First-principles Study On The Doping Modification Of Sodium Vanadium Phosphate As Cathode Material For Sodium Ion Batteries

Developing new efficient energy storage systems,featured by rich resources and low cost,has become a hot topic in related scientific and technological fields.Sodium has great cost and resource advantages over lithium.Its physicochemical properties are highly similar to those of lithium since they both belong to alkali metals.Hence,sodium batteries are considered to be the most promising alternative to LIBs.In the past decade,researchers have gradually moved their eyeballs to room-temperature sodium-ion batteries（RT-SIBs）.Sodium storage cathode materials are considered to be one of the key components of RT-SIBs.To improve the electrochemical performance of sodium storage cathodes,numerous efforts have been made and significant results have been achieved.Sodium vanadium phosphate Na₃V₂（PO₄）₃（NVP）,with the structure of a Na-superionic conductor（NASICON）,has a reversible specific capacity of up to 117 mAh g^-1 and a theoretical operating voltage or charge-discharge voltage of up to 3.4 V（vs.Na⁺/Na）,and its Na⁺diffusion coefficient is experimentally measured to be as high as 10^-11 cm² s^-1.NVP is considered to be a well-performing sodium storage anode material.Nevertheless,experimental research finds that the actual specific capacity of pure-phase NVP is lower than the theoretical level while the cycle stability often turns out poor.NVP’s electrochemical performance can therefore be effectively improved by nanocrystallization,carbon coating,and element doping.Adjusting the NVP composition and structure by element doping is one of the most effective ways to boost cyclical reversibility,reversible capacity,and kinetic performance of sodium ion diffusion.It is also suggested in theoretical calculations that doping can change the properties of the lattice to some extent and enhance its stability,electronic conductivity,and kinetic performance of sodium ion insertion and extraction.So far,there has been a lack of understanding of NVP at atomic and lattice levels.For example,the effects of multiple possible dopant atoms,doping sites and doping types on the NVP stability,conductivity,theoretical reversible capacity,charge-discharge platform and sodium ion migration and diffusion behavior are logically unclear.By use of first-principles calculation and simulation,this paper attempts to gain a systematic,in-depth understanding of how polyatomic doping modulates the key physicochemical properties,especially the electrochemical properties of the NVP system.The main research and new understandings and findings of this paper are as follows:1.The kinetic mechanisms for sodium ion migration and diffusion in the NVP system are dissected after a detailed analysis of the NVP lattice structure.There are four different migration routes:direct migration,step-by-step ion-exchange migration,detour migration,and the little-reported concerted ion-exchange migration,that the activation energies are 2.741 eV,1.362 eV,0.721 eV and 0.554 eV respectively.In the concerted ion-exchange migration route,adjacent Na（1）and Na（2）ions start migration simultaneously:while the Na（1）ion moves to the Na（2）site nearby,the Na（2）ion moves to the Na（1）vacancy,achieving energy coupling,making the activation energy markedly lower than other migration mechanisms.Arguably,the concerted ion-exchange migration is the most probable migration mechanism for sodium ions.2.The effects of Na（1）doping of different dopants on the NVP structure and electrochemical properties are investigated.Calculations show that the c-axis length of NVP increases in direct proportion to the radius of dopant atoms Li,“Na”and K,but decreases in inverse proportion to the radius of high-valence-state dopant atoms Ca,Mg and Al;the doped NVP systems have a smaller lattice change than the original NVP during the charge-discharge process,and K-NVP has the smallest lattice change,which is most conducive to more stable NVP charge-discharge structure;Li and K doping both boosts the NVP operating voltage,and K doping is most conducive to higher energy density of NVP as an anode material,while doping of high-valence-state elements leads to a significant voltage drop,and higher valence state goes with a sharper voltage drop.In the case of 5.6%K doping amount,the activation energy of sodium ion migration is0.510 eV,even lower than 0.554 eV of the original NVP,and the rate of sodium ion migration is 5.5 times of the original NVP rate at room temperature;further raising the K doping amount will increase the charge-discharge voltage but the increase is small and unfavorable to the sodium ion migration kinetics.In a nutshell,proper amount of K doping helps to improve the NVP electrochemical performance,which is consistent with the results reported in the literature.3.The effects of V-site doping of transition metals in the fourth period on NVP stability,electrochemical properties and sodium ion migration behavior are systematically examined.（1）The effects of doping on the NVP structural stability.Calculations show that except for Mn and Fe,the atomic size of doped elements has a certain linear relationship with the c-axis length of NVP cells;the Ni-NVP about 5.6%doping cohesive energy decreases with the increase of d-electrons of dopant atoms and at most by 0.7 eV relative to the original NVP,as much as 110.60 eV,so the effects of V-site doping of transition metals on the NVP structural stability are negligible;except for Mn ions producing quite strong Jahn-Teller effect,all the doped systems have a smaller structural change than the original NVP during the charge-discharge process,which helps to improve the structural stability of the NVP lattice during the charge-discharge process.（2）The effects of doping on the NVP electronic structure.Adding dopant elements will result in a significant change in the NVP band gap width.Of the various dopants,Ti,Cr,Co and Ni will significantly reduce the band gap value,which helps to improve the NVP conductivity.The main contribution of dopants is concentrated in the area of redox reaction around the Fermi level,indicating that dopants will take part in the redox process of electrodes so as to determine the electrochemical behavior of NVP.Bader’s charge analysis shows that adding dopants mainly changes the average charge density of V in NVP,and those where d-electrons outnumber Ti will increase the average charge density of V in NVP.The effects of doping on the V charge density are non-local and to some degree delocalized in the entire NVP skeleton.（3）The effects of doping on the NVP operating voltage（vs.Na⁺/Na）and sodium ion migration behavior.Calculations of Na as the battery cathode reveal that except for Ti-NVP with a slightly lower operating voltage,other transition metals have a higher operating voltage than that of the original NVP,of which Co-NVP and Ni-NVP have the highest and second highest operating voltage and their energy density is 17.31Wh kg^-1and 13.75 Wh kg^-1 higher than that of the NVP sodium ion battery;the effects of dopant elements on the sodium ions migration kinetics is non-local,which means the activation energy of the doped systems is far higher than the original NVP regardless of the distance of sodium ions to dopant atoms,which is not conducive to rate performance.Although Ni-NVP is second to Co-NVP in charge-discharge voltage,the activation energy of sodium ion migration in Ni-NVP is only 0.043 eV higher than that of the original NVP.In this regard,Ni doping can balance the operating voltage and sodium ion migration and thereby enhance the NVP electrochemical energy storage.4.The effects of P-site doping of B,Al and Ga on the NVP stability and electrochemical properties are investigated.The P-site doping of B with a smaller radius may lead to NVP lattice shrinkage,markedly increasing the NVP stability and improving the NVP charge-discharge voltage,but lattice shrinkage exerts adverse effects on the sodium ion migration kinetics,and lattice shrinkage caused by excessive amount of B doping will narrow the ion migration channel and make sodium ion migration more difficult.The P-site doping of Al and Ga will enlarge the lattice.Despite a slightly lower cohesive energy than the original NVP;doping can improve the NVP operating voltage to a certain extent,and with the increase of doping amount,the charge-discharge voltage will be further improved;the activation energy of sodium ion migration will decrease with the increase of the radius of dopant atoms,and in Ga-NVP with 5.6%doping amount,sodium ions migrate about 800 times faster than those in the original NVP.5.The effects of co-doping Ni and Ga at V and P sites respectively on the NVP structure and electrochemical properties are discussed.The NVP is doped with the best performing Ni in V-site doping and the best performing Ga in P-site doping.The cohesive energy of NiGa-NVP with 5.6%Ni amount and 5.6%Ga amount is 1.2%lower than that of the original NVP,with negligible adverse effects on structural stability,and its operating voltage is as high as 3.492 V,an increase of 260 mV over the original NVP,and its energy density is 30.80 Wh kg^-1 higher than 380.07 Wh kg^-1 of the NVP sodium ion battery;meanwhile,the activation energy of sodium ion migration in the co-doped system is only 0.103 eV,much lower than that of the original NVP,the sodium ion migration rate is about 37 million times that of the original NVP,exhibiting superb rate performance,and the band gap value is only 1.574 eV,displaying great electronic conductivity.In conclusion,this paper adopts the calculation and simulation of first-principles density functional theory for a systematic and comprehensive survey of the effects of doping heterogeneous elements at Na（1）,V and P sites on the sodium storage anode material NVP’s structure,electronic conductivity,operating voltage（vs.Na⁺/Na）,ion migration kinetics and other properties,which provides reliable theoretical guidance for debugging of dopants,doping forms and doping ratios in NVP experiments.At the same time,this paper gives feasible suggestions for developing NVP with high energy density and high stability,which promotes NVP research and development in sodium ion batteries,hybrid ion batteries,water batteries and hybrid supercapacitors and other areas of electrochemical energy storage.