Hierarchical multi-yolk-shell copper oxide@copper-1, 3, 5-benzenetricarboxylate as an ultrastable anode for lithium ion batteries.

The capacity attenuation of transition metal oxides (TMOs) and metal-organic frameworks (MOFs) is the obstacle for practical application in lithium ion batteries, due to the extensive volume variation upon charge/discharge cycles. Herein, a hierarchical composite material with copper oxide (CuO) multi-yolks and copper-1, 3, 5-benzenetricarboxylate (Cu-BTC) shell is synthesized by a facile method to study the effect of the hierarchical structure on the electrochemical performance. The porosity and pore volume of CuO@Cu-BTC composites are optimized to buffer the volume change and facilitate the infiltration of electrolytes by altering reaction conditions. The CuO@Cu-BTC (20 h) with the largest surface area and pore volume delivers an excellent reversible capacity of 780.7 mAh g-1 at 200 mA g-1 after 100 cycles, and ultrastable long-term performance with a specific capacity of 569 mAh g-1 at a current density of 1000 mA g-1 after 900 cycles. The corresponding full battery shows moderate capacity retention from 149.4 to 125.8 mAh g-1 after 70 cycles, with a specific capacity retention of 84.2%, based on the mass of lithium iron phosphate (LiFePO4) at 0.2 C (1 C = 170 mA g-1). This strategy applies copper oxide as the metal source of the coordination compound, as well as the internal yolks, which can be extended to the in-situ construction of other hierarchical composites, providing a new avenue for practical application of TMOs and MOFs as anode materials.

2D conductive MOFs with sufficient redox sites: reduced graphene oxide/Cu-benzenehexathiolate composites as high capacity anode materials for lithium-ion batteries.

As a superconductive metal-organic framework (MOF) material, Cu-BHT (BHT: benzenehexathiol) can exhibit outstanding electrochemical properties owing to the potential redox reactions of the cuprous ions, sulfur species and benzene rings of Cu-BHT, but its compact texture limits the specific capacity of Cu-BHT. To improve the dense feature of Cu-BHT, rGO/Cu-BHT (rGO: reduced graphene oxide) composite materials are fabricated via a facile route and they exhibit applicable conductivities, improved lithium ion diffusion kinetics compared to pristine Cu-BHT, and sufficient redox sites. The rGO/Cu-BHT composite materials maximize the potential capacity of Cu-BHT, and the rGO/Cu-BHT 1 : 1 material achieves outstanding reversible specific capacities of 1190.4, 1230.8, 1131.4, and 898.7 mA h g-1, at current densities of 100, 200, 500, and 1000 mA g-1, respectively, superior to those of pristine Cu-BHT and rGO. These results present the promising future of 2D conductive MOFs as functional materials for energy storage, based on the regulation of electronic conductivity, redox sites, and lithium ion diffusion kinetics.

Multiple Active Sites: Lithium Storage Mechanism of Cu-TCNQ as an Anode Material for Lithium-Ion Batteries.

Recently, carboxylate metal-organic framework (MOF) materials were reported to perform well as anode materials for lithium-ion batteries (LIBs); however, the presumed lithium storage mechanism of MOFs is controversial. To gain insight into the mechanism of MOFs as anode materials for LIBs, a self-supported Cu-TCNQ (TCNQ: 7,7,8,8-tetracyanoquinodimethane) film was fabricated via an in situ redox routine, and directly used as electrode for LIBs. The first discharge and charge specific capacities of the self-supported Cu-TCNQ electrode are 373.4 and 219.4 mAh g-1 , respectively. After 500 cycles, the reversible specific capacity of Cu-TCNQ reaches 280.9 mAh g-1 at a current density of 100 mA g-1 . Mutually validated data reveal that the high capacity is ascribed to the multiple-electron redox conversion of both metal ions and ligands, as well as the reversible insertion and desertion of Li+ ions into the benzene rings of ligands. This work raises the expectation for MOFs as electrode materials of LIBs by utilizing multiple active sites and provides new clues for designing improved electrode materials for LIBs.