Preparation And Electrochemical Properties Of Carbon-coated ZnO In Zinc-Ni Battery

Recently,much attention has been paid to the development of secondary battery which yields high energy density and safety,and benign to environment.Zinc-nickel battery with zinc as the negative active material and nickel hydroxide as the positive active material is a great potential candidate for substitution of Ni H battery and Ni Cd battery,due to the high theoretical energy density of 334 Wh/kg and low cost.The nickel electrode technology has been well developed.However,zinc electrode suffers from many problems such as electrode deformation and Zn-dendrite which seriously deteriorate the battery performance.Different from the traditional electrochemical reaction mechanism in many secondary batteries(for example,lithiation and delithiation in Li-ion batteries),the electrooxidation and electroreduction reactions at zinc electrode exhibit the typical liquid-solid reaction characteristics,leading to the uncertain active mass conversion path.Aiming at stabilization of electrochemical reaction zone at anode,a carbon-coated ZnO(ZnO@C)is developed.ZnO@C was prepared using glucose as the carbon source.Based on the high performance ZnO@C,a prismatic type battery(1.6V/8Ah)is developed.The main achievements are summarized as follows:(1)A scaleable production procedure of the precursor for ZnO@C synthesis is developed.Spray drying after wet ball milling for mixing ZnO with glucose provides the excellent precursor with homogeneous distribution of glucose on surfaces of ZnO particles.Under the optimized heating condition of 600 ^oC for 6 h in N₂ atmosphere,the obtained ZnO@C demonstrates uniform particle size and homogeneous C-coating layer.(2)The ZnO@C with different carbon contents of 2,4,8,12,16 and 20 wt%was prepared by increasing in the glucose amount in the mixture of ZnO with glucose solution.The increase in carbon content leads to formation of thicker carbon layer.The C-coating layer property determines the anti-corrosion capability,reaction reversibility,and ion migration of ZnO@C.It is found that the use of ZnO@C significantly stabilizes the electrode structure when the carbon layer thickness reaches2 nm(contains 4 wt%C in ZnO@C).(3)Zincate ion is a heavy anion,easy to accumulate at the button of battery.Increase in KOH concentration in electrolyte can suppress the zincate accumulation and enhance ZnO dissolution,leading to the homogenization of electrochemical reaction at anode.Taking the Ni electrode performance into account,we use 12 M KOH solution as the electrolyte to assemble the 1.6V 8Ah Zn-Ni battery,achieving high rate capacities,good low-temperature performance,and long cycle life.In summary,carbon coating effectively inhibits the growth of zinc dendrites during charging.The thickness of the carbon coating not only affects the conductivity of the electrode and the uniformity of electrode impedance distribution,but also demonstrates a significant impact on the mass transfer of hydroxide ions.The conductivity and impedance homogeneity of electrode can be improved by increasing the thickness of carbon layer,while thick carbon layer hinders the transfer of hydroxide ion.After optimizing of the composition and structure of the zinc electrode,the bottleneck problem of damage to electrode structure caused by instability of electrode reaction region was solved based on the research and development of functionalized carbon coated layer,leading to the improvement of performance cycling stability.We has established the key technology for fabrication of long-life zinc electrode with heavy Zn-loading for industrialization.