Capacity Fade Analysis of 18650 Li-ion Cells: A Methodology for Cycle Life Prediction

P. Ramadass, Bala S. Haran, R.E. White and B.N. Popov

Department of Chemical Engineering University of South Carolina, Columbia, SC

Introduction

There have been a variety of applications that use lithium ion batteries due to their high capacity, energy density as well as good reversibility. One of the problems associated with the performance of Li-ion batteries is capacity fade with cycling. Capacity decay is caused by various mechanisms, which depend on the electrode materials and also on the protocol adopted to charge the cell. Capacity fade in Lithium-ion cells can be attributed to unwanted side reactions that occur during charge, which cause electrolyte decomposition, passive film formation, active material dissolution and other phenomena.1

Our capacity fade studies on spinel based Li-ion

cells showed us the influence of the charging protocol on capacity fade.2 Our recent studies on Sony 18650 cells show3 that capacity fade increases with increase in temperature and discharge rate. Thus, it is critical to quantify the factors that influence the capacity loss with cycling. Our objective thus focuses on developing semi empirical correlation between capacity fade and cycle number for Sony 18650 cells and to use Li-ion diffusion model for predicting the nature of discharge curves with cycling. Results and Discussion:

Sony US18650S cells with a rated capacity of 1800 mAh were used for these studies. For cycling studies, the cell was charged at a constant current of 1 A until the potential reached 4.2 V. Subsequently the voltage was held constant at 4.2 V until the current drops to 50 mA. From the cycling studies of the 18650 cells and analysis of fresh and cycled positive and negative electrodes, we were able to identify three distinct factors involved in capacity fade of Li-ion cells. These factors are increase in cell resistance with cycling, loss of primary active material (Li+) and loss of secondary active material (LiCoO2/carbon).

The initial drop in cell voltage during beginning

of discharge keeps increases with cycling and the drop in cell voltage during the course of discharge is also found to be higher with cycling. This clearly indicates the increased internal resistance of the cell and is one of the critical parameter to be considered for simulating discharge curves after several cycles. T-cell studies of positive and negative electrode materials show a decrease in intrinsic capacity with cycling. Since State Of Charge (SOC) of electrode material is directly proportional to its intrinsic capacity, the second parameter to be considered for simulation is the SOC of positive and negative electrodes.

Li-ion diffusion model developed by Doyle et

al.4, has been used to simulate discharge curves after several cycles of charge and discharge. Based on the cycling data and the half-cell experiments, the parameters considered most significant for capacity losses are resistance and SOC of positive and negative electrode. An attempt was made to develop semi-empirical correlations

for these parameters with cycling. Discharge curves obtained through simulation based on the semi-empirical correlations were then compared with discharge performance obtained experimentally. From our initial results we could say that rigorous measurements of intrinsic capacity and impedance measurements of fresh and cycled electrode materials are critical for better cycle life predictions. The quantitative analysis of capacity fade of Sony 18650 cells with cycling and the semi-empirical correlations for critical capacity fade parameters (resistances and SOC) and the predicted cycle life based on the correlations would be presented in detail.

Acknowledgment

Financial support provided by National Reconnaissance Office for Hybrid Advanced Power Sources # NRO-00-C-1034 is acknowledged gratefully. References 1. P. Arora, R. E. White and M. Doyle, J.

Electrochem. Soc., 1998, 145, (3647). 2. Ramadass Premanand, Anand Durairajan, Bala

Haran, Ralph White and Branko Popov, J. Electrochem. Soc., 2001, 149, (A54).

3. P. Ramadass, Bala. S. Haran, R. E. White and B. N. Popov, Proceedings of the 17th Annual Battery Conference, Vol.1, 13 (2002).

4. M. Doyle, T.F. Fuller and J. Newman, J. Electrochem. Soc., 1993, 140, (1526).

Fig. 1 Comparison of experimental and simulated discharge curves of Sony 18650 cells cycled at RT. The simulation is based on semi-empirical correlations developed for resistances and SOC of positive and negative electrodes based on full cell and T-Cell studies.

ModelExptModelExpt