Graphene Anchored Metal Phosphides-based Composites As Anode Materials For Sodium-ion Batteries

Lithium ion battery is an important part of clean energy,but with the rapid growth of energy storage scale,its cost also rises rapidly,so it is very urgent to find a potential alternative with similar energy storage system.Sodium ion battery has attracted wide attention because of its low cost and similar rock-chair battery system,which is expected to replace lithium ion batteries in some fields.However,Na⁺is 55%larger than Li⁺in volume and twice as heavy as Li⁺,resulting in a terrible Na⁺storage performance in commercial grahite anode.Therefore,it is necessary to choose suitable sodium storage materials and design stable structures to store Na⁺more effectively.Due to the high specific capacity and facile preparation process,metal phosphides have attracted extensive attention.However,the huge volume expansion when sodiation of metal phosphides is very unfavorable to the cycle and rate performance,so the key is to construct a pretty stable electrode by combining other carbon based materials.In order to prepare metal phosphides based composites,this research was carried from the following aspects.Firstly,we preliminarily explored the synthesis technological parameters of Ni₂P by using Ni?OH?₂·H₂O and NaH₂PO₂ as raw materials,and we found no surface structures change of the Matel based precursors during the phosphorization process,which is also applicable to fabricate other metal phosphides.Secondly,we fabricated Ni₂P particles anchored on reduced grapheme oxide by solvothermal method and hexamethyltetramine?HMT?low-temperature solution deposition method respectively to improve the sodiation capacity and stability,then compared these two synthesis routes.The results showed that the particle size of Ni₂P synthesized by HMT low temperature deposition method was much smaller,with a size range of 20-100 nm,finally exhibited a much better electrochemical performance.In order to obtain metal phosphides anode materials with higher sodiation capacity,we further synthesized FeP nanoparticles anchored on graphene sheets?FeP@rGO?by HMT low-temperature solution deposition method and low-temperature phosphating process.Then we conducted the comprehensive structural characterizations and systematically investigated the electrochemical properties of the composites.The results showed that the FeP load in FeP@rGO is 95%,and the sodiation capacity still reached to 388.8 mAh g^-1,which was 106%capacity retention relative to the first-cycle charge capacity of 366.6 mAh g^-1 after 250 cycles at the current density of 100 mA g^-1.Remarkably,the specific capacities of 428.4,382.2,353.5,316.0,277.7,243.8 mAh g^-1 were obtained at the current densities of 0.05,0.1,0.2,0.4,0.8 and 1.6 A g^-1,respectively.The high performance can be attributed to the synergetic effects between stable rGO network and FeP nanoparticles.