【作者】 徐宇虹；

【导师】 尹鸽平；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2010， 博士

【摘要】 高容量负极材料已经成为锂离子电池发展的目标之一,而硅材料因具有高达4200mAh·g^-1的理论比容量而成为研究热点之一。但是硅负极材料在电化学循环过程中存在较大的体积变化,从而造成了电化学循环性能的迅速恶化。因此,有效抑制硅的体积变化造成的结构破坏、提高其电化学循环性能是本领域急待解决的问题。本文从晶体结构、电极组成、材料设计三方面研究了锂离子电池硅负极材料的失效行为及其电化学性能。采用电化学阻抗谱和X-射线光电子能谱（XPS）研究了硅电极表面固体电解质（SEI）膜的形成过程。结果表明,硅在0.5～0.8V之间形成SEI膜,且SEI膜在0.05V之上的稳定性较高。在0.05V以下,Li_xSi结构的变化造成了SEI膜逐渐增厚。硅表面SEI膜的成分中,有机物成分以醚类或醇类物质为主,而无机物以LiF为主。利用第一性原理计算方法研究了硅在合金化过程中形成的晶态Li_xSi的几何结构、电子结构等微观变化。结果表明,在硅与锂的电化学合金化过程中,晶态硅逐渐发生相变转化为Li_xSi。晶态硅中的Si-Si共价作用逐渐减弱,键长逐渐增大并最终断裂,四面体结构逐渐遭到破坏。随着Li_xSi中锂含量的增加,晶态硅的电子局域化程度逐渐降低,锂原子轨道上的p电子逐渐转移到硅原子的p轨道,这使得Li₁₅Si₄具有了金属导电性,电子共有化程度有所提高。锂与硅在合金化过程中逐渐形成Li-Si化学键,并从离子/共价混合键转变为弱共价键。在晶态硅向Li15Si4的转变过程中,体积膨胀为原来的369.2%。而晶体各向异性的特点使得材料在不同宏观方向上的体积膨胀程度有所不同,由此产生的内应力导致材料颗粒的破裂。结合电化学阻抗谱和电极形貌的变化可以将硅的电化学失效原因归结为“电子传输通道中断”,即微观晶体结构的变化导致宏观颗粒的破裂,进而导致颗粒间的电子传输路径中断,造成电化学性能的下降。研究了退火处理对硅电极电化学性能的影响。结果显示,在粘结剂（聚偏氟乙烯）熔点之上对电极进行退火处理可以显著提高电化学性能。剥离实验和电极形貌显示,退火之后,粘结剂的粘结力显著提高,电极结构更加致密,这些都有利于抑制硅的体积变化对电极结构稳定性造成的破坏,减小电池阻抗。研究了不同电极组成和结构对电化学性能的影响。结果表明,硅含量和电极密度对硅电极的电化学性能影响较大。当硅含量较小时,充放电循环初期硅的体积变化对电极结构稳定性的影响较小,储锂容量较高,这也使得循环后期的脱/嵌锂程度和相应体积变化增大,因此其容量以一定的速率持续衰减。而硅含量增加时,充放电循环初期硅的体积变化对电极结构稳定性的影响较大,衰减较快,这也造成后期循环过程中的脱/嵌锂程度较小,体积变化也相应减小,故在后期循环过程中能够保持较好的循环稳定性。由此可以推断:调节充放电电压范围、控制储锂容量可以改变硅的电化学循环稳定性。此外,在集流体/活性材料涂层界面加入导电碳层可以提高界面稳定性,降低界面接触电阻,提高电子传输效率,显著提高电化学性能。纳米硅容易发生团聚和电化学烧结现象,通过表面包覆技术可以显著改善此现象。以聚乙烯醇和聚偏氟乙烯为碳源制备得到的核/壳包覆结构可以显著提高纳米硅的电化学性能,其可逆容量在30次循环之后均保持在1000mAh·g^-1以上,容量保持率均在97%以上。其中,聚乙烯醇具有表面活性剂和碳源的双重作用,在液相混合过程中,聚乙烯醇可以吸附在纳米硅表面形成分子膜,分子膜经过碳化后可以形成碳层。而聚偏氟乙烯也可以在纳米硅表面形成致密的碳层。由于聚乙烯醇热解碳的导电性高于聚偏氟乙烯热解碳,因此,以聚乙烯醇为碳源得到的纳米复合材料具有更好的倍率性能,在电流密度为1000mA·g^-1时,其可逆容量可以保持在600mAh·g^-1以上。通过将微米尺度的硅分散、镶嵌到导电碳或金属纳米粒子中可以有效抑制硅的体积变化对材料结构造成的破坏、提高材料导电性和电化学循环稳定性。其中,以沥青热解碳包覆硅时,硅的可逆容量在经过100次循环后可以保持在1200mAh·g^-1以上。当硅表面形成以Co纳米粒子为主、并含有少量碳的包覆层时,复合材料的导电性得到了显著提高,其可逆容量在50次循环后可以保持在500mAh·g^-1以上,相对于未改性的硅材料有了显著提高。更多还原

【Abstract】 Exploring high-capacity anode material has been one of the targets for lithium ion batteries and silicon is one of the hot topics as anode candidate due to the high theoretical capacity of 4200mAh·g^-1. However, the huge volume change during electrochemical charge/discharge cycle decays the cycle performance of silicon. Suppressing the structural destruction caused by volume change and enhancing the cycle performance have been the research topics for silicon material. In this dissertation, crystal structure, electrode structure and material designs were investigated to from the electrochemical performance of silicon anode.Electrochemical impedance spectroscopy （EIS） and X-ray photoelectron spectroscopy （XPS） were employed to investigate the formation of solid electrolyte interphase （SEI） on silicon surface. SEI film was found to form in range of 0.8⁰.5V and was stable above 0.05V. Below 0.05V, SEI film became thick due to the volume change of Li_xSi. X-ray photoelectron spectroscopy （XPS） revealed that SEI film on the surface of lithiated silicon was comprised of organic compounds with aether-/hydroxyl groups and inorganic compound of LiF.The geometric and electronic structures of crystalline Li_xSi formed during lithiation were investigated by first-principle calculations. In processes of the phase transformation of crystalline silicon to crystalline Li_xSi, the results showed that the covalent interaction of Si-Si was weakened as the Si-Si bond length increased and the tetrahedron structure was destoryed. As the lithium contents in Li_xSi increased, Li-p electrons transferred to Si-p orbits, which generated the metallic conductivity of Li15Si4 and improved the communization degree of the electrons. Lithium was bonded with silicon and the bond type was changed from covalence/ionicity to weak covalence. The volume change was 369.2% from silicon to Li₁₅Si₄. As the volume was expanded, the anisotropy of the crystalline made the expansion degree of macro-particle in various directions different, resulting in internal stress acted on particle and the final cracking of the particle. Combined EIS with morphology change of the electrode, the fading mechanism of silicon could be ascribed to the model of“cutting off the path of electron transportation”, which demonstrated that the cracking of macro-particle caused by the volume change of microstructure cut off the path of electron transportation and decayed the electrochemical performance of silicon.The influence of annealing process was studied and it was found that the electrochemical performance was enhanced greatly by annealing the electrode at the temperature above the melt point of polyvinylidene fluoride （PVDF）. The peeling test and morphology change of electrode showed that the bond strength of PVDF binder was enhanced and the electrode structure became compact after annealing, which was good for restraining the destruction on electrode stability caused by volume change of silicon and reducing the cell impedance.The influence of electrode composites and structure was investigated and the results demonstrated that the electrochemical performance was affected by silicon amount and electrode density. In case of small silicon amount, the influence of volume change of silicon on electrode structure during earlier cycles was relatively low and thus the lithium storage performance was good, which made the lithium insertion/extraction degree and the volume change relatively large in later cycles. Therefore, the capacity was kept fading during cycles. In case of more silicon, the influence of volume change of silicon on electrode structure was relatively severe in earlier stage and thus the insertion/extraction degree and the volume change were relatively small in later cycles. Therefore, good cycle performance was obtained in later cycles. Based on above phenomenon, it was deduced that controlling the voltage range in cycles could vary the cycle stability of silicon anode. Besides, sandwiching a carbon layer at the current collector/actives coating interface was found to stabilize the interface and reduce the contact resistance, finally improving the electron transportation properties and the electrochemical performance.The coating technique is helpful to restrain the agglomeration and electrochemical sintering of nanosilicon. Here polyvinyl alcohol （PVA） and PVDF were employed as carbon source to construct the core/shell structure to enhance the performance of nanosilicon, both of which exhibited high capacity above 1000mAh·g^-1 for 30 cycles with retention above 97%. The PVA played the dual roles of surfactant and carbon source and could adsorb on the surface of silicon in the solution to form the molecular membrane. By pyrolysis process, the molecular membrane was turned into the carbon coating. Meanwhile, PVDF binder could also form a compact carbon coating on the surface of silicon. It was found that the PVA-pyrolyzed carbon had higher conductivity than PVDF-pyrolyzed carbon, which resulted in the enhanced rate capability compared with PVDF carbon source, i.e. 600mAh·g^-1 at the current density of 1000mA·g^-1.Dispersing or inlaying micron-scaled silicon in conductive carbon or nanosized metal matrix could restrain the destruction on material structure caused by volume change of silicon and enhance the conductivity and electrochemical stability effectively. For example, by coating silicon with pitch-pyrolyzed carbon, the silicon would exhibit the capacity above 1200mAh·g^-1 for 100 cycles. Meanwhile, by coating nanosized Co metal and trace carbon on surface of silicon, the conductivity of the prepared composite was improved remarkably and the capacity was remained above 500mAh·g^-1 for 50 cycles, which was much better than that of the untreated silicon.更多还原