Separation of the charge transfers and solid electrolyte interphase contributions to a battery voltage by modeling their non-linearities regarding current and temperature

One major aging phenomenon during the normal operation of a Li-ion battery is the thickening of its anode solid electrolyte interphase (SEI), which affects the battery state of health. However, the diagnosis of this SEI is difficult because its contribution to the battery voltage is mixed with those of other phenomena. In this article, a method is proposed to separate the contributions of charge transfers and anode SEI. It is based on EIS and GITT measurements and on a model of the charge transfers and SEI non-linearities regarding current and temperature, inspired by Butler–Volmer and Arrhenius equations. The proposed method is applied to a commercial LiFePO4/graphite cell. The activation energies of its SEI and charge transfers resistances are respectively 0.59 eV and 0.81 eV, which is in accordance with literature data. By construction, the method should be applicable to other battery chemistries, unless their cathode SEI has a significant resistive contribution. After an initial calibration, the proposed model can be embedded in a BMS. The latter may then be able to recalibrate the model online thanks to currents pulses at low temperatures and thus track the SEI growth of a battery during its entire operating life.