Fading Behavior And Performance Improvement Of Silicon Anode For Lithium Ion Batteries

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.