Preparation And Study On Sodium-storage Properties Of Sodium Vanadium Fluorophosphate@Graphene Aerogel Network Composites

The significant increase in energy consumption over the past decades has driven a shift in the world’s energy landscape and satisfied the needs of sustainable development.Lithium-ion batteries（LIB）have been commercialized as emerging power units.However,the range and charging speed are still not as good as those of fuel vehicles.Power batteries with higher energy and power densities are expected.Sodium and lithium are in the same main group with similar physical and chemical properties.And the element sodium is readily available and widely sourced.Sodium ion batteries（SIB）are recognized as a promising alternative to lithium-ion batteries.Their cathode materials are the key to the energy density,cost and electrochemical performance of sodium ion batteries.Therefore,it is important to develop cathode materials with high energy density and long cycle life.Sodium vanadium fluorophosphate,a polyanionic material,has attracted much attention due to its unique three-dimensional skeleton structure,long-term stable cycling capability and high Na⁺mobility.However,its imperfect conductivity and the uncontrollable morphology of sodium vanadium fluorophosphate materials still affect its electrochemical performance.In contrast,graphene aerogels have a three-dimensional porous mesh structure that not only provides a conductive network but also buffers the volume expansion of the material during the during the charging and discharging.To address the above issues of sodium vanadium fluorophosphate,this paper combines sodium vanadium fluorophosphate with functionalised graphene aerogel network as the cathode material for sodium ion batteries,and also uses microscopic morphology design and structural modulation to enhance the cycling and multiplicity performance of sodium ion batteries.Material characterization was used to analyze the material’s morphology and further searched the relationship between the structure and morphology of material and its electrochemical performance.The studies are as follows:（1）Functionalised graphene aerogels network（GN）was successfully prepared by hydrothermal and freeze-drying methods with graphene oxide（GO）as the raw material and ascorbic acid as the reducing agent.The test showed that GN has a loose three-dimensional structure with a higher degree of disorder and defects,in addition to more abundant pore.As a result,Na⁺has low charge transfer resistance and high ion diffusion in GN,which results in better cycle life(the capacity of GN is 34.1 m Ah g^-1 after 400 cycles at 5 C current density,and capacity retention of 92%).（2）In this paper,NVPF materials were obtained by wrapping GN（denoted as NVPF@GN）.The GN skeleton not only provided a transport channel for Na⁺but also improved the electrical conductivity of the material.By regulating the annealing temperature,it was found that the temperature affected the morphology of the material.Different morphologies of NVPF@GN had great influences on the electrochemical properties.By designing NVPF@GN at different annealing temperatures it was found that the NVPF@GN annealed at 480°C is a cuboid particle with high structural symmetry and homogeneous morphology.Tests have shown that NVPF@GN-480 exhibits long cyclability（0.01%capacity decay per turn in 300 cycles at 0.5 C）and optimized rate performance(107 m Ah g^-1with 99%capacity retention when the current density is reset to 0.5 C),which can be attributed to the high structural symmetry of NVPF@GN-480 and the high electrical conductivity of GN.（3）To further study the factors affecting the morphology of the material NVPF,the solvent ratio of the hydrothermal reaction was adjusted so that when water:ethanol=1:2,which was denoted as NVPF-1/2.The particle size is the smallest about 2μm than that of other materials.The smaller particle size makes shroter Na⁺diffusion distance within the NVPF and increases the diffusion rate.The discharge specific capacity of NVPF-1/2 was 82,81.2,80,73.5,63.8 and 55.3 m Ah g^-1 at 0.5,1,2,5,10 and 20 C,respectively.When the current density returned to 0.5 C,the capacity of NVPF-1/2 was still 82 m Ah g^-1 with a capacity retention rate of 100%.The above results indicate that GN can improve the conductivity of NVPF and the reduction of NVPF particle size can improve the storage performance of Na⁺.