First-principles Study Of Thermodynamic Function And Electronic Structure On Several Battery Materials

In recent years,all-solid-state batteries are considered to be one of the most promising new electrochemical energy storage systems due to their ultra-high energy density and excellent safety performance.With the rapid development of the all-solid-state battery field and the return of lithium metal anodes,on the one hand,there is an urgent need to find a matching lithium-free transition metal cathode material to break the current energy density limit of solid-state lithium metal batteries.On the other hand,the development of solid electrolyte materials with high ionic conductivity is essential for realizing the fast charge-discharge performance of solid-state lithium metal batteries comparable to traditional organic electrolyte batteries.In view of this,two types of typical cathode and solid electrolyte materials are seleted in this work.The lithium-free transition metal MX₂ family（M:transition metal elements,X:halogen elements）with a huge variety of compounds,and perovskite Li_3xLa_2/3-x□_1/3-2xTi O₃（LLTO,0<x<0.167,□=vacancy）with the highest ionic conductivity(10^-3 S·cm^-1)reported so far.Firstly,we use first-principles calculation and ab-initio molecular dynamics simulation to discuss key basic scientific issues such as structural evolution,charge compensation,and ion transport mechanism in the MX₂ of lithium-free cathode.Then image of the electronic regulation of electrode materials is obtained that to increase the insertion voltage.Secondly,lithium-free electrode materials usually have phase transitions in the voltage regulation process.The perovskite-type solid electrolytes generally have the problem of transitioning to low symmetry phases as the temperature decreases.From the perspective of thermodynamics,we discussed the essential reasons for the phase transition of the above electrode/solid electrolyte system by calculating the material formation enthalpy and vibration entropy based on thermodymamic functions.The main research contents and results are as follows:1.Two new 2H-phase MX₂ lithium-free cathode systems are designed(V_0.5Cr_1.5S₄and VCr S₄)based on the p-type doping regulation strategy to reduce the Fermi energy level of the electrode material,which is combined with the CALYPSO structure search software.First,the dynamic stability of the above two compounds ware determined by phonon spectrum calculation and ab-initio molecular dynamics simulation under high temperature conditions（400 K）.Secondly,the thermodynamic stability of V_0.5Cr_1.5S₄and VCr S₄ is verified by examining the ternary phase diagram of V-Cr-S under the 0 K ground state.Further thermodynamic function calculations results show that V_0.5Cr_1.5S₄and VCr S₄ undergo 2H to 1T phase transitions under 1050 K and 590 K,respectively.That indicating the stability of the 2H phase of the above system in the battery operating temperature range（about 250 K-350 K）.It is worth noting that the initial voltages of2H-VCr S₄ and 2H-V_0.5Cr_1.5S₄ are as high as 2.65 V and 2.48 V,respectively,which the corresponding energy densities can reach 461.75 Wh·kg^-1 and 397.49 Wh·kg^-1.That achieving a breakthrough in the voltage of the existing MX₂ electrode（such as Ti S₂,with an average voltage of 2.41V）,successfully.The NEB calculation results show that the Li⁺migration barrier of VCr S₄ and V_0.5Cr_1.5S₄ is not higher than 0.5 e V in the charge and discharge state structure,which the volume expansion rates are 17.7%and 17.9%,respectively.That proves their excellent rate performance and cycle performance as electrode materials.The effectiveness of p-type doping strategy for designing high-voltage lithium-free cathode materials based on the idea of electronic energy band structure regulation in this work.It has great potential to be extended to other ion desorption/intercalation type lithium-free cathode material designs based on rigid charge transfer.2.The perovskite family has always faced the problem of transforming the cubic phase to other low symmetrical phases with low ion conductivity under certain low temperature conditions.The essential relationship between the rotation/distortion of the Ti O₆ octahedron and the total energy of the system/atomic internal force in the typical LLTO perovskite structure was study by first principle calculations.It is determined that the unstable feature of Ti O₆ octahedron is the direct cause of the phase transition.Therefore,an A-site element doping strategy of LLTO structure was proposed based on the principle of increasing configuration entropy to stabilize the cubic phase structure of the perovskite system from the perspective of thermodynamics.The tolerance factor is used as the criterion.The influence of the valence state of the doping element is comprehensively considered on the configuration entropy.The 155 doping systems containing 78 doping elements were traverse in this study,which finally obtain 17doping elements that can potentially increase the system’s tolerance factor and stabilize the cubic phase.Six of these elements have been experimentally verified.Most importantly,Bi and Rb elements are considered to be the two most promising A-site doping elements for improving the ion conductivity of the LLTO system.This work provides a reference solution for thermodynamic doping for the future design of high ionic conductivity perovskite solid electrolytes.