ABSTRACT:We report on the synthesis of [2]rotaxanes driven by stabilization of the axleforming transition state. A bifunctional macrocycle, with hydrogen bond donors at one end and acceptors at the other, is used to stabilize the charges that develop during the addition of a primary amine to a cyclic sulfate.Most rotaxanes are formed by exploiting permanent recognition motifs in the components that subsequently 'live on' in the interlocked product. 1 Examples of rotaxanes formed through complexdriven effective molarity increases or solvation effects have also been described, 2 and active template synthesis, 3 in which metal ions act as both an organizing template and as a catalyst for the reaction used to covalently capture the threaded structure, enables rotaxanes to be assembled under kinetic control. Here we describe the reagent-less formation of a [2]rotaxane driven by transition state stabilization of an activated complex arising from the opening of a cyclic sulfate by a primary amine. 4
Vinylidene fluoride (VDF)-based copolymers bearing pendant perfluoroalkyl ether groups for potential application in gel polymer electrolytes were synthesized via conventional radical copolymerization of VDF with a 2trifluoromethacrylate monomer bearing pendant perfluoroalkyl ether groups [2,3,3,3-tetrafluoro-2-[1,1,2,3,3,3-hexafluoro-2-(perfluoropropoxy)propoxy]propyl 2-(trifluoromethyl)acrylate (MAF-PFE)]. Two kinds of initiators (hydrogenated and a perfluorinated highly branched persistent radical that released CF 3• radical from 80 °C) were used. Molar masses ranged between 35 000 and ca. 50 000 g•mol −1 . The synthesized poly(VDF-co-MAF-PFE) copolymers were evaluated in gel polymer electrolytes based on ionic liquids. With this aim, the poly(VDF-co-MAF-PFE) copolymers were mixed with lithium bis(trifluoromethanesulfonyl)imide and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Homogeneous gels with conductivity values at room temperature reaching up to 3 × 10 −3 S•cm −1 were obtained, while the transference number of lithium ions was 0.12 at 40 °C. Finally, such electrolytes displayed an electrochemical stability window between 1.5 and 4.4 V as determined by cyclic voltamperometry measurements.
Solid polymer electrolytes are prepared by mixing various amounts of lithium bis(trifluoromethanesulfonyl)imide with poly(vinylidene fluoride‐co‐vinyl dimethyl phosphonate) statistical copolymers with different compositions. Such copolymers are obtained by conventional radical copolymerization of vinylidene fluoride (VDF) with vinyl dimethyl phosphonate (VDMP) initiated by peroxides. A morphological study of the obtained solid polymer electrolytes (SPEs) shows that only samples prepared from the copolymer with the lower amount of VDMP (16 mol%) result in the formation of homogeneous electrolytes while aggregates of lithium salts are observed for the other copolymers. The best ionic conductivity values are accordingly observed for the copolymers with the lower VDMP amount and are reaching 5 × 10−3 mS cm−1 at 100 °C. The dependence of the ionic conductivity versus temperature suggests that the ionic conductivity is controlled by the motion of polymer segments. Indeed, the ionic conductivity can be increased by adding a small amount of trimethylphosphate plasticizer and can reach 1.9 × 10−2 mS cm−1 at 20 °C. Finally, the prepared SPEs exhibit a high electrochemical stability and a good resistance to flame because of the presence of fire‐retardant phosphate groups in their structure.
Lithium-ion batteries are today among the most efficient devices for electrochemical energy storage. However, an improvement of their performance is required to address the challenges of modern grid management, portable technology, and electric mobility. One of the most important limitations to solve is the slow kinetics of redox reactions associated to inorganic cathodic materials, directly impacting on the charging time and the power characteristics of the cells. In sharp contrast, redox polymers such as poly(2,2,6,6-tetramethyl-1-piperidinyloxy methacrylate) (PTMA) exhibit fast redox reaction kinetics and pseudocapacitors characteristics. In this contribution, we have hybridized high energy Li(NixMnyCoz)O2 mixed oxides (NMC) with PTMA. In this hybrid cathode configuration, the higher voltage NMC (ca. 3.7 V vs. Li/Li+) is able to transfer its energy to the lower voltage PTMA (3.6 V vs. Li/Li+) improving the discharge power performances and allowing high power cathodes to be obtained. However, the NMC-PTMA hybrid cathodes show an important capacity fading. Our investigations indicate the presence of an interface degradation reaction between NMC and PTMA transforming NMC into an electrochemically dead material. Moreover, the aqueous process used here to prepare the cathode is also shown to enable the degradation of NMC. Indeed, once NMC is immersed in water, alkaline surface species dissolve, increasing the pH of the slurry, and corroding the aluminum current collector. Additionally, the NMC surface is altered due to delithiation which enables the interface degradation reaction to take place. This reaction by surface passivation of NMC particles did not succeed in preventing the interfacial degradation. Degradation was, however, notably decreased when Li(Ni0.8Mn0.1Co0.1)O2 NMC was used and even further when alumina-coated Li(Ni0.8Mn0.1Co0.1)O2 NMC was considered. For the latter at a 20C discharge rate, the hybrids presented higher power performances compared to the single constituents, clearly emphasizing the benefits of the hybrid cathode concept.
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