Development and Validation of a ROM Incorporated By a Semi-Empirical Aging Model for a Large-Format LiFePO4/Graphite Cell
Authors: Zhao Xinchen , Choe Song-Yul 
Published: February 2020

Lithium iron phosphate (LFP) as cathode materials was firstly introduced by Padhi etc. in 1997. It has undergone remarkable developments and been widely used in commercial Li-ion batteries because of its good electrochemical and thermal stability. In order to overcome the low conductivity and high-current capability, carbon coating and nanoparticles are used to achieve a high rate capability with low cost. LFP as cathode materials has caused great interest in the field of electric and hybrid electric vehicles. The research aims to develop an online algorithms for LFP/Graphite cells to detailed describe lithiation and delithiation process during galvanostatic discharging and charging. The model is based on the electrochemical thermal principles.

A reduced order model (ROM) has been developed for both fresh and aged lithium iron phosphate (LFP) based cells (nominal capacity: 20Ah), in which plateau and path dependence phenomena caused by two-phase transition during charging and discharging have been taken into consideration. Two-phase transition during lithium intercalation/deintercalation into/from LFP particles is approximately modeled by a shrinking core with a moving boundary at the interface between two phases, lithium-rich phase and lithium-deficiency phase. Moreover, it is also described by the modified diffusion equations with consideration of multiple layers that formed in cathode particles. Polynomial approach, state space approach and some other reduction assumptions, such as linearization of Bulter-Volmer equations, are used for solid concentration, electrolyte concentration and potentials, separately.

For fresh cells, the physics-based ROM has been validated by experimental data obtained through galvanostatic charging and discharging at different C-rate (1C, 3C, 4C) and temperature (25, 45). Path dependence experiments are also conducted and then used to do comparable analysis with simulation results. The simulation results of single cycles, multiple cycles and pulse cycles show great consistent with experiment results. Furthermore, parameter sensitivity of ROM has also been analyzed. Therefore, the model is able to represent the characteristics of plateau and path dependence of LFP cells.

For aged cells, a semi-empirical degradation model has been incorporated into the former ROM. By fitting the semi-empirical degradation model, three important parameters, resistance, electrolyte diffusion coefficient and volume fraction of anode active materials, are extracted from the experimental voltage curve. Five cells are stored in a thermal chamber at 25 for six months at different SOC (10%, 30%, 50%, 70%, 90%). Five cells are cycled under 4C/4C charging/discharging procedure at different temperature (25, 45), SOC cycling range (5%-75%, 25%-95%) and number of cycles. The capacity fade of these five cells are 5.81%, 10.95%, 12.17%, 14.17%, and 23.54%, separately. Experiment results illustrate that high temperature and SOC could accelerate the battery aging. EIS and capacity measurements are conducted every 30 cycles during cycling. EIS results show that the impedance curve shifts rightwards as the increase of ohmic resistance and SEI resistance caused by side reactions. Post-mortem analysis, XRD and SEM, are used to analyze the change of morphology and composition for aged cells The impedances are increased as the number of cycles on the rise. It shows that there is no structure change in both cathode and anode. The model with semi-empirical degradation model incorporated is validated against cycling experiments and shows good consistency.