Researches On The Cathodes Of Nickel/Manganese Prussian Bule/RGO Composites For Sodium-ion Batteries

Sodium-ion batteries are suitable for applications in the fields of electric vehicles and large-scale energy storage with rich reserves and the low price of raw materials.It is the key to the development of sodium-ion batteries to explore better cathode materials with excellent sodium storage performance.Prussian blue analogues possess three-dimensional open frame structure and large ion channels which are suitable for the rapid migration of sodium ions and have attracted extensive attention and research as cathode materials for sodium ion batteries.Na₂MnFe?CN?₆ can realize the redox reactions of two electrons and provide a specific discharge capacity of up to 170 mA h g^-1 in theory,thus making it sutible for the cathode of sodium-ion batteries.However,Prussian blue analogues exhibite vacancy and coordination water defects as well as poor conductivity,and need to further improve the structure and electrochemical performance.In this dissertations,in-situ composites of Na₂MnFe?CN?₆/RGO and Na₂Ni_xMn_1-xFe?CN?₆/RGO were synthesized by co-precipitation method in order to obtain low vacancy and lattice water defects as well as highly conductive electrode materials.This main research results are listed as following:?1?Na₂MnFe?CN?₆ was synthesized by co-precipitation method.The sample with0.3 mol L^-1 H⁺and 0.8 g PVP was demonstrated as a regular cubic morphology and good crystallinity.The Na₂MnFe?CN?₆/RGO composite electrode was obtained by in-situ compound graphene and the graphene was able to uniformly coated with Na₂MnFe?CN?₆ particles.The graphene provided a fast electron transport channel for improving the conductivity of the composite.The obtained Na₂MnFe?CN?₆/RGO delivered excellent performance.The initial discharge specific capacity of Na₂MnFe?CN?₆/RGO was approximately 163 mA h g^-1 at a current density of 20 mA g^-1,and the first coulombic efficiency reached 95%.Even at a high current density of1000 mA g^-1,the specific discharge capacity can still reach 87 mA h g^-1.The ex-situ XRD analysis of the composite electrode material showed that a highly reversible phase transformation of rhombohedral-cubic in Na₂MnFe?CN?₆/RGO during charge and discharge process.?2?Ni²⁺was used to dop Na₂MnFe?CN?₆ and partially replaced Mn²⁺in the lattice,thus obtaining a series of Na₂Ni_xMn_1-xFe?CN?6 electrode materials with different Ni and Mn ratios.When the molar ratio of the reactant NiCl₂/MnCl₂ was 1:4,the electrode Na₂Ni_xMn_1-xFe?CN?₆ delivered the initial discharge specific capacity of 95mA h g^-1 at a current density of 20 mA g^-1 and the discharge specific capacity of the second cycle was increased to 118 mA h g^-1.The capacity retention capacity was 104mA h g^-1 after 100 cycles,indicating that the doping of Ni²⁺greatly improved the cycling stability of the electrode.?3?A composite of Na₂Ni_xMn_1-xFe?CN?₆/RGO was prepared on the basis of the reactant NiCl₂/MnCl₂ with 1:4.The electrode delivered an initial specific capacity of120 mA h g^-1 at a current density of 20 mA g^-11 and the specific capacity remained 96.7%after 100 cycles,exhibiting an excellent cycle stability.The first discharge specific capacities were about 107and 98 mA h g^-11 at the current density of 200 and 500 mA g^-11 with the coulombic efficiency above 85%.The composite exhibited excellent rate performace.Even at a high current capacity density of 1000 mA g^-1,a capacity of 85mA h g^-1 was achieved.