Electronics Protection - Summer 2014 - (Page 8)

Feature Passive Thermal Management of Lithium-ion Batteries Using Phase Change Materials Joe Kelly, Materials Scientist Outlast Technologies, LLC 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). Figure 1. Cycling performance of lithium-ion pouch cells at 25°C and 60°C.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 that 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 article is on passive thermal management systems that use these phase change materials (PCMs) to effectively mitigate large temperature escalation during discharge as well as charge, thereby relieving performance degradation over life of the battery and increasing the safety of the battery system. 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 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. 8 Summer 2014 * www.ElectronicsProtectionMagazine.com 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 lithium-ion 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 that negatively impact porosity of the layer, enhances irreversible reactions with lithium ions and lead 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. 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's 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 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 http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Summer 2014

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Electronics Protection - Summer 2014