(NCM 622) Cathodes and Fluoroethylene Carbonate Base

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07004 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Applied Materials & Interfaces

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,*,a Elena Markevich,*,a Michal Afri, a Yosef Talyosef , a Pascal Hartmann, b Joern Kulisch, b Yang-Kook Sun, c and Doron Aurbach*, a a

b

c

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

KEYWORDS Li batteries, NCM 622 cathodes, Li metal anodes, Li| NCM cells, fluoroethylene carbonate, high areal capacity, surface chemistry

ABSTRACT:

ACS Paragon Plus Environment

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We report on the highly stable lithium metal | LiNi0.6Co0.2Mn0.2O2 (NCM 622) cells with practical electrodes loading of 3.3 mAh 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 (DMC)). Li | NMC 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 SEI on the surface of the Li metal electrodes cycled in FECbased electrolyte solutions. The composition of the solid electrolyte interphase (SEI) on the Li and the NCM electrodes is analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). A drastic capacity fading of Li|NCM cells following by spontaneous capacity recovery during prolonged cycling is observed. This phenomenon depends on the current density and the amount of the electrolyte solution and relates to kinetic limitations due to SEI formation on the Li anodes in the FEC-based electrolyte solution.


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1. INTRODUCTION Lithium metal (Li) anodes with their extremely high theoretical specific capacity of 3860 mAh g−1 and low negative redox potential of −3.040 V vs. a standard hydrogen electrode are considerably superior compared to any other anode materials in term of specific capacity and positive impact on energy density [1]. These unique characteristics make Li anodes to be 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 Li surface once the Li is immersed in a non-aqueous 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 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 mAh cm−2 with a minimal amount of FEC-based 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 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 cathode operating at about 4V with parameters approaching practical level. Different strategies of surface modification of Li metal surface resulted in improved cycling performance of


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Li|cathode full cells with 4V LFP, NCM and LMO cathodes in LiPF6 containing organic carbonate electrolytes [6-9]. Skin grafting of Li surface with chemically and electrochemically active polymers on the Li metal surface enabled stable cycling of Li|NCM 532 cells (areal capacity 1 mAh cm−2, current density 0.3 mA cm−2) and Li|LiFePO4 (LFP) cells (areal capacity 2 mAh 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 β-PVDF thin film coating on the Li anode cycled with higher current rate of 0.9 mA cm−2 (areal capacity of about 1.8 mAh cm−2) in an electrolyte solution containing FEC as an additive. [6]. Micropatterned Li anodes enabled stable cycling of Li|LMO cells with areal capacity of 1.47 mAh cm−2 for 450 cycles but with lower current density (0.67 mA cm−2). [7] Replacing of LiPF6-containg electrolyte solution by LiTFSI-LiBOB dual salt electrolyte [10, 11], as well as the use of this electrolyte salts with the addition of small amount of LiPF6 [12] resulted in the improved performance of full cells with Li anode. Li|4V cathode cells with areal capacity of 1.75 mAh 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 cycling of Li|NCM cells with areal capacity up to 2.2 mAh cm−2 was demonstrated by Zheng et al. [13], and 500 stable cycles were achieved in standard LiPF6 EC-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 mAh 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.


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In the present paper we present the results of cycling lithium metal | NCM 622 cells in FECbased electrolyte solution at very high electrodes’ loading and depth of dischharge. FEC addition to EC-containing electrolyte solutions was shown to improve the performance of Li|cathode cells. Zhang et al. demonstrated stable cycling of LiNi0.5Co0.2Mn0.3O2 (NCM 532)|Li cells with an areal capacity of 1.9 mAh cm−2 at a current density of 2.16 mA cm−2 for about 100 cycles in 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 mAh cm-2 for more than 600 cycles at a current density of 1.5 mA cm−2. The areal capacity of 3.3 mAh 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. While Li anodes, NCM 622 cathodes and FEC solutions cannot not be consider 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 substantiates the previous reports about the positive impact of FEC as an important co-solvent in many kinds of rechargeable Li battery prototypes, due to the positive effect of its presence on the passivation of Li metal anodes. Most of 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.


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2. EXPERIMENTAL SECTION Cells Assembly and Electrochemical Tests. Li metal foil (FMC Chemicals Limited (U.K.) with the thickness of 0.25 mm which was kept in argon filled glovebox was used without any pretreatment. Cathode sheets comprising NCM 622 with the areal capacity of 3.3 mAh 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 NMC cathodes with diameter of 14 or 17 mm were assembled with 2 layers of polyethylene 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 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 2.8-4.3 V. For cyclic voltammetric and impedance measurements three electrode Li|NCM cells with Li reference electrodes were assembled with the use of standard coin-type cells as described in Ref.[17] Cycling votammogrames and impedance spectra in the frequency range of 100 kHz – 10 mHz (EIS) were measured using a potentiostat-galvanostat Model 128N Autolab (Eco Chemie). Characterization of Pristine and Cycled Electrodes. SEM images were obtained with Environmental Scanning Electron Microscope, Ouanta FEG 250 (FEI). Air sensitive samples were transferred from the Ar filled glove box to the chamber of the microscope with the use of homemade vacuum tight transferring cell. [4]


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X-ray diffraction (XRD) patterns were obtained with a D8 Advance system (Bruker Inc.) using Cu Ka radiation operated at 40 mA and 40 kV. To prevent the contact of NCM electrodes with the ambient atmosphere they were protected with 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 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 C1s peak at 285 eV. Air sensitive electrodes after cycling were transferred from the glove box to the X-ray photoelectron spectrometer without contact with ambient air using homemade devices, equipped with gate-vales 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 the Ar filled glove box, Li electrodes and NCM cathodes in the lithiated state were washed four times with pure dry DMC. Analysis of Electrolyte Solution and Gases from Cycled Cells.

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F-NMR spectra of the

electrolyte solutions were collected using a Bruker Avance-400 spectrometer (376.5 MHz for 19

F). Cycled cells were disassembled in a glove box and their components rinsed with pure dry


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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 inductive coupled plasma technique (ICP-OES, spectrometer Spectro Arcos, Ametek). For the FTIR analysis of the composition of gaseous products from cycled cells they were punctured under vacuum in an homemade metallic FTIR cell equipped with two KBr windows. 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/cm2 and charge/discharge capacity limited to 3.3 mAh/cm2 of Li anode are shown in Figure 1a and Figure S1.


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Figure 1. Galvanostatic cycling results obtained for symmetric Li|Li cells cycled with current density of 1 mA cm-2 and charge/discharge capacity limited to 3.3 mAh cm-2 (a) and for Li|NCM cells cycled with different current densities, as indicated. Red, blue, black and green curves – FEC-based and purple – EC-based electrolyte solutions. (b). (c) Voltage profile measured for Li|NCM cells cycled with a current density 1.5 mA cm-2, (d) Nyquist plots measured at 3.5V 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. The cells demonstrate 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/cm2 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


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cycled in same range of current densities as for the symmetric cells. It is clearly seen that the cells cycled with FEC-based electrolyte solution significantly outperform those cycled with ECbased electrolyte solution as we demonstrated earlier in Ref. [4]. Figure 1c and Figure 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 (what reduces the overall resistance and the total overpotential required to cycle the cells at constant current).[4] It is seen that Li|NCM cells with a commercial level of the areal capacity of the NMC cathodes demonstrate an excellent cycling performance in the FEC-based electrolyte solutions without any special pretreatment of the Li anodes. At a current density of 1.5 mA/cm2 and an amount of the electrolyte solution of 33 µl cm-2 the cells exhibit areal capacity of about 2 mAh/cm2 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/cm2 resulted in drastic capacity fading of the cells after about 100 cycles with the subsequent recovery of the cells 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 is obviously relates to the effect of cathode on the SEI which protects the Li anode in the full cells due to the


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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/cm2 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 vs. number of cycles was observed at current densities of 0.5 mA/cm2 (black curve) and 1 mA/cm2 (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/cm2 (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 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/cm2. 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 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


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0.25 mA cm-2, 22 µl cm-2

Capacity, mAh cm-2

0.5 mA cm-2, 33 µl cm-2

3 1 mA cm-2 0.5 mA cm-2, 22 µl cm-2

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Fresh Li anode and new portion of the electrolyte solution

3 1 mA cm-2, 22 µl cm-2

b

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Cycle number Figure 2. Cycling performance of Li | NCM cells with 22 – 33 µL electrolyte solution/ cm-2 of Li anode and NCM cathode at different current densities, as indicated. 30°C.


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resistance of the surface films on 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 fully discharged (lithiated) state at different stage 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 metals loss was 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 transition metals ions from NCM cathodes occurs during cycling with upper cut off voltages lower than 4.3 V.

Besides, according to SEM images of pristine and cycled

Figure 3. XRD patterns of pristine NCM 622 electrodes and of three NCM 622 electrodes in 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).


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cathodes (Figure 4 a-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 symmetric cell the surface of Li electrodes is much smoother than that of Li anode after 75 cycles in the full cell. Obviously, the speci The impedance spectra of two electrode Li|NCM cells in fully discharged state at 3.5V 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 following 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 impedance spectra of the NCM cathode and the Li anode were measured separately vs. Li reference electrode (blue and red dots in Figure 5c, respectively). Good agreement between the impedance response of these cells measured with the two electrode setup, NCM cathode vs. Li anode (black dots), and the calculated curve, obtained by the addition of the spectra of NCM cathode and Li anode (solid line) is indicative for correctness of the measurements. [27] Besides, cycling voltammogram measured with this 3 electrode cell after 20 cycles (Figure 5d) demonstrates typical behavior for NCM622 cathodes [28, 29]. The areal discharge capacity calculated from CV comprised about 3 mAh cm2. It is seen, that the impedance behavior of Li| NCM cells is determined mostly by the impedance of the Li anodes related to changes in the thickness, composition and morphology of the SEI formed on Li surfaces. Thus, it is clear that the reasons for the capacity fading from point 1 to point 2 is a fast formation of thick SEI on Li anode. The growth of the surface films on Li


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a

b

c

d

e

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Figure 4. SEM images of pristine NCM cathode (a, c), NCM cathode cycled in Li|NCM cell for 160 cycles (lowest point) (b, d) and Li electrodes cycled in Li|Li symmetric cell for 1050 cycles (e) and Li electrodes cycled in Li|NMC 622 cell for 75 cycles (f). anodes is responsible for the increase of impedance in point 2 compared to points 1 and 3. This


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assumption is in line with the conclusion made in Ref. [14] for Li|NCM cells cycled with ECbased 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.

a

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2 1.5 1 0.5 0 -0.5 -1 -1.5

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20 µV s

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Z', Ω

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3.5 E, V

th

20 cycle

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

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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 (dublet 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 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 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/discharge capacity as for full cells with NMC 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 contains sufficient amount of 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 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

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C NMR


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spectroscopy of the rinsed solutions, what means that the products of FEC transformation are insoluble in the electrolyte solution and present solid and/or gaseous species.

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F NMR a

Li|NCM cell after 40 cycles

FFEC/FPF6¯ = 1: 3.77

Li|Li cell after 1050 cycles

b

FFEC/FPF6¯ = 1: 3.12 -74.5 ppm, PF6¯

Standard electrolyte solution 1M LiPF6 in FEC/DMC (1:4)

FFEC/FPF6¯ = 1: 2.72

c -123.5 ppm, FEC

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


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1.8 DMC

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Gaseous phase of cycled Li|Li cell

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Absorbance, a.u.

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Absorbance, a.u.

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Wavenumber, cm-1 Figure 7. FTIR spectra of DMC (blue curve) and gaseous phase of typical Li|Li cell after 150 cycles (red curve).


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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 detected component. Typical FTIR spectrum collected from the gaseous phase of the cycled for 150 cycles Li|Li cell (Figure 7) displays also the additional peaks at 2356 cm-1 and 2336 cm-1 related to stretching vibration of CO2 and peak at 667 cm-1 corresponds to bending of CO2.[29, 30] 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 the result of transformation of the components of the electrolyte solution we performed XPS and FTIR analysis of the surface films formed on the cycled electrodes. Figure 8 compares 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 the cells is presented in Table1 (in light of the spectroscopic results). It is seen that 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 F1s 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 cathode. These results are indicative for 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.


20

Page 21 of 33

Li|Li cell LiF

F 1s

Intensity, cps

LixPOyFz

C-O-C

O 1s

LixPFyOz/

O2-

O-C=O

C=O,

C-C C-H

C 1s

POxFy

P 2p LixPFy

C-O CO-C(O)=O

LixPFy

O2-C=O Li2O

Li|NCM cell LiF

F 1s

C-O-C

O 1s

LixPFyOz/

Intensity, cps

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


C 1s

O2-C=O

C-C C-H

P 2p

LixPFy POxFy

O-C=O C-O CO-C(O)=O LixPOyFz O2-C=O

LixPFy Li2O

Binding energy, eV

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

Figure 9 presents FTIR spectra of Li electrodes 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


21


Page 22 of 33

same set of products presents 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 predominates over other carbonate species, what supports the conclusion about higher content of inorganic species in this case. Table 1

Composition of surface films on Li anodes cycled in 1M LiPF6/FEC/DMC electrolyte solution by XPS.

Element

Li | Li cell

Li| NCM cell

(atomic % )

(atomic %)

F

12.3

30.0

O

29.2

19.3

C

45.2

28.8

P

1.5

1.0

Li

11.8

20.9


22

Page 23 of 33

ROCO2Li

Li2C2O4 Li2CO3

R-O-CO-O-R

P(OR)3

0.08

Li2O, LiOR, LiF

Absorbance, a.u.

P-F 0.06

a

Li|Li cell, 1000 cycles

0.04 0.02

LiOH

0.00 0.04

Absorbance, a.u.

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


b Li|NCM cell, 40 cycles

0.02

0.00 3900

3400

2900

2400

1900

1400

900

400

Wavenumber, cm-1 (CH2OCO2Li)2

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


23


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 PEO-like polymer species. [39] These surface films contain also

Intensity, cps

carbonate species and products of LiPF6 decomposition.

Intensity, cps

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 24 of 33

Binding energy, eV

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


24

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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 of SiO2 filled PE separator

(Entek) placed between two layers of standard PE separator. 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 SiO2 filled separator, obviously, due to its scavenging effect. 4. Conclusions. In this work we unambiguously and strongly substantiated conclusions about the unique properties of solutions containing FEC as a co-solvent, 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 mAh cm2

), 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 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 is strongly depending on the current density and the amount of the electrolyte solution in them. We attribute this observation to the involvement of the products of


25


Page 26 of 33

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 due to the formation of too resistive surface films on the Li anodes causing changes in the impedance of the cells which, 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.

ASSOCIATED CONTENT Supporting Information. Galvanostatic cycling results obtained for symmetric Li|Li cells and voltage profiles measured for Li|NMC cells with different current densities. AUTHOR INFORMATION


26

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Corresponding Author *E-mail Gregory Salitra: [email protected], Elena Markevich: [email protected], Doron Aurbach: [email protected]

ACKNOWLEDGMENT 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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Table of Contents

Capacity, mAh cm-2

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


4

Li metal|NMC 622 0.5 mA cm-2

3 2

1 mA cm-2

1

1.5 cm-2

2 mA cm-2

mA

0 0

200

400

600

Cycle number

800


33

(NCM 622) Cathodes and Fluoroethylene Carbonate Base

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