As renewable energy with large output fluctuation increases, adoption of a large power storage system in which lithium ion secondary batteries are series-parallelized has been started for stabilization of grid power. However, stationary storage systems may be charged and discharged under more severe conditions than batteries for electric vehicles, and may deteriorate in a short period of time. In this research, while analyzing the change of capacity and internal resistance of the lithium ion secondary battery which uses LFP for the cathode and carbon anode. We estimated the life when charging and discharging under the conditions of ancillary service*, and considered the proposed measures for prolonging the life.

* ancillary service; A power adjustment method for stabilizing grid transmission

1. Experiment

A sample cell using an electrode (57 mm × 40 mm) including LiFePO₄ for the positive electrode and carbon for the negative electrode was used. As evaluation methods, temperature and charge / discharge cycle tests with different charge / discharge rates and calendar life tests were conducted to analyze cycle characteristics and time characteristics of capacity and internal resistance.

2. Results and Discussion

2.1 Cell performance

The LFP cell produced in this study had an average voltage of 3.05 V (during 1 C discharge), an initial capacity of 1.0 Ah (during 1 C discharge), and a DC resistance of 16 mΩ (SOC 50%). When 1 C charge and discharge were repeated in the initial cell, the temperature rose to 31.1 °C. under natural cooling conditions of 24.9 ° C. outside temperature.

2.2 Degradation characteristics

The capacity decrease is highly dependent on SOC, and in the state of 90% of SOC, the capacity decrease was about twice as fast as 50% of SOC. On the other hand, the increase in resistance was highly temperature-dependent, and the capacity decrease was about twice as fast as at 25 ° C. at 45 ° C. conditions. Also, multiple degradation mechanisms exist because at least two or more inflection points exist, not following the 1/2 power of time, which is the degradation tendency of a general lithium ion secondary battery.

2.3 Life prediction of LIB for grid scale energy storage system

There are many ways to operate the stationary storage system, but as a power adjustment method to stabilize power transmission of the system, the result of predicting the degradation behavior when the battery is operated under the test condition of ancillary service reported by PJM. It has been found that if the number of cells in parallel is reduced to increase the output power per unit cell, the deterioration is accelerated due to the influence of the temperature rise of the cells.