In-situ MnO₂/graphite Composites And Their Application In Electrochemical Devices

Manganese dioxide（MnO₂）has good electrochemical properties and is widely used in various electrochemical energy storage devices,but its low conductivity greatly reduces its practical application.In this paper,MnO₂ was grown on graphite paper by in-situ synthesis method.The obtained MnO₂/graphite composite materials were used as the electrode material of supercapacitors and aqueous zinc-Ion batteries and the separator of lithium-sulfur batteries,respectively.Their electrochemical properties were studied by cyclic voltammetry,electrochemical impedance spectroscopy and charge and discharge measurements.The main results of the research on manganese dioxide/graphite composites applied in electrochemical energy storage devices are summarized as follows:1.MnO₂ materials are widely used in supercapacitors.Its theoretical capacity is as high as 1370 F/g,but the actual specific capacity of MnO₂ is low because of its poor conductivity.Using graphite nanosheets（GNP）as the substrate,cellulose nanofibers（CNF）with a certain thickness were coated on both sides of GNP.And after oxidized by KMnO₄,MnO₂-based self-supporting electrode materials were prepared.The results show that the volume specific capacity of MnO₂ electrode materials fabricated by this method increases with increasing of the thickness of MnO₂.When the thickness of MnO₂ is 13.2μm,the volume specific capacity reaches 315.2 F/cm³ at a current density of 0.5 mA/cm².The sample was assembled into a symmetrical supercapacitor,and the volumetric energy density is significantly increased.When the power density is 0.11 W/cm³,the supercapacitor shows an extremely high volumetric energy density of 10.6 mWh/cm³.The capacitance retention rate is as high as 93%after 1000 cycles of charging and discharging,at a current density of 2 mA/cm².This suggests that the supercapacitor assembled with MnO₂ electrode material prepared by this method exhibits an excellent cyclic stability.2.Self-supporting electrode materials for secondary zinc-manganese dioxide batteries were produced by in-situ growth of MnO₂ on the surface of electrolytic graphite paper.The process of graphite paper electrolysis can improve the specific surface area of graphite paper and increase the growth point of its surface.This method not only retains the conductivity of graphite paper,but also increases the load of MnO₂ and the utilization of MnO₂ due to the increase of specific surface area.The optimum time and concentration of electrolyte for electrolyzing graphite paper were obtained.After that,the secondary zinc-manganese dioxide batteries were assembled with the electrode materials obtained under the optimum preparation conditions and their electrochemical properties were tested.The results show that its mass specific capacity reaches 279 mAh/g at a current density of0.2A/g.And the capacitance retention rate is 64%,after 200 cycles charging and discharging,at a current density of 0.3A/g,which indicates good cycle stability.Compared with disposable zinc-manganese battery,its performance has been improved.3.Celgard/graphite/manganese dioxide（Celgard/G/MnO₂）composite materials for lithium-sulfur batteries were fabricated by modifying the commercial membrane via simple chemical synthesis method.MnO₂ synthesized on graphite surface has nanowire-like porous structure and large specific surface area.The MnO₂ with porous structure can not only carry sulphur but also adsorb polysulfides.It is also conducive to the transport of electrolyte ions.The test results show that the specific capacity of lithium-sulphur batteries assembled with the composite material is 935 mAh/g at 0.5 C,which is much larger than that of traditional commercial membrane（588 mAh/g）.This kind of battery also exhibits good rate performance and cycle stability.After 100 cycles charge/discharge process,the specific capacity is 647 mAh/g and the capacitance retention rate is 69%.