High-Performance Cells Containing Lithium Metal Anodes, LiNi0

May 22, 2018 - Department of Chemistry, Bar-Ilan University , Ramat Gan 52900 , Israel ... Department of Energy Engineering, Hanyang University, Seoul...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19773−19782

High-Performance Cells Containing Lithium Metal Anodes, LiNi0.6Co0.2Mn0.2O2 (NCM 622) Cathodes, and Fluoroethylene Carbonate-Based Electrolyte Solution with Practical Loading Gregory Salitra,*,† Elena Markevich,*,† Michal Afri,† Yosef Talyosef,† Pascal Hartmann,‡ Joern Kulisch,‡ Yang-Kook Sun,§ and Doron Aurbach*,† †

Department of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel BASF SE, Ludwigshafen 67056, Germany § Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea ‡

S Supporting Information *

ABSTRACT: We report on the highly stable lithium metal| LiNi0.6Co0.2Mn0.2O2 (NCM 622) cells with practical electrodes’ loading of 3.3 mA h g−1, which can undergo many hundreds of stable cycles, demonstrating high rate capability. A key issue was the use of fluoroethylene carbonate (FEC)-based electrolyte solutions (1 M LiPF6 in FEC/dimethyl carbonate). Li|NCM 622 cells can be cycled at 1.5 mA cm−2 for more than 600 cycles, whereas symmetric Li|Li cells demonstrate stable performance for more than 1000 cycles even at higher areal capacity and current density. We attribute the excellent performance of both Li|NCM and Li|Li cells to the formation of a stable and efficient solid electrolyte interphase (SEI) on the surface of the Li metal electrodes cycled in FEC-based electrolyte solutions. The composition of the SEI on the Li and the NCM electrodes is analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A drastic capacity fading of Li|NCM cells is observed, followed by spontaneous capacity recovery during prolonged cycling. This phenomenon depends on the current density and the amount of the electrolyte solution and relates to kinetic limitations because of SEI formation on the Li anodes in the FEC-based electrolyte solution. KEYWORDS: Li batteries, NCM 622 cathodes, Li metal anodes, Li|NCM cells, fluoroethylene carbonate, high areal capacity, surface chemistry

1. INTRODUCTION

based ethylene carbonate (EC)-free organic carbonate electrolyte solutions which were shown to be the most promising electrolyte solutions for high-energy-density and high-voltage rechargeable Li batteries.5 It is very important, however, to achieve stable long-term operation of full Li|cathode cells with a commercial level of energy density per unit area. In the last years, there were many attempts to develop full cells with Li anodes and composite cathodes operating at about 4 V with parameters approaching the practical level. Different strategies of surface modification of the Li metal surface resulted in improved cycling performance of Li|cathode full cells with 4 V LiFePO4 (LFP), LiNi0.6Co0.2Mn0.2O2 (NCM 622), and LiMn2O4 (LMO) cathodes in LiPF6-containing organic carbonate electrolytes.6−9 Skin grafting of the Li surface with chemically and electrochemically active polymers on the Li metal surface enabled stable cycling of Li| LiNi0.5Co0.2Mn0.3O2 (NCM 532) cells (areal capacity 1 mA h cm−2, current density

Lithium metal (Li) anodes with their extremely high theoretical specific capacity of 3860 mA h g−1 and low negative redox potential of −3.040 V versus a standard hydrogen electrode are considerably superior compared to any other anode materials in terms of specific capacity and positive impact on energy density.1 These unique characteristics make Li anodes very promising components for the next generation of advanced, high-energy-density Li batteries. It was shown that a solid electrolyte interphase (SEI) is formed immediately on the Li surface once Li is immersed in a nonaqueous electrolyte solution and during initial plating/stripping processes.2,3 The main limitations for the commercialization of rechargeable batteries with Li metal anodes are the safety problems related to the dendritic growth of Li deposits and continuous side reactions, leading to the depletion of the electrolyte solution and the active metal.1 In our previous work,4 we demonstrated an excellent cycling performance of Li metal anodes in symmetric Li|Li cells for more than 1100 cycles at a current density of 2 mA cm−2 and an areal capacity of 3.3 mA h cm−2 with a minimal amount of fluoroethylene carbonate (FEC)© 2018 American Chemical Society

Received: April 29, 2018 Accepted: May 22, 2018 Published: May 22, 2018 19773

DOI: 10.1021/acsami.8b07004 ACS Appl. Mater. Interfaces 2018, 10, 19773−19782

Research Article

ACS Applied Materials & Interfaces 0.3 mA cm−2) and Li| LFP cells (areal capacity 2 mA h cm−2, current density 0.5 mA cm−2) for 400 cycles.9 Luo et al. demonstrated 200 galvanostatic cycles of Li|LFP cells with a βpoly(vinylidene difluoride) thin film coating on the Li anode cycled with a higher current rate of 0.9 mA cm−2 (an areal capacity of about 1.8 mA h cm−2) in an electrolyte solution containing FEC as an additive.6 Micropatterned Li anodes enabled stable cycling of Li|LMO cells with an areal capacity of 1.47 mA h cm−2 for 450 cycles but with a lower current density (0.67 mA cm−2).7 Replacing of LiPF6-containing electrolyte solution by lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)−lithium bis(oxalato)borate (LiBOB) dual-salt electrolyte10,11 as well as the use of this electrolyte salts with the addition of a small amount of LiPF612 resulted in the improved performance of full cells with the Li anode. Li|4 V cathode cells with an areal capacity of 1.75 mA h cm−2 demonstrated 450− 500 cycles without marked capacity fading at a current density of 1.75 mA cm−2 in these solutions.11,12 A profound effect of current density at the formation stage and discharge rate during the cycling of Li|NCM cells with an areal capacity up to 2.2 mA h cm−2 was demonstrated by Zheng et al.,13 and 500 stable cycles were achieved in standard LiPF6 EC/dimethyl carbonate (DMC) electrolyte solution when the discharge rate was around 1C. Recently, this group demonstrated 300 cycles of stable cycling of Li|NCM cells with the areal capacity in the range of 0.9−4.1 mA h cm−2 with a limited value of discharge current of 2 mA cm−2 and with charge current density which did not exceed 1 mA cm−2 in LiPF6-assisted LiTFSI−LiBOB dual-salt EC-based electrolyte.14 Higher charge current densities resulted in a very fast capacity fading. In the present paper, we present the results of cycling lithium metal|NCM 622 cells in FEC-based electrolyte solution at very high electrodes’ loading and depth of discharge. FEC addition to EC-containing electrolyte solutions was shown to improve the performance of Li|cathode cells. Zhang et al. demonstrated a stable cycling of NCM 532|Li cells with an areal capacity of 1.9 mA h cm−2 at a current density of 2.16 mA cm−2 for about 100 cycles in an EC-based electrolyte solution containing 5% FEC.15 In this work, we demonstrate highly stable cycling of Li| NCM cells with a loading of 3.3 mA h cm−2 for more than 600 cycles at a current density of 1.5 mA cm−2. The areal capacity of 3.3 mA h cm−2 complies with requirements of commercial lithium-ion batteries,16 and this level of areal capacity is a very important parameter for testing and evaluating full Li|cathode cells. Another key factor is the possibility of stable operation of Li|cathode cells at different current rates. We present cycling results obtained at different current densities and discuss the effect of the current density and the amount of the electrolyte solution per unit electrode area on the cycling performance of the cells. Although Li anodes, NCM 622 cathodes, and FEC solutions cannot be considered as new components, the novelty of this work is laid by the prolonged cycling with very high (fully practical) areal loading of the electrodes, while using high specific capacity Li metal anodes, the low amount of the electrolyte solutions required, and the very high depth of discharge of both electrodes in the cells used herein. The results presented fully substantiate the previous reports about the positive impact of FEC as an important cosolvent in many kinds of rechargeable Li battery prototypes because of the positive effect of its presence on the passivation of Li metal anodes. Most of the previous work has dealt with relatively low electrodes’ loading. Here, we show that the positive properties of the surface films formed on Li metal anodes in FEC/DMC/

LiPF6 solutions are fully relevant to cells comprising practically high areal-loaded electrodes.

2. EXPERIMENTAL SECTION 2.1. Cell Assembly and Electrochemical Tests. Li metal foil [FMC Chemicals Limited (U.K.)] with the thickness of 0.25 mm which was kept in an argon-filled glovebox was used without any pretreatment. Cathode sheets comprising NCM 622 with the areal capacity of 3.3 mA h cm−2 were obtained from BASF (Germany). An electrolyte solution of 1 M LiPF6 in FEC/DMC 1:4 was used. For galvanostatic tests of Li|Li symmetric cells and Li|NCM cells, Li metal foil disk electrodes and NCM cathodes with a diameter of 14 or 17 mm were assembled with two layers of a polyethylene (PE) separator (Tonen) in two-electrode configurations using coin-type cells (2325, NRC, Canada). The amount of the electrolyte solution used was 22−33 μL cm−2 of electrodes. Li|Li cells were cycled with a current density of 1 or 2 mA cm−2 and a charge/discharge capacity of 3.3 mA h cm−2. Li|NCM cells were cycled with a current density of 0.25−2 mA cm−2 between 2.8 and 4.3 V. For cyclic voltammetric and impedance measurements, threeelectrode Li|NCM cells with Li reference electrodes were assembled with the use of standard coin-type cells as described in ref 17. Cycling voltammograms and impedance spectra in the frequency range of 100 kHz to 10 mHz (electrochemical impedance spectroscopy) were measured using a potentiostat−galvanostat model 128N Autolab (Eco Chemie). 2.2. Characterization of Pristine and Cycled Electrodes. Scanning electron microscopy (SEM) images were obtained with an environmental scanning electron microscope, Ouanta FEG 250 (FEI). Air-sensitive samples were transferred from the Ar-filled glovebox to the chamber of the microscope with the use of a homemade vacuumtight transferring cell.4 X-ray diffraction (XRD) patterns were obtained with a D8 ADVANCE system (Bruker Inc.) using Cu Kα radiation operated at 40 mA and 40 kV. To prevent the contact of NCM electrodes with the ambient atmosphere, they were protected with the Kapton film of 20 μm (Goodfellow). X-ray photoelectron spectroscopy (XPS) was used for the surface analysis of Li anodes and composite NCM electrodes. Photoelectron spectra were obtained with a Kratos Axis-HS spectrometer (England) at a residual gas pressure of ∼5 × 10−10 Torr using monochromatized Al Kα radiation (hν = 1486.68 eV). High-resolution spectra were recorded with a pass energy of 40 eV (0.05 eV step). During the measurements, the vacuum in the analysis chamber was 2 × 10−9 Torr. Binding energies (BEs) were corrected with respect to the BE value of the C 1s peak at 285 eV. Air-sensitive electrodes after cycling were transferred from the glovebox to the X-ray photoelectron spectrometer without contact with ambient air using homemade devices, equipped with gate valves and magnetic manipulators. Diffuse reflectance Fourier transform infrared (FTIR) spectra from the Li electrodes were measured using a Magna 860 spectrometer (Nicolet) in a glovebox in an H2O- and CO2-free atmosphere. The cycled electrodes were washed three times with pure DMC, dried, and hermetically closed in a cell with a KBr window.18 Prior to the XPS, SEM, and FTIR measurements, the cycled cells were disassembled in an Ar-filled glovebox, and Li electrodes and NCM cathodes in the lithiated state were washed four times with pure dry DMC. 2.3. Analysis of Electrolyte Solution and Gases from Cycled Cells. 19F NMR spectra of the electrolyte solutions were collected using a Bruker AVANCE-400 spectrometer (376.5 MHz for 19F). Cycled cells were disassembled in a glovebox, and their components were rinsed with pure dry DMC. To prevent the reaction of HF with NMR tubes made of glass, special Teflon NMR probe tubes, equipped with screw caps (New Era, USA), were used for the measurements. Chemical analysis of the electrolyte solution from cycled Li|NCM cells was carried out using an inductively coupled plasma optical emission spectrometry (ICP-OES, Spectro Arcos, Ametek) technique. 19774

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Figure 1. Galvanostatic cycling results obtained for symmetric Li|Li cells cycled with a current density of 1 mA cm−2 and a charge/discharge capacity limited to 3.3 mA h cm−2 (a) and for Li|NCM cells cycled with different current densities, as indicated. Red, blue, black, and green curvesFECbased electrolyte solutions and purpleEC-based electrolyte solutions. (b,c) Voltage profile measured for Li|NCM cells cycled with a current density of 1.5 mA cm−2, (d) Nyquist plots measured at 3.5 V for Li|NCM cells before cycling (blue dots) after 1 cycle (black dots) and after 240 cycles (red dots). The amount of the electrolyte solution 33 μL cm−2. 30 °C. For the FTIR analysis of the composition of gaseous products from cycled cells, they were punctured under vacuum in a homemade metallic FTIR cell equipped with two KBr windows.

performance in the FEC-based electrolyte solutions without any special pretreatment of the Li anodes. At a current density of 1.5 mA cm−2 and an amount of the electrolyte solution of 33 μL cm−2, the cells exhibit an areal capacity of about 2 mA h cm−2 after more than 600 galvanostatic cycles (Figure 1b). At lower current rates, hundreds of cycles without marked capacity fading were observed as well. An increase of the current density up to the value of 2 mA cm−2 resulted in drastic capacity fading of the cells after about 100 cycles with the subsequent recovery of the cell capacity after prolonged repeated cycling for hundreds of cycles. Thus, from the results presented in Figure 1, one can conclude that symmetric Li|Li cells demonstrate much longer cycle life than Li|NCM cells cycled with the same current density and charge/discharge capacity. At the same time, it is well-known that graphite|NCM cells with commercial parameters stably operate for thousands of cycles.22,23 Thus, the shorter cycle life of Li|NCM cells obviously relates to the effect of cathode on the SEI, which protects the Li anode in the full cells because of the formation of the products of oxidative decomposition of the electrolyte solution on the cathode.24,25 It is remarkable that the phenomenon of fading/recovery of the capacity during repeated cycling of Li|NCM cells is observed not only at a high current rate of 2 mA cm−2 as is shown in Figure 1b but also at lower current densities at lower amounts of the electrolyte solution (Figure 2a). When the amount of the electrolyte solution comprised 22 μL cm−2, a pronounced minimum in the charge/discharge capacity curves versus number of cycles was observed at current densities of 0.5 mA cm−2 (black curve) and 1 mA cm−2 (blue curve). The higher the current density, the deeper was the minimum of the capacity curves. The decrease of the current density down to 0.25 mA cm−2 (red curve) when the amount of the electrolyte was 22 μL cm−2 resulted in stabilization of the cells and almost a full disappearance of the minimum. The effect of the amount

3. RESULTS AND DISCUSSION Typical galvanostatic cycling results obtained for symmetric Li| Li cells cycled with current densities in the range of 0.5−2 mA cm−2 and charge/discharge capacity limited to 3.3 mA h cm−2 of Li anode are shown in Figures 1a and S1. The cells demonstrate a very stable behavior for thousands of hours with a voltage profile typical of a stable homogeneous lithium plating/stripping process.19,20 The cells cycled with current densities of 1 and 2 mA cm−2 performed more than 1000 cycles during 5−10 months and were wet after disassembling. Figure 1b exhibits the cycling performance of Li|NCM cells with NCM 622 cathodes with the areal capacity equal to the charge/discharge capacity of symmetric Li|Li cells cycled in the same range of current densities as for the symmetric cells. It is clearly seen that the cells cycled with the FEC-based electrolyte solution significantly outperform those cycled with EC-based electrolyte solution as demonstrated earlier in ref 4. Figures 1c and S2 demonstrate voltage profiles of Li|NCM cells, which are typical for NCM 622 cathodes.21 Typical Nyquist plots measured for Li|NCM cells before cycling and after 1 and 240 cycles are presented in Figure 1d. Obviously, a decrease in the resistance of the surface films and the charge-transfer processes in the electrodes observed for two-electrode cells as cycling proceeds mainly relates to the changes in the morphology of the Li metal electrodes with increase in the effective surface area of the Li anodes (which reduces the overall resistance and the total overpotential required to cycle the cells at the constant current).4 It is seen that Li|NCM cells with a commercial level of the areal capacity of the NCM cathodes demonstrate an excellent cycling 19775

DOI: 10.1021/acsami.8b07004 ACS Appl. Mater. Interfaces 2018, 10, 19773−19782

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cathode (Figure 2b). The capacity of the new cell was recovered almost up to the initial value, and after several dozens of cycles, the minimum of the capacity curves was developed again. This simple experiment testify to the fact that not structural degradation of the cathode but rather a growth of the resistance of the surface films on the Li anode or the depletion of the electrolyte solution may be responsible for the capacity fading. Indeed, as one can see in Figure 3, the structure of cathodes taken out from the cycled cells in a fully discharged (lithiated) state at different stages of the cycling life (inset in Figure 3) remains unchanged and identical to that of pristine NCM cathodes. ICP analysis performed for the electrolyte solutions and the Li anodes revealed that the capacity fading of the cells does not correlate with the dissolution of transition metals from the cathodes. According to our ICP results, after 500 cycles, metal losses were 0.04, 005, and 0.009% for Ni, Mn, and Co, respectively. This observation is in line with the data presented in ref 26, showing that no marked dissolution of transitionmetal ions from NCM cathodes occurs during cycling with upper cutoff voltages lower than 4.3 V. Besides, according to SEM images of pristine and cycled cathodes (Figure 4a−d), the NCM cathodes we used preserve their morphology during cycling. At the same time, the morphology of Li anodes cycled in symmetric Li|Li cells and in full Li|NCM cells differs significantly (Figure 4e,f). Even after more than 1000 cycles in a symmetric cell, the surface of Li electrodes is much smoother than that of the Li anode after 75 cycles in the full cell. The impedance spectra of two-electrode Li|NCM cells in a fully discharged state at 3.5 V were collected in different periods of the cycling life of the cells at points 1−3 as shown in Figure 5a. An increase of the impedance from point 1 to point 2 followed by its decrease in point 3 correlates well with the trend of the capacity curve (Figure 5b). In order to evaluate the contribution of the cathode and the anode to the impedance of the full cells, three-electrode coin cells containing Li reference electrodes were prepared. The cells were cycled galvanostatically using the two-electrode scheme for 16 cycles. After that, the impedance spectra of the NCM cathode and the Li anode were measured separately versus Li reference electrode (blue and red dots in Figure 5c, respectively). Good agreement between the impedance response of these cells measured with

Figure 2. Cycling performance of Li|NCM cells with 22−33 μL electrolyte solution/cm2 of the Li anode and the NCM cathode at different current densities, as indicated. 30 °C.

of the electrolyte solution may be clearly demonstrated by the comparison of two charge/discharge capacity curves measured at the same current density of 0.5 mA cm−2. With 22 μL cm−2 of the electrolyte solution, a pronounced minimum of the capacity curves was always observed, but with the amount of the electrolyte solution increased up to 33 μL cm−2, the capacity fading/recovery phenomenon was not observed. To find out whether the Li anode or the cathode side of the full Li|NCM cells is responsible for a drastic capacity fading observed after the initial period of cycling, we disassembled the cell after a significant capacity fading and added a fresh Li anode and new portion of the electrolyte solution to the cycled

Figure 3. XRD patterns of pristine NCM 622 electrodes and of three NCM 622 electrodes in a fully lithiated state from Li|NCM cells cycled with a current density of 1 mA cm−2 and stopped in the points 1, 2, and 3 (inset). 19776

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after 20 cycles (Figure 5d) demonstrates a typical behavior for NCM 622 cathodes.28,29 The areal discharge capacity calculated from cyclic voltammetry comprised about 3 mA h cm2. It is seen that the impedance behavior of Li|NCM cells is determined mostly by the impedance of the Li anodes related to the changes in the thickness, composition, and morphology of the SEI formed on Li surfaces. Thus, it is clear that the reason for the capacity fading from point 1 to point 2 is a fast formation of thick SEI on the Li anode. The growth of the surface films on Li anodes is responsible for the increase of impedance in point 2 compared to that in points 1 and 3. This assumption is in line with the conclusion made in ref 14 for Li| NCM cells cycled with EC-based electrolyte solutions. A possible reason for the capacity increase starting from point 2 may be exfoliation or cracking of the resistive surface films with subsequent formation of thinner stable SEI films. The strong effect of the amount of the electrolyte solution on the voltage profiles of the charge/discharge curves of the Li| NCM cells suggests that the electrolyte solution is prone to chemical transformation and depletion during prolonged cycling. To check the composition of the electrolyte solutions, we performed NMR analysis of the pristine electrolyte solution and of solutions rinsed with pure dry DMC from the Li|NCM cell after 40 cycles and from the Li|Li symmetric cell after 1050 cycles. 19F NMR spectra of the rinsed electrolyte solutions are shown in Figure 6. According to our estimation obtained from the integration of the signals related to PF6− anions (doublet at −74.5 ppm) and FEC (multiplet at −123.5 ppm), the ratio of the number of F atoms corresponding to FEC molecules to the number of the F atoms related to PF6− anions decreases during the cycling of both types of cells. However, in the case of full Li| NCM cells, the consumption of FEC is much more pronounced compared to the case of Li|Li symmetric cells. Indeed, the ratio F(FEC)/F(PF6−) after 40 cycles in Li|NCM cells changes from 1:3.77 for the pristine electrolyte solution to 1:2.72, and for Li|Li, this ratio comprises 1:3.12 after more than 1000 galvanostatic cycles performed with the same charge/

Figure 4. SEM images of pristine NCM cathode (a,c), NCM cathode cycled in the Li|NCM cell for 160 cycles (lowest point) (b,d), Li electrodes cycled in the Li|Li symmetric cell for 1050 cycles (e), and Li electrodes cycled in the Li|NCM 622 cell for 75 cycles (f).

the two-electrode setup, NCM cathode versus Li anode (black dots), and the calculated curve, obtained by the addition of the spectra of the NCM cathode and the Li anode (solid line), is indicative of the correctness of the measurements.27 Besides, cycling voltammogram measured with this three-electrode cell

Figure 5. (a) Cycling performance of the Li|NCM cell, (b) Nyquist plots measured for the cell at the points 1, 2, and 3, (c) Nyquist plots after 16 cycles, and (d) cyclic voltammograms measured for a three-electrode Li|NCM 622 coin cell with the Li reference electrode. 30 °C. 19777

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detected component. Typical FTIR spectrum collected from the gaseous phase of the cycled Li|Li cell for 150 cycles (Figure 7) also displays additional peaks at 2356 and 2336 cm−1 related to the stretching vibration of CO2 and a peak at 667 cm−1 corresponding to the bending of CO2.31,32 According to the commonly accepted mechanism, CO2 is the product of decomposition of FEC.33−35 For the examination of the solid products formed during cycling as a result of transformation of the components of the electrolyte solution, we performed XPS and FTIR analyses of the surface films formed on the cycled electrodes. Figure 8 compares the XPS data collected for Li anodes cycled in symmetric Li|Li and Li|NCM cells, and the composition of the surface films on Li anodes cycled in these two types of cells is presented in Table 1 (in light of the spectroscopic results). It is seen that the surface films formed on Li anodes cycled in Li| NCM cells and symmetric Li|Li cells differ greatly in composition. The surface films formed on Li anodes in Li| NCM cells contain more F compared to O and C. From the F 1s spectra of the Li electrodes, one can conclude that the increased content of F on the surface of Li anodes cycled in Li| NCM cells relates to a markedly higher concentration of LiF in the surface films.36,37 Obviously, the sources of the high content of LiF in the surface films formed on Li anodes in Li|NCM cells are FEC and products of oxidative decomposition of FEC, which are transferred from the cathode. These results are indicative of the higher content of inorganic species in the surface films formed in full Li|NCM cells. In turn, SEI formed on Li anodes in symmetric Li|Li cells contains a significant amount of LiPF6 decomposition products, such as LixPOyFz and LixPFy. Figure 9 presents FTIR spectra of the Li electrode surface cycled in symmetric Li|Li cells and in full Li|NCM cells. From the general similarity between the spectra, one can recognize that the same set of products is present in the surface films formed on both types of Li anodes, including Li2CO3 and organic lithium alkylcarbonates, lithium oxalate, lithium oxide, hydroxide, and alkoxides, as indicated.38 However, in line with the XPS results, the surface films formed in the Li|NCM cells contain less P−F and P−O containing species, indicating lower content of LiPF6 decomposition products. Li2CO3 in the surface films formed on Li anodes in the Li|NCM cells

Figure 6. 19F NMR spectra of electrolyte solutions that were rinsed with dry DMC from the Li|NCM 622 cell after 40 cycles (a), Li|Li symmetric cell after 1050 cycles (b), and standard electrolyte solution 1 M LiPF6 in FEC/DMC (1:4) (c).

discharge capacity as for full cells with NCM cathodes. This observation points to the fact that the consumption of FEC is governed by the high potential of the cathodes in the Li|NCM full cells and results mainly from side oxidation reactions, which occur on the cathode during cycling. At the same time, Li|Li symmetric cells contain a sufficient amount of the FEC component during prolonged cycling for more than 1000 cycles during months of continuous cycling. The consumption of FEC was earlier observed by Jung et al.30 on silicon−carbon composite electrodes prepared from silicon nanoparticles in Li-ion batteries. The long cycle life of symmetric Li|Li cells with the FEC-based electrolyte suggests the formation of effectively protective SEI on Li anodes in this case. It is remarkable that no other products than DMC and FEC were detected by 1H and 13 C NMR spectroscopy of the rinsed solutions, which indicates that the products of FEC transformation are insoluble in the electrolyte solution and present solid and/or gaseous species. The presence of CO2 in the gaseous phase of the cycled Li|Li and Li|NCM cells punctured inside of the evacuated cell was revealed with the use of FTIR. In all cases, DMC was the main

Figure 7. FTIR spectra of DMC (blue curve) and gaseous phase of a typical Li|Li cell after 150 cycles (red curve). 19778

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Figure 8. XPS data measured from Li anodes cycled in symmetric Li|Li and Li|NCM cells.

Table 1. Composition of Surface Films on Li Anodes Cycled in 1 M LiPF6/FEC/DMC Electrolyte Solution by XPS element

Li|Li cell (at. %)

Li|NCM cell (at. %)

F O C P Li

12.3 29.2 45.2 1.5 11.8

30.0 19.3 28.8 1.0 20.9

predominates over other carbonate species, which supports the conclusion about the higher content of inorganic species in this case. It is worth noting that surface films which are formed on the NCM cathodes differ from those formed on the Li anodes. The composition of the surface films formed on NCM cathodes after 40 cycles in Li|NCM cells was measured by XPS (Figure 10). The signals of Ni 3p, Co 3p, and Mn 3p are attenuated by the surface films. The main components of the surface films formed on the cathodes are LiF and polyethylene oxide-like polymer species.39 These surface films also contain carbonate species and products of LiPF6 decomposition. All the results presented above are indicative of a strong detrimental influence of high-voltage cathodes on the properties of the surface films, which are formed on the Li anodes in Li|NCM cells. This influence is related to the transfer of products of the oxidative decomposition of the electrolyte solutions to the anode side and their participation in the formation of surface films on the Li anodes. To reduce the transfer and influence of the harmful species produced in the cathode side to the Li anode, we assembled cells with one layer

Figure 9. FTIR spectra of the Li electrode surface after 1000 galvanostatic cycles in symmetric Li|Li cells (a) and 40 cycles in Li| NCM cells (b).

of SiO2-filled PE separator (Entek) placed between two layers of standard PE separators. The effect of the presence of the SiO2-filled PE separator in the cells on the cycling performance of Li|NCM cells is demonstrated in Figure S3. Interestingly, the drastic capacity fading of the cells was prevented by the use of an SiO2-filled separator, obviously, because of its scavenging effect. 19779

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Figure 10. XPS data of pristine NCM cathodes and NCM cathodes cycled in Li|NCM cells.

4. CONCLUSIONS In this work, we unambiguously and strongly substantiated conclusions about the unique properties of solutions containing FEC as a cosolvent on the stability of all kinds of rechargeable Li cells containing Li metal anodes. We demonstrated that using FEC-based electrolyte solutions enables not just an extremely stable cycling of Li|Li symmetric cells (more than 1000 cycles for about 300 days at 1 mA cm−2 and charge/ discharge capacity of 3.3 mA h cm−2) but also very stable cycling performance of Li|NCM cells at different current densities (almost 600 cycles were achieved at 1.5 mA cm−2). However, full Li|NCM cells demonstrated a lower cycling stability than symmetric Li|Li cells. According to spectroscopic studies of solutions from cycled cells by NMR measurements, cycling Li|NCM is accompanied by a much faster consumption of FEC compared to the situation in symmetric Li|Li cells. Thereby, the performance of the full cells strongly depends on the current density and the amount of the electrolyte solution in them. We attribute this observation to the involvement of the products of the oxidative decomposition of the electrolyte solution at the positive electrodes in detrimental side reactions on the Li anodes, which worsen their passivation. These products are transferred further to the anodes being reduced on the anodes to form solid and gaseous products, which were detected by XPS and FTIR. These side processes lead both to a faster depletion of the electrolyte solution in the cycled cells and to the growth of the resistive surface films on the Li anodes, resulting in kinetics problems. A phenomenon of a drastic capacity fading after about 80 cycles with the subsequent gradual capacity recovery was observed with Li|NCM cells depending on the amount of the electrolyte solution and the current density. This behavior does not relate to the dissolution of transition metals or any

structural changes of the NCM cathodes. It corresponds to kinetic limitations because of the formation of too resistive surface films on the Li anodes causing changes in the impedance of the cells, which are dominated mostly by the impedance of the Li anodes. According to the XPS measurements, the content of LiF in the surface films on Li anodes, which are formed in Li|NCM cells, is markedly higher than that formed in symmetric Li|Li cells. Obviously, FEC is responsible for an increased content of LiF in the surface films formed on Li anodes in these Li|NCM cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07004.

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Galvanostatic cycling results obtained for symmetric Li|Li cells and voltage profiles measured for Li|NCM cells with different current densities (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.S.). *E-mail: [email protected] (E.M.). *E-mail: [email protected] (D.A.). ORCID

Elena Markevich: 0000-0002-6851-0475 Yang-Kook Sun: 0000-0002-0117-0170 Notes

The authors declare no competing financial interest. 19780

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ACKNOWLEDGMENTS A partial support for this work was obtained by the Israel High Committee of High Education and the Prime Ministry office in the framework of INREP (Israel National Research center for Electrochemical Propulsion).

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