A lithium-ion cell utilizing LiFePO₄/Li₄Ti₅O₁₂ electrodes has attracted interest as a result of its safe and fast charging performance. A commercially available software package, Battery Design Studio, was utilized to simulate the electrochemical performance of an LFP/LTO cell at different rates and temperatures to understand the charge transfer and diffusion processes of this electrode pairing. Open circuit potential curves for each electrode were obtained using galvanostatic intermittent titration technique (GITT) as a base for modeling. Simulation of the High Pulse Power Characterization (HPPC) test data on a 26650 cell showed that the pulse discharge was affected by six parameters, including ohmic resistance, capacitance, SEI resistance, electrode conductivity, electrolyte diffusion, and electrode diffusion. These parameters were obtained by comparing the experimental data with the model predictions at various SOC and temperatures to demonstrate their SOC and temperature dependence. Sensitivity of parameters was evaluated within the range of the reference values.

To further evaluate the dependence of these parameters on SOC and temperature, AC impedance measurements were performed on the full cell at different SOC and temperatures during both charge and discharge processes as shown in Figure 1. Analyses of impedance results indicated the charge and discharge processes undergo different charge transfer processes and diffusion in both the electrolyte and solid phase. AC impedance on half-cell coin cells was performed to understand the contribution from each electrode.

Finally, the electrochemical model was utilized to understand the performance limitations and failure mechanisms of the LFP/LTO cell though the current and concentration distributions throughout the electrode thickness.

Figure 1. AC impedance measurement on LFP/LTO 26650 cell at different SOC during charge and discharge processes at room temperature.

Reference:

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

[2] V. Ramadesigan, P. Northrop, S. De, S. Santhanagopalan, R. Braatz, and V. Subramanian, J Electrochem Soc, 159, R31 (2012).

Acknowledgement:

This work is funded by the Office of Naval Research under award N0001419WX01258.