Porosity Engineering of MOF‐Based Materials for Electrochemical Energy Storage

Abstract

energy storage (EES)-primarily supercapacitors and metal-ion batteries (MIBs, e.g., lithium-ion batteries (LIBs), sodiumion batteries (SIBs), and potassium-ion batteries (PIBs)).Both supercapacitors and batteries typically consist of current collectors, electrodes, electrolyte, and a separator. By applying a suitable potential between current collectors, the charge/discharge process takes place mediated by the electrode materials, and the electrolyte ions (i.e., the charge carriers) are accordingly driven to travel across the separator so as to connect the circuit. In recent years, solidstate electrolytes (SSEs) are also under popular development to enhance cell voltage (energy density) and safety of the devices. [1] Upon charging, electrical energy is converted to electrostatic potential for supercapacitors and to chemical energy for batteries, and vice versa. Therefore, each device has pros and cons. Supercapacitors usually provide a high power density (i.e., a fast charge/ discharge velocity) due to the fast physical charge-discharge process across the interfaces between electrode materials and the electrolyte solutions, while the energy density is usually small (≈5 Wh kg −1 ). In comparison, batteries often offer a large energy density (i.e., a large specific energy capacity of ≈200 Wh kg −1 ) attributed to the chemical redox reactions which take place throughout the whole volume of electrode materials, while the operation rate is relatively slow. [2] To construct desirable energy storage devices, porous materials have been widely adopted, particularly for electrodes and SSEs. These materials typically feature a large fraction of interconnected or reticulated porosity with a high specific surface area (SSA), offering numerous potential active sites and mass transfer channels. For example, porous materials based on nanocarbons (e.g., carbon nanotubes, graphene), conducting polymers, and conjugated microporous polymers with high electrical conductivity and large SSAs have been frequently adopted for supercapacitors, [3] while porous carbons, MoS 2 , and metal oxides have been used for LIBs because of the theoretically large stoichiometric lithium content in the respectively lithiated compound and an accelerated Li diffusion rate inside the pores. [4][5][6] Despite considerable progress made so far, these porous materials fall short of sufficiently large SSAs and readily tunable pores, which constrain the upper limit for the performance optimization and affect an accurate investigation of the structure-performance relationship.Metal-organic frameworks (MOFs) feature rich chemistry, ordered micro-/ mesoporous structure and uniformly distributed active sites, offering great scope for electrochemical energy storage (EES) applications. Given the particular importance of porosity for charge transport and catalysis, a critical assessment of its design, formation, and engineering is needed for the development and optimization of EES devices. Such efforts can be realized via the design of reticular chemistry, multiscale...

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“…[ 1–3 ] However, due to the uneven global distribution and limited reserves of the Li element, sodium ion batteries (SIBs) with similar working principles to LIBs, easy availability and abundant reserves of the Na element have attracted more attention. [ 4,5 ] The standard electrochemical potential of Na + /Na (−2.71 V vs SHE) is higher than that of Li + /Li (−3.04 V vs SHE). The larger ion radius of Na + (1.02 Å) compared with Li + (0.76 Å) means the slow kinetics and lower energy density for SIBs.…”