Cathode Materials Of Aqueous Secondary Zinc Ion Batteries Based On Nano Manganese Dioxide

Aqueous zinc ion batteries（AZIBs）are a new type of environmentally friendly and green battery systems.Compared with traditional lithium ion batteries,AZIBs have many advantages.On the one hand,zinc assumes a high theoretical capacity(819 m Ah g^-1),abundant resource,and is easy to be transported and stored.On the other hand,aqueous electrolyte is safer and cheaper than conventional organic electrolyte used in other secondary batteries.However,a challenge remained to be solved for AZIBs is lack of suitable cathode materials.Among the cathode materials for AZIBs studied so far,manganese oxide（MnO₂）has the advantages of higher working voltage and specific capacity.However,the dissolution of MnO₂ in electrolyte and the generation of irreversible by-products lead to the rapid decline of battery capacity,which hinders the practical application of MnO₂.In order to solve the above problems,in this work,MnO₂ was used as active material,graphene and carbon source generated from different sugars were used as carbon sources to coat MnO₂,to prepare new AZIBs cathode materials,the details are as follows:（1）MnO₂ was coated with carbon generated from sucrose and glucose,respectively,and two MnO₂/carbon composites,named as SCM（with sucrose as carbon source）and GCM（with glucose as carbon source）were prepared.The as-obtained composites were characterized and analyzed using different spectroscopic techniques and high-resolution microscopy.The AZIBs（coin half cells）with SCM and GCM as cathode materials were assembled and their electrochemical properties were studied.The results showed that when sucrose was used as the carbon source and the sucrose content was 5%（versus the generated MnO₂ theoretical mass in reactant）,the as-prepared SCM shows largest specific capacity of 400 m Ah g^-1,and the charge/discharge reaction mechanism of the battery seems closely related to the Zn²⁺insertion/extration mechanism.When 2.5%of glucose was used as carbon source,the specific capacity of the obtained GCM can be remained at 179.6 m Ah g^-1 even after 200 cycles,（2）Two kinds of MnO₂/graphene composites,MnO₂/G1 and MnO₂/G2,were obtained by coating MnO₂ with graphene through vacuum-rotary evaporation and hydrothermal method,respectively.Their structures and morphologies were characterized using scanning electron microscopy（SEM）and X-ray diffraction analysis（XRD）,and their electrochemical performances as cathode materials for AZIBs were investigated on coin cells with them as cathode materials.It was illustrated that the MnO₂/G2 shows better rate performance,manifest cycling stability,and its initial discharge specific capacity is as high as 236.41 m Ah g^-1,which increased by 202.69%in comparing with that of bare MnO₂.Meanwhile the overall electrochemical performances of MnO₂/G1 seem not significantly improved.There were two pairs of reduction oxidation peaks appeared on their voltammetry curves,indicating that the reaction mechanism related to the charge/discharge procedures of MnO₂/G1 and MnO₂/G2 follow a Zn²⁺/H⁺co-insertion/extration mechanism.