Rational Design Of Nitrogen,Phosphorus Co-doped Carbon Materials For High-performance Lithium-iodine Battery

On the basis of the redox reaction between iodine and lithium,lithium-iodine battery with high theoretical specific capacity(211 mAh g-1)and high energy density(612 Wh kg-1)have attracted increasing research attention.However,the application of lithium-iodine battery is hindered by the intrinsic problems including the poor stability of iodine,the shuttle effects of polyiodide and the poor cycling stability of lithium metal anode.These issues result in poor rate performances,fading capacity,low coulombic efficiency and safety hazards.To address these issues,carbon fibers doped with the nitrogen and phosphorus were prepared,aimed to regulate the surface composition and pore structures of porous carbon to improve electrochemical performances and enhance the battery performances.The main results are summarized as follows:(1)The electrodeposited manganese dioxide was used as the redox template for the in-situ polymerization of phytic doped polyaniline.Then,nitrogen,phosphorus co-doped porous carbon was obtained after pyrolysis.When used as the cathode material for lithium-iodine battery,the self-standing nitrogen,phosphorus co-doped carbon cloth with high Lil loading exhibited a large specific capacity(221 mAh g-1 at 1 C),excellent rate capability(95.8%capacity retention at 5 C)and superior long cycling stability(2,000 cycles with a capacity retention of 96%).Electrochemical kinetic analysis revealed the dominant contribution of capacitive effects at high scan rates,leading to the good high-rate performance.The improved electrochemical performance mainly stems from the unique structures of nitrogen,phosphorus co-doped porous carbon cloth,heteroatom doping provides extra active sites for strong adsorption of iodine species while the highly porous structure with large surface area is favorable to enhance the capacitive effects at high rates.(2)A three-dimensional(3D)stable lithium metal anode was prepared by loading of molten lithium on carbon cloth doped with nitrogen and phosphorous.The experiment observations and theoretical calculation firstly reveal that the N,P co-doping greatly improves the lithiophilicity of the carbon cloth,which not only enables the uniform loading of molten lithium but also facilitates the reversible lithium stripping and plating.Thus,the dendrite formation can be significantly suppressed at the 3D lithium electrode,leading to the stable voltage profiles over 600 h at a current density of 3 mA cm-2.When paired with lithium iodine cathode,the full cell showed impressive long-term stability with a capacity retention of around 100%for over 4000 cycles and enhanced high-rate capability.In summary,the cathode and anode as interrelated parts are optimized by the nitrogen and phosphorous co-doping,and porous structure regulation on carbon materials,leading to the significant improvement of the battery performances.It is believed this work will shed light on the design and development of carbon based materials for next generation rechargeable batteries.