Preparation,Properties And Ion Transport Calculation Of Electrode Materials For Phosphate Batteries

Abundant phosphate compounds are good sources of electrode materials for sodium ion battery systems.The representative NASICON-type phosphate owns advantages of open three-dimensional framework,large ion diffusion channel,high ion conductivity and the inherent shortcomings of low electronic conductivity which is constantly being modified by various methods.There is also a langbeinite-type phosphate,whose structure and composition are quite coincident with NASICON-type phosphate with superior ionic conductivity.Therefore,langbeinite-type phosphate is also a kind of potential super ion conductor.In this paper,we focus on the two kinds of materials:NASICON-type NaTi₂（PO4）₃doped Cr and langbeinite-type Na₂Cr Ti（PO4）₃.The electrochemical properties of the materials are studied by a series of material characterization and electrochemical test,and the D_Na⁺ and energy barrier of the materials are explored by molecular dynamics（MD）simulation.The specific research contents are as follows:（1）Na_1.25Cr_0.25Ti_1.75（PO₄）₃/C sample is synthesized by sol-gel method using sodium nitrate,chromium acetate,butyl titanate,and ammonium hydrogen phosphate as raw materials.It shows good rate performance,cycle performance and capacity retention after electrochemical testing.When the applied current returns to 1C after 1C-7C testing,the specific capacity returns to 128.1mAh g^-1 closed to the value of the first 1C,indicating quite outstanding rate capability.At 1C rate,the reversible capacity is as high as 96.4 mAh g-1 after ultra-long cycle life over 1000 cycles,corresponding to 65.1%capacity retention.（2）The MD method was used to simulate the sodium ion migration mechanism of pristine NaTi₂（PO₄）₃ and Na_1.25Cr_0.25Ti_1.75（PO₄）₃,and to calculate their energy barrier diffusion coefficients(D_Na⁺).On the one hand,it can be found that their migration paths of sodium ions are similar,while the sodium ions in Na_1.25Cr_0.25Ti_1.75（PO₄）₃ lattice are more active than those in NaTi₂（PO₄）₃ lattice.On the other hand,the calculated results show that energy barrier and the estimated diffusion coefficient at RT in the case of NaTi2（PO4）3 are 1.41 eV and 2.2×10-25cm2·s-1,and that of Na_1.25Cr_0.25Ti_1.75（PO₄）₃ are 0.32eV and 1.68×10-9cm2·s-1.The results tells us,doping Cr increases the diffusion coefficients of Na⁺by 16 orders of magnitude and reduces the energy barrier by 1.09 eV.This is supposed to be owing to the presence of the interstice Na⁺and the concerted migration of interstice Na⁺and the M1 site Na⁺.At the same time,the energy barriers of NaTi2（PO4）3 and Na_1.25Cr_0.25Ti_1.75（PO₄）₃ calculated by NEB method are 0.59eV and 0.315eV,respectively.（3）Using MD simulation to study the Na⁺transition mechanism of langbeinite Na₂Cr Ti（PO₄）₃and calculate the energy barrier,D_Na⁺.The sodium ion undergoes a three-dimensional,isotropic diffusion in the crystal lattice.The diffusion path of sodium ions is determined by the environment of large cage of metal ions framework.The sodium ions can only alternately hop between Na（1）and Na（2）,but is not allowed hopping between Na（1）-Na（1）or Na（2）-Na（2）.The energy barrier of Na₂Cr Ti（PO₄）₃ is 0.78 eV,and the D_Na⁺ is 2.16×10^-16 cm²·s^-1 at RT.Compared with the intensively investigated electrode materials of sodium ion battery or aqueous rechargeable sodium batteries,this material has more advantages over them in D_Na⁺.Therefore,we believe that Na₂Cr Ti（PO₄）₃ is also an excellent material for those batteries.