Hu Jian Zhi, Jaegers Nicholas R, Hahn Nathan T, Hu Wenda, Han Kee Sung, Chen Ying, Sears Jesse A, Murugesan Vijayakumar, Zavadil Kevin R, Mueller Karl T
Joint Center for Energy Storage Research, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States.
JACS Au. 2022 Mar 21;2(4):917-932. doi: 10.1021/jacsau.2c00046. eCollection 2022 Apr 25.
Efforts to expand the technological capability of batteries have generated increased interest in divalent cationic systems. Electrolytes used for these electrochemical applications often incorporate cyclic ethers as electrolyte solvents; however, the detailed solvation environments within such systems are not well-understood. To foster insights into the solvation structures of such electrolytes, Ca(TFSI) and Zn(TFSI) dissolved in tetrahydrofuran (THF) and 2-methyl-tetrahydrofuran were investigated through multi-nuclear magnetic resonance spectroscopy (O, Ca, and Zn NMR) combined with quantum chemistry modeling of NMR chemical shifts. NMR provides spectroscopic fingerprints that readily couple with quantum chemistry to identify a set of most probable solvation structures based on the best agreement between the theoretically predicted and experimentally measured values of chemical shifts. The multi-nuclear approach significantly enhances confidence that the correct solvation structures are identified due to the required simultaneous agreement between theory and experiment for multiple nuclear spins. Furthermore, quantum chemistry modeling provides a comparison of the solvation cluster formation energetics, allowing further refinement of the preferred solvation structures. It is shown that a range of solvation structures coexist in most of these electrolytes, with significant molecular motion and dynamic exchange among the structures. This level of solvation diversity correlates with the solubility of the electrolyte, with Zn(TFSI)/THF exhibiting the lowest degree of each. Comparisons of analogous Ca and Zn solvation structures reveal a significant cation size effect that is manifested in significantly reduced cation-solvent bond lengths and thus stronger solvent bonding for Zn relative to Ca. The strength of this bonding is further reduced by methylation of the cyclic ether ring. Solvation shells containing anions are energetically preferred in all the studied electrolytes, leading to significant quantities of contact ion pairs and consequently neutrally charged clusters. It is likely that the transport and interfacial de-solvation/re-solvation properties of these electrolytes are directed by these anion interactions. These insights into the detailed solvation structures, cation size, and solvent effects, including the molecular dynamics, are fundamentally important for the rational design of electrolytes in multivalent battery electrolyte systems.
为扩大电池的技术能力所做的努力引发了人们对二价阳离子体系日益浓厚的兴趣。用于这些电化学应用的电解质通常包含环状醚作为电解质溶剂；然而，此类体系内详细的溶剂化环境尚未得到充分理解。为深入了解此类电解质的溶剂化结构，通过多核磁共振光谱法（O、Ca和Zn核磁共振）结合核磁共振化学位移的量子化学建模，对溶解在四氢呋喃（THF）和2 - 甲基四氢呋喃中的Ca(TFSI)和Zn(TFSI)进行了研究。核磁共振提供了光谱指纹，可轻易与量子化学相结合，基于化学位移的理论预测值与实验测量值之间的最佳一致性，确定一组最可能的溶剂化结构。多核方法显著增强了人们对所确定的正确溶剂化结构的信心，因为对于多个核自旋，理论与实验之间需要同时达成一致。此外，量子化学建模提供了溶剂化簇形成能量学的比较，从而能够进一步优化优选的溶剂化结构。结果表明，在大多数这些电解质中存在一系列共存的溶剂化结构，结构之间存在显著的分子运动和动态交换。这种溶剂化多样性的程度与电解质的溶解度相关，Zn(TFSI)/THF在各方面表现出最低程度。对类似的Ca和Zn溶剂化结构的比较揭示了显著的阳离子尺寸效应，表现为阳离子 - 溶剂键长显著缩短，因此相对于Ca，Zn的溶剂键更强。环状醚环的甲基化进一步降低了这种键合的强度。在所有研究的电解质中，含有阴离子的溶剂化壳在能量上更有利，导致大量接触离子对以及因此的中性电荷簇。这些电解质的传输和界面去溶剂化/再溶剂化性质很可能由这些阴离子相互作用主导。这些对详细溶剂化结构、阳离子尺寸和溶剂效应（包括分子动力学）的见解对于多价电池电解质体系中电解质的合理设计至关重要。
