Accurate Consumption Analysis of Vinylene Carbonate as an Electrolyte Additive in an 18650 Lithium-Ion Battery at the First Charge-Discharge Cycle

The consumption via reductive and oxidative decompositions of vinylene carbonate (VC) molecules as an electrolyte additive at the anode and cathode in a 18650 lithium-ion battery was quantitatively analyzed with charge-discharge tests, NMR spectroscopy, and gas chromatography. It was unambiguously concluded that the molar number of VC reductively decomposed at the anode has the upper limit whereas that of VC oxidatively decomposed at the cathode depended highly upon the initial VC concentration. The excess amount of VC molecules causes the increase in oxidative decomposition at the cathode surface eventually reducing the Columbic efﬁciency at the ﬁrst cycle.

Vinylene carbonate (VC) is widely adopted as an electrolyte additive molecule in lithium-ion batteries. [1][2][3][4] The function of VC was explained as the formation of a solid electrolyte interphase (SEI) via reductive decomposition of VC molecules at the surface of anode. The SEI is supposed to prevent the further degradation of electrolytes resulting in preferable cycle characteristic or successful battery lifetime. A polymerized VC was thought of as the identity of the SEI. 5,6 Figure  1 indicates the HOMO-LUMO levels of carbonate esters concerning the electrolyte solvents. The LUMO level of VC is the lowest among those of the other carbonate esters such as ethylene carbonate (EC) and ethylmethyl carbonate (EMC) exhibiting that VC is reductively decomposed prior to others. The theoretical results are consistent with experimental results. VC also has the highest HOMO level expecting that VC will be oxidized prior to the others. So far poor attention has been paid to oxidative decomposition of VC at the cathode than reductive decomposition at anode and its influence for the cycle characteristic of lithium-ion batteries. [7][8][9] Herein we report on the accurate consumption analysis of VC at both anode and cathode in a 18650 lithium-ion battery at the first cycle.

Experimental
1,400 mAh 18650-type battery cells equipped with 6.1 g of 1.0 M LiPF 6 in EC:EMC possessing a ratio of 1:2 by volume with or without VC, graphite anode, and LiNi x Mn y Co z O 2 cathode were prepared and the charge-discharge tests were conducted with the CCCV mode at 23 mA (∼1/60 C) rate at 25 • C. The cutoff voltage was at 3.0-4.2 V. The concentrations of VC in electrolyte chosen were 0.0, 1.0, 2.0, 3.0, and 5.0 weight % and these electrolytes were abbreviated as VC0, VC1, VC2, VC3, and VC5 respectively unless otherwise noted. We have commenced the first charge at around 0.0 V and terminated it at 3.0 V right after the peak of reductive decompositions of EC and VC and then the electrolyte was centrifugally extracted from the cell for VC1. The electrolytes were also extracted from the cells for VC0-VC5 after charge-discharge tests. 1 H-NMR spectroscopy and gas chromatography (GC) of extracted electrolytes were measured with a JEOL 500-MHz NMR spectrometer and a HITACHI G5000 GC to provide quantitative analyses of carbonate esters including VC. Theoretical calculations to obtain the orbital coefficients and most stable conformations have been done by density functional theory z E-mail: tcytana@shizuok.ac.jp (DFT) with the B3LYP hybrid functional and the 6-311+G(d,p) basis set. These calculations were carried out with the Gaussian 09 suite of programs. 10

Results and Discussion
The charge-discharge curves at the first cycle for electrolyte V0-V5 were illustrated in Figure 2. The inset in the figure was an expanded drawing of the charge curves. All cells showed good reversibility with charge-discharge capacities at 1,600 and 1,400 mA respectively at the first cycle where the capacities were broadly independent from the initial VC concentrations. Table I summarizes charge-discharge capacities and Columbic efficiencies at the first cycle. Columbic efficiencies slightly decreased by increasing in VC concentration in electrolytes. The reason of the decrease of the efficiencies will be discussed below.
The differential type of Figure 2 was shown in Figure 3 as the dQ/dV curves. The peak at ca. 2.95 V in these two figures for VC0 could be attributed to the reductive decomposition of EC. 11,12 The peak was shifted lower potential as the VC concentration increased. This indicates that both EC and VC were reductively decomposed at the surface of anode presumably forming SEI. 9 The peak shift toward lower potential is also indicative of the increase in the molar number of decomposed VC over EC and the VC decomposition took place prior to EC. This is also corresponding to the theoretical results in Figure 1, the LUMO level of VC being lower than that of EC. These experimental observations conflict with the previous report discussed with theoretically calculated free energy changes in which the following mechanism is proposed that EC preliminarily suffered the one-electron reduction providing the EC anion radical and then the anion radical reacted with VC to form SEI. 7 The values of integral of these peaks in Figure 3 are equivalent to the electric capacity (= electric power, mAh) consumed during the reductive decomposition of EC and VC and these values were plotted in Figure 4. We have repeated the measurements with independent three cells for V0-V5 to secure the reproducibility. An intriguing phenomenon is that the electric capacity is nearly constant at ca. 20 mAh irrespective of initial VC concentrations ( Figure 4). These data exhibit that the total molar numbers of EC and VC decomposed at the graphite anode is constant during a single charging process so long as the one-electron reduction reaction mechanism were presupposed for both EC and VC.      The quantitative analyses of carbonate esters involving EC, EMC, and VC for VC1 were conducted with 1 H-NMR spectroscopy. The NMR's were measured for the extracted electrolytes form the cells before and after charge-discharge as well as after reductive decompositions of EC and VC at charging ( Figure 5). The inset in the figure was an expanded drawing of the area including VC signals at 6.6 ppm.  The signals in the figure were connected with protons marked in red via dotted lines as signal assignments. The values of integral derived from EMC signals were adopted as standard for quantitative analyses since the EMC molecules were supposed to be hardly involved in the electrochemical reactions. 6 When comparing the two spectra of before charge-discharge and after reductive decompositions of EC and VC in Figure 5 (a) the VC concentration decreased to roughly half and thus the molar number decomposed could be calculated to be approximately 0.35 mmol. The calculations to obtain the molar numbers of VC are as follows; thus the molar number of VC in the 18650 battery cell for VC1 before first charge is equal to 0.7 mmol ( 6.1 g × 1.0 wt%/86.04) where the weight of electrolyte in the battery cell and molecular weight of VC are 6.1 g and 86.04 g/mol respectively. 1 H-NMR spectra showed the VC concentration halved after reductive decompositions of EC and VC. Therefore the molar number of VC reductively decomposed is equal to 0.35 mmol (= 0.7 mmol/2). This number is equivalent to the electric capacity of 9.5 mAh. The calculation to provide the electric capacity for VC1 is as follows; the electric capacity equal to 0.35 mmol upon VC decomposition is 9.5 mAh (= 0.35 mmol × 26,800 mAh/mol) where 26,800 mAh/mol is Faraday constant. This also indicates that the electric capacity consumed during the reductive decomposition (20 mAh for VC1 in Figure 4) consists of VC (9.5 mAh) and EC (10.5 mAh = 20 mAh -9.5 mAh) decompositions. When comparing the two spectra of after the reductive decomposition of EC and VC and after charge-discharge in Figure 5 (b) the VC concentration further decreased to ca. 0.07 wt%. Therefore ca. 43 wt% (0.30 mmol) of VC seems to be oxidatively decomposed at the cathode for VC1 right after the VC consumption at the anode. 13 The VC decrease via electrochemical decomposition was also confirmed by gas chromatography analyses and these quantitative analyses with NMR and GC is consistent, VC concentration after chargedischarge for VC1 being at ca. 0.08 wt% (Table II). Table II also indicated that the oxidative decomposition of VC during first discharge largely increased when the initial VC concentration increased while the reductive decomposition of VC at the anode is limited at 20 mAh (= 0.78 mmol) at a maximum ( Figure 4). For instance ca. 0.40, 0.57, and 1.01 mmol of VC were oxidatively decomposed for VC2, VC3, and VC5 if it can be assumed that the 0.78 mmol of VC was always reductively decomposed at the anode. The calculations to afford the molar numbers of consumed VC at the cathode are as follows; thus [VC molar number before charge] -[VC molar number after charge-discharge] -[VC molar number consumed at the anode] = [VC molar number consumed at the cathode]. For example 3.55 mmol -1.76 mmol -0.78 mmol = 1.01 mmol for VC5. In particular the VC5 cell after charge-discharge test afforded the lowest Columbic efficiency among the other cells probably because of the high interfacial resistance at the cathode surface by forming VC's oxidative species (Table I).

Conclusions
We have found that the molar number of VC reductively decomposed at the anode is limited by a ceiling whereas that of VC oxidatively decomposed at the cathode depended highly upon the initial concentration of VC in a 18650 battery. The excess amount of VC molecules causes the increase in oxidative decomposition at the cathode surface eventually giving rise to the high interfacial resistance. The origin of an appropriate concentration of VC that is commonly referred as an electrolyte additive was derived to form characteristics of cathodic and anodic reactions above.