in situ TEM study on anode materials in lithium-ion batteries

Open Access
Author:
Liang, Wentao
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 11, 2013
Committee Members:
  • Sulin Zhang, Dissertation Advisor
  • Sulin Zhang, Committee Chair
  • Michael T Lanagan, Committee Member
  • Suzanne E Mohney, Committee Member
  • Barbara Shaw, Committee Member
  • Donghai Wang, Committee Member
Keywords:
  • in situ TEM
  • lithium ion battery
  • gallium nanodroplets
  • nanovoid
  • germanium nanoparticles
  • fracture
Abstract:
The growing demand for light-weight, high-capacity lithium-ion batteries (LIBs) for portable electronics, plug-in hybrid electric vehicles, and stationary energy storage systems has led to intensive research on developing new electrode materials with higher energy density, higher power density, and longer lifetime. However, a major issue with the high-capacity materials such as silicon (Si) is the rapid, irreversible capacity decay and poor cyclability due to the lithiation/delithiation induced mechanical degradation. A fundamental understanding of coupled electro-chemo-mechanical effects on the lithiation/delithiation of anode materials in LIBs is critical important for the development of advanced LIBs. In this thesis, we constructed solid cell and liquid cell nanobatteries inside high-resolution transmission electron microscopy (HRTEM) for electrochemical tests and mechanical degradation study of anode materials in LIBs. With this approach, in situ TEM electrochemical tests of crystalline Si nanowires (SiNWs), germanium nanoparticles (GeNPs), SiNPs, and α-Fe2O3 nanowires have been performed. Integrating in situ TEM study with multiscale modeling, the failure mechanisms of these anode materials are characterized. A set of interesting phenomena were discovered, ranging from ion and electron transport, electrochemical reactions, phase transformation, microstructural evolution, defect nucleation and growth, to chemo-mechanical failure inside battery materials. For SiNWs highly anisotropic swelling with huge volume expansion ~300% was observed upon first lithium insertion cycle. The highly anisotropic swelling was caused by the much faster lithiation rate along <110> directions than the other directions. The anisotropy of lithiation induced stress concentration at the adjacent {110} planes, causing the formation of cracks and size-dependent fracture. The high compression stress at the lithiation front also causes retardation of lithiation. A comparative study of electrochemical tests of GeNPs and SiNPs demonstrated that GeNPs exhibited tough lithiation/delithiation behavior without fracture in multiple cycles, in distinct contrast to the size-dependent fracture of crystalline SiNPs upon the first lithiation. GeNPs experienced large volume expansion similar to SiNPs, but showed isotropic lithiation behavior that differs to the highly anisotropic lithiation behavior of SiNPs. It turned out that lithiation anisotropy causes the non-uniform stress build-up in the hoop direction in lithiated SiNPs, leading to fracture in the well-defined planes. During lithiation/delithiation cycles gallium nanodroplets (GaNDs) underwent reversible liquid-to-solid and amorphous to crystalline phase transitions. During delithiation, void nucleation, growth, and annihilation were observed, exhibiting a self-healing behavior. A phase field modeling and theoretical analysis unraveled the void growth and annihilation mechanisms as well as the associated time laws. α-Fe2O3 as a typical transition metal oxide is potential anode candidate for next generation LIBs. Inside in situ TEM, the high spatial resolution observation of lithiation of α-Fe2O3 nanowires revealed the three-step lithiation process α-Fe2O3 -> α-LiFe2O3 -> cubic-Li2Fe2O3 -> BCC-Fe with FCC-Li2O. Our TEM results demonstrated the conversion reaction mechanism of lithiation in transition metal oxide electrodes.