Research On Structural Design And Electrochemical Performance Of The Cathode Materials For Zn-Ion Batteries

The cathode materials of Zn-ion batteries has a variety of oxide forms and crystal structures.During the intercalation/deintercalation process of Zn2+ ions,the cathode materials undergo different charge storage mechanisms and phase transition.resulting in various electrochemical performance of the cathode materials for Zn-ion batteries.Therefore,the structural design of the cathode materials will regulate their charge storage mechanism and phase transition,thereby achieving the advanced Zn-ion batteries with high capacity,large rate tolerance and long cycle life.Based on this,this research focused on the structural design of manganese-based oxides,including four levels:the crystal structure of manganese-based oxides,the material morphology of manganese-based oxides,the electrode structure of manganese-based oxides and the new structure of manganese-based oxides.In terms of experiments,this paper designed and prepared a series of manganese-based oxide cathode materials with different structures,including the crystal water interlayered?-MnO2-carbon cloth(?-MnO2/CC)composite electrode,the Na+ doped?-MnO2-layered graphene-like substrate(Na:MnO2/GCF)composite electrode,the urchin-like ?-MnO2 microspheres(AUM)cathode material and the spinel-type H+inserted HxMn2O4 porous microspheres cathode material.The details are as follows:(1)As for the crystal structure of manganese-based oxides,this paper designed and prepared the 8-MnO2/CC composite electrode containing interlayer crystal water.Experimentally,ultra-thin ?-MnO2 nanosheets were assembled on carbon cloth fibers in situ by a simple electrolysis method.Considering the degree of electrolyte degradation,redox peak position and intercalation/deintercalation current in different potential ranges,the optimal working potential window of the ?-MnO2/CC electrode were confirmed to be(0-1.1 V)for three electrolytes of(Li2SO4,Na2SO4 and K2SO4,0.5 mol/L).The specific capacitances of the 8-MnO2/CC electrodes were 318.3,272.2.and 276.1 F/g(2 mV/s),respectively,and the capacity retention after 5000 cycles(5 A/g)were 80.6%,83.8%and 83.5%,respectively.This experiment confirmed that the interlayer crystal water in the ?-MnO2 could help the transport of electrolyte ions between the layers.and enlarge the working potential window in the aqueous electrolyte,and further achieve advanced Zn-ion batteries with high capacity,large rate performance and long cycle life.(2)As for the material morphology of manganese-based oxides,this paper designed and prepared the urchin-like ?-MnO2 microspheres cathode material.Experimentally,porous MnOx microspheres with low crystallinity were prepared by a chemical precipitation,and then the MnOx template was transformed into urchin-like?-MnO2 microsphere after the hydrothermal reaction.Experiments confirmed that the urchin-like morphology of AUM featured larger specific surface area and better electrical conductivity,which effectively improved the electrochemical performance and cycle stability of ?-MnO2.The AUM-based Zn-ion batteries achieved a high initial capacity of 308.8 mAh/g(100 mA/g).After 1000 cycles of GCD at 1000 mA/g,the capacity remained at 170.2 mAh/g with a Coulomb efficiency of 99.9%.The maximum energy density of the device was 396.6 Wh/kg,and the maximum power density is 3813.8 W/kg.This experiment confirmed that during the charge and discharge process,the AUM electrode showed a high capacitive contribution(55.1%),and the crystal structure of ?-MnO2 underwent the reversible phase evolution along with the synergistic insertion/extraction of H+ and Zn2+ ions during the long-term cycling.(3)As for the electrode structure of manganese-based oxides,this paper designed and prepared a layered graphene-like carbon film substrate(GCF).As an application,a Na:MnO2/GCF composite positive electrode and a Zn/GCF composite negative electrode were fabricated.Experimentally,a simple and fast electrochemical peeling method was used to transform the raw graphite paper directly into the graphene-like carbon film,and then the Na:MnO2/GCF cathode and Zn/GCF anode were fabricated by the electrochemical deposition.The GCF substrate were constructed by a 2D—3D network of layered graphene sheets,and the Na:MnO2 and Zn materials were uniformly deposited on the graphene surface.This experiment confirmed that the GCF substrate showed a high capacitive contribution of 61.7%(0.1 mV/s).During the charge and discharge process,there is a reversible H+-Zn2+co-intercalation/deintercalation mechanism in the Na:MnO2/GCF electrode,and the interlayer spacing of ?-MnO2 underwent reversible expansion/contraction transtion.In the cycle test,the overpotential of the Zn/GCF negative electrode was smaller than that of the traditional metal zinc foil negative electrode.After 100 h of cycling at the current densities of 0.2,0.5,and 1.0 mA/cm2,the Zn/GCF electrode showed better cycle stability.The GCF-based Zn-ion batteries delivered a high initial discharge capacity of 381.8 mAh/g at 100 mA/g.After 1000 cycles of GCD at 1000 mA/g,the capacity remained at 188.0 mAh/g with the Coulomb efficiency of near 100%.Moreover,the maximum energy density of the device was 511.9 Wh/kg,and the maximum power density was 3908 W/kg.In addition,First-principles calculations are also carried out to investigate the effect of Na+ doping on the electrochemical performance of layered ?-MnO2 cathodes.The results demonstrated that Na+ ion doping could enhance the bonding of Zn2+ ions and ?-MnO2 layers,and promote the intercalation/deintercalation of Zn2+ ions between ?-MnO2 layers,and thus improve the electrochemical performance of ?-MnO2 electrodes in aqueous batteries.(4)As for the new structure of manganese-based oxides,this paper designed and prepared the spinel-type HxMn2O4 porous microspheres with H+ ions inserting.Experimentally,the spinel-type ZnMn2O4 microspheres were prepared by a chemical precipitation and a high-temperature calcination,and then a 0.5 M H2SO4 aqueous solution was used to exchange Zn2+ions with H+ions in the ZnMn2O4 crystals,and the HxMn2O4 porous microspheres were collected.Compared with the ZnMn2O4,the HxMn2O4 based Zn-ion batteries delivered a high capacity of 281.0 mAh/g at 100 mA/g,as well as a reversible capacity of 133.4 mAh/g after 1000 cycle at 1000 mA/g.Moreover,the maximum energy density of the device was 391.6 Wh/kg,and the maximum power density was 3900 W/kg.This experiment confirmed that the spinel-type HxMn2O4 porous microspheres are an excellent cathode material.During the charge/discharge process,there is a reversible H+-Zn2+co-insertion/deintercalation process in the HxMn2O4 cathode,and the crystal structure of HxMn2O4 undergoes a reversible phase transition during cycling.