High-resolution spatial profiling of structural and thermal properties of anode materials

Ji, Xiaoyang

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https://hdl.handle.net/2142/120496 Copy

Description

Title

High-resolution spatial profiling of structural and thermal properties of anode materials

Author(s)

Ji, Xiaoyang

Issue Date

2023-03-14

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

Cahill, David

Doctoral Committee Chair(s)

Cahill, David

Committee Member(s)

• Braun, Paul
• Chen, Qian
• Ertekin, Elif

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)

• Depth Profiling
• Mapping
• Anode
• Nuclear Reaction Analysis
• Thermal Conductivity

Abstract

Lithium-ion batteries (LIBs) are commonly applied in portable electronic devices and electrical vehicles due to the increasing need of renewable and clean energy. The traditional anode material is graphitic carbon, but it is limited to low theoretical capacity in commercial applications. The promising anode candidates such as silicon and lithium metal have been focused on for their ultimately high theoretical capacities. However, the degradation of silicon and lithium metal in the operation of lithium-ion batteries cannot be ignored and the mechanisms are not fully understood. The common reasons of failure for silicon anodes include the invasive growth of solid electrolyte interphase (SEI), lithium trapping in interior anode regions, and loss of contact with electrical conductors. The failure of lithium metal anodes is usually caused by the severe lithium dendrite growth in the cycling process of batteries. Therefore, the non-uniform and unstable growth of Li and SEI components, either in the depth scale or the lateral scale, are significant for understanding the degradation processes within these lithium-ion batteries. In this dissertation, I present the techniques I developed for depth profiling of silicon anodes in their cycling processes and lateral profiling on lithium metal batteries and thermal properties of battery materials. In Chapter 2, the physical principles and instruments of the main techniques are explained and discussed: nuclear reaction analysis (NRA), elastic recoil detection (ERD), lab-made thermally modulated cell voltage mapping, and time-domain thermoreflectance (TDTR) mapping. In Chapter 3, I apply non-destructive ion beam analysis techniques, NRA and ERD, to quantitatively determine the accumulation of lithium and SEI components, and their propagation in depth during the cycling of silicon anodes. The performance of silicon anodes prepared by electrodeposition and magnetron sputtering are studied and compared. The SEI growth can be observed in both silicon anodes. However, the quantity of trapped Li in electrodeposited Si increases gradually until the 20th cycle, while Li is mainly trapped and accumulated in the first cycle in magnetron sputtered Si. The different Li trapping process might be related to the initial compositions of these two silicon anodes, which means Li tends to interact with C and O in Si-O-C anodes and this process slows down the Li trapping process. In Chapter 4, I describe a laser setup for scanning spatial variation of lithium symmetric cells by detecting their voltage response under periodical heating. A design of lithium cells with optical access is applied for laser to directly heat the lithium foil from one side. The open circuit voltage (OCV) response is analyzed considering the heat flow in multiple layers containing of lithium and the separator soaked with electrolyte, and the Seebeck coefficient of the lithium cell is determined. Moreover, the galvo mirror in the setup controls the laser direction to scan the battery, which maps out the spatial variation of OCV or thermally modulated voltage with applied current, and speculates the inhomogeneity in the lithium battery. In Chapter 5 and 6, I apply TDTR mapping to determine spatial variations of thermal conductivities of carbon fibers and SiC/SiC composite, which can act as electrode additive materials. I measure the spatial variation of transverse thermal conductivity on different radial positions for two polyacrylonitrile (PAN)-based carbon fibers, the IM7 and AS4 fibers. Single-point TDTR measurements determine the longitudinal and transverse thermal conductivities of the carbon fibers. The transverse thermal conductivity of IM7 fibers is approximately uniform (2.0 W/m-K) at different radial positions. In contrast, the transverse thermal conductivity of the core (3.0 W/m-K) in AS4 fibers is higher than the shell (2.4 W/m-K). The longitudinal thermal conductivity of IM7 fibers is 7.5 W/m-K and the value of AS4 fibers is 6.9 W/m-K. The thermal measurements indicate a larger anisotropy of the IM7 fiber than the AS4 fiber. Furthermore, I use TDTR to map the thermal conductivities of the oxidized SiC/SiC composite with a spatial resolution of 3 μm. Heterodyne detection using a 50-kHz-modulated probe beam and a 10-MHz-modulated pump suppresses the coherent pick-up and enables faster data acquisition. The mapped thermal conductivity of the fibers was 15 W/m-K while that of the matrix was 50~90 W/m-K. The area near fibers has a medium thermal conductivity of 18~50 W/m-K. The effective thermal conductance of the oxide layer on the fibers was 27~40 MW/m2-K while that of the oxide layer on the matrix was 40~47 MW/m2-K. The differences of the thermal conductance of oxide layers on fibers and matrix are attributed to the different structures of the oxide layers on the fibers and the matrix. The higher thermal conductance of the oxide layer on the matrix possibly results from the less defective oxide composition and slightly smaller oxide thickness than the fibers.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Xiaoyang Ji

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