In summary, the state-of-the-art research breakthroughs in improving the rate capability and cycling stability of SCs using low cost transition metal oxides/hydroxides, sulfides, selenides, and phosphides have been reviewed. According to the different charge storage mechanisms, there are three types of the transition metal based compounds for SC application: surface redox pseudo-capacitance based materials, intercalation pseudo-capacitance based materials, and battery-type materials. The charge storage of all the materials involves faradaic redox reactions, which are responsible for the much larger energy density of these materials compared to carbonbased materials.
Through detailed analysis of the properties of electrode materials of these star-performing SC materials, it is clear that the intrinsic properties including the proper crystal structures, high electric conductivity, and charge storage mechanism determine the electrochemical performance of the electrode materials. Built upon
these, four approaches can be used to enhance the energy capacity, rate capability and cycling stability of the transition metal oxides/hydroxides, sulfides, selenides, and phosphides which including 1) material morphologies and nanostructure control; 2) composition tuning of using multiple metal elements in the compound to enhance electrical conductivity; 3) interface engineering to reduce barrier for charge transport and possible unwanted reactions; 4) hybridization by fabrication of material composites.
Construction of nanostructured material with controlled morphologies is a pre-requisite to obtaining good SC properties. Optimized nanostructures can provide a large surface area, shorten the diffusion paths of electrolyte ions and electrons, and provide abundant free spaces for buffering the large volume change of active materials during charge/discharge processes. Tuning the material composition by incorporation of multiple transition metal cations can create strong synergetic effects, resulting in much improved energy storage capacity, rate capability and cycling stability compared to single metal compounds. Incorporation of additional elements introduces more redox reaction active sites within the electrode materials. The foreign elements may also stabilize the crystal structure of host material against structural change because of insertion/extraction of electrolyte ions during charge/discharge process. The properties of interfaces of electrode active materials/electrolyte and of electrode active materials/substrates are critical to the electrochemical kinetics and charge collection of SCs. Three ways including element doping or introduction of vacancies, surface functionalization, and utilization of polymer based gel electrolyte
can effectively improve the electrode material/electrolyte interfaces. Meanwhile, methods including direct growth of electroactive material arrays on current collectors, a buffer layer introduction, and novel current collectors designing can greatly enhance the current collector/electrode interfaces. Composite materials made of low cost transition metal compounds combined with a conductive material matrix such as carbon materials, conductive polymers and other transition metal based materials can produce superior electrochemical performance due to their enlarged surface area, improved electrical conductivity, and enhanced morphological stability, which results in improved capacitance, rate capability and longer cycling stability.
The charge/discharge dynamic process gives rise to challenges to understanding the chemical reactions and change of material properties in the operation of SCs. Therefore, in-situ techniques such as EQCM, in-situ synchrotron XRD, in-situ XAS, and in situ/operando Raman spectroscopy combined with theoretical modeling have been employed to elucidate the change of metal valence state, phase transformation, and interfacial physical and chemical reactions including electrolyte ion absorption/desorption during the charge storage process. The results provide in-depth knowledge of charge storage mechanism involved in transition metal based SC electrode materials.
Even though some promising methods to achieve high SC performance using transition metal oxides/hydroxides, sulfides, selenides and phosphides have been demonstrated, there are still lots of challenges and research questions to be answered in this area. Among the thousands of publications on SCs using transition metal based
materials in the past few decades, only less than 200 papers demonstrated a long cycle life above 10 000 cycles. Furthermore, it is noted that the mass loading of the electroactive material in the reported work was generally much low (only a. 1 mg cm-2 or even less) compared to the requirement of mass loading for commercialized SCs (> 10 mg cm-2).[257] Given the availability of large pool of transition metal compounds and the advancement of material synthesis as well as improved understanding of the device system, it is expected that the SCs with high energy density, high power density, satisfactory capacity rate and cycling stability by using transition metal based electrode materials can be realised through innovative electroactive material design, morphological structure optimization, composition and interface engineering in the future. It is also possible to further increase the mass loading of active materials in order to provide sufficient energy density with commercial SCs through innovative material and device engineering which of course requires great effort. Some encouraging results in this aspect have been reported. For instance, Owusu1 et al.[53] reported SCs based on iron oxide hydroxide with a mass loading as high as 9.1 mg cm-2, which is close to the 10 mg cm-2 loading required in commercial devices. Even for the SCs with low mass loading of transition metal based materials but high cycling stability, they could still be potentially useful in areas such as micro-SCs, transparent devices[258], micro-sensors[259] and electrochromic SCs[224], which require high energy density delivered by low mass loading. Among all the transition metal based materials, composite materials are some of the most promising electrode materials for SCs owing to the complimentary properties of
transition metal based compounds and carbon based materials. Other than the materials that were discussed here, other recent developed transition metal based materials such as transition metal carbides, carbonitrides, nitrides (MXenes) also hold great potentials for SCs. The strategies that were proposed to optimize the electrochemical performance of the transition metal based electrode materials should also be applicable to MXenes materials to further boost the material performance in SCs. Current knowledge of the fundamental charge storage mechanism of transition metal compounds for SC applications is still very limited, which however is crucial for the design of next generation SCs materials with high electrochemical performance. Further research to fill this knowledge gap is urgently needed and should be the focus in the future as well.
Acknowledgments
This work was supported by the Australian Research Council (ARC) Future Fellowship program (FT120100674), ARC Laureate Fellowship Program (Project FL170100101), the Natural Science Foundation of Shandong Province (no. ZR2017BB042), China Postdoctoral Science Foundation (no. 2017M612184), QUT PRA scholarship, Queensland University of Technology (QUT) strategic research support fund, and the World-Class Discipline Program and the Taishan Scholar's Advantageous and Distinctive Discipline Program of Shandong Province.
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