Rapid,Microwave-Assisted Synthesis of a Composite of Mesoporous MnCo2O4 and Reduced Graphene Oxide(RGO) As a High Perfomance Anode for Li- Ion Batteries

Over the last two decades Li-ion batteries (LIBs) have been investigated to improve their electrochemical properties. LIBs have ushered wireless technology and are now employed in large scale applications such as electric vehicles, solar and wind electricity generation systems. The performances of LIBs depend on properties of electrode materials. Since the first commercialization of LIBs by Sony in 1991, graphitic carbons have been used as anode materials. However graphite suffers from challenges like limited capacity, lithium plating at high current densities and formation solid electrolyte interface (SEI). This has motivated researchers to develop alternate anode materials which could deliver high energy and power densities. Cobalt oxide, iron oxide, manganese oxide and tin oxide are some among them. But most of these materials show rapid capacity fade due to low electronic conductivity. Binary metal oxides such as ZnFe₂O₄, CuCo₂O₄, ZnCo₂O₄, ZnMn₂O₄, NiCo₂O₄ and MnCo₂O₄ have shown good capacity upon cycling because of their better electronic conductivity. MnCo₂O₄, belongs to cubic spinel structure have attracted much attention due to its characteristics inherited from manganese and cobalt based metal oxides.   

          One of the challenging issues in conversion reaction of a bulk electrode material is to maintain structural integrity during electrochemical cycling with associated issues such as volume changes, pulverization, etc., which affect the performance porous materials have been studied widely because of the easy percolation of electrolyte into core of particles which result in high rate capability and also by allowing volume expansion/contraction arising from intercalation/deintercalation of Li⁺ ions. Using templates become a general approach to synthesize these porous structures. However in most of the cases, the synthesis requires long duration and involves specially designed autoclaves.

             Here in we report a single step and rapid, microwave-assisted synthesis route to synthesize mesoporous microspheres of MnCo₂O₄ dispersed with low concentration (≤ 5 wt%) of reduced graphene oxide (RGO). In a typical synthesis, 0.02 g of graphite oxide (GtO) was added to 20 ml of ethylene glycol and sonicated for 20 minutes at room temperature to form graphene oxide (GO) dispersion.  0.5 mmol of Manganese (II)nitrate. tetra hydrate, 1.0 mmol of  cobaltous(III)nitrate.hexahydrate and 15 mmol of ammonium bicarbonate  were dissolved in 20 ml of  ethylene glycol and stirred for 30 minutes to obtain a homogeneous solution. Then this solution was added drop wise to already prepared solution of graphite oxide under sonication. After sonication, solution was taken into 80 ml glass vessel, heated to 200 ^oC by microwave irradiation in a single mode microwave reactor (2.45 GHz, Discover, CEM, USA) equipped with a pressure sensor, and an optical fibre temperature sensor, and held at the same temperature for 30 minutes. The system was then cooled down to room temperature naturally. The dark grey coloured product was collected by centrifugation and washed several times with distilled water and ethanol, followed by drying at 60 ^oC overnight. As prepared dark grey powder was heated to 600 ^oC at an average heating rate 5 ^oC per minute in a tube furnace under the flow of nitrogen and held at the same temperature for 5 h to convert it into MnCo₂O₄ and RGO composite. The phase purity and morphology of the final product were studied by XRD, SEM and TEM analysis. The microscopic image (figure.1) shows that mesoporous MnCo₂O₄ microspheres were attached on to RGO sheets. The charge-discharge profile is shown in Fig. 2. The specific discharge capacities of 841, 801, 832 and 765 mAh g^-1 were obtained for 1st, 2nd, 5th and 30th cycles, respectively, at a current density of 100 mA g^-1 in the potential range of 0.01 and 3.0 V vs Li/Li⁺. These values are comparable with the values that were reported previously. Results pertaining to the aforementioned studies including both physical and electrochemical investigations would be presented.