Surface Modifications And Electrochemical Performances Of Li_x（Ni,Mn,Co）O₂ Layered Cathode Materials For Lithium Ion Batteries

A continuous upgrading of various electronic equipment and the development of electric vehicles demand more and more lithium ion batteries with high energy density and high power density.As well known,the energy density and powder density of lithium-ion battery highly depend on the nature of cathode material.Finding cathode materials with high specific energy is particularly important for the development of lithium-ion batteries.Layered cathode materials Li_x（Ni,Mn,Co）O₂（1≤x≤1.2）have the advantages of high capacity,low toxicity and price,which are considered to be the most potential alternatives to the traditional LiCoO₂ cathode.The serious capacity loss and voltage decay as well as the poor rate performance,however,hinder the commercial applications of layered cathode materials.Previous studies have shown that the causes of capacity loss,voltage decay and the poor rate performance are as follows:（1）side reactions between the surface of layered cathode materials and the electrolyte;（2）irreversible phase transformation of the original layered structure during the cycle;（3）particle cracking.Analyzing the above reasons,it is found that these phenomena are closely related to the surface structure of cathode materials.On the one hand,the residual lithium species on the surface is easy to react with the electrolyte,forming a thick solid electrolyte interphase（SEI）which hinders the transport of ions.Moreover,the continuous erosion of the electrolyte on the surface of cathode materials will gradually deteriorate the phenomenon of particle cracking.On the other hand,the irreversible phase transition usually starts from the surface and gradually expands to the bulk phase.To sum up,the stable surface structure is very important for the maintenance of performances.Based on this,this dissertation adopts two methods of surface coating and optimizing the surface intrinsic features to stabilize the surface structure of these layered cathode materials Li_x（Ni,Mn,Co）O₂（1≤x≤1.2）.After detailed electrochemical study for these materials,the main research achievements are obtained as follows:1.Polypyrrole（PPy）,an electronic conductor,was coated on the surface of layered material LiNi_0.5Co_0.2Mn_0.3O₂ by chemical oxidation polymerization,and the effect of coating layer thickness on the electrochemical performance was studied experimentally.More detailed analysis of electrochemical impedance spectra and structures for the cycled electrodes show that the coating layer of about 3 nm endows the modified sample with the smallest resistance and the most stable structure,ensuring its excellent cycle stability and rate performance.Comparatively,coating layer thickness of about 1 nm is too thin to maintain the structural stability during a long-term cycle,and an over thick coating layer（approximately 5 nm）reduces the adhesion between the slurry and the current collector,causing the shedding of the electrode and the sharp increase of the resistance,which results in their poor electrochemical performance.2.Zeolitic imidazolate framework-8（ZIF-8）,one of porous metal-organic framework,was used as coating material for the first time to enhance the electrochemical performances of LiNi_1/3Co_1/3Mn_1/3O₂(NCM₃₃₃)layered cathode.As revealed by the detailed analyses of structure,morphology and electrochemical impedance spectra for the cycled electrodes,the unique properties of ZIF-8,such as permanent porosity,exceptional chemical and thermal stability as well as high ionic conductivity,reduce the transmission resistance of interfacial regions,inhibit the irreversible phase transformation of the structure and the cracking of particles,and improve the structural stability of NCM₃₃₃33 during the repeated charge and discharge cycles,leading to a superior cycling stability and rate performance of NCM₃₃₃@ZIF-8.At a high cut-off voltage of 4.6 V,after 300 cycles at 800 mA g^-1,the capacity retention ratio for NCM₃₃₃@ZIF-8 is as high as 81.4%,while that of NCM₃₃₃33 is only41.2%.Even at 1200 mA g^-1,it still delivers a high capacity of 122 mA h g^-1.Furthermore,the performances of NCM₃₃₃@ZIF-8 at high temperature of 55^oC was also effectively improved.3.ZnO was coated uniformly on the surface of LiNi_0.5Co_0.2Mn_0.3O₂（NCM523）by a simple wet chemical method using the ZIF-8 as an intermediate.The result of SEM-EDS shows that the Mn、Co、Ni and Zn elements are uniformly distributed on the surface of the NCM523@ZnO-ZIF-8.Compared with the composite NCM523@ZnO-Zn（OH）₂ prepared by the method reported in the literature,NCM523@ZnO-ZIF-8 shows better cycle stability and rate performance.NCM523@ZnO-ZIF-8 delivered high discharge capacities of 187 mA h g^-1 at 1C and150 mA h g^-11 at 10C（1600mA/g）.After 200 cycles at 2C,a stable capacity of 159 mA h g^-1 can be retained,corresponding to a capacity retention ratio of 82%.Moreover,the uniform coating of ZnO also makes the sample NCM523@ZnO-ZIF-8 have a smaller resistance and higher structural stability.4.A simple heat-treatment-assisted（HA）molten-salt（MS）strategy was initially introduced in the solvothermal process to optimize the surface intrinsic features of lithium-rich manganese-based oxide Li_1.2Mn_0.54Ni_0.13Co_0.13O₂（LMRO）,and a lithium-rich layered cathode HA-MS-LMRO with stable surface structure was obtained.Detailed analysis of the structure,valence state,and electrochemical impedance spectra shows that the heat-treatment-assisted molten-salt process plays an important role for improving the performance of HA-MS-LMRO.HA process enables the transition-metal ions in the synthesized samples to have stable surface valence states,which is conducive to maintaining structural stability and improving cycling performance.The following MS process facilitates the movement of lithium salt into the interior of the assembled microsphere precursors to reduce the lithium residue and prohibit the formation of lithium-containing amorphous compounds on the surface of LMRO particles during the lithiation process,thus enhancing the Li-ion kinetics and increasing the initial discharge capacity.Compared with the LMRO,the initial discharge capacity of HA-MS-LMRO increased by more than 40 mA h g^-1 at each current density.More importantly,after 100 cycles at 200 mA g^-1,the capacity retention ratio for HA-MS-LMRO is 75%,which is much larger than that of LMRO（23.06%）.