Synthesis Of Transition Metal Phosphides And Their Application In Electrochemical Energy Storage

In order to meet the urgent demand for high energy density secondary batteries,high energy lithium ion batteries（LIBs）and lithium sulfur batteries（LSBs）have been widely concerned.Lithium-ion batteries have the advantages of light weight,long cycle life and small self-discharge.It is considered to be a promising electrochemical energy storage device,but the traditional electrode material has a lower specific capacity,which limits its wide range of applications in high power devices.Lithium-sulfur（Li-S）batteries have the advantages of high theoretical specific capacity(1672 mAh g^-1),abundant resources,low cost and environmental friendliness,making them become an important energy storage device.However,due to the dissolution and shuttling effect of polysulfide of Li-S batteries,resulting in the loss of active material and changes in the structure of the cathode,thereby affecting its cycle life.Therefore,we will explore a new type of anode material with with high intrinsic conductivity and high specific capacity for lithium ion batteries,and a composite material capable of limiting polysulfide dissolution and shuttle effect,achieving high energy and high power input and output of lithium ion batteries and high performance lithium sulfur batteries.It is great significance to speed up the development of power batteries in china and the strategy of“curving overtaking”in the world.Therefore,in view of the defects of electrode materials of current lithium ion batteries and lithium sulfur batteries,a series of new electrode materials based on transition metal phosphide are designed and synthesized,and their structure,electrochemical properties and reaction mechanism are studied and analyzed.The specific research contents are as follows:1.Design and synthesize FeP@C/rGO composites with porous multi-channel three-dimensional（3 D）structure for high performance LIBs.It is found that the FeP@C/rGO composite electrode exhibits high capacity utilization and excellent rate performance,especially high rate performance,with a discharge capacity of 497 mAh g^-11 at a current density of 5 A g^-1.It also have high capacity utilization and good cycle performance,the reversible capacity is as high as 1080 mAh g^-11 at the current density of 0.1 A g^-1,and the average cycle decay rate is only 0.04%at the current density of1 A g^-11 in 500 cycles.The excellent electrochemical performance of the FeP@C/rGO composite can be attributed to its unique stable carbon octahedral multi-channel framework and N/P co-doped interconnected graphene conductive network,which significantly promotes the transition of lithium ions and electrons and well accommodate large volume changes during cycling.2.A novel 3 D porous CoP@C-CNTs composite with CNTs interwoven interpenetrating CoP@C nanocubes was designed and prepared.This material proved to be a potential LIBs anode material.The results show that the CoP@C-CNTs composite has a large specific surface area and rich porous structure resulting from the pyrolysis of ZIF-67.CoP nanoparticles are well encapsulated in the ZIF-67-derived nanocube carbon framework and can effectively accommodate volume changes during charge/discharge.In addition,the N/P co-doping induced during in situ synthesis help to improve its conductivity.Moreover,the interwoven CNTs network interspersed with the CoP@C nanocube enables the electrodes to achieve fast electron transport.Electrochemical results show that the CoP@C-CNTs electrode exhibits a high initial discharge capacity of 1254 mAh g^-11 at a current density of 0.1 A g^-11 and an excellent rate performance(The capacity is 532 mAh g^-11 at a current density of 5 A g^-1),as well as excellent cycle stability.At a current density of2 A g^-1,the capacity decay rate per cycle is⁰.02%in 500 cycles.3.A binder-free self-supporting CoP@CC anode is designed and synthesized to obtain high energy density LIBs.The prepared CoP@CC composite integrates the structural advantages of the one-dimensional CoP nanowire array and the excellent mechanical stability and flexibility of the two-dimensional CC substrate for efficient electrolyte infiltration and charge transport.The initial specific capacity of the CoP@CC electrode is 1283 mAh g^-1,and the initial coulombic efficiency（ICE）is83.4%,which is much better than the pure CoP electrode.It is worth noting that the CoP@CC electrode has an outstanding cycle performance and a high capacity retention of about 80.2%after 200 cycles at a current density of 0.1 A cm^-2.Meanwhile,it exhibits excellent rate performance,and the discharge capacity is still549 mAh g^-11 even at a current density of 5 A cm^-2.Our results show that 3 D CoP@CC composite has great potential as a high-performance LIBs binder-free self-supporting anode and next-generation flexible electronic device.4.A novel CoP modified commercial separator was designed and prepared,and applied to a highly efficient Li-S batteries.CoP has the effect of adsorbing and inhibiting the shuttle effect of polysulfide,while enabling polysulfide to achieve rapid electrochemical conversion on the electrode surface.The CoP modified separator also exhibits excellent rate performance and cycle stability in the Li-S batteries.At a low rate of 0.1 C,it exhibits a high specific capacity of 1237 mAh g^-1,and its discharge capacity is as high as 749 mAh g^-11 even at a high rate of 5 C.After 1000 cycles at a high rate of 1 C,the capacity still retains 857 mAh g^-1,and the average capacity decay cycle is as low as 0.034%rate per.The good electrochemical performance exhibited by CoP materials are attributed to the adsorption of polar polysulfides by polar CoP,and the Co-O-P contained in contact with LSP due to a certain degree of oxidation of CoP surface.This oxidizing substance activates the Co site to combine with the negatively charged S.