Preparation Of Phosphorus-Doped Boron Nitride/Carbon Composite And Its Application In Lithium-Sulfur Batteries

With the rapid development of portable electronic devices and electric vehicles,the traditional lithium ion battery is not enough to meet the current needs of people,the development of a new generation of secondary battery is urgently needed.Lithium-Sulfur(Li-S)batteries are considered as one of the most promising candidates due to their exceptional theoretical capacity(1675 mAh g-1),low cost and environmental friendliness.However,there are some inherent obstacles strongly impeding the practical applications of the Li-S batteries,such as the insulation of sulfur,the volume expansion of the discharge product,and the so-called "shuttle effect".The trickiest shuttle effect originated from the dissolved polysulfides intermediates diffusing between the cathode and anode during the charge/discharge process,leading to low coulombic efficiency and poor cyclability.In view of the above problems of lithium-sulfur batteries,the paper designs a multifunctional interlayer and a free current collector cathode,respectively.The main contents are as follows:(1)we perpared a multifunctional separator with few-layer graphene as a highly conductive network and stable scaffold to support P-doped borbon nitride(denoted as BN-P@GO)for Li-S batteries.The BN-P@GO was synthesised by freeze drying and high-temperature calcination,then applied to the commercial polypropylene separator via knife coating.The cell with the interlayer provides an initial discharge capacity as high as 1045.3 mAh g-1,and retains a high reversible capacity of 728.7 mAh g-1 at 1 C after 500 cycles with a capacity decay of 0.061%per cycle.Moreover,the rate capability is also superior to cells with BN-P@GO or BN-P interlayers,i.e.reversible capcity of 457.9 mAh g-1 even at 3 C.The excellent electrochemical performance is ascribed to the synergistic effect of physical barrier and chemical adsorption for dissolved polysulfides provided by the modified layer.Furhtermore,it also mitigates the polarization and promotes kinetic reactions of the cells.(2)Mesoporous carbon nanosheets loaded with cobalt-nickel bimetallic particles were prepared by hydrothermal synthesis and high-temperature sintering.The experimental results show that the CoNi bimetallic particles can promote the kinetic reaction of lithium-sulfur batteries and improve the utilization rate of sulfur.The larger specific surface area of the material is beneficial to the distribution of sulfur,and the mesoporous structure cushions the volume expansion of sulfur during discharge.Thanks to the above advantages,the battery capacity can still be maintained at 592.6 mAh g-1 after 400 cycles of 0.5 C,and the capacity attenuation rate is only 0.08%.In addition,the design of eliminating aluminum foil as a current collector objectively reduces the quality of the electrode and increases the energy density of the integral battery.