Center for Functional Nanomaterials, Brookhaven National Laboratory , Upton, New York 11973, United States.
Nano Lett. 2015 Feb 11;15(2):1437-44. doi: 10.1021/nl5049884. Epub 2015 Feb 2.
Nanoparticle electrodes in lithium-ion batteries have both near-surface and interior contributions to their redox capacity, each with distinct rate capabilities. Using combined electron microscopy, synchrotron X-ray methods and ab initio calculations, we have investigated the lithiation pathways that occur in NiO electrodes. We find that the near-surface electroactive (Ni(2+) → Ni(0)) sites saturated very quickly, and then encounter unexpected difficulty in propagating the phase transition into the electrode (referred to as a "shrinking-core" mode). However, the interior capacity for Ni(2+) → Ni(0) can be accessed efficiently following the nucleation of lithiation "fingers" that propagate into the sample bulk, but only after a certain incubation time. Our microstructural observations of the transition from a slow shrinking-core mode to a faster lithiation finger mode corroborate with synchrotron characterization of large-format batteries and can be rationalized by stress effects on transport at high-rate discharge. The finite incubation time of the lithiation fingers sets the intrinsic limitation for the rate capability (and thus the power) of NiO for electrochemical energy storage devices. The present work unravels the link between the nanoscale reaction pathways and the C-rate-dependent capacity loss and provides guidance for the further design of battery materials that favors high C-rate charging.
锂离子电池中的纳米颗粒电极既有近表面贡献，也有内部贡献，它们的氧化还原容量各有独特的倍率性能。我们使用电子显微镜、同步加速器 X 射线方法和第一性原理计算相结合的方法，研究了 NiO 电极中的嵌锂途径。我们发现，近表面的电活性（Ni(2+) → Ni(0)）位点很快就饱和了，然后在将相变扩展到电极（称为“收缩核”模式）时遇到了意想不到的困难。然而，在锂化“手指”成核并向样品体内部扩展后，可以有效地利用 Ni(2+) → Ni(0)的内部容量，但需要一定的潜伏期。我们对从缓慢的收缩核模式到更快的锂化手指模式的转变的微观结构观察，与对大型电池的同步辐射表征相吻合，可以通过高速放电时的传输的应力效应来解释。锂化手指的有限潜伏期设定了 NiO 用于电化学储能器件的固有倍率性能（因此也是功率）限制。本工作揭示了纳米级反应途径与 C 率相关的容量损失之间的联系，并为进一步设计有利于高 C 率充电的电池材料提供了指导。
