Studies on Capacity Fade of Spinel-Based Li-Ion Batteries

It is well known that the capacity of a lithium-ion battery de- creases during cycling and most of the loss can be associated with some unwanted side reactions that occur in these batteries during overcharge and over discharge conditions. 1 These reactions may cause electrolyte decomposition, passive film formation, active ma- terial dissolution, phase changes in the insertion electrode, and sev- eral other phenomena. Carbonaceous anode materials in lithium-ion rechargeable cells exhibit irreversible capacity loss in the first cycle, mainly due to reaction of lithium during the formation of passive surface films. 2 Passivation of the carbon electrode during the formation period and subsequent capacity loss are highly dependent on specific properties of carbon in use, such as degree of crystallinity, surface area, and so on. Positive electrode dissolution phenomena are both electrode and electrolyte specific and the factors that determine the positive elec- trode dissolution are structural defects in the positive active mate- rial, high charging potentials, and several other phenomena. 1 Oxy- gen defects in the electrode material may weaken the bonding force between the transition metal and oxygen resulting in the metal dis- solution. Previously, capacity fade studies were done on commercially available lithium-ion cells with LiCoO2 as the positive material. 3 These studies revealed that the positive electrode contributes more to the capacity fade of the lithium ion cells, when compared to the negative electrode and the increase in impedance of LiCoO2 elec- trode with cycling is the dominant factor for loss in capacity of the battery. In this paper an attempt was made to study the capacity fade of commercially available spinel-based lithium-ion batteries and also to optimize the charging current based on charging time and capacity fade. Commercially produced Li-ion cells include several features for safe operation under different conditions. During charging, to pre- vent electrolyte oxidation a potential limit ~charging to ultimate volt- age! is used with internal electrical circuitry ~cell voltage control and equalization circuit!. 4 However, different charging protocols lead to different charging times. Further, varying the charge protocol also affects the capacity fade during cycling. 1 One of the commonly used charging protocol for Li-ion cells is charging at constant cur- rent to a particular voltage and subsequently holding the potential constant. In this case, the total time for charging is held constant. One of the drawbacks of this process is that, since the total charging time is constant, the battery is held at a high constant voltage for longer than essential. In this case, holding the cell potential at high voltage can contribute to oxidation of the cathode leading to capac- ity decay during cycling. Optimization of charging protocol is essential to achieve superior performance for the Li-ion batteries. Objectives of this paper were to study the performance of lithium-ion batteries with spinel-based cathodes. First, we want to optimize the discharge capacity of the cell based on the charge current, end potential, and total charging time. Next, we compare the capacity fade of cells charged at differ- ent rates to a common end potential and discharged at the same current. The goal here is to minimize the capacity loss with cycling by choosing an optimum charging current. Finally, we study the causes for the capacity fade in spinel based Li-ion batteries.