Preparation And Electrochemical Properties Of CoP/carbon Based Materials

Cobalt phosphide（CoP）is an inactive transition metal phosphide based on a conversion-type mechanism,with the advantages of high theoretical specific capacity,low polarization and low voltage plateau,as an ideal anode material for lithium-ion batteries（LIBs）is a hot topic of research in recent years.However,the low electrical conductivity and large volume change during cycling of CoP prevent its excellent lithium storage performance.To address the above problems,this paper constructs structurally stable CoP/carbon-based composites by compounding CoP nanoparticles with different conductive carbon materials through a simple one-step method to improve ion/electron conductivity and alleviate volume expansion to obtain high-capacity and long-life lithium storage properties,which are studied as follows.（1）CoP@NPC+CNTs anode materials with a nitrogen,phosphorus co-doped tri-carbon synergistic cross-linked network structure were prepared by a one-step self-templating method.The material relied on the hydrogen bonding of phytate to melamine and the coordination ability to Co²⁺for self-assembly into two-dimensional sheet precursors,which was dried and then simultaneously carbonized and phosphatized at900°C to produce a carbon-coated CoP homogeneously embedded in a composite carbon skeleton of NPC and CNTs.It is shown that the tri-carbon synergistic network structure can effectively suppress the volume change of CoP,promote the structural stability,and provide multi-dimensional channels for rapid ion/electron transport.In addition,the N,P co-doping synergy can enhance the material conductivity and provide a large number of reactive sites to enhance the lithium ion storage capacity.Compared with carbon nanotube-free CoP@NPC and pure CoP anode materials,CoP@NPC+CNTs have smaller particle size,better structural integrity,high capacity(up to 1074.5 m Ah g^-1 after 280cycles at 0.1 A g^-1)and long cycling performance(701.7 and 581.5 m Ah g^-1 at 1.0 A g^-1 and2.0 A g^-1 after 1000 cycles).（2）CoP@Nitrogen-Phosphorus-Sulfur triple-doped honeycomb porous carbon（CoP@NPSC）anode materials were prepared by a one-step foaming method using maltose with in situ carbonization ability as the carbon source,thiourea as the nitrogen and sulfur source,and phytate as the phosphorus source.It is shown that N,P,and S elements contribute to enhance the lithium storage performance in charge transfer kinetics,ion transport capacity and pseudocapacitance contribution,respectively,while the synergistic effect of the three elements further optimizes the CoP@NPSC pore structure and electrochemical reaction active sites.Moreover,CoP is stably embedded in the pore channel by forming surface chemical bonds with honeycomb porous carbon,and the carbon sheets are connected by triangular branches to enhance structural stability and effectively suppress CoP volume expansion.The electrochemical performance study shows that CoP@NPSC delivered an ultra-high reversible capacity of 1237.7 m Ah g^-1 at 0.5 A g^-1,and the capacity remained stable of 280.3 m Ah g^-1(5.0 A g^-1)and 241.8 m Ah g^-1(10.0 A g^-1)after 5000 cycles at high current densities,exhibiting ultra-long cycle stability.（3）To address the problems of weak connection strength and non-uniform distribution between CoP and graphene,highly stable three-dimensional porous structure of CoP/N,P co-doped graphene（CoP/NPG）anode material was prepared by amino-functionalized graphene in a one-step method.After functionalization of graphene with ethylenediamine,the negatively charged phytate adsorbs Co²⁺uniformly on the graphene surface by electrostatic attraction,and the CoP nanoparticles are uniformly wrapped by NPG sheets with high electrical conductivity and abundant active sites.Meanwhile,C-P and Co-O-C surface chemical bonds were established between CoP and NPG,which improved the structural strength and promoted fast electron/ion transport,effectively alleviating the volume expansion of CoP.Compared with CoP/PG and pure CoP anode materials,CoP/NPG has better uniformity and bond strength,exhibits excellent lithium storage capacity(917.9 m Ah g-1 at 0.5 A g^-1 after 600 cycles),ultra-long cycle stability(434.8 and350.5 m Ah g^-1 at 3.0 and 5.0 A g^-1 after 3000 cycles)and fast pseudocapacitive charge storage capability(up to 97%pseudocapacitive contribution at a scan rate of 2.0 m V s^-1).All of the above CoP/carbon-based materials exhibit excellent lithium storage properties,and their unique highly stable structural designs and simple synthesis methods provide an effective strategy for the development of high-capacity and long-life LIBs anode materials.