Tech Briefs Magazine - April 2024 - 20

Manufacturing Microbatteries
a
b
e
f
cd
Stainless Steel ComponentsLithium Anode
Kapton Insulator
LiFePO4/C cathode
RTIL/BN Electrolytes & Separator
Figure 1. Microbattery (2 mm) fabricated and sealed in combination with a Kapton gasket/insulator and room-temperature curing epoxy. Note: epoxy
adhesive is shown in (e) as the black material on the external casing of the battery.1
(Image: Masterbond)
tinuously wet-out and to strongly bond
to the substrate. Care should also be
taken to assure that shrinkage of the
epoxy or mismatch of CTE between the
adhesive and substrate do not cause any
gaps to form at the interface during
cure or after cooldown. Shrinkage is
primarily related to adhesive chemistry,
but it may also be affected by the cure
schedule.
(a)
(b)
(c)
(d)
(e)
Figure 2. Fabrication steps used by PNNL researchers in the construction of a CFx/Li, 6-mm long by
3-mm diameter cylindrical microbattery hermetically sealed in combination with a neoprene gasket
and a two-part epoxy. The neoprene gasket is shown as the black oval in (b). The epoxy was applied
at a thickness of <0.5 mm in (e) as a secondary, non-permeable sealing layer.2
(Image: Masterbond)
high degree of adhesion with the battery
casing, usually metal, and it must
be applied appropriately to a cleaned
surface to ensure strong adhesion and
the absence of any weak boundary layers.
Here, the adhesive must appropriately
wet the substrate surface to form
a continuous interface between the adhesive
and the substrate. Further, rheological
considerations such as flow and
self-leveling are critical in providing a
sealing layer free of defects, cracks, and
channels.
As the battery may experience large
temperature changes over its operating
cycle, it is important to consider factors
such as the coefficient of thermal expansion
(CTE) of the adhesive. Stresses may
accumulate at the bond line as a result of
differential thermal expansion, and this
may potentially lead to bond failure and
a loss of hermeticity. Epoxy-based systems
can be tailored to include various
mineral fillers to exhibit a wide range of
CTE values to minimize thermal stresses
during operation.
20
Additional factors such as modulus,
the strain response of a material under
applied stress, must be balanced such
that the adhesive maintains a high degree
of rigidity and dimensional stability,
while also yielding appropriately to dissipate
accumulated stress and to resist
cracking.
Processing Considerations
As the epoxy adhesive must serve as a
hermetic seal and barrier to both atmospheric
ingress and loss of electrolyte,
the formation of a seamless interface
with the metal surface to be bonded is
critical. Regardless of the permeability
or chemical resistance of the adhesive, if
the interfacial region is not continuous,
the integrity of the hermetic seal will be
compromised.1 Control of the viscosity,
metering, and the adhesive application
method will assist with consistency in the
process and in assuring seal integrity.
The rheology, viscosity, and surface
tension properties of the adhesive can
be tailored to improve its ability to conwww.techbriefs.com
Care
should be extended to the mixing
and degassing of the epoxy: accuracy,
precision, and adherence to the
manufacturer's guidelines with respect
to mix ratios is critical to assuring proper
cure. Further, degassing of the epoxy
prior to application is critical as micro-pockets
of air or channels present
within the cured epoxy will reduce the
integrity of the hermetic seal. Higher
cure temperatures will provide a greater
degree of chemical resistance and crosslink
density; this is important for minimizing
leaching as well as minimizing
permeability of the adhesive layer. Optimization
and control of the cure temperature
is then critical for assuring the
integrity of the seal.
Dual-Sealing Method Using
Adhesives in Combination
with Polymeric Gasket
A study conducted by a team at Wayne
State University reported promising
results in the fabrication of a 2-mm
high-temperature resistant microbattery
using an epoxy that cured at room
temperature in combination with a Kapton
gasket/insulator.1 Their battery exhibited
a discharge capacity of 62µAh at
120 °C with excellent capacity retention.
A sensor duty cycle test was carried out
at 120 °C, and the sensor powered by the
Tech Briefs, April 2024
High-temperature rechargeable
2-mm battery

http://www.techbriefs.com

Tech Briefs Magazine - April 2024

Table of Contents for the Digital Edition of Tech Briefs Magazine - April 2024