Modification Of Cathode Materials And New Method Of Testing For Lithium Ion Batteries

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.