| Enhancing the energy density,cycle life,and safety performance and reducing the cost are key factors to further extend the application of lithium-ion batteries(LIBs).The cycling performance of cathode materials is closely related with the crystal structure,particle size distribution,and the surfacial or interfacial properties.Irn this dissertation,a variety of Ni-rich cathode materials with different compositions are synthesized from core-shell structured precursors,and the variation of precursors’ structures during high-temperature calcination and its effect on electrochemical performance are investigated;The CSTR-Batch series connection precipitation technique is developed to prepare Ni-rich precursors with narrow particle size distribution;Based on the difficulty of metal ions with larger ionic radius to diffuse into the inner particles of LiMn2O4 materials,the interfacial modifications of LiMn2O4 is realized using one-time sintering process,thus the energy density and cycling performance are enhanced.The specific research contents are shown as below:(1)Three LiNi0.88Co0.12O2 materials were synthesized from three different structured precursors,namely the completely coated 0.88Ni(OH)2@0.12Co(OH)2,the semi-coated&semi-doped 0.94Ni0.936Co0.064(OH)2@0.06Co(OH)2,and the completely doped Ni0.88Co0.12(OH)2,respectively.Unexpectedly,the core-shell structures of the completely coated and semi-coated&semi-doped precursors disappears,and the radial concentration distributions of Ni and Co are almost uniform in spherical LiNi0.88Co0.12O2 particles obtained by sintering with LiOH-H2O,no matter the precursor is coating type or doping type.Even so,the LiNi0.88Co0.12O2 material synthesized from the 0.88Ni(OH)2@0.12Co(OH)2 precursor still has the best electrochemical properties.The reason is that the cobalt hydroxide coating layer on the surface of precursor acts as a barrier to reduce the diffusing rate of lithium ions into the core material,which leaves enough time to make Ni2+ be oxidized to Ni3+ as many as possible and reduce the Li+/Ni2+ cation mixing.This provides valuable references to synthesize Ni-rich trinary cathode materials with excellent cycling performance.(2)The difference of electrochemical properties in the LiNi1-xCoxO2 materials synthesized fr-om the coating-type and doping-type precursors as a function of the cobalt content is systematically investigated.The testing results of the electrochemical performance demonstrate that the difference of electrochemical properties of the final products obtained from different structured precursors gradually diminished with the increasing of the the cobalt content.When x<0.12,the electrochemical performances of LiNi1-xCoxO2 materials from the coating-type precursors are better than the materials fr-om the doping-type precursors.When x>0.12,the electrochemical performances of LiNi1-xCoxO2 materials from the doping-type precursors are better than the materials from the coating-type precursors.(3)The LiNi0.88Co0.12-xMnxO-2(x = 0.0,0.06,0.12)materials were prepared from Ni(OH)2@Co1-yMnyCO3(y = 0.0,0.5,1.0)precursors.Ni and Co are almost uniform in LiNi0.88Co0.12O2 particles,while Mn exhibits concentration gradient from the top surface into the core in spherical LiNi0.88BMn0.12o2 particles,which demonstrates that the inter-diffusion rate between Ni3+/Co3+ is bigger than that between Ni3+/Mn4+.Besides,it is found that the LiNi0.88Mn0.12 2O2 material synthesized from the doping precursor has better electrochemical properties than that from the core-shell precursor.(4)The Ni0.88 Co0.1 Mn 0.1(OH)2 precursor was synthesized using the joint precipitation technique of continuous over-flowing and intermittent precipitation methods,then the final LiNi0.8 Co0.1Mno0.1O2 material was prepared by high-temperature solid state reaction.The particle size distribution of as-synthesized material is more uniform,which favors the enhancement of long-term cycling performance.Through the study of the sintering process,the LiNi0.8CoO.1Mn0.1O2 material synthesized at 750 ℃ for 15 h has the best electrochemical properties.In the voltage range of 2.75～4.3 V(vs.Li+/Li),the specific discharging capacity at 0.1 C is 197.7 mAh·g-1 with the 100th capacity retention of 91.6%.(5)Using the simple and mass production preferred solid-state reaction,surficial Nb-doped LiMn2O4 materials made up of the truncated octahedral or spherical-like primary particles are successfully synthesized.The structural characterizations confirm that most of Nb5+ enrich in the surficial layer of the particles.The reason is that the ioinic radius of Nb5+(0.064 nm)is 20.7%bigger than that of Mn4+(0.053 nm).Due to the fusion effect of Nb205 to Mn-based materials,the crystalline properties of LiMn204 are greatly increased,so that the compacted density of LiMn204 electrodes is increased.Contrast with the pristine LiMn2O4,the volumetric energy density of LiMn1.99Nb0.01O4-bascd 18650R-type battery increases is increased by～13.9%,which benefits from the joint increments of the specific discharge capacity and the compacted density.(6)The diffeirent amounts of ZrO2-coated LiMn204 materials are successfully prepared by one-step sintering Zr02-coated Mn3O4 and Li2C03.The three key factors to realize ZrO2-coated LiMn204 materials via one-step sintering method is as follows:i)the Mn3O4 precursor is coated by ZrO2 in advance;ii)the ionic radius of Zr4+ is much larger than that of Mn3+ and Mn4+;iii)the pre-calcination temperature is set in the reaction temperature range between Li2C03 and Mn3O4 and lower than that between Li2C03 and ZrO1.Compared with the two-step sintering method,the one-step sintering process to synthesize Zr02-coated LiMn2O4 materials is very simple,low-cost,environmental friendly,and easy for large-scale industrial production,which also provides valualble reference to prepare other coated cathode materials for lithium-ion batteries.In this dissertation,the layered Ni-rich cathode materials and spinel LiMn2O4 materials are modified from three perspectives of the stability of crystal structure,uniformity of particle size distribution,and interficial properties,which greatly enhance the cycling performance.The research results can provide valuable references for the modification of other cathode materials for LIBs. |
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