Graduate School of Energy Environment Water Sustainability, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea.
J Am Chem Soc. 2013 Jul 3;135(26):9733-42. doi: 10.1021/ja4016765. Epub 2013 Jun 20.
Lithium-oxygen chemistry offers the highest energy density for a rechargeable system as a "lithium-air battery". Most studies of lithium-air batteries have focused on demonstrating battery operations in pure oxygen conditions; such a battery should technically be described as a "lithium-dioxygen battery". Consequently, the next step for the lithium-"air" battery is to understand how the reaction chemistry is affected by the constituents of ambient air. Among the components of air, CO2 is of particular interest because of its high solubility in organic solvents and it can react actively with O2(-•), which is the key intermediate species in Li-O2 battery reactions. In this work, we investigated the reaction mechanisms in the Li-O2/CO2 cell under various electrolyte conditions using quantum mechanical simulations combined with experimental verification. Our most important finding is that the subtle balance among various reaction pathways influencing the potential energy surfaces can be modified by the electrolyte solvation effect. Thus, a low dielectric electrolyte tends to primarily form Li2O2, while a high dielectric electrolyte is effective in electrochemically activating CO2, yielding only Li2CO3. Most surprisingly, we further discovered that a high dielectric medium such as DMSO can result in the reversible reaction of Li2CO3 over multiple cycles. We believe that the current mechanistic understanding of the chemistry of CO2 in a Li-air cell and the interplay of CO2 with electrolyte solvation will provide an important guideline for developing Li-air batteries. Furthermore, the possibility for a rechargeable Li-O2/CO2 battery based on Li2CO3 may have merits in enhancing cyclability by minimizing side reactions.
锂-氧化学作为一种“锂空气电池”，提供了可再充电系统的最高能量密度。大多数锂空气电池的研究都集中在证明在纯氧条件下的电池操作；因此，从技术上讲，这种电池应该被描述为“锂-二氧化物电池”。因此，锂-“空气”电池的下一步是了解反应化学如何受环境空气中的成分影响。在空气中的成分中，CO2 因其在有机溶剂中的高溶解度以及与 O2(-•)的积极反应而特别有趣，O2(-•)是 Li-O2 电池反应中的关键中间体。在这项工作中，我们使用量子力学模拟结合实验验证，研究了在各种电解质条件下 Li-O2/CO2 电池中的反应机制。我们最重要的发现是，影响势能面的各种反应途径之间的微妙平衡可以通过电解质溶剂化效应来改变。因此，低介电常数电解质往往主要形成 Li2O2，而高介电常数电解质则有效地电化学激活 CO2，仅生成 Li2CO3。最令人惊讶的是，我们进一步发现，像 DMSO 这样的高介电常数介质可以使 Li2CO3 在多个循环中可逆反应。我们相信，目前对锂空气电池中 CO2 化学和 CO2 与电解质溶剂化相互作用的机制理解，将为开发锂空气电池提供重要的指导。此外，基于 Li2CO3 的可再充电 Li-O2/CO2 电池的可能性可能通过最小化副反应来提高循环性能。
