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Synthesis and properties of CoO2, the x = 0 end member of the LixCoO2 and NaxCoO2 systems

We report here the synthesis of single-phase bulk samples of CoO2, the x = 0 end member of the AxCoO2 systems (A = Li, Na), from a pristine LiCoO2 sample using an electrochemical technique to completely de-intercalate lithium. Thus, synthesized CoO2 samples were found to be oxygen-stoichiometric and possess a crystal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetic susceptibility of the CoO2 sample was revealed to be relatively large in its initial value and then level off as the temperature increases, suggesting that CoO2 is a Pauli-paramagnetic metal with itinerant electrons.


Introduction
The layered cobalt oxide system, Na x CoO 2 , has attracted increased interests due to a variety of unconventional transport and magnetic properties. About x = 0.7, Na x CoO 2 exhibits an unusually large thermoelectric power and a metallic conductivity [1]. It was revealed that, when x is increased only by 0.05, i.e. at x = 0.75, thermoelectric power is significantly enhanced [2,3] and a spin-density-wave state appears below T m = 22 K [4].
In the lower Na-content region, the x ≈ 0.35 member readily absorbs water and the water derivative Na 0.35 CoO 2 ⋅ yH 3 O ⋅ y'H 2 O becomes a superconductor (with T c = 4.5 K) [5]. It was also found [6] that the magnetism of the Na x CoO 2 system is of the Curie-Weiss type for x > 0.5, while paramagnetic for x < 0.5, and for x = 0.5 electrical conduction is poor due to the appearance of a charge-ordered state.
The variety in the property with respect to the Na content (x) in the Na x CoO 2 system is likely stemmed from some involved spin interactions. It was reported that the x = 1.0 end member, i.e. NaCoO 2 , is a non-magnetic insulator containing low-spin (S = 0) Co III [7]. As x decreases, the average valence of cobalt should gradually increase toward +4 such that the concentration of magnetic Co IV species may increase. In the Na x CoO 2 system, antiferromagnetic ordering is suppressed since the cobalt atoms form a triangular lattice with geometrical frustration that may cause intriguing electronic/magnetic properties. Thus one may anticipate that the other end member, i.e. CoO 2 , would form an ideal magnetically-frustrated system in which every site in the triangular lattice is occupied by magnetic Co IV . Recent theoretical works based on (LDA+U) band calculations suggested [8,9]  and LiCoO 2 contain the CdI 2 -type CoO 2 layers in common, the x = 0 end members of the two systems, i.e. Na 0 CoO 2 and Li 0 CoO 2 , are considered to be identical [11]. Note that LiCoO 2 is one of representative cathode materials for the lithium-ion secondary battery since the capability of electrochemical de-intercalation of Li is large enough.
Synthesis of CoO 2 samples through electrochemical oxidation was previously reported by Amatucci et al. [12] Their in-situ x-ray diffraction analysis of a Li x CoO 2 cathode that had been placed in a dry plastic cell while being electrochemically charged revealed that Li ions were completely extracted and thus pure CoO 2 was obtained.
Nonetheless, no physical property data of CoO 2 were reported. Therefore, it is highly desirable to obtain bulk samples of pure CoO 2 for the study of its physical properties.
Here we report synthesis and some properties of pure CoO 2 bulk as materialized through electrochemical de-intercalation of Li from a LiCoO 2 bulk. Approximately 50 mg of CoO 2 bulk allowed us to successfully measure its dc-magnetic susceptibility with reliable accuracy.

Experimental
First stoichiometric LiCoO 2 samples were synthesized by a conventional solid-state reaction technique [13]  Since high-valent metal oxides tend to experience chemical instability when exposed to atmospheric moisture, sample handling and characterization were made such that the samples might be exposed to moisture as little as possible. After the electrochemical procedure, the sample was washed with anhydrous acetonitrile or dimethyl carbonate in an argon-filled glovebox and encapsulated into a Pyrex ampoule.
XRPD analysis was carried out for the electrochemically treated sample which was set in an airtight sample holder filled with Ar gas. The oxygen content of the sample was determined based on hydrogen reduction experiments carried out in 5% H 2 /Ar gas flow with a thermobalance (Pyris 1; Perkin Elmer). Magnetic susceptibility measurements were performed with a SQUID magnetometer (MPMS-XL; Quantum Design). An as-encapsulated sample was put in a cryostat for magnetic measurements: the influence of the ampoule on the measurement was calibrated accordingly.

Results and discussion
Single-phase bulk samples of the CoO 2 phase were successfully obtained through the aforementioned electrochemical de-intercalation of lithium from a stoichiometric LiCoO 2 bulk pellet. Figure 1 [14]. The V-vs-x curve is in good agreement with that previously reported by Amatucci et al. [12]. The only difference between the two curves is the voltage value at x = 0.0, which is due to the different anode materials. The actual Li content (x) of the resultant CoO 2 sample was determined to be below the detection limit of the ICP-AES apparatus, i.e. smaller than 0.01. This ensures that the resultant CoO 2 samples are indeed the x = 0 end member of the Li x CoO 2 system.
To understand the phase evolution process during the course of Li de-intercalation from LiCoO 2 , the derivative of the potential curve, i.e. dV/dx, is plotted as a function of Li content (x) in Fig. 1(b). A characteristic "dip-peak-dip" structure is clearly seen about x = 0.5. Similar features were reported in previous works [12,15], attributing the structure to the appearance of a Li-ion ordered phase at x = 0.5 as predicted by a first-principle calculation [16]. This marking for x = 0.5 supports the accuracy of the x value as calculated from Faraday's law. Furthermore, two broad peaks are seen at x ≈ 0.33 and 0.15 in the dV/dx vs x plot, which would indicate the appearance of another Li ordering phenomenon. 7 The CoO 2 bulk sample was shiny gray in color and easily collapsed into fine powder.
In Fig. 2, XRPD patterns for both end members, pristine LiCoO 2 (x = 1.0) and CoO 2 (x = 0.0), are shown. For LiCoO 2 diffraction peaks were readily indexed based on space group R-3m with the lattice parameters, a = 2.814 Å and c = 14.05 Å: the a and c values are in good agreement with those previously reported [12,13]. Now the diffraction pattern for CoO 2 was indexed based on space group P-3m1 with the lattice parameters, a = 2.820 Å and c = 4.238 Å. As the use of an airtight sample holder had significantly deteriorated the resolution of diffraction pattern for the CoO 2 phase, refinement of the CoO 2 crystal structure was unsuccessful. The LiCoO 2 phase crystallizes in a so-called "O3-type" structure [17] in which Li ions occupy the octahedral site with three CoO 2 layers per unit cell, while the CoO 2 phase possesses an "O1-type" structure consisting of a single CoO 2 layer only per unit cell (Fig. 3) [12,17]. Accordingly the interlayer distance is much shorter for the CoO 2 phase (4.24 Å) than the Li  Fig. 4 are a thermogravimetric (TG) curve and the corresponding differential TG (DTG) curve taken for a CoO 2 sample in 5% H 2 /Ar gas flow. In this experiment, approximately 3.3 mg of the CoO 2 sample was heated with a heating rate of +2°C/min up to 600°C, followed by a rapid cooling to room temperature. Three distinct peaks and steps are seen in the DTG and TG curves below 500°C. The first DTG peak at ~100°C may be attributed to the vaporization of residual electrolyte. The second and third peaks correspond to the following reductive reactions, respectively:

Shown in
For Reactions (1) and (2), both of which accompany weight losses, the amounts of oxygen evolved during individual reactions are obtained to be 0.98 and 1.00, respectively. Thus, the value of (2−δ) of the starting material is determined at 1.98±0.02.
Thus the resultant CoO 2 sample is essentially oxygen stoichiometric. Previously, Tarascon et al. [17] reported based on their synchrotron x-ray diffraction analysis that their "CoO 2 " sample prepared by electrochemical de-intercalation of lithium contained both oxygen-stoichiometric CoO 2 and oxygen-deficient CoO 1.92 phases. On the other hand, Venkatraman et al. [17] reported a large amount of oxygen deficiency (2−δ = 1.72 − 1.88) for their "CoO 2 " sample obtained with a chemical oxidation technique.
Therefore, it is considered that the oxygen content of CoO 2 samples depends strongly on the synthesis procedure such that the more equilibrated synthesis and/or more highly oxidizing conditions tend to suppress the formation of oxygen vacancies.
Magnetic susceptibility (χ) data for the resultant CoO 2 sample (solid circles) and the starting LiCoO 2 sample (open triangles) are given in Fig. 5. The χ value for the LiCoO 2 sample is small in magnitude and little dependent on temperature, as the constituent Co III is in non-magnetic low-spin state. This result is in good agreement with those previously reported [19]. Surprisingly, the CoO 2 sample also exhibits temperature-independent susceptibility in a temperature range between 50 and 300 K.
This is in sharp contrast to that for Na x CoO 2 with x > 0.5 where a Curie-Weiss behavior is clearly observed [4,20]. The χ vs T plots were fitted with the following formula: is the Pauli-paramagnetic contribution [21], the density of states at the Fermi level, D(ε F ), is calculated at 13 electrons / eV. This value is three times larger than the theoretical value for D(ε F ), i.e. ∼4 electrons / eV for non-magnetic CoO 2 according to LDA calculations [9]. This may be attributed to an electron-mass enhancement. We note that some unconventional magnetic models such as a spin liquid picture in a frustrated spin lattice [22] may not be ruled out only with the present dc-magnetic susceptibility data. Further detailed characterizations are highly desirable to gain the deeper insight into the electronic structure of the CoO 2 phase by means of , e.g. 59 Co NMR.

Conclusions
Single-phase bulk samples of CoO 2 were successfully obtained through electrochemical de-intercalation of lithium from a pristine LiCoO 2 sample. The CoO 2 phase was found to possess a crystal structure consisting of triangular lattice layers of CoO 2 stacked infinitely. Thermogravimetric analysis revealed no significant oxygen deficiency such that the CoO 2 sample may be concluded to be essentially oxygen stoichiometric. Magnetic susceptibility of the CoO 2 phase little depends on temperature.    The upper panel shows the corresponding differential TG (DTG) curve, revealing two major steps for the compound to decompose completely to Co metal.