| As the key components of lithium ion batteries,anode and cathode materials are very important for improving the performance of the fabricated lithium ion batteries.High temperature solid state sintering is a common method for preparing electrode materials on a large scale.However,there are some issues to be addressed in high temperature solid state sintering method:1.Solid state method needs high sintering temperature leading to large energy consumption.2.Solid phase diffusion is difficult to achieve trace and precise ion doping.3.Solid state method has limited control on the microstructure of electrode materials.In order to address these problems,the development of the electrospinning method has received widespread attention due to its controllability of electrode material morphology,realizability of trace and precise ion doping and low sintering temperature during preparation process,however,due to the spinning principle limit of electrospinning,the high proportion of polymer and inorganic salt in the precursor solution and low solution injection rate result in low production efficiency,which is not conducive to large-scale preparation for practical applications.Secondly,there is high voltage static electricity in the process of electrospinning,which is a potential safety hazard for the material preparation.In present work,we developed a new type solution blow spinning method for electrode preparation,which integrates the advantages of high temperature solid-state sintering method and electrospinning.Based on our developed solution blow spinning technique,the high speed air flow to make the shear force to overcome the surface tension on the surface of the solution,the spinning process is no longer restricted to dielectric constant of the solvent,which expands the precursor solution selection range.Furthermore,the obtained fibers can be collected in any material collector,enabling efficient large-scale preparation.In addition,the reduction of sintering temperature also reduces production costs.Moreover,ion-level precursor mixing can achieve trace and precise doping of elements,which is of great significance for scientific research and product consistency.Based on the new solution blow spinning method,we are committed to the assembly of high energy density and high power density lithium ion batteries,and systematically study the high voltage lithium cobalt oxide cathode,the new type of fast charging niobium titanium oxide anode and 5 V high voltage spinel LiNi0.5Mn1.5O4.The main research contents are summarized as follows:1.The traditional sol-gel method and solid state reaction method have the problems of uneven doping and uncontrollable doping amount.Based on this,we use a new solution blow spinning method to prepare the Mn,La precisely co-doped and Ti-riched layer coated lithium cobalt oxide(LiCoO2,LCO)cathode material.Firstly,manganese acetate,lanthanum nitrate and tetrabutyl titanate were directly added into the solution to stir and form transparent and uniform precursor solution.Then,by adjusting the flow rate of compressed air and the pushing speed of precursor solution,the precursor fibers with uniform size distribution were obtained.Finally,Mn,La precisely co-doped and Ti-rich layer coated lithium cobalt oxide cathode material(MLT-LCO)was obtained by sintering in air.In order to verify that our co-doping strategy improves the performance of high voltage lithium cobalt oxide,we test its cycle stability and rate performance.The cycle performance of the co-modified LCO cathode has been greatly improved,and the capacity retention is 83%after 300 cycles at 0.3C.Moreover,it has excellent rate performance(1.85 mAh cm-2 at 2C).Our solution blow spinning co-modification strategy will provide an effective way to prepare high performance cathode materials.2.With the development of solution blow spinning electrode material preparation strategy,we further preparaed multi-scale designed niobium titanium oxide anode,from crystal structure modification to nanometer carbon surface carbon coating,and to the whole electrode scale 3D fast ion/electronic conduction network design,which achieved the fast charging lithium ion batteries.The introduction of O2-vacancy in the TiNb2O7-x crystal structure resulted in the formation of low-value cations(Ti3+and Nb4+ions)with larger radius,which resulted in the expansion of cell volume and significantly increased the lithium ion diffusion coefficients of TiNb2O7-x and TiNb2O7-x@C samples.On the scale of 1D fibers material,uniform carbon coating with nanometer thickness can greatly improve the intrinsic conductivity of the material.At a larger electrode layer,1D TiNb2O7-x@C fibers synthesized by a novel solution blow spinning technique were bonded together during electrode preparation process to form a high-speed ion/electron transport network,which effectively reduces the polarization within the electrode.Finally,the multi-scale designed TiNb2O7-x@C anode and the previously prepared co-modified MLT-LCO cathode were assembled into a full cell,which showed excellent fast charging performance(3C rate time surface capacity of 1.55 mAh cm-2)and cycle stability(1C cycle 50 surface capacity of 2.21 mAh cm-2).3.In view of the problem of low output voltage(～2.3 V)of TiNb2O7-x@C/LiCoO2 full cell,we further selected the cathode of 5 V high voltage lithium nickel manganate(LiNi0.5Mn1.5O4)as the research object to carry out the preparation optimization work of related materials.Based on the solution blow spinning method,Cr,Fe and Cu were co-doped in to LiNi0.5Mn1.5O4(CFC0.5-LNMO).On the one hand,the selective doping of elements(octahedral 16C and tetrahedral 8A sites)ensured the stability of the crystal structure of LiNi0.5Mn1.5O4 and protected the surface of the material during the charge-discharge process.Meanwhile,the intrinsic lithium ion diffusion and electronic conductivity of LNMO were improved.On the other hand,the thermal stability of the material has also been improved.In order to verify that our co-doping strategy improves the performance of LiNi0.5Mn1.5O4 at 5 V high voltage,we test its cycle performance and rate performance.CFC0.5-LNMO electrode showed excellent rate performance(1.75 mAh cm-2 at 1C)and long cycle stability(78%at 300 cycles at 0.5C)at high surface capacity of 3 mAh cm-2.In summary,based on the advantages of precise doping,morphology regulation and large-scale preparation of the solution blow spinning method,we started the thesis work from the precise doping optimization of the electrochemical performance of LiCoO2 at 4.5 V high voltage,and then designed and prepared the multi-scale designed TiNb2O7-x@C anode,which was further matched with the co-modified LiCoO2 cathode to fabricate high power density full cell.Then,based on the problem of low output voltage of TiNb2O7-x@C/LiCoO2 full cell,we further studied the 5 V LiNi0.5Mn1.5O4,and improved its electrochemical performance at high areal capacity by precise co-doping modification.The study fully demonstrated the universality and unique advantages of the solution blow spinning method in the preparation of anode and cathode materials,and provided a new route for the development and preparation of new electrode materials. |
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