【作者】 黄思玉；

【导师】 刘心宇；

【作者基本信息】 中南大学 ， 材料加工工程， 2010， 博士

【摘要】 可以作为锂电池或锂离子电池的正极材料的FeS2和NiS2,由于具有组成元素无毒、价格低廉、充放电容量高和可以大电流放电等优点一直备受关注,在军用热电池和一次高能原电池方面都有应用,但是由于在室温条件下这类正极材料的循环性能差,所以作为二次电池的正极材料应用研究开展的很少。本文采用溶胶-凝胶法制备了Fe2O3薄膜和NiO薄膜,然后把Fe2O3薄膜和NiO薄膜与气体硫进行气-固反应合成了FeS2和NiS2薄膜材料。本文的研究内容为使用origin软件和QBasic程序计算和绘制了相图,从绘制的相图选择了合理的合成方法,实验上主要研究了合成工艺条件对FeS2和NiS2薄膜材料的相结构及电化学性能的影响,并在此基础上对Li+离子在FeS2和NiS2薄膜材料中的扩散系数和首次放电时产生的电化学极化机理等进行了研究,主要结果如下：使用XRD和电化学方法研究证实金属Au耐高温硫化,并且在1.0-2.5V(vs Li+／Li)范围内电化学性能稳定,可以用作制备黄铁矿薄膜时的基体及可用作FeS2薄膜电极在锂电池中的正极集流体。使用计算机绘制了Fe-S-O、Ni-S-O的lgPs2-lgPO2优势区位图和lgPs2-lgPSO2优势区位图,从理论上证明Fe2O3、NiO在硫气体中热处理可以分别得到相应的二硫化合物FeS2、NiS2。在金属氯化物无水乙醇溶胶-凝胶体系中,使用提拉-热处理方法,制备铁氧化物薄膜和镍氧化物薄膜,XRD光谱证明制备的铁氧化物薄膜在500℃热处理30分钟后为Fe2O3相结构(赤铁矿结构)薄膜,而镍氧化物薄膜在600℃热处理30分钟后为NiO相结构(绿镍矿结构)薄膜。制备的Fe2O3薄膜和NiO薄膜表面平整、均匀,细腻且与基体结合良好。系统研究了硫化温度、硫化时间和硫化时硫气体压力等合成条件在Fe2O3、NiO薄膜转化为FeS2、NiS2薄膜过程中对薄膜相结构、薄膜的晶粒尺寸及制备的薄膜电极的电化学性能的影响。Fe2O3薄膜与0.4克硫一起密封于30cm3的石英管中,并于350-800℃热处理硫化20小时热硫化处理后可以使F2O3薄膜转变成为FeS2薄膜,在SEM显微图谱上可以看到,薄膜的晶粒尺寸随硫化温度的升高而增大,尤其当硫化温度在800℃后出现颗粒达7000nm大小的FeS2薄膜。综合电化学性能较好的硫化工艺条件为：硫化温度为500℃,硫化时间20小时,硫化时最佳硫气体压力为13大气压。在这个优化条件下制备的FeS2薄膜的Li／FeS2电池中,其电化学性能最佳,首次电池放电容量大约在900mAh g-1,第二次电池放电容量大约530 mAh g-1,经30次循环充放电后的放电容量达410 mAh g-1。NiO薄膜与0.4克硫一起密封于30cm3的石英管中,并于400-600℃热处理硫化20小时后可以转变成为NiS2相结构薄膜,而当硫化温度在700-800℃,由于气化而使基体材料上的Ni损失且不能得到NiS2相结构薄膜。对于NiS2相结构薄膜,电化学性能最好的硫化工艺条件为：硫化温度为600℃,硫化时间20小时,硫化时最佳硫气体压力为15大气压。在这个优化条件下制备的NiS2薄膜的Li／NiS2电池中其电化学性能最佳,首次电池放电容量大约在880mAh g-1,第二次电池放电容量大约600 mAh g-1,经30次循环充放电后的放电容量为400 mAhg-1。由于电极表面含有S、O2等杂质,使得新装配的Li／FeS2、NiS2／Li电池的开路电压高于Li／FeS2、NiS2／Li电池的理论电压。在Li／FeS2电池和Li/NiS2电池中,使用0.4C电流放电到1.0V时,FeS2可以100%还原为Fe+2Li2S,而NiS2可以100%还原为Ni+2Li2S。第一次放电结束后接着的充电过程中,Fe可以几乎100%可以电化学氧化成Li2FeS2,而Ni几乎100%可以电化学氧化成Ni3S2。电位阶跃法测定得到Li+离子在FeS2中的扩散系数为1.4×10-16cm2S-1,Li+离子在NiS2中的扩散系数为5.3×10-17cm2S-1。FeS2和NiS2薄膜电极在1M LiPF6的EC／DMC(1：1,v／v)溶液中循环伏安、恒电流充放电及电位阶跃实验结果证明,首次放电时FeS2和NiS2薄膜电极存在严重的Li+离子扩散极化,使得电池放电电位低于理论电池电位。从0.4C恒电流时电池的恒流循环充电电化学性能来看,NiS2薄膜电极的第二到第十次放电容量比FeS2薄膜电极要大,而第十一次循环以后,NiS2薄膜电极的放电容量比FeS2薄膜电极略小。但从FeS2和NiS2薄膜电极的电位阶跃法测得的Li+离子在薄膜材料中的扩散系数结果都表明,室温条件下,在1M LiPF6的EC/DMC(1:1,v/v)溶液中FeS2薄膜电极的大电流电化学性能要优于FeS2薄膜电极的电化学性能。更多还原

【Abstract】 FeS2 and NiS2 are promising candidates for cathode materials in lithium batteries or lithium ion batteries due to their high theoretical capacity, low cost (abundant mineral), non-toxicity and high high current densities. They have been used as cathode materials in thermal batteries and primary cells in the past. Because of the poor cycle performances of electrochemistry in room temperature, FeS2 and NiS2 have not used as cathode materials in second batteries. In this paper, Fe2O3 films and NiO films were prepared by sol-gel techniques, then the FeS2 and NiS2 films were synthesized on gold substrates after the metal oxide films being heat-treated in sealed quartz tubes in gaseous sulfur.The effects of technological conditions on the phase structures and the electrochemical performances of films have been studied. And the diffusion coefficient of Li+ion in FeS2 and NiS2 Materials have been determined by potential step method.After metal Au being heat-treated at 400-800℃in sealed quartz tubes in gaseous sulfur, the phase structure of Au has not been changed according to the result of XRD diffraction. And It is determined that the metal Au has excellent electrochemical stability in 1M LiPF6 of EC/DMC (1:1, v/v) solution in the range of 1.0-2.5 V (vs Li+/Li) potential by cyclic voltammetry and constant current charge-discharge techniques. So the Au is adequate substrate on which the FeS2 and NiS2 were prepared and the Au can be used as current collector for cathode of Li batteries in the range of 1.0-2.5V (vs Li+/Li) potential. The predominance area diagrams of 1gPS2-1gPO2 and 1gPS2-1gPSO2 for Fe-S-0 and Ni-S-0 were drawn by the improved component activity term method of base element. And the influences on the purity of pyrite by the residual oxygen impurity in the process of synthesis have been characterized according to these drawn predominance area diagrams. The predominance area diagrams show that the high-purity FeS2 and NiS2 can be prepared by heat-treatment of iron/ iron oxides and nickel/nickel oxide with sulfur in sealed quartz reactor. Iron oxide films and nickel oxide films were prepared on gold substrates by sol-gel method with Both Acetylacetone and Ferric chloride and chlorepoxy-propone dissolved in ethanol solution. The Iron oxide films and nickel oxide films were crack-free, transparent and uniform. These metal oxide film exhibit good bonding with the Au matrixs.The effects of sulfurzing temperature, sulfurizing time and the gaseous sulfur pressure on the phase structures and the electrochemical performances of films have been studied systematically. The experimental results show that the FeS2 films can be prepared by the F2O3 films being heat-treated at 350-800℃in 13atm gaseous sulfur for 20hours. The SEM determined that the grain size of the films will increase as the heat-treatment temperature increases. Especially after being heat-treated at 800℃, the grain sizes of the FeS2 films reach 8μm. While the FeS2 films is prepared at 500℃in 13atm gaseous sulfur for 20hours, the prepared FeS2 films exist the best electrochemical performances:the first discharge capacity is 900mAh g-1, and 30th discharge capacity is 410 mAh g-1。Meanwhile the experimental results show that the NiS2 films can be prepared by the NiO films being heat-treated at 400-600℃in 15atm gaseous sulfur for 20hours. After NiO films being heat-treated at 700-800℃for 20hours, the NiS2 phase structure films cannot be obtained and the nickel on the matrix will loss Owing to evaporation. While the NiS2 films is prepared at 600℃in 15atm gaseous sulfur for 20hours, the prepared NiS2 films exist the best electrochemical performances:the first discharge capacity is 880mAh g-1, and 30th discharge capacity is 400 mAh g-1.The open-circuit potential (OCV) of the fresh Li/FeS2、Li/NiS2 cell is 2.2-3. IV, which is larger than that of theory. This is caused by FeS2、NiS2 electrodes containing impurities of S、O.While the Li/FeS2 or Li/NiS2 cell is discharged to 1.0V wtith 0.4C current at the first discharging,100% of the FeS2 in cathode can be reduced to Fe+2Li2S and 100% of the NiS2 in cathode can be reduced to Ni+2Li2S. Subsequently at the first charging,100% of the reduced Fe can be oxidized to Li2FeS2 and 100% of the reduced Ni can be oxidized to Ni3S2.The diffusion coefficient have been determined by potential step method:the diffusion coefficientof Li+ion in is 1.4×10-16 cm2 S-1 and the diffusion coefficient of Li+ion in the prepared NiS2 film is 5.3×10-17 cm2 S-1. It is determined that the diffusion polarization of Li+ion into crystals of FeS2 or NiS2 film is one of the reasons that causes the discharging potential is low at the first discharging 1M LiPF6 EC/DMC (1:1, v/v) solution by cyclic voltammetry and constant current charge-discharge techniques.Moreover, electrochemical performance of the NiS2 film is better than that of the FeS2 film between the second and the 10th cycle at 0.4C dishcharging, but electrochemical performance of the NiS2 film is worse than that of the FeS2 film after the 10th cycle.更多还原