The results of a study of anodes obtained by carbonization of silicon monoxide by means of a reaction with solid-phase fluorocarbon CF0.8 are presented. Charge/discharge voltage profiles were studied at different currents depending on the composition and temperature of the synthesis of composites. The irreversible losses of the 1st cycle and the contribution to them of intrinsic losses due to the formation of lithium oxide and its silicates and losses associated with the formation of SEI are analyzed. A difference has been established in the behavior of anodes made of SiO carbonized by annealing with CF0.8 at T = 800°C (SiO/C composite) and silicon monoxide annealed with CF0.8 at T > 1000°C at which disproportionate occurs simultaneously with the carbonization of SiO (d-SiO/C composite). The difference consisting in a higher discharge capacity, a higher Coulomb efficiency, and better rate capability of d-SiO/C is explained by a change in the composition of the SiOx matrix that occurs during the disproportionation process. The effect of the formation of d-SiO/C anodes by preliminary lithiation with a low current, after which the electrodes can be charged and discharged with much higher currents, has been discovered. The effect is explained by the amorphization of silicon crystallites and the increasing diffusion coefficient of lithium.

The properties of titanium oxyfluoride‐based anodes are studied by galvanostatic cycling and galvanostatic intermittent titration technique in a TiOF 2 —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 Li 4 Ti 5 O 12 . The chemical diffusion coefficient of Li + in TiOF 2 varies by two orders of magnitude in the range from 2.3 × 10 −13 to 2.5 × 10 −15 cm 2 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.

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 <0.7 mg cm−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 processes of lithium insertion into TiOF2 and TiF3 electrodes were studied using electrochemical impedance spectroscopy. The measurements were carried out at different degrees of lithiation during the first two charge/discharge cycles. Electrical equivalent circuits were proposed to model the resulting impedance spectra. They reflect the processes of Li+ migration through SEI, lithium ion insertion/extraction stages with the change of titanium oxidation degree and lithium ion diffusion in the material volume. A comparative evaluation of kinetic parameters of TiF3 and TiOF2 electrodes depending on the degree of lithiation was carried out for the first time. It is shown that the main process of SEI formation takes place at the first cycle. SEI has a lower impedance value on TiF3 electrodes. The concentration dependence of the charge transfer resistance and diffusion coefficient for TiF3 and TiOF2 materials is observed: with increasing lithium content the interfacial charge transfer resistance decreases and the Li+ diffusion coefficient increases. The concentration dependences of these parameters for TiOF2 electrodes are more pronounced.

Composite SiO/C anodes for lithium-ion batteries (LIB) obtained by carbonization of silicon monoxide with fluorocarbon were investigated. Preliminary modification of the initial silicon monoxide was carried out by disproportionation and subsequent treatment in hydrofluoric acid. The study of the elemental composition of modified SiO and the electrochemical performance of composite anodes made from it showed that the oxygen content in the oxide matrix played a decisive role in changing their behavior. Depletion of its oxygen as a result of treatment in HF had a beneficial effect, leading to higher stability of the electrodes during cycling, an increase in their capacity, coulombic efficiency and rate capability.

Composite SiO/C anodes for lithium-ion batteries (LIB) obtained by carbonization of silicon monoxide with fluorocarbon were investigated. Preliminary modification of the initial silicon monoxide was carried out by disproportionation and subsequent treatment in hydrofluoric acid. The study of the elemental composition of modified SiO and the electrochemical performance of composite anodes made from it showed that the oxygen content in the oxide matrix played a decisive role in changing their behavior. Depletion of it with oxygen as a result of treatment in HF had a beneficial effect, leading to higher stability of the electrodes during cycling, an increase in their capacity, coulombic efficiency and rate capability. Keywords: composite SiO/C anodes, lithium-ion batteries, treatment in HF.

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 Ti4+/3+, and the second stage is the reversible reaction with a change in the charge state Ti3+/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.

Disperse composite materials based on silicon monoxide and carbon (SiO/C) have been obtained by thermal treatment of a powder mixture consisting of 40 wt % SiO and 60 wt % CF0.8. Annealing has been performed in the argon atmosphere at temperatures 1000–1250°C. It has been established using electron microscopy and Raman scattering that, at T ≥ 1100°C, silicon carbide is formed in the solid-phase product, including in the form of nanowhiskers of cubic modification. Based on the data on the decrease of the reaction mixture weight, the composition of the formed products is calculated as a function of the annealing temperature. The anodes prepared from the composites obtained at a temperature above 1100°C demonstrate a sharp decrease in the capacitance and in the Coulomb efficiency. It is shown that the observed changes are determined by an increase in the oxygen concentration in the matrix surrounding silicon precipitates, which have been formed as a result of SiO disproportionation, rather than by the formation of SiC. It is established that an optimal annealing temperature provides the highest values of the electrode capacitance, the Coulomb efficiency of the first cycle, and the ability to operate at high current densities is T = 1050°C.

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.

The subject of this study was Si-C composites for lithium-ion battery (LIB) anodes obtained by carbonization of nanodispersed silicon with carbon monofluoride. To determine the possibility of increasing the degree of graphitization of nanodispersed carbon forming shells around the silicon particles at lower temperatures, nickel in the form of an alcoholic solution of Ni(NO3)2 was introduced as a catalyst into the pellets of the resulting composite. The XRD, Raman scattering and EDS methods were used to investigate changes both in the phase and elemental composition of materials resulting from the annealing of the Ni-containing Si-C composite over the temperature range of 500–1100 °C. It was found for the first time that nickel silicides that emerged during the annealing became catalysts and, at the same time, intermediate products, of cubic silicon carbide (β-SiC) synthesis, which reduced its temperature formation from ~1100 °C to ~800 °C. The same compounds had a catalytic effect on the carbon atom association, leading to an increase in the degree of its graphitization. The influence of changing the composition of the investigated material on the electrochemical characteristics of the obtained negative LIB electrodes was traced.

This work investigated the possibility of increasing the cycle life and rate capability of silicon anodes, made of macroporous membranes, by adding fluoroethylene carbonate (FEC) to the complex commercial electrolyte. It was found that FEC leads to a decrease in the degradation rate; for a sample without FEC addition, the discharge capacity at the level of Qdch = 1000 mAh/g remained unchanged for 220 cycles and the same sample with 3% FEC added to the electrolyte remained unchanged for over 600 cycles. FEC also improves the power characteristics of the anodes by 5–18%. Studies of impedance hodographs showed that in both electrolytes (with 0% and 3% FEC, respectively) the charge transfer resistance grows with an increasing number of cycles, while Solid Electrolyte Interphase (SEI) parameters, such as its resistance and capacitance, show little change. However, the addition of FEC more than halves the overall system impedance and reduces the resistance of the liquid electrolyte and all current carrying parts as well as the SEI film and charge transfer resistances.

The results of a study of anodes obtained by carbonization of silicon monoxide by means of a reaction with solid-phase fluorocarbon CF0.8 are presented. Charge/discharge voltage profiles were studied at different currents depending on the composition and temperature of the synthesis of composites. The irreversible losses of the 1st cycle and the contribution to them of intrinsic losses due to the formation of lithium oxide and its silicates and losses associated with the formation of SEI are analyzed. A difference has been established in the behavior of anodes made of SiO carbonized by annealing with CF0.8 at T=800°C (SiO/C composite) and silicon monoxide annealed with CF0.8 at T>1000°C, at which disproportionation occurs simultaneously with the carbonization of SiO (d-SiO/C composite). The difference consisting in a higher discharge capacity, a higher Coulomb efficiency, and better rate capability of d-SiO/C is explained by a change in the composition of the SiOx matrix that occurs during the disproportionation process. The effect of the formation of d-SiO/C anodes by preliminary lithiation with a low current, after which the electrodes can be charged and discharged with much higher currents, has been discovered. The effect is explained by the amorphization of silicon crystallites and the increasing diffusion coefficient of lithium

The processes of the disproportionation of solid-phase silicon monoxide, accompanied by the formation of nanocrystalline silicon precipitates in the medium of amorphous suboxide SiOx (initial composition of SiO0.9), are studied. Based on the data of X-ray diffraction analysis and transmission electron microscopy, the dynamics of changes in the amount, concentration, and size of phase precipitates of silicon with the temperature of isochronous annealing increasing from 800 to 1200°C is traced. It is found that, with the total mass of the precipitated silicon steadily increasing, the number of its crystallization centers per unit volume nonmonotonically depends on temperature. The activation energy of the diffusion of silicon atoms in the SiOx matrix is determined to be Ea1 = 1.64 eV and the activation energy of their transfer from precipitates formed to the SiOx growth medium is Ea2 = 2.38 eV. The anisotropic deformation of silicon crystallites precipitated during the disproportionation of SiO is revealed for the first time. This phenomenon is attributed to the difference between the specific volumes of the phases being separated and to the anisotropy of the growth rate of silicon precipitates formed in a solid amorphous medium.

In this work, the processes of disproportionation of solid-phase silicon monoxide, accompanied by the formation of nanocrystalline silicon precipitates in the medium of amorphous SiOx suboxide (initial composition SiO0.9), have been studied. Based on the data of X-ray diffraction analysis and transmission electron microscopy, the dynamics of changes in the amount, concentration and size of phase precipitates of silicon with an increase in the temperature of isochronous annealing from 800 °C to 1200 °C is traced. It was found that with a monotonic increase in the total mass of the precipitated silicon, the number of its crystallization centers per unit volume nonmonotonically depends on temperature. The activation energy of diffusion of silicon atoms in the SiOx matrix was determined to be Ea1= 1.64 eV, and the activation energy of their transfer from the formed precipitates to the growth medium of SiOx was Ea2 = 2.38 eV. Anisotropic deformation of silicon crystallites precipitated during the disproportionation of SiO has been revealed for the first time. This phenomenon is associated with the difference in the specific volumes of the separated phases and the anisotropy of the growth rate of silicon precipitates formed in a solid amorphous medium.

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.

Dry-sintered macroporous Si electrodes for Li-ion batteries are studied for the first time using spectral impedance measurements. The spectra obtained in the lithiated and delithiated states are simulated with an equivalent electrical circuit, the parameters of which reveal the role of electrochemical processes caused by the surface layer of a solid-phase electrolyte (SEI), electric double layer, and diffusion in the solid phase of the electrode material. It is shown that the effective diffusion coefficient of Li in Si grows with increasing degree of lithiation from D = 6 . 5 × 10 –12 to 2 . 6 × 10 –10 cm 2 /s. The effect of carbonization via saccharose pyrolysis, which led to a decrease in the impedance and an increase in the diffusion coefficient of lithium to D = 2 . 2 × 10 –10 –1 . 7 × 10 –9 cm 2 /s, is studied.

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.

It is suggested to form porous silicon-carbon nanocomposites via thermal reduction of carbon monofluoride by silicon. For this purpose a mixture of powders of nanocrystalline silicon and fluorocarbon is subjected to cold compaction and the resulting pellets are annealed in an inert atmosphere at T = 800 degrees C. The density, porosity, structure, composition, and electrical resistivity of thus produced Si-C materials have been studied in detail in relation to the content of the monofluoride in the starting mixture. It was shown that the materials obtained have a hierarchical porous structure constituted by silicon nanoparticles in a shell of finely dispersed carbon. The shells contacting with each other form a carbon matrix providing a high electrical conductivity of the material. The composite was used to fabricate negative electrodes of lithium-ion batteries with increased storage capacity. The electrochemical characteristics of Si-C nanocomposite anodes of varied composition were analyzed and those with high carbon content demonstrated the best performance. (C) 2020 Elsevier B.V. All rights reserved.

Influence exerted by the temperature of annealing in the atmosphere of argon on the ability of Si‒C nanocomposites to enable a reversible introduction of lithium has been studied. It was found that the higher the annealing temperature in the formation of a composite, the lower the capacity of the electrode fabricated from this composite. X-ray diffraction analysis and scanning electron microscopy demonstrated that the capacity decreases because silicon carbide of cubic modification β-SiC inactive toward formation of lithium alloys or intercalates is formed at T ≥ 1100°C.