A QUICK METHOD OF MEASURING THE CAPACITY VERSUS DISCHARGE RATE FOR A DUAL LITHIUM-ION INSERTION CELL UNDERGOING CYCLING

During extended cycling of an experimental cell, it is common to test periodically the rate behaviour of the cell by performing a series of discharges at various rates and measuring the capacity to a cutoff potential. The fastest way to obtain this information is to carry out successive discharges at decreasing rates with a brief relaxation period between each discharge but no charging step. The capacity obtained at a given rate is assumed to be the cumulative capacity up to that point. We have simulated this testing procedure to determine when the method is valid and have developed criteria for the optimum number of discharges and relaxation times to use. It is found that a series of seven discharges with a 5 min relaxation period gives an accurate prediction of the cell capacity with less than 1% error. The mechanisms which cause erroneous results to arise are described. The system chosen for simulation is the LixC6/propylene carbonate +1 M LiClO4/LiyMn2O4 dual lithium-ion insertion cell. Experimental discharge curves are provided that verify the results of the simulations.