Study On Dry Synthesis And Mechanism Of Li-Rich Fe-Mn Cathode Materials

With the rapid development of new energy vehicles,the Lithium-ion power battery market has grown dramatically,and the lithium,cobalt,nickel and other resources is in great demand.The price of cobalt,nickel and other raw materials required for the production of lithium-ion batteries has risen rapidly in recent years.The resources of cobalt and nickel in China are poor and mainly imported from abroad,which directly affects the development of new energy industry in China.In terms of resources and performance,lithium-rich Fe-Mnbased cathode material x Li₂MnO₃·（1-x）LiFe O₂has high theoretical specific capacity（～300 m Ah/g）,without cobalt and nickel,low cost and environment-friendly.It will be a great application potential material for lithium-ion batteries in power and energy storage area for future.However,the low solid solubility of Fe-Mnsystem,Li/Fe mixing and the variation of valence states of Fe in Li-rich Fe-Mnmaterials lead to its complex microstructure,difficult control of synthesis conditions,poor electrochemical properties and structural stability.At present,the current methods include co-precipitation-hydrothermal-calcination three-step method,low-temperature molten salt method,and the modified Pechini method;however,there exist complex systhesis steps and control conditions.Therefore,combined with high-energy ball milling and solid-state synthesis methods,this thesis explores the simple and practical dry synthesis route and studies related mechanism of lithium rich Fe Mnmaterials.Firstly,using Li₂MnO₃and LiFe O₂synthesized at medium low temperature as precursors,Li-rich Fe-Mncathode materials were directly prepared by dry synthesis through high-energy ball milling and heat treatment.Studies have found that with the calcination temperature increases,the crystallization and ordered layered structure are improved,but some peaks of LiFe₅O₈spinel phase appears in XRD patterns from600℃.The results of selected area electron diffraction（SAED）,element mapping（EDS）and vibrating sample magnetometer（VSM）demonstrate convincingly the growth progress of LiFe₅O₈as the temperature rises.In addition,the material calcined at low temperature shows high activity but poor cycle stability.Increasing temperature,due to the growth of LiFe₅O₈spinel phase,the reversible capacity decreases greatly,but the cycle stability is improved.Subsequently,the synthesis temperature of precursor Li₂MnO₃was increased to 650℃and 750℃,which is expected to improve the crystallinity,inhibiting the aggregation of Liand Fe and thus hinder the growth of spinel phase.XRD and VSM tests show that the improvement of crystallinity of Li₂MnO₃can effectively inhibit the formation of LiFe₅O₈spinel phase and improve the material properties:there is no spinel phase at 600℃,and the specific capacity of initial discharge is up to 230 m Ah/g,but the capacity decays to 120 m Ah/g after 50cycles.For the sample calcined at 650℃,there is a small amount of LiFe₅O₈,the initial capacity is 160 m Ah/g,but stabilizes at about 160 m Ah/g after 50 cycles.Therefore,calcination temperature is a key factor can control the phase growth and balance the electrochemical activity and stability.At the same time,Fe-Mnbased Li-rich material with acceptable electrochemical properties were prepared successfully by dry synthesis.It makes the practical application of Fe-Mnbased Li-rich material more achievable.Based on the above research,in order to avoid the formation of LiFe₅O₈phase,improve the cycle stability and activity,we further studied the effect of low Fe content on synthesis and electrochemical properties of Fe-Mnmaterials.Using dry synthesis,a series of（1-x）Li₂MnO₃·x LiFe O₂with Fe/Mnratio of 3/7,2/8 and 1/9 were synthesized.The Fe/Mnratio decreased from 3/7 to 2/8 and 1/9,the calcination temperature increased from 600℃to 650℃and 700℃,LiFe₅O₈spinel phase did not appear both in XRD and VSM tests.Moreover,the crystallinity and electrochemical cyclic stability of Fe-Mnmaterials has been improved.It is noteworthy that the reduction of Fe content does not significantly reduce the initial capacity of the materials,and 3/7,2/8 and 1/9 samples have the best electrochemical properties at600℃,650℃and 700℃,respectively.The initial discharge specific capacities are225 m Ah/g,210 m Ah/g and 206 m Ah/g.Electrochemical test shows the sample with higher Fe content had smaller voltage drop,with low initial irreversible capacity loss and high initial coulomb efficiency.There exists less Mndissolution after 100 cycles by ICP test.The Fe content of 1/9 samples is very small;however,the charge-discharge capacity is 110 m Ah/g higher than that of pure Li₂MnO₃.The d Q/d V plots show that the peak intensity of O oxidation of a small amount of Fe doped sample is higher than that of Li₂MnO₃sample,and with increasement of Fe/Mnratio,the potential of O oxidation moves to low potential.Therefore,Fe doping is conducive to activating O^2-to participate in redox.Finally,based on density functional theory（DFT）calculation,the charge compensation mechanism of Li₂MnO₃and Fe-Mnmaterials and the activation effect of Fe doping on lattice O were investigated.The results show that in Li₂MnO₃system,removing 6%lithium,corresponding charging to 4.4 V,there is intensive DOS of O2p crosses the Fermi level.In Fe-Mnsystem,when remove 6%lithium,corresponding to about 4.17 V,the DOS of Fe 3d and O 2p cross the Fermi level at the same time.At33%delithiation,corresponding to about 4.4 V,the DOS of Fe 3d does not change,the DOS of O 2p continues to cross the Fermi level,and the intensity of DOS increases significantly,indicating that after the oxidation of Fe³⁺,more O^2-participates in the oxidation reaction.In addition,with different charging cut-off voltages,the d Q/d V plots show that the reduction peak of O^n-/2-in Li₂MnO₃sample appears at cycle of 4.65 V cut-off voltage.While the O^n-/2-in Fe-Mnsample appears at cycle of 4.4 V cut-off voltage.Proving that Fe doping makes O^2-activated below 4.4 V and participates in charge compensation.In addition,the differential charge density calculation at delithiation is carried out to study the charge change of each element.Compared with Li₂MnO₃,there are charge flow of O^2-not only in Li-O-Liconfiguration,but also in Li-O-Fe configuration in Fe-Mnsystem.Moreover,O^2-bonded with Fe has charge flow.DFT calculation explained the reason for Fe doping improves the activity and cyclic stability of materials,clarifies the electrochemical reaction mechanism of Fe-Mncathode materials.