LiCoO2/graphite pouch cells with 1.0M LiPF6 in EC:DEC (1:2 v/v) electrolyte were used to study the effect of succinonitrile (SN) as an electrolyte additive on the cell impedance during cycling. Pouch cells containing no additives, 2 wt% vinylene carbonate (VC), 2 wt% SN and 2 wt% VC + 2 wt% SN were studied for comparison. In order to investigate which electrode contributed to impedance changes during charge and discharge cycling, positive and negative electrode symmetric cells, fabricated using electrodes extracted from the parent pouch cells, were studied using electrochemical impedance spectroscopy (EIS). Two wt% VC added to the electrolyte suppressed impedance growth of the positive electrode during cycling, while the addition of 2 wt% SN greatly increased the impedance growth of the positive electrode during cycling. The addition of both 2 wt% VC and 2 wt% SN leads to intermediate behavior. In all cases, the negative electrode impedance decreased during cycling when VC, SN or VC+SN additives were present. The dominant contribution to the impedance growth of pouch cells containing SN during cycling comes from the positive electrode.

The effect of the nitrile electrolyte additives, succinonitrile (SN), adiponitrile (AN) and pimelonitrile (PN) was studied using High Precision Charger at Dalhousie University, automated cell storage and AC impedance. Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite (NMC111/graphite) cells containing the nitrile electrolyte additives showed no significant effect on the cycling between 2.8 V and 4.2 V and storage at 4.2 V at 60. ± 0.1 • C. However, when the high cutoff voltage increased to 4.5 V in Li [Ni 0.4 Mn 0.4 Co 0.2 ]O 2 /graphite (NMC442/graphite) cells, the addition of 2 wt% SN and 2 wt% vinylene carbonate (VC) reduced the reversible capacity loss and gas generation during storage experiments at 60. ± 0.1 • C compared to 2 wt% VC alone. The addition of 2 wt% SN also caused a large impedance growth during the 4.5 V storage period, but that could be suppressed by the addition of 1 wt% ethylene sulfite (ES) + 1 wt% tris(trimethlysilyl) phosphite (TTSPi) or 2 wt% trimethylene sulfate (TMS) + 2 wt% TTSPi. This work shows that SN leads to meaningful improvements in storage properties at voltages higher than 4. The incorporation of electrolyte additives is one of the approaches for improving the cycle life and calendar life of Li-ion cells.1-2 Recently, Burns et al. suggested that the addition of one or more electrolyte additives can lead to longer cycle life by suppressing electrolyte oxidation at the positive electrode.3 Burns et al. reported that electrolyte oxidation products created at the positive electrode migrate to the negative electrode and are reduced on the surface of the negative electrode as a solid layer of undesired material. As the layer becomes thicker during the cycle test, ion transport to the bulk of the negative electrode is reduced, which leads to "cycle life failure".Based on the results of reference 3, it is possible that nitriles as electrolyte additives can be potential candidates to improve cycle life because results in the literature showed that EC:DMC mixed with nitrile co-solvents has high electrochemical stability up to around 5.0 V and starts to be oxidized at around 5.5 V vs. Li/Li + , while pure EC:DMC starts to be oxidized suddenly at around 4.5 V. 4 Duncan et al. showed that dinitriles with shorter carbon chain length between nitrile groups gave higher capacities in Li-ion cells than dinitriles with longer carbon chain between nitrile groups because the shorter dinitriles showed higher ionic conductivity than the longer dinitriles. 5 The possibility of using adiponitrile and glutaronitrile as electrolyte solvents was investigated thoroughly.6-7 However, the electrochemical characteristics of Li-ion cells, when nitriles are used as electrolyte additives (less than 5 wt%), have not been thoroughly investigated yet.Here, the possibility of using 2 wt% of succinonitrile (SN), adiponitrile (AN) or pimelonitrile (PN) as an electrolyte additive is considered. Storage and cycling experiments at 60• C using Li [Ni 0.33 8 A paper about the effect of SN on * Electrochemical Society Fellow. z ...

The effect of electrode density on safety was investigated with Li[Ni0.8Co0.15Al0.05]O2 (NCA) electrodes in the presence of 1.0M LiPF6 EC:DEC (1:2 v/v ratio) using accelerating rate calorimetry (ARC). The densities of the electrodes were 1.6 g/cm3, 2.0 g/cm3, and 2.3 g/cm3, respectively. Denser electrodes showed inferior thermal stability compared to porous electrodes. ARC tests with ground electrode powder in the presence of EC:DEC (1:2 v/v) solvent, with and without LiPF6 salt, show that the denser electrode has lower thermal stability because it contains less LiPF6. A charged NCA electrode in 0.7M LiBOB EC:DEC (1:2 v/v) electrolyte shows worse thermal stability than in 1.0M LiPF6 EC:DEC (1:2 v/v) electrolyte. These results may be useful for researchers and engineers interested in designing safe electrodes using NCA positive electrode material.

Using accelerating rate calorimetry (ARC), the effect of lithium bis(oxalato)borate (LiBOB), vinylene carbonate (VC) and succinonitrile (SN) electrolyte additives and LiPF 6 salt on the reactivity between electrolyte and charged positive electrode material has been investigated. The ARC samples were made with varying concentrations of LiBOB, VC and SN as electrolyte additives and delithiated Li 1-x [Ni 1/3 Mn 1/3 Co 1/3 ]O 2 (NMC) and Li 1-x [Ni 0.8 Co 0.15 Al 0.05 ]O 2 (NCA) charged to 4.2 V as positive electrode materials. The thermal stability of NMC decreases slightly with the addition of the electrolyte additives but the thermal stability of NCA increases with the addition of the electrolyte additives except in the case of VC. The thermal stability of NMC decreases as the concentration of LiPF 6 increases while the thermal stability of NCA increases as the concentration of LiPF 6 increases. The ARC results shown here suggest that additives and LiPF 6 salt can play a different role in thermal stability depending on the positive electrode material. Therefore careful considerations about safety are required to incorporate new electrolyte additives in Li-ion cells.

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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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).