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Acoustic Emission (AE) technique is employed to detect degradation events inside of lithium-ion batteries. A commercial 18650 type battery and a model half cell (Li/organic electrolyte /LiCoO 2 ) are monitored with AE technique during charge and discharge cycles. In the commercial battery, AE events are detected during every discharge process at SOC of around 35% and 15%. These AE can be attributed to the fracture of cathode material by phase transition. In the model half cell, AE events are found early cycles in charge process owing to the SEI formation, however in discharge process detected only under high C rate.

In this study, the effects of charge/discharge cycling on the thermal stability of LiNi 0.8 Co 0.1 Mn 0.1 O 2 , a high-Ni cathode material, are systematically investigated. X-ray diffraction measurements show that there is almost no change in the bulk structure of the cathode after cycling. However, X-ray absorption fine structure measurements indicate that Ni in the surface layer is reduced and stable rock-salt structures are formed. Differential scanning calorimetry (DSC) measurements show that the heat generation at the lowest temperature, which can trigger thermal runaway in batteries that use high-Ni cathodes, decreases significantly with the formation of rock-salt structures on the active material surface. This finding indicates that the rocksalt layer on the surface enhances the thermal stability of a high-Ni cathode. The change in the total heat generation with degradation, indicated by DSC measurements, is similar to that in the K-edge of Ni (i.e., the Ni valency), suggesting a strong correlation between the heat generation and crystal structure changes during cycling.

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A nondestructive, simple detecting method of internal Li-metal plating in the Lithium-ion battery (LiB) is essential for diagnosing the safety of battery systems for realising the long-life use of the LiB. We show a direct Li-metal detection technology that is dependent on the electromagnetic response of the battery at a high-frequency (MHz). By the unique response of the high-frequency electromagnetics based on the Maxwell’s law, a negative correlation is observed between the real-part impedance and the internal Li-metal plating deposited on the anode surface by the partial electrochemical potential overstress. This finding makes the diagnostic process much simpler than conventional methods such as data-driven analytical approach, because this finding measures a direct response from the electronic behaves to the metal deposition, rather than symptoms from the ionic behaves. The sufficient decrease in the real-part impedance was confirmed only in the Li-metal plated commercial batteries (LFP-18650, 1500 mAh, representatively) that are degraded by repeated rapid charging. Notably, this finding can be assembled as a battery monitoring sensor that diagnoses the growth of Li-metal plating during real-field operation. This finding is expected to contribute to the solutions for the new global environmental challenges by supporting the safe and long-term use of LiBs both in the classification process and its reuse.