The dynamics of individual components in 1,4-polyisoprene/poly(vinylethylene) miscible blends are studied using two-dimensional deuteron exchange NMR. The rate of the backbone reorientation process near the glass transition is quantitatively determined for each species in a miscible blend as a function of temperature. We demonstrate that the broad glass transition arises both from a wide distribution of segmental motional rates for each species and from intrinsic differences in the motional rate between the two species.In addition, the temperature dependence of their motional rates in the blend DSC glass transition region suggests that the two components undergo distinct effective glass transitions, which is consistent with previously observed thermorheologically complex behavior. The origins of dynamic heterogeneity are examined further by comparing the experimental results with a simple model calculation that takes into account the effect of composition variations in an ideal miscible blend. This comparison suggests that the observed dynamic heterogeneities can be explained only by including two distinct contributions: local composition variations in the blend and intrinsic differences in chain mobilities.

The segmental motion of each species in polyisoprene/poly(vinylethylene) (PI/PVE) miscible blends is studied at three different compositions using two-dimensional deuteron exchange NMR (2D 2H NMR). The individual species exhibit widely different mean mobilities and broad mobility distributions near the glass transition of each blend. As the PVE content increases, both the difference in mean mobilities between the two species and the width of the mobility distribution for both components increase. The change in these two types of dynamic heterogeneity with PVE content appears to produce the anomalous broadening of the glass transition. The mean reorientational correlation times of each component can differ by 2 orders of magnitude under identical conditions. This difference can be described in terms of distinct effective glass transition temperatures, Tg*, for the two species. The separation between the two effective glass transition temperatures increases almost monotonically with PVE content, consistent with the more pronounced thermorheological complexity of blends rich in PVE. The individual Tg*' s also exhibit a different compositional dependence from that of the calorimetric Tg of the blend observed by differential scanning calorimetry (DSC). This behavior can give rise to the complex compositional dependence of individual mobilities, apparent when the mobilities are compared at the same T -Tg with respect to the DSC Tg of the blend.

The high capacity of the layered Li−excess oxide cathode is always accompanied by extraction of a significant amount of oxygen from the structure. The effects of oxygen on the electrochemical cycling are not well understood. Here, the detailed reaction scheme following oxygen evolution was established using real-time gas analysis and ex situ chemical analysis of the surface of the electrodes. A series of electrochemical/chemical reactions involving oxygen radicals constantly produced and decomposed lithium carbonate during cell operation. Moreover, byproducts, including water, affected the cycle life and rate capability: hydrolysis of the electrolyte salt formed hydrofluoric acid that attacked the surface of the electrode. This finding implies that protection of the electrode surface from damage, for example, by a coating or removal of oxygen radicals by scavengers, will be critical to widespread usage of Li−excess transition metal oxides in rechargeable lithium batteries.

To elucidate the origin of graphite exfoliation, we have investigated the influence of various material parameters relevant to solvent co-intercalation, such as the cation, the electrolytic solvents, and the structure of graphite, on the solvent decomposition behavior. By electrochemically probing changes in the electrode, we demonstrated that a large increase of surface area accompanies the decomposition of propylene carbonate (PC). Furthermore, such a change in surface area is dramatically amplified when Li ϩ is replaced by tetrabutylammonium ion. A slight structural modification of PC exerts a profound influence on the solvent decomposition behavior, as demonstrated with cis-and trans-2,3-butylene carbonate. These reaction behaviors are also altered significantly by the choice of graphite. Such an influence of graphite structure is particularly surprising for t-BC electrolyte, in which SFG44 graphite undergoes extensive exfoliation, whereas SFG6 graphite and MCMB25 can be cycled reversibly. These results can be best explained by incorporating the co-intercalation of cyclic carbonate as a critical process in the solid electrolyte interphase formation mechanism.

A key challenge hindering the mass adoption of Lithium-ion and other next-gen chemistries in advanced battery applications such as hybrid/electric vehicles (xEVs) has been management of their functional performance for more effective battery utilization and control over their life. Contemporary battery management systems (BMS) reliant on monitoring external parameters such as voltage and current to ensure safe battery operation with the required performance usually result in overdesign and inefficient use of capacity. More informative embedded sensors are desirable for internal cell state monitoring, which could provide accurate state-of-charge (SOC) and state-of-health (SOH) estimates and early failure indicators. Here we present a promising new embedded sensing option developed by our team for cell monitoring, fiber-optic

In order to understand the structural effect on irreversible capacity, the electrochemical lithium intercalation of various graphitic carbons was studied in 1 M LiPF6 solution of ethylene carbonate/propylene carbonate/diethyl carbonate (EC/PC/DEC), as a function of PC content. Because the irreversible capacity was increased by PC only for the carbons with a considerable edge fraction, it was possible to selectively amplify and monitor the reaction on the edge surface. The important role of the edge surface was confirmed qualitatively by high resolution transmission electron microscope images of the surface structure and quantitatively by further analysis on the results, using a simple model that explicitly considered the different reactivity of EC and PC toward the basal and edge surfaces. Exfoliation behavior, emerging as the PC content increased above a certain structure‐dependent threshold, was examined also. The structural effect could be explained by assuming that the cointercalation of PC gave rise to exfoliation. Two controlling factors were suggested: the structural integrity affecting the expansion of layer spacing and the geometry of the edge surface affecting the cointercalation of PC. © 1999 The Electrochemical Society. All rights reserved.

No abstract

No abstract