The Effect of Intentionally Added Water on Gas Evolution in Li-Ion Pouch Cells

Introduction

    Water is widely regarded as having a detrimental effect on cell performance so Li-ion battery electrolyte normally has a water content below 50 ppm (1). This stringent water content specification results in increasing manufacturing cost.  Burns et al. found that prismatic cells with intentionally added water function well and, in fact, sometimes better than cells without added water (2, 3). However, the added water caused cell swelling. In this study, pouch cells were been chosen to study gas evolution from intentionally added water.

Experimental

    323036-size LiCoO₂ (LCO)/graphite (nominal 300 mAh) pouch cells were received from BAK Co. (Shenzhen, China). 1 M LiPF₆in EC:DEC (BASF, USA) (1:2 v/v) was used as control electrolyte. 2 wt % vinylene carbonate (VC) (99.97% BASF, USA) and/or 2 wt % HQ115 (99.9% purity, 3M, USA) were used as additives. About 1 g of electrolyte was added to the pouch cells. The first charge-discharge cycle (called the formation cycle here) was: hold at 1.5 V for 24 h, charge at 2 mA for 10 h and then at 15 mA (corresponding to C/20) to 4.2 V followed by a discharge to 3.8 V at 15 mA. The total volume of gas generated in the cells after the formation cycle was measured using Archimedes’ principle by weighing the cells while submerged in nano-pure water (18 MΩ) and comparing the weight before and after formation. Two cells with 0.84 g of control electrolyte + 1750 ppm water and 0.79 g of control electrolyte + 1750 ppm water + 2% VC were held at 1.5 V for 45 h and 1.9 V for 10 h and measured by in-situ gas equipment (4).

 

Results and discussion

    Figure 1 shows the gas volume evolved during formation as a function of the amount of added water. The slope of the gas volume versus water content curve for cells without VC is roughly the same as that for cells with VC. This may suggest that the gas generation mechanism due to added water is the same whether the cells contain VC or not.  VC has been shown to limit gas generation in cells without water (4) by presumably changing the distribution of products of electrolyte reduction from gases to liquids or polymers.

     Figure 2 shows gas evolution versus time for cells with and without VC in the presence of added water. It shows that the gas volume gradually increases during the two-step potential hold at 1.5 V and 1.9 V where no electrolyte reduction or oxidation occur. After the two-step potential hold, the gas volume from added water in both electrolytes roughly match the theoretical calculation that one mole of water produces one mole of hydrogen. This may suggest that added water first reacts with LiPF₆to produce HF and then the HF reacts with lithium and produces hydrogen.

     The above results show that more gas is generated when more water is added to the electrolytes. At both 1.5 V and 1.9 V potential holds, water, or HF derived from water, can react with lithiated graphite and can be eliminated, which may be the reason why added water does not have a detrimental effect on cell performance as shown in references 2 and 3.

References

[1]http://www.targray.com/documents/DMMP-Electrolyte-Solution.pdf, last accessed Feb. 11th, 2014.

[2] J. C. Burns et al. J. Electrochem. Soc., 160, A2281-A2287 (2013)

[3] J. C. Burns et al. J. Electrochem. Soc., 161 A247-A255 (2014)

[4]  C. P. Aiken, J. Xia, David Yaohui Wang, D.A. Stevens, S. Trussler and J. R. Dahn, An apparatus for the study of in situ gas evolution in Li-ion pouch cells, submitted to J. Electrochem. Soc., Feb. 14, 2014.