Study of the Impact of Electrolyte Additives on High Voltage LiNi_0.4Mn_0.4Co_0.2O₂/Graphite Cells by Electrochemical Impedance Spectroscopy

Introduction Materials such as LiNi0.4Mn0.4Co0.2O2 have been proposed as good candidates for high energy density cells. Even though these types of materials show reversible cycling up to 4.7 V, the stability of typical carbonate based electrolytes towards such high voltage proves to be a problem. Electrolyte additives can improve capacity retention at high voltage, however very little is known about how these additives work. The effect of three additive combinations on the cycling of LiNi0.4Mn0.4Co0.2O2/graphite cells cycled to different upper voltage cut-offs has been studied using electrochemical impedance spectroscopy (EIS). Experimental Dry pouch LiNi0.4Mn0.4Co0.2O2/graphite cells balanced for 4.7 V with a nominal capacity of 240 mAh were obtained from a reputable cell manufacturer. The pouch cells were filled with ~0.9 g of 1M LiPF6 EC:EMC (3:7) based electrolyte with different additive combinations. The additive combinations tested were 2% vinylene carbonate (VC), 2% prop-1-ene-1,3-sultone (PES), and 2% VC + 1% Methylene Methanedisulfonate (MMDS) + 1% additive A. Cells were vacuum sealed in an argon-filled glove box. The cells then underwent a formation protocol during which they were opened and re-vacuum sealed at 3.5 V and 4.5 V to remove any gas generated during the first charge. Cells were then moved to a charger to be cycled at 40°C to various upper potential cut-offs. After a period of 400 – 500 h of cycling, the impedance of the cells was measured at 3.8 V and at 10°C. Results and discussion Figure 1 shows the discharge capacity as a function of time for cells containing various additive combinations and cycled to various upper voltage cut-offs. Figure 1 shows that all cells exhibit capacity loss when cycled to potentials above 4.4V. Cells containing 2% VC + 1% MMDS + 1% A have encouraging cycling performance at both 4.4 and 4.5 V. Figure 1 also shows that cells containing any of the additive combinations show substantial capacity fade when cycled at to 4.5 V and above. Figure 2 shows the Nyquist representation of the impedance of LiNi0.4Mn0.4Co0.2O2/graphite cells cycled for 400 - 500 h to different upper voltage cut-offs. Figure 2 shows that the additives have a dramatic impact on the impedance of cells cycled to high voltage. Figure 2 shows that the additive combination, 2% VC + 1% MMDS + 1% A, yields a very small impedance compared to other more conventional additives such as 2% VC. Figure 2 also shows that the shape of the EIS spectrum of cells containing any additive combination dramatically changes when increasing the higher voltage cut-off. Other measurements (not shown) showed that the EIS spectra of cells cycled to upper cut-offs of 4.0 V to 4.3 V are very similar. This seems to indicate the activation of various surface reactions when increasing the upper voltage to 4.4 V and above. These surface reactions seem to be dramatically hindered when the electrolyte additive ,2% VC + 1% MMDS + 1% A is used Conclusion The effect of different additive combinations on the cycling performance of LiNi0.4Mn0.4Co0.2O2/graphite cells cycled to various upper voltage cut-offs and up to a maximum of 4.7 V has been evaluated. Results showed that 2% VC which is a very useful additive in 4.2 V cells yields very poor cycling performance for high voltage applications. Other additives combinations such as 2% VC + 1% MMDS + 1% A yield dramatically improved performance. This study also showed that the impedance spectra of cells cycled to 4.4 V and above dramatically change as the upper cutoff potential is increased.. Reference 1. J. C. Burns et al., J. Electrochem. Soc., 160, A1668–A1674 (2013).

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