The Impact of Intentionally Added Water to the Electrolyte in Li-Ion Cells

Electrolyte formulation is crucial to extending the lifetime and increasing the performance of lithium ion cells.  Keeping moisture levels low (< 15 ppm [1]) has always been a standard in commercial lithium ion cell production which also increases the cost of electrolyte and cell manufacturing.  The purpose of this study is to show the impact of small amounts of intentionally added water (100 – 2000 ppm) to the electrolyte impacts cell performance.

Recently it has been shown that short term measurements of the coulombic efficiency and charge endpoint slippage rate coupled with storage and impedance measurements can give good indications of long term cycling performance [2-4]. In this study, wound LiCoO₂/graphite, Li[Ni_0.42Mn_0.42Co_0.16]O₂/graphite and LiCoO₂/Li₄Ti₅O₁₂cells were made (similar to those in references 2-4) and filled with electrolytes containing additives such as vinylene carbonate [3, 5] and LiTFSI [4, 6] with and without intentionally added water to the electrolyte.  Duplicate cells were made for both cycling on the High Precision Charger [7] and storage on an automated cycling/storage system [8] both built at Dalhousie University.  Along with these short term measurements, impedance spectra were collected and long term cycling performance was evaluated. 

This presentation will discuss the impact of adding water with and without the presence of these additives in different cell chemistries along with other studies done on pouch cells containing intentionally added water in the electrolyte.

One outcome of this work is that there appears to be no negative impact, and perhaps a positive impact in adding 1000 ppm water to LCO-graphite cells in the presence of the additives, VC and LiTFSI.  This suggests that in the presence of these additives, water content specifications could perhaps be relaxed somewhat leading to an avenue for cost reduction.  Long term cycling results on many of the cells are available to compare to the short-term, precision, measurements.

Figure 1.  A summary of data collected for LiCoO₂/graphite cells, with and without both additives and water in the electrolyte, including charge transfer resistance (Rct), voltage drop during storage (V Drop), coulombic efficiency (shown as 1-CE) and charge endpoint slippage rate (Ch. Slippage).

Figure 2.  The charge endpoint capacity (top), discharge capacity (middle) and coulombic efficiency (bottom) versus cycle number for LiCoO₂/Li₄Ti₅O₁₂cells at both 30 (left) and 60°C (right) containing various amounts of added water in the electrolyte from 200 – 2000 ppm.

References:

[1] http://www.targray.com/documents/DMMP-Electrolyte-Solution.pdf, last accessed April 9, 2013.

[2] J.C. Burns, G. Jain, A.J. Smith, K.W. Eberman, E. Scott, J.P. Gardner, and J.R. Dahn,  J. Electrochem. Soc., 158, A255 (2011).

[3] J.C. Burns, N.N. Sinha, D.J. Coyle, G. Jain, C.M. VanElzen, W.M. Lamanna, A. Xiao, E. Scott, J.P. Gardner, and J.R. Dahn, J. Electrochem. Soc., 159, A85 (2012).

[4] J.C. Burns, N.N. Sinha, G. Jain, H. Ye, C.M. VanElzen, W.M. Lamanna, A. Xiao, E. Scott, J. Choi, and J.R. Dahn, J. Electrochem. Soc., 159, A1095 (2012).

[5] B. Simon and J.-P. Boeuve, U.S. Patent No. 5626981 (6 May 1997).

[6] M. Armand, M. Gauthier, and D. Muller, U.S. Pat. 5,021,308 (1991)

[7] A.J. Smith, J.C. Burns, S. Trussler, and J.R. Dahn, J. Electrochem. Soc., 157, A196 (2010).

[8] N.N. Sinha, A.J. Smith, J.C. Burns, G. Jain, K.W. Eberman, E. Scott, J.P. Gardner, and J.R. Dahn, J. Electrochem. Soc., 158 A1194 (2011).