Desenvolvimento de supercapacitores híbridos empregando materiais nanoestruturados e líquido iônico

Abstract

Lithium-ion hybrid supercapacitors (HSC-Li) are devices based on a Li-ion battery electrode and other of a capacitive (or pseudocapacitive) material. Once these systems combine different storage mechanisms, they can present intermediate electrochemical properties (energy and power densities) of both batteries and supercapacitors. Considering this aspects, new devices were developed, called hybrid supercapacitors. In these devices, different types of energy storage mechanisms are implemented in the different electrodes. Hybrid battery-type SCs are those consisting of two different electrodes: a battery-type and a capacitive (or pseudocapacitive) one, and can meet the need for the energy density of the SCs and supplant the low power density of the batteries. Based on this, the present study aimed to prepare hybrid battery- supercapacitors containing different Li+ insertion electrodes, such as the cathodic materials, LiMn2O4, LiFePO4 and LiMnPO4. In addition, the behavior of complete cells was studied facing the preparation of physical mixtures with mesoporous carbon; doping of the materials: LiNixMn2-xO4 and LiMnxFe1-xPO4 (x = 0.01, 0.03, 0.05, 0.08 and 0.10), and the preparation of a LiMn0.5Fe0.5PO4 solid solution. All devices were built using the EMITFSI/LiTFSI (1 mol L-1) mixture as electrolyte and mesoporous carbon (MESO) as negative electrode. The LiMn2O4 + 20% MESO // MESO (2.0V) cell was able to provide energy density in the range of 28-12.3 W h kg-1 and power density 12.2-366.5 W h kg-1. Its cycling at the highest studied current density (400 mA g-1) was impaired, reaching 43% after 2500 cycles, due to the irreversible distortion of the Jahn Teller type. This effect was minimized with the doping of the material, presenting a superior result for LiNi0.01Mn1.99O4, which besides the improvement in the cycling stability with a retention of 71% after 2500 cycles, also allowed an improvement in the electronic conductivity of the oxide and consequently, on the other cell parameters such energy and power: 40.9-21.7W h kg-1 and 29-440.1W kg-1, respectively. The limitation in energy density of the system is justified by the specific capacity of LiMn2O4 (148 mA h g-1). In this way, the nanostructured LiFePO4, with superior specific capacity (170 mA g-1) was tested. The LiFePO4 // MESO (2.1V) system had limited performance, reaching 35.9-15.4 W h kg-1 and 23.2-356.6 W kg-1 due to its low intrinsic conductivity, and of one-dimensional path for Li+ diffusion. This limitation was overcome due to the use of doped materials, LiMnxFe1-xPO4, with LiMn0.05Fe0.95PO4 being the most prominent material with 43.1-18.9W h kg-1 and 25-9-358.8W kg- 1. Finally, the energy limitation of the hybrid device was overcome once again, using the LiMn0.5Fe0.5PO4 solid solution. This was possible through the application of a 2.3 V potential, which led the system to delivery 51.2-22.9W h kg-1 and 42-613.5W kg-1. It can be seen that the doping and preparation of solid solutions showed to be efficient strategies in the improvement of cathodic materials and in the improvement of the performance of complete devices, rarely reported in the literature. The developed study showed that despite the viscosity, the electrolyte, associated with the synthesized materials allowed the construction of hybrid supercapacitors with similar electrochemical properties (and in some cases superior) to the systems prepared with aqueous and organic electrolytes described in the literature.