Tech Briefs Magazine - April 2024 - 18

How Adhesives Address
the Challenges of
Manufacturing
Microbatteries
S
ince the inception of integrated circuits (IC) and their rapid proliferation,
Moore's Law has led to increasingly powerful ICs while also
enabling miniaturization. As the battery size decreases, the relative
weight proportion of the packaging envelope increases due to surface-area-to-volume
factors requiring innovative means to preserve an
appropriately high energy density. Thus, the twin constraints of size
and the maximization of energy density leads to a complex host of
engineering challenges.
At the small manufacturing scales needed for microbatteries,
existing sealing technologies like thermal sealing are impractical.
Currently, researchers are exploring the use of
adhesives in combination with laser-cut gasket materials
to deliver maximum energy density, miniaturization,
and while maintaining a high degree of hermetic integrity
to assure long-lived operation and efficiency.
Adhesive Specifications for Hermetic
Sealing of Microbatteries
An adhesive used for the hermetic sealing
of batteries must meet a host of criteria:
chemical,
temperature,
pressure, and
electrical resistance, minimal outgassing
during cure, low permeability
to atmospheric moisture, high adhesion
to the packaging envelope
materials, and appropriate
modulus and thermal expansion
properties. Overall, the
adhesive must avoid contaminating
the sensitive
electrolyte solution
either because of cure-byproducts
or because of chemical degradation
over the operating life of
the battery. Further, rheology, wetting,
and adhesion properties are critical
for assuring a continuous, non-permeable
seal.
An appropriate adhesive must have chemical
resistance to the corrosive salts and the organic
electrolyte present within the battery; further,
during operation the cell experiences high
temperatures and pressures which encourage chemical
attack on the adhesive. The chemical breakdown
of the adhesive poses three risks: loss of the hermetic seal
with concomitant loss of the electrolyte through volatilization,
the introduction of deleterious moisture to the cell internals,
and the introduction of adhesive breakdown products
into the battery itself.
It is then critical that the adhesive provides the utmost in chemical
resistance while also engineering the battery such that the surface of
the adhesive exposed to the internal electrolyte solution is minimized.
Generally, to minimize exposure of the adhesive to the electrolyte, a gasket
material such as Kapton or neoprene is used.1,2 Further, the adhesive must
be capable of electrically isolating the cathode and anode; thus, the adhesive
must withstand the chemical and electrical environment posed by the cell.
Permeability is another critical factor in providing hermeticity. The adhesive
must have a low permeability to atmospheric moisture; silicone adhesives offer
exceptional temperature resistance; however, they suffer from high oxygen permeability.1
Polydimethylsiloxanes (PDMS)-based adhesives are then precluded from
use. Epoxy-based adhesives systems are then preferred as they offer a high degree of
chemical and temperature resistance while possessing suitable permeability properties
to limit the ingress of air and moisture.
To some degree, permeability depends upon the chemical structure and polymeric
structure of the adhesive. However, increased cross-link density, polymer microstructure,
and a glass transition temperature (Tg) will influence the degree of permeability. Further,
softening above the glass transition temperature and an increase in the mobility of polymer
chains at elevated temperatures may increase the permeability of the adhesive. Epoxy
adhesives can be formulated to provide a high Tg, high softening point, and to possess a
high temperature stability and resistance.
To maintain a hermetic seal, the adhesive must also possess properties that assure the
integrity of the bond over a wide temperature range. First, the adhesive must have a
18
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