Preparation Of Hierarchically Porous Transition Metal Oxides Monoliths And Their Li-Storage Properties

Lithium-ion batteries(LIBs)are devices that convert electrical energy and chemical energy by means of lithium ion intercalation and deintercalation between positive and negative electrode,and have been widely applied in portable digital products,smart grids and electric devices.However,with the development of electric vehicles industry,the energy density and power density of traditional anode materials are inadequate to satisfy the continuous growth in demand.Considering the high energy equipment,it is essential to explore anode materials with excellent rate capability and long cycle life.Compared with the graphite electrode with a low theoretical capacity(372 mAh g-1),the transition metal oxide based on the conversion mechanism with a higher specific capacity(500-1000 mAh g-1),environmrntal benignity and low cost have received extensive attention.Meanwhile,due to the large volumetric variation,gradual aggregation,low Li+ diffusion rate and poor electronic conductivity,how to improve the ion diffusion rate,conductivity and nano-structure will be a research hotspot.Based on the comprehensive review on the research progress of anode materials for lithium ion batteries,this study points out the advantages of multi-composite porous transition metal oxides as anode materials for lithium ion batteries.The controllable structure of the transitional metal hydroxide is carried out to construct the hierarchical porous nanostructure,and a series of transition metal hydroxide porous monoliths are successfully synthesised.The composite component of hierarchical porous structure transition metal alloy and the transition metal oxide are obtained by different thermal treatment methods.Meanwhile,the preparation mechanism and construction strategy of hierarchical porous composites were also studied,and the efficient lithium storage mechanism was analyzed.By means of the construction of hierarchical porous structure,active material nanocrystallization and multi-component compounding to solve the main problems,resulting in important theoretical basis for application of porous transition metal oxide monolith in lithium ion batteries.The specific research mainly includes the following four parts:(1)Chontrollable preparation of three dimensional(3-D)porous transition metal hydroxide monoliths.Synergic control of phase separation and sol-gel transition allows the formation of interconnected macropores and co-continuous solid skeletons by the divalent transition metal chloride(Mn,Fe,Co,Ni,Zn)as the precursor,water and glycerin as solvents,polyacrylic acid and propylene oxide used as phase separation and gel promoter,respectively.The effects of the amount of polyacrylic acid,the ratio of mixed solvent,the amount of gel promoter and the amount of inorganic salt precursor on its structure were investigated in detail.The results show that polyacrylic acid has the function of inducing phase separation,enhancing the strength of the skeleton and promoting gelation.The pore size distribution of the porous monolith can be controlled by varied the solvent composition.Expanded to the binary composite system,it is judged whether the composite binary transition metal hydroxide porous monolith can be prepared according to the difference in hydrolysis rate constant(pKa).(2)Preparation and porous structure of 3-D NiCo alloy/C monolith.NiCo hydroxide porous monolith was prepared by sol-gel accompanied with phase separation,and the NiCo alloy/C composites was obtained at 800 ℃ in an inert atmosphere.The prepared NiCo alloy/C dispalys the characteristics of interconnected macropores and co-continuous solid skeletons with a pore diameter of-0.4 ym and a particle size of 100-250 nm.NiCo alloy nanoparticles are evenly distributed on the surface of skeleton.Its porosity and specific surface area are as high as 77.2%and 149.72 m2 g-1,respectively.(3)Preparation and Li-storge of 3-D ZnFe2O4/C monolith composites.The porous monolith of ZnFe hydroxide was synthesised by sol-gel with accompanied phase separation method,and three-dimensional porous ZnFe2O4/C composite was obtained after being coated by carbon.The composite material shows the features of hierarchical porous structure with the large pore size of～0.32 μm,the hollow structure skeleton(35 nm)constructed of ZnFe2O4(16 nm),and the specific surface area of 29 m2 g-1.The results of electrochemical analysis show that the 3-D co-continuous and hollow structure ZnFe2O4/C has excellent lithium storage performance.The specific capacity can reach 970 mAh g-1 at current density 0.2 A g-1 after 100 cycles,and the specific capacity can reach 710 mAh g-1 at the current densityl A g-1 after 400 cycles with high coulombic efficiency.(4)Preparation and Li-storge of 3-D ZnFe2O4/C monolith composites.The Mn hydroxide porous monolith was prepared by sol-gel with phase separation,and the 3-D co-continuous MnO/C composites were obtained by heat treatment under two different atmospheres.The in-situ pyrolyzed carbon of polyacrylic acid forms a 3-D carbon skeleton,and MnO particles with the size of the 60 nm are uniformly distributed on the 3-D co-continuous carbon skeleton.Benefiting from the advantages of hierarchical porous structure,the lithium storage performance of MnO/C composite is completely limited by the storage of pseudocapacitive charge storge instead of the diffusion rate of Li+.The anode material for lithium ion batteries has excellent electrochemical performance.The specific capacity can reach 480 mAh g-1 after 1000 cycles at current density 1 A g-1.