A Systematic and Comparative Study of Electrolyte Additives on LiCoO2/Graphite and Li[Ni1/3Mn1/3Co1/3]O2/Graphite Pouch Cells

Introduction
              Electrolyte additives are used to improve the properties and performance of Li-ion cells [1]. However, the way that electrolyte additives and combinations of additives function in Li-ion cells has not been well explained in the literature.

              The ultra high precision charger (UHPC) at Dalhousie University, which can measure the coulombic efficiency (CE) to an accuracy of ± 0.003% [2], was used to investigate the effects of electrolyte additives singly or in combination on LiCoO₂ (LCO)/graphite and Li[Ni_1/3Mn_1/3Co_1/3]O₂(NMC)/graphite pouch cells. It is believed that precision measurements of CE and other factors during the first weeks of cycling can point to the best additive combinations.

Experimental
                 Machine-made LiCoO₂/graphite and Li[Ni_1/3Mn_1/3Co_1/3]O₂/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 0.75 g (for LCO/graphite cells) or 0.85 g (NMC/graphite cells) of 1 M LiPF₆in ethylene carbonate (EC):ethylmethyl carbonate (EMC) (3:7 in weight ratio, BASF) as control electrolyte. Vinyl ethylene carbonate (VEC), vinylene carbonate (VC), lithium bis(oxalato) borate (LiBOB), fluoroethylene carbonate (FEC), trimethoxyboroxine (TMOBX), ethylene sulfate (DTD), 1,3-Propanediol cyclic sulfate (TMS), propylene sulfate (PLS) and methylene methanedisulfonate (MMDS) were used as electrolyte additives.

                 The cells were cycled using the UHPC between 2.8 and 4.2 V at 40.0 ± 0.1°C using currents corresponding to C/15 for 15 cycles where comparisons were made. After the UHPC cycling, electrochemical impedance spectroscopy was used to measure the combined charge transfer resistance (R_ct) of both electrodes in each cell. Before the impedance tests, cells were held at 3.8 V until the current dropped below the corresponding C/1000 current, so that all cells were measured under the same conditions. All impedance data were collected at 10.0 ± 0.1°C, in order to separate the impacts of the various additives better.

                 An automated storage system [3] was used to measure the self-discharge of cells stored under open circuit conditions at 4.2 V. The open circuit potential of each cell was automatically measured every 6 hours (for 500 hours) at a fixed temperature of 40.0 ± 0.1°C.

Results and discussion
                   The coulombic inefficiency per hour, charge endpoint capacity slippage, charge transfer resistance at 10°C after the UHPC cycling (600h) and voltage drop during storage are four important parameters that can be used to predict the lifetime of Li-ion cells. Cells with low CIE/h, low charge slippage, low R_ctand low voltage drop are much more desired and believed to have long lifetime. Much more detailed information about each of the four parameters will be presented in the lecture. In order to easily distinguish and compare the effectiveness of electrolyte additives, we use one formula to combine the effects of the four parameters into a “Figure of Merit” (FOM). Figure 1 shows the FOM as a function of electrolyte additives. The smaller the FOM, the better the overall performance that additives can bring to cells. There are many interesting things to note in Figure 1 which will be discussed in the lecture. Most notable is that DTD is the only single additive that has a similar FOM as VC.

Figure 1. Figure of merit (FOM) consisting of CIE/h (A), charge slippage (B), R_ct (C) after UHPC cycling and voltage drop (D) using the formula for (a) LCO/graphite pouch cells FOM=2×10⁵A+50B+C/16 (b) NMC/graphite pouch cells FOM=2×10⁵A+10×B+2×0.05C+40D. The bars without labels are for proprietary additives.

References

[1] S.S. Zhang, J. Power Sources, 162, 1379 (2006).
[2] T. M. Bond, et. al., J. Electrochem. Soc, 160, A521 (2013).
[3] N.N. Sinha, et al., J. Electrochem. Soc. 158 A1194 (2011).