EIS and Structual Analysis of LiCoO2 Cathode Degradation Behavior in Libs at Initial Stage of Charge-Discharge Cycles

Abstract

Lithium ion batteries (LIBs) are widely applied not only in portable electronic devices but also electric vehicles and stationary energy storages. Electric vehicles and stationary energy storages require more safeness and higher durability of LIBs. Therefore, a dynamic and static degradation mechanism of LIBs required to be understood. Electrochemical impedance spectroscopy (EIS) is powerful for understanding of LIBs degradation mechanism. Impedance response of LIBs are complicated because various elemental steps, such as electron migration with inductive response, ion migration in bulk and electrode, and charge transfer of chemical reaction of cathode and anode, are included. In addition, some elemental steps overlap in close frequency region. Therefore, the impedance responses of cathode and anode have to be measured separately for detailed analysis. We have demonstrated impedance separation approaches for LIBs by using temperature control1, separable cell2,3 and micro reference electrode4. Among them, we reported a static degradation of LiCoO2 cathode observing impedance increase of LiCoO2 cathode under storage in the presence of HF.3 In present paper, we focused a dynamic degradation of LiCoO2 cathode caused by initial charge-discharge cycles. Impedance increase of LiCoO2 cathode in 20 charge-discharge cycles and the characteristics change of the active material are discussed. We evaluated a lab-made pouch-type LIB which was composed of a cathode whose active material was commercially available LiCoO2, an anode whose active material was commercially available graphite, and a reference electrode of Al wire (φ 25 μm, 4 mm) which was inserted between the cathode and anode with separation provided by a commercially available polyethylene separator. The electrolyte was 1 M LiPF6/ ethylene carbonate (EC)- diethyl carbonate (DEC) (50:50 vol.%). The EIS was carried out at the open circuit voltage of a state of charge in the frequency range of 100 kHz - 10 mHz and an AC amplitude of 10 mV0-p, while applying the defined voltages via a multi-channel potentiostat and a frequency response analyzer system. The charge-discharge test for degradation was carried out with a constant current of 35.0 mAh (ca. 0.5 C against design capacity) and a cut-off voltage of 4.2 V and 3.0 V via the charge-discharge system. Figure 1 shows impedance responses of LiCoO2 cathode separated from that of LIB by LiAl micro reference electrode after 0, 10, and 20 charge-discharge cycles at the SOC of ca. 60%. The Nyquist plot and Bode plots clearly show two semicircles whose top frequencies are ca. several kHz and several mHz. The latter semicircle is attributed to the charge transfer resistance. The former semicircle would be attributed to a kind of film on the cathode active material like solid electrolyte interphase on anode active material. The radius of those semicircles increased with charge-discharge cycles. Especially, the radius of the semicircle at several kHz increased. To reveal the reason, the crystal structures of the surface of the LiCoO2 particle were evaluated by transmission electron microscopy. The results imply the change of crystal structure with charge-discharge cycles. On the other hand, LiF formation was not confirmed. The surfaces of the LiCoO2 cathode were analyzed by Raman spectroscopy. As the result, the formation of CoO after charge-discharge cycles was confirmed. Therefore, those results suggest that the increase of impedance at several kHz attributed to the surface crystal structure rather than film formation on the active material. We will present the characteristics change with charge-discharge cycles in detail. Acknowledgement This work was supported by “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING)” from New Energy and Industrial Technology Development Organization (NEDO) of Japan.