Investigating the Various Composite Cathode Configurations Combining Battery and Capacitor Materials in an Internal Hybrid Capacitor

Abstract

In our recent work, we have demonstrated a new type of energy storage device, hybrid lithium-ion battery-capacitor (H-LIBC) [1-2]. The H-LIBC technology integrated a lithium-ion battery (LIB) with a lithium-ion capacitor (LIC), in the form of a hybrid composite cathode, hence making it possible to combine as much as possible, the advantages of LIB and LIC in one single device. The gap between the high energy densities offered by LIBs and the high-power densities seen in LICs can thus be bridged by an optimal utilization of these materials and electrode/cell designs. The schematic of an internal hybrid capacitor with a composite cathode is shown in Fig. 1a, where the anode consists of a battery type intercalation material, while a separator electronically isolates it from a composite cathode, which is a combination of a battery type cathode material (storage by intercalation) and a supercapacitor type activated carbon (surface charge storing) material. The composite cathode can be fabricated to possess different configurations, which can potentially influence the performance characteristics of the assembled H-LIBC device. Four of such possible configurations are shown in Fig. 1 b-d: (a) a cathode with a mixture of lithium-intercalating metal oxide battery material and activated carbon capacitor material; (b) a cathode with two overlapping layers, namely a layer of lithium-intercalating battery material and a layer of activated carbon capacitor material. This configuration can further be assembled in two variations, namely: (i) a capacitor material layer over a battery material layer, Fig. 1c (i), and (ii) a battery material layer over a capacitor material layer, Fig. 1c (ii); and (d) a segmented cathode with side-by-side layers, namely a layer of lithium-intercalating metal oxide battery material beside a layer of activated carbon capacitor material. In our present study, we present the morphological characteristics of these configurations, along with their performance characteristics in a comparative manner in the H-LIBC format. Many of the battery materials are metal oxides (e.g. LiFePO4) and hence are insulating in nature and require additional processing, to improve their electronic conductivity. On the other hand, the capacitor type activated carbon materials are inherently highly conductive. Hence, the type of combination of these two materials can result into different distribution, availability and accessibility of electronically active sites for electrochemical reactions, potentially resulting into different performance characteristics. Moreover, in a layered configuration, the material (battery or capacitor) directly in contact with the current collector, will experience lower electronic resistance, but at the same time, may also be subjected to higher ionic resistance than the other material due to longer diffusion lengths, as it is further away from the electrolyte soaked separator.