Metal-organic framework derived porous ternary ZnCo₂O₄ nanoplate arrays grown on carbon cloth as binder-free electrodes for lithium-ion batteries

Highlights

• Porous ternary ZnCo₂O₄ nanoplate arrays (NPAs) grown on carbon cloth are synthesized.

• The porous ternary ZnCo₂O₄ NPAs are derived from metal-organic frameworks.

• The ZnCo₂O₄ NPAs exhibit excellent performance as binder-free electrodes for lithium-ion batteries.

Abstract

Metal-organic frameworks (MOFs) derived ternary transition metal oxide nanostructures have attracted considerable attention for their important applications in energy-related fields. However, most of the present reports on MOFs-derived materials are in the powder form, which should be further mixed with polymer binder and conductive agents before they are coated on current conductor. In this work, we demonstrate a facile solution method to grow the bimetallic zinc/cobalt zeolitic imidazolate frameworks nanoplate arrays (Zn/Co-ZIF NPAs) on carbon cloth (CC) substrate and then they are well converted to porous ternary ZnCo₂O₄ NPAs on CC after an annealing treatment in air. When investigated as binder-free electrodes for lithium-ion batteries (LIBs), the ZnCo₂O₄ NPAs/CC electrode exhibits a high area capacity of 3.01 mA h cm⁻² (corresponding to 1341.7 mA h g⁻¹ for the ZnCo₂O₄ NPAs) at the current density of 0.24 mA cm⁻² for 100 cycles, which is much higher than that of the Co₃O₄ NPAs/CC electrode (1.93 mA h cm⁻²). Moreover, at a high current density of 1.20 mA cm⁻², the ZnCo₂O₄ NPAs/CC electrode can still maintain a high capacity of 2.05 mA h cm⁻². The excellent electrochemical performance could be attributed to the unique morphology, structure and composition of the porous ternary ZnCo₂O₄ NPAs, as well as directly grown on current collector.

Introduction

Energy storage issue has become one of the most important concerns in the twenty-first century. Among various energy storage systems, advanced lithium-ion batteries (LIBs) have drawn great research attentions from both academia and industry because of their merits including high energy density, long lifespan, safety, and good environmental compatibility [1], [2], [3]. However, the current commercial graphite anode can only deliver a low specific capacity of 372 mA h g⁻¹, which can not fulfill the demands for future high-performance LIBs with large energy and power density. Therefore, numerous research endeavour have been devoted to exploring new high-performance electrode materials to displace the conventional graphite anode.

Since Tarascon et al. first discovered that some transition metal oxides can reversibly react with Li⁺ through the conversion reaction, various transition metal oxides such as MnO_x, CoO_x, FeO_x and NiO have been extensively studied owing to their high theoretical capacities about 2–3 times larger than that of graphite anode [4], [5], [6], [7], [8], [9]. In recent years, ternary metal oxides (AB₂O₄; A, B = Co, Fe, Mn, Ni, Zn, etc.) have stimulated great research attention for their richer redox reactions, better mechanical stability and higher electrical conductivity originating from the synergy effects from two different metal species [10], [11], [12]. Among them, ternary ZnCo₂O₄ is particularly attractive compared with the binary Co₃O₄ because of its high theoretical capacity, low-cost and environmental benignity [13], [14], [15], [16], [17], [18], [19], [20], [21]. ZnCo₂O₄ possesses a spinel structure with the trivalent Co-ions occupying the octahedral sites and the bivalent Zn-ions occupying the tetrahedral sites [16], [17]. Up to now, various nanostructured ZnCo₂O₄ materials including nanotubes, nanowires, nanosheets and microspheres have been reported as advanced anode materials for LIB [13], [14], [17], [20], [21]. Despite the great progress that has been obtained, researches on the synthesis of ZnCo₂O₄ anodes is still at an early stage. Besides, the rate performance and cycling stability of ZnCo₂O₄ is still unsatisfied because the large volume expansion during the repeated lithiation/delithiation process and the low electrical conductivity.

Metal-organic frameworks (MOFs) are a new class of crystalline porous structures consisting of metal centers coordinated to organic linkers with adjustable chemical composition, which have been regarded as appealing templates/precursors for construction of hollow/porous ternary transition metal oxides [22], [23], [24], [25], [26], [27], [28], [29], [30]. Generally speaking, ternary transition metal oxides derived from MOFs had obvious advantages including high surface area, inter-connected pores and uniform elemental distribution, leading to enhanced performance compared to those obtained via other methods in various applications [22]. Recently, hollow porous CoFe₂O₄ nanocubes, multilayer CuO@NiO hollow spheres, ZnO/ZnFe₂O₄ sub-microcubes, Zn_xCo_3–xO₄ hollow polyhedras, multishelled Ni-Co oxide onions and mesoporous Ni_xCo_3–xO₄ nanorods derived from different types of MOFs have been reported and exhibited enhanced electrochemical performance as anode materials for LIBs [31], [32], [33], [34], [35], [36]. However, most of the present reports on MOFs-derived materials were in the powder form, which should be further mixed with polymer binder and conductive agents before coated on the current conductor to form compact films [37]. In such binder involved electrodes, the interfacial impedance greatly increased due to the unwanted polymer binder, and some electroactive sites were blocked for electrochemical reactions [38], [39]. On the contrary, direct growth of well-aligned arrays of electrode materials on current collectors can not only simplify the fabrication process of the electrodes, but also supply multiple advantages such as tight electrical contact with substrates, facile diffusion of electrolyte, enlarged electrode-electrolyte contact area and adequate electrochemical reaction of each nanostructures [40], [41]. Therefore, it would be of great interest to directly grow MOFs arrays on current collectors and then transform them into porous ternary metal oxide arrays as binder-free electrodes for LIBs, which might provide outstanding performance owing to the large surface area and abundant accessible active sites [42], [43]. However, the facile synthesis of desirable MOFs-derived ternary metal oxide arrays on current collector still remains a major challenge.

In this work, we demonstrated a facile solution method to first grow the bimetallic zinc/cobalt zeolitic imidazolate frameworks nanoplate arrays (Zn/Co-ZIF NPAs) on the carbon cloth (CC) at room temperature and then convert them to porous ternary ZnCo₂O₄ NPAs on CC after an annealing treatment in air. Benefiting from the multiple advantages of this electrode design, the ZnCo₂O₄ NPAs/CC electrode exhibited excellent performance including high specific capacity, excellent cycling stability and good rate performance when investigated as binder-free electrodes for LIBs. Herein, the chosen of CC as the substrate was attributed to its high conductivity, lightweight and excellent mechanical flexibility and strength. Moreover, the CC substrate can also act as the lithium host to make a contribution to the total capacity [40]. Impressively, the ZnCo₂O₄ NPAs/CC electrode can exhibit a high discharge capacity of 3.01 mA h cm⁻² (corresponding to 1341.7 mA h g⁻¹ for the ZnCo₂O₄ NPAs) at the current density of 0.24 mA cm⁻² after 100 cycles, which confirmed the promising application of the ZnCo₂O₄ NPAs/CC electrode for LIBs.