Variously Engineered Alloying Reaction Si hybrids and Their Electrochemical Analysis for Commercially-oriented High Energy LIB Anodes

Department of Energy Engineering(Battery Science and Technology)In order to keep pace with the increasing energy demands for advanced electronic devices and to achieve commercialization of electric vehicles (EVs) and energy storage systems (ESSs), improvements in high energy battery technology are required. In this regard, Si anodes have been studied as one of the most promising alternative electrode materials for next-generation high energy LIBs systems. Pristine Si exhibits high reversible capacities of 3579 mAh g-1 which is 10 times higher than conventional graphite anode at room temperature. However, even though Si has high gravimetric and volumetric capacities, it undergoes intrinsic drawbacks such as low initial Coulombic efficiency, electrical disconnection and fracture as well as continuous formation of unstable solid electrolyte interphase (SEI) caused by huge volume changes during cycling, leading to rapid capacity fading and increase in internal impedance. Although intensive researches with variously engineered Si have provided some enhanced electrochemical performances, they could not be a comprehensive solution, which prevent practical applications in high energy LIB anodes. In this study, to overcome this barrier, novel structure characterization methods and fundamental understanding are proposed in this dissertation. 1) Amorphous (a) silicon nanoparticles backboned-graphene nanocomposite (a-SBG) for high-power lithium-ion battery anodes allows improved electrical conductivity and stable electrode deformation with sustaining interconnectivity between electrode components. The a-SBG provides ideal electrode structures-a uniform distribution of amorphous silicon nanoparticle islands (particle size <10 nm) anchored on both sides of graphene sheets-which address the improved kinetics and cycling stability issues of the silicon anodes. Especially, a-Si in the composite shows elastic behavior during lithium alloying and dealloying reactions: the pristine particle size is restored after cycling, and the electrode thickness decreases during the cycles as a result of self-compacting. This noble architecture facilitates superior electrochemical performance in Li-ion cells, with a specific energy of 468 Wh kg-1 and 288Wh kg-1 under a specific power of 7 kW kg-1 and 11 kW kg-1, respectively. 2) With developing an industrial-relevant equipment using chemical vapor deposition (CVD) process, Si nanolayer-embedded graphite/carbon hybrid (SGC) has succeeded in achieving high energy density (1043 mAh l-1), and unprecedented Coulombic efficiency (92%) at the 1st cycle. Moreover, even with industrial electrode density (> 1.6 g cc-1), areal capacity loading of > 3.3 mAh cm-2 with limited binder composite under 4 wt. % of SBR-CMC binder, SGC hybrid shows 96% of capacity retention after 100th cycles and rapid increase of cycling efficiency upward of 99.5% after only 6th cycles, which is quite comparable to conventional graphite anode counterpart. Moreover, with allowing favorable cyclability and high rate capability, this electrode entirely overcomes the huge volume expansion of Si anode, exhibiting 38% of electrode thickness increase after 50 cycles with retaining the electrode integrity in comparison with that of pristine graphite electrode.clos