Abstract
The combustion accidents caused by the thermal runaway of lithium-ion batteries
(LIBs) have heightened people's concerns and impeded development. Currently, the
internal physical and chemical changes, as well as the potential reasons, during the
thermal runaway process of LIBs are not yet fully understood. This PhD project aimed
to investigate this in three pouch LIBs: LiCoO2 (LCO)|Graphite, Li(Ni0.6Mn0.2Co0.2)O2
(NMC622)|Graphite, and LiFePO4 (LFP)|Graphite (all the electrolyte is composed of
LiPF6 dissolved in ethylene carbonate (EC) and diethylcarbonate (DEC)), under various
abusive conditions.
Chapters 3 and 4 study NMC622 batteries during overcharge abuse. Results
indicate that with the collapse of the cathode structure and the material's decomposition,
along with significant lithium deposition at the anode, a series of side reactions occur.
In this process, the creation of Ni0 and LiH speeds up side reactions, leading to thermal
runaway. Overcharge decreases the thermal stability of the battery, which decreases
with the increase of state of charge (SOC). Overcharge also changes the thermal
contribution ratio of the components in the thermal runaway process. At SOC 100%
and 120%, cathode's exothermic reaction contributes ≥ 80%. At SOC ≥ 140%, anode
dominates (about 60%). Overcharge accelerates the phase transition of the cathode
crystal structure from a layered rhombohedral to a disordered spinel, reducing thermal
stability. Chapter 5 compares thermal runaway in LCO, NMC622, and LFP batteries
during overcharge, focusing on flame traits. LFP batteries are least tolerant to
overcharging but poses the lowest thermal runaway risk, causing only smoke. LCO and
NMC622 batteries show sparking and jet flames as main types, with larger flame range
as charging rate rises. Specifically, the highest flame goes up to 341.7 mm and spreads
across 0.3 m². Chapter 6 compares and analyses failure of LCO, NMC622, and LFP
batteries due to overheating and nail penetration. LFP batteries demonstrate the poorest
heat resistance but the lowest risk of thermal runaway. However, under nail penetration
conditions, NMC622 batteries exhibit the poorest tolerance to internal short circuits,
while LFP batteries perform the best. LCO batteries produce a new phase, LiAlCo0.8O2.
To sum up, this PhD thesis uncovers how thermal runaway happens in LIBs,
providing insights for battery storage, transportation, and usage. It also offers data for
safety assessment and design.