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层状LiMnO2正极材料的制备及电化学性能研究
Study on Preparation and Electrochemical Performance of Layerd LiMnO2 Cathode Material
【作者】 李浩;
【导师】 刘兴泉;
【作者基本信息】 电子科技大学 , 工程硕士(专业学位), 2021, 硕士
【摘要】 层状LiMnO2正极材料由于其高理论比容量(285 m Ah g-1)、高能量密度、低成本引起了学者们的广泛注意,但由于其难以制备,且循环性能差,难以满足现今社会新能源汽车对高循环稳定性正极材料的需求。为了能制备出稳定的层状LiMnO2正极材料,并获得优良的循环性能,本文首先通过改进的高温固相法初步制备出了稳定的层状LiMnO2正极材料,然后为了进一步改善其电化学性能,利用不同的合成方法(液相法及水热法)控制前驱体Mn2O3形貌,分别制备出了纳米颗粒LiMnO2正极材料以及微立方LiMnO2正极材料,探究了微观形貌对LiMnO2正极材料电化学性能的影响,最后通过碳包覆的手段对纳米颗粒LiMnO2正极材料进行改性处理,进一步提高了LiMnO2正极材料的循环稳定性。本文采用了XRD、SEM、TEM、TGA和电化学测试等手段对材料的晶体结构组成,表面形貌,反应温度以及电化学性能进行了测试表征,研究结果如下:(1)采用高温固相反应的方法成功合成了具有层状结构的LiMnO2正极材料,考察了最终烧结温度分别为700℃、750℃及800℃时对材料电化学性能的影响。对其不同温度下合成的材料进行形貌测试以及电化学性能测试,发现不同温度下制备的材料形貌不规则,750℃制备得到的材料纯度最高,且在0.1C放电倍率下,具有最高的放电比容量131 m Ah g-1,经过60次循环后,材料放电比容量为100 m Ah g-1,容量保持率为76.3%。(2)通过采用控制前驱体Mn2O3形貌的方法成功制备出了纳米颗粒LiMnO2正极材料以及微立方LiMnO2正极材料。经过测试发现,纳米颗粒LiMnO2正极材料,粒径分布均匀,最高放电比容量达到182.8 m Ah g-1,经过60次循环后,放电比容量维持在137.6 m Ah g-1,容量保持率为75.3%;而微立方LiMnO2正极材料是由很多纳米颗粒堆积而成,其放电比容量最高达到217.7 m Ah g-1,经过60次循环后,放电比容量仍为183.5 m Ah g-1,容量保持率达到84.3%;两种不同微观形貌的LiMnO2正极材料的电化学性能均远优于固相制备得到的LiMnO2正极材料,探究得到不同微观形貌对材料的电化学性能有着明显的改善作用。(3)采用液相包覆法对纳米颗粒LiMnO2正极材料进行了碳包覆研究,包覆量分别为1wt.%、3wt.%、5wt.%,测试倍率为0.5C。实验结果表明,没有包覆过的材料其初始放电比容量最高,为137.8 m Ah g-1,经过60圈循环后,材料的放电比容量降低仅为109.7 m Ah g-1,容量保持率为79.6%;而通过不同包覆量的碳包覆的LiMnO2材料初始放电比容量均有所降低,分别为128.6 m Ah g-1、126.5 m Ah g-1、120.1 m Ah g-1,而经过60圈循环后放电比容量分别仍为122.9 m Ah g-1、121.5 m Ah g-1、117.5 m Ah g-1,容量保持率分别为95.6%、96%、97.8%,容量保持率明显提高,但碳包覆对材料的倍率性能并没有提升作用;对材料进行Ni以及Cr的掺杂处理,材料循环60圈后的容量保持率分别为84.9%以及89.6%,得出经过Cr掺杂处理过的材料具有更高的循环稳定性。
【Abstract】 The layered LiMnO2cathode material has attracted wide attention from scholars due to its high theoretical specific capacity(285 m Ah g-1),high energy density,and low cost.However,because of its difficulty in preparation and poor cycle performance,it is difficult to meet the requirements of new energy automotive demand for high cycle stability cathode materials.In order to prepare a stable layered LiMnO2cathode material and obtain excellent cycle performance,in this paper,we prepared a stable layered LiMnO2cathode material through an improved high-temperature solid phase method,and then used different methods(liquid phase method and hydrothermal method)to control the morphology of the precursor Mn2O3,prepared nano-spherical LiMnO2cathode materials and microcubic LiMnO2cathode materials,and explored the influence of micro-morphology on the electrochemical performance of LiMnO2cathode materials.The positive electrode material was coated to further improve the cycle stability of the LiMnO2positive electrode material.In this paper,XRD,SEM,TEM,TGA and electrochemical tests were used to test and characterize the crystal structure,surface morphology,reaction temperature and electrochemical performance of the material.The results of the study are as follows:(1)The LiMnO2cathode material with a layered structure was successfully synthesized by high-temperature solid-phase reaction method,and the effect of different final sintering temperatures such as 700℃,750℃and 800℃were investigated.The morphology test and electrochemical performance test of the materials synthesized at different temperatures found that the materials prepared at different temperatures have no special morphology.The material prepared at 750℃has the highest purity,and it has the highest discharge specific capacity 131 m Ah g-1under 0.1C discharge rate.After 60cycles,the material discharge specific capacity is 100 m Ah g-1,and the capacity retention rate is 76.3%.(2)By controlling the morphology of the precursor Mn2O3,the nanoparticles LiMnO2cathode materials and microcubic LiMnO2cathode materials were successfully prepared.After testing,it is found that the particle size distribution is uniform,and the highest discharge specific capacity reaches 182.8 m Ah g-1.After 60 cycles,the discharge specific capacity remains at 137.6 m Ah g-1,and the capacity retention rate is75.3%;while the microcubic LiMnO2cathode material is made up of many nanoparticles,and its discharge specific capacity is up to 217.7 m Ah g-1.After 60 cycles,the discharge specific capacity is still 183.5 m Ah g-1,and the capacity retention rate is reached 84.3%;the electrochemical performance of the two different micro-morphology LiMnO2cathode materials are far better than the LiMnO2cathode material prepared by solid phase,and the different micro-morphology has a significant improvement on the electrochemical performance of the material effect.(3)By means of liquid phase coating,the carbon-coated nano-spherical LiMnO2cathode material was successfully prepared,and the coating amount was 1wt.%,3wt.%,5wt.%,the test rate was 0.5C.The experimental results show that the uncoated material has the highest initial discharge specific capacity of 137.8 m Ah g-1.After 60 cycles,the discharge specific capacity of the material decreases to 109.7 m Ah g-1,and the capacity retention rate is 79.6%;The initial discharge specific capacity of LiMnO2materials coated with different amounts of carbon are reduced to 128.6 m Ah g-1,126.5 m Ah g-1,120.1 m Ah g-1,and the specific discharge capacities after 60 cycles are 122.9 m Ah g-1,121.5 m Ah g-1,117.5 m Ah g-1,and the capacity retention rates are 95.6%,96%,and97.8%,which displyed remarkably higher capacity retention.However,the carbon coating does not improve the rate performance of the material;The materials are doped with Ni and Cr,and the capacity retention rate after 60 cycles of the material are 84.9%and 89.6%,respectively.It is concluded that the material treated with Cr doping has higher cycle stability.
【Key words】 LiMnO2; High temperature solid phase method; Nanoparticles; Micro-cubic; Modification research;