The Optimal Synthesis And Surface Modification Of Li_1.2Mn_0.6Ni_0.2O₂

Compared with the anode,the low capacity of commercial cathode materials is the bottleneck to limit the capacity and energy density of lithium-ion battery.Li-rich Mn-baselayeredoxidesLi_1+xNi_aCo_bMn_cO₂（x+a+b+c=1）or xLi₂MnO₃·（1-x）LiNi_aCo_bMn_cO₂（a+b+c=1）have attracted extensive attention because of their high capacity originated from the activation of a electrochemically inert Li₂MnO₃ component at≥4.5V,especially the Co-free Li-rich cathode material which does not contain expensive and high toxicity Co element.However,some problems must be solved before the commercial application of these Li-rich Mn-base layered oxides.For example,the two-phase integration mode of Li-rich materials is still controversial.How to identify these integration structure is facing challenge.Moreover,the excellent electrochemical performance of electrodes can only be judged by a long-term electrochemical testing,which waste too much experimental resource and personnel energy.On the other hand,the electrochemical performance of Li-rich materials need to be improved,including severe capacitance loss,rapid voltage decay caused by structure phase transition and inferior rate performance attributed to their inherent 2-Dimensional layered structure.In order to solve the above problems,this thesis focus on investigating the relation between the integrated structure complexity and electrochemical performance of Li_1.2Mn_0.6Ni_0.2O₂,and seeking the method for enhancing their electrochemical performance based on the optimal synthesis process and surface modification.The main research findings can be summarized as:（1）Raman Spectroscopic investigations of Li-rich materials to predict their electrochemical performances.Li_1.2Mn_0.6Ni_0.2O₂ layered materials were synthesized by five synthesis methods,namely sol-gel method,solid phase method,coprecipitation method,hydrothermal method and solvent thermal method.The comparison and analysis of Raman spectra for five Li-rich samples,Li₂MnO₃ and LiNi_0.5Mn_0.5O₂ show that the integrated structures of the five samples are different.The co-precipitation sample（CP）forms a solid solution.The sol-gel sample（SG）and high temperature solid-state sample（HS）are nano-composite.The hydro/solvent thermal samples（HT and ST）exhibit composite structure feature.Combining this work with previous report about the size of Li₂MnO₃-domain effects on the electrochemical performance,the electrochemical performances of the five materials are predicted.The sample CP exhibits high capacity and the best electrochemical performance because of the smallest Li₂MnO₃-domain in its integrated structure can activate fully during the first charging.SG and HS samples with nano-composite structure are in second place.The performances of HT and ST with composite structure are worst.Electrochemical results show that sample CP gave the maximum contribution of Li₂MnO₃ to the actual charge capacity and the difference between the theoretical and observed valuesΔC_Li2MnO3 is only 47.88mAh·g^-1,being consistent with the expected results.Therefore,it is feasible to predict the electrochemical performance of Li-rich materials by using Raman spectroscopy to distinguish its integrated structures.（2）Optimized synthesis of{010}oriented Li_1.2Mn_0.6Ni_0.2O₂ nanosheets by hydroxide coprecipitation.Basing on above experimental results,The synthesis conditions in the co-precipitation procedure were adjusted.In addition to NaOH,LiOH and KOH were used as precipitating agents to synthesize Li_1.2Mn_0.6Ni_0.2O₂.The results show that the sample using LiOH as precipitant gives a good cyclic stability and excellent rate performance.Further characterization demonstrated that the sample contains nanoplates with（-110）exposed plane.The large size nanoplate can ensure stable structure to some degree,and（-110）exposed plane can facilitate Li⁺conduction.After100 cycles at 0.5C and 5C,its capacity retentions are as high as 88.3%and78.1%,respectively,and the rate performance is also significantly improved,especially at high rate（>4C）.（3）Li_1.2Mn_0.6Ni_0.2O₂/C composites were constructed simply to improve the electrochemical performance of lithium ion battery.A simple dry chemical method is designed to realize the surface modification of Li_1.2Mn_0.6Ni_0.2O₂ by conductive acetylene black（CAB）.By optimizing the synthesis process,the Li_1.2Mn_0.6Ni_0.2O₂/C composites was successfully constructed.The excellent conductivity of CAB and the strong chemical interaction between CAB and Li_1.2Mn_0.6Ni_0.2O₂ formed in the heat treatment process inhibit the structural phase transition and reduce the electrochemical impedance.The modified Li_1.2Mn_0.6Ni_0.2O₂ with 5%CAB（CAB-5）shows the optimal performance,which possesses the most stable structure,thermal stability and fast charge transfer ability.After 100 cycles,the capacity retention can reach 74.3%at 5C.More importantly,the voltage attenuation is only 0.5 V at 1C,and the rate performance of CAB-5 can reach 93.0 mAh·g^-1 at 10C.