A new family of anion receptors based on boron compounds has been synthesized. These compounds can be used as anion receptors in lithium battery electrolytes. This family includes various borane and borate compounds with different fluorinated aryl and fluorinated alkyl groups. When these anion receptors are used as additives in 1,2-dimethoxyethane (DME) solutions containing various lithium salts, the ionic conductivities of these solutions are greatly increased. The electrolytes tested in this study were DME solutions containing the following lithium salts: LiF, LiC1, LiBr Lii, CF3COOLi, and C2F5COOLi. Without the additive, the solubility of LiF in DME (and all other nonaqueous solvents) is very
Using a commercial available LiCoO 2 as starting material, surface-modified cathode material was obtained by coating its surface with a nanosize layer of amorphous Al 2 O 3 . Electrochemical performances and structural evolutions of the modified cathode material were characterized and compared with that of pristine LiCoO 2 . Specific capacity of 190 mAh/g was obtained in Li/ (Al 2 O 3 -coated LiCoO 2 ) experimental cells when charged to 4.5 V. The relationship between the structural evolution and the electrochemical performances at overcharged state was investigated using in situ synchrotron X-ray diffraction ͑XRD͒. After charging the half-cell using uncoated LiCoO 2 as cathode, the variation of the c value in hexagonal structure is significantly reduced during subsequent cycles. This reduction in c variation range is concurrently related to the capacity fading. In contrast, the variation range of c is preserved in the Al 2 O 3 -coated cathodes, and so is capacity. Based on these results, a capacity fading mechanism of LiCoO 2 -type cathode materials after overcharge is proposed, and an explanation of the surface coating effects is given.
The possibility of using a strong anion coordinate agent, tris͑pentafluorophenyl͒ borane ͑TPFPB͒, to suppress the thermal decomposition of LiPF 6 -based electrolyte was studied. Cyclic voltammogram measurements showed that addition of 0.1 M TPFPB maintained electrochemical stability of a LiPF 6 -based electrolyte at 55°C for a week, while under the same conditions severe degradation in electrochemical stability was observed in the same LiPF 6 -based electrolyte without the TPFPB additive. A Li/LiMn 2 O 4 cell with a composite LiPF 6 -based electrolyte containing 0.1 M TPFPB additive also exhibited superior capacity retention and cycling efficiency at 55°C than a cell with an electrolyte without additive. These data demonstrate the excellent effect of TPFPB additive in improving the thermal stability of LiPF 6 -based electrolyte.
A new family of anion receptors, based on aza-ether compounds, has been synthesized. These compounds can have either cyclic or linear aza-ether structures in which the amine hydrogen is substituted with electron withdrawing groups such as CF 3 SO2. When these compounds are added to lithium salt nonaqueous electrolytes, such as LiCl/tetrahydrofuran (THF) or LiBr/THF solutions, the ionic conductivity can be dramatically increased. Near-edge x-ray absorption fine structure spectroscopy studies show that C1-anions are complexed with the nitrogen groups in these compounds. The increase in ionic conductivity, the degree of complexation, and the stability of the complexes are related to the anion size and the structure of the aza compound. Unlike conventional anion complexing agents these compounds are not based on positively charged sites, Lewis acid metal centers, or hydrogen bonding, so they have the potential to be used as additives in lithium battery electrolytes. Details of the synthesis of several of these compounds are given. They can be synthesized with various geometries to accomodate polyatomic anions.
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