Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
This is why LIBs have several levels of fail-safe internal
Introduction to Li-ion Battery Technology
cell level and external protection circuitry, which shuts
down the battery pack when parameters go out of
range. The addition of this protection circuitry takes up
useful space in the battery pack and cell, thereby
reducing the available capacity. It also causes a small
current drain on the pack and contributes to potential
points of failure, which can permanently disable the cell
or pack.
Lithium-ion batteries (LIB) have now become part of
the standard battery pack of choice used in most
notebook, smartphone, e-reader, and tablet designs.
The LIB chemistry produces optimal characteristics
with regard to high energy density, low self-discharge,
light weight, long cycle life, lack of memory effect, and
low maintenance. LIBs are now gaining popularity in
other market segments such as electric vehicles,
power tools, and military/aerospace applications. Since
the technology was developed in the 1970s, LIBs have
improved dramatically in terms of energy density, cost,
durability, and safety.
Internal cell protection consists of a shut-down separator
(for over-temperature), tear-away tab (for internal
pressure), vent (pressure relief), and thermal interrupt
(over-current/over-charging) (Figure 1).
Current
Collector
Sealing Plate
The three main functional components in a lithium-ion
battery cell are the anode (typically graphite), the
cathode (typically lithium cobalt oxide), and a non-aque-
ous electrolyte (typically a lithium salt or organic
solvent containing complexes of lithium ions). The
material choices affect a cell’s voltage, capacity, life,
and safety.
Vent Plate
Positive Cap
Negative Teminal
Gasket
Gas Release Vent
Current Interrupt Device
Insulation Plate
Spacer
Positive
PTC
Tab
Device
Separator
Sealing Cap
Inlet
Gasket
Separator
Positive
Electrode
Insulator
Positive
Electrode
Negative
Electrode
Casing
Negative
Electrode
Negative
Tab
Case
(Positive Polarity)
Li-ion cells are available in a cylindrical solid body,
prismatic semi-hard plastic/metal case, or pouch form,
which is also called Li-polymer. Although pouch cells
and prismatics have the highest energy density, they
require some external means of containment to
prevent an explosion when their State of Charge (SOC)
is high (see Figure 1).
Anode tab
Cathode tab
Negative
Tab
Separator
Top insulator
Anode
Tab Sealant
Positive Tab
Positive
Negative
Electrode
Cathode
Electrode
Tab Sealing
Area
Barcode
Al laminate ꢀlm
Side Folding
Overheating is the main safety concern for lithium-ion
cells. Overheating causes thermal runaway of the
cells, which can lead to cell rupture, ꢀre, or explosion.
A deep discharge event could cause internal shorts in
the cell, which would cause a short circuit upon
charging.
Laminated
Foil
Figure 1. Various Li-ion cell conꢀgurations
Over-charging and deep discharge/short-circuit events
create heat (generated by the anode of the cell) and
oxygen (created by the cathode). Both of these effects
can be dangerous to the cell and cause bloating (in the
case of Li-polymer pouch cells), rupture, ꢀre, or even
an explosion.
Over the last ꢀve years, LIBs have been the subject of
highly publicized recalls of notebook and cell phone
battery packs, as a result of instances of overheating,
ꢀre, and rupture. Several new standards from IEC, UL,
and the DOT/UN have emerged to specify required
safety measures and testing.
©2012 Littelfuse, Inc
1
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