Preparation Of Molybdenum-based Composites And Their Lithium Storage Properties

Graphite,as the commercial anode materials for Lithium-ion batteries(LIBs), can not meet the requirements of next-generation high-capacity LIBs because of its low theoretical capacity(372 m Ah g-1). Nowadays, the development of electrode materials with satisfactory properties in terms of high capacity, long cycle life and good rate capability, have been the hot topic for next-generation LIBs.For the purpose of the development of high-performance electrode materials, a variety of molybdenum-based nanocomposites(four distinct morphologies of Mo O2/Graphene composites, two distinct morphologies of Cr2Mo3O12/Graphene composites) are prepared in this thesis. Their microstructures and electrochemical properties as the anode materials for LIBs are explored.(1) This thesis develops a facile one-step thermolysis method in acetylene for fabricating carbon-coated Mo O2 nanosheets erected on graphene(Mo O2@C/Graphene sheet-on-sheet) when hexaammonium heptamolybdate(AHM) tetrahydrate and graphene as raw materials. The flat-lying Mo O2 nanosheet on graphene, Mo O2/graphene rod-on-sheet and particle-on-sheet composites can be obtained by varying experimental conditions such as gas atmosphere and the Mo relative precursor. The morphology structures and electrochemical properties of the material are discussed in detail. The Mo O2@C/Graphene sheet-on-sheet composite shows a unique three-dimensional porous structure and excellent electrochemical properties. At a small current of 100 m A g-1, it delivers a large capacity of 752 m Ah g-1 after 100 cycles with a capacity retention rate of 99.2 % compared to the first-cycle value. An average capacity fading of 0.008 % per cycle can be calculated. Reversible capacities of 586, 502 and 374 m Ah g-1 can be retained for the composites at 1 C, 5 C and 10 C after 200 cycles, respectively. The excellent high-rate cycling performances of the standing Mo O2 nanosheets on graphene composite should be ascribed to the 3D porous hierarchical nanostructures. The increased electrical contact, the improved mechanical stability, and the facilitated lithium diffusion, the alleviation of the large volume change, can all be delivered for carbon coating and graphene support. Besides, these void spaces among erect Mo O2 nanosheets are also useful to accommodate the large volume expansion during cycling. The restacking of graphene is also prevented more efficiently by the erect Mo O2 nanosheets between two neighboring graphene nanosheets. The resultant 3D porous hierarchical networks in the electrodes can introduce more open and porous space and therefore facilitate the charge transfer and electrolyte infusion.(2) For the synthesis of Cr2Mo3O12 nanorods on graphene composites, a mixed solution of AHM and Cr Cl3?6H2O with fixed p H value is introduced as the precursor, followed by a one-step hydrothermal method. The effect of different reaction conditions such as reaction temperature, reaction time, and reactant concentration on the products is explored. The Cr2Mo3O12/Graphene sheet-on-sheet composite is prepared in a similar method by reducing the weight amount of Cr Cl3?6H2O in the preparation process. The microstructures and electrochemical properties of these two composites are discussed in detail. The Cr2Mo3O12/Graphene rod-on-sheet composite shows excellent electrochemical properties as anode materials. A large reversible capacity of 988 m Ah g-1 after 50 cycles at a current density of 100 m A g-1 can be obtained. For comparison, the charge capacity of the Cr2Mo3O12/Graphene sheet-on-sheet electrode after 50 cycles is evaluated to be 721 m Ah g-1. The charge capacities of sheet-like Cr2Mo3O12 and rod-like Cr2Mo3O12 are 243 and 482 m Ah g-1, respectively. The rod-like Cr2Mo3O12 with unique one-dimension nanostructures may shorten the distance for mass and charge transport. Graphene, as an excellent conductive substrate, can improve the electrical contact of the anode, reduce the impedance and improve its mechanical stablity. The composite with a large specific surface area can provide more active sites for lithium storage. Meanwhile, the drastic volume change of Cr2Mo3O12 during cycling can be buffered and the restacking of graphene can be prevented effectively in the composite. As a consequence, the Cr2Mo3O12/Graphene rod-on-sheet composite with a large specific capacity and good cycling stability may be a promising anode alternative for lithium ion battery.