Structurae Design And Energy Storage Mechanism Of High-performance Phosphorus-based Electrode Material

Exploring and developing high performance electrode materials with high energy density and long cycle stability is an important research direction to promote the wide application of lithium/sodium ion batteries.Due to the high theoretical capacity,abundant resources and low cost,phosphorus-based materials are considered as promising anode materials for lithium/sodium ion battery.However,phosphorus-based materials suffer from low rate performance and fast capacity fading due to the serious volume expansion during charge-discharge process.In this paper,we have introduced new synthetic methods and designed appropriate nanostructures in attempt to solve the above problems.Furthermore,the electrochemical energy storage mechanism of phosphorus-based materials is explored by in-situ transmission electron microscope technology,electrochemical performance test and density functional theory（DFT）calculations and some significant results are obtained.These works can provide a new strategy to promote the practical application of high-performance phosphorous based materials for lithium/sodium ion batteries.The main research results are as follows:（1）We report a novel covalent heterostructure with monodisperse Ni₂P immobilized on N,P-co-doped carbon nanosheets（Ni₂P@NPC）by one-step carbonization/phosphatization method.The monodisperse nature of Ni₂P nanoparticles prevents them from intermediate contact,thus avoiding the stress concentration among nanoparticles and ensuring the structural integrity upon cycling.In addition,the Ni₂P nanoparticles are immobilized on carbon buffer trough strong chemical bonding,forming a covalent heterostructure,whereby realizing an agglomeration free charge storage process.As a result,the as-prepared Ni₂P@NPC exhibits a remarkable reversible discharge capacity and outstanding long-term durability for sodium storage（the specific capacity is 361mAh/g at the current density of 100mA/g after 300 cycles,and the reversible capacity is still at181mAh/g at 500mA/g after 1200 cycles）.Furthermore,a strong bonding between Ni₂P and NPC is evidenced by density functional theory（DFT）calculations,which helps to stabilize the Ni₂P NCs and meanwhile enables the synergistically enhanced charge storage by offering exponentially increased electronic states around Fermi levels.Significantly,the agglomeration free charge storage process and energy storage mechanism are visualized by means of in-situ transmission electron microscopy（TEM）.This work provides a new design strategy for metal phosphide as anode material of sodium/lithium ion secondary battery.（2）We report a self-confined growth of FeP quantum dots on CNT-grafted P-doped 3D octahedral carbon framework（FeP@OCF/CNT）through an in situ reductive phosphatization/carbonization of metal organic framework（MOF）that with porous structure formed by self-assembly between metal ions（atoms）and organic ligands.This unique architecture design offers various advantages for high rate and stable sodium storage:i)carbon framework confines the growth of FeP quantum dots（<10 nm）,thus shortens the charge transfer length（L）within the TMP particle;ii)the 3D carbon octahedral framework with highly porous structure work like sodium ion reservoirs and facilitates the Na⁺ions diffusion,and meanwhile buffer the volume change,and prevents the Fe P nanoparticles from aggregation upon cycling;iii)the 3D P-doped carbonoctahedra-grafted with 1D CNTs can work as an electrical highway for fast electron transportation.Particularly the as prepared FeP@OCF electrode exhibits a reversible specific capacity of 674 mAh/g at0.1A/g and demonstrates a record high-rate capacity with 262 mAh/g at 20 A/g.In addition,the evolution process of microscopic characteristic and intercalation/conversion reaction storage mechanism of FeP@OCF have been identified through in-situ TEM observation and ex situ X-ray diffraction（XRD）patterns.A sodium full-cell FeP@OCF//Na₃V₂（PO₄）₃ is constructed with an outstandingly high energy density of 185 Wh/kg at the power density of 54W/Kg（based on the total mass of active materials in both electrodes）.This result presents that the unique self-confined growth strategy has great potential applications in advanced phosphide electrode materials for next-generation energy storage.（3）We report red phosphorus spherical nanoparticles（RPNP）by facile liquid-phase reduction reaction.The evolution of microscopic morphology has been visually observed by in-situ TEM.It demonstrated that there was a large volume expansion for red phosphorus nanoparticles in the repeated sodiation/desodiation process but without any fracture or fragmentation phenomenon.Combined with the selected area diffraction electron（SADE）image,the sodium storage mechanism of the multiple alloying reaction for red phosphorus particles was proved.During the process of sodium storage,the multiple alloying reaction mechanism of red phosphorus particles from P to Na₅P₄ intermediate to Na₃P,and the reversible desodiation of Na₃P can be realized in the charge process.Furthermore,form the electrical properties of red phosphorus in the sodiation process,we can find that the conductivity of red phosphorus in the discharge process was gradually increased.As a result,the as-prepared red phosphorus nanoparticles can realize a high reversible capacity of 775.4mAh/g at1A/g.And the specific capacity can be maintained at 931 mAh/g after 120 cycles at the current density of 0.1 A/g,demonstrating an excellent cycling stability.This work proved the sodium storage mechanism and transport property of red phosphorus and provided a theoretical basis of red phosphorus for the practical application as the high-performance electrode material for sodium ion battery.（4）We report a new design of FeP nanoparticles wrapped in 3D interconnected N,P-codoped carbon nanofiber film（FeP@NPC）by electrospinning FeOOH nanorods and subsequent heat treatment/phospharization process for flexible sodium ion batteries.During the synthesis,the growth of FeP nanoparticles is confined by the polyacrylonitrile（PAN）nanofiber,which can efficiently prevent the agglomeration of FeP nanoparticle.In addition,the PAN-derived N,P codoped 3D connected carbon fiber network facilitates electron/ion transport which accelerated reaction kinetics effectively.The as-prepared material as free-standing anode material exhibits high reversible capacity of 557 mAh/g and long-term cycling stability of 391 mAh/g capacity can be achieved at 0.1A/g after 1000 cycles.Particularly,a reversible capacity of 250 mAh/g can be remained when the current density is up to 5 A/g and long-term cycling stability of 94%capacity retention after 300 cycles can be achieved at1A/g.Furthermoer,the Na₃V₂（PO₄）₃//FeP@NPC full cell was assembled with Na₃V₂（PO₄）₃ as cathode and FeP@NPC film as anode that can constantly light up three LEDs at various bending angles.This work paves a new strategy to construct the phosphides-based high-performance anode material for the potential application in flexible energy storage systems.