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Lithium battery disassembly illustrated in detail

2023/06/16 14:37:24

The following situations are all based on some kind of battery negative sheet condition. Graphite negative electrodes gradually turn golden yellow when lithium ions enter, while positive electrodes are difficult to observe by colour change. We usually judge the battery charging and discharging mechanism and how good the battery is by dismantling the battery and observing the state of the negative electrode sheet interface.

SOC=7%, you can see a ripple in the centre of the electrode, the electrode does not change colour.

SOC=14%, ripples can be seen in the centre of the electrode. Slight discolouration.

SOC=26%, slightly turning blue.

SOC=53.9%, turning blue, lines of bubbles can be seen.

SOC=70.0%, turning a dark purple to red colour, black uncharged areas can be seen. No lithium ion precipitation is observed.

SOC=81.5%, turns golden and black uncharged areas can be seen. White lithium precipitation can be seen.

According to the above pictures, the state of the electrodes does not change much at the stage where the SOC is less than 50%. From 50% onwards a change in the electrode can be observed. According to picture 4, traces of a large number of bubbles can be seen at around 3.6V. The SOC only changes by 10%, but the state of the electrodes is completely different.

The state of the cell electrodes at SOC = 100%. The white part is mostly lithium precipitation.

The surface of the electrode below the white part does not turn golden after stripping, which means that the lithium ions are not embedded in the negative electrode material. In conclusion, this part of the negative material is preventing the embedding of lithium ions and therefore lithium ion precipitation onto the surface is occurring. It is speculated that the binder (CMC, SBR) may be unevenly distributed on the electrode surface and encapsulated by the carbon of the negative electrode material, resulting in the lithium ions not being embedded.

At SOC=100%, the black dots are, more often than not, uncharged areas. More black dots are an important reason for the low capacity of the battery. The main factors leading to black dots are the uniformity of the paste, the precision of the control of the density of the coating surface, the tension control of the winding, and the gas production of the chemical formation.

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