Effect of LiTFSI Solvation on Ionic Conductivity of Polyester-Based Solid Electrolytes
Zhenzhen YangJiahui CaiZitong ShenJiaqi BianJiashang ChenYao XuZecheng FangCongkun DuXing XiangJuan WangPeng YuRuiyao CuiSiwen Bi
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Carbonyl-containing polymers have been considered promising candidates as hosts in solid polymer electrolytes (SPEs) due to the reasonable chelating coordination with Li+, better antioxidation for high-voltage cathodes, and higher ion transference number compared with polyether SPEs. In this work, four polyesters of poly(octamethylene succinate), poly(hexmethylene succinate) (PHS), poly(butylene succinate), and polycaprolactone were investigated. In these SPEs with different −C═O/–CH2– ratios, PHS had the highest conductivity (σ) of 1.24 × 10–4 S/cm because of the excellent ability to deionize bis(trifluoromethane)sulfonimide (LiTFSI) up to 88.3 ± 3.2% at 70 °C and the lowest activation energy of Li+ ionic conduction. The effect of Li+/–C═O ratios (r) on the ionic conductivity can be clarified into low-, middle-, and high-concentrated regions. The decrease of PHS crystallinity due to LiTFSI solvation provided ion transport paths and mainly contributed to the improvement of ionic conductivity in the middle-concentrated region, while the solvation degree dominantly facilitated ionic conduction in the high-concentrated region and at higher temperatures. By combining the DFT simulation and polymer thermal analysis, we found the transition of Li+ coordination from multichain to single-chain bindings provided more flexible segment movement. It also proved that the sequence design of active groups in a polymer chain would be a promising strategy for stable and high-performance SPEs.Cryptand
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Ionic transport behaviors of silver chloride (AgCl) have been revealed with impedance spectra measurement under high pressures up to 20.4 GPa. AgCl always presented ionic conducting under experimental pressures, but electronic conduction can coexist with ionic conduction within the pressure range from 6.7 to 9.3 GPa. The ionic conductivity of AgCl decreases by three orders of magnitude under compression, indicating that Ag+ ion migrations are suppressed by high pressure. A parameter, fW, was defined as the starting frequency at which Ag+ ions begin to show obvious long-distance diffusion in AgCl. fW showed a similar trend with the ionic conductivity under high pressures, indicating that the speed of Ag+ ion diffusion slows down as the pressure increases. Unlike AgI, Ag+ ion diffusion in AgCl is controlled by the indirect-interstitial mechanism. Due to stronger ionic bonds and larger lattice deformation, Ag+ ion diffusion in the rigid Cl− lattice is more difficult than in the I− lattice under high pressures.
Silver chloride
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Dependence of the lattice parameter and Li+ ionic conductivity on the B-site ion substitution in perovskite-related compounds (Li0.1La0.3)yMxNb1 − xO3 (M = Zr, Ti, Ta) has been investigated. According to the calculation based on the classical ionic crystal model, ionic conductivity is expected to increase under the following two conditions: (i) a smaller average charge of B-site cations; and (ii) a larger unit cell. From the powder XRD results, Zr4+ substitution was satisfied with these two conditions. While in the case of Ti4+ substitution the former condition was satisfied, the latter one showed opposite tendency. In the case of Ta5+ substitution, both of the conditions are not changed. Therefore Zr4+-substituted samples were expected to increase the ionic conductivity. However, the ionic conductivity of all the B-site substituted samples decreased with the amount of substitution, in particular, Zr4+-substituted samples showed the lowest ionic conductivity. This disagreement indicates that there is an additional factor affecting the ionic conductivity. We suggest three possible explanations: (1) local distortion introduced by cation substitution; (2) the change of B–O bond covalency; and (3) formation of short-range ordering with B-site substitution.
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