## 3 Results and Discussion  
To understand the effect of the rapid charging at 3C with a conventional constant current profile (3C-CC) and the rapid charging at 6C with a pulse current profile (6C-PC) on the performance of the commercial cylindrical LIB, the charging and discharging conditions were kept the same as explained above. Figure 3 shows the discharge capacity and Coulombic efficiency according to the number of charge and discharge cycles. In the case of the 3C constant current charging, the capacity retention and the Coulombic efficiency were maintained stable during the first 50 cycles, and then gradually decreased over the next 150 cycles. Considering the fact that the rapid charging was carried out over the range from 20% SOC to 50% SOC, which is believed to be a rather robust, safe, and stable range against charging stress and therefore does not significantly affect the  
Figure 2: Charging current-time profile of constant current and pulse current procedures. battery performance, there was a clear fluctuation in both the capacity and Coulombic efficiency after 50 cycles. It was believed that the high charging current value of 8.4 A for relatively longer time might cause battery deterioration. In contrast to the constant current charging, the pulse charging, which was constructed with 0.5 s of 6C current charging and 0.5 s of resting time, astonishingly showed stable capacity and Coulombic efficiency over 450 cycles. To clarify the difference between the two charging methods, the capacity-voltage profiles were investigated, as shown in Figure 3(a) for the case of 3C constant current charging and Figure 3(b) for the case of 6C pulse current charging. In the case of 3C constant current charging, the initial slope of the charging voltage curve became steep as the cycle progressed, and the increase in voltage magnitude at the end of charging was noticeably observed. In particular, on the 150 th charging voltage curve, it can be seen that a very small capacity was obtained during constant current charging by the charge voltage limit of 4.2 V. This might come from the over-potential formation and lithium plating that occurred on the surface of the graphite anode, while the insertion process took place incompletely in the graphite structure due to the high C-rate charging [8,9]. It is known that the plated lithium interacts with the electrolyte to form the SEI, which increases the internal resistance of the battery. In addition, continuous SEI formation would result in lithium consumption, formation of \"dead lithium\" and lithium dendrites,  
Figure 4: Voltage-capacity profile at selected cycles. Figure 3: Discharge capacity and Coulombic efficiency. electrode cracking, and loss of electrical contact in electrodes [10, 11], which increase the internal resistance of the battery and further decrease its capacity. Particularly in the current research, even though the repetitive charging and discharging were performed only within a portion (20-50% SOC) of the total battery capacity, the capacity decrease and the increase in charging start voltage (10-100 th cycle, 0.185 V) clearly revealed the limitation of high C-rate constant charging on typical lithium ion batteries. On the other hand, the 6C pulse current charging showed a voltage-capacity curve that was quite different from the 3C constant current charging one, as illustrated in Figure 4b. Charging was performed by switching between the high voltage and low voltage parts according to the pulse current application and the pause. In the current study, the pulse current was applied for 0.5 s and the pause was made for 0.5 s in sequence, as shown in Figure 2. Both high and low voltage curves were obtained from the battery under the current application and the pauses.