Annotation of the results obtained in 2020
Within the framework of the third stage of the project, it was planned to develop approaches for obtaining a solid electrolyte. From a number of metals (Al, Ta, Ti), tantalum was chosen, since an analysis of the literature data showed the success of using the Li-Ta-O system as a solid electrolyte and correspond to the tasks set for the project. Films obtained on silicon are smooth, homogeneous, without defects visible on an electron microscope. The density of the films is close to that of crystalline Ta2O5, as is the chemical composition (XPS) of the surface. Further, a series of experiments were carried out to obtain lithiated tantalum oxide and the ratios of the number of pulse LiOtBu / Ta(OEt)5 in the supercycle: 1: 2, 1: 3, and 1: 7. The ratios were chosen on the basis of literature data, according to which, with an increase in the lithium content from x = 0.32 to 0.98 in LixTaOy, the lithium-ion conductivity improves by two orders of magnitude, but a higher concentration of lithium (x = 1.73) leads to to a decrease in conductivity. LiTaO-1/2 coatings on silicon, homogeneous, uniform, without visible defects (SEM). Chemical composition studies (XPS) showed that the sample surface contains: lithium, tantalum, oxygen, and carbon. After etching off the surface layer, carbon disappears, which indicates its absence in the coating volume. The resulting films are X-ray amorphous, but with the possible presence of nanocrystallites. It is shown that films of the Li-Ta-O system do not contribute to the electrochemical capacity. A study of the impedance of films of the Li-Ta-O system in comparison with a steel substrate showed that the resistance of the SEI film on steel is lower than on Li-Ta-O. To obtain cathode materials based on lithium nickelate, a general approach was developed: the deposition of a crystalline layer of transition metals with their initially specified ratio, the subsequent deposition of a layer of Li-Ni-O (traces of nickel) and the use of heat treatment (HT) to create the crystal structure of the cathode material. To improve the electrochemical characteristics of the developed cathodes, new TO approaches were tested: two-stage TO and a decrease in the treatment time to 1 minute. The samples were multilayer films with a layered structure: the lower layer of transition metal oxide (Ni-Co-O), on which a lithium-containing layer of the Li-Ni-O type was subsequently deposited. On multilayer samples on a silicon substrate before heat treatment, electron diffraction with TEM showed amorphousness of the upper lithium-containing layer, but the presence of a cubic NiCoO phase with the Fm-3m structure in the lower Ni-Co-O layer. These results agree with the XRD data, according to which the NiO · CoO phase was found in the composition of the films. The results of chemical mapping of the sample on silicon before HT showed a uniform distribution of Co and Ni over the film thickness using transmission electron microscopy. But the border of the Ni map with the Li-O layer is not as clear as in the case of the cobalt map. Before maintenance, the surface looks uniform; at high magnifications, the roughness of the sample surface and the presence of single inclusions 10–20 nm in size are seen. After HT (800 ° C, 5 minutes), the silicon sample is covered with rounded crystallites with a size of 50-150 nm. The boundary of the silicon substrate is blurred, and the layer between the substrate and crystallites is amorphous. Chemical TEM mapping showed that as a result of HT, nickel diffuses to the surface and concentrates in crystallites, and also partially diffuses into the substrate. The distribution of cobalt atoms during HT does not change significantly. In both layers, the content is about 12%. In spherical crystallites, the nickel content is increased and amounts to 21%. In addition to the diffusion of nickel, silicon actively diffuses from the substrate into the bulk of the film, while its content in the amorphous layer is 17%, and in crystals on the surface it is about 6%. EELS analysis of the spectra of the region with spherical crystals shows an increase in the low-energy front in the region of 58-60 eV, which may indicate the presence of lithium in these crystal structures. On steel substrates, the analysis of the morphology of the coatings after HT showed that, upon rapid cooling, the samples deposited on the steel have bulk defects such as cracks and delamination of the coating from the substrate. Slow cooling avoids such defects. After the HT, the morphology of the films undergoes significant changes. As a result, particles from 50 to 200 nm in size with pronounced crystal faces appear on the surface of the samples. HT for 5 minutes leads to the formation of a uniform crystal structure over the entire area of the substrate. In addition to changes in the morphology of coatings, as a result of HT, an active change in the chemical composition of the film occurs. With a decrease in the duration of HT, a natural tendency towards a decrease in the iron content in the films is observed. It should be noted that chromium, unlike iron, does not diffuse into the synthesized cathode material, and thus an additionally deposited layer of metallic chromium can serve as a barrier to prevent the diffusion of iron into the synthesized cathode material. As a result of the performed electrochemical studies of the cathodes of the Li-Ni-Co-O system, it can be concluded that it is possible to use HT at 800 ° C from 5 to 1 minute for multilayer samples. In this time interval, high capacities of 4 μAh and 3 μAh and acceptable Coulomb efficiencies of more than 90 and 95%, respectively, were obtained, which indicates the reversibility of the charge-discharge processes. It was found that with an increase in the HT time, the specific capacity of the thin-film cathode material increases, which is most likely associated with a more complete lithiation of the layer of transition metal oxides, but the reversibility of the charge-discharge processes (Coulomb efficiency) decreases due to the diffusion of impurity elements from the substrate into electrode. The greatest reversibility of electrochemical processes occurs at a heat treatment time of 1 minute (Coulomb efficiency is more than 95%). The Li-Ta-O system was deposited on the cathode surface as a solid electrolyte. The deposition of films was carried out by sequentially alternating one cycle of obtaining lithium oxide and two cycles of tantalum oxide. Based on the XPS data, it can be concluded that in the samples obtained by successive deposition of Li-Ta-O on the cathode formed after HT, the composition changes as follows: on the sample surface there is a thin layer containing lithium carbonate / lithium oxide / lithium hydroxide / tantalum oxides. When investigating in the bulk of the film, the concentration of tantalum increases. With further etching of the film, lithium practically disappears. The concentration values obtained after 60 seconds of etching may correspond to the lower film edge of the lithium oxide structures. Based on the results of measurements of the effect of the current on the discharge capacity and the retention of the discharge capacity during cycling, it was shown that the chromium sublayer contributes to an increase in the discharge capacity regardless of the electrode composition. The Coulomb efficiency of the samples obtained on chrome plating is about 95%. Increasing the coverage of the Li-Ta-O electrolyte also allows an increase in the measured discharge capacity, although by itself does not add additional capacity to the system; perhaps this prevents unwanted interaction between the cathode and the liquid electrolyte. Electrolyte application slightly increases the resistance of the SEI film. The reason for this, apparently, is the other conditions of its formation. The application of the Cr substrate allows the resistance to be reduced. In addition, within the framework of stage 3, studies were carried out to obtain systems Al-O, Li-Al-O, Li-Ti-O and Ti-O. The data obtained allow us to draw some conclusions about the features of growth, optimal synthesis conditions, morphology and composition of the obtained systems. As part of the third stage of the project, 3 articles were published in journals included in the Scopus and Web of Science list.Detailed studies of foams of the Ni-Co-O system obtained using NiCp2, CoCp2 and oxygen plasma are published in the special issue Nanostructured Cathode and Anode Materials: Synthesis and Applications of the journal Nanomaterials, MDPI (https://www.mdpi.com/2079- 4991/11/4/907)

 

Publications

---

Annotation of the results obtained in 2018
At the first stage of the work, an analytical review of the literature was carried out which made it possible to select the most promising approaches for performing the experimental part of the work. In particular, source reagents were chosen for the synthesis of films of lithium oxides (bis (trimethylsilyl) amide lithium (LiHMDS)), nickel (cyclopentadienyl nickel (NiCp2), methylcyclopentadienyl nickel (Ni (MeCp)2)) and complex structures containing lithium nickel oxides by atomic layer deposition method (ALD). Water, oxygen plasma and ozone were tested as counterreagents for the synthesis of lithium-oxygen systems using LiHMDS. The use of water turned out to be the least promising, since its excess can lead to the hydrolysis of the formed lithium oxide to form hydroxide. The use of ozone and remote oxygen plasma resulted in reproducible results and a rather high value of the increase in film thickness per cycle MN (0.27-0.36 nm). However, when stored in air, the films degraded due to the interaction with water vapor and carbon dioxide. To stabilize the films, nanoscale alumina with a thickness of several nanometers was deposited on their surfaces by the ALD method immediately after the preparation of lithium oxide films. However, such coatings could not completely protect the film from the interaction with water vapor and carbon dioxide, so their color changed during prolonged (several weeks) storage in air. It was found that sequential treatment of the substrate with LiHMDS vapors, oxygen plasma and trimethylaluminum vapors leads to the formation of a stable lithium oxide with inclusions of the aluminum oxide phase. This observation suggested that with further work on the preparation of complex lithium-nickel oxides, the problem of degradation of lithium oxide will be solved. Further work was carried out to determine the optimal conditions for the synthesis of nickel oxide films. To determine the time of inlet, which is necessary to achieve saturation, the dependences of the growth per cycle on the time of inlet of NiCp2 were studied. It was found that after NiCp2 pulse time was increased for more than 5 seconds, the average value of the growth per cycle did not increase. Thus, to saturate the surface of the substrate, it is sufficient to conduct the vapor of the nickel-containing reagent for 5 seconds. To determine the temperature window of synthesis for NiCp2, the dependences of the increase per cycle on temperature were studied. With an increase in temperature from 150 ° C to 200 ° C, an increase in the growth per cycle is observed, which indicates an insufficient reactivity of the reagents in this temperature range. In the range of 200-300 ° С, it practically does not change and is equal to approximately 0.011-0.012 nm. This interval can be considered as optimal and which is the "ALD window". A further increase in temperature in the range of 300–400 ° C leads to an increase in the growth rate of NiO films, which is apparently due to the partial decomposition of NiCp2. Similar optimal parameters were determined for the Ni(MeCp)2 reagent. However, for this reagent, the optimal synthesis temperature range was much narrower (250–300 ° C), and the reagent was less stable during long-term operation. In this connection, LiHMDS, NiCp2 and oxygen plasma were used as reagents for further work on the preparation of complex lithium-nickel oxides. During the study of the growth and development of methods for the synthesis of thin-film cathode materials based on the Li-Ni-O system, obtained by the ALD method, for solid-state lithium current sources, the influence of the sequence of reactants supply (Ni-O-Li-O or Li-Ni -O) and additional heat treatment on the morphology, structure, phase, chemical composition of thin films in order to obtain structurally and chemically homogeneous coatings. Li-Ni-O films were deposited at a substrate temperature of 300 ° C. The evaporator precursor LiHMDS and nickel Ni (Cp) 2 evaporator temperatures were 160 ° C and 110 ° C, respectively. Remote oxygen plasma was used as a co-agent. Different ratios of the amount of lithium-containing and nickel-containing reagents (1/1, ½, 1/3, 1/5, 1/10) were used. Samples of films of nickel oxides, lithium and complex lithium-nickel oxides were studied by the methods of spectral ellipsometry, scanning electron microscopy, atomic force microscopy, X-ray phase analysis, X-ray photoelectron spectroscopy, time-of-flight secondary ion spectrometry. The films obtained were characterized by a slight roughness (less than 1 nm). An extremely small amount of nickel was found in the surface layer of films of complex lithium-nickel oxide (less than 1%), however, the NiO phase appeared on X-ray diffraction patterns after annealing (700 ° С). In addition, the complex oxide films contained much more nitrogen and silicon (ligands of the LiHMDS molecule) than samples of pure lithium oxide. The effects of film thickness, discharge voltage, and current density (up to 50 C) on specific capacitance and cyclic stability were studied. Using the method of Electrochemical impedance spectroscopy (EIS), the electronic resistance and charge transfer resistance of the obtained cathode materials were determined. The curves of cyclic voltammetry (CVA) of samples obtained without heat treatment, there are no maxima characteristic of nickel oxidation / reduction processes. When studying a series of annealed (600 ° C, 15 minutes) samples obtained as a result of a deposited with a different ratio of pulses of lithium and nickel in the super-cycle composition, it was found that for samples with a ratio of Li / Ni 1/3 and Li / Ni 1/1, a high irreversible charging capacity, so it was decided to conduct further studies of samples obtained as a result of the largest amount of nickel-containing reagent (LNO-1/5 and LNO-1/10). Considering the results of the study of CVA curves for a series of samples heated at 600 ° C, it can be assumed that the efficiency increases with increasing nickel content in the coating composition and to a lesser extent depends on thickness (thickness of the LNO-1/5 and LNO-1/10 films was 90 and 60 nm, respectively). On the CVA curves of the samples heated at temperatures of 600–700 ° C, the anodic (4.2 V) and cathodic (3.72 V) samples indicate the presence of LiNiO2 in the electrochemically active phase of the samples. There are also peaks (3.41 V and 2.97 V) of weak intensity, corresponding to electrochemical processes, which may indicate cracking of the film as a result of heating. The specific discharge capacity (volumetric, mass) is lower than the nominal characteristic of the bulk phase LiNiO2, which may be due to large discharge currents (4-40С) or a reduced fraction of the electrochemically active phase in the coating composition With an increase in the annealing temperature (700–900 ° C), an increase in the discharge capacity was observed. The shape of the discharge curves for a series of LNO-K samples was similar to the shape of the curves for LiNiO2. As a result of the project, an article was published in a high-rating journal, which is available for download by the link - https://www.mdpi.com/2079-6412/9/5/301

 

Publications

---

Annotation of the results obtained in 2019
The work is a continuation and development of research of the previous stage on the development of thin-film cathode materials based on lithium-containing transition metal oxides. In this reporting period, the possibility of synthesis by the method of ALD of thin-film cathode systems based on the Li-Ni-O system containing metal oxides from the series Al, Mn, Co, as well as studies of the electrochemical characteristics of such systems, was investigated. As a result of a detailed analysis of the data obtained in the previous reporting period, it was found that samples of lithium-containing cathode materials synthesized by the ALD method contained an admixture of silicon (depending on the synthesis conditions from several units to 20 at.%), and it can reduce electrochemical activity. It was shown that the source of silicon is insufficiently reactive under the conditions of the ongoing synthesis of the ligands of the initial reagent bis(trimethylsilyl)lithium amide (LiHMDS). The possibilities of using alternative precursor which do not contain silicon for the synthesis of lithium-containing cathode materials were studied. Based on published data, lithium butoxide (LitOBu) was chosen as the most suitable reagent. Using this reagent and oxygen plasma as an oxidizing agent and co-reagent, Li-O systems were synthesized, and the optimal conditions for MN were studied in detail (reactor and initial reagent temperatures, reagent purge, and recharge times, reagent supply mode). The highest growth rate and the best uniformity were shown by samples obtained with LitOBu pulse time of 3 s, purge for 10 s, reactor temperature of 300 ° C, and the maximum possible evaporator temperature (218 ° C). The average growth rate was 1.1 ± 0.15 Å / cycle, which is significantly less than when using LiHMDS - 1.56 ± 0.49, however, the uniformity of the coating thickness when using LiOtBu is much better. Subsequent studies were aimed at studying the possibility of obtaining and determining optimal synthesis conditions by the ALD method of Li-Co-O and Li-Ni-O systems using LiOtBu, oxygen plasma, and CoCp2 or NiCp2 as reagents, respectively. Both systems were characterized by a lower average growth per supercycle compared with the expected values calculated on the basis of the average growth rates of Li-O and Co-O or Ni-O systems. For the Li-Co-O system, significant compositional heterogeneity was found. According to X-ray photoelectron spectroscopy (XPS), lithium-containing phases (Li2O, LiOH, and Li2CO3) predominated in the surface layer. CoO prevailed in the sample volume, however, according to the X-ray phase analysis (XRD), the LiCoO2 phase was also detected. The Li-Ni-O system is characterized by low nickel content (less than 1%) and the samples obtained are predominantly Li2O / LiOH / Li2CO3. An extremely small amount of nickel was also noted for the similar Li-Ni-O system obtained using LiHMDS in the previous reporting period. A more detailed study of the structure, morphology, phase, and chemical composition, and also electrochemical characteristics were carried out for these samples. It was found that annealing of these samples at 800 ° С leads to the local formation of the LiNiO2 phase. Cyclic voltammograms (CVA) showed the presence of peaks that confirm a change in the oxidation state of nickel for both samples obtained using LiHMDS and LiOtBu. These samples are also comparable in absolute capacitance values. The calculated values of the specific volumetric capacity for Li-Ni-O-1/30 samples (where 1 to 30 is the ratio of the number of LiHDMS and NiCp2 pulses in one ALD supercycle) after annealing at 800 ° С has the highest values: at a cycling current of 20 μA - 35 μA⋅ h / μm⋅cm2. Of the least importance is the series of LNO-M samples (multilayer structures) after heat treatment at 800 ° C, current density 20 μA - 20 μA⋅h / μm⋅cm2. The remaining samples obtained using different ratios of the number of pulses of LiHDMS and NiCp2 in one supercycle are at the level of 25 μA⋅h / μm⋅cm2 with a cycling current of 20 μA. To doping of the Li-Ni-O system with metals, the synthesis features of the Mn-O and Co-O systems by the MN method were studied. Bis (2,2,6,6-tetramethyl-3,5-heptandionato) manganese - Mn(thd)3 and cobalt cyclopentadienyl (CoCp2) were used as starting reagents. As a reagent - oxygen plasma. Optimum synthesis temperatures are 270-300 °C with an average increase per cycle of 0.22 ± 0.04 and 0.1 ± 0.01 Å / cycle, respectively. The obtained samples mainly contain CoO and Mn2O3 with a small amount of Co2O3 and MnO2, which indicates a low oxidative activity of the co-reagent. In addition, the Al-O system was synthesized using trimethylaluminum (TMA) and oxygen plasma, which is also planned for use for doping. Features and optimal synthesis parameters for this system are studied in detail and described in the literature, so we limited ourselves to reproducing them on existing equipment. At the next stage, the possibility of synthesizing Ni-Co-O, Ni-Al-O, and Ni-Mn-O complex oxides by the ALD method was shown and the main parameters of their growth were determined (growth per supercycle, growth rate, uniformity of thickness). The growth rate per supercycle for the Ni-Al-O system was 1.59 Å, which is comparable to the calculated one - 1.6 Å. For the Ni-Co-O and Ni-Mn-O systems, the experimental values turned out to be significantly lower than calculated, which is caused by a decrease in the growth rate of the NiO phase, the content of which is much lower than should be based on the growth rate of the nickel oxide system. In order to study the possibility of controlling the composition of the samples, we obtained samples of the Ni-Co-O series with different ratios of the number of puffs of NiCp2 and CoCp2 in the supercycle (5/1, 3/1, 1/1). Samples were studied by XPS, XRD, and EDX. Using the XPS method, it was shown that a change in the number of pulses of NiCp2 and CoCp2 in the supercycle does not affect the qualitative chemical composition of the samples, however, a regular proportional change in the quantitative composition of the samples depending on the ratio of pulses of NiCp2 and CoCp2 was found, which made it possible to construct a calibration curve for precision control the composition of the Ni-Co-O system. Further, for these samples, the possibility of their electrochemical lithiation using cyclic voltammetry (CVA) in the range of potentials up to 3 V was studied, however, as further cycling showed at various current densities in the potential range from 3 V to 4.3 V, the lithiation of transition metal oxide films did not observe. To obtain the cathode materials of the Li-Ni-Co-O system, Ni-Co-O samples were used on the surface of which Li-O type coatings were deposited by the ALD method, and then annealing was performed at 800 ° С. The obtained CVA curves have a similar appearance, pronounced peaks confirming a change in the oxidation state of nickel and/or cobalt were not detected. Resource tests of Li-Ni-Co-O samples obtained from the different composition of Ni-Co-O samples at different final voltages and discharge currents showed similar results, the absolute capacity is much lower in comparison with Li-Ni-O samples. Detailed studies of Li-Ni-O system films obtained using LiHMDS as a lithium precursor are published in a special issue Advanced Nanomaterials for Li- and Na-Ion Batteries, open-access journal Energies, MDPI. The publication is available for download at the link - https://www.mdpi.com/1996-1073/13/9/2345

 

Publications

---