Kazemiabnavi Saeed, Dutta Prashanta, Banerjee Soumik
School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, USA.
Phys Chem Chem Phys. 2015 May 7;17(17):11740-51. doi: 10.1039/c4cp06121g.
The unique properties of ionic liquids such as a relatively wide electrochemical stability window and very low vapor pressure have made them promising candidates as electrolytes for improving the cyclic performance of lithium-air batteries. The local current density, which is an important parameter in determining the performance of lithium-air batteries, is a function of the rate constant of the electron transfer reactions at the surface of the cathode. In this study, a novel method based on Marcus theory is presented to investigate the effect of varying the length of the alkyl side chain of model imidazolium based cations and the operating temperature on the rates of electron transfer reactions at the cathode. The necessary free energies of all the species involved in the multi-step reduction of oxygen into the peroxide ion were calculated using density functional theory (DFT). Our results indicate that the magnitude of the Gibbs free energy for the reduction of oxygen into the superoxide ion and also for the reduction of superoxide into the peroxide ion increases with an increase in the static dielectric constant of the ionic liquid. This trend in turn corresponds to the decrease in the length of the alkyl side chain of the ionic liquid cation. Furthermore, the change in Gibbs free energy decreases with increase in the operating temperature. The inner-sphere reorganization energies were evaluated using Nelsen's four point method. The total reorganization energies of all reduction reactions increase with decrease in the length of the alkyl side chain and increase in the operating temperature. Finally, the rate constants of the electron transfer reaction involved in the reduction of oxygen were calculated. The logarithm of the reaction rate constants decreases with increase in the static dielectric constant and increases with increase in the operating temperature. Our results provide fundamental insight into the kinetics and thermodynamics of the electron transfer reactions at the cathode that will help in the identification of appropriate electrolytes for enhanced performance of lithium-air batteries.
离子液体具有诸如相对较宽的电化学稳定性窗口和极低的蒸气压等独特性质，这使其成为有望用于改善锂空气电池循环性能的电解质候选材料。局部电流密度是决定锂空气电池性能的一个重要参数，它是阴极表面电子转移反应速率常数的函数。在本研究中，提出了一种基于马库斯理论的新方法，以研究改变基于咪唑鎓的模型阳离子的烷基侧链长度和操作温度对阴极电子转移反应速率的影响。使用密度泛函理论（DFT）计算了将氧气多步还原为过氧离子过程中所有相关物种的必要自由能。我们的结果表明，将氧气还原为超氧离子以及将超氧离子还原为过氧离子的吉布斯自由能大小会随着离子液体静态介电常数的增加而增加。这种趋势反过来对应于离子液体阳离子烷基侧链长度的减小。此外，吉布斯自由能的变化随着操作温度的升高而减小。使用尼尔森四点法评估了内球重组能。所有还原反应的总重组能随着烷基侧链长度的减小和操作温度的升高而增加。最后，计算了氧气还原过程中涉及的电子转移反应的速率常数。反应速率常数的对数随着静态介电常数的增加而减小，随着操作温度的升高而增加。我们的结果为阴极电子转移反应的动力学和热力学提供了基本见解，这将有助于识别用于提高锂空气电池性能的合适电解质。
