Synthesis of a zinc ferrite effectively encapsulated by reduced graphene oxide composite anode material for high-rate lithium ion storage

Abstract

Effectively immobilizing nano-sized electrochemical active materials with a 3D porous framework constituted by conductive graphene sheets brings in enhanced lithium ion storage properties. Herein, a reduced graphene oxide (RGO) supported zinc ferrite (ZnFe₂O₄) composite anode material (ZnFe₂O₄/RGO) is fabricated by a simple and effective method. Firstly, redox reaction takes place between the oxygen-containing functional groups on few-layered graphene oxide (GO) sheets and controlled quantity of metallic Zn atoms. ZnO nanoparticles are in-situ nucleated and directly grow on GO sheets. Secondly, the GO sheets are completely reduced by abundant Fe atoms, and corresponding γ-Fe₂O₃ nanoparticles are formed neighboring the ZnO nanoparticles. In this step, 3D porous RGO supporting framework are constructed with γ-Fe₂O₃@ZnO nanoparticles effectively encapsulated between the RGO layers. Finally, the well-designed γ-Fe₂O₃@ZnO/RGO intermediate product undergoes a thermal treatment to allow a solid-state reaction and obtains the ZnFe₂O₄/RGO composite. At a high current rate of 1.0 A·g⁻¹, the ZnFe₂O₄/RGO composite exhibits an inspiring reversible capacity of 1022 mAh·g⁻¹ for 500 consecutive cycles as anode material for lithium ion batteries. And the insight into the attractive lithium storage performance has been studied in this work.

Introduction

The current commercialized graphite anode materials of rechargeable lithium ion batteries have a low maximum specific capability of only 372 mAh·g⁻¹, which is far from satisfying the requirement from the electric/hybrid vehicles and smart grid storage, etc. [1] There have been urgent demands for the exploration of new-type anode candidates possessing higher energy density as well as better lithium ion storage properties. Amongst the current studied anode materials, ZnFe₂O₄ is a prospective binary transition metal oxide (TMO) anode material, featured with high theoretical capacity (1000 mAh·g⁻¹) due to its combination of conversion and alloy lithium ion storage mechanism, cost-effectiveness, abundance, eco-friendliness, low discharging voltage plateau of about 1.5 V. [2], [3] Moreover, ZnFe₂O₄ crystal possesses an AB₂O₄ spinel structure, where an oxygen is cubic-close packed to form a 3D framework for lithium ion diffusion. As a result, higher electrochemical activities can be achieved for the binary ZnFe₂O₄ anode than the unitary TMO counterparts, mainly due to the synergistic work from its complex components. [4], [5]

The practical application of ZnFe₂O₄ is hampered from the rapid reversible capacity fading upon cycle, resulting from both the pulverization caused by large volume change and poor inherent electrical conductivity of the ZnFe₂O₄ crystals. [6] Therefore, hybridizing nanostructured ZnFe₂O₄ with conductive and flexible carbon supporting framework is one of the most effective strategies to enhance its electric conductivity and structure stability of ZnFe₂O₄ composite anode. [7] Graphene and its derivates, e. g. GO and RGO, have been regarded as a promising carbonaceous supporting material for nano-sized electrochemical active materials. [8], [9], [10] Since the first time reported successful synthesis of ZnFe₂O₄/RGO composite in 2011 using a hydrothermal method [11], enhanced lithium ion storage performances have been witnessed compared with bare ZnFe₂O₄ electrodes in recent years. [12], [13], [14]

3D porous RGO framework could effectively improve the charge/electrolyte transfer and sustain the integrity of composite materials during electrochemical reaction, thus improved battery performances can be expected for composite materials. Moreover, as a precursor for RGO, 2D GO sheets are easy to be processed and the rich surface functional groups are agreeable to capture metal ions through electrostatic interaction, which usually function as metal source of TMO nanocrystals. [15], [16], [17] Therefore, nano-sized TMOs will be nucleated and grow on the surface side of RGO layers after further treatments. However, the previously reported preparation methods for TMO/RGO composites are always involved of expensive equipment, rigorous fabrication condition, toxic raw materials or poor scalability, which increases fabrication and/or environment cost. Particularly, the construction of 3D RGO framework and formation of TMOs are commonly independent. Consequently, the TMO nanocrystals will not be well encapsulated by the RGO sheets or excessively anchor on the exterior side of the 3D RGO skeleton, which is unbeneficial for fully realization of electrochemical reaction during charge/discharge.

Based on the above considerations, it is reported that some reductive metals are capable to reduce GO in moderate aqueous condition to obtain corresponding TMO/RGO composites. [18], [19] And a series of TMO/RGO composites with distinctive structures have been successfully prepared using metal Fe, Zn and Sn as both TMO precursor and GO reducing agent in our group. [20], [21], [22], [23] This type of TMO/RGO composite preparation methods is more facile, effective and eco-friendly. Particularly, the reduction of flexible GO sheets to construct mechanical strong RGO architecture and transformation of reductive metals to electroactive TMO nanoparticles are mutual dependent. As a result, good lithium ion storage performances have been delivered from these TMO/RGO composite electrodes. In this work, a ZnFe₂O₄/RGO composite is further designed and prepared on the above foundation. Few-layered GO sheets are first deoxygenated in sequence by metal Zn and Fe in ambient condition to obtain the corresponding ZnO and γ-Fe₂O₃ nanoparticles, both of which are anchored on the RGO supporting layer to construct a well-designed intermediate product (γ-Fe₂O₃@ZnO/RGO). Then the γ-Fe₂O₃@ZnO/RGO undergoes a thermal treatment to obtain the final ZnFe₂O₄/RGO, during which solid-state reaction takes place between the ZnO and γ-Fe₂O₃ nanoparticles. The ZnFe₂O₄ nanoparticles are well immobilized by the constructed 3D porous RGO supporting matrix, which accelerates both electron transportation and electrolyte infiltrations. As a result, the ZnFe₂O₄/RGO electrode delivers attractive lithium ion storage performances. Particularly, at high current rate of 1.0 A·g⁻¹, it exhibits a reversible capacity of 1022 mAh·g⁻¹ for 500 cycles. And the insight into the attractive lithium storage performance has been studied. The overall fabrication process for the ZnFe₂O₄/RGO composite is novel and facile, which has good potential to find wider applications beyond lithium ion storage.