Prehal Christian, Samojlov Aleksej, Nachtnebel Manfred, Lovicar Ludek, Kriechbaum Manfred, Amenitsch Heinz, Freunberger Stefan A
Institute for Chemistry and Technology of Materials, Graz University of Technology, 8010 Graz, Austria;
Department of Information Technology and Electrical Engineering, ETH Zürich, 8092 Zürich, Switzerland.
Proc Natl Acad Sci U S A. 2021 Apr 6;118(14). doi: 10.1073/pnas.2021893118.
Electrodepositing insulating lithium peroxide (LiO) is the key process during discharge of aprotic Li-O batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether LiO grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for LiO packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the LiO phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li-O cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a LiO surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin LiO platelets potentially forming large toroidal particles. LiO solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller LiO particles, but there is no transition to an electronically passivating, conformal LiO coating. Hence, mass transport of reactive species rather than electronic transport through a LiO film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li-O reaction mechanism ought to be reconsidered.
电沉积绝缘过氧化锂（Li₂O₂）是非质子锂氧电池放电过程中的关键步骤，它决定了电池的速率、容量和可逆性。目前的认识是，表面吸附的超氧化锂和溶解的超氧化锂之间的分配决定了Li₂O₂是以保形表面膜还是更大的颗粒形式生长，分别导致低容量或高容量。然而，要更好地理解Li₂O₂堆积密度和容量的控制因素，需要结构敏感的原位计量学方法。在此，我们建立了原位小角和广角X射线散射（SAXS/WAXS）方法，作为一种合适的手段，用于在定制的原位锂氧电池循环过程中，以原子到亚微米分辨率记录Li₂O₂相的演变。结合复杂的数据分析，SAXS能够从复杂的多相系统中获取丰富的定量结构信息。令人惊讶的是，我们发现即使在被认为是表面生长典型的不良溶剂化电解质中，也没有指向通过连续两次电子转移形成Li₂O₂表面膜的特征。所有散射数据都可以用可能形成大环形颗粒的薄Li₂O₂薄片堆叠来建模。旋转环盘电极测量和电子显微镜进一步证明了Li₂O₂的溶液生长。更高的放电过电位会导致更小的Li₂O₂颗粒，但不会转变为电子钝化的保形Li₂O₂涂层。因此，活性物种的质量传输而非通过Li₂O₂膜的电子传输限制了放电容量。如果物种迁移率和碳表面积较高，即使在弱溶剂化电解质中也能实现高放电容量。目前被接受的锂氧反应机理应该重新考虑。
