【作者】 于利红；

【导师】 邱新平；

【作者基本信息】 清华大学 ， 化学， 2006， 硕士

【摘要】 为了提高锂离子电池正极材料LiMn₂O₄的电池容量和循环稳定性,前人实验过的方法不计其数,其中用元素Ti对LiMn₂O₄进行掺杂的方法,由于使结构更不稳定而被否定。但TiO₂结构中有允许锂离子迁移的通道,用TiO₂对LiMn₂O₄表面进行修饰,可以避免整体掺杂Ti元素所带来的弊端。本文用TiO₂以溶胶凝胶法对LiMn₂O₄进行了表面修饰,在LiMn₂O₄颗粒的表面形成了一种LiTixMn2-xO4固熔体。所得的产物TiO₂修饰的LiMn₂O₄（以下称TiO₂-LiMn₂O₄）作为正极材料在高温以及高电压下有明显改进的电化学性能:在55 oC下循环30周后,容量仍可保持初始容量的90%;而相同实验条件下LiMn₂O₄在55 oC下循环25周容量只能保持初始容量的30%。在较高电压范围3.0⁴.8 V之间工作时,TiO₂-LiMn₂O₄材料循环60周后容量可以保持为初始容量85%,而LiMn₂O₄材料容量循环40周后容量为初始容量的70%.用交流阻抗法测试电池的阻抗,发现TiO₂-LiMn₂O₄电池较LiMn₂O₄电池阻抗得到了很大的降低.电化学阻抗除了用在评价两种正极材料性能之外,还用于评价18650型商业锂离子电池性能.对18650商业电池进行了阻抗随循环次数、工作温度以及工作电压变化的测试。结果表明大量次数循环会引起阻抗的增加,引起阻抗增加的主要原因是电解质电阻（Re）的增加,而不是SEI电阻（Rsl）,或者电荷转移电阻（Rct）的增加.低温下（<-20 oC）锂离子电池无法正常工作,不论是充电过程还是放电过程,这一点与以往的研究结果不同。低温下电池有较差性能,主要因为正极电荷转移电阻（Rct）值增加了两个数量级。另外,论文还对荷电状态对阻抗的影响进行了研究,并对影响的原因进行了探索。更多还原

【Abstract】 In order to improve the capacity and cycling stability of LiMn₂O₄ cathode of Lithium ion batteries, much effort was given by other researchers. The method of doping element Ti into spinel LiMn₂O₄ was abandoned for thorough doping Ti harms the structure stability by enlarging the electrostatic repulsion. However, TiO₂ consists of channels allowing Li ion to travel which reminds to modify the surface of LiMn₂O₄ using TiO₂ avoiding thorough doping Ti. The surface of spinel LiMn₂O₄ was modified with TiO₂ by a simple sol-gel method to improve its electrochemical performance at elevated temperatures and higher working potentials. Compared with pristine LiMn₂O₄, surface-modification improved the cycling stability of the material. The capacity retention of TiO₂-modified LiMn₂O₄ was more than 85% after 60 cycles at high potential cycles between 3.0 and 4.8 V at room temperature and near to 90% after 30 cycles at elevated temperature of 55°C at 1 C charge-discharge rate. Electrochemical impedance spectroscopy （EIS） was used to study the materials and impedance of TiO₂-modified LiMn₂O₄ is decreased greatly compared to the pristine LiMn₂O₄.Electrochemical impedance spectroscopy （EIS） was also used to study the electrochemical performance of 18650 Li-ion batteries vs cycling, low temperatures and state of charge. Results show that the increase of battery resistance after long cycling is mainly from the Re（electrolyte resistance） but not Rsl or Rct.At low temperatures （<-20°C）, neither the discharge nor charge process can be carried out normally, which is inconsistent with the former belief. The poor performance of batteries at low temperatures is linked to the resistance increase of Rct by 2 magnitude grade. We also found that the SOC （state of charge） has impact on Rct, and additionally, gave the reasonable explanation.更多还原