【作者】 王刚；

【导师】 申泮文；

【作者基本信息】 南开大学 ， 无机化学， 2009， 博士

【摘要】 锂离子二次电池由于具有高能量密度、高输出电压、环境友好和无记忆效应等优点,广泛用于各种便携式电子设备的同时,在电动汽车等大型电动设备上也有着广阔的应用前景。开发高容量、高循环稳定性的负极材料以取代传统的炭材料成为近年来锂离子二次电池研究的热点。本论文综述了锂离子二次电池负极材料的研究进展及其存在的主要问题。在此基础上,针对锡基材料比容量高,但循环性能较差的特点,采用各种不同合成法制备了一系列具有新型结构的锡基负极材料。利用XRD,SEM和TEM等技术对这些材料的微观结构和形貌进行了分析,采用恒电流充放电、循环伏安（CVs）和电化学阻抗谱（EIS）等技术测试其电化学性能。本论文主要开展了以下几个方面的研究工作:锡基合金负极材料:由于锂离子二次电池有限的内部空间,寻找高容量、高振实密度的负极材料有着非常重要的现实意义。本工作利用La-Co-Sn三元合金具有高晶体密度的特点,通过电弧熔炼法制备了一系列富Sn的La-Co-Sn三元合金。在深入研究Co/Sn比率对合金结构、形貌影响的基础上,研究了其电化学性能。结果表明,所有的La-Co-Sn三元合金均以Yb₃Rh₄Sn₁₃结构类型的La₃Co₄Sn₁₃金属间化合物作为主相。随着三元合金中锡含量的增加,可逆容量增大,但循环性能变差。由于铸态La-Co-Sn三元合金颗粒较大,在首次循环中活性成分利用率低,导致首次不可逆容量损失比较大。其中,铸态LaCoSn₄合金显示了最好的电化学性能。在此基础上,进一步对铸态LaCoSn₄合金进行高能球磨处理,发现球磨处理导致La₃Co₄Sn₁₃主相的逐步分解和结晶度的降低。与铸态LaCoSn₄合金相比,所有球磨的LaCoSn₄合金表现出了改善的电化学性能。特别是,球磨16h的LaCoSn₄合金在循环40周后放电容量仍然在500 mAh/g以上。球磨过程有利于合金的电荷转移电阻和Warburg阻抗明显减小,这是导致球磨LaCoSn₄合金循环性能改善的直接原因。虽然需要进一步提高首次库仑效率,具有高晶体密度的La-Co-Sn三元合金仍然能够被考虑作为有希望的锂离子二次电池的负极材料。Co₂SnO₄负极材料:对锂离子二次电池而言,锡基复合氧化物是最具吸引力和最具竞争力的电极材料,然而其锂合金化过程中严重的体积效应导致电极材料逐渐粉化,从而引发循环性能下降。针对锡基复合氧化物的体积效应,通过减小锡基复合氧化物的晶粒尺寸来实现良好循环性能是一个有效的途径。本工作通过水热方法成功地制备了立方尖晶石结构的Co₂SnO₄纳米晶,系统地研究了碱溶液浓度、水热温度和水热时间对产物结构和形貌的影响。结果表明,在2.0M的NaOH溶液中,240℃反应48h可以制备出结晶性良好的Co₂SnO₄纳米晶。电化学性能测试表明,Co₂SnO₄纳米晶的首次可逆容量（充电容量）达到了1088.8 mAh/g,循环50周后可逆容量仍然维持在555.9 mAh/g,远优于由高温固态法制备的体相Co₂SnO₄的循环性能。通过CVs和XRD分析表明,Co（?）CoO的可逆转化是影响Co₂SnO₄的循环稳定性的关键因素。虽然减小粒径尺寸可以改善锡基氧化物的循环性能。然而,氧化物的导电性对其循环性能,尤其是倍率性能有着重要影响。本工作通过水热法将Co₂SnO₄纳米晶与碳纳米管复合,不仅有助于缓冲合金化过程中巨大的体积变化引发的机械应力,提高其循环稳定性,而且还能充分发挥碳纳米管的优良的导电性,从而最大程度地发挥纳米尺寸效应,促进电极材料倍率性能的改善。当Co₂SnO₄与MWCNTs重量比为5:1时,Co₂SnO₄-MWCNTs复合物的首次可逆容量（充电容量）高达986 mAh/g。在循环50周以后,其可逆容量仍然稳定在898 mAh/g,容量保持率达到了91.1%,平均每次循环容量仅损失0.18%。而且该复合材料还具有比较良好的倍率性能,即使在1000 mA/g的电流密度下可逆容量仍然达到了100-120 mAh/g左右。这种优异的性能表明了碳纳米管载体在增强电子导电率,维持热力学/动力学稳定性和降低电荷转移电阻方面所起的关键作用。锡-炭复合材料:针对金属锡在深度嵌锂过程中严重的体积效应,采用具有较高导电性和适当孔隙率的多孔炭材料作为金属锡纳米粒子的缓冲材料制备了新型锡-多孔炭复合材料。因该锡-多孔炭复合材料具有特殊的异质结构,优势互补,具有协同效应,进而导致增强的电化学性能。其中,锡含量为45.6%的锡-多孔炭复合材料在循环60周后可逆容量仍然维持在503 mAh/g。即使在1000mA/g电流密度下可逆容量仍然达到180 mAh/g。复合材料的电化学结果表明,所制备的多孔炭材料是金属锡的良好的缓冲材料,它既能保证复合材料的导电性,又能对其内部的金属锡在充放电过程中起到有效的缓冲作用。更多还原

【Abstract】 Recently,lithium-ion secondary batteries have been widely used in portable electronic devices due to their high energy density,high voltage and non-pollution. Meanwhile,they have great potential for using in electric vehicles（EVs）.The development of anode materials with the high-capacity and good cycle stability to replace the conventional carbon-based materials has attracted more attention for the new generation of lithium-ion secondary batteries.In this dissertation,the research and development of anode materials of lithium-ion secondary batteries were reviewed. In order to improve the electrochemical performance of tin-based anode materials, tin-based anode materials were prepared and characterized by XRD,SEM and TEM measurements.The electrochemical performance of tin-based anode materials was evaluated by means of various techniques including galvanostatic method,cyclic voltammetry（CVs） and electrochemical impedance spectroscopy（EIS）.The main content is presented as following:Because the limited inner space in lithium-ion secondary batteries,it is important to find the materials with a high capacity and a high tap density as anode materials. Based on the high crystal density of alloys,Sn-rich La-Co-Sn ternary alloys were prepared by arc melting.The relationship between the morphology,microstructure and electrochemical properties were analyzed.In particular,the effect of the crystallinity in Sn-rich La-Co-Sn ternary alloys on electrochemical properties was elucidated.On the basis of obtained results,all the as-cast La-Co-Sn ternary alloys have the same main phase of La₃Co₄Sn₁₃ and low electrochemical capacities due to its large particle sizes.Among these alloys,the as-cast LaCoSn₄ alloy exhibits the best electrochemical performance.The ball-milling process results in the reduced crystallinity,and the enhanced electrochemical capacities as compared to the as-cast alloy.In particular,the LaCoSn₄ alloy,obtained after ball-milling for 16 h,provides the higher discharge capacity of 500 mAh/g after 40 cycles.Both the charge-transfer resistance and the Warburg impedance of the ball-milled alloy are much lower than that of the as-cast alloy,which are beneficial to the great improvement of the electrochemical performance.Although further studies are required to understand and increase the coulombic efficiency in the initial cycle,La-Co-Sn ternary alloys with the high crystal density can be considered as promising anode materials for lithium-ion secondary batteries.Tin-based oxides are good candidates as new electrode materials for lithium-ion secondary batteries.Unfortunately,the large volume change generated during the lithium alloying processes causes serious mechanical damage to the electrode, resulting in a large capacity loss.It was reported that small and uniform distribution of electrode materials may minimize the dimensional changes of the active materials and provide the improved performance of the electrode.In this work,spinel Co₂SnO₄ nanocrystals were successfully synthesized by one-pot hydrothermal method.The hydrothermal conditions,such as alkaline concentration,reaction temperature,and duration time on the structures and morphologies of the resultant products were investigated.The well crystallinity and pure phase of Co₂SnO₄ nanocrystals can be obtained with 2.0M of NaOH solution at 240℃for 48h.The as-prepared Co₂SnO₄ nanocrystals exhibit good electrochemical performance with initial coulombic efficiency of 71%,high reversible capacity of 1088.8 mAh/g and relatively good capacity retention.It is the first report about spinel Co₂SnO₄ nanocrystals prepared by hydrothermal method as anode materials for lithium-ion secondary batteries.Although the significant improvement in the electrochemical performance is obtained for nanocrystalline active materials,the poor electronic conductivity of Co₂SnO₄ nanocrystals still is retained.To resolve the problem,Co₂SnO₄-MWCNTs nanocomposites were prepared in this work.In the nanocomposites,3D conducting networks of MWCNTs would enhance the electronic conductivity of the nanocomposites.Moreover,MWCNTs may also provide an elastic buffer for releasing the strain of Co₂SnO₄ nanocrystals during the electrochemical processes.On the basis of obtained results,it is demonstrated that the Co₂SnO₄-MWCNTs nanocomposite（5:1） has highly reversible capacity（about 900 mAh/g） and capacity retention of 91.1%after 50 cycles,and improved rate capability.These results suggest that carbon nanotubes are beneficial for increasing electronic conductivity,and decreasing charge-transfer resistance in Co₂SnO₄-MWCNTs nanocomposites.Taking into account of the large volume change of tin during the electrochemical cycle,the novel tin-highly porous carbon composites were prepared using highly porous carbon materials as the matrix of tin nanoparticles.The tin-highly porous carbon composite with optimized tin contents（45.6%） has the high reversible capacity of 503 mAh/g and good capacity retention of 83.1%after 60 cycles,and good high rate discharge ability.The results indicate that the highly porous carbon is the suitable matrix for supporting tin nanoparticles because the highly porous carbon can ensure the high electronic conductivity of composites and buffer the volume change of tin nanoparticles during the electrochemical processes.更多还原