SYNTHESIS OF LITHIUM AND COBALT OXIDES SYSTEMS BY THE ALD METHOD TO OBTAIN LITHIUM COBALT OXIDE CATHODE FOR THIN-FILM LIBS

The development of microand nanoscale power sources for micro size electronic devices such as wireless sensors, biomedical implants, and smart cards is an important task. In this work, the processes of synthesis uniform lithium oxide, cobalt oxide, and lithium cobalt oxide thin films using atomic layer deposition (ALD) method were studied. Lithium tert-butoxide (LiOtBu) and cobaltocene (Co(Cp)2) were used as precursors. Remote oxygen plasma was used as a counter-reagent. The synthesis was carried out at 300 °C. The growth rate and uniformity of the films were controlled by adjustment the precursor's evaporation temperature, reagent pulse time, and purge times. Monocrystalline silicon and stainless steel (SS316) were used as substrates. The films were studied by spectral ellipsometry, energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction analysis (XRD). Results showed that an increase the purge time to 10 s after the reagent pulse positively affects the uniformity of the films. The growth rates of Li-O and Co-O systems were 0.080±0.010 nm/cycle and 0.022±0.003 nm/cycle, respectively. XRD data showed the presence of the crystalline CoO phase for the CoO and the presence of the LiCoO2 for Li-Co-O samples. The optimized synthesis parameters determined for Li-O and Co-O were subsequently used to synthesize Li-Co-O (LiCoO2-x) system. According to the growth rates of Li-O and Co-O, the ratio of Li and Co pulses was set as 1/5. XRD data showed well crystalline LiCoO2 textured in (003) direction for as-deposited Li-Co-O thin films. According to cyclic voltammetry (CV), there is a pronounced peak characterizing of the change in the oxidation state of cobalt from 3+ to 4+. Based on cyclic tests, the Li-Co-O films have high coulombic efficiency.


INTRODUCTION
Today lithium-ion batteries (LIBs) are widespread for power supply in modern portable electronics, sensors, medical devices, etc. The downscale trend in portable electronics requires the on-board power supply device to be of smaller dimensions, higher energy, and power density. Therefore, the production of thin-film anode and cathode materials is one of the most important task for the development modern portable electronic devices. Atomic layer deposition (ALD) is the most promising technology for synthesis inorganic thin films due to the ability to control the composition and thickness of the films with high precision. ALD is based on selflimited chemical reactions of gas-phase reagent with substrate surface [1,2]. Some examples of synthesis electrode materials by ALD method and their electrochemical properties can be found somewhere [3][4][5]. However, most of the work on the use of ALD for preparation of active materials is devoted to the synthesis of thin film anodes. There is much less work to synthesize cathode materials. The problem is the cathode materials contain lithium, which is difficult to produce by ALD due to hydrolysis when using water as a counterreagent. However, the use of oxygen plasma as a counter-agent can solve this problem. LiCoO2 is the most commonly used cathode material in commercial LIBs due to its high energy density, low self-discharge and excellent cyclability. Therefore in this work we synthesized the lithium cobalt oxide thin films by ALD using oxygen plasma as counter-reagent and studied its electrochemical performance. The thicknesses of the films were measured by spectral ellipsometry (vawelength range 350-1000 nm) using Ellips-1891 SAG ellipsometer (CNT, Novosibirsk, Russia). X-ray diffraction (XRD) studies were performed using a Bruker D8 ADVANCE (Cu-Kα). The obtained results were processed by the Rietveld method using TOPAS 5 software (XRD). The morphology and chemical composition of the films was studied by energydispersive X-ray spectroscopy (EDX) using Mira-3M (Tescan, Quanta 200, FEI).
Electrochemical studies were performed in CR2032 coin cells. The samples of Li-Co-O deposited on the 316SS surface were used. Lithium foil, polyolefin porous film 2325 (Celgard, Charlotte, NC, USA), and TC-E918 (Tinci, Guangzhou, China) solution were used as the counter electrode, separator, and electrolyte, respectively. Coin cells were assembled in an argon glove box OMNI-LAB (VAC). Cyclic voltammetry (CV) was performed using a potentiostat PGSTAT302N+ (Autolab, Utrecht, the Netherlands) in the range of 3.0 -4.3 V with a scan rate of 0.5 mV/s. Cyclic charge/discharge was performed using the battery testing system CT-3008W-5V10mA (Neware, Shenzhen, China) at room temperature in the range of potential 3.0 -4.3 V, at current 20 -80 µA.

Li-O system
Lithium butoxide is the most commonly used reagent for the synthesis of lithium-containing thin films by ALD [8]. The temperature of evoparation of lithium butoxide varies over a wide range from 90 to 180 °С [4,9]. Such a large temperature difference depends on the design features of the setups [4,9]. Our results showed that the Li-O growth of the films occurs only at the maximum possible temperature of the evaporator (218 °C) on the installation used (Picosun R-150) with a pulse duration of 3 seconds. In this regard, experiments were carried out to study the uniformity of coatings at various purge times and two precursor pulse modes: line boost mode (Picohot™ Boosting) and under standard mode. Picohot™ Boosting is provided by the manufacturer for the low volatile reagent to reduce the temperature of the evaporator. In this work, the lithium reagent pulse time is 1.6 s (Boosting) and 3.0 (Standard mode). The temperature of the evaporator at Standard and Boosting modes was 218 °С.
It should be noted that the reagent is low volatile and stable up to 250 °C [10], which imposes restrictions on the minimum temperature for the synthesis of lithium-containing films. Based on the literature data and taking into account the physicochemical characteristics of the reagents, the reactor temperature was set at 300 °C.
All films, in accordance with our previous studies [7] on the preparation of lithium-containing films, were coated with a protective layer of alumina (50 ALD cycles -TMA+H2O) and stored in an inert atmosphere of argon.
The effect of purge time on the growth rate and uniformity of the films using two types of reagent pulse modes (Standard and Boosting) shown in Figure 1. With increasing purge time, uniformity of coatings increases and the growth rate decreases for both modes. Acceptable growth rate and uniformity of the films was obtained by pulse 10 -15 s. To optimize the synthesis time, a purge of 10 s was chosen for subsequent experiments. It is worth noting that when using Boosting, the growth rate more than 2 times lower than with Standard pulse mode. To increase the growth rate of the films, the conditions with the highest growth rates with similar uniformity were chosen for Li-Co-O deposition. XRD data didn't show presence of any crystalline phase.

Co-O system
The literature provides information on the synthesis of CoOx films by the ALD in a wide temperature range using cobaltocene and ozone or oxygen plasma as a counter-reagent. The synthesis is carried out in a wide temperature range (140 -350 °С) [11,12]. However, to synthesize the lithium cobaltate, it is necessary to combine the deposition temperature of cobalt and lithium oxides (for Li-O the temperature is at least 300 °C). Therefore to optimize the deposition conditions of cobalt oxide, we used a temperature of 300 °C.

Figure 2
The change of growth per cycle (GPC) with an increase in the evaporation temperature of Co(Cp)2 Based on our previous work [13], when the nickelocene (NiCp2) was used as ALD precursor for NiOx thin film synthesis we used 1.0 second for pulse and 10 second for purge. As we found out earlier, the temperature of the source is of great importance when using metallocenes. In this regard, we studied the effect of evaporator temperature on growth rate and uniformity Co-O films (Figure 2). The optimal temperature range was from 120 to 150 °C. At higher temperatures, the growth rate increases, but the gradient of film thickness also increases significantly, which may be caused by the decomposition of cobaltocene.
EDX data showed the presence of cobalt in the film. Figure 3 shows the diffraction pattern of Co-O sample deposited on silicon surface. Three peaks at 33, 55 and 57 degree are associated with the imperfection of the crystal structure of the silicon substrate. The other peaks correspond to the CoO phase (Fm-3m). The coincidence of the intensities of these peaks with the table value indicates the absence of texture in the film.   [14]. The difference in the lattice parameter "a" for a thin film may be due to surface effects.
The resulting samples were investigated by cyclic voltammetry (Figure 5). An intense peak in region 4V is characteristic of the process of cobalt oxidation from 3+ to 4+. [15,16], while there is no pronounced peak for the reverse process (reduction processes from 4+ to 3+). Also, there is a decrease in the anode peak and its displacement in the region of lower potentials. All of the above can be associated with the involvement of the substrate material in the process [13]. The areas of the anodic and cathodic curves on cycles 2 and 3 are comparable, which may indicate good reversibility of the process. Figure 5b shows the dependence of the discharge capacitance on the number of cycles at various currents. A stepwise increase in the discharge current from 20 to 80 μA leads to a marked decrease in the discharge capacitance, which is similar for all electrode materials. Coulombic efficiency is about 95 %. (Figure 6) The average discharge capacity was 0.85μAh. The average specific discharge capacity is 37 mAh/g, which is less than the values obtained in practice for powder analogues (practical capacity of LiCoO2 is only ∼137 mAh/g, while the theoretical capacity of ∼274 mAh/g [15]), this may be due to the involvement of the substrate material in the process.

CONCLUSION
ALD-growth of Li-O, Co-O, and Li-Co-O films was carried out using lithium butoxide, cabaltocene, and remote oxygen plasma. The growth rates of Li-O and Co-O systems were 0.080 ± 0.010 nm/cycle and 0.022 ± 0.003 nm/cycle, respectively. Results showed that an increase the purge time to 10 s positively affects the uniformity of the films. The presence of cobalt in the films was confirmed by the EDX method. In contrast to the results presented in literature [3,11] the synthesized Co-O and Li-Co-O films contain well crystalline CoO and LiCoO2 phases respectively. For as-deposited Li-Co-O thin films, LiCoO2 phase textured in (003) direction. According to CV study, there is a pronounced peak characterizing of the change in the oxidation state of cobalt from 3+ to 4+. Based on cyclic tests, the Li-Co-O films have high coulombic efficiency.