Detecting Internal Short Circuits in Commercial Sodium Ion Batteries By Measuring Their Cryo-Resistance
Authors: Rehm Mathias , Jossen Andreas , Schaeffler Stefan 
Published: December 2024

Sodium-ion batteries (SIB) are considered a more cost-effective and environmentally friendly alternative to the commonly used lithium-ion batteries. They are expected to reach energy densities up to 150 Wh/kg and thus to be suitable for applications where currently lithium iron phosphate (LFP) batteries are used. While research on SIBs started over 50 years ago, only recently have SIBs been commercially available. We systematically investigated two different cell types and, among others, cycled these cells with different load rates. Both cell types use hard carbon as anode and layered oxide, namely NaFe1/3Ni1/3Mn1/3, as cathode material. We classified these cells as high-power (SIBHP), and high-energy (SIBHE) types based on their maximum charge currents.

We found that some of these cells showed a sudden increase in temperature accompanied by a voltage drop during charge. The charge capacity was strongly increased, while the discharge capacity showed no, or only a very small difference compared to the previous discharge capacity. Thus, this end-of-charge temperature rise leads to a strong decrease in coulombic efficiency, which was measured to be decreased by only a few percent points up to 40%. This effect happens at lower state-of-charges when the environment temperature decreases or the charge rate increases.

While we could detect this effect during higher charge rates close to the limits of the datasheets for both cell types, for SIBHP, we could detect that pausing before the test can have a strong negative influence. We found several cells which, after being stored for at least 24h at 25°C, have shown this temperature rise already at a charge current of 0.5C, which is only 10% of the maximum allowed value.

To evaluate the cause of this effect, we opened the cells under Argon atmosphere. We could see that the cells which have shown this end-of charge temperature rise had, in contrast to cells which have not shown such behavior, deposits on the anode which we interpret as sodium. These were more clearly recognizable than similar pictures of lithium plating presented in literature. Furthermore, some of the anodes have already shown structural changes. We thus assume that the sodium plating leads to sodium dendrites which causes internal short circuits. This theory is supported by the fact that cells can show for instance over 10 cycles a difference in charged and discharged capacity which is 300% of the initial capacity of the cell (measured for the SIBHP). Thus, a sodium consuming effect could not sufficiently explain this phenomenon.

To proof our theory, we have chosen 3 cells per type which were known to show the effect of sudden temperature rise. These cells were at rest at room temperature for more than 24h. We put these cells into liquid nitrogen for at least 10 minutes. We monitored their voltage and could see that it drops to 0V within the first 2 minutes. Then, directly after removing the cells from the liquid nitrogen, we measured their resistance using a multimeter. Results have shown that the cells which have shown the temperature rise, had significant lower cryo-resistances (for instance 10 kΩ versus 1 MΩ in the case of SIBHP). Then, we charged the SIBHP with 0.5C at 25°C, while the SIBHE were charged with 1C at 20°C. Now, we stopped the test when the temperature rise was more 2K/min. Directly after end of the test, we freezed the cells again as described above. Now, the measured cryo-resistance was strongly decreased down to 30 Ω (in case of SIBHP). After pausing for 1h at 25°C we measured cryo-resistance again and found values in between those before and after the charging of the cells. This can be explained such that during charging internal short circuits due to dendrites build. However, when the cell is stored at room temperature, this short circuits degrade over time. As a result, the cryo-resistance increases again, as the internal short circuits reduce.

Concluding, we could show that the commercially available sodium-ion cells we tested show sodium plating, which can lead to a sudden temperature increase during charging. This effect could be shown to be a result of internal short circuits. This was done by freezing the cells electrolyte using liquid nitrogen and subsequently measuring their cryo-resistance. Lastly, we measured that these internal short circuits degrade over time when storing the cell at room temperature.

Figure 1