(Invited) Faradaic Charge Storage and Supercapattery Explained

Abstract

Merit-merge of rechargeable batteries with supercapacitors is desirable because the two have complementary charge storage properties, namely larger storage capacity in the former, but higher power capability, greater efficiency and longer durability in the latter. It can be easily achieved via external connections, but this approach sacrifices both volumetric and gravimetric storage capacity. The more space-material efficient approach is internal combination, which can be achieved either at the material or device level [1]. In 2007, two new hybrids were proposed, namely lithium-ion capacitor [2] and supercapattery [3]. Whilst both terms describe internal hybridization, lithium-ion capacitor exemplifies at the device level, but supercapattery encompasses both material and device combinations. Because of its similarities to lithium-ion battery, lithium-ion capacitor has attracted fast growing R&D interest and investment, followed by various other ion capacitors [4]. Attention to supercapattery has grown only in the past few years, largely because the ion capacitor concept is not applicable to many new materials that are capable of Faradaic charge storage, but not specific for a particular ion. Another reason for the increased attention to supercapattery is related to the concept of pseudocapacitance which has long been identified to result from Faradaic charge storage. However, there are two Faradic storage mechanisms, namely Nernstian storage and capacitive storage, which were not acknowledged by many, leading to confusions and misleading claims in the literature. The past five years have seen much improved understanding of these issues, which will be summarized and selectively analyzed in this presentation, in relation with supercapattery. An example is to experimentally differentiate between electric double layer capacitance and pseudocapacitance by electron spin resonance (ESR) spectroscopy [5,6], as shown in Figure 1.