Synthesis and compositional investigations of electrodeposited silicon for 3D structured lithium-ion battery anodes

Fritz, Nathan J.

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https://hdl.handle.net/2142/117638 Copy

Description

Title

Synthesis and compositional investigations of electrodeposited silicon for 3D structured lithium-ion battery anodes

Author(s)

Fritz, Nathan J.

Issue Date

2022-10-31

Director of Research (if dissertation) or Advisor (if thesis)

Braun, Paul V

Doctoral Committee Chair(s)

Braun, Paul V

Committee Member(s)

• Krogstad, Jessica A
• Wang, Pingfeng
• Perry, Nicola H

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Lithium-ion Batteries
• Silicon Anodes
• Electrodeposition
• Chemical Vapor Deposition
• Energy Storage

Language

eng

Abstract

Silicon anodes have been heralded as critical enablers for high energy density and fast charging lithium-ion batteries (LIBs) due to their 10 times higher theoretical capacity compared to graphite and low stable lithiation potential (0.3 V vs. Li) which significantly reduces the risk of dendrite formation at high currents. Nanoengineering of Si-coated porous 3D-structured metal scaffolds has demonstrated extended cycling stability of Si-dominant active materials with high loadings by reducing the critical form factor of the Si layer. However, a key challenge is that Si is predominantly grown via methodologies like chemical and physical vapor deposition that are more suited to the microelectronics industry and will not scale for battery electrodes. Electrodeposition is an extremely promising route to deposit Si for lithium-ion batteries, but the process is known to introduce significant impurities (halides, carbon, hydrogen, and oxygen). The presence of these species has not been well understood with respect to electrochemical cycling. This thesis systematically examines the synthesis, materials properties, and electrochemical behaviour of electrodeposited Si (EDEP-Si) for application with 3D structured anode technologies. The structure and properties of the anode scaffold directly influence performance through volume accommodation via pore-size, Si loading of surface area, and structural integrity of the metal filling fraction. Characterization of the porous 3D-structured Ni scaffold is performed as a basis for full anode performance, including foam geometry, surface area, porosity, and composition. Multiphysics simulations developed by Dr. Zhuoyuan Zheng reveal adverse mechanical implications of pore geometry and non-uniformity, which lead to mechanical failure through localization of stress during lithiation. X-ray diffraction and subsequent Rietveld refinement are demonstrated as a pathway to study the aging and in situ expansion-induced strain of the substrate an quantify proposed improvements. Parameterization of the synthesises processes for electrodeposited Si (EDED-Si) and Si grown by static chemical vapor deposition (CVD-Si) is vital to understand distinctions in the resulting deposits measured via thorough characterization of morphology, composition, and bonding structure. EDEP-Si is found to be significantly less dense than CVD-Si with considerable compositions of oxygen and carbon that are uniformly distributed in the layer and bonded directly to constituent Si atoms and in residual molecular structures from the solvent and salt. This suggests inclusion or reduction of bath molecules are also responsible for observed contamination. The impact of oxygen and carbon species on cycling behaviour of EDEP-Si with Li is evaluated via electrochemical analysis and post-cycling materials characterization. Despite their compositional differences, the reversible specific capacity of EDEP-Si are remarkably similar to CVD-grown Si (CVD-Si) on a silicon basis (~2400 mAh/g–Si after 100 cycles). However, early cycles of EDEP-Si indicate additional reactions of Li with species other than Si, which require 10-20 cycles to stabilize reversibility. A mechanism for the irreversible conversion of oxygen within the film is developed to explain this early cycle behaviour. Nevertheless, high utilization of the Si in the EDEP layer is demonstrated. Moreover, high-current performance of both deposits on both planar and structured substrates shows no significant rate limitations related to the conversion of EDEP-Si contamination other than the lower baseline capacity due to the reduced Si content. Our observations suggest ultra-pure Si is not necessary for high electrochemical access to reversible charge storage, although limiting or removing incorporated impurity species would improve important metrics such as energy density and first cycle efficiency. Identifying and controlling the sources of oxygen from the synthesis process would improve deposit purity. At the same time, post-deposition treatments to remove the species after film growth are attractive, because successful prevention of contamination has not been reported in the literature. In the first effort, we show that changing the bath solvent to acetonitrile slightly reduction of oxygen is measured (~15 at%) which is attributed to the absence of bonded oxygen in this solvent compared to propylene carbonate. However, complete prevention of oxidation is not achieved even after careful removal of dissolved gasses and trace water. In the second effort, thermal analysis of the electrodeposit up to 600 ˚C reveals significant events which lower the deposit mass and increase coordination of the Si network after electroreduction. Nevertheless, the oxygen content is unchanged and cycling of the annealed material shows decreased capacity commensurate with increasing process temperature. Lastly, magnesiothermic reduction is explored to remove constituent oxygen through selective reaction with Mg and subsequent removal of MgO with HCl. In the literature, this method has successfully produced high purity Si from silica precursors in a variety of formats. However, our investigations on 3D structured Ni foams and planar substrates demonstrate the challenges associated with reactant proximity and uniform distribution of the Mg source. We measure significant reactions of the Ni substrate with Mg which interfere with reduction of the Si layer.

Graduation Semester

2022-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/117638

Copyright and License Information

Copyright 2022 Nathan Fritz

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