Fluorinated Electrolyte for High Voltage Li(Ni0.4Mn0.4Co0.2)O2 (NMC442)/Graphite Pouch Cells

Introduction

One of the best ways to increase the energy density of Li-ion batteries is to utilize high voltage cathode materials. Li(Ni_0.4Mn_0.4Co_0.2)O₂ (NMC442) has been proposed as a promising cathode material for high voltage Li-ion batteries. However, the instability of conventional organic carbonates towards high voltage is a great challenge [1, 2]. Searching for an appropriate electrolyte system is essential for application of these materials at high potential.

In this presentation, fluorinated carbonate mixtures which are composed of fluoroethylene carbonate (FEC) and bis(2,2,2-trifluoroethyl) carbonate (TFEC) are shown to be viable in NMC442/graphite pouch cells that can be cycled to at least 4.5 V. The introduction of proper electrolyte additives has been shown to improve the cell performance. Long-term cycling results were also made to compare with 1 M LiPF₆ EC/EMC (3:7 wt.%) with and without additives.  

Experimental

The pouch cells employed in this study were Li[Ni_0.4 Mn_0.4Co_0.2]O₂ (NMC442)/graphite cells with a capacity of 240 mAh and coated NMC442/graphite cells with a capacity of 180 mAh. The positive electrode material in the coated cells has been coated with 3 wt% of LaPO₄ which appears as nanoparticles on the NMC particle surfaces.  Both types of cells were balanced for 4.7 V operation. 1 M LiPF₆ EC/EMC (3:7 wt.% ratio, BASF, 99.99%) was used as the control electrolyte.  The NMC442/graphite pouch cells were filled with 0.75 mL of electrolyte (0.90 g for EC:EMC=3:7 based electrolyte and 1.15 g for FEC:TFEC=1:1 based electrolyte). The cells then underwent a formation protocol during which they were opened and re-vacuum sealed at 3.5 V and again at 4.5 V to remove any gas generated during the first charge. In-situ gas measurements were made using the apparatus and procedure described in reference [3].  After formation, cells were then moved to the Ultra High Precision Charger (UHPC) for cycling.  Storage tests were also used as were frequency response analyzer/cycling (FRA) experiments to measure the impedance growth during cycling [4]. The detailed testing method used in the FRA experiments is described in ref [3]. After these tests, cells were charged or discharged to 3.8 V where impedance spectra and gas volume produced were measured. 

Results and discussion

Figure 1a shows the cell voltage versus time of two NMC442/graphite pouch cells with control and 2% VC in EC:EMC 3:7 electrolyte.  Data for three other cells with, 2% PES, 2% PES + 1% MMDS and 2% PES + 1% TTSPi in FEC:TFEC 1:1 electrolyte are also shown.  Figure 1b shows the in-situ gas volume versus time during the first two cycles for the same cells as shown in Figure 1a.  Figures 1a and 1b show the FEC:TFEC electrolyte system containing PES as an electrolyte additive produces much less gas during the first a few cycles compared to the EC:EMC electrolyte system when charged to 4.5 V. However, a steady increase in gas volume produced occurs.  Figure 1c shows the resistance, R_ct, as a function of voltage for the NMC442/graphite pouch cells with 2% PES in FEC:TFEC 1:1 electrolyte at 40. ± 0.1ºC measured using the FRA. Figure 1c shows that R_ct of the cells with FEC:TFEC:PES electrolyte does not increase when charged from 3.8 V to 4.5 V in each separate cycle. However, R_ct increases as the cycle number increases.   This suggests that FEC:TFEC electrolytes will not solve the problems associated with high voltage NMC/graphite cells.

References

1. L. Ma, J. Xia and J. R. Dahn, J. Electrochem. Soc., 161, A2250–A2254 (2014).
2. L. Ma, J. Xia, and J. R. Dahn, Promising sulfur-containing electrolyte additives or ternary Electrolyte additive mixtures for Li-ion cells that promote long lifetime and better safety, 2015, submitted to J. Electrochem. Soc
3. C. P.Aiken, J. Xia, D. Y. Wang, D. A. Stevens, S. Trussler and J. R. Dahn, J. Electrochem. Soc., 161, A1548–A1554 (2014).
4. K. J. Nelson, G.L. d’Eon, A.T.B. Wright, L. Ma, J. Xia and J. R. Dahn, Studies of the effect of high voltage on the impedance and cycling performance of Li[Ni_0.4Mn_0.4Mn_0.2]O₂/graphite lithium-ion pouch cells, 2015, submitted to J. Electrochem. Soc.

Figure caption

Figure 1. (a) Cell voltage versus time during the first 260 h for the 240 mAh NMC/graphite pouch cells in FEC:TFEC 1:1 electrolyte with different additives cycled at C/20 (12 mA) and 40.0ºC; (b) Gas volume versus time measured using the Archimedes’ in-situ gas analyzer during the first ~260 h (2 continuous cycles and 2 cycles with 10 hour holds at 4.5 V). (c) R_ct as a function of voltage measured every 5 cycles from 3.8 to 4.5 V on FRA. R_ct increases with cycle number