In-situ TEM Investigation Of The Lithiation/Delithiation Behavior Of Core-Shell StructureNanomaterials For Lithium-ion Batteries

With the rapid development of electric vehicles and smart grids, lithium ion batteries (LIBs) with high energy densities, high power densities, long cyclic life and low cost have been considered as the most promising energy storage devices to satisfy those demanding applications. Many anodes materials with high theoretical capaticy for LIBs have been widely investigated to replace the current commercial graphite anode with a low theoretical capacity. Transition metal oxides (like MnO2) and alloy type anodes (like Sn) have been extensively studied due to their higher specific capacities. One of the main obstacles that these materials suffer from is the rapid capacity degradation and poor cyclic performace arising from large volumetric change. The influences of core-shell structure (TiO2@MnO2, Sn@C) as one of the strategies to circumvent these obstales above have been explored using in-situ transmission electron microscopy (TEM) method in this thesis.MnO2, a kind of transition metal oxide, has a high theoretical capacity of 1232 mAh g-1 as anodes for LIBs, while it has a poor conductivity and cyclic stability. Nanostructure is believed to produce great benefits for anode materials in LIBs by enhancing lithium ion transfer, accommodating large volume change and increasing surface area. Whether the nanostructure (especially the porous nanostructure) could be well held during charging/discharging process is one of the most commonly concerned issues in LIBs research. The dynamic evolution of birnessite manganese dioxides nanosheets during lithiation process was investigated by in-situ TEM for the first time. The TiO2@MnO2 core-shell nanowires were used as the anode and Li metal as the counter electrode inside the TEM. Interestingly, the lithiation process was confirmed as MnO2 and Li converting to Li2O and Mn. And the original porous structure of the nanosheets was hard to preserve during lithiation process due to lithiation-induced contact flattening.Coating is an effective method to improve the energy capacity and rate performance of anode materials for lithium ion batteries. However, it easily leads to the fracture/rupture of the anode materials in the lithiation process due to the tight/compact core-shell structure and different expansion coefficient especially with the diameter increasing. A chemical vapor deposition (CVD) process has been developed to synthesis Sn@void@C core-shell nanospheres. Some of the Sn would diffuse to the outer space and creat a void in the carbon shell during the preservation process. The influences of the reaction temperature and the reaction time were investigated. It was found that the thickness of the carbon shell would increase with both the increase of the reaction temperature and time. The sample synthesized at 750℃ for 60 min showed the perfect nanosphere morphology with a thickness of carbon shell at 15-30 nm and a diameter of the Sn core at 50-200 nm. The preservation time has a crucial imapact on the void volume of the Sn@void@C core-shell nanospheres. Sample preserved at 1000℃for 180 min resulted in the Sn@void@C core-shell nanostructure with a void volume of 50%. When the preservation temperature is 750 ℃, the nanostructure will remain tight as Sn fulfilling the carbon shell. On the other hand, the carbon shell will rupture when the preservation temperature is higher than 1100℃.The high-cycle (-200 cycles) ultrafast (cycle period of 2 s) reversible lithiation and delithiation was realized in the synthesized Sn@void@C core-shell nanospheres. The lithiation rate reached 100 nm/s without rupture. For the Sn@void@C with a VSn/VC (VSn is the volume of Sn and VC is the volume of core including the void volume) above 0.7, the carbon shell would rupture due to the huge volume expansion of the lithiation process. For the Sn@void@C with a VSn/VC below 0.3, the Sn could not fulfill the shell even when it was fully lithiated. The constraint effect of the carbon shell is also discussed by comparing of the intact Sn@void@C core-shell nanospheres and the broken ones in the reversible lithiation and delithiation process. The research results will provide a useful guide for choosing, designing, fabricating and optimizing new anode materials for LIBs.