Structural Evolution during Cycling

5 Surface Chemistry and Solid Electrolyte Interphase of SiNWs 

In general, because the negative electrode operates at low potentials close to metallic Li, the electrolyte is outside its thermod5mamic 

Stability region. Electrochemical reduction of both the solvent and salt of the electrolyte can cause the formation of a film on the surface of the anode. This film is composed of insoluble reduction products of the electrolyte. The presence of this film, the SEl, has been known to be an important feature of graphite anodes that allows for reversible cycling and long-term stability due to surface passivation. The components of the SEl on graphite electrodes have been well studied and have shown that the decomposition products of the ethylene carbonate solvent, namely Li allg l carbonates and Li carbonate, dominate the SEl layer. - 

Because analysis of the SiNW electrodes showed that the NWs did not pulverize during cycling, irreversible capacity loss due to lost or disconnected material seems unlikely for low cycle numbers, but it may rather be due to side reactions and SEl formation. For high-surface area materials such as NWs, there may be increased reactivity of the electrode with the electrolyte, which may have implications for long-term cycling, depending on which species are being formed. 

To further understand the SEl formation on SiNWs, several SiNW samples were cycled to different states ofcharge or discharge, washed to remove excess electrol)fte, and then characterized for changes in SEl composition and morphology using X-ray photoelectron spectroscopy (XPS) and SEM.  

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Ex situ Studies on Structural Evolution during Cycling... 

Limits of Time-Resolved and Simultaneous Measurements. Structure evolution studies are based on the ability to carry out time-resolved scattering experiments. The power of this scattering technique is a function of the minimum cycle time during which a scattering pattern with sufficient signal-to-noise ratio can be recorded. As cycle times for anisotropic 2D SAXS patterns have fallen below a value... 

Some experiments are aiming at the study of structure evolution. In general, the studied material is isotropic or exhibits simple anisotropy (e.g., fiber symmetry). Most frequently the material is irradiated in normal-transmission geometry. A synchrotron beamline is necessary, because in situ recording during the materials processing is requested with a cycle time of seconds between successive snapshots (time-resolved measurements). 

Figure 6.2 Schematic representation of the structural evolution in sol-gel cross-linked PCL during one- (a) and two-way (b) SM cycles. Note filled gray circles represent the sol-gel induced cross-links, while black domains mark the crystalline PCL...

Figure 2.58 shows the presence of two regimes of lubricaiion with Mo-S-1 nanowires. During the first 200 cycles, the friction coefficients are low and stable. Then, an increase in the friction is observed. A more precise analysis of the curves shows that ouly a few cycles are necessary to reach the low friction coefficient. This supposes that a structural evolution of nanowires occurs inside the contact area, rather than a chemical reaction for which an induction period, which can be long, is required. 

Generally speaking, the chance to establish autocata-lytic cycles during chemical evolution (Orgel 2003) with or without external selecting influences is rather small. On the other hand, some (not all of the) similarities between product patterns of prebiotic simulation experiments and contemporary biochemistry suggest that the principal biochemical structures kept constant throughout the existence of life on Earth. Hence auto-catalytic feedback loops came into existence only later. 

Various additives are used for the Ni-Cd batteries to improve the battery performance. Additives are selected based on their special functions to improve the electrode structure and/or electrode chemical and electrochemical properties. For example, cadmium hydroxide Cd(OH)2 is added to the cathode to prevent phase segregation and to help maintain a single phase of the solid solution during the transfer between Ni(OH)2 and NiOOH in charge and discharge processes. Because Cd(OH)2 is isomorphous with both Ni(OH)2 and NiOOH, this structural functionality can improve the cycle life of the battery. Cd or CdO can increase the overpotential of oxygen evolution... 

In this chapter we consider the development and evolution of modem biosphere during its geological history. The reader will find a brief description of the formation of the lithosphere, atmosphere, and hydrosphere and their chemical composition. Creation and evolution of the biogeochemical structure of the biosphere and hydrosphere is also a subject of discussion. Finally, we show the role of biogeochemical cycles in formation of biogenic depositions (oil and gas). 

We have mentioned before the possibility of combining chemical evolution with self-replication. In principle, chemical evolution can be associated to self-reproducing micelles or vesicles. There are in principle two ways to conceive this in this case on the one hand, the surfactants of the self-reproducing vesicles could be chemically transformed during their reproduction cycles into compounds which may give rise to more efficient cell-like compartments. This possibility has been discussed theoretically some time ago. On the other hand, the supramolecular structure can help and determine the evolution of internalized compounds—i.e. permitting certain reactions and avoiding others thanks to the semipermeable character of the membrane. As already mentioned, studies of this type with vesicles still remain to be initiated. 

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