The energy storage performance of Li-ion batteries has continuously improved in recent years, reaching over 250 Wh/kg with an annual growth rate of 5.5 Wh/kg [1, 2]. The improved performance is due to the adoption of new active materials such as nickel rich layered oxides, and the maximization of active material fraction in batteries by: I) reducing the thickness of electrochemically inert cell components such as current collectors, separators, and packaging; II) diminishing the contents of inactive materials (e.g., carbon black and binder) in composite electrodes; and III) reducing electrode porosity which minimizes electrolyte amounts. Approaches I and II are physically limited in order to maintain safety and electrochemical performance; increasing the active material loading level or electrode thickness is a promising option that could achieve higher energy densities in the near future.

Increasing the thickness of the electrode, however, generally results in electrochemical performance deterioration. Past modeling studies conclude that the high rate performance of porous electrodes is limited by liquid-phase ion transport due to concentration gradients and ion depletion. Experimentally, however, the effects of ionic conductivity and electronic conductivity of thick electrodes on their rate performance have not been examined simultaneously. In this work, composite electrodes with LiNi_0.8Co_0.1Mn_0.1O₂ as the active material with various microstructures are prepared to understand the electrochemical performance limiting factors. The electrode composition (80:10:10 weight ratio of active material: carbon black : binder), loading (~ 25 mg/cm²) and porosity (30%) are maintained as the same. On the other hand, electrode microstructure is tuned by controlling the electrode slurry compositions such as solvent and solids content ratio. Such variation in processing conditions sometimes leads to intentional crack formation in the electrodes which serves as a mechanism to influence electrode tortuosity. The contact resistance at the current collector/electrode interface (R_c) and the effective ionic conductivity (κ_eff) are estimated by measuring the electrochemical impedance spectroscopy (EIS) of a symmetric coin-cell with two identical composite cathodes and electrolytes composed of a non-intercalating salt (TBAClO₄), as shown in Figure 1 [3]. The rate and cycling performances are assessed by using galvanostatic charging/discharging tests. For these thick electrodes, R_c, in addition to ionic transport in the electrode, has been found to have a significant influence on electrode rate performance. Based on these observations, potential approaches to improve rate performance of thick electrodes will be discussed.

Figure 1. Schematic diagram of the electrochemical impedance spectroscopy (EIS) of the cathode symmetric coin-cell with non-intercalating salt. Note that the real impedance values of the high frequency intercept, the semi-circle, and the 45-degree slope are corresponded to the electrolyte bulk resistance (R_sol), contact resistance at the current collector/electrode interface (R_c), and the ionic resistance in pores (R_ion).

References

[1] H. Sasaki, T. Suzuki, M. Matsuu, Y. Kono, H. Takahashi, R. Yanagisawa, K. Watanabe, A. Fujisawa, Y. Yamamoto, K. Takeda, M. Matsumura, S. Hirakawa, C. Amemiya, N. Hamanaka, N. Oda, iMLB 2016 Meeting Abstracts, MA2016-03 (2016) 99.

[2] C.-X. Zu, H. Li, Energy & Environmental Science, 4 (2011) 2614-2624

[3] J. Landesfeind, J. Hattendorff, A. Ehrl, W.A. Wall, H.A. Gasteiger, J. Electrochem. Soc., 163 (2016) A1373-A1387.