| The olivine structure LiMPO4(M=Mn,Fe)have many advantages,such as high capacity,good safety,long cycle life,environmental friendliness,etc.,and it is considered to be one of the new generation lithium ion battery cathode materials with the most development prospects and industrial application potential.This type of material is especially suitable for electric vehicles and energy storage equipment that have high requirements for the safety and long-cycle stability of the power battery system.At present,LiFePO4 has been realized commercial application.However,it still needs to be improved in terms of high-rate fast charge and discharge performance and energy density(voltage platform 3.4 V),which is also difficult to meet the current energy and large the demand for rapid charge and discharge rate.On the basis of high stability and safety,LiMn PO4(voltage platform 4.1 V)can theoretically provide an energy density of more than 20%higher than that of LiFePO4.However,the extremely poor electrical conductivity(almost insulating)makes its high energy density difficult to be sufficient released,and it leads to poor rate performance.Therefore,a solid solution method is designed,which uses a part of Mn is added to LiFePO4 to form LiMnxFe1-xPO4 is used to increase the energy density of the material.Due to the particle surface morphology,dispersibility,effectiveness of carbon coating,crystallinity and crystal structure of the material play a crucial role in the electronic conductivity of the material and the diffusion performance of lithium ions.Therefore,the energy density,conductivity and rate performance of the material are improved.These are achieved by using LiMnxFe1-xPO4calculation of energy density utilization rate,optimization and design of synthesis process conditions,improvement of particle dispersion,and a combination of multiple synthesis methods.In this study,LiMnxFe1-xPO4 was used as the research object,from the perspective of improving the dispersion of the material and then the conductivity of the material,to improve the energy density and rate performance of the material,and the high temperature solid phase method was used as the basic synthesis methods:The well-dispersed LiMnxFe1-xPO4/C(x=0.1,0.3)was synthesized by conventional solid-phase method,multi-step temperature-programmed solid-phase method,vacuum-assisted solid-phase method,and co-precipitation-solid-phase combined method.A horn-shaped stacked LiMnxFe1-xPO4/C(x=0.2)with excellent crystallinity and exposed 101 crystal plane LiMnxFe1-xPO4/C(x=0.5)material was obtained.In LiMnxFe1-xPO4/C material,the energy ratio distribution of Mn(Mn3+/Mn2+electric pair discharge specific energy)and Fe(Fe3+/Fe2+electric pair discharge specific energy)and the law of material energy utilization rate have been systematically studied;And the influence of a multi-step temperature program setting based on the solid-phase reaction process on the dispersion and conductivity of the material has also been studied;At the same time,vacuum-assisted solid-phase synthesis of special horn-shaped stacked morphology materials and morphology evolution history are analyzed;In addition,the precursor structure and morphology of the material prepared by the coprecipitation solid phase combination method and the performance of the cathode material have been paid attention.The main work carried out is as follows:1.The main parameters of the conventional solid-phase synthesis process were optimized,and the effects of sintering temperature,sintering time and the ratio of Mn and Feon the material properties were explored.It was found that the material obtained at sintering temperature of 650?and sintering time of 10 h has good dispersibility and small particle size,and the material has the best electrochemical performance.The discharge specific capacity be promoted to 140.9 and 107.3 m Ah g-1 at current densities of 0.1C and 5C.Rietveld method was used to refine the XRD diffraction patterns of materials with different ratios of Mn and Fe,and it was found that the unit cell parameter values increased with the increase of the ratio of Mn,that show a linear relationship with the value of x.It can be seen that the synthesized material has a solid solution structure.The d V/d W differential method is used to divide the energy contribution value of Mn and Feto the material,and the distribution law of the energy value is analyzed.It is found that the energy utilization rate of Fe(83.4-94.4%)is much higher than that of Mn(54.6-61.1%),which is better the dispersibility of the particles can improve the energy utilization rate of both at the same time.2.According to the changing characteristics of the reaction system with temperature,a multi-step temperature program system is set up to improve the dispersion of the synthetic material,and the carbon coating effect,and also increase the electrical conductivity of the material,In addition,it increases the energy density and rate performance of the material.In this paper,XRD,TG/DTA and other analytical methods are used to study the phase structure and thermal decomposition characteristics of the mixed raw materials at different temperatures.The synthesis process of the solid-phase reaction is summarized,that is,the reactants are dehydrated and decomposed at a specific temperature to form LiMnxFe1-xPO4.The temperature control program in the synthesis conditions is optimized according to the reaction process,and the original one-step rapid heating system is adjusted to a multi-step slow heating system to synthesize the required cathode material.Through the FESEM and HRTEM test,it is found that the dispersibility of the synthetic material after the adjustment method is significantly improved,and the surface of the material is smooth and the particle size is smaller,the carbon coating effect and its crystallinity are better,and the good carbon coating effect is beneficial to the improvement of electrical conductivity.The higher crystallinity and small particle size are conducive to the diffusion of lithium ions inside the material,so the material shows excellent electrochemical performance.After optimizing the process,the specific discharge capacity of the synthesized LiMn0.3Fe0.7PO4/C at 0.5 and 5C are 145.3 and129.7 m Ah g-1.The specific discharge energy at 2C is also increased from 467.0 Wh kg-1by 506.0 Wh kg-1.3.The LiMn0.2Fe0.8PO4/C(V-Ar/V-V)composite material was synthesized by vacuum-argon and two-step vacuum-assisted solid-phase method,and the morphology evolution law of the product was analyzed in detail using FESEM and HRTEM methods.Different from the typical spherical shape,the secondary particles of V-V material prepared under vacuum are composed of unique layered stacked angular particles,and have many scattered connection through holes.Because the vacuum has a special effect on the gas generation reaction,and the material has a special morphology.A detailed analysis of the structure of different materials,carbon layer structure and carbon content,morphology,electrochemical performance and electrode reaction kinetics and other related properties through a variety of characterization methods.The results show that this special morphology and structure can effectively immerse the electrolyte into the material.This is because the diffusion resistance of lithium ions and electrons at the interface between the material and the electrolyte is reduced,so that the electrochemical reaction activity is improved.The calculation results show that the specific discharge capacity of the materials obtained under 10 C current density of Ar-Ar and V-V are 35.0 and 88.6m Ah g-1,respectively,and the discharge specific energy at 2C current density are 361.0and 429.6 Wh kg-1,respectively.The energy utilization rates of the two materials prepared under Ar-Ar and V-V are 81.2%and 85.1%,respectively,and the apparent conductivity is 1.87×10-5 S cm-1 and 3.35×10-5S cm-1,respectively.4.The?-phase oxalate precursor Mn0.5Fe0.5C2O4·2H2O,which belongs to the monoclinic system,was synthesized by co-precipitation,which proved the existence of Mn-O-Febond in the precursor material and its low-temperature sintered oxide.Compared with the conventional solid-phase method,the rate performance of the precursor is significantly improved when the precursor is used to prepare the cathode material,especially at a high rate of 10 C current density,the specific discharge capacity of the two are 62.4 and 108.6 m Ah g-1,the specific discharge energy is 137.6 and 350.6Wh kg-1,respectively,the conductivity of the material and the lithium ion diffusion coefficient are respectively from 2.32×10-5 S cm-1 and 4.46×10-14 cm2 s-1 to 2.46×10-5 S cm-1and 5.09×10-14 cm2 s-1.The electrochemical kinetics test results of the material show that,based on perfect crystallinity,the material prepared by the combined method has excellent electrochemical reaction reversibility.The chemical kinetic test showed that the material prepared by the combined method with good crystallinity has good electrochemical reaction reversibility,the electrochemical reaction overpotentials of Fe3+/Fe2+and Mn3+/Mn2+pairs are 0.20,0.25 V(combined method)and 0.27,0.30 V(solid phase method),respectively.It can be seen from the HRTEM analysis results that the material prepared by the co-precipitation method to synthesize the precursor exposes the(101)crystal plane.This crystal structure material can inhibit the mixed arrangement of Fe(Mn)-Liand It can make the transmission of lithium ions inside the material more smoothly.The scientific issues in the influence of Mn doping and material's dispersion on the electrochemical performance have been thoroughly and systematically studied,which provides a reference for the research on the preparation of high-performance lithium ion battery cathode materials. |
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