Non-Flammable and Highly Concentrated Carbonate Ester-Free Electrolyte Solutions for 5 V-Class Positive Electrodes in Lithium-Ion Batteries.

Non-flammable and highly concentrated electrolyte solutions were designed using tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a main solvent toward a radical improvement in the safety and energy density of lithium-ion batteries. Unlike conventional carbonate ester-based solutions, simple TFEP-based electrolyte solutions were not intrinsically compatible with 5 V-class LiNi0.5 Mn1.5 O4 positive electrodes, even at high concentrations. Based on the degradation mechanism that was analyzed by Raman spectroscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, a fluorinated diluent of methyl 3,3,3-trifluoropropionate (FMP) was introduced to suppress the decomposition of LiBF4 and TFEP at high potentials. A nearly saturated LiBF4 /TFEP+FMP electrolyte solution with a specific composition improved the charge and discharge performance of a LiNi0.5 Mn1.5 O4 electrode, and the solution structure was studied by pulsed-field-gradient NMR spectroscopy.

Effect of Lithium Silicate Addition on the Microstructure and Crack Formation of LiNi0.8Co0.1Mn0.1O2 Cathode Particles.

The microstructure of LiNi0.8Co0.1Mn0.1O2 cathode materials was controlled by the addition of lithium silicate, and the influence on the cycle performance and the rate capability was investigated. Si was not included within the lattice, but localized at the grain boundaries of the primary particles and the pores inside the secondary particles. The addition of the lithium silicate greatly decreased the density of the pores between the primary particles and improved the density of the secondary particles. The capacity retention was successfully improved for lithium silicate-added LiNi0.8Co0.1Mn0.1O2. When lithium silicate-free LiNi0.8Co0.1Mn0.1O2 was charged to 4.3 V, many cracks were formed along the grain boundaries even in the first cycle, while crack formation was remarkably inhibited for lithium silicate-added LiNi0.8Co0.1Mn0.1O2. Moreover, lithium silicate-added LiNi0.8Co0.1Mn0.1O2 particles were almost free from visible microcracks even after 100 cycles at the discharged state. These results suggest that the lithium silicate reinforces the grain-adhesion at the grain boundaries, inhibiting crack formation and electrolyte decomposition inside the cracks.

Artificial lithium fluoride surface coating on silicon negative electrodes for the inhibition of electrolyte decomposition in lithium-ion batteries: visualization of a solid electrolyte interphase using in situ AFM.

The solid electrolyte interphase (SEI), which is a surface layer formed on the negative electrode, plays an important role in inhibiting the reductive decomposition of the electrolyte solution in a lithium-ion battery. However, it has not been understood well which components are important for the SEI to prevent the electrolyte decomposition. Lithium fluoride (LiF), as an artificial SEI, was formed on an amorphous-Si thin film by physical vapor deposition. Changes in the surface morphology of the Si electrode with potential sweeping were investigated using in situ atomic force microscopy (AFM). Although large amounts of non-uniform surface deposits that originate from electrolyte decomposition emerged on the bare Si-film electrode during the first lithiation process, few surface deposits were observed on the LiF-coated Si-film electrode even after two cycles in an ethylene carbonate-based electrolyte solution without additives. It is clear that LiF is a required SEI component that inhibits electrolyte decomposition on Si negative electrodes.

In situ Scanning Electron Microscopy of Silicon Anode Reactions in Lithium-Ion Batteries during Charge/Discharge Processes.

A comprehensive understanding of the charge/discharge behaviour of high-capacity anode active materials, e.g., Si and Li, is essential for the design and development of next-generation high-performance Li-based batteries. Here, we demonstrate the in situ scanning electron microscopy (in situ SEM) of Si anodes in a configuration analogous to actual lithium-ion batteries (LIBs) with an ionic liquid (IL) that is expected to be a functional LIB electrolyte in the future. We discovered that variations in the morphology of Si active materials during charge/discharge processes is strongly dependent on their size and shape. Even the diffusion of atomic Li into Si materials can be visualized using a back-scattering electron imaging technique. The electrode reactions were successfully recorded as video clips. This in situ SEM technique can simultaneously provide useful data on, for example, morphological variations and elemental distributions, as well as electrochemical data.

Analysis of cationic structure in some room-temperature molten fluorides and dependence of their ionic conductivity and viscosity on hydrofluoric acid concentration.

To understand the ionic and nonionic species in (CH(3))(4)NF·mHF, (CH(3))(3)N·mHF, (C(2)H(5))(4)NF·mHF, and (C(2)H(5))(3)N·mHF melts, the structures of these melts were investigated by infrared spectroscopy, NMR, and high-energy X-ray diffraction. Infrared spectra revealed that three kinds of fluorohydrogenate anions, (FH)(n)F(-) (n = 1, 2, and 3), and molecular hydrofluoric acid (HF) are present in every melt. Ionic conductivity and viscosity of these melts were measured and correlated with their cationic structure. The ionic conductivity of the R(4)N(+)-systems was higher than that of corresponding R(3)NH(+)-systems because a strong N-H···F(HF)(n) interaction prevents the motion of R(3)NH(+) cations in the R(3)N·mHF melts. (CH(3))(4)N(+) and (CH(3))(3)NH(+) cations gave higher ionic conductivity than (C(2)H(5))(4)N(+) and (C(2)H(5))(3)NH(+) cations, respectively, because the ionic radii of former cations were smaller than those of latter. It was concluded that these effects on ionic conductivity can be explained by the cationic structure and the concentration of molecular HF in the melts.

Preparation of core/shell and hollow nanostructures of cerium oxide by electrodeposition on a polystyrene sphere template.

Core/shell nanostructures of polystyrene (PS)/CeO2 have been prepared on conductive glass substrates by using a novel electrochemical route consisting of (i) the electrophoretic deposition of a PS sphere monolayer on the substrate and (ii) the following potentiostatic electrodeposition of CeO2 on the PS sphere template in Ce(NO3)3 aqueous solutions. The structural morphologies of the deposit changed drastically depending on the Ce(NO3)3 concentration; i.e., spherical and needlelike shells were deposited. The deposit was formed only on the PS sphere surface because of an interaction between cationic cerium species and a sulfate group that was immobilized on the PS sphere surface. The spherical shell layer was assigned as CeO2, and the needlelike shells were composed of Ce(OH)3 needles formed on the CeO2 layer surface, indicating that the deposit species changes from CeO2 to Ce(OH)3 during electrodeposition only in a 1 mM Ce3+ solution. Deposition of Ce(OH)3 would begin when electrogenerated hydrogen peroxide was consumed by decomposition under reductive conditions and could no longer oxidize Ce3+ ions. The corresponding CeO2 hollow shells were obtained by thermal elimination of the PS sphere core and transformation of Ce(OH)3 into CeO2 while keeping their original shapes.

Atomic force microscopy study on the stability of a surface film formed on a graphite negative electrode at elevated temperatures.

The stability at elevated temperatures of a solid electrolyte interphase (SEI) formed on a graphite negative electrode in lithium ion batteries was investigated by storage tests and in situ atomic force microscopy (AFM) observation. When the fully discharged graphite electrode was stored at elevated temperatures, the irreversible capacity in the following cycle increased remarkably. On the other hand, when the electrode was stored at the fully charged state at elevated temperatures, it was severely self-discharged during storage. AFM observation of the SEI layer formed on a model electrode of highly oriented pyrolytic graphite revealed two important facts on the stability of the SEI at elevated temperatures: (i) dissolution and agglomeration of the SEI layer at the discharged state and (ii) serious SEI growth at the charged state. These phenomena well explain the results of the charge and discharge tests. It was also shown that the addition of vinylene carbonate greatly improves the stability of the SEI at elevated temperatures, and gives good charge and discharge performance after storage.