<p>Rechargeable lithium
ion batteries (LIBs) are widely used in various daily life applications
including electronic portable
devices,
cell phones, military
applications, and electric vehicles throughout the world.
The demand for building a safer and
higher volumetric/gravimetric energy density LIBs has increased exponentially
for electronic devices and electric vehicles. With the high energy density and
longer cycle life, the LIBs are the most prominent energy storage system for electric vehicles. Researchers are further
exploring for new materials with a high specific capacity, the MXene
has been a promising new anode material for LIBs. The typical MXene material Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub>
has 447mAh/g theoretical capacity, which is higher than traditional graphite
(372 mAh/g for LiC<sub>6</sub>) based anode.</p>
<p>Though LIBs are used in most of the
portable energy storage devices, LIBs are still having thermal runaway safety
concern, which is caused by three main reasons: mechanical,
electrical, and thermal abuse. The
thermal runaway is caused by the initiation of solid electrolyte interface
(SEI) degradation above 80 °C on the anode surface, generating exothermic heat,
and further increasing battery temperature. The SEI is a thin layer formed on
anode due to electrolyte decomposition during first few charging cycles. Its
degradation at low temperature generates heat inside the LIBs and triggers the
thermal runaway. The
thermal runaway follows SEI degradation, electrolyte reactions, polypropylene
separator melting, cathode decomposition and finally leads to combustion. The thermal
runaway mechanism of graphite, which is the most common and commercialized
anode material of LIBs, has been studied for years. However,
the thermal safety aspects of the new MXene material has not been investigated
yet. </p>
<p>In
this thesis, we primarily used differential scanning calorimetry (DSC) and specially
designed multi module calorimetry (MMC) to measure exothermic and endothermic
heat generated at <a>Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub> </a>anode,
associated with multiple chemical reactions as the temperature increases. The <i>in-situ</i> MMC technique is employed to
study the interactions and chemical reactions of all the components (separator,
electrolyte, cathode and MXene anode) in the coin cell for the first time,
while the <i>ex-situ</i> DSC is used to investigate the reactions happened on
anode side, including electrolyte, PVDF binder, MXene, SEI and intercalated Li.
Along with other <a>complementary </a>instruments and methods, the morphological, structural and compositional
studies are carried out using X-ray
diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM),
energy-dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET) surface area measurement and electrochemical
measurement to support the thermal analysis. The electrochemical and thermal runaway mechanism of conventional graphitic anode is studied and used for comparison with MXene<sub> </sub>anodes.</p>
<p>The
Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub> thermal runaway is triggered by SEI
decomposition around 120 °C analogous to conventional graphite. The thermal behavior of Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub>
anode is highly dependent on
electrode material, surface area, lithiation states, surface morphology,
structure and surface-terminating functional groups on Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub>, which provides more active lithium
sites for exothermic reactions with the electrolyte. Especially
the terminal groups (-OH, -F, =O, etc.) from the etching process affect the
lithium ion intercalation and thermal runaway mechanism. With annealing
treatment, the surface-terminating functional groups are modified and can
achieve less exothermic heat release. By normalizing the total heat generation
by specific capacities of the anode materials, it is observed that Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub>
(2.68 J/mAh) generates slightly less exothermic heat than graphite (2.72 J/mAh)
indicating slightly safer nature of Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub>
anode. The <i>in-situ</i> thermal analysis
results on the Ti<sub>3</sub>C<sub>2</sub>T<sub>z</sub> half-cell exhibited
less total heat generation per mass (1.56 kJ/g) compared to graphite (1.59 kJ/g)
half-cell. </p><br>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12722114 |
| Date | 29 July 2020 |
| Creators | Lirong Cai (9174149) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/THE_THERMAL_SAFETY_UNDERSTANDING_OF_MXENE_ANODES_IN_LITHIUM-ION_BATTERIES/12722114 |
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