Cell self-heating rate

The PHI-TEC or VSP bench scale apparatus can be employed to determine information about the self-heat rate and vapor disengagement when this is not readily available. Additionally, the VSP equipment can be used for flashing flow characteristics using a special bottom vented test cell. Here, the flowrate, Gq (kg/sm ), is measured... 

By this technique, the temperature directly tracks the exothermal process under pseudo-adiabatic conditions. Pseudo, because a part of the heat released in the sample serves to heat the cell itself. Nevertheless, essentially in the USA, it became a very popular method as a screening technique. Concerning its sensitivity, for a well-tuned instrument, able to detect a self heating rate of 0.01 Kmin"1, with a sample mass of 2g, the sensitivity is as low as 0.5 W kg"1. 

FIGURE 19.2 Self-heat rates of fully charged 18,650-type cells with spinel, layered LINIo.33Coo.33Mno.33O2 and olivine cathodes and graphite anodes In 1.2 mol/l LIPFo In ethylene carbonate (EC) and ethyl methyl carbonate (EMC)... 

IiCo02 is still the cathode of choice for the majority of Li-ion cells produced today. However, it is the most reactive and has poorer thermal stability than the other cathodes. A calorimetric measurement of cells with different cathodes tells the story. Accelerating Rate Calorimetry (ARC) is a common technique to measure reactivity of combinations of materials, and is particularly well suited to the characterization of batteries. The technique places the cell in a nearly adiabatic environment, and slowly increases the temperature of the cell. At specific intervals, the ARC measures the heat output of the cell. If the self-heating rate is above a specific value (0.2 °C/min is typical), the ARC stops applying heat and follows the temperature of the cell. The result is a measurement of self-heating rate vs cell temperature. Data for full Li-ion cells fabricated with different cathodes is shown in Figure 27.4. Key parameters measured from ARC are onset temperature and maximum self-heating rate. 

Figure 27.5 Onset of self-heating in ARC experiment on Li-ion cells. LiFeP04 olivine cathodes show the greatest reduction in self-heating rate and increased onset temperature for runaway (from Ref [35]).

Fig. 14.12 Self-heat rates of fully charged 18650 cells with spinel, layered, and olivine cathodes measured in ARC. Self-heat rates of olivine cathode overcharged to 4.2 V are also shown...

Figure 14.12 shows the self-heat rate (SHR) released from the 18650 cell (LiFePOV/graphite) fully charged to 1.42 Ah capacity, when heated up to... 

Processes which generate heat in organic materials are reviewed. At ordinary temperatures, respiration of living cells and particularly the metabolism of microorganisms may cause self-heating, while at elevated temperatures pyrolysis, abiotic oxidation, and adsorption of various gases by charred materials drive temperatures up whenever the released heat is unable to dissipate out of the material. The crucial rate of pyrolytic heat release depends on exothermicity and rates of the pyrolysis process. 

Both liquid and powdery chemicals of the TD type are, however, the same to the effect that their exothermic decomposition reactions are accompanied with no phase transition. Therefore, when charged in the open-cup cell, or confined in the closed cell, in accordance with the self-heating property of the chemical, and subjected to the adiabatic self-heating test started from a r, 2 cm each of a liquid chemical, or a powdery chemical, of the TD type continues to self-heat over the at a very slow, but virtually constant, rate depending on the value of Ts in accordance with the Arrhenius equation, after its having been warmed up to the Ts. 

Figure 22, Dependence of the rate of increase in temperature of the resistor-thermocouple composite set in the draft cell and subjected to the adiabatic self-heating test started from a 7) near 60 t under statie eonditions on the rate of electric heat generation, (tW,...

On the other hand, it is apparent to us in Fig. 22 that, as the temperature of the composite rises away from the 7, at each rate of electric heat generation, the adiabatic control becomes gradually broken. As stated in Subsection 4.4.6, die glass closed cell used in the recorder is not perfcclly explosion-proof besides. Therefore, the adiabatic self-healing process recorder is not appropriate for the record of the whole self-heating process up to the thermal explosion of 2 cm of... 

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