Abstract
Internal short circuit (ISC) is one of the most common causes of thermal runaway (TR) for lithium ion batteries. Several causes of the ISC have been identified including mechanical defects, small impurities blended during manufacturing, lithium dendritic growth and mechanical deformation brought by nail penetration. In order to study the characteristics of ISC, substitute ISC test is an effective way. When designing an ISC substitute test, the ISC should be triggered with practicability, repeatability and reproducibility. Alternative triggering approaches by mechanical deformation, such as nail penetration test and blunt rod test, are under investigation. Unfortunately, as for the most dangerous case – spontaneously induced ISC, these kinds of approaches are unable to simulate the exact thermal-electrical behaviors. The spontaneously induced ISC still cannot be eliminated in practical operation, and threatens the safety of lithium ion batteries.
To best simulate the spontaneously induced ISC, the substitute ISC triggering approaches need to meet following requirements: 1) simultaneously triggering ISC with voltage drop and temperature rise; 2) high repeatability; 3) ISC triggering under control with scheduled triggering fine and specific value of short resistance; 4) the heat generation by ISC completely absorbed by the cell; 5) the discharge caused by ISC diminish the state of charge of the cell; 6) the damage to cell materials similar with practical condition.
Five substitute ISC triggering approaches were evaluated in this work: 1) phase change material (PCM); 2) shape memory alloy (SMA); 3) dendrite growth (DG); 4) equivalent resistance (ER); 5) nail penetration. Identical pouch cells with NMC cathode were selected in all test. The evaluations are carried out from the above six requirements mentioned above. Table 1 The evaluation results of different ISC triggering approaches
| PCM | SMA | DG | ER | Nail Penetration | |
| 1) Thermal-electrical characteristics | Relatively obvious | Relatively obvious | Obvious | Obvious | Obvious |
| 2) Repeatability | High | Middle | High | High | Middle |
| 3) Controllability | ISC occurrence under control ,termination not, TR controlled by SOC and ISC type | ISC occurrence under control ,termination not, TR controlled by SOC and ISC type | ISC occurrence under control , termination not, TR controlled by ISC type | ISC occurrence and termination under control, no TR | ISC occurrence under control , termination not, TR controlled by SOC |
| 4) Heat generation absorption | Complete | Complete | Complete | Partial | Partial |
| 5) Discharge effect | Equal | Equal | Equal | Equal | Equal |
| 6) The damage to the cell materials | Half similar | Half similar | Very similar | No damage | Half similar |
The PCM triggering approach follows the work from NREL with minor difference. PCM are embed PCM during fabrication before electrolyte filling and SEI formation. 4 types of ISC, including aluminum-anode, copper-cathode, aluminum-copper and cathode-anode, can be all triggered by the PCM approach with an ISC temperature at 50oC.
The SMA triggering approach embeds a controllable component SMA in cell during fabrication. It is also capable to trigger 4 types of ISC, with a triggering temperature of 70-80oC, which is the transformation temperature of SMA. Sample perpetration of the SMA triggering approach is shown in Fig. (a).
The DG triggering approach embeds metal particles at positive electrode during fabrication. The growing dendrite intrudes the separator and ISC finally occurs during cycling. This approach is most promising to trigger real spontaneously induced ISC.
The ER triggering approach puts an equivalent resistance between cell layers and a switch is used to control the occurrence and termination of ISC, as shown in Fig. (b). Results show that the voltage drop and the temperature rise are well simulated.
In summary, the PCM and SMA triggering approaches embed controllable components inside the cell to trigger ISC under specific controllable conditions, and both are effective to simulate the thermal-electrical coupled characteristics. The DG triggering approach has great potential to simulate spontaneously induced ISC, which is of significance for the mechanism investigation of ISC. The ER triggering approach could not cast damage to the cell, but is beneficial to establish an ISC model. However, the repeatability and controllability of all approaches require further improvement.
Figure 1