HALIDE PEROVSKITES FOR FLEXIBLE SUPERCAPACITOR APPLICATIONS

Abstract

Metal halide perovskites (MHPs) has received a lot of attention in the field of photovoltaics since 2009 because of their excellent optoelectronic characteristics. These materials have been used extensively in solar cells during the past several years, with the most efficient perovskite solar cell (PSC) realizing outstanding power conversion efficiencies as high as 25.2%. Other than solar cells, the halide perovskite materials are excellent candidates for a variety of energy conversion devices, including light-emitting diodes, photodetectors, scintillators, and field-effect transistors. A variety of optoelectronic and solar energy conversion applications have been made possible by the halide perovskite materials' distinctive features, which includes adjustable bandgap, outstanding solar energy harvesting capability, and ambipolar features with cost effective fabrication procedure. However, current-voltage (J-V) hysteresis is present in PSCs, mainly due to ion diffusion in vacancy and/or interstitial defects induced by high ionic conductivity of perovskites. Furthermore, the perovskite crystal is tolerant to hosting extrinsic ions, and its diffusion properties make it ideal for energy storage applications. Powering automobiles, buildings, and portable electronic devices with clean energy requires energy storage materials. Lithium-ion batteries, which have a high energy density, a low self-discharge property, little memory effect, a high open circuit voltage, and durability, are the most common type of secondary batteries on the market. In addition, technological advancements have been made in lithium-ion battery derivatives like the lithium-air battery, lithium-sulfur battery, and organic electrode battery. Despite this, each of these batteries is designed for a particular application, and none of them has the same level of popularity as the lithium-ion battery. Supercapacitors are regarded as another significant class of energy storage technologies that provide remarkable performance in addition to lithium-ion batteries. The supercapacitors have high power densities, quick charge/discharge rates, and extended cycle life. Supercapacitors have a capacity that is significantly higher than normal parallel-plate capacitors (< 30 Wh/ kg) and a discharge rate that is significantly higher than that of batteries, allowing for quick charging of an electric vehicles. The superior ion diffusion characteristics present in halide perovskite materials are primarily responsible for the fact that these materials, which were originally developed for solar cells, can also function as an energy storage device. It is interesting to note that the ion diffusion in halide perovskite materials believed to have a negative side effect in perovskite solar cells and to adversely affect the performance of the entire device. Perovskite solar cells exhibit hysteresis and variable power conversion efficiency due to unfavourable ion diffusion in halide perovskite materials. In spite of this, the ion diffusion ability of the halide perovskite materials is advantageous for the applications of supercapacitor and batteries. However, the halide perovskite materials have at least two significant limitations. First, the inclusion of harmful lead species raises significant environmental concerns. Second, the instability problems with the devices are caused by the extreme degradation of the halide perovskite materials in the humid and air environment. As a result, new classes of lead-free perovskite materials have been developed to replace lead (Pb) with alternate elements as antimony (Sb), bismuth (Bi), germanium (Ge), indium (In), tin (Sn), and double halide perovskite, which maintain original perovskite features. In recent years, there has been a lot of interest in integrated solar energy conversion and storage systems, such as photorechargeable batteries and photorechargeable supercapacitors that use halide perovskites to photocharge the battery or supercapacitor component. In this work, halide perovskite based supercapacitors have been studied in details in three electrode system. Furthermore, two terminal perovskite based have been fabricated with quasi solid state gel electrolye as electrolyte medium. Different kind of materials have been used to fabricate the supercapacitors. For this, we have stuctured this dissertation into following six chapters: In chapter 1, a brief introduction of the theoretical and experimental research on the chemical composition and crystal structure of halide perovskites, as well as challenges and applicaions in various fields has been presented. After a more thorough analysis of ion migration, which is one of the main causes of the long-term instability of perovskites, the literature on the potential uses for ion migration is reviewed. Futhere, application of lead free materials like Bi, Sn etc. for sustainable energy storage devices has been discussed. State of the art materials for high energy density supercapacitors and battery has been discussed. A mechanistic picture of the halide perovskite-based energy storage is provided by the explanation of the interactions between the lithium ions and in halide perovskite materials. Role of Halide Substitution in Perovskite-based Asymmetric Hybrid Supercapacitor We have also shown that porous electrodes made from mixed halide perovskite powder obtained from single crystals can be used to fabricate efficient asymmetric supercapacitors. While power density is nearly stable throughout a range of bromine ratios, increasing iodine concentration up to 75% to that of bromide ion can boost energy density beyond 20 Wh/kg. The increased ionic conductivity and lower charge transfer resistance at the perovskite/electrolyte interface are responsible for the composition demonstrated to have a higher charge storage capacity. However, increasing iodide content increases the instability, especially at higher bias voltage. In chapter 2, we have initially used tetrabutylammonium tetrafluoroborate (TBTF), or tetrabutylammonium perchlorate (TBPC) in dichloromethane (DCM) as electrolyte which has the voltage window of ~1.5 V. Due to the bulky nature of TBTF and TBPC the ion intercalation in the perovskite structure is less likely to occur. Therefore, the major contribution to the charge storage is due to EDLC along with some ion accumulation at the porous active sites of the halide perovskites. To increase the energy density of the supercapacitors we have taken Li-based electrolyte such as lithium tetrafluoroborate (LiTFB) for the supercapacitor application. The energy density is significantly improved to 88 Wh/kg due to the Li-ion intercalation into the perovskite electrodes with MAPbBr3 as active material. Also, Li-ion intercalation does not affect the electronic band structure of the perovskite material. Further energy density also improved by using LiTFSi as electrolyte with an optimized concentration. Interestingly perovskite electrodes degrade to 80% in TBTF electrolyte only after 1000 cycles of operation while they are stable over 90% after 2500 cycles of operation in Li-based electrolytes. To further investigate the role of cations and to improve the overall energy storage capacity we have performed the electrochemical measurement for inorganic Cesium lead tri-halide (CsPbBr3) based perovskite supercapacitors. To improve the stability of the electrode as well as to increase the specific surface area of the materials we have have also used CsPbBr3 nanocrystals as active material for perovskite electrodes. We have then fabricated halide perovskite-based pouch cells using filter paper as separator soaked in electrolyte and sandwiched between two porous electrodes. The device is stable only for a few cycles of operation. We have then prepared LiTFSi-PVA based quasi-solid-state gel electrolyte for device application. We have then fabricated symmetric device by sandwiching layer of gel electrolyte between two perovskite electrodes in case of symmetric device.Room Temperature Phase transition in FAPbI3 perovskite We have fabricated hybrid halide perovskite as well as inorganic halide perovskite supercapacitors with chemical formula APbI3 (where A can be MA, FA, Cs) to study the role of A-site cations on the charge storage and stability of these devices. It is found that FAPbI3 has the highest specific capacitance due to higher ionic mobility as well as higher specific surface area. We have further fabricated thin-film solid-state supercapacitors using quasi-solid-state gel electrolyte. We have achieved over 9 μWh cm−2 energy density at 0.25 mA cm−2 current density which, to the best of our knowledge, is the highest energy density for halide-perovskite-based thin-film supercapacitors. Areal capacitance is also over 72 mF cm−2 for FAPbI3 sample. This is almost 600% more than the CsPbI3-based supercapacitor. We have studied the charge-storage mechanism in all three samples which is dominated by the diffusion of electrolytic ions. There are structural changes during the electrode preparation for all three perovskite samples. However, not much of phase transition is observed during charging or discharging cycles in MAPbI3 and CsPbI3 perovskites. On the contrary, FAPbI3 goes under several phase transition such as non-perovskite δ phase to perovskite α phase during charging. This type of phase transition is generally observed due to temperature variation. Therefore, this may be the first report on non-perovskite to perovskite phase transition upon lithiation. In chapter 3, we have compared the electrochemical performance of lead free and lead based supercapacitor devices. The growing interest in smart portable electronic devices demands a flexible and lightweight power supply. Among all the rechargeable energy storage devices thin film-based supercapacitors are the best alternatives for high-power and high-energy-density applications. Very recently hybrid halide perovskites have gained a lot of attention in energy harvesting and storage applications due to their superior electronic and ionic conductivity. However, one of the challenges in these materials is lead toxicity which may be the bottleneck for commercialization in portable and wearable electronics. In this article, we have demonstrated that the lead-free double perovskite could be the best choice for thin-film-based supercapacitors due to their higher energy and power density compared to lead-based perovskite supercapacitors. We have compared the electrochemical performance of organic-inorganic hybrid perovskite such as methylammonium lead trichloride (MAPbCl3), inorganic perovskite such as cesium lead trichloride (CsPbCl3), and lead-free perovskite such as cesium bismuth chloride (Cs3Bi2Cl9). Lead free perovskite based asymmetric thin film supercapacitor device fabricated from quasi-solid-state gel electrolyte shows an areal capacitance of 64 mF/cm2 which is 8 – 10 times higher than leadbased supercapacitors due to the higher specific surface area (26.4 m2/g) and ionic conductivity in Cs3Bi2Cl9 electrode. The estimated energy density is over 6.6 mWh/cm2 which is also 3 – 4 times higher than the lead-based devices. X-Ray diffraction analysis after first charging cycle reveals the reappearance of orthorhombic peak of Cs3Bi2Cl9 while lead-based perovskites do not show any change during charging or discharging cycles. Lead-free perovskite supercapacitors also show improved stability over lead-based perovskite supercapacitors. These devices are also flexible over wide angle for bending and twisting with ~90% capacity retention. In chapter 4, we have used vacancy ordered tin-based perovskite for supercapacitor applications. Lead halide perovskites having ABX3 structure attracted a lot of attention in various optoelectronic and energy applications owing to their extraordinary photophysical properties. These distinctive materials have exceptional ionic response because of halide ions as well as A-site cations. However, the lead-toxicity issue as well as ion kinetics in lead halide perovskites leading to structural deformation and instability under illumination and external bias prevents these materials from commercialization. In this article, we have explored the possible application of vacancyordered tin-based lead-free double perovskite (A2BX6) in energy storage devices. We have further tuned the X-site anion (X= I, Br, Cl) to see the effect of halide ions on the supercapacitor properties. Cs2SnI6 based electrode has the highest specific capacitance (~ 420 F/g) and energy density (~200 Wh/kg) as estimated from the three-electrode half-cell measurement system. Solidstate devices were also fabricated by using gel electrolytes. We have achieved areal capacitance over 10 mF/cm2 while energy density and power density are 3.2 Wh/cm2 at 0.2 mA/cm2 and 670 W/cm2 at 0.75 mA/cm2 current density, respectively. We have also achieved maximum voltage window over 1.8 V for Cs2SnI6 sample. Various fundamental characterizations such as Ex situ XRD, FTIR, and XPS-measurements are carried out to understand the charge storage mechanism in vacancy-ordered perovskite structures. In chapter 5, we have first fabricated asymmetric and stacked device for supercapacitor applications. Then, we have fabricated flexible supercapacitors on 50 m and 200 m stainless steel and flexible graphite substrates to study the substrate effect on the device performance. We have used home built customizable programmable linear and angular stage for bending/twisting angle and bending/twisting cycle measurements. Therefore, during bending or twisting measurement delamination of the film occurs and the micro-cracks are observed. On the other hand, improved surface wettability leads to a better contact of the perovskite film to the flexible graphite substrate. Therefore, the probability of film delamination is also reduced. The contact angle of solvent on different substrates was also measured. We have also performed EIS measurement as a function of bending angle measurement for stainless steel substrate and flexible garaphite substrate to know about the change in charge transfer resistance. In chapter 6, we provided a brief summary of all the research findings from earlier chapters and suggest possibilities of future research.