UC Riverside

Lithium ion batteries, Lithium ion batteries (LIBs) have for several years dominated the market for cell phones, laptops, and several other portable electronic devices. In order to match the necessity of increasing need for higher energy density storage devices, for example, hybrid/electric vehicles. Higher energy density lithium ion batteries have to be investigated. Anode as one of the most important components of in LIBs has been intensively studied in recent years. Silicon, tin and metal oxide etc. based materials are very promising high energy density candidates for anode active materials. Yet, silicon/tin suffer from volume expansion during lithium ions insertion and lead to fast capacity fading upon cycling. This hinders the them from commercialization.

We synthesized and fabricated silicon/tin based nanostructure, and studied their electrochemical performance as anode materials in LIBs. Engineering empty space on the nanometer scale can be one solution for silicon based anode materials. We demonstrate the synthesis of yolk (SiNPs)–shell (NiO) particles using spray-pyrolysis, a technique with proven scalability to industrial production level. After coating and annealing in the presence of polyvinylpyrrolidone, the nickel oxide shell is converted into a porous nickel cage enclosing the silicon particles. The polymer decomposition leads to the formation of an amorphous carbon layer surrounding the nickel cage, the SiNPs-aC-Ni york-shell structure were achieved. This structure maintains a high specific discharge capacity after more than 100 cycles (~ 1400 mAh/g at the 110th cycle with a 0.5 C discharge rate, on a silicon basis) when used as anode for lithium-ion batteries.

Additionally, instead of compositing with carbon, SnNPs was used as conductive materials in silicon based anode materials. The uniformly dispersed tin nanoparticles provide an electronic conductivity in the active materials, which facilitates conduction of electrons in the system. Beside conductive, tin also has a high theoretical energy capacity. Thus, this SiNPs-SnNPs electrode exhibits a stable storage capacity exceeding 1100mAh/g with ~80% first cycle coulombic efficiency. This performance is superior to that of the control samples produced using silicon nanoparticles alone and tin nanoparticles alone.

Furthermore, uniform dispersion of small silicon nanoparticles in electric conductive matrix, which can accommodate silicon volume expansion were fabricated. For the first time, silicon quantum dots synthesized using non-thermal plasma CVD have been utilized as anode materials. SiQDs were successfully synthesized using non-thermal PECVD and surface functionalized. These quantum dots, after specific structure engineering, formed uniform SiQDs-aC agglomerations mixed with carbon nanotubes. This structure has good electrical conductivity and has a carbon-based coating preventing the direct contact between the silicon particles and the electrolyte. This structure maintains a specific charge capacity of approximately 1000 mAh/g-1 for 200 cycles and reaches a coulombic efficiency of 99.8%. The proposed process is based on commercially available carbon nanotubes, on silicon quantum dots which are produced using a scalable plasma-enhanced chemical vapor deposition technique, and is compatible with large area coating and processing techniques. The fabrication protocol described in thesis represents a step towards the successful commercial utilization of silicon-based nanomaterials for energy storage applications.