Graphene-enhanced hybrid phase change materials for thermal management of Li-ion batteries
Introduction
Development of Lithium-ion (Li-ion) batteries enabled progress in mobile communications, consumer electronics, automotive and aerospace industries. Li-ion batteries are an essential part of the hybrid electric vehicles (HEV) owing to their high energy densities and low weight-to-volume ratios [1]. One of the most significant factors negatively affecting Li-ion battery performance is a temperature rise beyond the normal operating range. If overheated due to short-circuiting or fast charging/discharging processes the Li-ion battery can suffer thermal runaway, cell rupture or even explosion [2]. A fire in the Li-ion battery results in the emission of dense irritating smoke which could present a serious health and environmental risk [2], [3]. Combining multiple Li-ion cells close together in a battery pack in order to provide higher electric power makes the thermal management of the batteries even more challenging. The severity of the potential thermal issues with the battery packs is exemplified by a recent incident with the overheating and fire in the batteries on-board the Boeing 787 Dreamliner [4].
A common approach for thermal management of Li-ion battery packs is based on the utilization of phase-change materials (PCM). The latent heat stored in PCM, as its phase changes over a small temperature range, allows one to reduce the temperature rise inside the battery [5], [6], [7]. By varying the chemical composition of PCM one can tune its melting point and the temperature range in which it can operate as a heat absorber. It is important to note that common PCMs are characterized by very low thermal conductivity, K, with typical values in the range of 0.17–0.35 W mK−1 at room temperature (RT) [8]. For comparison, the RT thermal conductivity of silicon and copper are ∼145 W mK−1 and ∼381 W mK−1, respectively. PCMs store heat from the batteries rather than transfer it away from the battery pack. The use of PCM in battery cells also serves the purpose of buffering the Li-ion cell from extreme fluctuations in ambient temperature. This is a different approach from what is used in the thermal management of computer chips. In order to reduce the temperature rise in a computer chip one uses thin layers of thermal interface materials (TIMs) or heat spreaders that transfer heat from the chips to heat sinks and outside packaging [9], [10], [11]. The thermal conductivity of TIMs is in the range of 1–25 W mK−1 while that of solid graphite-based heat spreaders can be on the order of 103 W mK−1 [12].
Here we show that these two different approaches for thermal management can be combined via introduction of the hybrid PCM with graphene acting as filler for increased thermal conductivity. Graphene is known to have extremely high intrinsic thermal conductivity [13], [14] and form excellent binding with a variety of matrix materials [11], [15], [16]. The graphene-enhanced hybrid PCM reveals thermal conductivity that is two orders of magnitude higher than that of conventional PCM while preserving its latent heat storage ability. Utilization of the hybrid PCM results in substantial decrease of the temperature rise inside battery packs as demonstrated under realistic conditions.
Section snippets
Preparation and characterization of graphene-enhanced composites
In order to demonstrate possible enhancement of thermal properties with graphene we selected paraffin wax (IGI-1260) as the base PCM. Paraffinic hydrocarbons, or paraffins, are straight-chain or branching saturated organic compounds with the composition CnH2n+2. The term paraffin wax refers to mixtures of various hydrocarbon groups, particularly paraffins and cycloalkanes that are solid at ambient temperature [17]. Paraffin waxes are commonly used PCMs owing to their availability, chemical
Thermal conductivity of graphene-enhanced phase change materials
The thermal conductivity of the hybrid composites was measured using the transient planar source (TPS) technique (Hot Disk TPS2500) [23]. This method is best suited for the examined class of materials and was previously used for the investigation of thermal properties of other PCM [24] and thermal greases [25]. We calibrated our TPS system by measuring reference samples with known thermal conductivity. We also compared the results of our measurements with those obtained by other experimental
Thermal management of battery packs with graphene phase change materials
In order to directly prove that the developed hybrid graphene–PCM composites can significantly improve the thermal management of Li-ion batteries we performed the battery testing under realistic conditions. Fig. 4 shows the experimental setup for the battery testing. We used six 4-V Li-ion cells with the capacity of 3000 mAh each placed in a standard aluminum battery pack. The measurements were performed with the charger–discharger setup (HYPERION EOS 720i) and the temperature probes (Applent
Acknowledgments
This work was supported in part by the Center for Function Accelerated nanoMaterial Engineering (FAME). FAME Center is one of six centers of STARnet–a Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA. AAB also acknowledges partial support from the Winston Chung Energy Research Center at UC Riverside.
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