Improving energy and power densities of three-dimensional next generation secondary battery electrodes

3D mesostructured electrodes have been intensively investigated since the beginning of this century, based on the concept of shortening diffusion length of Li ions and electrons using either structured current collector or carbon-based additives, e.g. carbon nanotubes, reduced graphene oxides, etc. Majority of these work focus on improving electrical conductivity and thus rate performance of the electrodes. However, this strategy is not panacea, there are various issues associated with specific next generation electrochemical systems when moving towards practical applications. This thesis talks about problems that were often overlooked by researchers while exploiting the 3D mesostructuring methodologies. A conversion compound Fe2O3 electrode was fabricated using self-assembly, colloidal crystal templating and electrodeposition methods. A good rate performance was obtained as well as a reduced voltage hysteresis was observed for the first time on the Fe2O3 system. The underlying mechanism behind the reduced voltage hysteresis was proposed and confirmed by impedance measurement and post cycling characterization, which was found to be ascribed to the capability of the porous electrode to accommodate lithiation induced strain. Further this thesis pointed out that the use of relatively heavy Ni inverse opal as 3D current collector was problematic in the sense of compromising active material loading and electrode-based capacity. A quick and scalable colloidal templating method was developed with doctor blade casting and a real 3D (~ 50 μm) bicontinuous porous carbon current collector was obtained. Sulfur and selenium was incorporated with the 3D BPC, achieving much higher loadings than the Ni inverse opal case and meanwhile demonstrating good electrochemical properties. The porous feature of the 3D BPC was versatile so that a hierarchical structure could be obtained by controlled heat treatment. S could be encapsulated within the mesopores in the 3D BPC, alleviating the “shuttle effect”. While for Se, even without intentionally confinement of Se which was mostly demonstrated in literatures, the 3D Se/BPC electrode could still deliver a capacity approaching theoretical value at slow rate. Moreover, the combination of carbon current collector and vinyl carbonate additive in the electrolyte resulted in stable SEI formed during cycling and thus stable long term cycling properties. Lastly but actually most importantly, emphasis was put on full electrode-based capacity which was compromised no matter using Ni inverse opal or bicontinuous porous carbon as current collector. A template-free Fe3O4/C electrode was fabricated hydrothermally, removing the 3D porous template completely and wisely. Loading as high as commercial electrode was achieved with both good full-electrode gravimetric capacity and volumetric capacity