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3,3'-(ethylenedioxy)dipropiononitrile (EDPN) as an Electrolyte Additive for 4.5V LiNi1/3Co1/3Mn1/3O2/Graphite Cells Chengyun Wang, Le Yu, Weizhen Fan, Jiangwen Liu, Liuzhang Ouyang, Lichun Yang, and Min Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16220 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 25, 2017

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ACS Applied Materials & Interfaces

3,3'-(ethylenedioxy)dipropiononitrile (EDPN) as an Electrolyte Additive for 4.5V LiNi1/3Co1/3Mn1/3O2/Graphite Cells †,‡

Chengyun Wang,

Le Yu,⊥ Weizhen Fan,⊥ Jiangwen Liu,

†,‡

Liuzhang Ouyang,*,

†,‡,§

†,‡

Lichun Yang,

†,‡

and Min Zhu †

School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, People’s Republic of China. ‡ China-Australia Joint Laboratory for Energy & Environmental Materials, Guangzhou, 510641, People’s Republic of China. § Key Laboratory of Fuel Cell Technology of Guangdong Province, Guangzhou, 510641, People’s Republic of China. ⊥ Guangzhou Tinci Materials Technology Co. Ltd., Guangzhou, 510760, People’s Republic of China. Abstract 3,3'-(ethylenedioxy)dipropiononitrile (EDPN) has been introduced as a novel electrolyte additive to improve the oxidation stability of the conventional carbonate-based electrolyte for LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries cycled under high voltage. Mixing 0.5 wt% EDPN into the electrolyte greatly improved the capacity retention, from 32.5% to 83.9%, of cells cycled for 100 times in the range 3.0-4.5V with a rate of 1C. The high rate performance (3C and 5C) was also improved, while the cycle performance was similar to that of the cell without EDPN when cycled between 3.0 to 4.2 V. Further evidence of a stable protective interphase film can be formed on the LiNi1/3Co1/3Mn1/3O2 electrode surface due to the presence of EDPN in the electrolyte. This process effectively suppresses the oxidative decomposition of electrolyte and the growth in the charge-transfer resistance of the LiNi1/3Co1/3Mn1/3O2 electrode and greatly improves the high-voltage electrochemical properties for the cells. On the contrary, EDPN has no positive effect on the cyclic performance of the LiNi0.5Co0.2Mn0.3O2-based cell under high operating voltage.

Keywords: 3,3'-(ethylenedioxy)dipropiononitrile; high voltage; surface passivation; LiNi1/3Co1/3Mn1/3O2/graphite. * Corresponding author. E-mail address: [email protected] (Liuzhang Ouyang).

ACS Paragon Plus Environment

electrolyte additive; cathode


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1. Introduction With the growing demand by society for electrical energy storage, rechargeable batteries with superior energy and power density are needed urgently. Lithium-ion batteries (LIBs) hold a tremendous potential as a clean power source used in powering electric vehicles.1,2 Unfortunately, the present-day LIB technology is characterized by a low energy density, which limits its use to small portable electronics. Developing high operating voltage cathode materials is an effective approach to improve power or energy density ofLIBs.3-6 Currently, layered-structured materials based on Ni-Mn-Co mixed oxides have been attracted increasing interest as the high voltage cathode materials for commercial power LIBs by virtue of their high specific capacity operated at high voltage.7 However, today’s commercial electrolytes based on lithium hexafluorophosphate (LiPF6) and organic carbonate solvents are prone to decomposition when the operated voltage approaches or exceeds 4.5V (vs. Li/Li+). This potential issue will lend a direct consequence of serious electrolyte oxidation decomposition on the cathode, causing poor cyclic stability. There are mainly two ways to suppress electrolyte oxidation at high voltage conditions, thus improving performance. One method consists of using more stable solvents to enhance the oxidation stability of electrolyte solutions. It was recently reported that the discharge capacity retention of LiNi1/3Co1/3Mn1/3O2 can be improved from 41.3% to 82.7% by using fluoroethylene carbonate (FEC) as co-solvent over 100 cycles when the cell operated at 4.6 V with 0.2C rate.6 Wang et al. demonstrated

that

the

LiNi1/3Co1/3Mn1/3O2/graphite

1H,1H,5H-Perfluoropentyl-1,1,2,2-tetrafluoroethylether

pouch (F-EAE)

cells in

with the

40

wt%

conventional

carbonate-based electrolyte, the discharge capacity retention of cell after 100 cycles is significantly improved, from 28.8% to 86.8% (charged to 4.5 V at 1.0C rate).8 On the other hand, introducing functional additives in the electrolytes to modify the components of the protective layer on the positive electrode surface improves the electrode/electrolyte interface stability. For example, Yu et al. investigated trimethylboroxine (TMB) as a functional additive to enhance the properties of LiNi1/3Co1/3Mn1/3O2 and found that the cathode benefited from the formation of a surface passivation film derived from the degradation of TMB.9 Wang et al. reported that the 10 wt % triethylborate (TEB)-containing electrolyte improves the electrochemical performances of the LiNi1/3Co1/3Mn1/3O2 cathode as a result of TMSB preferentially oxidized and the subsequent generation of interface protective film on the positive electrode surface.10 Chen et al. demonstrated that adding 1 wt %


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succinonitrile (SN) into the electrolyte, an excellent cyclability of the Li1.2Ni0.2Mn0.6O2/Li cell can be achieved, which is attributable to the formation of a uniform interface between the cathode and the SN electrolyte.11 These studies clearly indicate that employing an appropriate electrolyte co-solvent or additive effectively enhances the electrochemical properties for the high-voltage cathode material. Nitrile compounds have increasingly gained attention for their excellent physical and thermodynamic stability. In particular, dinitriles, such as adiponitrile (ADN),12 sebaconitrile (SBN),13 glutaronitrile (GLN)14 and SN11, 15-17 have been reported as additives or co-solvents thatare suitable for high energy/power LIBs and can suppress the impedance and protect the stability of cathode.18 Dinitriles usually have low compatibility with the electrodes and cannot form a stable SEI on the graphite-based or lithium metal-based anode materials; however, Yaser demonstrated that this incompatibility can be solved by adding SEI-forming solvents component, such as EC or other additives, in the case of the graphite anode.14 Additionally, Wang’s group synthesized alkoxy-substituted nitriles (3-methoxypropionitrile or MPN and 3-ethoxypropionitrile or EPN) and their positive effect on the high rate capabilities of Li4Ti5O12-based cells19,20 was evaluated. According to the examples mentioned above, we can speculate that alkoxy-substituted dinitrile may be able to serve as a bifunctional electrolyte additive for LIB. Analkoxy-substituted dinitrile compound, 3,3'-(ethylenedioxy)dipropiononitrile (EDPN), was developed as a novel electrolyte additive with the aim of improving the stability of SEI film on the electrode and suppressing electrolyte degradation during LiNi1/3Co1/3Mn1/3O2-based repeated cycling under

high

operating

voltage

conditions.

The

electrochemical

properties

of

LiNi1/3Co1/3Mn1/3O2/graphite pouch cells with EDPN-based electrolytes were measured at a cut-off charge potential of 4.5V. The contribution of EDPN to the improved interfacial stability and the cyclic stability for 4.5V LiNi1/3Co1/3Mn1/3O2 cathode was also proposed by the combination of electrochemical and spectroscopic methods.

2. Experimental section 2.1. Electrodes and Electrolytes Preparation All experimental data were performed at room temperature of 25 ° C. Electrochemical cyclic and C-rate performance were evaluated using LiNi1/3Co1/3Mn1/3O2/graphite pouch cells. The electrochemical behaviors of the separate electrodes were studied by using two-electrode



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LiNi1/3Co1/3Mn1/3O2/graphite and three-electrode LiNi1/3Co1/3Mn1/3O2 /Li /graphite pouch cells. The LiNi1/3Co1/3Mn1/3O2 composite electrode consisted of cathode materials (obtained from Tianjiao Technology Co. Ltd, Shenzhen, China), carbon black, and poly(vinylidene difluoride) (PVDF) in a weight ratio of 93:3:4, casted on aluminum foil, dried at 120 ° C for 30 min and then the dried positive composite electrode was compressed by a roll press. The average loading level, thickness, and density of the prepared positive electrode were 31.18 mg/cm2, 106 µm, and 2.94 g/cm3, respectively. The graphite composite electrode contained anode materials (obtained from BTR Battery Materials Co., Ltd, Shenzhen, China), Super-P, carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) in a weight ratio of 95:1:1.5:2.5, casted on copper foil, dried at 100 ° C for 30 min and then the dried negative composite electrode was compressed by a roll press. The average loading level, thickness, and density of the prepared negative electrode were 16.50 mg/cm2, 116 µm, and 1.42 g/cm3, respectively. The positive and negative composite electrodes were heat treated at 120 ° C for 10h in a vacuum oven prior to cell assembly. For the two-electrode pouch cells, in which the LiNi1/3Co1/3Mn1/3O2 electrode and graphite electrode were used as cathode and anode, respectively. For the three-electrode pouch cells, in which the LiNi1/3Co1/3Mn1/3O2 electrode, lithium foil, and graphite electrode were used as the cathode, reference electrode, and anode, respectively. For the cells operated at the charge cut-off voltage of 4.2 V and 4.5 V, the capacities of these cells are about 535 mAh and 632 mAh, and the capacity ratios of graphite electrode to LiNi1/3Co1/3Mn1/3O2 electrode (N/P ratios) are about 1.36 and 1.15, respectively. In the experiment, the production process followed for the LiNi0.5Co0.2Mn0.3O2/graphite cell was similar to the one described for the LiNi1/3Co1/3Mn1/3O2-based

cell.

Additionally,

the

electrolyte

additive,

3,3'-(ethylenedioxy)dipropiononitrile (EDPN) (99.5%) was obtained from Fujian Chuangxin Science and Develops Co., Ltd, China. Battery-grade carbonate solvents and salt, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and lithium hexa-fluorophosphate (LiPF6) were produced by Guangzhou Tinci Materials Technology Co., Ltd, China. These chemicals were utilized without further purification. The 1.0 M LiPF6 electrolyte, dissolved in EC-DEC (1:3, by wt), was used as the control electrolyte. The additive-containing electrolytes were prepared simply by mixing EDPN with the prepared electrolyte in different weight contents. The ionic conductivity measurements were performed using a conductivity meter (S230, Mettler Toledo, Switzerland). The H2O and free acid (HF) contents in these electrolytes were controlled at below 20 ppm and 50 ppm,


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respectively. In the test, the injection weight of electrolyte for each battery is maintained at about 3.65±0.05g. The electrolyte contents for the LiNi1/3Co1/3Mn1/3O2/graphite pouch cells operated at the charge cut-off voltage of 4.2 V and 4.5 V are around 6.82 mg/mAh and 5.78 mg/mAh, respectively.

2.2. Electrochemical tests Linear sweep voltammetry (LSV) was performed on a CHI660 Instrumental Electrochemical Station (Shanghai, China) in a three-electrode configuration with platinum electrode as the working electrode and lithium foil as the counter and pseudo reference electrodes. The LSV measurement was conducted between 3.0 to 6.0 V (vs. Li/Li+) with a scanning rate of 1 mV/s. The electrochemical charge-discharge cycling was conducted by changing the charging cutoff voltage of 4.2 V and 4.5 V at 1.0 C rate over 100 cycles, following the three initial cycles: the 1st cycle at C/10, the 2nd cycle at C/5 and the 3rd cycle at C/2 rate. The rate capabilities of the cells were evaluated at different discharging currents, varying from C/5 to 5 C at a constant current mode, while keeping a charging current at 1.0 C using a constant current/constant voltage mode in a voltage range between 3.0 and 4.5 V. Measurements at each C-rate were repeated over 3 cycles. The cyclic performance and C-rate performance were measured by a computer-controlled battery charger test (CT-3008W, Neware, China). The electrochemical impedance spectroscopy (EIS) analysis was tested on a frequency response analyzer (FRA, Solartron1455A, Solartron, England) with an amplitude potential of 10 mV and operated between 0.04 Hz and 100 kHz. The spectra of the two-electrode cells were recorded at the 1st and 100th cycle and those of three-electrode cells were recorded after the 3rd, 20th, 50th, 70th and 100th cycle. All cells were conducted at a nearly full state of charge condition after a potentiostatic equilibration step at 4.5 V.

2.3. Morphological and Compositional Analysis After 100 discharge/charge cycles, the two-electrode cells were dismantled in an argon glovebox. The collected cathodes were washed thoroughly in purified DMC to remove the residual electrolyte and dried under a transition chamber in Ar atmosphere at room temperature. All electrodes were stashed in argon-filled plastic bottles and vacuum-packed in an aluminum foil before analysis, to avoid contamination with the atmosphere. The surface morphology of the fresh and



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high-voltage cycled electrodes was examined with a field emission scanning electron microscopy (SEM, ZEISS Ultra 55, Carl Zeiss, Germany) and the chemical components concentration was determined by energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM, JEM-2100HR, JEOL, Tokyo, Japan); TEM samples were scraped from the surface of electrodes and then placed in a small test-tube with ethyl alcohol as solvent for ultrasonic dispersion. Surface composition of the fresh and cycled LiNi1/3Co1/3Mn1/3O2 cathodes was analyzed using X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos).

3. Results and discussion 3.1. Electrochemical stabilities of electrolytes with and without EDPN To determine the electrochemical oxidation stabilities of the electrolytes with and without EDPN, the oxidation potential at 1 M LiPF6-EC:DEC (1:3, wt %) was measured by LSV. As depicted in Figure 1, the oxidation decomposition potential of the electrolyte containing 0.5 wt % EDPN is around 3.8 V, suggesting that EDPN is preferential oxidized than the carbonate solvents and maybe employed as a cathode additive. Additionally, the current density of the electrolyte with 0.5 wt % EDPN was small, below 3.8V, and started to increase sharply at a potential of 5.2 V. By contrast, the electrolyte without additive decomposed at ~4.6V and the electrochemical stability was poorer than that of the electrolyte containing EDPN. All results indicate that EDPN may take part in the formation of a passivation film on the positive electrode and also improve the oxidation stability of the conventional carbonate-based electrolyte. Therefore, the electrolyte containing EDPN may be more appropriate to use under high voltage conditions.

3.2. Electrochemical properties of EDPN in LIBs The electrochemical properties of the two-electrode LiNi1/3Co1/3Mn1/3O2/graphite pouch cells with different electrolytes were investigated under different operating conditions. Figure2a shows the cyclic performance of full cells with different EDPN ratios in electrolytes for voltage in the range of 3.0-4.5V. It was found that the cell with 0.5 wt % EDPN achieved the best cyclic stability, delivering an initial reversible discharge capacity of 156.2 mAh/g and maintaining a value of 131.0 mAh/g over 100 cycles. The discharge capacity retention was improved from 32.5% to 83.9% by adding 0.5 wt % EDPN into the electrolyte (Table 1), while the discharge capacity retentions all decreased when the


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cells containing lower or higher EDPN. Figure S1 shows the effect of EDPN on the EIS of LiNi1/3Co1/3Mn1/3O2/graphite cells for the first cycle charged to 4.5 V (Figure S1a) and the ionic conductivity of the electrolyte at 25 ° C (Figure S1b). The excessive additive will lead to large film-forming impedance and low ionic conductivity, which will promote the polarization and degrade the performance of the cells to some extent. Therefore, 0.5 wt % was selected to be the appropriate additive amount in the electrolyte. Interestingly, the cells with and without EDPN exhibited similar cyclic performances when operated at the 4.2V charge cut-off voltage, their discharge capacity retentions were all beyond 90%, irrespective of the role of EDPN, as presented in the Figure2b and Table 1. It is obvious that a certain amount of EDPN in the electrolyte can greatly improve the discharge capacity retention for the cells operated at a high charge cut-off voltage. The initial voltage profiles of the LiNi1/3Co1/3Mn1/3/graphite full pouch cells (with and without EDPN) cycled at C/10 rate are portrayed in Figure S2. It can be seen a large polarization occurred when the additive EDPN was added,3 which may be ascribed to the decomposition reaction of EDPN and formation of SEI with large internal resistance on electrodes. As presented in Figure 2c, the coulombic efficiency in the first cycle was equal to 81.4% in the cell with 0.5 wt % EDPN and 83.6% without EDPN. Additionally, the coulombic efficiency of the cell with 0.5 wt % EDPN was also low in the initial four cycles, which is most likely due to an irreversible capacity loss consequent to the formation of the SEI layer. From the 5th to 70th cycle, the coulombic efficiencies of all the cells exhibited a similar performance and were over 99%. After a greater number of cycles, the coulombic efficiency of the EDPN-containing cell remained above 99%, but that of the cell without EDPN was reduced below 94%, with a discharge capacity retention of only 32.5% after 100 cycles (Table 1), implying severe electrochemical degradation of electrolyte on the surface of electrode during cycling. The improvement in stability of the SEI layer and cyclic performance under high operating voltage may benefit from the presence of EDPN in the electrolyte. Figure 2d shows the effect of EDPN on the discharge capacities of LiNi1/3Co1/3Mn1/3O2/graphite cells with various discharge rates. At low discharge rate (C/5, C/2 and 1C), the discharge capacities of the cell with EDPN was lower than that of the cell without EDPN. The reason could be the large SEI film impedance, which also causes an additional irreversible capacity loss. Interestingly, the discharge capacity gap of cells with and without EDPN decreased with the increase of discharge rate; even when the rate reached 3C and 5C, the capacity of the cell containing EDPN was higher than that of the cell without EDPN. Specifically,



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the cell with 0.5 wt % EDPN presented a discharge capacity of ~83.0 mAh/g at 5C, while the cell without EDPN delivered only ~62.3 mAh/g. The result suggests that EDPN improves the cell capacity under high discharge rates. A similar conclusion had been reached by Wang et al. regarding the influence of nitriles on the high rate performance of Li4Ti5O12-based cells.19,20 Figure 3a and b show the impedance measurement results of the LiNi1/3Co1/3Mn1/3O2/graphite cells after one cycle and after 100 cycles. According to literature, the high frequency semicircle (Rf) is normally attributed to the solid electrolyte interphase resistances on anode or cathode,21 and the medium frequency semicircle (Rct) stands for the charge-transfer resistance at the surface of the active material.22,23 The equivalent circuit data of the LiNi1/3Co1/3Mn1/3O2/graphite cells without and with 0.5 wt% EDPN after 100 cycles is presented in Table 2. As indicated in Figure 3a, after the first cycle, the AC impedance of the EDPN-containing unit cell was slightly higher than that of the cell without EDPN, which implies that a film layer with a higher impedance value was formed due to the electrolyte containing nitrile components. Figure 3b and Table 2 show that both the Rf and Rct of the cells were affected by the presence of EDPN in electrolytes after 100 cycles. In particular, a significant increase of the Rct value was observed for the reference cell (Table 2). This implies that the EDPN can effectively suppress the growth in the impedance by forming a stable interface film during the cycles. Additionally, the solid electrolyte interphase resistances on the electrodes of the cell with EDPN exhibited a lower impedance value, which suggests that the EDPN-containing solid electrolyte interphase layer can inhibit the contact between the electrolyte and electrode interface and protect the integrity of the electrolyte to some extent during the cycling process. The above electrochemical measurement indicates that the EDPN electrolyte additive can effectively improve the AC impedance and cyclic stability of cells at high operating voltage. To further understand the separate electrode/electrolyte interface behavior occurring inside the LIBs, electrochemical impedance spectroscopy (EIS) measurements were carried out by the three-electrode LiNi1/3Co1/3Mn1/3O2/Li/graphite pouch cells. Figure 4 shows the Nyquist plot of LiNi1/3Co1/3Mn1/3O2 vs. graphite, LiNi1/3Co1/3Mn1/3O2 vs. Li, and graphite vs. Li, which were collected from the three-electrode pouch cells with different electrolytes charged to 4.5V after 3, 20, 50, 70 and 100 cycles, respectively. Figure 4a-b shows that the impedance spectra of the full cell and cathode vs. Li without EDPN. It can be seen that the Rct value increased negligibly during the first 70 cycles and then dramatically until the 100th cycle. Thus, both the full cell and the positive electrode without


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EDPN in the electrolyte have large impedance range abilities from cycle 70 to cycle 100. Figure 4c shows the impedance spectra of anode vs. Li without EDPN, highlighting that both the increasing trend and the range ability of impedance are not as large as those of the positive electrode. The change in the graphite electrode impedance may be related to the transition metal elements migrating from the positive electrode surface to the negative electrode, altering the SEI.24 All results of Figure 4a-c suggest that the cathode is the main electrode causing the AC impedance value rises of the whole battery. Figure 4d shows the effect of EDPN on the impedance spectra of LiNi1/3Co1/3Mn1/3O2 vs. graphite. Compared with Figure 4a, the impedance of the cell with EDPN, unlike that of the cell without EDPN, increased much during cycling, especially at the subsequent 30 cycles (Figure 4d). Additionally, the positive resistance value at the 100th cycle of the cell containing EDPN was significantly lower than that of the reference cell, as presented in Figure 4e. This result suggests that the interface film provided by EDPN is more stable and compact, and effectively enhances the interfacial stability between electrolyte and cathode surface. Also the negative electrode impedance of the cell with EDPN increased slightly during the initial 20 cycles and exhibited an inappreciable increase from the 50th cycle to the 100th cycle as shown in Figure 4f. A possible reason might be that the stable SEI on the cathode could suppress the migration of the transition metal cation from the cathode surface to anode. In order to characterize the electrode surface, the cells without and with EDPN, were dismantled after 100 cycles. It was clearly observed that the active material was separated from the aluminum current collector (marked with a red circle) on the surface of the positive electrode tab for the cell without EDPN, as shown in Figure 5b. Differently, as displayed in Figure 5c, the surface of the positive electrode containing EDPN was similar to the pristine state (Figure 5a) with a good adhesion between the active material and the current collector. These results, combined with the two-electrode and three-electrode AC impedance measurements results (Figure 3 and Figure 4), show that the electrolyte additive could inhibit the contact between the electrolyte and electrode interface and protect the integrity of the electrolyte. Therefore, the above-mentioned electrochemical tests confirmed that the addition of EDPN is an effective way to improve the cyclic stability and high C-rate performance for the cell cycled under high operating voltage via an enhancement of the interfacial stability between cathode and electrolyte.



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3.3. SEM measurement of fresh and harvested electrodes in LiNi1/3Co1/3Mn1/3O2/graphite full cells The improvement of the electrochemical performance for the cell under high operating voltage is beneficial from the effect of EDPN on the LiNi1/3Co1/3Mn1/3O2 cathode interface, which was verified by electrochemical techniques. To further characterize the roles of EDPN in the LiNi1/3Co1/3Mn1/3O2 electrode morphology at 4.5 V, the SEM, TEM and XPS spectroscopic techniques were employed. Figure 6 shows the SEM images of the cathodes, performed after the charge/discharge cyclic measurement for the cells. From the SEM results, clean and unobstructed edges can be observed in the fresh electrode, as shown in Figure 6a. After 100 cycles, the outlines of the cathodes with or without EDPN were all covered by a surface layer, which was ascribed to the formation of a solid interface film on the cathode surface. Compared to the fresh electrode, the presence of disordered and uneven decomposed material, accumulated on the electrode surface of the cell in which EDPN was not added, did not allow distinguishing an obvious edge between two particles after cycling. This suggests extensive electrolyte decomposition on the cathode of the reference cell. In contrast, a uniform interfacial film was found on the cathode in the system with 0.5 wt % EDPN (Figure 6c), which indicates that the interfacial film proved by EDPN can greatly restrain the oxidative decomposition of the electrolyte on the electrode surface, and improve the positive AC impedance at high operating voltage. 3.4. TEM measurement of fresh and harvested electrodes in LiNi1/3Co1/3Mn1/3O2/graphite full cells The TEM images of the fresh LiNi1/3Co1/3Mn1/3O2 cathode and of the same electrode over 100 cycles are presented in Figure 7. A smooth surface on the fresh LiNi1/3Co1/3Mn1/3O2 electrodes can be observed (Figure 7a), while surface films formed on the cycled cathode surface (Figure 7a-b). In comparison, a stable and compact film grew on the cycled cathode without EDPN, as shown in Figure 7b; however, the TEM micrograph of the surface film originated from EDPN is quite different from of the one formed in the absence of that additive in the electrolyte. The loose passivation film is observed in Figure 7c. As described above in Figure 2c, the coulombic efficiency of the cell with EDPN remained higher than that of the cell without EDPN after 100 cycles, which suggests that the loose film is helpful for the reversible Li+ intercalation into the LiNi1/3Co1/3Mn1/3O2 electrode. Also


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as proved by the impedance spectra discussed in Figure 3b, the protective layer derived from EDPN has a lower Rct value than the one obtained without EDPN after 100 cycles.

3.5. Effect of EDPN on the surface composition of LiNi1/3Co1/3Mn1/3O2 Compositional analysis of the passivation layer on the LiNi1/3Co1/3Mn1/3O2 electrodes after 100 cycles in the electrolyte with and without EDPN was carried out by XPS, and the results are shown in Figure 8. For the C 1s spectra, a peak located at ~284.8 eV can be attributed to the C-C bond in the conductive carbon black,25 the peaks at around 285.5 eV and 291 eV are related to C-H and C-F bonds in PVDF, respectively. A noticeable feature for the cathode cycled with EDPN is the appearance of a C-N peak around 286.0 eV, which was only present on the cathode obtained from the cycled cell with EDPN (see Figure 8a, c). The above result indicates that EDPN was oxidized and participated in the formation of a passivation film at the surface of LiNi1/3Co1/3Mn1/3O2 electrode. The peaks at ~286.5 eV and 290.1 eV on the cycled cathodes assign to C-O and C=O, respectively. These groups correspond to the electrolyte decomposition products (ROLi, ROCO2Li and Li2CO3). Additionally, the cathode without EDPN exhibits a slightly higher C=O peak compared to that of the cathode with EDPN, demonstrating that the formation of the SEI film with EDPN can effectively restrain the oxidative decomposition for the carbonate-based electrolytes. The O 1s spectrum of the fresh cathode exhibits two major peaks at ~529.5 eV and ~532.0 eV (Figure 8b), attributed to Me-O and C=O, which due to the presence of a metal oxide and Li2CO3, respectively. The peak broadens significantly by the presence of new peaks C-O (533.2 eV) that might result from different electrolyte decomposition products. Compared with the fresh cathode, the peak for Me-O was greatly reduced on the cycled cathodes, and was only present on the cathode obtained from the cycled cell without EDPN. Meanwhile, in the case without EDPN, the concentrations of transition metal on the cathode and anode surface were higher than that of the fresh and EDPN-based cells, as shown in Figure 8f. The result implies that the transition metal cations migrate to the cathode surface, dissolving in the electrolyte and possibly migrating to the anode surface.26-28 This assumption is consistent with the discussion of the negative electrode AC impedance; the graphite electrode displayed decreased impedance after 100 cycles as a consequence of EDPN addition. Compared to the cathode obtained from the cycled cell EDPN-free in electrolyte, additional peaks characteristic of N (1s, 400 eV) are presented on the cathode obtained from the



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cycled cell with EDPN-based electrolyte, further confirming that EDPN is incorporated into the SEI film on theLiNi1/3Co1/3Mn1/3O2 particles. As can be seen from the F1s spectra of Figure 8d, the peaks located at around 688.1 eV, 684.9 eV, and 687.1 eV corresponded to PVDF binder, LiF, and the residual LixPFy/LiPF6 on the surface layer, respectively.29 As reported, LiF is normally one of the major chemical compositions of the SEI on the surface of cathode.30 The F1s spectra exhibits that the intensity of the LiF peak is slightly enhanced as EDPN is added. Combined with the previously mentioned EIS data, the result suggests that a proper LiF content reduces the interfacial resistance and improves interfacial behavior.31,32 In the P2p spectra of Figure 8e, two peaks appeared around 136.3 eV and 134.3 eV, characteristic of LixPFy and LixPFyOz. It is easily noticed that the peak area of LixPFyOz is reduced for the cathode cycled in the presence of EDPN, which implies that the side decomposition of LiPF6 has been suppressed by EDPN addition. All results indicate the role of EDPN in the formation of a protective SEI layer, which effectively stabilizes the electrode/electrolyte interface. Finally, we attempted to evaluate whether the EDPN component could also have a positive effect on the LiNi0.5Co0.2Mn0.3O2/graphite pouch cells under high operating voltage. Figure 9 shows the cyclic performances of LiNi0.5Co0.2Mn0.3O2/graphite cells under various conditions. As presented in Figure 9a, it can be clearly seen that the capacities of the cells with or without EDPN all faded dramatically and exhibited low discharge capacity retention (Table 3) when the cells are charged to 4.5V. On the other hand, when the cells were cycled in the normal voltage range (between 3.0 and 4.2 V), they showed similar cycling performance (Figure 9b) and discharge capacity retentions were all above 90% after 100 cycles (Table 3). These results suggested that EDPN does not affect the performance of the LiNi0.5Co0.2Mn0.3O2/graphite cells cycled in the voltage ranges of 3.0-4.5V or 3.0-4.2V. The reason why EDPN can work on the LiNi1/3Co1/3Mn1/3O2 material but has no effect on the LiNi0.5Co0.2Mn0.3O2 material may be related to the differences in elemental proportions and surface properties of the two materials.

4. Conclusions The use of EDPN as a cathode protecting additive in a carbonate electrolyte to enhance the electrochemical properties of LiNi1/3Co1/3Mn1/3O2/graphite 4.5V batteries was demonstrated. At a voltage range of 3.0-4.5V, the addition of 0.5 wt % of EDPN to the electrolyte was proved to yield


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the best performance ofLiNi1/3Co1/3Mn1/3O2 batteries, which exhibited a better capacity retention, higher coulombic efficiency and better high rate performance compared to values obtained with commercial electrolytes. The LSV data suggested that the oxidation stability of the electrolyte containing 0.5 wt % EDPN had been greatly enhanced. EDPN can sacrificially decompose to form an effective SEI on the LiNi1/3Co1/3Mn1/3O2electrode, which contributes to prevent the side reaction between the electrolyte and the LiNi1/3Co1/3Mn1/3O2 surface, reducing the accumulation of product on the cathode, and ultimately decreasing the growth in cell impedance. The results were confirmed by EIS, SEM and TEM analyses. The study also showed that EDPN has no positive effect on the cyclic performance of LiNi0.5Co0.2Mn0.3O2/graphite pouch cells under high operating voltage. All results

suggested

that

EDPN

is

a

promising

electrolyte

additive

for

high

voltage

LiNi1/3Co1/3Mn1/3O2-based cells.

Acknowledgements This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. NSFC51621001) and by the Project Supported by Natural Science Foundation of Guangdong Province of China (2016A030312011 and 2014A030311004) and 2014GKXM011. Author Ouyang also thanks Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014).



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1H,1H,5H-Perfluoropenty1-1,1,2,2-tetrafluoroethylether as a Co-Solvent for High Voltage LiNi1/3Co1/3Mn1/3O2/Graphite Cells. J. Power Sources 2016, 307, 772-781. (9) Yu, Q. P.; Chen, Z. T.; Xing, L. D.; Chen, D. R.; Rong, H. B.; Liu, Q. F.; Li, W. S., Enhanced High Voltage Performances of Layered Lithium Nickel Cobalt Manganese Oxide Cathode by Using Trimethylboroxine as Electrolyte Additive. Electrochim. Acta 2015, 176, 919-925. (10) Wang, Z. S.; Xing, L. D.; Li, J. H.; Xu, M. Q.; Li, W. S., Triethylborate as an Electrolyte Additive for High Voltage Layered Lithium Nickel Cobalt Manganese Oxide Cathode of Lithium Ion Battery. J. Power Sources 2016, 307, 587-592. (11) Chen, R. J.; Liu, F.; Chen, Y.; Ye, Y. S.; Huang, Y. X.; Wu, F.; Li, L., An Investigation of Functionalized Electrolyte Using Succinonitrile Additive for High Voltage Lithium-Ion Batteries.


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(23) Ogihara, N.; Kawauchi, S.; Okuda, C.; Itou, Y.; Takeuchi, Y.; Ukyo, Y., Theoretical and Experimental Analysis of Porous Electrodes for Lithium-Ion Batteries by Electrochemical Impedance Spectroscopy Using a Symmetric Cell. J. Electrochem. Soc. 2012, 159, A1034-A1039. (24) Petibon, R.; Madec, L.; Abarbanel, D. W.; Dahn, J. R., Effect of LiPF6 Concentration in Li[Ni0.4Mn0.4Co0.2]O2/Graphite Pouch Cells Operated at 4.5 V. J. Power Sources 2015, 300, 419-429. (25) He, M. N.; Su, C. C.; Peebles, C.; Feng, Z. X.; Connell, J. G.; Liao, C.; Wang, Y.; Shkrob, I. A.; Zhang, Z. C., Mechanistic Insight in the Function of Phosphite Additives for Protection of LiNi0.5Co0.2Mn0.3O2 Cathode in High Voltage Li-Ion Cells. ACS Appl. Mater. Interfaces 2016, 8, 11450-11458. (26) Yang, L.; Ravdel, B.; Lucht, B. L., Electrolyte Reactions with the Surface of High Voltage LiNi0.5Mn1.5O4 Cathodes for Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2010, 13, A95-A97. (27) Martha, S. K.; Markevich, E.; Burgel, V.; Salitra, G.; Zinigrad, E.; Markovsky, B.; Sclar, H.; Pramovich, Z.; Heik, O.; Aurbach, D.; Exnar, I.; Buqa, H.; Drezen, T.; Semrau, G.; Schmidt, M.; Kovacheva, D.; Saliyski, N., A Short Review on Surface Chemical Aspects of Li Batteries: A Key for a Good Performance. J. Power Sources 2009, 189, 288-296. (28) Talyosef, Y.; Markovsky, B.; Lavi, R.; Salitra, G.; Aurbach, D.; Kovacheva, D.; Gorova, M.; Zhecheva, E.; Stoyanova, R., Comparing the Behavior of Nano- and Microsized Particles of LiMn1.5Ni0.5O4 Spinel as Cathode Materials for Li-Ion Batteries. J. Electrochem. Soc. 2007, 154, A682-A691. (29) Liao, L. X.; Cheng, X. Q.; Ma, Y. L.; Zuo, P. J.; Fang, W.; Yin, G. P.; Gao, Y. Z., Fluoroethylene Carbonate as Electrolyte Additive to Improve Low Temperature Performance of LiFePO4 Electrode. Electrochim. Acta 2013, 87, 466-472. (30) Martha, S. K.; Nanda, J.; Veith, G. M.; Dudney, N. J., Surface Studies of High Voltage Lithium Rich Composition: Li1.2Mn0.525Ni0.175Co0.1O2. J. Power Sources 2012, 216, 179-186. (31) Profatilova, I. A.; Kim, S. S.; Choi, N. S., Enhanced Thermal Properties of the Solid Electrolyte Interphase Formed on Graphite in an Electrolyte with Fluoroethylene Carbonate. Electrochim. Acta 2009, 54, 4445-4450.


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(32) Gmitter, A. J.; Plitz, I.; Amatucci, G. G., High Concentration Dinitrile, 3-Alkoxypropionitrile, and Linear Carbonate Electrolytes Enabled by Vinylene and Monofluoroethylene Carbonate Additives. J. Electrochem Soc. 2012, 159, A370-A379.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure Captions: Figure 1. Linear sweep voltammograms of Pt electrodes in 1mol L-1 LiPF6-EC: DEC (1:3, wt %) with and without EDPN additive with a scan rate of 1.0 mV s-1 in the voltage of 3.0 to 6.0V (vs. Li/Li+). Figure 2. Cycle performances of the LiNi1/3Co1/3Mn1/3O2/graphite pouch cells containing different ratios of EDPN in electrolytes with 1.0 C of charge/discharge rate at different voltage ranges of (a) 3.0-4.5 V and (b) 3.0-4.2 V; (c) Coulombic efficiencies of cells with 1.0 C of charge/discharge rate and (d) rate performances of the cells cycled between 3.0 and 4.5 V. Figure 3. EIS of LiNi1/3Co1/3Mn1/3O2/graphite cells with and without EDPN in the electrolyte and charged to 4.5 V (a) after one cycle and (b) after 100 cycles. Figure 4. Electrochemical impedance spectra of the three-electrode pouch cells with and without EDPN and charged to 4.5 V after different cycles, full cell: (a) without EDPN and (d) with 0.5 wt % EDPN; LiNi1/3Co1/3Mn1/3O2 vs. Li: (b) without EDPN and (e) with 0.5 wt % EDPN; graphite vs. Li: (c) without EDPN and (f) with 0.5 wt% EDPN. Figure 5. The pictures of (a) pristine LiNi1/3Co1/3Mn1/3O2 cathode, (b) high-voltage cycled cathodes without EDPN and (c) with 0.5 wt % EDPN in the electrolyte. Figure 6. SEM micrographs of LiNi1/3Co1/3Mn1/3O2 electrodes: (a) fresh; (b) without and (c) with 0.5 wt % EDPN after 100 cycles. Figure 7. TEM images of (a) LiNi1/3Co1/3Mn1/3O2 particles prior to the test, the same material from cycled cells (b) without and (c) with 0.5 wt % EDPN in the electrolyte. Figure 8. XPS date of the fresh and cycled LiNi1/3Co1/3Mn1/3O2 electrodes in the 1 M LiPF6-EC: DEC (1:3, wt %) with and without EDPN: (a) C1s, (b) O1s, (c) N1s, (d) F1s and (e) P2p; (f) Comparison EDS analysis of the transition metal concentration (at. %) on the surface of cathode and anode. Figure 9. Cycle performances of the LiNi0.5Co0.2Mn0.3O2/graphite pouch cells containing different ratios of EDPN in electrolytes with 1.0 C of charge/discharge rate at different voltage ranges of (a) 3.0-4.5 V and (b) 3.0-4.2 V.


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Table captions Table 1. Discharge capacity retention of the LiNi1/3Co1/3Mn1/3O2/graphite cells with different ratios of EDPN in the electrolyte at 4.5 and 4.2 V charge cut-off voltages after 100 cycles. Table 2. Equivalent circuit data of the LiNi1/3Co1/3Mn1/3O2/graphite cells without and with 0.5 wt % EDPN after 100 cycles Table 3. Discharge capacity retention of the LiNi0.5Co0.2Mn0.3O2/graphite cells with different ratios of EDPN in the electrolyte at 4.5 and 4.2 V charge cut-off voltages after 100 cycles.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Linear sweep voltammograms of Pt electrodes in 1mol L-1LiPF6-EC: DEC (1:3, wt %) with and without EDPN additive with a scan rate of 1.0 mV s-1 in the voltage of 3.0 to 6.0V (vs. Li/Li+).


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Figure 2. Cycle performances of the LiNi1/3Co1/3Mn1/3O2/graphite pouch cells containing different ratios of EDPN in electrolytes with 1.0 C of charge/discharge rate at different voltage ranges of (a) 3.0-4.5 V and (b) 3.0-4.2 V; (c) Coulombic efficiencies of cells with 1.0 C of charge/discharge rate and (d) rate performances of the cells cycled between 3.0 and 4.5 V.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. EIS of LiNi1/3Co1/3Mn1/3O2/graphite cells with and without EDPN in the electrolyte and charged to 4.5 V (a) after one cycle and (b) after 100 cycles.


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Figure 4. Electrochemical impedance spectra of the three-electrode pouch cells with and without EDPN and charged to 4.5 V after different cycles, full cell: (a) without EDPN and (d) with 0.5 wt % EDPN; LiNi1/3Co1/3Mn1/3O2 vs. Li: (b) without EDPN and (e) with 0.5 wt % EDPN; graphite vs. Li: (c) without EDPN and (f) with 0.5 wt % EDPN.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The pictures of (a) pristine LiNi1/3Co1/3Mn1/3O2 cathode, (b) high-voltage cycled cathodes without EDPN and (c) with 0.5 wt % EDPN in the electrolyte.


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Figure 6. SEM micrographs of LiNi1/3Co1/3Mn1/3O2 electrodes: (a) fresh; (b) without and (c) with 0.5 wt % EDPN after 100 cycles.



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Figure 7. TEM images of (a) LiNi1/3Co1/3Mn1/3O2 particles prior to the test, the same material from cycled cells (b) without and (c) with 0.5 wt % EDPN in the electrolyte.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. XPS date of the fresh and cycled LiNi1/3Co1/3Mn1/3O2 electrodes in the 1 M LiPF6-EC: DEC (1:3, wt %) with and without EDPN: (a) C1s, (b) O1s, (c) N1s, (d) F1s and (e) P2p; (f) Comparison EDS analysis of the transition metal concentration (at. %) on the surface of cathode and anode.


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Figure 9. Cycle performances of the LiNi0.5Co0.2Mn0.3O2/graphite pouch cells containing different ratios of EDPN in electrolytes with 1.0 C of charge/discharge rate at different voltage ranges of (a) 3.0-4.5 V and (b) 3.0-4.2 V.



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Table 1. Discharge capacity retention of the LiNi1/3Co1/3Mn1/3O2/graphite cells with different ratios of EDPN in the electrolyte at 4.5 and 4.2 V charge cut-off voltages after 100 cycles. Discharge capacity retention/% Potential range

0%

0.1%

0.3%

0.5%

0.7%

1.0%

3.0-4.5 V

32.5%

61.2%

71.5%

83.9%

71.2%

44.7%

3.0-4.2 V

93.4%

—

—

91.0%

—

—


Page 31 of 34

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Table 2. Equivalent circuit data of the LiNi1/3Co1/3Mn1/3O2/graphite cells without and with 0.5 wt % EDPN after 100 cycles Samples

Rb(mΩ)

Rf(mΩ)

Rct(mΩ)

EDPN

102

193

823

with 0.5% EDPN

104

142

278

without



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

Table 3. Discharge capacity retention of the LiNi0.5Co0.2Mn0.3O2/graphite cells with different ratios of EDPN in the electrolyte at 4.5 and 4.2 V charge cut-off voltages after 100 cycles. Discharge capacity retention/% Potential range

0%

0.1%

0.3%

0.5%

0.7%

1.0%

3.0-4.5 V

43.2

41.2

34.8

47.8

32.0

50.2

3.0-4.2 V

96.2

97.5

96.5

94.1

93.4

92.6


Page 33 of 34

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Graphical abstract



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

143x68mm (300 x 300 DPI)


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(Ethylenedioxy)dipropiononitrile as an Electrolyte ... - ACS Publications

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