Study On Doping Modification Of Cathode Of Zinc-Nickel Single Flow Battery

Ni?OH?₂ is widely used in the field of commercial energy storage batteries because it has the advantages of excellent electrochemical performance,simple structure and environmental friendliness.There are two main goals for the modification of Ni?OH?₂,one is to reduce the cost of the electrode,and the other is to increase the specific discharge capacity of the zinc-nickel single-flow battery and improve the electrochemical performance of Ni?OH?₂.Both of these points can be achieved by doping metal ions.In this paper,the buffer solution method was used to prepare Mn-doped and Mn-Mg co-doped samples.The samples were tested by scanning electron microscopy and X-ray diffraction analysis to observe the structure and morphology.The samples were subjected to cyclic voltammetry and constant current charge and discharge tests to analyze the discharge specific capacity,cycle stability and other electrochemical properties of the samples.In addition,theoretically analyze the state density of?-Ni?OH?₂,and discuss the influence of doped metal ions on the open circuit potential of the electrode.The main conclusions of the full text are as follows:?1?The models of?-Ni?OH?₂ and?-NiOOH were established and theoretically calculated using first-principles.The calculation results showed that the distribution of the state density of?-Ni?OH?₂ and?-NiOOH were between-23 eV?10 eV,and the state density of?-NiOOH splited into two peaks around-20 eV,mainly because the two oxygen atoms in the crystal structure of?-NiOOH were in different environments.The intensity of the peak in the?-NiOOH state density diagram was significantly lower than that of?-Ni?OH?₂,because?-NiOOH lost electrons during the electrode reaction.In the analysis of the electrode potential,it was found that Mg and Mn have a higher electrode potential value than Ni,so they have a stronger reducing ability,they were more likely to lose electrons in the electrode reaction,and the battery energy storage were also higher.?2?Ni_1-xMn_x?OH?₂?x=0.15,0.18,0.22,0.25?cathode materials with different Mn contents were prepared to study the effect of Mn content on the performance of?-Ni?OH?₂.X-ray diffraction analysis showed that the samples doped with Mn were all in?phase,but when x=0.18,0.22,0.25,a small amount of impurities Mn₃O₄ appeared in the samples.The half-peak width of the diffraction peak of the Mn-doped sample was narrow,indicating grain refinement.Scanning electron microscope observation revealed that the Mn-doped sample was finer and more porous than the commercial?-Ni?OH?₂ particles.This phenomenon was confirmed by the specific surface area test.Cyclic voltammetry test showed that after doping Mn,the redox potential difference of the sample was small,and the electrode reversibility was good.The constant current charge and discharge test showed that the specific discharge capacity of the Mn-doped sample increases first and then decreases with the increase of Mn content.When x=0.18,the specific discharge capacity of the sample was 314.5 m Ah/g at a current density of 800 m A/g.The commercial?-Ni?OH?₂ discharge specific capacity under the same conditions was 195 m Ah/g.It showed that the doping of Mn can improve the discharge specific capacity of nickel electrode and make it have better charge and discharge performance of large current.When the Mn content was x=0.18,the sample shows good cycle stability.After the samples were cycled for 30 cycles under the same conditions,the discharge specific capacity of sample x=0.18 was not decayed,while the commercial Ni?OH?₂ was 6.4%decayed,and other samples were decayed by 1.9%?x=0.15?and 0.49%?x=0.22?,3.1%?x=0.25?.?3?Based on Ni_1-xMn_x?OH?₂?x=0.15,0.18,0.22,0.25?cathode materials,Ni_0.82Mn_0.18?OH?₂ had the best electrochemical performance.Partial substitution of Mn with Mg element was used to prepare Ni_0.82Mn_0.18-yMg_y?OH?₂?y=0.06,0.09,0.12?,and the effect of Mn-Mg co-doped on the performance of?-Ni?OH?₂ was studied.In X-ray diffraction analysis,the samples Ni_0.82Mn_0.18-yMg_y?OH?₂ and?-Ni?OH?₂ diffraction peaks had the same peak shape.However,the samples Ni_0.82Mn_0.18-yMg_y?OH?₂ peak intensity were weak,indicating that the particles were refined.Scanning electron microscopy found that the sample?-Ni?OH?₂ was spherical particles with smooth surface.The surface of the Ni_0.82Mn_0.18-yMg_y?OH?₂ particles of the samples were very rough,and there were more fine particles.This morphology could make the active material fully contact with the electrolyte and improve the utilization rate of the active material.In the cyclic voltammetry test,the redox potential difference between Ni_0.82Mn_0.09Mg_0.09?OH?₂ and Ni_0.82Mn_0.06Mg_0.12?OH?₂ was significantly smaller than?-Ni?OH?₂,indicating that the reversibility of the electrode reaction of these two samples was better.In the constant current charge and discharge test,when the current density was 150 m A/g,the discharge specific capacity of Ni_0.82Mn_0.09Mg_0.09?OH?₂ was the highest,which was 263.5 m Ah/g.Under the same conditions,the discharge specific capacity of sample?-Ni?OH?₂ was slightly lower than that of Ni_0.82Mn_0.09Mg_0.09?OH?₂ sample,which was 252.9m Ah/g.The specific discharge capacities of samples Ni_0.82Mn_0.12Mg_0.06?OH?₂ and Ni_0.82Mn_0.06Mg_0.12?OH?₂ were lower,192.2 m Ah/g and 225.5 m Ah/g,respectively.After doping Mn and Mg elements,the discharge potential of the sample was increased.The samples?-Ni?OH?₂ and Ni_0.82Mn_0.18-yMg_y?OH?₂?y=0.06,0.09,0.12?were charged and discharged for 30 cycles at a current density of 800 m A/g.The discharge specific capacity of commercial?-Ni?OH?₂ was attenuated by 7%,the discharge specific capacity of Ni_0.82Mn_0.12Mg_0.06?OH?₂ was attenuated by 14%,Ni_0.82Mn_0.09Mg_0.09?OH?₂ and Ni_0.82Mn_0.06Mg_0.12?OH?₂ did not see attenuation.It showed that Mn-Mg co-doped can improve the cycle stability of zinc-nickel single-flow battery.Doping Mn is helpful to improve the discharge specific capacity of?-Ni?OH?₂,and co-doping Mn-Mg is helpful to improve the cycle stability of?-Ni?OH?₂ and prolong the service life.On the other hand,the nickel content in the positive electrode material after doping is reduced,and the cost of the nickel electrode is reduced.Studies have shown that doping appropriate amounts of metal ions can effectively improve the electrochemical performance of nickel hydroxide cathode materials,so that it has a stronger market competitiveness and better application prospects.