Thermal Effects in Supercapacitors

Thermal management of electrochemical energy storage systems is essential for their high performance over suitably wide temperature ranges. An introduction of thermal management in major electrochemical energy storage systems is provided in this chapter. The general performance metrics and critical thermal characteristics of supercapacitors, lithium ion batteries, and fuel cells are discussed as a means of setting the stage for more detailed analysis in later chapters.

Energy loss in the form of heat generation is inevitable in supercapacitors because coulombic efficiencies are always less than 100 %. The rate of heat generation depends on structural design, power profiles (e.g., charge/discharge rates), and other factors such as voltage imbalances among individual cells within a module. This heat generation causes a temperature rise within the cells. For instance, voltage imbalances can occur in a series string of supercapacitor modules, resulting in temperature differences among the cells. Reliability issues arise when some cells with higher temperatures fail sooner than others, since high temperature generally causes shorter life for the cells. Thus thermal management of supercapacitor systems is important for practical applications. This chapter provides a general discussion of thermal management in supercapacitors, including different practical applications, thermophysical properties of supercapacitor components, thermal transport mechanisms, thermal characterization techniques, performance metrics, and cooling systems. This chapter paves the way for the following chapters that address thermal influences on supercapacitor components and performance.

Thermophysical properties of supercapacitor components determine the thermal behavior of supercapacitors at different application temperatures. A fundamental understanding of the influence of temperature on these properties is necessary to design supercapacitors with high performance for practical applications. Major supercapacitor elements include electrolytes, electrodes (active electrode materials, current collectors, and binders) and separators. As discussed in Chap. 2, supercapacitor electrolytes can be broadly classified into two types: liquid electrolytes and solid-state/polymer gel electrolytes (Xiong et al. in Electroanalysis 26:30–51, 2014 [24]). Conventional liquid electrolytes include: (i) aqueous electrolytes, (ii) organic electrolytes and (iii) ionic liquid electrolytes. The commonly used solid-state polymer gel electrolytes are water-containing (proton-conducting/alkaline), organic solvent-containing, and ionic liquid-containing polymer electrolytes. Active electrode materials for supercapacitors are broadly classified into three categories (Xiong et al. in Electroanalysis 26:30–51, 2014 [24]): (1) carbon materials, (2) conducting polymers, and (3) transition metal oxides. The importance of these electrolytes, electrode materials and separators has been addressed in prior reviews (Xiong et al. in Electroanalysis 26:30–51, 2014 [24], Simon and Gogotsi in Nat Mater 7:845–854, 2008 [39], Ye et al. in J Mater Chem A 1:2719–2743, 2013 [84], Zhang in J Power Sources 164:351–364, 2007 [193], Huang in J Solid State Electr 15:649–662, 2011 [194]). This chapter discusses the effects of temperature on the thermophysical properties of these components.

The previous chapter considered the influence of temperature on different supercapacitor components, including electrolytes, electrodes and separators. The thermophysical properties of these components dictate the electrochemical performance of a supercapacitor at different temperatures, which is reflected by two crucial metrics-capacitance and ESR—and also others such as aging, self-discharge and leakage. For instance, the high ionic conductivity and high dissociation rate of the electrolytes at elevated temperatures facilitates ion migration towards the electric double layer [1], leading to a low ESR. Capacitance depends on the amount of ions aggregated at the interface between electrodes and electrolytes, which is determined by the effective specific surface area of the electrodes. Higher temperature promotes the migration of ions to the innermost pores of electrodes, leading to an increase in effective surface area, and thus a higher capacitance. Energy and power densities are directly related to capacitance and ESR. Aging and self-discharge are also important parameters to evaluate the performance of supercapacitors in practical applications. In this chapter, the influence of temperature on electrochemical performance including extreme-temperature performance is discussed.

The previous chapter reviewed the experimentally observed variations in electrochemical performance with temperature. The performance of supercapacitors depends strongly on operating temperature; therefore it is necessary to model temperature variations inside a supercapacitor. The major advantage of theoretical models is that they provide an opportunity to avoid time-consuming and expensive experiments by predicting performance in a wide range of applications and then building guidelines based on those predictions (Ike et al. in J Power Sources 273:264–277, 2015 [13]). Models can be used to study the thermal behavior of supercapacitors and thereby to develop new thermal management strategies. In this chapter, fundamentals of thermal modeling and various modeling approaches for temperature evolution are discussed from a theoretical standpoint.

Thermal management of supercapacitors is a key issue influencing their practical performance. Knowledge of temperature effects on performance is of great importance in the design of all energy devices operating over a wide temperature range. This chapter provides a summary of the discussion in the preceding chapters as well as pointers toward important future work.