Steinrück Hans-Georg, Cao Chuntian, Veith Gabriel M, Toney Michael F
SSRL Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
J Chem Phys. 2020 Feb 28;152(8):084702. doi: 10.1063/1.5142643.
To understand the origins of failure and limited cycle life in lithium-ion batteries (LIBs), it is imperative to quantitatively link capacity-fading mechanisms to electrochemical and chemical processes. This is extremely challenging in real systems where capacity is lost during each cycle to both active material loss and solid electrolyte interphase (SEI) evolution, two indistinguishable contributions in traditional electrochemical measurements. Here, we have used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) x-ray reflectivity measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in half-cell geometry using a carbonate electrolyte with LiPF salt. This approach allowed us to quantify the capacity lost during each cycle due to SEI evolution. Combined with electrochemical analysis, we identify SEI growth as the major contribution to capacity fading. Specifically, the continued SEI growth results in increasing overpotentials due to increased SEI resistance, and this leads to lower extent of lithiation when the cutoff voltage is reached during lithiation. Our results suggest that SEI grows more with increased time spent at low voltages where electrolyte decomposition is favored. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our methodology allows for the quantitative determination of lithium-ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and the SEI and offers a powerful method of quantitatively understanding LIB loss mechanisms.
为了理解锂离子电池（LIBs）失效的根源以及有限的循环寿命，必须将容量衰减机制与电化学和化学过程进行定量关联。在实际系统中，这极具挑战性，因为在每个循环中，容量损失既源于活性材料的损失，也源于固体电解质界面（SEI）的演变，而在传统电化学测量中，这两种贡献难以区分。在此，我们使用了一个模型系统，并结合（1）通过电化学实验对整体库仑效率进行精确测量，以及（2）对活性材料损失进行X射线反射率测量。该模型系统由在碳化硅上的515 Å厚非晶硅（a-Si）薄膜组成，采用半电池结构，使用含LiPF盐的碳酸盐电解质。这种方法使我们能够量化由于SEI演变在每个循环中损失的容量。结合电化学分析，我们确定SEI生长是容量衰减的主要原因。具体而言，持续的SEI生长由于SEI电阻增加导致过电位增加，这使得在锂化过程中达到截止电压时锂化程度降低。我们的结果表明，在有利于电解质分解的低电压下停留时间增加时，SEI生长得更多。最后，我们根据抛物线生长规律提取了SEI生长的比例常数。我们的方法通过分别跟踪活性材料和SEI中的锂离子，能够定量确定LIBs中的锂离子损失机制，并提供了一种定量理解LIBs损失机制的有力方法。
