The effect of the charging protocol on the cycle life of a Li-ion battery
Introduction
Li-ion batteries have been extensively used in many portable electronic devices such as laptop computers, camcorders and cellular phones. Most of these batteries in the current market employ a graphite-based anode, into which Li+ ions are able to intercalate and de-intercalate in a narrow potential range of 0.05–0.3 V versus Li+/Li through multistage phase transitions [1], [2]. In particular, the potential of the last phase transition from LiC12 to LiC6 is known to be as low as 0.065 V versus Li+/Li. This implies that the maximum over-potential for the graphite anode during charging a Li-ion battery cannot exceed 0.065 V. Otherwise, metallic lithium plating inevitably occurs on the anode, which not only destroys solid electrolyte interface (SEI) on the surface of graphite and reduces cycling efficiency, but also raises safety concerns due to the internal short circuiting caused by the formation of dendritic lithium. In order to alleviate the unwanted lithium plating, Li-ion batteries are required to be charged through two continuous steps. That is, the battery is charged at a constant current until its voltage reaches the pre-determined limit (4.1 or 4.2 V) followed by a constant voltage charging until the current declines to a pre-determined low value. This method is called constant current–constant voltage (CC–CV) charging. In our recent work [3], however, we found that metallic lithium plating can occur near the end of CC-charging step during a normal CC–CV charging as long as the charging current rate reaches or exceeds a certain value. In such cases further increasing of the current rate does not shorten the total charging time significantly, instead it greatly increases the CV-charging time and the battery chemistry deteriorates as a result of the unwanted reactions between the plated metallic lithium and the electrolyte components. Therefore, searching for an appropriate charging protocol is essential for Li-ion batteries to achieve a balance between a short charging time and a long cycle life.
In addition to using electrode active materials with small particle sizes and modifying the cell design (such as by reducing the electrode loading/thickness and increasing the content of conducting carbon), several efforts have been devoted to study the charging techniques and charging protocols. The techniques that have been reported for fast charging or healthy charging include boost charging [4], multistage CC-charging [5], [6], current decay charging [7], [8], [9], and dynamic pulse charging [10], [11], [12], [13]. Regardless of the charging techniques, a short charging time is achieved always at the expense of cycle life with few exceptions [13]. There must be an optimized charging protocol that can balance fast charging and healthy cycling for the Li-ion batteries. In this work, two other charging protocols were designed to compare the conventional CC–CV method. The correlation between charging protocol and cycle life of the Li-ion batteries was studied and discussed by using these three methods with the same charging rate.
Section snippets
Experimental
Two charging protocols, constant power–constant voltage (CP–CV) and multistage constant current–constant voltage (MCC–CV), were designed to study the effect of the charging protocol on cycle life of the Li-ion cells by comparing them with conventional CC–CV charging. In all protocols, a CV-charging was applied to achieve the fully charged state after the cell voltage reached 4.2 V until the current declined to 0.05 C. The CP-charging features a current that starts with a high value and gradually
Characteristics of the charging protocols
Three charging protocols with the same charging rate were, respectively, used to charge the Li-ion cells until the voltage reached 4.2 V followed by CV-charging until the current declined to 0.05 C. Fig. 1 shows the characteristics of these protocols having an averaged charging rate of 0.5 C. A common characteristic is that with each protocol the cell could be charged to 0.82–0.84 SOC at the point where the voltage reached 4.2 V and that the rest SOC was a top-up by the subsequent CV-charging.
Conclusions
From the results of this work, it may be concluded that cycle life of Li-ion cells is significantly affected by the charging protocol even if the same charging rate is applied. A good charging protocol should strictly follow the cell chemistry. Impedance analysis shows that the cell with a low SOC (<0.1) has a much higher resistance, which suggests that a low charging rate be desirable for charging the initial 10% capacity. Potential monitoring indicates that due to electric polarization, the
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