Alesya V. Parfeneva

The work presents the results of the formation and structural properties study of aluminum nitride (AlN) nanostructures on the hexagonal boron nitride (h-BN) flakes transferred onto a silicon dioxide/silicon (SiO2/Si) substrate. For the first time, the possibility of aluminum nitride nanowire growth on h-BN flakes using plasma-assisted molecular beam epitaxy through a self-induced mechanism is demonstrated. The experimental dependence of the length of synthesized nanowires on their diameter is provided. The dependence corresponds to an analytical solution of the length versus diameter for nanowires grown by a self-induced mechanism. Raman spectroscopy studies reveal the wurtzite crystal phase of the aluminum nitride nanowires formed on h-BN flakes. The full width at half maximum of the E2(high) peak is 5 cm⁻¹, suggesting high crystalline quality of nanowires. These results have implications for the development of deep-ultraviolet optoelectronic devices and the advancement of III-nitride optoelectronics integrated with silicon technology.

The properties of lithium‐ion battery (LIB) anodes fabricated from nanoscale silicon Si and polyaniline (PANI) as a binder are reported. PANI is prepared by in situ polymerization of aniline in the presence of phytic acid, which serves both as dopant and as a gel‐forming agent. PANI pellets obtained by dry compression are used to investigate the morphology and to measure the resistivity of PANI and Si/PANI composites. The anodes are fabricated using the slurry technique. Their properties as a function of precursor ratio are studied in the half‐cell cells by charge–discharge characteristics, cyclic voltammetry, electrochemical impedance spectroscopy and cyclic lifetime. It is shown that stable cycling (>350 cycles at a current of 300 mA g⁻¹) is inherent only to thin Si/PANI layers with composite loading −2. The discharge capacity in this case is as high as 500–800 mAh g⁻¹.

The work is devoted to a comparative study of the electrochemical behavior and molecular level transformations in TiF₃ and TiOF₂ electrodes of Li‐ion batteries at charge‐discharge processes. Based on analysis of the electrode voltage profiles, we propose possible redox reactions which occur with the change of Ti oxidizing state. According to these models, titanium trifluoride has a reversible capacity of 600 mAh/g, TiOF₂ of 395 mAh/g. It has been shown that titanium oxyfluoride is characterized by a long cycle life while specific capacity of the titanium trifluoride rapidly degrades. The degradation of TiF₃ is associated with a temporal molecular ordering and decrease in the chemical activity of titanium and lithium fluoride formed during the reversible introduction of lithium. The related effect lies under the superreversibility phenomenon in TiOF₂ electrodes (Coulomb efficiency more than 100%). The dependences of the equilibrium voltage and chemical diffusion coefficient of Li on the concentration of Li were determined from GITT analysis.

The properties of titanium oxyfluoride‐based anodes are studied by galvanostatic cycling and galvanostatic intermittent titration technique in a TiOF₂—Li half‐cells. This anode material is shown to have a high degradation resistance and a specific gravimetric capacity comparable to that of carbon and Li₄Ti₅O₁₂. The chemical diffusion coefficient of Li⁺ in TiOF₂ varies by two orders of magnitude in the range from 2.3 × 10⁻¹³ to 2.5 × 10⁻¹⁵ cm² s⁻¹ depending on the degree of lithiation and the direction of charge or discharge process. The electrode electrical resistance changes with cycling and its value is always lower in the lithiated state than in the delithiated state. The phenomenon of superreversibility inherent in this material has been studied. This phenomenon consists in the excess of the discharge capacity (Li extraction) over the charge capacity (Li insertion), that is, the Coulomb efficiency (CE) exceeds 100%. In the first few tens of cycles, the phenomenon is due to the extraction of excess lithium, which was previously stored in the electrode during the first cycle. On subsequent cycles, the CE stabilizes at a level above or below 100%, depending on the ratio between the charge and discharge currents, and is not related to the accumulation or spending of Li.

A study of the electrochemical characteristics of titanium oxyfluoride obtained with the direct interaction of titanium with hydrofluoric acid is reported. Two materials T1 and T2 synthesized under different conditions in which some TiF₃ is formed in T1 are compared. Both materials exhibit conversion-type anode properties. Based on the analysis of the charge–discharge curves of the half-cell, a model is proposed according to which the first electrochemical introduction of lithium occurs in two stages: the first stage is the irreversible reaction resulting in a reduction in Ti^4+/3+, and the second stage is the reversible reaction with a change in the charge state Ti^3+/1.5+. The difference in material behavior is quantitative: T1 has a higher reversible capacity but lower cycling stability and a slightly higher operating voltage. The Li diffusion coefficient determined from the CVA data for both materials averages 1.2–3.0 × 10⁻¹⁴ cm²/s. A distinctive feature of titanium oxyfluoride anodes is the asymmetry in kinetic characteristics that revealed themselves during lithium embedding and extraction. In the long cycling regime, the excess of Coulomb efficiency over 100% was found in the present study.

Here, we study an effect of FEC addition to TC-E918 electrolyte on the electrochemical performance of Si/C negative electrodes. The anodes were fabricated from nanosilicon powder coated with a carbon shell by means of a standard slurry technique. The low-temperature reduction of fluorocarbon on the surface of Si nanoparticles was used to form the shell. It was shown that the presence of FEC in the electrolyte increases the cyclic stability of the electrodes and maintains a 1.5-fold higher discharge capacity during 300 cycles. Impedance measurements were used to study changes in the electrode parameters during long-term cycling with and without FEC additives.

GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiO_x/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N₂ flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N₂ flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements.

Dispersed composite materials based on silicon monoxide and carbon (SiO/C) were obtained as a result of heat treatment of a powder mixture consisting of 40 wt% SiO and 60 wt% CF0.8. Annealing was carried out in an argon atmosphere at temperatures 1000 − 1250°C. Using electron microscopy and Raman scattering, it was found that at Т>=1100°C, silicon carbide appears in the solid-phase product, including in the form of cubic nanowhiskers. Based on the data on the weight loss of the reaction mixture, the composition of the resulting products was calculated as a function of the annealing temperature. Anodes made of composites obtained at temperatures above 1100°C demonstrate a sharp drop in capacitance and Columbic efficiency. It is shown that the observed changes are caused not so much by the formation of SiC as by an increase in the oxygen content in the matrix surrounding the silicon precipitates, which were formed as a result of disproportionation of SiO. It was found that the optimal annealing temperature, which provides the highest values of capacity, the initial coulombic efficiency and the ability to operate at high speed current densities is Т =1150°С.

The effect of annealing temperature in argon atmosphere on the ability of Si-C nanocomposites to reversibly insert lithium was investigated. It was found that the higher the annealing temperature during the formation of the composite, the lower is the capacitance of the electrode made from it. X-ray diffraction analysis and transmission electron microscopy reveal that the reason of the capacitance decrease is formation at T  1100°C of silicon carbide of cubic modification -SiC, inactive with respect to the formation of lithium alloys or intercalates.

A new method for formation of porous silicon-carbon nanocomposites is proposed. It is based on reduction of carbon monofluoride by silicon. The resulting composite materials consist of silicon nanoparticles enclosed in a carbon shell. The contacts of such particles ensure the flow of current through the arising carbon matrix. Density, porosity and resistivity on the composition of the Si-C tablets obtained by the proposed method are determined. The materials under the study are of interest for high-capacity negative electrodes of lithium-ion batteries.