Synthesis Of Graphene-Based Materials For Rechargeable Lithium Batteries

Positive and negative electrode materials with high specific capacities are the key for the R&D of high-performance rechargeable batteries.However,these materials generally present high volumetric change of the active material during charge and discharge,leading to the large loss of active material arising from pulverization,and rapid capacity degradation due to the electrode expansion.Particularly,cathode materials with high specific capacities usually show poor conductivity.These problems become the bottle-neck for commercialization of high-performance batteries.Graphene has excellent electrical conductivity,high specific surface area,and excellent mechanical properties.It is an excellent support material for synthesis of high-performance electrode materials.A green and inexpensive graphene-based electrode material synthesis method with high-efficiency is of great significance for the commercialization of high-performance lithium-ion batteries.To address above problems,this paper studies and develops a series of graphene-based anode materials and graphene-based sulfur cathodes,on the basis of the new green and low-cost graphene synthesis method which is developed in this work.In this paper,regarding to the strong polar ion environment of KCl/NaCl eutectic molten salt which promotes the transformation from three-dimensional sp3 C-C or C-X bonds to two-dimensional aromatic sp2 C-C bonds,glucose can be converted to graphene material(MS-Gr)directly.On the basis of the above-mentioned graphene synthesis method,the nitrogen-doped graphene(N-Gr)is successfully prepared according to the Maillard reaction of NH4CI with glucose.Compared with the MS-Gr,much higher specific surface area of N-Gr is achieved,because the doped N presents a π-conjugated structure in graphene,favorable for graphene formation.The specific surface area of N-Gr increases with elevating carbonization temperature.However,a temperature higher than 1050 ℃ leads to the decreased specific surface area because the aromatic ring segments generated in the molten salt tends to aggregate and stack.The N-dopping creates more defects in the graphene sheet,providing more active sites.The N-Gr demonstrates high enhancement effect,thereby exhibits higher performance,compared with the MS-Gr.Furthermore,on the basis of the nitrogen-doped graphene process to improve the specific capacity of the anode materials,a series of high capacity composite materials are prepared via adding precursors of Sn,Co9S8,Ni3S2.By the synergy of graphene and the blended second anode materials,the obtained graphene-based hybrids of(Sn,Co9S8,Ni3S2)exhibit good electrochemical performance.Among above-mentioned hybrids,the Sn@Gr material exhibits the most excellent performance.The in-situ formed Sn nanoparticles not only expand the interlayer space of graphene to increase the lithiation capacity of graphene but also bridge the graphene sheets to enhance the electrical conduction in longitudinal direction of graphene sheet,leading to the excellent rate performance.The mechanical toughness of the graphene coupled with the firm connection between nano-Sn and graphene sheet contributes the enhanced durability of the Sn@Gr anode.Both lithiation and delithiation capacities of 671 mAh g-1 remain even after 700 cycles at a current of 800 mA g-1.A stable rate capacity as high as 606 mAh g-1 has been achieved at a current of 1000 mA g-1.Mechanism analysis of test results provide a new idea for the preparation of more practical high-performance graphene-based anode materials.For the cathode materials in Li-S battery,the Ni@Gr is prepared for enhancement of polysulfide(PS)adsorption capacity to suppress PS shuttle.Taking the advantages of strong adsorption by Ni towards polar PS species,and by graphene towards weakly-polar PS species,the Li-S battery with Ni@Gr exhibits more stable cycleability than that with N-Gr.By the composition optimization,Ni@Gr-5S composite sulfur electrode with 5 wt%Ni shows the best performance.Based on the Ni@Gr,the carbon nanotube grafted graphene(CNT-g-Gr)composite with three-dimensional network structure with ultra-high specific surface area is fabricated by CVD.Compared to N-Gr samples,CNT-g-Gr has higher specific surface area(2994 vs.603 m2 g-1)and a total pore volume(4.76 vs.0.04 cm3 g-1),as well as stronger polysulfide adsorption capability.The in-situ formed Ni nanoparticles on graphene sheet not only serve as the catalytic sites for CNT growth,but also function as the anchor-sites for polar PS absorption.The CNT-g-Gr contributes a superb PS adsorption capability arising from graphene and CNT absorbing weakly-polar PS species,and Ni nanoparticles absorbing the species with stronger polarity.The resultant Li-S battery with the CNT-g-Gr shows excellent cycleability and rate performance.A stable discharge capacity of 900 mAh g-1(with low capacity degradation rate)and a rate capacity of 260 mAh g-1 at 30 C discharge rate have been achieved,far higher than the rate performance of other similar materials,demonstrating the feasbility for power batteries.