Date on Master's Thesis/Doctoral Dissertation

12-2024

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Degree Program

Mechanical Engineering, PhD

Committee Chair

Wang, Hui

Committee Co-Chair (if applicable)

Sunkara, Mahendra

Committee Member

Berfield, Thomas

Committee Member

Narayanan, Badri

Committee Member

Cohn, Robert

Author's Keywords

Silicon anode; NMC cathode; silicon nanotubes; silicon on copper foil; NMC single crystal; high nickel NMC

Abstract

Growth of the EV industry has created an increased demand for Li-ion battery electrode materials, and support of a robust lithium-ion battery supply chain is a national imperative. For mainstream acceptance in the American market, the range of EVs must be improved, i.e., the energy density of the battery must be increased. Increasing the specific capacity of the electrode materials is the most direct strategy for increasing the energy density of the battery. Silicon anode materials have a theoretical capacity of over 3700 mAh·g-1 - an order of magnitude beyond that of the graphitic materials which are currently implemented. The primary challenge of silicon is that it incorporates lithium into its microstructure as an alloying process as opposed to direct intercalation, like graphite; as such, a volume change of 300% causes mechanical degradation upon cycling if conventional electrode processing is used. Currently, the highest silicon content in commercialized large scale anode materials is 8-12% in a particulate composite with graphite. Typical strategies to utilize higher silicon content include nanosizing and engineering porosity within the structure. Presented here is the developed and patented process which used these two fundamental concepts to demonstrate a silicon nanotube layer bonded directly to a copper film- all in one streamlined, low temperature, liquid-free process- which is a functional anode. The three cathode materials used in large scale Li-ion batteries are LiCoO2 (LCO), LiFePO3 (LFP), and LiNixMnyCo(1-x-y)O2 (NMC). NMC is simply a modified LCO- a strength of NMC is that toxicity and expense are reduced from LCO due to reduced cobalt content. NMC also has the highest specific capacity of these materials- theoretically up to 275 mAh·g-1 compared with 170 mAh·g-1 in LFP or just 140 mAh·g-1 in LCO. Although many lab scale methods of synthesis are available, the conventional processing of NMC involves coprecipitation followed by a long high temperature secondary calcination, which is high in complexity, waste, and cost. Many technical issues still present barriers for economical, large-scale production of NMC, including difficulties with reproducibility and control of morphology. Offered here is a novel method of NMC synthesis which achieves either a polycrystalline “meatball” morphology, or a single crystal particle morphology in a matter of seconds. The main objective for this project was to develop potentially commercialize-able synthesis methods for both high performance anodes and cathodes. Plasma enhanced chemical vapor deposition (PECVD) was used to coat a sacrificial substrate with silicon and an atmospheric microwave plasma reactor was used to make various compositions of NMC. PECVD process can be tuned for active material loading, and the microwave plasma process can be adjusted to produce either polycrystalline or single crystal NMC particles. Preliminary characterization and testing prove that the current respective processes have created electrode materials with reliable structure, morphology, and composition, as well as performance metrics such as high capacity and durability. The anode films with thin-walled tubes, 5-20nm, had silicon loading of 0.3 mg·cm-2 and exhibited durability over 100 cycles starting at 1345 mAh·g-1 (after first charge irreversible cycle), with Coulombic efficiency >97%. Tubes with wall thicknesses >100 nm resulted in degraded performance. Results demonstrate that higher loading and durable performance of Si anode on copper foils is possible with film thicknesses >20 microns, but with tubes having wall thicknesses (< 150 nm). Durable morphology at a high loading of 1.2 mg·cm-2. was produced with first discharge capacities on par with the theoretical capacity, ~3700 mAh·g-1. Tuning of silicon nanotube wall thickness was utilized to increase the silicon loading, and a stable interfacial layer between the nanotube layer and the copper foil was formed. Polycrystalline powders were produced using the plasma method to form a mixed metal oxide precursor and mixing lithium source secondarily with high temperature calcination. The conventional polycrystalline particle morphology performed with initial specific capacity 159.0, 181.5, and 211.5 mAh·g-1 for compositions 622, 811, and 955, respectively- capacity retention 98.4%, 97.7%, and 90.9% over 40 cycles. Single crystalline material was produced; these crystals had ordered layered crystal structure with minimal mixing between metal cation and lithium layers; structure and particles developed optimally at 5s plasma exposure, as evidenced by the high 2.06 I-ratio. LiNi0.8Mn0.1Co0.1O2 single crystal powders exhibited capacity 221.89 mAh·g-1 with 15% better capacity retention than the agglomerated, sintered polycrystalline NMC lithiated with a long calcination step.

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