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Introduction Development of high voltage Li-ion cells is critical for the improvement of energy density of Li-ion cells [1]. Electrolyte additives are used to improve the properties and performance of Li-ion cells [2]. However, the way that electrolyte additives function and the impedance changed that occurs with voltage in Li-ion cells have not been well-explained in the literature. A Maccor 4000 series charger, combined with a frequency response analyzer (Maccor FRA 0356), was used to investigate the effects of electrolyte additives on cell impedance changes with voltage in Li[Ni0.42Mn0.42Co0.16]O2 (NMC)/graphite and LiCoO2(LCO)/graphite pouch cells. Experimental Machine-made Li[Ni1/3Mn1/3Co1/3]O2/graphite and LiCoO2/graphite dry pouch cells (402030 size, 220 mAh) were supplied by reputable manufacturers and were filled and sealed at Dalhousie University. Cells were filled with 1 M LiPF6in ethylene carbonate (EC):ethylmethyl carbonate (EMC) (3:7 by weigh, BASF) as control electrolyte. Vinylene carbonate (VC, BASF, 99.97%) was used as an electrolyte additive. After electrolyte filling and vacuum sealing (MTI Corporation, MSK-115A) in an argon-filled glove box, a 24 h hold at 40.0 ± 0.1°C and 1.5 V was used to ensure complete wetting of the cell coil. The first charge cycle (called the formation process here) consisted of charging at 11 mA (corresponding to C/20 current) to 3.5 V. Then cells were degassed in the glove box and vacuum sealed again. Subsequently, cells were charged to 4.5V using the same current, followed by degassing and vacuum sealing in the glove box. The cells were cycled using a Maccor 4000 series charger between 2.8 and 4.7 V at 40.0 ± 0.1°C using currents corresponding to C/20 while the cell impedance was measured at every 0.1 V interval between 3.6 and 4.7 V with a Maccor frequency response analyzer (FRA 0356). The FRA unit and cells during testing were in temperature controlled environments (21°C for NMC/graphite pouch cells, and 30°C for LCO/graphite pouch cells) with variations in temperature of an amplitude less than 2°C. AC impedance spectra were collected from 10 kHz – 10 mHz with an amplitude of 2 mV. Ten data points per decade were measured. Results and discussion Figure 1 shows selected Nyquist plots for NMC cells containing 2 % VC during the first charge and the first discharge. The charge transfer resistance (Rct) was marked in Figure 1a and taken to be the diameters of overlapping semicircles from the Nyquist plots [3]. Cells with a small Rct are much more desirable. Figure 2 shows summary of Rctversus voltage for NMC/graphite and LCO/graphite pouch cells with 2 % VC. The impedance of NMC/graphite cells increases a lot as the cells are charged above 4.3 V. This impedance change is almost reversible over one cycle but the impedance slowly increases cycle by cycle. By contrast the LCO materials do not show any real impedance increase after charging to 4.4V. There are many interesting things to note which will be discussed in the lecture. References [1] M. Hu, X. Pang and Z. Zhou, J. Power Sources, 237, 229 (2013). [2] S.S. Zhang, J. Power Sources, 162,1379 (2006). [3] D.Y. Wang, N.N. Sinha, R. Petibon, J.C. Burns, and J.R. Dahn, J. Power Sources, 251, 311 (2014).

Introduction Electrolyte additives can be used to improve the lifetime of a Li-ion cell [1]. Electrolyte additives are believed to function by forming or modifying a solid electrolyte interface (SEI) layer on the surface of the positive or negative electrode thus impacting the cycle life, calendar life and safety of Li-ion cells. Ethylene sulfite (ES) has been widely studied by many researchers and has been regarded as an effective SEI-forming additive, especially in PC based electrolytes. Wrodnigg et al. [2] found the introduction of 5 vol % ES to a PC-based electrolyte could suppress or even prevent PC co-intercalation into graphite. Ota et al [3] suggested that when ES was used as an electrolyte additive, the SEI film on the graphite anode contained both inorganic materials like Li2SO3 and organic materials like ROSO2Li. The bulk of the studies on ES have focused upon its effect on the carbon electrode. In this presentation, a detailed study of ES and/or VC as electrolyte additives for Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells was investigated using UHPC [4] and a precision storage system at Dalhousie University [5]. Gas evolution during formation and cycling, coulombic efficiency (CE) and charge endpoint capacity slippage during cycling as well as charge transfer resistance before and after cycling were examined and compared. Experimental Dry Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite pouch cells (225 mAh) were obtained from Whenergy (Shandong, China). Before electrolyte filling, the cells were cut just below the heat seal and dried at 80°C under vacuum for 14 h to remove any residual water. Then the cells were transferred immediately to an argon-filled glove box for filling and vacuum sealing. Cells cycled using the UHPC were tested between 2.8 and 4.2 V at 40.0 ± 0.05 °C using currents corresponding to C/20 for 15 cycles where comparisons were made. Electrochemical impedance spectroscopy (EIS) measurements were conducted on NMC/graphite pouch cells before and after cycling on the UHPC. Results and discussion Figure 1 shows data collected from the UHPC during cycling. Figure 1 shows cells containing 2% VC + ES (1 % or 2 %) can provide similar performance in delta V, coulombic efficiency and charge endpoint capacity slippage to cells containing 2% VC. Figure 2 shows the Nyquist plots for NMC/graphite pouch cells with different amounts of ES and/or VC after formation and after UHPC cycling measured at 3.80 V and 10°C. Figure 2 shows that cells containing only ES show obvious impedance growth during cycling. When ES (1% or 2%) is used in combination with VC, the impedance was dramatically decreased both before and after cycling. Cells with 2% VC + 2% ES have the lowest impedance after cycling, only half of that of cells with 2% VC. Therefore, there appear to be significant benefits of the combination of VC and ES for high power cells. References 1. S. S. Zhang, J. Power Sources, 162, 1379 (2006). 2. G. H. Wrodnigg, J. O. Besenhard and M. Winter, J. Electrochem. Soc., 146, 470 (1999). 3. H. Ota, T. Akai, H. Namita, S. Yamaguchi and M. Nomura, J. Power Sources, 119-121, 567 (2011). 4. T. M. Bond, J.C. Burns, D.A. Stevens, H.M. Dahn, and J.R. Dahn, J. Electrochem. Soc., 160 , A521 (2013). 5. N. N. Sinha, T. H. Marks, H. M. Dahn, A. J. Smith, D. J. Coyle, J. J. Dahn and J. R. Dahn, J. Electrochem. Soc., 159, A1672 (2012).

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