Safety of Li-ion Batteries for Electric Vehicles

Description

Abstract

Recent Li-ion battery technology enables mass production of affordable electric

vehicles. Typical sizes of battery packs for new models of electric cars are 45 kW h to

77 kW h (VW ID.3.). The energy density of the battery pack reached 160 W h kg−1

(Tesla Model 3). The packs have a typical voltage of 400 V. Those packs give a

driving range of 300 km to 500 km making the car practical, but when such a pack

fails, the large amount of stored energy might be released inside the battery, causing

excess heat, gas release and possibly a fire.

The process of unwanted energy release of a Li-ion battery is called thermal runaway.

The aim of this thesis was to quantify the different characteristics of the thermal

runaway and to generate a foundation to derive appropriate safety measures in

battery packs.

At the beginning of the doctoral work a test stand for testing of smaller cells

(<5 A h) was build. It was used to test easily available cylindrical cells at different

state of charge. In the test series the cells were exposed to over temperature and

it was recorded if thermal runaway occurred, at which temperature exothermic

reaction started, the temperature rate, the maximum temperature as well as the

composition and amount of released gas. Three cell types with different cathodes

were compared.

Based on experience with smaller cells a second much bigger test stand was build

to test larger automotive Li-ion cells which are used by European car manufac-

turer. The second test stand can be used with Li-ion cells up to 420 A h, it can

accommodate cell holder for different cell types and allows testing of different

cell-failure cases. It includes a significantly upgraded gas analysis equipment and

more channels for temperature measurement.

Test results from two types of larger cells are included in this work. The first large

test cell (prismatic metal can, 50 A h) was tested with different over temperature

modes. Different failure mechanisms were observed. The second cell type (pouch

cell, 42 A h) was tested extensively at different state of charge and with different

heating modes including localized hot-spots on the cell surface. Critical parameters

to trigger a thermal runaway and the propagation of the exothermic reaction along

the cell were recorded.