Innovative Active Materials for Hybrid and High Power Devices
Authors: Balducci Andrea , Schroeder Matthias , Zhang Xiaofei , Kühnel Ruben-Simon 
Published: February 2020

High power, hybrid devices have attracted an increased attention in the last years. These hybrid devices are realized combining a lithium-ion battery (LIB) electrode with a supercapacitor (SC) one [1-3]. This electrode combination might lead to a device able to display a higher power output compared to LIBs, a higher energy output compared to SCs, and a cycling stability in between these two systems. Although many combinations are possible, the use of a carbonaceous LIB anode together with an activated carbon (AC) positive electrode appears till now the most convenient. As a matter of fact, such an electrode combination allows devices with a high cell voltage (up to 4 V), high energy and high power. These devices are often addressed as lithium-ion capacitors (LICs). Two processes are simultaneously taking place during the charge-discharge of LICs: lithium insertion-extraction within the bulk of the carbonaceous anode and formation-depletion of a double layer on the AC positive electrode.

Similar to LIBs, LICs use mostly graphite as anode material both in academia and industry. This selection appears somehow surprising, as many works showed that graphite displays rather low performance during high current densities charge processes, thus limiting the performance of these devices [4]. Taking this point into account, the introduction of a material able to guarantee the same insertion potential as graphite, but with improved performance at high current densities could be therefore beneficial for high performance LICs.

In this work we report about the use of two alternative materials, soft carbon and lithium vanadium phosphate, for the realization of hybrid, high power devices.

Soft carbon features different structural and microstructural properties compared to graphite. We showed that, as a consequence of these properties, the capacity retention of soft carbon at high current densities is significantly higher than that of graphite (see Fig.1). Recently we realized soft carbon-based LIC containing organic as well gel-polymer based electrolytes, and we showed that these devices display remarkable cycling stability (100.000 cycles) and energy and power higher than those of graphite-based LIC [5-8].

Lithium vanadium phosphate (LVP) is currently considered as one of the most interesting materials for the realization of electrochemical energy storage devices. LVP displays high specific capacity, excellent thermal stability and good performance during test carried out at high current densities. Moreover, since it is an amphoteric material, it can be conveniently used as anodic and cathodic material. Recently, we showed that LVP is an interesting material candidate for the realization of innovative and advanced high-power systems [9]. Furthermore, we report about a novel ionic-liquid assisted sol-gel synthesis for nano-structured carbon-coated LVP particles [10]. The LVP nanoparticles obtained with this synthesis can be successfully used for the realization of composite electrode able to display high performance in term of specific capacity and capacity retention at high current. Using these electrodes we realized and tested different types of high power devices [10]. The result of this work indicates LVP as a very interesting candidate for the realization of innovative hybrid, high-power devices.

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