Fundamental Limitations on Fast Charging of Li-Ion Batteries

Long charging time has been a major hurdle of battery-powered electric vehicles (EV). Compared to traditional combustion engine vehicles whose fuel tank can be filled up in less than 5 minutes, EV requires hours to get a full recharge.

A few attempts on reducing charging time of Li-ion batteries have been made in the literature. Notten et al.¹ suggested that batteries be charged with very high currents for a short period of time, followed by CC-CV protocol. Purushothaman and Landau² used pulse-charging sequences. Both their works end up with reduced charging time. However, it is still not clear as to, what the fundamental principles underlying fast charging are, what the fastest charging protocol should be, what the fundamental limits on charging time are, and what the most effective optimizations in cell design and material selection are to enable fast charging.

This study aims to shed light on the above questions as well as to reduce the charging time of Li-ion batteries by means of: (1) charging protocol development, (2) cell design and material optimization. Li-ion cell behaviors are simulated by using a one-dimensional electrochemical-thermal (ECT) model, which has been validated against 18650 cells over a wide range of rates and temperatures.³

A new protocol CC-CS (const current – const stoich) has been proposed based on analysis of experimental data. Large charging current is employed during CC period until graphite particle surface stoich reaches a threshold level, above which permanent capacity loss might be induced. The fastest charging protocol is to maintain the surface stoich at that threshold level without incurring lithium plating. As shown in Figure 1, the CC-CS protocol takes less time than the traditional CC-CV method, and also avoids Li plating.

On cell design and material optimization, the rate-limiting mechanisms for fast charging are identified by both theoretical analysis and modeling parameter studies. It is found that solid phase diffusion and electrode transport are the two limitations. More importantly, this study also illustrates how these limitations are related to cell design parameters and material transport properties quantitatively. By alleviating these limitations, charging can be reduced to within 5 minutes, comparable to the time of refilling a fuel tank in gasoline vehicles.

Acknowledgements
Support of this work by DOE CAEBAT project awarded to EC Power and industrial collaborators of ECEC is greatly acknowledged.

References

1.  P. H. L. Notten, J. H. G. O. H. Veld and J. R. G. van Beek, J Power Sources, 145, 89 (2005).

2.  B. K. Purushothaman and U. Landau, J Electrochem Soc, 153, A533 (2006).

3.  Y. Ji, Y. C. Zhang and C. Y. Wang, J Electrochem Soc, 160, A636 (2013).