| Secondary batteries,such as Lithium-ion and Sodium-ion batteries,have been realized to be the most promising choices for large-scale stationary energy storage.For large-scale stationary energy storage,beyond the requirement of high energy density,long operation life,high rate capability as well as low deployment cost are always demanded.Low-cost and environment-friendly transition metal cyanides,which allow Na+ and Li+ fast migration,have been well regarded as a promising candidate for cathode materials secondary batteries.In this thesis,we were aimed at enhancing the redox voltage of nickel hexacyanoferrate(Ni HCF)to improve its energy density,and exploring more cheaper K3Fe(CN)6 as a new cathode material for sodium-ion batteries.The main results and progress in this thesis are summarized as follows:1.Higher redox voltage of rhombohedral nickel hexacyanoferrate(r-Ni HCF)as the cathode material of sodium-ion batteries.By controlling and optimizing homogeneous co-precipitation method,we synthetized a kind of rhombohedral nickel hexacyanoferrate,cyclic voltammetry results suggest that the operation voltage of r-Ni HCF is 120 m V higher than c-Ni HCF,which is also the highest operation voltage of Ni HCF for sodium-ion batteries.Electrochemical charge/discharge cycle tests suggest that Na-r-Ni HCF delivers a specific capacity of 66.8 m Ah g-1 at the current density of 80 m A g-1,the energy density is 229 Wh kg-1 which is approximate to the theoretical capacity(230 Wh kg-1)and 26% higher than Na-c-Ni HCF.The capacity retention of 96% can be achieved after 200 cycles indicating an excellent stability which can be assigned to the absence of rhombohedral-cubic phase transition and negligible volume variation during electrochemical redox as proven by the ex situ XRD patterns.As the current density increases to 480 m A g-1,the specific capacity retains 58.9 m Ah g-1,and could retain 83% after 200 cycles,indicating superior rate capability.First principles density functional theory(DFT)simulations suggest that Na-r-Ni HCF delivers 70 m Vhigher operation voltage than Na-c-Ni HCF,which agrees well with the experimental results,qualitatively.In addition,the mechanism of high operation voltage of Na-r-Ni HCF is identified that Na+ ions prefer to stay asymmetrically at the N-coordinated corners with much closer Na-N distances,leading to dramatic electron polarization particularly for the adjacent N atoms and resulting in the iron ions are more positive.As consequence,the lower electron density of iron ions should benefit gaining electrons during the discharge process,leading to improvement on the discharge potential.2.Higher redox voltage of rhombohedral nickel hexacyanoferrate(r-Ni HCF)as the cathode material of lithium-ion batteries.Based on the mechanism of higher operation voltage of r-Ni HCF in sodium-ion batteries,we conjecture that the r-Ni HCF will delivers more obvious higher-operation voltage effect when serving as cathode materials of lithium-ion batteries,as Li+ ions with smaller ionic radius and more concentrated positive charge than Na+ ions.Our DFT simulations validate this conjecture that the theoretical discharge potential of Li-r-Ni HCF is 350 m V than that of Li-c-Ni HCF.The experimental results further confirmed that Li-r-Ni HCF delivers 3.47 V(vs.Li+/Li)discharge potential,270 m V higher than Li-c-Ni HCF,and 80 m Ah g-1 specific capacity,which is also higher than c-Ni HCF(70 m Ah g-1).As a consequence,the energy density of r-Ni HCF can reach as high as 280 Wh kg-1,25% higher than that of c-Ni HCF.Both of r-Ni HCF and c-Ni HCF encounter ignorable capacity decay in 100 cycles at a low current density of 10 m A g-1,indicating excellent cycle stability.Nevertheless r-Ni HCF exhibits feeblish rate capability than c-Ni HCF,as the current density increasing to 600 m A g-1,c-Ni HCF could maintain 76% of the capacity at 10 m A g-1,however r-Ni HCF will encounter sharp dropping to 23%.The DFT simulations of ion migration in framework suggest that the stronger Na-N or Li-N interreaction in r-Ni HCF compared to c-Ni HCF increases migration resistance result in a poorer rate capability of r-Ni HCF than c-Ni HCF.Therefore,smaller grain size of r-Ni HCF is necessary to improve the electrochemical performance especially the rate capability.By controlling the concentration of raw materials,a smaller grain size with good dispersion of r-Ni HCF was synthetized,and the cycle and rate performance tests suggest that r-Ni HCF delivers 54% and 50% capacity retention of low current density as current density increasing to 600 m A g-1 and 800 m A g-1,respectively,and there is ignorable capacity decay in 200 cycles at a current density of 300 m A g-1,realizing excellent rate and cycling performance.3.Lower-cost K3Fe(CN)6 material is explored as new cathode materials for sodium-ion batteries.In view of synthesizing prussian blue analogues always need K3Fe(CN)6 or Na4Fe(CN)6·10H2O and transition metal salts as raw materials,and the necessary water removal of Na4Fe(CN)6·10H2O when serving as cathodes of sodium-ion batteries,we propose to investigate K3Fe(CN)6 who have no crystalliferous water as the cathodes of sodium-ion batteries.Our experimental results suggest that K3Fe(CN)6 delivers reversibility Na+ ions insertion/deinsertion in solid phase K3Fe(CN)6 at 3.3/3.4 V(vs.Na+/Na)as well as at 2.65/2.76 V which is very weak and should be attribute to the dissolved ions [Fe(CN)6]3-/4-,however it needs more conductive agents(Super P).K3Fe(CN)6/Super P(6:4,w:w)simple composite delivers excellent cycling stability,however,the utilization rate of active materials and rate capability need to be improved further.In order to decreasing the grain size of K3Fe(CN)6 and increasing the degree of mixing between K3Fe(CN)6 and Super P,a series of ball milling strength were carried out to K3Fe(CN)6/Super P(6:4,w:w)composite.The rate and cycling test results suggest the K3Fe(CN)6/Super P composite based on facile strength ball-milling exhibit the best electrochemical performance.There is almost solid phase sodium storage with a theoretical specific capacity(81 m Ah g-1),and encounter ignorable capacity decay in 200 cycles at a low current density of 0.1 C,the capacity could maintain 34% of the capacity at 0.1 C when the current density increasing to 12 C,indicating an excellent cycle stability and good rate capability.In addition,the composition and crystal structure of active material in electrode were characterizated.As Na+ ions insertion/deinsertion,K+ ions in K3Fe(CN)6 have been replaced into electrolyte gradually,and as consequence the active material in electrode turn into Na2.88K0.12Fe(CN)6.The Na2.88K0.12Fe(CN)6 has a new crystal structure compared to K3Fe(CN)6 and Na3Fe(CN)6,and the volume variation is small during Na+ ions insertion/deinsertion proven by the ex situ XRD patterns,indicating an excellent stability.Our results indicate that K3Fe(CN)6 could be a lower-cost cathode materials for sodium-ion batteries. |
