Structure And Electrochemical Properties Of LiVPO₄F Cathode Materials For Li-ion Batteries

SLiVPO₄Fce1980s, when the LiCoO2as the first possible rechargeable lithium-ionbattery cathode material was reported, the transition metal LiVPO₄Ftercalation oxides havecaught the major reseach LiVPO₄Fterests as lithium-ion battery cathode. Categorized bystructure, the conventional cathode materials LiVPO₄Fclude LiMO2layered structure （M=Co,Ni, Mn, etc.）, LiM2O4spLiVPO₄Fel structure （M=Mn, etc.） and LiMPO4olivLiVPO₄Fe structure （M=Fe, Mn, Ni, Co, etc.）. Most of the researches are performed on these materials andtheir derivatives. The new materials, such as silicates, borates and fluorophosphateshave also been a growLiVPO₄Fg number of developments. The new cathode materials needto consider energy density, rate capability, cycle performance, security, performanceand cost etc. In recent years, the fluorophosphates （LiMPO4F） material as a newcathode material typical has attracted much attentions. From the poLiVPO₄Ft of view elements,LiMPO4F is only more than LiMPO4with one F atom. The LiVPO₄Ftroduction of the F-ionschanges the crystal structure of materials, the results make it possible for Li+ions LiVPO₄F the[100],[010] and [101] three-dimensional diffusion channel. AccordLiVPO₄Fgly predicted,LiMPO4F have the higher ionic conductivity and diffusion coefficient of Li+ions thanLiMPO4.LiVPO4F represents a typical LiMPO4F material, which is similar to natural mLiVPO₄FeralalumLiVPO₄Fum phosphate rock （LiAlPO4F） crystal structure. LiVPO4F have good ionicconductivity, thermal stability and capacity retention etc. As we all know that theelectrochemical properties of the electrode material subject to joLiVPO₄Ft constraLiVPO₄Fts,LiVPO₄FcludLiVPO₄Fg crystal structure, electronic structure, morphology, microstructure, and theelectrode structure etc. multi-level structrure factors. So this thesis based on LiVPO4Fas the center, through first-prLiVPO₄Fciples calculations, we obtaLiVPO₄Fed crystal structure,electronic structure and molecular dynamics; Through electrochemical test andanalysis, we studied the cycle performance and rate performance, and by comparLiVPO₄Fgthe situation of high temperature （55℃） to room temperature （²5℃）, we studied the influence of the temperature on electrochemical performance; For researching phasetransition of LiVPO4F during charging and discharging, we used in-situ HEXRD, in-situXANES and evolving factor analysis（EFA） methods; Through in-situ HEXRD and DSCtests, we studied the thermal stability of LiVPO4F, and concluded the main cause ofgood thermal stability.Firstly, we use the first-principles calculations based on density functional theory,detailed calculations of the crystal structure, electronic structure, electrochemicalproperties and kinetic properties of LiVPO4F/VPO4F systems. LiVPO4F/VPO4F systemson the crystal structure have very stable tetrahedral PO4and octahedral VO4F2withJohn-Taylor effect. The tetrahedron and octahedron connected with sharing O atoms.Lithium ion sited on3D channel surrounded with tetrahedron and octahedron. As John-Taylor effect, there were two kinds of V-O bond length. With moving out Li, the V-O tendto be more uniform and the octahedral distortion effect became weak. The P-O bondlength was no change basically, indicating a stable structure of PO43-ions; it is also thereason of its good cycling performance.The microstructure has a relationship with transition metal ions V3+/V4+electronicconfiguration on LiVPO4F/VPO4F systems. LiVPO4F low spin ferromagnetic state, itstransition metal ions V3+electronic configuration is: t2g（↑）2. When the delithiated, VPO4Fis transformed into antiferromagnetic state at this time of transition metal ions V4+electronic configuration is: t2g（↑）1.Using GGA+U method allows us to successfully get more in line with theexperimental system of LiVPO4F/VPO4F average insertion voltage V3+/V4+:4.29V.By molecular dynamics simulation, LiVPO4F have a good Li-ion diffusion ability.Other atoms under heating conditions, in addition to the tiny expansion of the lattice,are unchanged. It is again illustrated that the structural stability of the material LiVPO4Fis good. And by computational analysis Li ions derived LiVPO4F along the c-axisdirection is to do a one-dimensional diffusion, and the diffusion abilities of the two Liatoms in the unit cell is also different.Secondly, we used solid-phase carbon thermal reduction method to synthesisLiVPO4F, studied the influence of electrochemical perporties by controlling differentanode （lithium metal, Li4Ti5O12） and different temperature （²5℃,55℃）. LiVPO4Fmaterial particle size was about2μm, clusters existed between grains. After removallithium, the particle size was reduced to about1-1.5μm. At room temperature （²5℃）, Li/LiVPO4F discharge plateau was4.2V, and had good cycle performance and poorrate performance; Li4Ti5O12/LiVPO4F discharge plateau decreased to about2.6V. Therate capability and cycle performance were more excellent than Li/LiVPO4F. At hightemperature （55℃）, the viscosity of the electrolyte was affected by temperature,increased the ionic mobility and further increased the utilization of the active lithium,and there has been a tendency to increase the capacity of the first cycle, and evenhigher than the theoretical capacity. Li/LiVPO4F have the good cycle performance atlow rate. The decay rate of capacity was increased with increasing the rate;Li4Ti5O12/LiVPO4F were excellent in cycle characteristics at high rate and poor cycleperformance at low rate. With the increase of the rate, the capacity retention rateincreased. So Li4Ti5O12as anode, Li4Ti5O12/LiVPO4F reduced the voltage plateau （from4.2V to2.6V） and in further to reduce the energy density. But Li4Ti5O12/LiVPO4Fimpoved the charge and discharge capacity, cycle performance and rate performance,and at high rate the influence of cycle performance by temperature was weak.Thirdly, combining the first-principles calculations and the results ofelectrochemical test, we use a variety of methods, including high-energy X-raydiffraction and X absorption spectroscopy, to analyse the existing intermediate phaseLixVPO4F on the process of charging and discharging. And we found a new analysismethod from these, evolving factor analysis （EFA） of XANES data. From the results offirst-principles calculations, the Li atom diffusion abilities on two positions of crystalstructure were different. It could lead to the generation of intermediate phase. From theresults of electrochemical test, there were two plateaus on process of charging. Wespeculated that the intermediate phase might be Li0.67VPO4F. By in-situ HEXRD results,we found the intermediate phase was Li0.67VPO4F on process of charging. By in-situXANES linear combination fitting analysis with fitting error analysis, we found theintermediate phase was Li0.5VPO4F. Because of the great differences between HEXRDand XANES, we made further to EFA of XANES data. And the results were shown thatthe stoichiometric ratio of Li is in the range, LixVPO4F phase, wherein x is0.80-0.25.The conclusionof the above analysis, Li0.67VPO4F and Li0.5VPO4F are within this range.Finally, by DSC and in-situ HEXRD combination, we analysed the structuralchanges in the thermal stability of the cathode material LiVPO4F, and found the reasonthat was better than the lithium transition metal oxide on thermal stability. DSC resultsshowed that the solvent and solute of the electrolyte affected the thermal stability of Li1- xVPO4F. But in comparison, this kind of influence were smaller than the situation oflithium transition metal oxide, and with less heat release. HEXRD results provideevidence that the delithiated material, Li1xVPO4F, reacted with the solvent or theelectrolyte. Like H+or Li+back to delithiated material, it formed a phase like-LiVPO4F.The heat release of this reaction was much smaller than the situation of release O2onlithium transition metal oxide. And LiVPO4F in the presence of electrolyte seems to stillhave the good thermal stability as high as400℃.In short, through this study, we learn more about the structure and electrochemicalproperties, the reaction mechanism of cathode material LiVPO4F charge-discharge andgood thermal stability reasons, which played a good role in guiding the development ofanionic polymer cathode material.