Graphite has dominated the market for anode materials of lithium ion batteries (LIBs) for more than two decades owing to its low working potential, excellent cyclic stability, low cost and environmental friendliness. Nevertheless, the capacity delivered by graphite with a theoretical specific capacity of 372 mAh g^-1 is far from sufficient to satisfy today’s demanding and emerging applications like electric vehicles (EVs) and smart grids, which require much higher energy/power densities, longer cyclic life and lower costs than current LIBs can offer. Exploring and developing alternative electrodes to meet the above requirements become exigent and imperative.

Si has long been identified as one of the most promising alternatives to graphite owing to its high theoretical capaci...[ Read more ]

Graphite has dominated the market for anode materials of lithium ion batteries (LIBs) for more than two decades owing to its low working potential, excellent cyclic stability, low cost and environmental friendliness. Nevertheless, the capacity delivered by graphite with a theoretical specific capacity of 372 mAh g^-1 is far from sufficient to satisfy today’s demanding and emerging applications like electric vehicles (EVs) and smart grids, which require much higher energy/power densities, longer cyclic life and lower costs than current LIBs can offer. Exploring and developing alternative electrodes to meet the above requirements become exigent and imperative.

Si has long been identified as one of the most promising alternatives to graphite owing to its high theoretical capacity of ~4200 mAh g^-1, a low working potential of ~370 mV and abundance in nature. Despite these intriguing benefits, Si electrodes suffer from important drawbacks, like fast capacity degradation, potential pulverization and the formation of unstable solid electrolyte interface (SEI) along with huge volume expansion of ~300 % during discharge. This thesis is dedicated to mitigating the above challenges and developing advanced Si/C composites with excellent electrochemical property. The research objectives include not only designing novel nanostructures but also understanding the lithiation mechanisms by special techniques like in-situ microscopy.

Carbon nanofibers (CNFs) containing uniformly dispersed Si nanoparticles (NPs) are synthesized via one-pot electrospinning and carbonization as free-standing electrodes for LIBs. As a prerequisite, monodispersion of Si NPs in aqueous solution is obtained via amino-silane functionalization followed by F ion mediation. The graphene oxide sheets and graphene-covered Ni particles are further introduced to improve the ionic/electronic conductivities of Si/CNF composites, the resultant Si/CNF/G, Si/CNF/Ni electrodes deliver remarkable electrochemical performance with high capacities of 872 and 1045 mAh g^-1 at 0.1 A g^-1 after 50 cycles, respectively. The Si/CNF structure is also modified to obtain nanocavity-engineered Si NPs which are encapsulated within the porous graphitic CNFs, which is designated as C-Si/F-CNF. The nanocavity and graphitic carbon spheres function not only as buffer to accommodate the volume expansion of Si but also as conducting networks for fast ion/electron transport. The C-Si/F-CNF shows exceptional high rate capacities of 770 and 580 mAh g^-1 at 2 and 5 A g^-1 after 70 cycles, respectively.

The effect of ultrathin carbon coating on lithiation mechanisms of commercial Si particles is evaluated using in-situ transmission electron microscopy (TEM). It is revealed that the carbon-coated Si particles undergo an isotropic to anisotropic transition during the initial lithiation with the lithiation rate 3 to 4.5 times faster than the bare Si. Mesoporous Si/C microspheres are also prepared by magnesiothermic reduction of porous silica and chemical vapor deposition of a thin carbon layer. The Si/C microspheres present anti-pulverization capability with significantly reduced volume expansion of ~85 % as confirmed by in-situ TEM observation. The Si/C electrodes deliver an extraordinary high capacity of 1500 mAh g^-1, cyclic stability of 90 % capacity retention after 1000 and 2500 cycles and an areal capacity of ~1.44 mAh cm^-2 after 500 cycles. Upon completion of this thesis, the deleterious materials pulverization and poor cyclic stability of Si/C electrodes have been successfully addressed.