Effects of cathode impedance on the performances of power-oriented lithium ion batteries

Since power batteries have different requirements than traditional energy-oriented lithium ion batteries (LIBs) and their design concept is also different than energy-type cells, some new problems not encountered in energy-oriented LIBs must be carefully considered. This study illustrates that cathode impedance, a contributor to total cell impedance which can be ignored in the traditional energy-type LIBs, plays a very important role in power cells. This study uses 18650 cylindrical power cells consisting of a LiMn2O4 cathode and graphite anode with a basic electrolyte of PC/EC/DMC = 1/3/6 by weight containing 1.2 M LiPF6 as model power LIBs. This study also investigates the charge-discharge performance of these model batteries made from cathodes with the same recipe but dried at different oven temperatures. The high impedance cathode produced under a high drying temperature causes the cell to fail during high-power applications. Cell heating during extreme high rate discharging periods not only causes pore closure in the porous separator, but also cathode peeling from the substrate. These phenomena, increasing the cell resistance and reducing the transfer rate of charged species, are believed to be the main causes for the poor cycle life of model batteries in high rate discharge tests.