As lithium-ion batteries have been extensively used in many applications, for example, mobile devices, electric vehicles, and so on, it is necessary to adopt the most suitable charge protocol to meet the general requirements: a short charge duration and a long cycle life. However, in a real situation of the battery operation, it is difficult to satisfy both of the two requirements because a shorter charge duration represents the need of a larger current flowing through the cell, which further causes a larger degree of degradation of cells. The aim of this study is to investigate the correlation between the degree of mechanical degradation and the charge protocol imposed on cells.

Mechanical degradation during discharge-charge cycle consists of two major parts for commercialized cells. One is the SEI cracked by the cyclic diffusion-induced stress due to the lithium intercalation/deintercalation¹. The other is the fracture of active material caused by the unstable crack growth when the stress intensity factor calculated by diffusion-induced stress exceeds the fracture toughness of electrode particles. These two characteristics show that the diffusion-induced stress is an important measure to assess the degree of degradation.

By constructing isothermal one-dimensional porous electrode model of Lithium Manganese Oxide (LMO) cell and three-dimensional electrode particle stress analysis, we simulate the time evolution of the diffusion-induced stress at the center of the three-dimensional particles in a one-dimensional cell under various charge modes. The charge protocols under consideration include Constant Voltage (CV)²,  Linear Current Decay² (LCD), Constant Current (CC), and Constant Current-Constant Voltage (CC-CV). As shown in Figs. 1 and 2, we specify the values of the slope of current decay, the constant charge rate and the voltage such that at the same cut-off charge time, the state of charge (SOC) approaches unity.

Figure 3 shows the radial stress evolution for various charging protocols, indicating that the electrode particle suddenly experiences highest peak stress during the CV charging mode. The high stress, which further gives rise to unstable crack growth, will lead to the failure of the electrode particle after a few discharge-charge cycles. The peak stress produced by LCD is larger than that generated by CC and CC-CV mode. According to the Paris law, the larger peak stress would accelerate the subcritical crack growth. Consequently, the crack on the surface of the electrode particle exposed to electrolyte enhances the SEI formation and growth¹, giving rise to a further loss of lithium ion. Besides, comparing with the CC and CC-CV modes, more degradation of the cell for the LCD mode is expected since a larger amount of irreversible Li-ion would be accumulated.

REFERENCES

[1]  R. Deshpande, M. Verbrugge, Y.T. Cheng,  J. Wang, P. Liu, Battery cycle life prediction with coupled chemical degradation and fatigue mechanics, Journal of Electrochemical Society, 159, A1730 (2012)

[2]  G. Sikha, P. Ramadass, B.S. Haran, R.E. White, B.N. Popov, Comparison of the capacity fade of sony US 18650 cells charged with different protocols, Journal of Power Sources, 122, 67 (2003)