碳纳米管纤维电化学锂化机理的揭示
Revealing the Mechanism of Electrochemical Lithiation of Carbon Nanotube Fibers
摘要：
碳纳米管（CNTFs）的连续纤维织物是用于多功能能量存储装置的有吸引力的材料，无论是作为集流体还是作为活性材料。尽管具有相似的化学组成，但CNTF中的锂化/脱锂作用与传统的石墨电极有很大不同。在CNTFs中，这个过程主要是表面过程，插入束间隙，电化学掺杂和经常被忽视的sp2晶格在循环时的部分降解。通过广泛的电化学分析，结合原位拉曼光谱测量，我们分析了高度结晶的碳纳米管纤维的复杂锂化行为。CNTF可以可逆地存储锂，具有高的比容量和倍率性能，这要归功于大的电容贡献。在锂化后，它们经历电化学掺杂，纵向电导率增加多达100%，伴随着拉曼光谱的大幅下移。然而，CNTF也受到高的首次循环不可逆容量、电压滞后和循环时的非晶化的影响。电化学分析证实，SEI的形成是负责的第一个循环的不可逆容量。电压滞后主要归因于堆叠纳米管之间的间隙中的锂离子的捕获。另一个主要特征是预先存在的缺陷，这促进了电容存储，但导致CNTF的渐进非晶化。事实上，已经证明，在没有预先存在的缺陷的超纯CNTF中阻碍了不期望的非晶化。
英文摘要：
Fabrics of continuous fibers of carbon nanotubes (CNTFs) are attractive materials for multifunctional energy storage devices, either as current collector, or as active material. Despite a similar chemical composition, lithiation/delithiation in CNTFs is substantially different from traditional graphite electrodes. In CNTFs this process is dominated by surface processes, insertion in the bundles interstices, electrochemical doping and often-overlooked partial degradation of the sp2 lattice upon cycling. Through extensive electrochemical analysis, together with in situ Raman spectroscopy measurements, we analyzed the complex lithiation behavior of highly crystalline fibers of CNTs. CNTF can store lithium reversibly with high specific capacity and rate capability, thanks to a large capacitive contribution. Upon lithiation, they undergo electrochemical doping, with longitudinal conductivity increasing by as much as 100 %, concomitant with large downshifts in Raman spectra. However, CNTF are also affected by high first-cycle irreversible capacity, voltage hysteresis and amorphization upon cycling. Electrochemical analysis confirms that SEI formation is responsible for the first-cycle irreversible capacity. Voltage hysteresis is attributed primarily to the trapping of lithium ions in the interstices between stacked nanotubes. Another dominant feature is pre-existing defects, which promote capacitive storage but lead to progressive amorphization of the CNTFs. Indeed, it is evidenced that undesired amorphization is hindered in ultra-pure CNTF without pre-existing defects.
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