Li-ion Batteries

1. BACKGROUND & OBJECTIVE

Li-Ion Batteries (LIB) are much expected for various application ways like cell phone, PC battery, electric car, emergency battery of airplane and power plant and so on because of high energy density. However, LIB often uses Ethyl Methyl Carbonate (EMC) as a main component of electrolyte, which is flammable organic carbonate solvent. This feature brings quite higher combustion energy of electrolyte than electro-chemical energy which LIB has. Consequently, suppressant of EMC flammability is very important to improve the fire safety of LIB. Therefore, we focus on the application of Organophosphorus Compounds (OPC), which is known as a fire suppressant to solve LIB's sensitive problem. In this research, the effect of OPC on the flammability of EMC has been studied. Especially, relationship between the amount of OPC addition and extinction limit (=Limit of Oxygen Concentration: LOC) of EMC has been investigated experimentally. The main aim is to evaluate the OPC addition influence quantitatively and reveal critical flame retardant mechanism. Final goal is supply useful data to LIB industry to realize more safety battery.

 

 

2. EXPERIMENTAL OVERVIEW

In this research, we've been used EMC for “fuel” and Trimethyl Phosphate (TMP: (CH3O)3P=O) and Triethyl Phosphate (TEP: (CH3CH2O)3P=O) for “fire retardant” for the experiment. Figure 2 shows the experimental apparatus. Some kinds of mixture fuel like only EMC, EMC + 5wt%OPC, etc. are tested with wick combustion method known as alcohol lamp to evaluate extinction phenomena of organic solvents mixture. In the experiments, fuel rate is kept constant by overflow system. We changed O2%, flow velocity and OPC addition%, then specified the LOC, direct image, chemical luminescence intensity, fuel consumption rate, flame temperature.

 

 

3. RESULTS & DISCUSSION

   From LOC detecting experiment, we found TMP’s retardant effect becomes higher than TEP at the same addition amount, on the other hand the trend of LOC regarding to flow velocity doesn’t change not so much. Furthermore, the difference of LOC between additive-free and 5wt% addition condition is very large but that between 5 and 10wt% addition condition is not so large. From Figure 3, we could observe flame retardant effect, but we are also able to see flame promotion effect from temperature measurement: increase temperature by OPC addition. That is to say, there exist two opposite phenomena in terms of LOC decision on the effect of OPC: chemical retardant and physical promotion effect.At this point, it's very difficult to consider about chemical kinetics and heat balance at the same time, so we separated. Now chemical kinetics study is conducting by using CHEMKIN and we try to predict flame retardant mechanism. EMC or TEP's reaction is veiled so suggestion of EMC and TEP reaction mechanism is much challenging. Heat balance modeling study is just started so still unclear but to discuss about retardant effect and near LOC condition, heat balance is absolutely important.Finally we are going to combine the two effects and realize quite detail discussion and prediction.

 

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