope (SEM).

3. Results and Discussion

3.1. The Study of the Ordered Cobaltite “Li0.5Na0.5CoO₂” Formation

There are essentially two ways to prepare the ordered mixed layered cobaltite “Li0.5Na0.5CoO₂”: from presynthesized LiCoO₂ and NaxCoO₂ and from a Co₃O₄, Na₂CO₃, and Li₂CO₃ powder mixture. According to previous works, it is not easy to obtain a pure phase no matter which method is employed. Most often LiCoO₂ is found as an impurity in the final product [30]. In order to evaluate possible sources of discrepancy between the chemical compositions of the final product and the raw mixture, we examined the following processes: formation of NaxCoO₂, formation of LiCoO₂, and preparation of the mixed cobaltite Li0.5Na0.5CoO₂ from carbonates and Co₃O₄ and from pre-synthesized LiCoO₂ and NaxCoO₂.

3.1.1. Formation of NaxCoO₂ from a Mixture of Co₃O₄ and Na₂CO₃

Let us consider processes occurring during the interaction of the Co₃O₄ oxide and the sodium carbonate Na₂CO₃ taken in the ratio required for the obtaining of the hexagonal cobaltite γ-Na0.7CoO₂ (Sp. gr. P6₃/mmc) [12]. As follows from the thermal analysis data (Figure 1(a)), both the decomposition of the sodium carbonate Na₂CO₃ (1) and the formation of the NaxCoO₂ cobaltite (2) start at °C and are accompanied by mass changes:

(a)

(b)

(c)

(d)

Figure 1: Thermal analysis of cobaltites formation (heating rate 5°C/min, in air). (a) Formation of the γ-NaxCoO₂ from a mixture of Co₃O₄ and Na₂CO₃: 1-TG, 2-DTG, 3-DTA. Dotted lines—adsorbed water removal; (b) formation of the LiCoO₂ from a mixture of Co₃O₄ and Li₂CO₃: 1-TG, 2-DTA; (c) formation of the mixed lithium-sodium cobaltite “Li0.5Na0.5CoO₂” by the reaction (6): 1-TG, 2-DTG, 3-DTA; (d) formation of the mixed lithium-sodium cobaltite “Li0.5Na0.5CoO₂” from a mixture of LiCoO₂ and γ-Na0.7CoO₂: 1-TG, 2-DTA.

Melting of the undecomposed part of the sodium carbonate intensifies reaction (2), which is evident from a more abrupt way of the TG curve. The endothermic effect at 860°C is indicative of the melting. If reaction (2) is not completed, unreacted residues of the cobalt oxide can decompose (3):

The endothermic effect observed at 911°C corresponds to this process.

Both DTG and DTA curves reveal peaks at 1020°C, which can be ascribed to melting and evaporation of the formed sodium cobaltite. Na0.7CoO₂ was found [35] to melt incongruently. Due to the decomposition of the cobalt oxide (3) and evaporation of molten sodium cobaltite, the total weight loss is almost 20% higher than that resulting from reaction (2). The possibility of these processes must be taken into account in synthesis of the mixed lithium-sodium cobaltite “Li0.5Na0.5CoO₂.”

3.1.2. Formation of LiCoO₂ from a Mixture of Co₃O₄ and Li₂CO₃

The TG-DTA curves of a mixture of the Co₃O₄ oxide and the lithium carbonate Li₂CO₃ taken in the ratio necessary for the obtaining of the hexagonal cobaltite LiCoO₂ are shown in Figure 1(b). In comparison with Na0.7CoO₂, the lithium cobaltite LiCoO₂ is a more stable phase, and there are no effects on the curves associated with melting or evaporation. Decomposition of Co₃O₄ via reaction (3) does not occur either, since the formation of LiCoO₂ proceeds with a greater speed and terminates to 900°C, when this reaction is possible.

Weight losses are revealed beginning with the temperature 400°C. Initially, they are caused probably by the decomposition of the lithium carbonate:

It should be noted that although Li₂CO₃ is stable in air to 750°C [36], in reaction mixtures with several oxides it can decompose at much lower temperatures [37, 38]. This process is accompanied by a diffuse endothermic effect at about 500°C.

At higher temperatures, mainly reactions of lithium cobaltite formation take place:

In this region, the TG curve becomes more flattened. The endothermic effect observed on the DTA curve at 723°C may be attributed to melting of the unreacted Li₂CO₃ residue. As is seen from Figure 1(b), the weight loss near the melting point becomes greater probably owing to an increased rate of reaction (5) with participation of the molten carbonate. Total losses are ≈16% of the initial weight, which corresponds to the theoretical value.

3.1.3. Formation of the Mixed Lithium-Sodium Cobaltite “Li0.5Na0.5CoO₂” from a Mixture of Carbonates and Co₃O₄

As follows from [28], the mixed layered cobaltite has the composition Li0.43Na0.36CoO1.96. The formation of this particular composition from a mixture of carbonates and the cobalt oxide is described as

Figure 1(c) clearly shows that the process is of a two-stage character. The first stage (300–680°C) is similar to the lithium cobaltite formation (Figure 1(b)). However, the real weight loss at this stage is by about 13% higher than that in reaction (5). This evidently means that the sodium carbonate also participates in the first stage of the mixed cobaltite formation. To confirm parallel formation of lithium and sodium cobaltites at the first stage of the process, a mixture of carbonates and Co₃O₄ taken in the ratio corresponding to reaction (6) was annealed at 686°C independently of the thermogravimetric experiment and then quenched. X-ray diffraction analysis of the quenched sample revealed the presence both of LiCoO₂ and γ-Na0.7CoO₂ phases (Figure 2(a)). In addition, small traces of Co₃O₄ were fixed (strong diffraction lines for Co₃O₄ are within a different angle interval and therefore were not present in the given pattern).

(a)

(b)

Figure 2: Fragments of powder diffraction patterns of the annealed and quenched mixture of carbonates and Co₃O₄ taken in the ratio corresponding to reaction (6): (a) °C; (b) °C.

As it is seen from the thermogram (Figure 1(c)), the second stage of the ordered mixed cobaltite synthesis begins at the temperature ≈780°C, which is significantly higher than that for the Na0.7CoO₂ formation (Figure 1(c)). Therefore, it can be assumed that the second stage is described mainly by the following reaction:

The formation of the mixed lithium-sodium cobaltite was confirmed by XRD data. Figure 2(b) demonstrates a part of the X-ray diffraction pattern of the annealed at 895°C and quenched mixture of carbonates and Co₃O₄ taken in the ratio corresponding to reaction (6). After quenching from 895°C, the sample contained a significant amount of LiCoO₂ in addition to the ordered cobaltite “Li0.5Na0.5CoO₂.”

The DTA curve (curve 3, Figure 1(c)) reveals a number of endothermic effects: the effects at ≈500°C and ≈837°C correspond to active decomposition of lithium and sodium carbonates, respectively, while the thermal effect at 866°C is associated with melting of residual undecomposed sodium carbonate. A weak endothermic effect at 930°C points to the formation of the mixed lithium-sodium cobaltite.

3.1.4. Formation of “Li0.5Na0.5CoO₂” from a Mixture of LiCoO₂ and Na0.7CoO₂

Numerous and therefore difficult-to-control chemical processes occurring during synthesis of “Li0.5Na0.5CoO₂” from carbonates and Co₃O₄ complicate the obtaining of a single-phase product. So, it is reasonable to consider the thermal behavior of a mixture of LiCoO₂ and γ-Na0.7CoO₂ prepared in advance and taken in the ratio required to obtain Li0.43Na0.36CoO₂. The mixture was heated both in air and in oxygen flow. According to XRD, this process actually leads to the obtaining of the desired mixed cobaltite “Li0.5Na0.5CoO₂.” Figure 1(d) shows the weight changes and the corresponding thermal effects accompanying the interaction between lithium and sodium cobaltites. The weight changes below T≈ 1000°C are not accompanied by any thermal effects and according to [39] reflect the oxygen stoichiometry variation in γ-Na0.7CoO2±z. The endothermic effect at °C (Figure 1(d), curve 2) indicates that it is exactly at this temperature that the formation of the ordered phase becomes possible. The main feature of the ordering process is a significant weight loss (curve 1). As is evident from Figure 1(d), this weight loss is at least partly reversible and therefore can be associated only with oxygen liberation. During a relatively slow cooling under thermogravimetric experiment conditions, the ordered phase “Li0.5Na0.5CoO₂” decomposes, which is attested by the exothermic effect at ≈940°C and the weight increase. XRD measurements of a slowly cooled sample reveal the presence of significant amounts of LiCoO₂ and γ-Na0.7CoO₂. Therefore, in order to prepare a pure mixed cobaltite “Li0.5Na0.5CoO₂,” it is necessary to quench the sample from –1025°C. Heating to higher temperatures turned out to result in an irreversible loss of sodium in the form of γ-Na0.7CoO₂. The final weight decrease observed on the thermal curve (1—heating, Figure 1(d)) corresponds to this process. Crystals of the sodium cobaltite were found on the walls of the crucible (Figure 3, insert). In this case, the reaction product after cooling contains an admixture of lithium cobaltite. Sodium loss can be suppressed if annealing is carried out in oxygen flow. Figure 3 shows the diffraction patterns of samples obtained by annealing of LiCoO₂-γ-Na0.7CoO₂ mixtures in oxygen flow and in air. As is seen, the first pattern reflects only the target single-phase formation, whereas the second pattern contains diffraction lines inherent to the LiCoO₂ impurity.

Figure 3: Powder diffraction patterns of samples obtained by annealing of LiCoO₂-γ-Na0.7CoO₂ mixtures at °C in oxygen flow, and in air with following quenching. Insert: crystals of the sodium cobaltite found on the walls of the crucible after annealing of the mixture in air.

3.2. Characterization of the Mixed Lithium-Sodium Cobaltite “Li0.5Na0.5CoO₂”

Based on foregoing DTA-TG studies of the lithium-sodium cobaltite formation, we chose the following conditions for its synthesis. Stoichiometric amounts of LiCoO₂ and γ-Na0.7CoO₂ were mixed thoroughly in an agate mortar, pressed into a pellet, annealed at 1050°C for 6 hours in oxygen flow, and then quenched onto a thick copper plate. According to X-ray analysis, the obtained sample had a structure identical to that described in [28] and did not contain any impurities (Figure 3). The hexagonal cells parameters of “Li0.5Na0.5CoO₂” were  Å and  Å. The contents of alkaline elements determined by ICP-AES analysis were found to be 2.86 and 8.10 wt% for Li and Na, respectively. This allowed us to write the formula of the lithium-sodium mixed cobaltite as Li0.42Na0.36CoO₂. According to TG data (Figure 1(d)), the weight loss during the synthesis of the mixed cobaltite was about 3 wt%. This corresponds with the liberation of ≈10% of the initial oxygen, and hence the stoichiometric oxygen index in the mixed cobaltite close to the formation temperature is ≈1.8. During decomposition of the Li-Na phase into LiCoO₂ and γ-Na0.7CoO₂, the oxygen content increases (Figure 1(d)). So, the appearance of a rather great amount of oxygen defects may be considered as the necessary condition for the mixed lithium-sodium cobaltite formation. In order to estimate the oxygen content in the mixed Li-Na cobaltite at room temperature, the weight of the single-phase sample obtained by annealing and following quenching of the lithium and sodium cobaltites mixture in oxygen flow was compared with that of the initial mixture. The measured weight loss turned out to be 1.18%, which corresponds with the oxygen index 1.92 in the mixed cobaltite. Slow cooling under the same conditions does not lead to any weight changes.

Figure 4 displays the scanning electron microscopy (SEM) images illustrating the typical microstructure of the synthesized polycrystalline Li0.42Na0.36CoO₂. The grain sizes range from 1 to 30 μm. The material clearly shows features of a layered structure. The crystal structure of this material is based on close packed layers of edge-sharing CoO₂ octahedra, which are perpendicular to the c-axis and are separated by Na- or Li-containing sheets. The crystals can be cleaved easily between the CoO₂ layers and give rise to noticeable layered crystalline fragments, as displayed in the SEM images, especially in Figure 4(c). Another feature of the observed microstructure is a partly “fused” form of the grain surface (Figure 4(a)). It can be assumed that this is due to partial decomposition of the near-surface zone of the quenched Li0.42Na0.36CoO₂ and to incongruent melting of the liberated sodium cobaltite [35].

(a)

(b)

(c)

Figure 4: Scanning electron micrograph of the Li0.42Na0.36CoO₂: (a) frontal surface, (b) side surface, (c) chip.

The mixed lithium-sodium cobaltite was proved to be stable only at high temperatures and can therefore be obtained only by quenching. Heating of the quenched sample leads to its decomposition. An admixture of LiCoO₂ is detected starting from 250°C and then at about 750°C Li0.42Na0.36CoO₂ completely decomposes into mixture of LiCoO₂ and γ-Na0.7CoO₂ (Figure 5). Taking this into account, we assume that Li-Na cobaltite can be considered as thermoelectric material only for low-temperature application. Both resistivity and thermopower of Li0.42Na0.36CoO₂ measured at low temperatures are given in Figure 6. The thermopower increases at first to a value ≈220 μV/K and then remains practically constant (curve 1). The existence of nonstoichiometry in the oxygen sublattice and a sublattice of an alkali metal in this compound may lead to the formation of ions as Co²⁺ and Co⁴⁺. The positive sign of Seebeck coefficient reflects the majority of hole-like contribution to charge transfer associated with Co⁴⁺. It is a common feature for other cobaltites [40, 41].

Figure 5: Fragments of powder diffraction patterns of the Li0.42Na0.36CoO₂ heated to the different temperatures in the O₂ flow.

Figure 6: Temperature dependence of the thermopower (S) and electrical resistivity (ρ) for the Li0.42Na0.36CoO₂: curve 1-thermopower; curve 2-electrical resistivity of the single-phase Li0.42Na0.36CoO₂; curve 3-electrical resistivity of the partly decomposed Li0.42Na0.36CoO₂ contained 3% LiCoO₂.

The resistivity of Li0.42Na0.36CoO₂ exhibits the semiconductor behavior in this temperature range (curve 2). The room temperature resistivity is about 0.4 Ohm*cm. Detailed analysis of the transport properties is beyond the scope of this study, but it should be noted that obtained values are comparable to that of the other layered cobaltites considered as perspective thermoelectric materials. It is important that precipitation of only 3% of LiCoO₂ leads to a sharp deterioration in the conductivity (curve 3). It seems to be due to the structural peculiarities of the Li0.42Na0.36CoO₂ decomposition.

3.3. Electron Microscopic Study of Li0.42Na0.36CoO₂ Decomposition

In order to investigate the structural features of the long-period phase decomposition, we performed an electron microscopic study of Li0.42Na0.36CoO₂, which was subjected to thermal treatment at 500°C. According to X-ray analysis, upon thermal treatment the single-phase sample of Li0.42Na0.36CoO₂ partially decomposed, and the diffraction pattern contained lines both of LiCoO₂ and γ-Na0.7CoO₂ in addition to the reflections of the main phase. Precipitation of LiCoO₂ and γ-Na0.7CoO₂ inevitably leads to a variation in the alkaline elements content in the mixed cobaltite. Taking in account this uncertainty, hereinafter we shall designate the mixed cobaltite as “LixNayCoO₂.”

A typical electron diffraction pattern obtained from 0.5–2 μm anisotropic crystal aggregates of a mixed cobaltite sample is shown in Figure 7(a). Electron beam is perpendicular to mostly elongated planes of the aggregates. It contains not Debye rings but nets of point reflections, which may be indexed as individual cross-sections of the reciprocal lattice (rl) of the corresponding phases. The main feature of the diffraction pattern is the presence of several nets of reflexes with very close interplanar distances and close angles between the directions to the reflexes. As a rule, the angles are about 57–63°. These nets are close to the (001)* cross-sections of the reciprocal lattice associated with the hexagonal structure of the mixed lithium-sodium cobaltite. The series of close cross-sections shows first of all that in the studied textured aggregates there are individual crystallites, which are polar misoriented relative to the direction. The mixed lithium-sodium cobaltite is characterized by a long-period structure in the “c” direction, and therefore its diffraction pattern has dense rows of reflections directed along the c*-axis of the reciprocal lattice. Another feature of the diffraction pattern obtained for the lithium-sodium cobaltite is that all the diffraction effects cannot be indexed within the P6₃mc structure only.

Figure 7: Electron diffraction pattern obtained from crystal aggregate of Li0.42Na0.36CoO₂ subjected to ultrasonic dispersion (a) and its schemes (b, c). Black lines and white lines in (a) show nets of the (101)* cross-sections of LiCoO₂ and γ-Na0.7CoO₂, respectively. Indexing of the reflections for LiCoO₂ (R), γ-Na0.7CoO₂ (hex) and Li0.42Na0.36CoO₂ (hex’) is shown: for (101)* cross-sections (b); for other Laue zones (c). Notations: bullet—reflections of (101)*NaCoO₂; white circle—reflections of (101)*LiCoO₂; gray circle, cross (numbers from 1 to 10)—reflections of NaCoO₂ and LiCoO₂ that not belonging to (101)* cross-section (see Table 1).

The overwhelming majority of additional reflections can be indexed as a set of two phases—LiCoO₂ (R-3m,  nm,  nm) and γ-Na0.7CoO₂ (P6₃/mmc,  nm,  nm). Besides, Na0.75CoO₂ (C2/m,  nm,  nm,  nm, °) and lithium carbonate were also considered. Figure 6 shows a scheme of the (101)* cross-sections of the reciprocal lattices belonging to the mutually oriented γ-Na0.7CoO₂ and LiCoO₂ phases, which most closely coincide with the experimental reflections. The scheme is punched on the electron diffraction pattern too. Indexes of the reflections as well as interplanar spacing of the considered phases are given in Table 1. It should be noted that, in order to demonstrate indexing, we chose only one group of (101)* cross-sections, which is azimuthally misoriented relative to another group by several degrees.

Table 1: Indexing of reflections for electron-diffraction pattern (Figure 7).

d exp, Å
Symbol
d hex, Å (γ-NaCoO₂, P6₃/mmc)
, Å (LiCoO₂, R3m)
d hex’ , Å (Li0.42Na0.36CoO₂, P6₃/mc)

The reflections of (101)*cross-sections

2.44
●    ○
2.46 (010)
2.43 (010), I = 0
2.46 (010)

2.43
●    ○
2.40 (10-1)
2.40 (1-1-1)
2.44 (10-1)

2.40 (1-1-1)
2.40 (1-1-1), twin. reflect.
2.44 (1-1-1)

0.821
○
0.821 (030)
0.811 (030)
0.820 (030)

The reflections of adjacent Laue zone

2.31
1

2.299 (10-2)
2.31 (10-3)

2.299 (10-2), twin. reflect.

2.08
2
2.033 (1-1-3)

1.97
3

2.00 (1-1-4)

2.00 (1-1-4), twin. reflect.

1.41
4

1.405 (1-20)
1.416 (1-20)

1.405 (1-20), twin. reflect.

1.368
5
1.375 (1-2-2)

1.367 (1-2-4)

1.348
6

1.346 (1-2-3)

1.347
7

1.346 (2-1-3)

1.215
8
1.220 (022)

1.215
9

1.212 (021)
1.210 (02-3)

1.153
10

1.150 (-204)

In accordance with the performed calculations, as one of the basic directions of the reciprocal lattices coincides for LiCoO₂ (R) and γ-Na0.7CoO₂ (hex), while the * and * directions are misoriented within one degree. In addition to these reflections, the electron pattern contains a whole series of reflections that do not belong to the (101)*-type cross-section of for these phases but are due to a deviation from the exact orientation of the (101)* plane or to elastic distortions in the lattice (Figures 7(b) and 7(c)). These are as a rule reflexes of the series directed along the c*-axis and belonging to the LiCoO₂ phase (marked with a cross in the diffraction pattern scheme in Figure 7(c)). For example, in accordance with the extinction law, the (10-1)R reflections must be absent, but there are (10-2)R reflections from another zone, or, along with the (1-1-1)R reflections, observed are the reflections of the (1-1-4)R type belonging to another zone, the intermediate reflections (1-1-2)R, (1-1-3)R being prohibited and having zero intensity (see Table 1). One more peculiarity of the diffraction pattern associated with LiCoO₂ is the presence of two different orientations of the lattice with a twinning arrangement relative to each other.

Note that some of the reflections, which we indexed as LiCoO₂ and γ-Na0.7CoO₂, may belong also to Li0.42Na0.36CoO₂. The lattice orientation of this phase is also close to the (101)* cross section (see Table 1). As is seen from this Table, the appearance of reflections from adjacent Laue zones is a characteristic feature of the diffraction pattern of the mixed lithium-sodium cobaltite. The same analysis can be performed for the second group of (101)* nets azimuthally misoriented relative to that described above. These two groups of reflections found in the crystalline aggregates seem to be associated with two blocks of Li0.42Na0.36CoO₂ separated by a small-angle grain boundary.

A characteristic feature of individual diffraction spots on the electron diffraction pattern is not only the above-mentioned polar blurring but also the azimuthal blurring also associated with the presence of misoriented crystallites in the aggregates. As follows from the dark-field TEM images obtained in the group of reflections (1-1-1), (1-1-1), and (10-2), these crystallites are rather small (Figure 8). The size of the crystallites can be estimated from moiré contrast observed on the electron microscopic images. Both parallel moiré contrast (moiré fringes are perpendicular to the vectors of the reflections for the dark-field image) and mixed moiré (fringes are inclined to the vectors) are observed in the extinction contour (Figure 8). The former is due to the crystallites with different interplane distances, while the latter is produced by azimuthally misoriented regions also having different values of interplanar distances. The sizes of individual crystallites correspond to the sizes of domains with the same periodicity and direction of moiré fringes and are as small as 20–70 nm.

Figure 8: Dark-field TEM image in the reflections (1-1-1)hex, (1-1-1)hex’’, and (10-2)R for crystal aggregate containing crystallites of the mutually oriented γ-Na0.7CoO₂, LiCoO₂, and “LixNayCoO₂” phases. Vector of reflections for dark-field TEM image and area with fringe moiré contrast (white circle) are shown.

Thus, the electron microscopic studies showed that in the crystal aggregates formed as a result of partial decomposition of the mixed cobaltite Li0.42Na0.36CoO₂ the initial phase (hex’) coexists with LiCoO₂ (R) and γ-Na0.7CoO₂ (hex). It is essential that the lattices of the initial phase and of the phases formed upon decomposition demonstrate mutual orientation. The orientation relations can be written as

In other words, close packed hexagonal planes and close packed directions of these planes coincide for the three coexisting phases. Significant parts of the cells have an identical type and sequence of the atomic layers. This can be seen from Figure 9 where the structures of the three phases with the selected common fragments are shown. Note that the observed features of the ordered mixed lithium-sodium cobaltite decomposition come from its “nanoblock” structure built up from weakly coupled γ-Na0.7CoO₂ and LiCoO₂ blocks [33].

Figure 9: Crystal structures of the LiCoO₂, “Li0.5Na0.5CoO₂,” and γ-Na0.7CoO₂.

We assume that the decomposition of Li0.42Na0.36CoO₂ begins with the precipitation of LiCoO₂ on the base of the corresponding fragment of the mixed cobaltite. This supposition is proved by the fact that according to X-ray data only the LiCoO₂ impurity is observed at the initial stages of thermal decomposition of Li0.42Na0.36CoO₂. The formation of LiCoO₂ requires redistribution of Li and Na ions in one half of appropriate layers of the initial structure, the positions of the layers along the c-axis direction) in LiCoO₂ and Li0.42Na0.36CoO₂ being very close to each other. As to the rest of lithium, cobalt, and oxygen ions, they remain in the same layers and will shift by distances close to interatomic ones in the case of LiCoO₂ formation. The sodium cobaltite γ-Na0.7CoO₂ forms on the basis of the remaining fragment of the initial structure. A required redistribution of Li and Na ions can be achieved via their diffusion mainly in the (001) planes. As a result of the mixed cobaltite decomposition to LiCoO₂ and γ-Na0.7CoO₂, the grains in the initial crystal aggregates decrease not only in the direction but also within the layers. The fine grain structure of the partly decomposed mixed cobaltite is confirmed by the electron microscopic study.

4. Conclusion

Formation and decomposition of the ordered lithium-sodium cobaltite “Li0.5Na0.5CoO₂” were investigated. The ordered phase is stable in narrow temperature interval. It forms at °C and partly decomposes at °C due to an irreversible loss of sodium in the form of γ-Na0.7CoO₂. The main feature of the mixed cobaltite formation is a significant weight loss associated with oxygen liberation. The appearance of a rather great amount of oxygen defects is considered as the necessary condition for the “Li0.5Na0.5CoO₂” formation.

In order to investigate the structural features of the phase decomposition, an electron microscopic study was performed. It was shown that the lattices of the initial phase and of the LiCoO₂ and γ-Na0.7CoO₂ formed upon decomposition demonstrate mutual orientation: close packed hexagonal planes and close packed directions of these planes coincide for the three coexisting phases.

Acknowledgment

The authors would like to acknowledge the financial support from the Russian Foundation for Basic Research (Grant no. 10-03-00203-a, Grant no. 11-02-00166-a) and Programm no. 12-M-23-2032.

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