【作者】 李琳；

【导师】 杨勇；

【作者基本信息】 厦门大学 ， 物理化学， 2009， 硕士

【摘要】 正交结构正硅酸盐聚阴离子型化合物由于其高的理论容量和突出的安全性能等优点成为很有发展潜力的新一代锂离子电池正极材料。本文通过水热辅助溶胶凝胶法,合成了Li₂MnSiO₄/C和Li₂Mn_0.5Fe_0.5SiO₄/C复合正极材料,测试了其结构和电化学性能。初步研究了其电化学性能差异的原因,并采用非原位FTIR和非原位XRD的实验手段研究了其电化学过程中结构的变化与其容量衰减的原因。研究表明,水热辅助溶胶凝胶法所合成Li₂MnSiO₄/C和Li₂Mn_0.5Fe_0.5SiO₄/C复合材料均具有相同的晶体结构,均属于正交晶系空间群Pmn2₁,并且都具有高的首次放电容量,其首次放电容量分别为205.8 mAhg^-1（可逆的嵌脱1.24个锂）和215.6 mAhg^-1（可逆的嵌脱1.30个锂）。而此时Li₂Mn_0.5Fe_0.5SiO₄的Mn的电子交换数≥1.6,也就是说事实上,铁的取代使锰的实际电化学活性得到了很大的提高。GITT计算出的数据得出,Fe取代Mn后,使得Li₂MnSiO₄材料的锂离子扩散系数增加,因此传输改善,也证实了这一点。虽然水热辅助溶胶凝胶法所合成Li₂MnSiO₄/C和Li₂Mn_0.5Fe_0.5SiO₄/C复合正极材料循环性能均较差,但是Fe取代Mn后,前五圈的循环性能得到改善。非原位FTIR数据显示,Li₂MnSiO₄的对应于[SiO₄]的吸收谱带,随循环次数的增加,逐渐向高波数方向偏移。而Li₂Mn_0.5Fe_0.5SiO₄的[SiO₄]的特征吸收峰在循环过程中没有发生偏移。非原位XRD图数据显示,Li₂MnSiO₄材料在开始充放电后就变为非晶态,而Li₂Fe_0.5Mn_0.5SiO₄的结构得到部分保持。说明Fe部分取代Mn部分稳定住了材料的晶体结构,还稳定住了材料中的[SiO₄]基团。这表明通过阳离子取代来提高Li₂MnSiO₄框架结构稳定性的可能性。锂一次电池和锂离子二次电池在现代生活中发挥着同样重要的作用。CuCrO₄是一种具有多电子交换反应的锂一次电池正极材料,其理论容量高达744mAh/g。本文报道了一种合成CuCrO₄的改进方法,并对材料的结构和电化学性能进行了表征。为了验证可能的放电产物,采用非原位XRD和FTIR的方法研究了其放电机理。实验结果表明,改进的沉淀方法制备的CuCrO₄一次锂电池正极材料,具有放电容量高、倍率性能好等优点。以10mA/g和200mA/g的倍率放电到1V,比容量分别达到706mAh/g和610 mAh/g。该材料的棒状形态和亚微米级的粒子大小,以及单质铜在放电过程中的析出,使该材料倍率放电特性优异。非原位XRD和IR的研究结果揭示了CuCrO₄在放电过程中可能转变为了其他化合物,如Cr³⁺化合物,并形成反应产物单质Cu和Cu₂O。更多还原

【Abstract】 Orthosilicates are promising candidates for next generation of lithium ion batteries, due to their high theoretical capacity and excellent safety performance.In this work, Li₂MnSiO₄/C and Li₂Mn_0.5Fe_0.5SiO₄/C electrode materials were prepared by hydrothermal assisted sol-gel process.The structure character and electrochemical performance of the prepared materials were studied.Structure stability and the reason of the poor cyclic performance of the materials were investigated by ex-intu FTIR and ex-intu XRD.The Li₂MnSiO₄/C and Li₂Mn_0.5Fe_0.5SiO₄/C composite material prepared through hydrothermal assisted sol-gel process were iso-structure and the space group is orthorhombic system Pmn2₁.The Li₂MnSiO₄/C and Li₂Mn_0.5Fe_0.5SiO₄/C composite material shows a discharge capacity as high as 205.8 mAhg^-1（1.24 Li reversible exchange per unit formula） and 215.6 mAhg^-1（1.30 Li reversible exchange per unit formula）.But more than 1.6 electrons exchange per unit Mn occurred in Li₂Mn_0.5Fe_0.5SiO₄.Actually,electrochemical performance of Mn was improved after the mixing Fe and Mn.And the GITT data showed the diffusion of lithium in Li₂Mn_0.5Fe_0.5SiO₄ was improved.The cyclic stability of Li₂MnSiO₄/C and Li₂Mn_0.5Fe_0.5SiO₄/C composite material were not good.But the cyclic stability in the first five cycles was improved after the mixing Fe and Mn.Ex-intu FTIR showed that the characteristic bands of[SiO₄]in Li₂MnSiO₄ shift to high wavenumber with the increasing of cycle numbers.Compared to Li₂MnSiO₄, the characteristic bands in Li₂Mn_0.5Fe_0.5SiO₄ were not changed.Ex-intu XRD showed that after the first cycle,the crystalline Li₂MnSiO₄ changed to an amorphous state,but the characteristic peaks in Li₂Fe_0.5Mn_0.5SiO₄ were partly stable.We conclude that after the mixing Fe and Mn,the structure stability of Li₂MnSiO₄ were improved.It proved that a better structure stability probably be obtained by the mixing Fe and Mn.Primary lithium batteries and lithium ion batteries play the same important role in modern life.CuCrO₄ was one of multistep reductions cathodes for primary lithium batteries;its theoretical capacity is about 744mAh/g.Here,we report an improved method to synthesize the CuCrO₄.In addition,a possible mechanism of the CuCrO₄ material upon discharging was discussed.A high capacity CuCrO₄ composite cathode material with good rate performance has been successfully prepared through an improved precipitation method.The material delivered high discharge capacities of 706 and 610 mAh/g at 10 and 200 mAg^-1,respectively.The sub-micron rod-like morphology and discharging product Cu are responsible for its good rate performance. Furthermore,the ex-intu FTIR and ex-intu XRD measurements demonstrated that during the reductions of CuCrO₄ accompanied with the intercalation of lithium ions, the crystalline CuCrO₄ transformed into an amorphous state or decomposed into other amorphous compounds during the first discharge process;and the newly formed discharging products include Cu₂O and Cu.更多还原