Lithium Difluorophosphate as a Promising Electrolyte Lithium Additive

Article Cite This: ACS Appl. Energy Mater. 2018, 1, 2647−2656

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Lithium Difluorophosphate As a Promising Electrolyte Lithium Additive for High-Voltage Lithium-Ion Batteries Chengyun Wang,†,‡ Le Yu,∥ Weizhen Fan,∥ Jiangwen Liu,†,‡ Liuzhang Ouyang,*,†,‡,§ Lichun Yang,†,‡ and Min Zhu†,‡

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†

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 ‡ SUNWODA-SCUT Joint Laboratory for Advanced Energy Storage Technology, South China University of Technology, Guangzhou, 510641, People’s Republic of China § Key Laboratory of Fuel Cell Technology of Guangdong Province, Guangzhou, 510641, People’s Republic of China ∥ Guangzhou Key Laboratory of New Functional Materials for Power Lithium-Ion Battery and Guangzhou Tinci Materials Technology Co. Ltd., Guangzhou, 510760, People’s Republic of China S Supporting Information *

ABSTRACT: Lithium difluorophosphate (LiDFP), the decomposition product of LiPF6, was evaluated in high-voltage LiNi1/3Co1/3Mn1/3O2/ graphite pouch cells. We report that conventional carbonate-based electrolytes containing 1 wt % LiDFP can notably enhance the cyclability and rate capability of the battery at 4.5 V. Its capacity retention maintained 92.6% after 100 cycles, whereas it is only 36.0% for the additive-free battery. Even after 200 cycles, the capacity retention remained 78.2%. The EIS measurements performed by three-electrode graphite/Li/LiNi1/3Co1/3Mn1/3O2 pouch batteries indicate that LiDFP can efficiently restrain the breakdown of the electrolyte on the LiNi1/3Co1/3Mn1/3O2 electrode surface and relieve the increase of cathode resistance. Additionally, a uniform and stable SEI film modified by LiDFP on the anode can effectively remit the electrode/ electrolyte interfacial reaction and relieve the increase of anode resistance during cycling. Further evidence for the beneficial effect of LiDFP in inhibiting the dissolution of transition metal from the cathode under a high operating voltage is also found. On the basis of electrochemical methods and spectroscopic techniques, the enhancement in the high-voltage performance of the cell attributed to the LiDFP component can simultaneously modify the cathode and anode surfaces. Consequently, LiDFP is a promising electrolyte lithium additive for practical applications in highenergy lithium-ion cells. KEYWORDS: lithium difluorophosphate, LiPF6 decomposition product, high voltage, electrolyte additive, LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries

1. INTRODUCTION With the rapid development of intelligent digital and electric vehicle markets, high energy density lithium-ion batteries (LIBs) have been widely considered in recent years as the most promising candidate for energy storage devices.1−4 The positive electrode, as one of the major components, plays an important part in determining the battery capacity. Presently, layered LiNixCoyMnzO2 is the commonly used commercial cathode material. Although these cathode materials deliver an excellent cyclic stability when operated at a normal working potential of 4.2 V, their low capacity cannot meet the market requirements for high-energy LIBs. As reported, the extra capacity could be achieved by raising the working voltage, but to the detriment of cyclic stability.5,6 The severe capacity fade is mainly caused by unstable carbonate-based electrolyte breakdown and transition metal ion dissolution on the positive electrode surface when cycling under a high working potential (beyond 4.5 V),7−10 © 2018 American Chemical Society

which hinders their high energy density applications. For this reason, developing an electrolyte solution with high electrochemical stability to match the application of these positive electrode materials is imperative. So far, significant works have been dedicated to exploiting high electrochemical stability electrolytes, including the search for novel electrolyte solvents with intrinsic stability and functional additives with good film-forming properties. As a result, different classes of cosolvents, including sulfones,11−13 nitriles,14−16 and fluorinated organic carbonates,17−20 have been reported in relation to their beneficial effect in enhancing the oxidation stability of electrolyte systems. Unfortunately, the improvement of the voltage window is accompanied by a Received: March 4, 2018 Accepted: May 30, 2018 Published: May 30, 2018 2647

DOI: 10.1021/acsaem.8b00342 ACS Appl. Energy Mater. 2018, 1, 2647−2656

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ACS Applied Energy Materials

All electrochemical measurements and some spectroscopic techniques suggest that LiDFP can simultaneously modify the interface between the electrolyte and electrode (anode and cathode). Therefore, there is substantial evidence that LiDFP could be a promising lithium salt for high-voltage LIB applications.

decrease in bulk ionic conductivity and an increase in viscosity for the mixed electrolyte, resulting in poor low temperature and rate capability performance. Compared to the use of cosolvents, the utilization of additives seems to be a more economical and practical path, which has little negative influence on the physical properties of the mixed electrolyte and a noticeable impact on the improvement of the cell electrochemical properties. Many additives have been investigated to boost the high-voltage properties by modifying the cathode electrolyte interface film or the anode solid electrolyte interphase (SEI). Cathode protection additives usually enhance the stability of the cathode-electrolyte interface, reduce interfacial resistance, and suppress the oxidative breakdown of electrolytes and transition metal dissolution under high operating voltages.21−31 For example, LiNi0.5Co0.2Mn0.3O2 with tris(2,2,2-trifluoroethyl) phosphite,21 methyl (2,2,2-trifluoroethyl) carbonate,22 and tris(trimethylsilyl)phosphate;23 LiNi1/3Co1/3Mn1/3O2 with dopamine,24 methyl 3,3,3-trifluoropropanoate and ethyl 3,3,3trifluoropropanoate,25 tris(2,2,2-trifluoroethyl) phosphite,26 3,3′-sulfonyldipropionitrile, 27 and 3,3′-(ethylenedioxy)dipropiononitrile;28 LiCoO2 with poly[bis(ethoxyethoxyethoxy)phosphazene]29 and methylene methanedisulfonate,30 and Li1.2Ni0.2Mn0.6O2 with succinonitrile31 all delivered excellent high voltage cyclic stability with the presence of additive components in the electrolyte. In comparison, anode protection additives can usually reduce transition metal deposition and weaken the reduction decomposition of electrolytes on the anode surface, thereby reducing the capacity fade during cycling under high voltages.32 Additionally, some bifunctional additives, like lithium tetrafluoroborate,33 glutaric anhydride,34 lithium bis(2-methyl-2fluoromalonato) borate,35 diphenyl disulfide,36 and lithium difluoro(oxalato)borate,37 can sacrificially decompose and form passivation films on both electrodes simultaneously. These materials are considered ideal high-voltage electrolyte additives. Interestingly, during the process of surface modification, researchers found that some intermediate decomposition products of LiPF6, such as LixPFyOz-type compounds,38 dimethyl fluorophosphate, and diethyl fluorophosphate,39,40 have a positive effect on cell cyclability. In contrast, H(PO2F2), a phosphoric acid derivative that is also the hydrolysis product of LiPF6, is an unwanted product because of its harmful effect on battery performance. Song et al.41 revealed that phosphitebased additives presented in the electrolyte can effectively control the formation of H(PO2F2) and enhance the electrochemical property of Li/LiNi0.5Mn1.5O4 coin cell. It is also envisaged that if additional LiDFP is mixed into the conventional electrolyte (LiPF6 as the main lithium salt), [PO2F2]− can enhance the electrochemical stability window of the electrolyte by inhibiting the hydrolysis of LiPF6 and the formation of HPO2F2. In a similar manner, as a lithium salt with the same anion ([PO2F2]−) as H(PO2F2), lithium difluorophosphate (LiPO2F2, LiDFP) has a beneficial effect on the cyclability of batteries cycled under different measurement conditions, such as high rates,42 low temperatures,43 and a cyclic voltage range of 3.0−4.3 V.44 Inspired by these benefits, LiDFP is explored as a high-voltage electrolyte additive for LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries in this work. We demonstrate that the pouch full cells containing LiDFP can achieve a long lifespan when operated under high-voltage conditions. Three-electrode graphite/Li/LiNi1/3Co1/3Mn1/3O2 pouch cells are employed to track the impedance variations of LiNi1/3Co1/3Mn1/3O2 and graphite electrodes during cycling.

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials and Electrolyte. Graphite and LiNi1/3Co1/3Mn1/3O2 powders were available from Shenzhen BTR Battery Materials Co., Ltd. and Shenzhen Tianjiao Technology Co. Ltd., respectively. The Li metal foil (purity >99.9%, diameter 15.8 mm, thickness 0.2 mm) was purchased from China Energy Lithium Co., Ltd. The cathode composed of LiNi1/3Co1/3Mn1/3O2, carbon black, and a polyvinylidene fluoride binder (93:3:4, by weight) was doublelayer coated on aluminum foil. The anode composed of graphite, Super P carbon, styrene butadiene rubber, and carboxymethyl cellulose (95:1:2.5:1.5, by weight) was double-layer coated on copper foil. LiDFP (99%) was commercially available from Nippon Shokubai Co., Ltd., Japan. Electrolyte-grade carbonate solvents and electrolyte-grade lithium salt were provided by Guangzhou Tinci Materials Technology Co., Ltd., China. Detailed experimental descriptions about the preparations of electrolytes and three-electrode and two-electrode pouch batteries have been reported in our previous publication.45 Selected information regarding the electrode materials and electrolyte injection weight for the LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries is presented in Table 1.

Table 1. Selected Parameters of the LiNi1/3Co1/3Mn1/3O2/ Graphite Pouch Cells potential range/V

capacity/ mAh

N/P ratio

electrolyte injection weight/g

3.0−4.2 3.0−4.5

546 639

1.36 1.16

3.65 ± 0.05 3.65 ± 0.05

2.2. Electrochemical Measurements. Linear sweep voltammetry was carried out by a three-electrode system recorded from the open circuit potential to 0 and 6.00 V at 1.0 mV s−1 (CHI660 Instrumental Electrochemical Workstation). The three-electrode system with platinum wire as the working and lithium metal as both counter and reference electrodes. The influence of the additive on the cyclability and rate capability was characterized by LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries (Neware battery test system). Direct current (DC) impedance of the cells after different cycles was performed by a BlueKey battery internal resistance test analyzer. Alternating current (AC) impedance spectra obtained from the two- and three-electrode cells were recorded over different cycles. The detailed experimental descriptions about the activation steps, the cyclability, the rate capability test, and DC and AC impedance analysis of cells can be found in our previous publication.45 The impedance analysis of the cells was carried out with about a 100% state of charge. The aforementioned electrochemical measurements were obtained at ∼25 °C. 2.3. Characterization. The cathode and anode samples for surface characterization were obtained from the two-electrode cells (∼0% state of charge) in different electrolytes operated at 4.5 V over 100 cycles. The cycled electrode samples were rinsed by dimethyl carbonate (DMC) and after dried were separately loaded in an argon-filled 50 mL plastic bottle and then vacuum packed with an aluminum plastic bag. The surface morphologies of the fresh and cycled electrodes were examined using the JEM-2100HR transmission electron microscope (TEM) and the Carl Zeiss Supra 40 fieldemission scanning electron microscope (SEM) instrument, respectively. Energy dispersive spectrometry (EDS) was carried out to probe the sample surface chemical compositions and concentrations. 2648


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Figure 1. (a) Photos of the 1 mol L−1 LiPF6-EC/DEC (1:3, wt %) electrolytes containing different weight ratios of LiDFP. (b) The dependence of ionic conductivity on additive concentration in the electrolytes at 25 °C.

3. RESULTS AND DISCUSSION 3.1. Effect of LiDFP on Electrolyte Ionic Conductivity and Electrochemical Stability. Figure 1a displays photographs of nine (1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, and 9#) differently prepared electrolytes, corresponding to LiDFP contents of 0%, 0.25%, 0.5%, 0.75%, 1.0%, 1.25%, 1.5%, 1.75%, and 2.0% (by wt %), respectively. From the photograph of the electrolyte with 0% LiDFP, it can be seen that the electrolyte solution is clear and transparent. Also, the electrolytes containing 0.25%, 0.5%, 0.75%, and 1.0% LiDFP show similar transparency to the 1# sample, which implies that the addition of the LiDFP lithium salt can be completely dissolved in the EC/DEC mixed solution. When the content of LiDFP exceeds 1.0%, the electrolytes become unclear. In particular, when the additive concentration is up to 2.0%, the sample solution shows white turbidity. Thus, the solubility measurements suggest that the content of LiDFP in electrolyte should be controlled, as excessive addition will affect the phase characteristics of the electrolyte. In addition, we also investigated the influence of LiDFP on the electrolyte ionic conductivity. The corresponding conductivity data of each electrolyte are depicted in Figure 1b, and the ionic conductivity decreased with increasing LiDFP concentration from 0.0% to 2.0%. The sample with 0.0% LiDFP delivered an ionic conductivity of ∼6.34 mS cm−1, while ∼5.72 mS cm−1 for the sample containing 2.0% was achieved. Therefore, the solubility and conductivity measurements indicate that LiDFP can only be used as an electrolyte additive in the EC/DEC system. The impact of LiDFP on the electrochemical stability of the commercial electrolyte was investigated by linear sweep voltammetry measurement. The anodic behaviors are given in Figure 2a. For the LiDFP-free electrolyte, the current rose drastically at a potential of ∼4.7 V, indicating solvent decomposition of the electrolyte beginning to oxidize. In contrast, there is an additional anodic current that can be found at a potential of ∼4.3 V for the electrolyte with 1 wt % LiDFP, implying that the component is preferentially oxidized and might produce a protective film on the platinum electrode.46 Interestingly, the current increased sluggishly between 4.3 and 5.5 V and then sharply increased when the potential was over 5.5 V for the LiDFP-containing electrolyte. This phenomenon is similar to the electrochemical behaviors of LiODFB,47 LiBOB,48 and LiBF449 with the characteristics of a passivated positive electrode current collector and inhibition of the

Figure 2. Electrochemical window of 1 mol L−1 LiPF6-EC/DEC (1:3, wt %) electrolyte without and with 1.0 wt % LiDFP employing linear sweep voltammetry at Pt electrode with a scan rate of 1.0 mV s−1 (a) oxidative potential (OCV-6.0 V), (b) reductive potential (OCV-0.0 V).

electrolyte oxidative breakdown. Therefore, combined with the above study, we could speculate that the protective layer formed by the participation of LiDFP can effectively suppress the subsequent oxidation decomposition and then enhance the oxidation stability of the bulk electrolyte. Additionally, the effect of LiDFP on the electrochemical reduction of electrolyte was also studied. There are no remarkable differences in the two electrolytes from the reduction curves of Figure 2b, which suggested that the LiDFP-containing electrolyte displays good compatibility with the anodes of LIBs.20,30 As a consequence, the LiDFP component was considered a possible useful electrolyte additive for cells worked at high-voltage. 3.2. Electrochemical Performances of LiDFP in TwoElectrode Full Cells. After three precycles of activation, the cycling performances of cells containing different amounts of LiDFP were investigated under the working voltages of 3.0−4.5 V and 3.0−4.2 V with a 1.0 C rate. As displayed in Figure 3a and Figure S1, for the LiDFP-containing cells, the cycling stability and discharge capacity retention are significantly enhanced. The capacity of the cell without LiDFP decayed sharply and with a capacity retention of 36.0% (discharge capacity: 56.0 mA h g−1) over the 100th cycle (Table 2). In contrast, especially when the content of LiDFP in the cell reaches 0.5% and 1%, the corresponding capacity retention is maintained at 94.4% (discharge capacity: 149.6 mA h g−1) and 92.6% (discharge capacity: 146.5 mA h g−1) over 100 cycles, respectively. After the subsequent 100 cycles, the capacity retention of 78.2% of the cell with 1.0% LiDFP was higher than the 68.8% of the cell with 0.5% additive after 200 cycles. Intriguingly, when the charge cutoff potential drops down to 2649


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Figure 3. Cycling performance of the LiNi1/3Co1/3Mn1/3O2/graphite pouch cells without and with different amount of LiDFP operated under the voltage ranges of (a) 3.0−4.5 V and (b) 3.0−4.2 V. Comparison of (c) Coulombic efficiencies and (d) discharge C-rate performances between the cells without and with 1.0% LiDFP cycled at a charge cutoff voltage of 4.5 V.

process, which is consistent with the behavior in Figure 3a. The above results suggest that the addition of LiDFP in the electrolyte is beneficial for a stable SEI layer formation, which can effectively alleviate electrolyte decomposition during cycling and improve the cyclic stability under high operated potential. The impact of LiDFP on the C-rate performance of the cells was also investigated by working under different discharge rates. As displayed in Figure 3d, the discharge capacities have a negligible difference between the cells with and without LiDFP under the current densities of 0.2, 0.5, and 1 C. However, when the current density reached up to 3 and 5 C, the discharge capacity of the cell containing additive was obviously higher than that of the cell without the additive. This could be primarily attributed to the surface film modified by LiDFP with a low interfacial resistance and small polarization at a large current density, which is helpful in facilitating Li-ion migration and delivering a high discharge capacity.3,24,26,37 The impact of LiDFP on the interfacial resistance will be discussed in the following EIS measurements. Consequently, the above electrochemical measurements indicate that LiDFP has an obvious effect on the cyclic stability and C-rate performance for the cells cycled under the charge cutoff voltage of 4.5 V. The impact of LiDFP on the high-voltage electrochemical property for the cells can also be reflected in the behaviors of the AC and DC impedance. As indicated in Figure 4a−c, the electrochemical impedance spectra of the LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries using various electrolytes were obtained over different cycles. After the third cycle, the unit battery containing LiDFP delivered a slightly lower impedance than the one without the additive. A

Table 2. Discharge Capacity Retention of Cells Containing Different Amounts of LiDFP in Electrolyte at 4.2 and 4.5 V Charge Cut-off Voltages after 100 and 200 Cycles discharge capacity retention/% potential range

0.0%

0.5%

1.0%

1.5%

2.0%

3.0−4.2 V (100 cycles) 3.0−4.5 V (100 cycles) 3.0−4.5 V (200 cycles)

94.2 36.0

96.3 94.4 68.8

95.0 92.6 78.2

90.1 85.1 58.7

91.5 75.6

4.2 V, all cells show a slight difference in the cyclability and the discharge capacity retention can maintain beyond 90% (Table 2) of the initial discharge capacity over 100 cycles (Figure 3b). Therefore, the LiDFP component shows a noteworthy effect on the cyclability of the batteries cycled under 4.5 V but not at 4.2 V. Figure 3c presents the comparison of Coulombic efficiencies (including the first three precycles) of the cells without and with 1.0% LiDFP over 100 cycles worked under the potential range of 3.0−4.5 V. In the initial first cycles, both cells deliver a low Coulombic efficiency (LiDFP-free cell: 83.8%; additivebased cell: 84.4%), which is likely due to the formation of an interface film on the electrodes during the first charge/ discharge cycle and causes irreversible capacity loss, which also can been verified by the voltage profiles of the first three cycles (see Figure S2). In a subsequent long-term cycling test, the Coulombic efficiency for the battery without additive was only ∼93.3% after the 100th cycle. Conversely, the Coulombic efficiency can be averagely maintained at more than 99.8% for the 1% LiDFP-based cell throughout the cycle of the testing 2650


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Figure 4. Electrochemical impedance spectroscopy measurement of LiNi1/3Co1/3Mn1/3O2/graphite pouch cells with and without LiDFP obtained after (a) the third, (b) the 50th, and (c) the 100th cycle and (d) the DC-IR data of cells obtained during different cycles (each of the cells was tested with a full charge state of 4.5 V).

morphology and composition, spectroscopic techniques, such as SEM, EDS, and TEM, were performed. Figure 5 displays the SEM morphologies of the fresh and cycled LiNi1/3Co1/3Mn1/3O2 electrodes obtained from the cells

possible reason for this was that LiDFP was one intermediate decomposition product of LiPF6, which can inhibit the decomposition of LiPF6 on the electrode surfaces to a certain extent. Additionally, the cell containing LiDFP with a low impedance value also can be verified by the voltage profile of the initial cycle (see Figure S2). After further cycling, the difference in the total resistance of each cell became more distinct, as displayed in Figure 4b and c. The impedance value of the additive-free cell increased observably, which might be attributed to the LiDFP-free electrolyte with poor electrochemical stability on the electrode surface. Meanwhile, the DC impedance values were also collected over 3, 20, 50, 70, and 100 cycles, as shown in Figure 4d. The DC-IR value for the LiDFP-containing cell increased from 52.0 mΩ initially to 94.4 mΩ after 100 cycles, while a significant value variation was observed for the LiDFP-free cell, which sharply increased from 51.9 mΩ initially to 237.5 mΩ after 100 cycles. The behavior of the significant difference in DC impedances was similar to that of the EIS measurements, which further implies that LiDFP has the beneficial impact of improving the stability of the electrode surface passivating layer. Consequently, the cell using the electrolyte with 1% LiDFP can deliver a better high-voltage electrochemical performance than that of the additive-free cell, as shown in Figure 3. 3.3. Surface Characterization of Different Cathodes. The aforementioned electrochemical measurements intuitively indicated that the high-voltage electrochemical properties of LiNi1/3Co1/3Mn1/3O2/graphite pouch cells can be observably improved by adding 1% LiDFP into the electrolyte. Thus, to better elucidate the impact of LiDFP on the electrode interface

Figure 5. SEM images of different LiNi1/3Co1/3Mn1/3O2 electrodes: (a) fresh; (b) without and (c) with 1.0 wt % LiDFP after 100 cycles; (d) comparison EDS analysis of the Ni, Co, and Mn concentration (atom %) on the surface of cathode.

operated at 3.0−4.5 V after 100 cycles. The particle edge of the fresh electrode was distinct, and the crystal faces were clean (Figure 5a). Compared to the fresh one, a layer film covering the cycled electrodes and the particle edge became cambered, as presented in Figure 5b and c. The variation of surface 2651


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the positive electrode is conducive for the cell to achieve prominent electrochemical performance when operated at a high charge cutoff voltage. 3.4. Surface Characterization of Different Anodes. After 100 cycles in the two-electrode pouch cells, SEM micrographs of the graphite electrodes were also recorded, as shown in Figure 7. In the SEM image of the fresh anode, a

morphologies for the cycled electrodes is attributed to electrolyte decomposition during cycling. Noticeably, there are some white granules evenly distributed on the LiDFPcontaining cathode surface (Figure 5c), which were absent for the LiDFP-free surface (Figure 5b). Thus, the uniform surface layer film with white particles was modified by the LiDFP component present in the electrolyte. Moreover, the transitionmetal concentrations of different cathode surfaces were also collected by EDS. As shown in Figure 5d, the contents of Ni, Co, and Mn (atom %) on the fresh LiNi1/3Co1/3Mn1/3O2 surfaces were approximately 6.12%, 6.25%, and 6.38%, respectively. In contrast, a noteworthy transition-metal distribution for the cycled cathodes can be observed in Figure 5d. For the cathode without an additive, each of the transitionmetal concentrations was higher than that of the fresh one, while that of the additive-containing one was not. The difference in transition-metal concentrations between the cycled cathodes with and without LiDFP also can be supplied by the Me−O peak of O 1s spectra from XPS analysis (see Figure S3). The results obtained from the SEM and EDS implied that the cell using LiDFP-containing electrolyte delivered a thicker film on the positive electrode surface.25,27 Figure 6 presents TEM images of the pristine and cycled positive electrodes obtained from the cells operated at 3.0−4.5

Figure 7. SEM micrographs of graphite electrodes: (a) fresh, (b) without and (c) with 1.0 wt % LiDFP after 100 cycles, (d) comparison EDS analysis of the transition-metal concentration (atom %) on the surface of anode.

clean and flat graphite flake substrate can clearly be seen (Figure 7a). Nevertheless, the clean graphite surface was easily altered by the electrolyte reductive breakdown during cell cycling. After cycling, the surface of the graphite electrode without LiDFP is rough and uneven (Figure 7b), suggesting that the poor quality of the SEI layer could not availably alleviate the electrode/electrolyte interface reaction. The phenomenon of significant decomposition products accumulating on the graphite electrode will deliver a high resistance for Li+ diffusion and lead to rapid cell impedance changes during charging and discharging cycles, as supported by the EIS measurements (Figure 4). In comparison, a uniform and dense SEI layer was covered on the additive-containing sample surface, as presented in Figure 7c. The conspicuous surface morphological variations were attributed to the role of the LiDFP component in modifying the SEI layer. The previous publications have reported that the transition-metal ions can easily dissolve out from the LiNixCoyMnzO2-based cathode and possibly migrated to the negative electrode surface while cycling under high working voltage conditions.7−10,50 Therefore, the transition-metal ion content on the different graphite electrodes surface was probed by EDS, and the data are presented in Figure 7d. There are no transition-metal ions existing in the fresh sample, while the noteworthy variation in surface chemistry component was observed for the cycled samples. The concentrations of transition-metal ions on the cycled sample without LiDFP were higher than that of the one containing additive, which further confirmed that the anode SEI film modified by LiDFP can effectively reduce the direct contact of transition-metal ions with the graphite interface and remit the reductive decomposition of the electrolyte catalyzed by these ions. Additionally, to clarify the role of LiDFP in the modification of the graphite electrode surface, the different graphite samples

Figure 6. TEM analysis of LiNi1/3Co1/3Mn1/3O2 electrodes: (a) fresh and (b) without and (c) with 1.0 wt % LiDFP after 100 cycles.

V after 100 cycles. From Figure 6a, it can be observed that the pristine cathode surface was clean and smooth, which is consistent with the description from SEM displayed in Figure 5a. Unlike the fresh cathode, as Figure 6b and c show, a surface layer covered on the cycled cathode samples can be observed. For the cycled cathode without LiDFP (Figure 6b), the surface layer was uneven and weak. Differently, for the sample of Figure 6c shown, a uniform and complete surface layer can be obtained by the additive component presented in the electrolyte, which can availably isolate the contact of the positive electrode/electrolyte. Therefore, combined with the discussion of Figure 5, the LiDFP modified protective layer on 2652


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protective layer covered the cycled LiDFP-containing sample surface (Figure 8c), which suggests that the stable SEI modified by LiDFP would effectively remit the electrolyte decomposition that occurred on the surface of the negative electrode. These results dovetail with the morphologies of the cycled graphite electrodes obtained by SEM (Figure 7). Consequently, the LiDFP component is apparently beneficial for stabilizing the graphite electrode surface. 3.5. Electrochemical Properties of LiDFP in ThreeElectrode Full Cells. Overall, the results obtained by spectroscopic techniques indicated that LiDFP simultaneously has a beneficial effect on the cathode and anode while the cell is working under a high operating potential. To further understand the interfacial behaviors for each electrode during cycling, EIS was implemented to record the impedance variation by the graphite/Li/LiNi1/3Co1/3Mn1/3O2 three-electrode full cells. Figure 9 displayed the EIS spectra of cathode vs Li and anode vs Li using various electrolytes over different cycles, respectively. In the EIS spectra curve, the mediumfrequency (Rct, the charge transfer resistance) and highfrequency semicircles (RSEI, the passivating surface film resistance) are usually focused on by electrolyte researchers. It can be seen from Figure 9a and d that only the highfrequency semicircle was presented in the spectra for both electrodes, implying that Rct was small and negligible at the initial cycles. Figure 9b and c show the total resistance of cathode vs Li was noticeably changed over 100 cycles, a rapid increase for the cathode without LiDFP, while that of the sample with additive was not with cycling. This result further confirmed that a certain amount of intermediate decomposition product of LiPF6 presented in the electrolyte could effectively relieve the increase of the LiNi1/3Co1/3Mn1/3O2 electrode resistance by suppressing the breakdown of electrolyte on the cathode surface. From Figure 9e and f, both the RSEI and Rct can be clearly observed in the spectra curves of the anodes using two electrolytes. For the LiDFP-containing anode, the variation of RSEI was negligible, and Rct increased marginally from the 50th to the 100th cycle. Differently, both the RSEI and Rct of the LiDFP-free graphite electrode increased observably, and the

were also characterized by TEM. Figure 8 presents the TEM images of the pristine and cycled negative electrodes using

Figure 8. TEM analysis of graphite electrodes: (a) fresh and (b) without and (c) with 1.0 wt % LiDFP after 100 cycles.

different electrolytes. From Figure 8a, it can be observed that the pristine anode displays a clean surface. In comparison, there are film layers covered on the graphite electrodes surface after 100 cycles, as displayed in Figure 8b and c. The protective layer of the cycled LiDFP-free sample was incomplete and nonuniform, and as expected, some graphite surface was exposed (Figure 8b). This unstable SEI would not completely separate the electrolyte from the graphite electrode surface, which in turn causes the continuous reductive decomposition of the electrolyte during cell cycling. In contrast, a uniform

Figure 9. Electrochemical impedance spectra of cathode and anode of the three-electrode pouch cells with and without LiDFP in the electrolytes operated at a rate of 1.0 C and charged to 4.5 V. Cathode vs Li: after the (a) third, (b) 50th, and (c) 100th cycles. Anode vs Li: after the (d) third, (e) 50th, and (f) 100th cycles. 2653



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ACKNOWLEDGMENTS This work was supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. NSFC51621001 and 51771075), the Science and Technology Projects of Guangzhou (201604016131). L.O. also thanks Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014).

corresponding values were larger than that of the LiDFPcontaining sample. The comparison of the anode resistance suggested that the SEI modified by LiDFP is relatively more stable and uniform, which could effectively suppress the electrode/electrolyte interfacial reaction during cycling. Consequently, both the electrochemical method and spectroscopic techniques illustrated that LiDFP is a benefit for enhancing the high-voltage cyclic stability by simultaneously modifying the cathode and anode surfaces during cycling.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00342. Figure S1 shows the retention of discharge capacity for the LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries containing various amounts of LiDFP operated under the working voltages of 3.0−4.5 V and 3.0−4.2 V; comparison of charge/discharge profile of the first three cycles for LiNi1/3Co1/3Mn1/3O2/graphite pouch batteries using the electrolytes with 0% and 1% LiDFP shown in Figure S2; Figure S3 displays the O 1s XPS spectra for the LiNi1/3Co1/3Mn1/3O2 electrodes (the pristine and the cycled cathode without and with LiDFP) (PDF)

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REFERENCES

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4. CONCLUSIONS In summary, LiDFP, as an intermediate decomposition product of LiPF6, was assessed in high-voltage LiNi1/3Co1/3Mn1/3O2/ graphite pouch cells. The 1 wt % LiDFP presented in the conventional carbonate-based electrolytes can notably enhance the cyclability and rate capability of batteries worked under 4.5 V. Its capacity retention was maintained at 92.6% over 100 cycles, which is better than that of the additive-free battery being 36.0%. Even after 200 cycles, the capacity retention remained 78.2%. For the positive electrode, a uniform and complete protective layer modified by LiDFP can efficiently restrain the oxidative breakdown of the electrolyte on the LiNi1/3Co1/3Mn1/3O2 electrode surface, especially the LiPF6 component. For the negative electrode, a stable and dense SEI provided by LiDFP can guard against the graphite electrode surface exposed to electrolytes and efficiently remit the reductive decomposition of electrolytes. Furthermore, the EIS results obtained by the graphite/Li/LiNi1/3Co1/3Mn1/3O2 three-electrode pouch battery demonstrated that the LiDFP additive can separately suppress the increase of the negative and positive electrode impedance while cycling under high voltages. Accordingly, LiDFP is a promising electrolyte lithium additive for practical applications in high-energy LIBs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liuzhang Ouyang: 0000-0003-2392-2801 Min Zhu: 0000-0001-5018-2525 Notes

The authors declare no competing financial interest. 2654


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ACS Applied Energy Materials Review on Surface Chemical Aspects of Li Batteries: A Key for a Good Performance. J. Power Sources 2009, 189, 288−296.

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Lithium Difluorophosphate as a Promising Electrolyte Lithium Additive

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