A Study of Gas Evolution at Elevated Temperature Using Pouch Cells and Pouch Bags
A lithium-ion pouch cell cycled at elevated temperature may experience volume expansion due to gas production leading to rapid capacity fade. This gas production may be caused by impurities (LiOH and Li2CO3) on the surfaces of electrodes,1 the instability of LiPF6,2 interactions between the electrolyte and the negative electrode or interactions between the electrolyte and the positive electrode. 3, 4 Recently, Xia et al. found that prop-1-ene-1,3-sultone (PES) suppresses gassing especially for NMC/graphite cells cycled at elevated temperature.5 Therefore, studies were made using pouch cells and pouch bags to determine the main factor which causes gas production and understand why the addition of PES can suppress gassing at elevated temperature.
The LiNi1/3Mn1/3Co1/3O2/graphite [NMC (111)] pouch cells (220 mAh) were filled with 0.9 g of 1M LiPF6 in EC:EMC (3:7 v/v) (BASF, 99.99%) with 2% PES. After electrolyte filling, they were placed in a temperature box at 40.0°C at 1.5 V for 24 h. After they were charged to 3.8 V at C/20, they were transferred to a glove box for degassing. After degassing, they were charged to either 4.2 or 4.4 V, then discharged to 2.8 V and charged back to the same cutoff voltage until the current dropped to C/2000. Two pouch cells at each cutoff voltage, either 4.2 V or 4.4 V, were moved to a 60°C temperature box for storage. Their voltage and gas volume were measured using a voltmeter and ex-situ gas equipment, respectively. The other cells were transferred to an argon-filled glove box and dissembled there. The delithiated NMC (111) electrodes collected from the full cells were inserted into different pouch bags (made of the same material as pouch cells) with 0.7 g of control electrolyte with 2% PES. In order to study the effect of the LiPF6 on gas evolution at elevated temperature, some of the delithiated NMC (111) electrodes collected from the full cells were first washed with DMC three times and then inserted into different pouch bags with 0.7 g of EC/EMC with 2% PES. After these pouch bags were vacuum sealed, they were transferred to the same 60oC temperature box for storage. The volume changes of all pouch bags were measured using the ex-situgas equipment.
Results and discussion
Figure 1 shows that the pouch cells and pouch bags containing 2% PES continuously produce gas during storage at 60°C. The rate of gas production for these pouch cells and pouch bags during the 500 h storage period is slower than that for the corresponding pouch cells and pouch bags without 2% PES shown in Figure 1a. This suggests that the addition of PES can slow down electrolyte oxidation at the delithiated NMC (111) electrode maybe due to a better SEI formed at the electrode. However, there is still a large amount of gas generated in the pouch bags even with the addition of PES, which is much larger than that in the corresponding pouch cells. This suggests that the addition of PES does not significantly suppress some gaseous products which can be consumed at the lithiated graphite electrode but suppresses some gaseous products which cannot be consumed at the lithiated graphite electrode.
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Figure 1 (a) Gas volume versus time for: pouch cells having an initial voltage of 4.2 V ( black cross) and 4.4 V (black diamond), pouch bags containing the delithiated NMC (111) electrode taken from pouch cells having an initial voltage of 4.2 V (red cross) and 4.4 V (red diamond) and pouch bags containing the delithiated NMC (111) electrode (DMC washing) taken from pouch cells having an initial voltage of 4.2 V (blue cross) and 4.4 V (blue diamond); (b) Open circuit voltage versus time for pouch cells