Xu Chen, Ahmad Zeeshan, Aryanfar Asghar, Viswanathan Venkatasubramanian, Greer Julia R
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125;
Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213.
Proc Natl Acad Sci U S A. 2017 Jan 3;114(1):57-61. doi: 10.1073/pnas.1615733114. Epub 2016 Dec 19.
Most next-generation Li ion battery chemistries require a functioning lithium metal (Li) anode. However, its application in secondary batteries has been inhibited because of uncontrollable dendrite growth during cycling. Mechanical suppression of dendrite growth through solid polymer electrolytes (SPEs) or through robust separators has shown the most potential for alleviating this problem. Studies of the mechanical behavior of Li at any length scale and temperature are limited because of its extreme reactivity, which renders sample preparation, transfer, microstructure characterization, and mechanical testing extremely challenging. We conduct nanomechanical experiments in an in situ scanning electron microscope and show that micrometer-sized Li attains extremely high strengths of 105 MPa at room temperature and of 35 MPa at 90 °C. We demonstrate that single-crystalline Li exhibits a power-law size effect at the micrometer and submicrometer length scales, with the strengthening exponent of -0.68 at room temperature and of -1.00 at 90 °C. We also report the elastic and shear moduli as a function of crystallographic orientation gleaned from experiments and first-principles calculations, which show a high level of anisotropy up to the melting point, where the elastic and shear moduli vary by a factor of ∼4 between the stiffest and most compliant orientations. The emergence of such high strengths in small-scale Li and sensitivity of this metal's stiffness to crystallographic orientation help explain why the existing methods of dendrite suppression have been mainly unsuccessful and have significant implications for practical design of future-generation batteries.
大多数下一代锂离子电池化学体系都需要一个能正常工作的锂金属（Li）负极。然而，由于循环过程中枝晶生长不可控，其在二次电池中的应用受到了限制。通过固体聚合物电解质（SPEs）或坚固的隔膜对枝晶生长进行机械抑制，在缓解这一问题方面显示出了最大的潜力。由于锂的极端反应性，在任何长度尺度和温度下对锂的力学行为进行研究都很有限，这使得样品制备、转移、微观结构表征和力学测试极具挑战性。我们在原位扫描电子显微镜中进行了纳米力学实验，结果表明，微米尺寸的锂在室温下具有高达105 MPa的极高强度，在90°C时为35 MPa。我们证明，单晶锂在微米和亚微米长度尺度上表现出幂律尺寸效应，室温下的强化指数为-0.68，90°C时为-1.00。我们还报告了从实验和第一性原理计算中得出的弹性模量和剪切模量与晶体取向的函数关系，结果表明，在熔点之前，弹性模量和剪切模量具有高度的各向异性，在最硬和最软的取向之间，弹性模量和剪切模量相差约4倍。小尺寸锂中出现如此高的强度以及这种金属的刚度对晶体取向的敏感性，有助于解释为什么现有的枝晶抑制方法主要未取得成功，并且对下一代电池的实际设计具有重要意义。
