Abstract
With the rapid growth of electric vehicles (EVs), light electric vehicles (LEVs), energy
storage systems, and portable electronics, lithium-ion batteries (LIBs) have become
increasingly important in modern energy applications. However, LIBs may undergo thermal
runaway (TR) under thermal, electrical, or mechanical abuse, leading to serious fire and
safety hazards. Among these abuse conditions, mechanical damage caused by road
impacts, traffic accidents, chassis scratching, or localized indentation is difficult to predict
and may directly induce internal short circuits (ISCs) and subsequent TR. Current
mechanical safety standards mainly focus on conventional loading conditions and short-
term failure responses, while the effects of temperature, off-axis loading, and long-term
cycling after mechanical abuse remain insufficiently understood.
This thesis investigates the mechanical abuse safety of pouch LIBs through specially
designed experimental setups. Hemispherical indentation tests were conducted under
different initial temperatures and off-axis loading angles, and long-term cycling tests were
performed under constant local indentation loads. The mechanical response, internal
structural damage, ISC behavior, performance degradation, and TR characteristics of LIBs
under these conditions were systematically analyzed.
The results show that increasing temperature significantly reduces battery hardness
and strength. At low temperatures, the layered structure mainly undergoes multilayer shear
fracture, forming loose ISCs with relatively slow and uniform heat generation. At elevated
temperatures, deformation becomes localized near the indenter, producing close-contact
ISCs that rapidly generate hot spots and trigger TR. Initial temperature also strongly affects
TR propagation and flame behavior. Higher temperatures reduce the critical indentation
depth for TR initiation, intensify gas and high-temperature particle ejection, and increase
jet-flame severity and mass loss.
Off-axis hemispherical indentation induces more complex structural damage than
conventional on-axis indentation. Four dominant damage modes were identified: shear–
tension combined inclined fracture, tensile fracture, wrinkle accumulation, and top-layer
crushing. Among these, inclined fracture and top-layer crushing are most likely to induce
severe ISCs. As the off-axis angle increases, the tangential load component rises sharply,
making LIBs more vulnerable to ISC and TR at lower overall loads. Once a localized ISC
occurs, the resulting hot spot rapidly triggers local TR and propagates throughout the cell.
High-temperature particles ejected from the indentation side can ignite surrounding
flammable gases and form jet flames.
Long-term cycling under local hemispherical indentation further reveals delayed safety
risks after local indentation. Sustained local stress causes non-uniform electrochemical
reactions within LIBs. Lithium intercalation is suppressed beneath the indentation center,
while lithium-rich deposition preferentially forms at the indentation edge. This mechanically
induced heterogeneity accelerates capacity fading, reduces active material utilization,
increases internal resistance, and decreases thermal stability. Local indentation
significantly reduces the critical temperature induced battery TR, and the energy release
rate and jet flame intensity during TR also increase significantly with increasing load and
aging cycles.
Overall, this thesis clarifies the coupled effects of temperature, loading direction, and
long-term cycling on mechanical abuse-induced failure of LIBs. The findings guide
improving mechanical safety testing standards, evaluating post-abuse battery risks, and
designing safer battery structures for EVs and energy storage systems.