<p>Li-ion batteries is a
system that dynamically couples electrochemistry and mechanics. The electrochemical
processes occurring during battery operation induces a wealth of elemental
mechanics such as deformation, plasticity, and fracture. Likewise, mechanics
influences the electrochemical processes via modulating the thermodynamics of
Li reactions and kinetics of ionic transport. These complex interrelated
phenomena are far from being well understood and need to be further explored.
This thesis studies the couplings between the mechanical phenomena and
electrochemical processes in Li-ion batteries using integrated theories and
experiments. </p>
<p>A continuum model coupling
the kinetics of Li diffusion and kinematics of large elasto-plastic deformation
is established to investigate the coupling between Li transport and stress
evolution in electrodes of Li-ion batteries. Co-evolutions of Li distribution,
stress field and deformation in the electrodes with multiple components are
obtained. It is found that the Li profile and stress state in a composite
electrode are significantly different from <a></a><a>that </a>in
a free-standing configuration, mainly due to the regulation from the mechanical
interactions between different components. Chemomechanical behaviors of the
heterogeneous electrodes in real batteries are further explored. Three-dimensional
reconstructed models are employed to investigate the mechanical interactions of
the constituents and their influence on the accessible capacity of batteries. </p>
<p>Structural disintegration of the
state-of-art cathode materials LiNi<sub>x</sub>Mn<sub>y</sub>Co<sub>z</sub>O<sub>2</sub>
(x+y+z=1, NMC) during electrochemical cycling is experimentally revealed. Microstructural
evolution of different marked regimes in electrodes are tracked before and after
lithiation cycles. It is found that the decohesion of primary particles
constitutes the major mechanical degradation in the NMC materials. Electrochemical
impedance spectroscopy (EIS) measurement confirms that the mechanical
disintegration of NMC secondary particle causes the electrochemical degradation
of the battery. To reveal the reasons for particle disintegration, the dynamic
evolution of mechanical properties of NMC during electrochemical cycling is
explored by using instrumented nanoindentation. It is found that the elastic
modulus, hardness, and interfacial fracture strength of NMC secondary particle
significantly depend on the lithiation state and degrade as the electrochemical
cycles proceed, which may cause the damage accumulation during battery cycling.</p>
<p>Corrosive fracture of electrodes in
Li-ion batteries is investigated. Li reaction causes embrittlement of the host
material and typically results in a decrease of fracture toughness. The
dynamics of crack growth depends on the chemomechanical load, kinetics of Li
transport, and the Li embrittlement effect. A theory of coupled diffusion,
large deformation, and crack growth is implemented into finite element program
and the corrosive fracture of electrodes under concurrent mechanical and
chemical load is simulated. The competition between energy release rate and
fracture resistance as crack grows during both Li insertion and extraction is
examined in detail, and it is found that the corrosive fracture behaviors of
the electrodes rely on the chemomechanical load and the supply of Li to the
crack tip. The theory is further applied to model corrosive behavior of
intergranular cracks in NMC upon Li cycles. The evolving interfacial strength
at different states of charge and different cycle numbers measured by in-situ
nanoindentation is implemented in the numerical simulation.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7410392 |
| Date | 17 January 2019 |
| Creators | Rong Xu (5930426) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Theories_and_Experiments_on_the_Electro-Chemo-Mechanics_of_Battery_Materials/7410392 |
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