Chemical synthesis and steady state characterization of a nanocrystalline lithium cobalt oxide

Lithium cobalt oxide (LiCoO2) is one of the most relevant components in lithium-ion batteries. The array of sought-after features of LiCoO2 depends on its synthesis method. In this work we synthesized and characterized a nanocrystalline LiCoO2 oxide obtained with a wet chemistry synthesis method. The oxide obtained was a homogeneous powder in the nanometric range (5-8 nm) and exhibited a series of improved properties. Characterization by FTIR and UV-Vis techniques led to identifying citrate species as main products in the first step of the synthesis process. X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) characterizations led to identifying a pure crystalline phase of the synthesized LiCoO2 oxide. Steady state electrical characterization and solid-state impedance spectroscopy determined the high conductance of the synthesized oxide. All these features are desirable in the design of cathodes for lithium ion batteries.


Introduction
Lithium cobalt oxide (LiCoO 2 ) is a structural material with a laminar configuration that efficiently stores energy. Because of its structural stability and long life in the processes of load cycling, LiCoO 2 is used in the design of positive electrodes in lithium ion (Li-ion) batteries [1][2][3] and electronics, including modern mobile devices [4].
Several works on LiCoO 2 obtention confirm that adequate nanometric structures provide an effective pathway to promote Li + ions and generate a high charge capacity, even at high discharging rates [7][8][9][10]. LiCoO 2 cathodes synthesized from metal oxides exhibit temperature associated evaporation of lithium species. Consequently, adjustments in the process of LiCoO 2 chemical synthesis could improve its effectiveness. These improvements involve changes in the material's microstructure, as in previous works in which it has been possible to insert active materials in glassy matrices [11][12][13][14].
Steady-state electrical characterization of compact pellets of LiCoO 2 facilitates the analysis of alternating current in simple circuits and the solving of DC circuits. Furthermore, the ability of the system to return to its steady after electrical disturbance can be analyzed in these materials [14].
In this work we synthesized and evaluated the properties of LiCoO 2 . Our synthesis method is based on a chemical route of combustion, starting from corresponding metallic nitrates. Our aim is to develop materials with morphological, superficial, and structural properties ideal in the design of advanced cathode components in lithium-ion batteries.

Materials and Methods
The synthesis of lithium cobalt oxide LiCoO 2 required LiNO 3 anhydrous and Co(NO 3 ) 2. 6H 2 O, both from Merck (99.99 %). Each reactant was dissolved in deionized water (18 MΩ cm) to a concentration of 1.0 Mol L −1 . Subsequently, a citric acid solution (1.0 Mol L −1 ) was added in a 0.5:1.0 molar ratio, in relation to the total content of metalions, following Gómez and Valencia [24][25].
The system remained in reflux for 2 hours, under a nitrogen atmosphere of 50 mmHg at 80 • C, and magnetic stirring (500 rpm). The obtained dissolution formed a solid gel, which was subsequently treated on a hot plate at 120 • C to obtain a solid porous foam. The solid precursor of LiCoO 2 was collected, grounded in agate mortar, and sieved through a 400 US standard mesh for the FTIR analysis in a PerkinElmer Paragon 1000 FTIR equipment and Raman spectroscopic characterization. This characterization was made in a Raman spectrometer HR-UV 800 infinity microprobe with a CCD detector (-70 • C), and a laser power of 10.7 mW equipment.
The solid precursor solid of LiCoO 2 were calcined at 850 • C for 2 hours in a tubular furnace under nitrogen flow (50 mL.min −1 ), ensuring the consolidation of the crystalline phase. The characterization of the calcined material was performed by X-ray diffraction (XRD) in a PANAlytical X'per PRO MPD diffractometer, using an ultra-fast X'Celerator detector in a Bragg Brentano arrangement, using the CuKα radiation (Kα = 0.154186 nm). The diffractogram pattern was evaluated between 10 and 90 • 2θ¸with steps of 0.02, and the results obtained were analyzed with the X'Pert High Score Software. The crystal size of LiCoO 2 sample was calculated using the Equation 1, as follows: Where D is the average crystallite size in nm, λ (∼ 0.154056 nm) is the wavelength of X-ray radiation, θ is the Bragg angle and β is the full width at half of the maximum (FWHM) signal.
The detailed structural aspects of the LiCoO 2 oxide were analyzed with transmission electron microscopy (TEM). A JEOL 2100 microscope-equipped with a LaB 6 thermionic cannon, an acceleration voltage of 200 kV, and a CCD camera-was employed for this purpose.
The textural characteristics of the LiCoO 2 were established via N 2 adsorption isotherms at 77 K in a conventional volumetric equipment, Autosorb 3B, Quantachrome. Further UV characterization was performed in a Mapada 1000 equipment at a wavelength range of 190 nm-1 100 nm, to obtain the corresponding band-gap.
Electrical property measurements of LiCoO 2 were performed on an Autolab potentiostat-galvanostat equipment, at room temperature between 10 mHz and 1 MHz, using a copper electrode and a system for pressure coupling. Measurements were performed ten times, in complete frequency cycles, from which an average result was obtained. Bode curves were also obtained from these measurements. To corroborate the values of electrical conductance (G) from resistance values (Z) the Eq. (2) was employed as follows:

FTIR spectroscopic features
The characterization by infrared spectroscopy (FTIR) of the solid precursor of LiCoO 2 , shown in Fig. 1, allowed to determine the formation of citrate species. This is a fundamental aspect for the obtention of ceramic oxides under the studied reaction conditions. We identified absorption bands of carboxyl groups attached to the COO-M bond, located at 1 409 cm −1 . We observed a citrate-type precursor-related deformation signal, which is consistent with carboxyl terminal groups (-COO-) associated with the used citric acid as Wavenumber (cm -1 )

Transmittance (%)
Universitas Scientiarum Vol. 25  We observed a band located at 2 345 cm −1 associated with the vibration mode of the O=C=O bond. The occurrence of this band was attributed to the presence of CO 2 in the structure of the material due to the combustion process in the synthesis method. The tension band related to C=O and C-H bonds was observed at 1 048 cm −1 and 1 055 cm −1 . This band is associated with the presence of alcohol and ketonic groups, as well as to the type of vibration tension of the C-H bond, which is present in the precursor. The bands observed around 1 600 cm −1 -1 750 cm −1 , correspond to carbonyl groups and can be related to the formation of ketones, by a process related with the decomposition of citric acid and associated to C=O and O-C=O groups [26,27].
The band observed at 584 cm −1 -554 cm −1 is related with a vibration bending (m) of the MO bond, which corresponds to an oxygen bond and a Co 3+ cation. Their vibrational modes υ(OMO) are a function of pH, favoring the formation of metal-ligand bonds. Based on previous information and data reported by Gómez and Valencia [24], we can relate the presence of citrate-species in our synthesis process to the main functional groups indicated in Table 1.

X-ray diffraction characterization
The relationship between the indexed and the experimental XRD patterns of LiCoO 2 oxide after combustion is shown in Fig. 2. The analysis via the X'Pert High-Score software classified our LiCoO 2 material with ICSD code: 00-016-0427, and placed it in the rhombohedral geometry of spatial group R-3m (166).
Based on the most intense signals from the diffraction pattern along facets (0 0 3), (1 0 1), and (1 0 4), the sample's crystallite sizes ranged between 5 nm-8 nm. This agrees well with the results of Chinarro et al. [28], in which citrate precursors provide a way to obtain homogeneous and nanometric materials. The combustion reaction led to a strong a brief heat release that was absorbed by the synthesized oxides. The gases from the combustion dissipated, conforming a material with large surface area and manometric crystal size. These are relevant features of the obtained oxide, consequence of the employed synthesis method [29,30].   (1 0 4) facets indicate an increase of the LiCoO 2 phase. Furthermore, the presence of pyrite-marcasite oxide, derived from the synthesis procedure, has been observed for many LiCoO 2 -based materials [31,32].
Simulation of the LiCoO 2 system unit cell, resulted in the estimation of the main cell parameters: a = 2.8166 , b = 2.8166 , c = 14.045 , with α = β = 90 • and γ = 120 • . This set of estimated parameters corroborates an oxide structure with the geometry shown in Fig. 3: the O 2− ions are located along the (0 0 3) facet, and Li + and Co 3+ ions are placed in the octahedral sites of the alternate layers. This geometry is related with low consolidation temperatures during the synthesis process in accordance with Cabrera and Levi [33,34].

Raman characterization
Raman characterization of the synthesized LiCoO 2 (Fig. 4) revealed two active vibration modes, A 1 g and E g , located in the region of 400 cm −1 to 700 cm −1 . These are due to the vibrations of the oxygen atoms present in the oxide structure [35]. The A 1 g mode corresponded to the higher frequency Raman band resulting from the vibration of two oxygen atoms. The E g mode (double degeneration d z, d x − y) was revealed by a frequency band at 486 cm −1 . In this E g mode, lithium atoms, together with cobalt cations, vibrate parallel-but in opposite direction-to oxygen atoms [36].  The signals that dominate the Raman spectrum were observed at 486 cm −1 and 596 cm −1 . These correspond to the vibrational flexion modes attributed to the A 1g and E g vibrational modes, associated with the presence of LiCoO 2 [8,37,38].
According to Santiago et al. [13], these Raman results are consistent with a structure, devoid of spinels or secondary phases, resulting from a calcination process. The calcination temperature of 850 o C promoted the final evolution of the precursor to a pure oxide form. The qualitative analysis of the main signals is consistent with previous works in which the LiCoO 2 was obtained in a single phase and exhibited bands attributed to oxygen vibrations involving mainly Co-O stretching (A 1 g ) and O-Co-O bending (E g ) vibrations [39,40]. The spectrum lacks the so-called defect band (D band), related to C-C stretching modes related with graphite or carbonaceous species [41].
The active bands observed in the range of 500 cm −1 -580 cm −1 , with the presence of vibration modes for the metal-oxygen bonds, corresponds to a lithium deficiency, as previously reported by Julien and Escobar [42,43], According to the irreducible representations derived from the molecular symmetry, the E g -A 1 g signals are sites of octahedral configuration and corroborate the obtained results of X-ray diffraction.

Raman shift (cm -1 )
Universitas Scientiarum Vol. 25  Transmission electron microscopy TEM micrographs of a population of 150 LiCoO 2 particles revealed nanometric crystallite sizes with a Gaussian distribution and a median size of 6.2nm -7.2 nm (Fig. 5). The observed size range is in accordance with that estimated for LiCoO 2 particles under XRD measurements.
TEM-derived data showed that our synthesized LiCoO 2 crystals grew towards the preferential (1 0 4) and (0 0 3) facets, revealing a highly crystalline structure with interplanar d distances of 0.04 microns (Fig. 6A-6F). The general view of images between 0.2 µm and 0.5 µm confirms a homogeneous distribution of oxide particles (Fig. 6B-6E). This agrees with Okubo et al. results [44,45], in which the electron diffraction patterns revealed the characteristic geometry of an oxide structure along atomic layers.
Additionally, the absorption isotherms of the material entailed a surface area of 115 m 2 g −1 , corroborating that solids of small sizes feature large contact areas. This is crucial in the development of electrolyte materials for Li-ion batteries. Material large contact areas improve carriers' interchange charge capacity [46].

UV-Vis spectroscopy
We subjected the oxide to ultraviolet visible UV-Vis spectroscopic characterization and determined the oxide's features at an electronic transition frequency of 9.96×10 −11 Hz. The signal depicted in Fig. 7, is consistent with a n → π * transition that confirms the presence of an unsaturated group in the molecule providing p electrons. This, in turn, agrees with a previous observation that the active cation in the absorption process is located in the B position of the oxide [47].
The UV-Vis study also allowed measuring the transmittance levels of the LiCoO 2 oxide. The transmittance value observed was of 62 %. This value is at least 3 % lower than the theoretical value from similar materials [48]. This finding highlights the possibility of designing transparent Li-ion cells.

LiCoO 2 sample electrical properties
Steady state electrochemical evaluation of the LiCoO 2 sample revealed a logarithmic trend of negative slope (Fig. 8). This indicates resistance in the equivalent circuit (horizontal plateau before 100 Hz). The slope after 100 Hz is associated with the behavior of a capacitor system, indicating a faster charge transfer in the LiCoO 2 sample.  Our observations are consistent with the arrangement of the atoms in the LiCoO 2 structure and its mode of ion transport. Regarding the latter, the double-layer capacitance in parallel promotes the accumulation of ions near the electrode surface and induces the attraction of opposite sign charges, thus forming a double electrical layer. This behavior has been observed in type p semiconductor materials during the process of electric transport [49,50].
The Bode diagram depicted in Fig. 8 represents the cutoff frequency of LiCoO 2 oxide. The process occurs in the active sites of insertion and disintegration of lithium ions in the laminar structure of the oxide. Such process, depends mainly on the atomic disposition of Li ions and the defects of the material [51].
The frequency values are expressed by means of an equivalent circuit. An equivalent circuit consists of an interface of different electrical elements in which the resistance alternates with a frequency of 5.00 Hz passes through the material. In Fig. 8, R s o l stands for the resistance of the medium, C the double layer capacitance, and R the resistance to load transfer with a value of 140 Ω. This resistance value is caused by the load transport reactions in the borders and inside the grains of the material [52].  The electrical conductance (G), obtained from resistance values (Z) of 159.3 Ω was G = 6.27×10 −3 Ω −1 cm −1 . Such a value, is consistent with a conductance process that occurs at the grain boundaries. Furthermore, the observed low temperature and the concentration of electronic holes inside the grain has been previously reported [53].

Conclusions
The employed LiCoO 2 synthesis method favors the conformation of intermediate citrate species. They correspond with characteristic compounds derived from initial synthesis process between nitrates and citric acid. These are crucial to the consolidation of crystalline pure LiCoO 2 .
Our LiCoO 2 characterization results showed that it is possible to design materials with a high degree of purity, crystallinity, and homogeneity. The compound we synthesized and characterized has a nanometric crystalline structure and a high surface area. This makes it a desirable component of Li-ion battery electrodes.
Finally, the enhanced conductance of our LiCoO 2 solid is its most prominent feature. This is due to the formation of conducting interfacial regions that facilitate the passage of electric current along nanoscopic grains. Furthermore, this feature confirms the effectiveness of our proposed synthesis method.

Jairo Alberto Gómez Cuaspud
Chemist and Doctor in Chemical Sciences with extensive experience in the synthesis of advanced solid state materials for different types of applications related with power generation. Full time full professor at the Universidad Pedagógica y Tecnológica de Colombia and Associate Researcher MINCIENCIAS (2020), belonging to the Institute for Research and Innovation in Materials Science and Technology (INCITEMA) and the materials integrity and evaluation group (GIEM).

Ariatna Yizel Neira Guio
Master in Chemistry from the Universidad Pedagógica y Tecnológica de Colombia, Chemistry graduated from the same university, with experience in research on the analysis and synthesis of advanced materials such as nanomaterials applied to the development of lithium-ion batteries, a topic in which I have some publications of the national order. orcid: 0000-0003-4150-9308