Synthesis And Electrochemical Performance Of NaV₆O₁₅ And FeFe?CN?₆ Cathode Materials For Sodium-ion Batteries

Sodium-ion battery?SIB?,as a potential alternative for rechargeable lithium-ion battery?LIB?,has gained more and more concern recently because of large resource availability,relatively low price of raw material and high safety.In particular,it is a key issue on the development road of SIBs to look for proper cathode materials which are non-toxic,low cost and high capacity with robust structure.In light of this,two cathode materials for SIBs,including NaV₆O₁₅ and FeFe?CN?₆,were studied in this dissertation.As one of the oxide cathode materials,NaV₆O₁₅ is of special interest to researchers in view of its outstanding electron mobility and large embedded void for Na+.In order to further improve the electrochemical performance of NaV₆O₁₅,the highly uniform NaV₆O₁₅ nanorods were obtained via PVP-modulated hydrothermal process,while the NaV₆O₁₅/C composite was also synthesized through the carbon coating technology in one-step hydrothermal synthesis route.Prussian blue?PB?and its analogues?FeFe?CN?₆?,a new type of cathode materials for SIBs,is particularly attractive because of its unique 3D open framework structure,a theoretical two-electron redox reaction and facile synthetic procedure.Therefore,the solution co-precipitation and single iron-source method were performed respectively in this study,to compare the electrochemical properties of FeFe?CN?₆ based on two various synthesis routes.Moreover,the influence of concentration of Na+ on electrochemical performance was also investigated.The main research results obtained were shown as follows:1.The PVP-modulated hydrothermal method was used to synthesize NaV₆O₁₅ nanorods and the synergistic effect of PVP on the formation of NaV₆O₁₅ nanorods was systematically investigated.The samples with 0.1 g PVP as the surfactant were demonstrated as rod-like uniform morphology as well as a smooth and clean surface with length of 1-2 ?m and width of 100 nm.The obtained NaV₆O₁₅ nanorods delivered excellent performance.The initial discharge capacity was approximately 157 mAh g-1 for potentials ranging from 1.5 to 3.8 V at the current density of 20 mA g-1,with a capacity retention of 71.38%after 20 cycles.Even when increasing the current density to 200 mA g-1,a specific capacity of 121 mA h g-1 still can be achieved by NaV₆O₁₅ nanorods.Moreover,the oxidation and reduction peaks of the electrode made from this sample were located at 2.69 V and 2.40 V?vs Na+/Na?respectively under reducing discharge depth at 2.0-3.8 V.The initial discharge capacity of the NaV₆O₁₅ nanorods was 113 mA h g-1 at the current density of 20 mA g-1 and remains 97.88 mA h g-1 after 50 cycles,all of which showed excellent rate and cycling performance of NaV₆O₁₅ cathode materials.In addition,EIS analysis indicates that the diffusion coefficients DNa of the before and after 1st cycle are 3.46 × 10-12 and 2.71 × 10-12 cm2 s-1 respectively.Combining with the calculated result of density states of NaV₆O₁₅,all the results suggested that the NaV₆O₁₅ nanorods possess decent ionic conductivity and electrical conductivity.2.NaV₆O₁₅/C composite with the size of 500 nm was synthesized using low-cost glucose as the carbon source via one-step hydrothermal method.The effect amounts of C on the properties of products were also discussed in this study.The experimental results indicate that the electrochemical performance tend to increase firstly and change to decreases subsequently with the increasing of C content.And notably a best energy-storage performance can be achieved by the sample with 15.67%C content,benefiting from its amorphous carbon shell layer uniformly coated with?ca.4 nm in thickness?,which not only retarded the excessive growth of NaV₆O₁₅ nanorods,but also enhanced its electronic conductivity.The initial discharge capacities of carbon-coating NaV₆O₁₅ samples at the current densities of 20 and 200 mA g-1 were 169.03 and 145.8 mA h g-1,respectively,with the coulombic efficiency were above 80%.3.FeCl₃ and K₃Fe?CN?₆ were used to synthesize 500 nm FeFe?CN?₆ nanoparticles by solution co-precipitation.Although the capacity of FeFe?CN?₆ nanoparticles was only 19%of the theoretical specific capacity with poor rate performance at high rate??5C?,it was worth noting that the initial discharge capacity of 115.476 mA h g-1 and 63%of its initial specific capacity was retained after 200th cycle at 0.1 C rate,indicating its excellent cycling performance.On the other hand,high-quality FeFe?CN?₆ nanocubes with a low crystal water content and the size of 150-250 nm were also obtained via a facile Na2S2O3-assisted precipitation process,employing K₃Fe?CN?₆ as the only iron-source precursor.The discharge/charge capacities of FeFe?CN?₆ nanocube were 124.72/125.12 mA h g-1,corresponding to the high initial coulombic efficiency of 95.6%and have a high 94%capacity retention over 100 cycles at the current rate of 0.5 C.The initial discharge capacities for the sample at the current rates of 5,10 and 20 C were 103.24,100.81,and 85.87 mA h g-1 respectively,which indicating excellent discharge rate and cycle capability.Furthermore,it can be found in our work that increasing the concentration of Na+ in the original solution can increase the discharge capacity up to 134.689 mA h g-1.4.The electrochemical reaction mechanism of localized 3d electron configuration of Fe₃+ was investigated through DFT calculations and ex-situ XRD tests.The results suggest that the whole redox reaction process can be divided into two steps.The first step is characterized by the 3.68/3.46V redox couple with reactions FeHS3+[FeLs3+?CN?₆]???NaFeHS3+[FeLS2+?CN?₆]for the insertion of Na ions.The second step is associated with the 3.08/2.81V redox couple with reactions NaFeHs3+[FeLS2+?CN?₆]???Na2FeHs2+[FeLS2+?CN?₆]for the insertion of Na ions,where Fe₃+ connected with C is low spin?FeLs3+?while that connected with N is high spin?FeHs3+?.Ex-situ XRD tests results implied that the lattice structure change is highly reversible during Na insertion and extraction.