【作者】 张新龙；

【导师】 胡国荣； 彭忠东； 杜柯；

【作者基本信息】 中南大学 ， 电化学工程， 2012， 博士

【摘要】 近年来,锂离子电池在能量密度,倍率性能等方面取得了极大的进步,从而逐渐占据了便携式电子设备的消费市场,并且被认为是最有希望成为电动汽车、混合动力汽车的首选电源体系。本论文旨在于制备高比容量、循环稳定、性能优良的锂离子电池的关键电极材料,主要采用了室温充放电、循环伏安等测试方法,以及XRD、 SEM对材料进行表征。钛酸锂Li4Ti5O12充放电过程中体积变化小,循环性能非常优异,但由于本身电位较高,约为1.5V,作为负极材料时,正极材料电位要求尽可能高为好。尖晶石型5V正极材料LiNio.5Mn1.5O4电位约为4.7V,以LiNio.5Mn1.5O4和Li4Ti5O12组成电池,电池电压适中,循环性能好。本文选取Li4Ti5O12和LiNio.5Mn1.5O4为研究对象,对其合成与改性进行系统研究,并将其组成电池体系,研究其电化学性能。本文针对Li4Ti5O12材料倍率性能差这一瓶颈问题,以降低材料合成温度和时间,减小材料粒径为目标,研发了湿式及固相煅烧合成工艺。首先通过锂源和钛源的筛选得出以LiOH和Ti02为原料能够获得最好的性能。考察了不同烧结温度、烧结时间对钛酸锂结构和性能的影响,750℃下烧结8h获得最佳的电化学性能。研究了Li4Ti5O12的倍率性能和循环性能。在0.5C充放电倍率下,Li4Ti5O12可逆容量达165.5mAh/g,在20C、30C的大倍率充放电条件下,材料仍能拥有131mAh/g、118mAh/g的可逆容量。接下来,以通过提高电子导电率改善材料倍率性能为目标,研究了以Mg、Ta为原料的Li4Ti5O12掺杂改性研究。考察了不同Mg含量掺杂对Li4Ti5O12结构的影响,XRD分析表明,在Mg掺入量较高时,Li4Ti5O12中会有Li2MgTi308的存在,说明Mg存在时阻止了高温过程中Li盐的扩散,通过对比不同掺入量材料的晶胞参数发现掺入Mg后引起Li4Ti5O12晶胞变大。通过SEM形貌分析发现掺Mg后Li4Ti5O12颗粒变化不大。充放电测试表明,Mg掺入量越多材料的充放电性能越差,而且掺Mg后材料循环性能变差,说明Mg会引起Li4Ti5O12较大的晶格畸变,会带来不利的影响。掺杂Mg0.05的LTO (Li4Ti5012)20C高倍率放电容量为152.1mAh/g;掺杂Mg0.02和Mg0.1的20C放电容量为分别为138.3mAh/g和120.6mAh/g。考察了不同Ta含量掺杂对Li4Ti5O12结构的影响,XRD分析表明,选用Ta5+离子取代Ti4+,有效掺杂后增大晶胞常数,但未引起尖晶石结构的变化；掺杂少量Ta5+离子后材料充放电过程中的电荷转移阻抗显著减小,有利于克服充放电过程的动力学限制,降低电池极化,提高了材料的可逆容量和循环性能。掺杂少量离子半径较Ti4+大的Ta5+离子,有利于形成半径较大的空隙,使Li+可快速嵌入和脱出,同时提高了材料的离子导电性和电子导电性,掺杂后获得了较好的可逆容量和循环性能。尖晶石型的锂离子正极材料LiNi0.5Mn1.5O4具有4.7V的高电压放电平台,且具有较高的充放电比容量,表现出了优良的电化学性能,逐渐成为当今正极材料研究的一个热点。采用氢氧化物控制结晶法合成Ni0.25Mn0.75(OH)2时,主要探讨前躯体的合成条件对形貌、粒度、振实比重、比表面积等物理性能的影响。pH值、氨水含量对前躯体微观晶粒形貌影响显著,提高pH值和合成温度,增加反应溶液中的氨水含量,有利于提高前躯体的振实比重,减小比表面积。将前驱体Ni0.25Mn0.75(OH)2与Li2CO3混合,焙烧温度为850℃,焙烧保温时间24h,Li/(Ni+Mn)=0.55。此条件下合成的LiNi0.5Mn1.5O4材料在3.5-5.0V的电压区间,0.2C的充放电倍率下的首次放电比容量达128mAh/g,30次循环后容量保持率为98.4%。对LiNi0.5Mn1.5O4正极材料进行A13+和F双掺杂,并研究了双掺杂对LiNi0.5Mn1.5O4性能的影响。实验结果表明,A13+和F-双掺杂对材料的微观结构与表面形貌均没有影响。在阳离子总数不变的前提下,进行双掺杂能显著提升材料的循环稳定性；而引入了阳离子空位后不仅改善了循环稳定性而且大大提高了其倍率性能。倍率性能提升的原因在于空位的存在,减少了锂离子脱嵌时的阻力,增大了电化学反应能力,提高了固相扩散系数。组装成新型的电池体系LiNi0.45Mn1.45Al0.1O3.95F0.05/Li3.95Mg0.05Ti5O12,并测试了其电化学性能。20C以下倍率充电具有很强的实际应用价值,放电容量高达140mAh/g。以1C的倍率对LiNi0.45Mn1.45Al0.1O3.95F0.05/Li3.95Mg0.05Ti5O12电池体系进行恒电流充放电测试,以L13.95Mg0.05Ti5O12的活性物质计算,以20C的倍率放电时,放电比容量为141mAh/g。循环200次后,容量保持率为90%,是比较有前途的电池体系之一。更多还原

【Abstract】 During recent years, lithium ion battery technology has made great progress in terms of energy density and power capability even higher than primary batteries. Thus, lithium ion batteries have rapidly conquered the consumer market of advanced portable electronics and are now considered as the most promising power sources for future electric vehicles (EVs), hybrid EVs and plug-in hybrid EVs. The purpose of the experiment is to synthesize high voltage materials with high specific capacity, steady cyclability, and good high rate cycle performance. The main testing methods include constant current charge-discharge test, cycle voltammagram (CV) and X-ray Diffraction (XRD) as well as scan electron microscope (SEM).Lithium titanate Li4Ti5O12due to the small volume change during the charging and discharging cycle performance is very execellent, but because of its high potential of about1.5V, as the anode materials, cathode materials for potential requirement as high as possible as well.5V spinel LiNi0.5Mn1.5O4is one of the most promising and attractive cathodes because of its acceptable stability, good cycling performance and high dominant potential plateau at around4.7V. It has been expected that the3V LiNi0.5Mn1.5O4/Li4Ti5O12cells exhibited good cycling performance, flatness in operating voltage and high rate capability.Due to the poor rate performance of Li4Ti5O12materials, and aiming at decreasing calcined temperature and reducing particle size, the wet type and solid state calcination process was developed, the effect of type of raw materials, calcined temperature and alcined time on the structure and properties of materials was studied. The Li4Ti5O12with small particle size and good rate performance was calcined. Aiming at improving the electronic conductivity of Li4Ti5O12, Mg and Ta doping Li4Ti5O12was prepared and studied.The LiOH and anatase TiO2were chosen as the raw material to get the best performance. The influence of sintering temperature and time were studied and the results showed that the optimal performance could be achieved by sintering at750℃for8hours. Its reversible capacity was165mAh/g(0.5C),131mAh/g (20C) and118mAh/g(30C).Mg and Ta were adopted to improve electrochemical performance of Li4Ti5O12. The X-Ray Diffraction test indicated that Mg doping causesd the lattice parameters of Li4Ti5O12to become larger and produce Li2MgTi3O8which has negative effect on material conductivity and Li ion diffusion.The capacity of Li3.95Mg0.05Ti5O12was152.1mAh/g at20C rate and Li398Mgo.o2Ti5O12and Li3.9Mg0.1Ti5O12was138.3mAh/g and120.6mAh/g. Mg and Ta doping can create Ti3+/Ti4+which increase material conductivity.As the cathode material of lithium ion batteries, cubic spinel LiNi0.5Mn1.5O4shows excellent electrochemical performance, such as high discharge plateau at4.7V and high energy density, and it is emerging as an active research topic. Ni0.25Mno.75(OH)2was also synthesized by co-precipitated metal hydroxide by controlling crystallization. The effect of precursor operating conditions on morphology, particle size, tap density and specific surface area was investigated. PH and ammonia content significantly influenced micro-morphology of precursor. With increasing pH, preparing temperature and ammonia content in reaction solution, tap density of precursor was increased and specific surface area was decreased. Meanwhile, influence of sintering temperature and sintering atmosphere of lithiation sintering on material performance was investigated. The optimized sintering conditions were obtained as follows:At the elevating rate of100℃/h, the material was sintered at850℃for24h. The discharge capacity can be reached128mAh/g (0.2C) in the range of3.5V-5.0V and the capacity retention reached98.4%after30cycles.It is found that the Al and F can enter into the lattice of material. The method of Al3+and F co-doping was used to improve the cycling stability of LiNi0.5Mn1.5O4cathode material and the effects of substitution of different transition metal elements by Al3+and Al3+content were investigated. The studies showed that Al3+and F co-doping did not change the structure of material, and significantly improved the cycling stability and rate capability. CV and EIS measurements showed that the enhancement of rate performance was due to the existence of vacancies, which reduced the resistance of lithium ion deintercalation and improved solid diffusion coefficient and the electrochemical activity.The LiNi0.45Mn1.45Al0.1O3.95F0.05/Li3.95Mg0.05Ti5O12full cell system was also investigated. When the mass ratio of the initial discharge capacity under1C rate was141mAh/g, and after200cycles, with the capacity retention rate about90%.更多还原