Metal-organic frameworks for lithium ion batteries and supercapacitors
Graphical abstract
MOFs with large surface area and high porosity can offer more reaction sites and charge carriers diffusion path. Thus MOFs are used as cathode, anode, electrolyte, matrix and precursor materials for lithium ion battery, and also as electrode and precursor materials for supercapacitors.
Introduction
As the economies develop and prosper, we will face challenge to feed the demand of more than two-fold energy increase in this century. The increasing demand created fierce worldwide competition for the gradually depleting fossil fuel reserves. Meanwhile, increasing greenhouse emission results in global climate change. All of these situations require our society to move towards sustainable and renewable energy resources [1], such as solar and wind energy. However, due to intermittent feature of solar and wind source, energy storage systems play a key role to the proper utilization of solar and wind energy. Effective chemical storage (e.g. lithium-ion battery and supercapacitors) thereby becomes imperative in the future energy technologies to provide electrical transportation power for commuters and to store energy from intermittent solar or wind power. Currently, neither lithium-ion batteries (LIBs) nor supercapacitors has yet meet the demand of electric vehicles because of the following challenges: for LIBs, current primarily used anode and cathode materials exhibit a low energy density and poor rate performance. Although the supercapacitors have high-rate performance, the energy density is too low to power future electric vehicles. Due to this current status, large research efforts are devoted to design and fabrication of novel electrode materials with both high energy densities and high power for next-generation batteries and supercapacitors [1b].
Porous materials have been successfully applied in newly developed energy storage and conversion systems, such as fuel cells, LIBs, supercapacitors, solar cells. They can provide large surface area for reaction and interfacial transport, and short diffusion paths [2]. Zhao et al. explored a controllable one-pot method to synthesize N-doped highly ordered mesoporous carbon, which can deliver a specific capacity of 262 F g−1 (in 1 M H2SO4) as the supercapacitor electrode [3]. Since Nazar and co-workers reported mesoporous carbon for Li–S battery with improved cycling performance, the Li–S batteries have gained ever-increasing attention as the next-generation energy storage system [4]. Our previous work also showed porous structures can improve the cycling life and high rate performance of Sn-based materials for LIBs [5].
With the rapid development of porous materials, a lot of novel porous materials with different structures emerged, among which metal–organic frameworks (MOFs) have experienced explosive development during the past decade. MOFs are prepared by linking inorganic and organic ligands through strong bonds to create open crystalline frameworks with permanent porosity [6]. The largest surface area of MOFs is as high as 7000 m2 g−1, which exceeds most of traditional porous materials. The pore size can be tuned by changing the length of the organic ligands, with the largest pore aperture of 9.8 nm [6]. Based on properties of huge surface area, high porosity, low density, controllable structure, tunable pore size, and controllable functional ligands, MOFs could be used in a wide range of potential areas including gas separation and storage, energy storage, catalysis, and drug delivery, in particular, applications in energy storage and conversion systems such as fuel cells, LIBs, supercapacitors. The topic of this mini review focus on the application of MOFs in LIBs and supercapacitors. We present the recent progress investigation, discussion and challenges related to the applications of MOFs in the fields of LIBs and supercapacitors. We expect to promote the knowledge transfer from previous studies to generate new ideas for the future developments of MOFs in the electrochemical energy storage.
Section snippets
MOFs for lithium ion batteries
A typical LIB is based on two redox couples at the anode and cathode [7]. During the charging process, lithium ions move from the cathode (e.g. LiCoO2) to anode (e.g. graphite) through an lithium ion conducting electrolyte (e.g. LiPF6 in organic solvents). The half-cell reaction at the positive electrode side (charge being forward):LiCoO2↔Li1−xCoO2+xLi++xe−
At the negative electrode side (charge being forward):6C+xLi++xe−↔LixC6
The overall reaction becomes:LiCoO2+6C↔LixC6+Li1−xCoO2
LIBs are
MOFs for supercapacitors
Supercapacitors, also called electrical double-layer capacitors (EDLCs), store energy through either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors) [1]. It consists of electrodes, electrolyte, and a separator. Among them, the performance of electrode materials is a crucial factor for the supercapacitors. Most of the work has focused on electrode materials, especially, carbon and metal oxides. Carbon electrodes include carbon
Conclusion and future
The gradually depleting fossil fuels and climate change require our society to move on to sustainable and renewable resources, such as solar and wind energy, as well as the development of electric vehicles to decrease greenhouse emission. All of these require the develop of highly effective electrical energy storage systems, such as LIBs and supercapacitors. The performances of LIBs and supercapacitors depend on the electrode materials. Developing new materials with high specific capacity and
Acknowledgments
This work was supported by the National Key Basic Research Program of China (No. 2014CB239203), National Thousand Young Talent Plan and New Faculty Startup Fund (No. 203273002) and the Young Faculty Fund (No. 2042014kf0044) of Wuhan University.
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