Electrochemical properties and morphology of composite fibers based on silicon and carbon obtained by electroforming

Pyrolized Si/C composite fibers and films were studied in the processes of lithium intercalation-extraction during the operation of rechargeable lithium-ion batteries and supercapacitors. It was found that their electrochemical characteristics differ from those of pure silicon and pure carbon. Analysis of these parameters under charge-discharge conditions allowed us to determine the optimum composition of precursors: FM-1 and TEOS mixtures used as silicon-containing components. The optimum modes and temperatures of fiber pyrolysis were found to have a Si/C ratio of 3/2 in the pyrolyzed fibers and an oxygen content not much greater than 20 at %. The prospects for applying fibers in the form of film electrodes (the best stability in cycling) and adhesives with developed structural networks are shown.     

Electrode materials for lithium-ion batteries of new generation

The studies of fundamentally new electrochemical systems for lithium-ion batteries of new generation, which were performed at the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, are briefly reviewed. The results of investigation of lithium insertion into negative electrodes based on silicon and silicon-carbon composites and operation of positive electrodes of nano-structured materials based on vanadium oxides are described.     

Lithium insertion into silicon films produced by magnetron sputtering

Using the method of magnetron-plasma sputtering of polycrystalline silicon target, amorphous silicon films 32–214 nm thick were produced on various (copper and titanium, polished and rough) substrates. A study of their charge-discharge characteristics under the galvanostatic conditions showed that all thin-filmed electrodes are capable of reversible lithium insertion. The amount of lithium inserted in the first cycles is close to the theoretical one. An analysis of composition and morphology of surface layer and also the behavior of reversible and irreversible capacities during cycling showed that the degradation of capacity is caused by the exfoliation of films from the substrate (the effect is more pronounced for the specimens with polished substrates) and somewhat breaking (cracking) of films. The thicker are the films, the severer is the disruption of silicon films in the cycling. The adhesion of films to the substrate surface is favored by the film roughness. At sufficiently high adhesion of films, their electrochemical properties only slightly depend on the nature (copper or titanium) of substrate.     

Electrochemical intercalation of lithium into nanocrystalline ceria

The specifics of electrochemical lithium intercalation into nanocrystalline ceria were studied. The lithium capacity of CeO_{2 − x} is discovered to increase systematically as the nanoparticle size shifts down, indicating the potential of nanocrystalline ceria for use in electrochromic applications.     

The impedance of lithium-ion batteries

Cylindrical and flat one-layer lithium-ion batteries, as well as symmetrical cells with like electrodes, are studied using electrochemical impedance spectroscopy. Equivalent circuit is suggested, its parameters found, and the activation energies estimated for different stages of the electrode process.     

57Fe Mössbauer study of Li x Fe1-y Co y PO4 (y = 0, 0.1, 0.2) as cathode material for Li-ion batteries

Lithium iron phosphates LiFe_1-y Co _y PO₄ (y = 0, 0.1, 0.2) exposed to a charging process were studied by ⁵⁷Fe Mössbauer spectroscopy taking into account XRD and SEM data. Hyperfine parameters of the spectra were determined above and below the magnetic ordering temperature for all the samples. It was shown that the presence of Co impurity atoms in lithium phosphates gives no effect on the hyperfine interaction of ⁵⁷Fe²⁺ cations. However, Co atoms in the nearest cation environment of Fe atoms lead to a significant change of the hyperfine interactions of ⁵⁷Fe³⁺ cations. The Co impurity atoms distribution over the positions of the iron atoms in the structure is found not to be statistical,but correlated.     

Minimal irreversible capacity caused by the lithium insertion into materials of negative electrodes in lithium-ion batteries

The minimal irreversible capacity of negative electrodes of lithium-ion batteries, necessary for their stable operation, is theoretically evaluated. The theoretical values are compared with real ones reported in literature. It is shown that the real values of the irreversible capacity for electrodes made of carbonaceous materials exceed several times the minimal required values; the real irreversible capacity of silicon-based electrodes exceeds the minimal values by 2 to 3 orders of magnitude.