由于其高比容量和相当低的操作潜力,在过去的20年中,用于锂离子电池的硅基负极材料一直是广泛研究的主题。尽管对硅的(去)锂化行为的理解已大大增加,几个重大挑战尚未解决,阻碍了其广泛的商业应用。一个主要问题是低的初始库仑效率和硅电极始终存在的自放电。自放电本身会影响电化学存储系统的长期稳定性,此外,必须考虑不可避免的预锂化方法。结晶Li15Si4相的影响是非常令人感兴趣的,因为结晶(c)相和非结晶(a)相之间的相变不仅增加了比表面积而且引起巨大的极化。此外,由于缺电子的Li15Si4相,有可能对Li15+aSi4相进行电化学过锂化,可以对电解质高度反应。这提出了关于c-Li15Si4相与其无定形对应物相比对自放电行为的影响的问题。这里,用作模型电极的硅薄膜被锂化至相对于Li|Li(U10mV和U50mV)的截止电位为10mV和50mV,以便通过开路电位(UOCP)测量系统地研究其自放电机理,并通过扫描电化学显微镜观察固体电解质相间(SEI)的生长。我们表明,在U10mV电极上形成了c-Li15Si4相,而对于U50mV电极则没有发现。反过来,U50mV电极显示几乎线性的自放电行为,而U10mV电极在大约达到UOCP平台。380mV对Li|Li+,这是由于从c-Li15Si4到a-LixSi相的相变。在这个高原潜力下,在Si|电解质界面处的相变导致电子更绝缘和更均匀的SEI(U10mV电极),而U50mV电极显示较不均匀的SEI层。总之,硅电极的自放电机理,因此,电解质的不可逆分解和相应的SEI形成过程在很大程度上取决于下面的锂-硅相的结构性质。
Because of their high specific capacity and rather low operating potential, silicon-based negative electrode materials for lithium-ion batteries have been the subject of extensive research over the past 2 decades. Although the understanding of the (de)lithiation behavior of silicon has significantly increased, several major challenges have not been solved yet, hindering its broad commercial application. One major issue is the low initial Coulombic efficiency and the ever-present self-discharge of silicon electrodes. Self-discharge itself affects the long-term stability of electrochemical storage systems and, additionally, must be taken into consideration for inevitable prelithiation approaches. The impact of the crystalline Li15Si4 phase is of great interest as the phase transformation between crystalline (c) and amorphous (a) phases not only increases the specific surface area but also causes huge polarization. Moreover, there is the possibility for electrochemical over-lithiation toward the Li15+aSi4 phase because of the electron-deficient Li15Si4 phase, which can be highly reactive toward the electrolyte. This poses the question about the impact of the c-Li15Si4 phase on the self-discharge behavior in comparison to its amorphous counterpart. Here, silicon thin films used as model electrodes are lithiated to cut-off potentials of 10 mV and 50 mV versus Li|Li+ (U10mV and U50mV) in order to systematically investigate their self-discharge mechanism via open-circuit potential (UOCP) measurements and to visualize the solid electrolyte interphase (SEI) growth by means of scanning electrochemical microscopy. We show that the c-Li15Si4 phase is formed for the U10mV electrode, while it is not found for the U50mV electrode. In turn, the U50mV electrode displays an almost linear self-discharge behavior, whereas the U10mV electrode reaches a UOCP plateau at ca. 380 mV versus Li|Li+, which is due to the phase transition from c-Li15Si4 to the a-LixSi phase. At this plateau potential, the phase transformation at the Si|electrolyte interface results in an electronically more insulating and more uniform SEI (U10mV electrode), while the U50mV electrode displays a less uniform SEI layer. In summary, the self-discharge mechanism of silicon electrodes and, hence, the irreversible decomposition of the electrolyte and the corresponding SEI formation process heavily depend on the structural nature of the underlying lithium-silicon phase.