PASSIVE THERMAL MANAGEMENT OF LITHIUM-ION
BATTERIES USING LATENT HEAT STORAGE MATERIALS
WHITE PAPER | Joe Kelly - Materials Scientist, Outlast Technologies LLC | March 2015 | Rev: 2
INTRODUCTION
As demand steadily grows for more powerful portable electronics, battery powered tools, and electric
vehicles there is a desirable need for battery systems to effectively meet these necessary power and
energy density requirements for operation. Because of their energy density, higher voltage, and negligible
memory effects, lithium-ion batteries are the popular choice for a wide range of applications, especially
in portable electronics. However, larger power demands and increasing cell density of lithium-ion battery
packs result in higher operating temperatures, especially under peak loads. Because of the susceptibility
of most commercial lithium-ion cell chemistries to degrade or age at or above 60°C, this leads to rapid loss
of capacity over subsequent charge/discharge cycles as well as reduced overall power output (figure 1).
In order to address these concerns, numerous studies
into both active and passive thermal management
systems for batteries have been undertaken for many
applications which use lithium-ion batteries. An area
of interest that shows great promise in reducing
detrimental thermal effects is through the use of
latent heat storage materials that absorb and store
thermal heat during a change in material phase. The
focus of this paper is on passive thermal management
systems that use these phase change materials (PCMs)
to effectively mitigate large temperature escalation
during both discharge and charge, thereby relieving
performance degradation over life of the battery and
increasing the safety of the battery system.
1
J. Shim; R. Kostecki; T. Richardson; X. Song; K.A. Striebel, J. Power Sources, 112, 222-230 (2002)
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FIGURE 1: CYCLING PERFORMANCE OF LITHIUM-ION POUCH
CELLS AT 25°C AND 60°C. 1
1
THERMAL DEGRADATION MECHANISMS
In order to understand how a thermal environment affects lithium-ion batteries it is necessary to recognize
the major components of a battery, as each are a major player in the overall degradation mechanisms.
A battery or a single cell is composed of two electrodes: anode and cathode, which are separated by a
polymer membrane. Ionic conduction between the electrodes is achieved through an electrolyte, which
can be liquid, solid, or polymeric. It is the interactions at the electrolyte/electrode interface that account
for a large percentage of the thermal degradation of a battery.
Anode Degradation
Anode/electrolyte interactions at elevated temperatures, especially with carbon anodes have been widely
studied to determine aging effects over battery lifetime. Typically, during the first discharge of a lithiumion cell, there is a certain amount of electrolyte decomposition and irreversible lithium ion loss at the
anode/electrolyte interface due to unstable operating voltages at the anode. The decomposition of the
electrolyte forms a protective solid-electrolyte interphase (SEI) layer on the electrode surface that is
permeable to lithium ions but inhibits further electrolyte decomposition and electrode corrosion. The
formation, composition, and morphology of the SEI layer are critical for effective anode performance.
Change in any of these aspects can negatively affect battery capacity and life. Elevated temperatures
greatly favor both SEI formation and growth, which can result in morphological and compositional
changes. This can negatively impact porosity of the layer, enhancing irreversible reactions with lithium
ions and leading to increased cell impedance, mobile lithium loss, resulting in power and capacity fade.
Cathode Degradation
Typically lithium-ion cathodes are composites containing a lithiated metal oxide as an active material,
conductive additive(s) to increase overall electrical interconnectivity and binders coated together on an
aluminum current collector. Therefore, degradation mechanisms of these cathodes are complex and
are highly material dependent. Elevated temperatures can adversely affect the inactive components of
the cathode, such as increase decomposition reactions of the binder, enhance oxidation of conductive
additives, and intensify corrosion of the current collector from the electrolyte. Depending on the
composition of both the metal oxide active material and electrolyte, elevated temperatures can drastically
increase decomposition and facilitate structural changes, adversely affecting phase changes during
lithiation/delithiation processes. These degradation pathways result in overall loss of capacity, increasing
cell impedance, and power fade.
2
J. Vetter; P. Novak; M.R. Wagner; C. Veit; K.-C. Moller; J.O. Besenhard; M. Winter; M. Wohlfahrt-Mehrens; C. Vogler; A. Hammouche, J.
Power Sources, 147, 269-281 (2005)
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2
This summary of the major thermal degradation mechanisms in lithium-ion batteries shows that such
processes are complex and decidedly material dependent. In order to achieve thermal stability of one or
all components of the battery system requires intensive material research and development that is both
costly and time consuming. Passive thermal management of cells and battery packs shows promise in
being able to maintain thermal stability of current commercial cell chemistries, circumventing extensive
material development and reducing expensive new product lines.
PASSIVE THERMAL MANAGEMENT
Thermal management systems rely on thermal transfer of heat away from the cell surface, thereby
inhibiting core temperature rise and limiting material degradation. The effectiveness of regulating core
temperatures is both a function of the ability to efficiently transfer heat away from the cell’s surface
and the inherent thermal properties of the battery materials. The decision in using active or passive
thermal management systems at the cell level or in a pack is application dependent. While active cooling
methods are effective in shuttling heat away from a surface, their size and complexity are prohibitive
in applications such as portable electronics and battery-power tools. Passive systems offer simplicity in
design that eliminates the need for mechanical equipment and additional capacity to power them. One of
the most promising passive systems for managing heat dissipation involves the use of latent heat storage
materials.
Latent Heat Storage Materials (LHS)
Latent heat storage materials depend on changing states of matter or phase changes in order to absorb
and release heat while maintaining a constant temperature. Latent heat, otherwise called enthalpy of
fusion, is defined as the amount of thermal energy required for a certain mass of material to undergo
a change in phase, typically quantified as joules per gram (J/g). These phase change materials (PCMs)
can have solid-solid, solid-liquid, or liquid-gas phase transitions and can be organic, inorganic, or eutectic
compounds. When a phase change temperature is reached, the energy absorbed is utilized in transforming
the material to another phase, thereby keeping the temperature relatively constant (figure 2). At a constant
thermal flux the length of the temperature plateau (phase change) is directly related to both the latent
heat properties of the PCM and its applied mass.
Most PCM type materials considered for managing of thermal stress for battery systems are either organic
or inorganic complexes. While inorganic PCMs such as salt hydrates exhibit high latent heat and high
thermal conductivity, they suffer from limited melt/freeze cycles, are corrosive, electrically conductive,
3
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and are prone to supercool. Organic PCMs,
such as paraffin waxes, are more appropriate
Sensible
Heat
as they are able to achieve >1,000 melt/freeze
cycles without serious degradation-similar to
Latent
Heat
Sensible
Heat
for current batteries. Organic PCMs also exhibit
high latent heat values and can be “tuned” for
specific temperature ranges by modifying alkyl
TEMP (ºC)
the necessary charge/discharge cycles needed
chain lengths to match appropriate melting
Melting Time
Melt Temperature
point temperatures. They are chemically stable,
non-reactive, and compatible with many
polymer and resins used in packaging and
construction. Conversely, they suffer from low
TIME (MIN)
FIGURE 2
thermal conductivity and need to be properly contained. However, with proper formulation to address
these issues, organic PCMs have shown great success in being able to mitigate surface temperature
escalation during high current discharges with commercial lithium-ion batteries.
Thermal Performance of LHS Materials with Li-ion Batteries
Studies using commercial 18650 (1.95Ah) format lithium-ion cells encased in polymer sleeves containing
Outlast’s Latent Heat Storage (LHS) materials have shown a remarkable ability to maintain cell surface
temperature below 50°C during high current discharges compared to control cells (figure 3). Specifically,
during discharge currents of 20A Outlast observed that the surface temperature for the LHS-sleeved cells
were on average 18.5°C lower than control cells across >600 charge/discharge cycles (figure 4 and 5) . This
ability to maintain optimal thermal management effectiveness across the lifetime of the cells highlights
FIGURE 3: LHS SLEEVES (LEFT) FOR 18650 LI-ION CELLS. IR IMAGES OF A) CONTROL AND B) LHS SLEEVE AFTER 15 MIN. DISCHARGE AT 20A.
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the inherent reliability of the LHS material’s energy storage and dissipation capabilities. Also demonstrated
is the LHS material’s excellent ability to resist mechanical deformation during thermal cycling thereby
maintaining intimate contact between the sleeve and cell surface throughout the calendar life of the
battery. This minimizes interface thermal resistance and preserves thermal storage potency of the LHS
sleeves.
Significantly, LHS-sleeved cells remained above 75% of the rated cell capacity for greater than 150 cycles
when compared to control cells (figure 6). Through repeatedly efficient thermal absorption at the cell
surface during discharge and effective dissipation of this heat away from the cell while charging, the LHSsleeves are able to drastically minimize thermal stress experience by the cell. Consequently, LHS-sleeved
cells are able to operate at higher discharge rates for approximately 40% longer, enhancing capacity and
power capabilities of commercial cells.
In order to facilitate a better understanding of the LHS sleeve/battery system, Outlast developed a finite
difference, lump parameter thermal model using SINDA/FLUINT software. Comparisons between the
average experimental temperature profiles and the thermal model show strikingly similar behavior with
an average data point discrepancy of 0.61°C (figure 7).
FIGURE 4: SURFACE TEMPERATURE COMPARISON BETWEEN LHS AND CONTROL 18650 CELLS FOR
FIRST 3 CYCLES OF 1.95A CHARGE/ 20A DISCHARGES.
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FIGURE 5: COMPARISON OF HIGHEST CELL SURFACE TEMPERATURES AFTER EACH DISCHARGE CYCLE
BETWEEN LHS-SLEEVED 18650 CELLS AND CONTROL CELLS.
FIGURE 6: COMPARISON OF DISCHARGE CAPACITY RETENTION BETWEEN LHS-SLEEVED 18650 CELLS
AND CONTROL CELLS.
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FIGURE 7: GRAPHICAL COMPARISON BETWEEN OUTLAST’S THERMAL MODEL WITH EMPIRICAL DATA FOR
LHS-MODIFIED 18650 CELLS (1.96AH) BEING DISCHARGE AT 20A.
SUMMARY
Outlast LHS Battery Sleeves have been specifically engineered to meet critical thermal management
needs for devices utilizing 18650 Li-ion cells by moderating cell surface temperatures during high current
charge and discharge operations in order to enhance capacity and power performance while reducing
potential thermal runaway issues. With this simple passive thermal management system utilizing LHS
sleeves, 18650 cell surface temperatures can be reduced by approximately 19°C compared to unmodified
cells depending on the discharge regime. Consequently, LHS-modified cells experienced notable increases
of approximately 40% in charge/discharge cycles before reaching 75% of rated capacity compared to
control cells, suggesting substantial decrease in thermal stress. Extensive cycling of the LHS sleeves show
that they can operate over 600 cycles or greater without loss of thermal absorption properties while
also maintaining mechanical integrity. Overall, these LHS materials demonstrate promise in preventing
escalating capacity and power fade over operational lifetimes or a variety of Li-ion functions in which
thermal stability is critical to device performance. The availability of a wide range of latent heat PCMs and
the ability to modify their content permit more effective balancing between pack design versus application
centered thermal management, especially for large thermal dissipative systems such as battery backup
units. With their relative abundance, high latent heat, and ease of processability, PCM materials are
becoming the forerunner in effective, simple, and cost conscious thermal management designs for power
intensive lithium-ion battery applications.
7
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outlast.com/thermalsystems