Doping Structure Manipulation Of BaSnF₄ Solid-State Electrolyte For Fluoride-Ion Batteries Based On First-Principles Calculations

BaSnF₄,as a typical solid electrolyte material for fluoride-ion batteries,has attracted extensive attention of researchers due to its special layered structure,good room temperature ionic conductivity and mechanical properties.However,the room temperature ionic conductivity(10^-4S/cm)of BaSnF₄is still lower than the practical standard(10^-3S/cm),which limits its application in fluoride-ion batteries.Cationic doping is an effective method to improve the ionic conductivity of materials.However,there exist randomness and complexity in experiment,and the cationic doping mechanism of BaSnF₄needs to be further elucidated.Therefore,in this paper,the mechanism of cationic doping of BaSnF₄has been systematically studied from the atomic scales by first-principles calculations.The specific content is as follows:（1）Mechanism of Nd doping on the crystal structure of BaSnF₄.Firstly,four doping models of Ba_1-xNd_xSnF_4+x（x=0.02,0.0625,0.125,0.25）were constructed.From the formation energies,we know that Ba_1-xNd_xSnF_4+x（x=0.02,0.0625,0.125,0.25）was the easiest to be synthesized with Nd₅Sn₃as Nd doping material.And Ba_0.875Nd_0.125SnF_4.125has the best ductility among Ba_1-xNd_xSnF_4+x（x=0,0.02,0.0625,0.125,0.25）.By calculating the fluoride-ion diffusion barriers and band gaps of Ba_1-xNd_xSnF_4+x（x=0,0.02,0.0625,0.125,0.25）,it is found that Ba_0.875Nd_0.125SnF_4.125not only has the lowest diffusion barriers,but also has good electronic insulation.Then,the thermal stability and diffusion kinetics of Ba_0.875Nd_0.125SnF_4.125at different temperatures were further studied by AIMD.The results showed that Ba_0.875Nd_0.125SnF_4.125not only has good thermal stability,but its fluoride-ion conductivity increases exponentially with the increases of temperature;More importantly,the fluoride-ion conductivity of Ba_0.875Nd_0.125SnF_4.125is 5.35×10^-3S/cm at room temperature（300K）,much higher than that of BaSnF₄(3.40×10^-4S/cm).In addition,Ba_0.875Nd_0.125SnF_4.125has good electrochemical stability.（2）Mechanism of Gd doping on the crystal structure of BaSnF₄.Firstly,Gd Sn₂is an optimal Gd source after calculating the formation energies of Gd doped BaSnF₄.Moreover,among Ba_1-xGd_xSnF_4+x（x=0.0625,0.125,0.25）,Ba_0.875Gd_0.125SnF_4.125has the best ductility and the lowest fluoride-ion diffusion barrier.Then AIMD calculation analysis was performed,it was found that Ba_0.875Gd_0.125SnF_4.125has higher fluoride-ion conductivity than BaSnF₄at room temperature(4.03×10^-3S/cm vs 3.40×10^-4S/cm).In addition,the voltage platform of Ba_0.875Gd_0.125SnF_4.125is good.（3）Mechanism of Tb doping on the crystal structure of BaSnF₄.First of all,by comparing the formation energies,it can be found that Ba₃Tb can be used as raw material to introduce Tb element into BaSnF₄crystal more effectively.And the Ba_0.875Tb_0.125SnF_4.125has the maximal B/G value.The electronic structure and diffusion barriers of Ba_1-xTb_xSnF_4+x（x=0.0625,0.125,0.25）were analyzed to screen out the optimal doping structure as Ba_0.875Tb_0.125SnF_4.125.Meanwhile,Ba_0.875Tb_0.125SnF_4.125has good fluoride-ion conductivity at room temperature(4.37×10^-3S/cm).At the same time,Ba_0.875Tb_0.125SnF_4.125has a stable voltage platform when the anode materials is Baor Tb.（4）Mechanism of Dy doping on the crystal structure of BaSnF₄.Firstly,Ba₃Dy,with the lowest formation energies,can provide the optimal Dy source for synthesis of Ba_1-xDy_xSnF_4+x（x=0.0625,0.125,0.25）.By calculating the mechanical properties,it was found that Ba_0.875Dy_0.125SnF_4.125has the best ductility.Further comparing the electronic structure and fluoride-ion diffusion barrier of Ba_1-xDy_xSnF_4+x（x=0.0625,0.125,0.25）,the optimal Dy doping concentration was determined to be 12.5%.At the same time,fluoride-ion conductivity of Ba_0.875Dy_0.125SnF_4.125at room temperature is better than that of BaSnF₄(4.72×10^-3S/cm vs 3.40×10^-4S/cm).Furthermore,Ba_0.875Dy_0.125SnF_4.125possesses good electrochemical stability.