Fluoro-Ether as a Bifunctional Interphase Electrolyte Additive with

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Fluoro-Ether as a Bi-Functional Interphase Electrolyte Additive with Graphite/LiNi0.5Co0.2Mn0.3O2 Full Cell Shuai Heng, Yan Wang, Qunting Qu, Ruitian Guo, Xiaojian Shan, Vincent S. Battaglia, Gao Liu, and Honghe Zheng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00972 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Energy Materials

Fluoro-Ether as a Bi-Functional Interphase Electrolyte Additive with Graphite/LiNi0.5Co0.2Mn0.3O2 Full Cell Shuai Henga, Yan Wanga, Qunting Qua,* Ruitian Guoa, Xiaojian Shana, Vincent S Battagliab, Gao Liub, and Honghe Zhenga,* aCollege

of Energy & Collaborative Innovation Center of Suzhou Nano Science and

Technology, Soochow University, Suzhou, Jiangsu 215006, PR China, bEnergy

Storage & Distributed Resources Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd. Berkeley, CA 94720, USA

Corresponding authors: *[email protected] (Qunting Qu) *[email protected](Honghe Zheng)

KEYWORDS: lithium ion batteries; fluoro-ether; electrolyte additive; solid electrolyte interphase; graphite/LiNi0.5Co0.2Mn0.3O2 full cell

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Abstract: In this paper, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is adopted as a bi-functional electrolyte additive, which is identified to effectively stabilize the surface for both graphite anode and LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode within graphite/NCM523 full cell. The overall electrochemical performances of the full cell are significantly enhanced with 3 wt% TTE additive in conventional organic electrolyte. A combination of studies of scanning electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy show that a uniform, compact and stable solid electrolyte interphase (SEI) with improved mechanics on the graphite anode and an effective cathode electrolyte interphase (CEI) on the NCM523 cathode is developed.

The electron with-drawing C-F group contributes to the stable

and compact surface film, which explains the electrochemical enhancement of the NCM523 /graphite full cell.

1. Introduction Compared to other commercialized battery chemistries, lithium-ion batteries (LIBs) have high volumetric and gravimetric energy density, which makes them suitable for expanding use in consumer electronics, electric vehicles (EVs) and energy storage systems (ESSs).1-4 The ever-growing aggressive requirements for high performance LIBs are always stimulating the development of key materials. New cathode materials of high working voltage and reversible capacity such as LiNi0.5Mn1.5O4 spinel, LiMn2O3, and LiNi0.8Co0.1Mn0.1O25-7 have been extensively investigated and optimized. However, the widely adopted electrolyte systems, i.e. 1


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mol L-1 LiPF6 dissolved into ethylene carbonate (EC) based mixing solvents, greatly hinder the wide use of NCM cathode materials due to it poor passivation ability on the electrode surface, especially in the full cells with graphite or Si-graphite composite anodes.8-11 As the bridge connecting the positive and negative electrodes and a media for Li ion transportation, electrolyte plays a crucial role affecting many electrochemical properties of the battery system including reversible capacity, rate capability, cycling performance, and operating temperature range etc.12-17 Structurally, electrolyte is a macroscopic homogeneous system with short-range order and long-range disorder characteristics. In an electrolyte with multiple solvents, Li ion is always preferentially or selectively solvated.18-22 The size and the structure of the solvation layer not only influence the migration of Li ion in the electrolyte, but also affects the interfacial reaction mode and kinetics.23-28 Additives are commonly used ingredients in electrolyte solutions.29-32 Although the dosage is not large, additives are always quite effective contributing to a significant electrochemical improvement of a cell. During cell formation, the surface passivation relating to the formation of solid electrolyte interphase (SEI) on the graphite anode and the cathode electrolyte interphase (CEI) on the cathode surface is one of the most fundamental issues in LIBs.33-37 At a certain potential, irreversible interfacial reactions of the electrolyte components produce insoluble organic and inorganic substances precipitated on the active material surface. The ultra-thin passivation film features with ionic conductive and electronic insulation properties. The film plays a crucial role protecting the active material from further electrolyte decomposition and impurity corrosion.38-40 Therefore, high quality and stability of the passivation film are very important for the overall electrochemical performances of the cell, particularly during long-term cycles.



Ni-rich layer structured cathodes (LiNixCoyMnzO2, NCM with x+y+z=1) have attracted tremendous attention for their high reversible capacity of ca. 200 mAhg−1.41-43 However, its wide application is seriously hindered by the rapid capacity fading due to the well-known Mn/Ni dissolution-migration-deposition and the serious active lithium consumption on the graphite surface. As the matter of fact, both the transition metal cation dissolution at the cathode and the active lithium loss at the anode are closely related to the quality and stability of the passivation surface film developed on the active material surface.

The introduction of functional electrolyte

additives is one of the effective and cost favorable approach to enhance the quality and the stability of the electrode surface film. So far, many kinds of compounds have been reported including inorganic salts such as difluoro-(oxalato) borate (LiDFOB), lithium bis(oxalato)borate (LiBOB) and lithium terakis-(trimethylsiloxy)aluminate (LiTMSA) and even more organic substances such as 1,3,2-Dioxathiolane-2,2-dioxide (DTD), trimethylene sulfate (TMS),ethylene sulfate, methylene methanedisulfonate (MMDS), prop-1-ene-1,3-sultone (PES) and tris(trimethylsilyl) phosphate (TTSP) and fluorine-containing solvent (FCS) etc.44-45 By analogy to FCS, fluorinated ethers have also attracted some attention in recent years. It has been found that fluorinated ether additives are effective suppressing polysulfide dissolution and shuttle effect in lithium-sulfur batteries. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) has been attempted as a co-solvent in Li-S batteries, which is effective to reduce polysulfide dissolution and mitigate the self-discharge by forming a stable SEI on sulfur surface.46 However, rare work has been carried out for the fluorinated ether in LIBs, especially relating to its bi-functional interphase role with both NCM cathode and graphite anode. In this work, the fluorinated ether additive (TTE) as an electrolyte additive, is


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systematically studied in the graphite/NCM full cells. On the one hand, the substitution of hydrogen atom by fluorine atom brings about a high electronegativity. On the other hand, fluorine substitution contributes to a higher oxidation stability and a lower reduction stability. The bi-functional interfacial role on the graphite anode and on the NCM cathode is achieved.

At an optimized amount of 3 wt%, the first

colombic efficiency, rate capability and long-term cycling stability of the graphite anode is greatly enhanced. More importantly, similar electrochemical improvements are also obtained in the graphite/LiNi0.5Co0.2Mn0.3O2 (NCM523) full cell. With the help of SEM, XRD and XPS studies, the underlying mechanisms for the electrochemical improvement are discussed.

2. Experimental 2.1 Raw materials A commercialized natural graphite powder (average particle size of ca. 12 μm) was purchased from Shenzhen Beiterui new energy Materials Group Co., Ltd. China. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) additive was purchased from J&K Scientific Ltd.

The purity is 99.99% and the water content is determined

to be less than 20ppm by Karl-fischer titration. Acetylene black (AB, particle size of 40 nm) was obtained from Denka Singapore Private Co. Ltd.) Polyvinylidene fluoride (PVDF)

binder

was

purchased

from

Kureha

Battery

Materials

Inc.

N-methylpyrrolidone (NMP, anhydrous, from Adamas-beta Reagent Co. Ltd.) solvent was used as the dispersant in the electrode processing. 2.2 Preparation of the electrolytes and electrodes 1 mol L-1 LiPF6 dissolved in ethylene carbonate (EC), ethyl methyl carbonate



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(EMC) at a weight ratio of 1:1 was used as the control electrolyte. TTE additive was added into the control electrolyte at different weight percentages (0 wt%, 1 wt%, 3 wt%, and 5 wt%) in an argon-filled glove-box under room temperature. The graphite electrode was prepared by casting the uniform slurry consisting NG (88.8%), AB (3.2%) and PVDF (8%) in N-methy1-2pyrrolidone (NMP) solvent. The obtained slurry was spread onto 15 mm-thick Cu foil. After drying for 2 h at 80 oC, the electrode laminates were roll-pressed and punched into electrode discs (14 mm diameter).

The NCM cathode was obtained from Pylon Technologies Co. Ltd., in

which NCM active material, PVDF and AB account for 94%, 3% and 3%, respectively.

The cathode laminate is cut into discs of 13mm diameter.

All the

electrode discs were further dried under vacuum at 120 oC for 12 h before cell assembly. 2.3 Electrochemical measurements CR2032 coin-type cells were assembled in an argon-filled glove box (OMNI-LAB, VAC, dew point≤ -80 oC).

In the half cells, graphite was used as the working

electrode with lithium foil as the counter electrode. In the full cells, the NCM laminate was used as the positive electrode while graphite laminate as the negative electrode. Before electrochemical test, all the assembled cells were left to stand for 10 h to ensure sufficient wetting of the electrolyte to the electrodes. Afterwards, three formation cycles at C/20 charge and discharge rate are applied for the half cells between 0.01 and 2.0 V (vs. Li/Li+) on Maccor S4000 battery cycler (Maccor Instruments, USA) at 30 oC condition. Rate test was conducted by discharging the


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graphite anode (delithiation) at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, 20 C, 30 C and 50 C, respectively.

Cycling performance was tested at 0.5C charge-discharge

rate for 200 cycles. For the full cells, after three formation cycles at 0.1 C between 2.8 and 4.3 V, long-term cycling test was performed at 0.5C charge-discharge rate for 200 cycles. AC impedance was determined on a Zahner Elektrik IM6 electrochemical workstation with an alternating amplitude of 5 mV over the frequency from 105 to 100 mHz at 60% depth of discharge (DOD). 2.4 Structural and morphological characterizations Morphologies of the electrodes were observed by transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN, 200 KV) and scanning electron microscope (SEM) (Hitachi S-800, 10 or 15 kV). After electrochemical cycles, the electrodes were retrieved from the dismantled cells and washed with DMC solvent for 3 times to remove the residual electrolyte. The crystalline phase of the electrodes was characterized by using XRD (Rint2000, Rigaku with Cu Ka) at a scan rate of 3o min-1 from 10o to 80o. X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher, America, Pass Energy 30.0 eV) was used to analyze the chemical composition of the electrode surface at different electrochemical stages. The Young,s modulus of the surface film was measured with an atomic force microscope (AFM, Dimension Icon, Bruker, Germany). The tip spring constant and radius were calibrated before each test. The DMT model was used to calculate Young,s modulus of the SEI and the poisson was set to be 0.4.

3. Results and discussion



As the dosage of film-forming additive in organic electrolyte is very low (less than 5 wt%), its impact on the conductivity and viscosity is always neglected. Nevertheless, its structure-reactivity relationship and the decomposition compared to the control electrolyte are important. The energy level of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for EC, EMC and TTE calculated by DFT are provided in Table 1. Clearly, there are no significant difference in the energy level of LUMO and HOMO between TTE, EC and EMC. This means that the reduction and oxidation potential between them may not differ greatly. However, it should be stressed that the strong electron-withdrawing of the -CF2 group makes the C-O bond more active. Moreover, in presence of fluorinated group, the passivation layer developed on the electrode surface may become more stable and ionic conductive.47-48 In these senses, the introduction of TTE could contribute to a modification of the electrode/electrolyte interphase. The first discharge-charge profiles of the graphite anode in 1mol L-1 LiPF6/EC+EMC (1:1) electrolyte with various contents of TTE at 0.05C are presented in Fig.1a. Basically, no significant difference is discerned between the shape of the charge-discharge curves. Table 2 summarizes the charge-discharge capacity and the first coulombic efficiency of the graphite anode. Without addition of TTE additive, a reversible capacity of 362.3 mAh g-1 was obtained with the first coulombic efficiency of 87.7%. In the presence of 3 wt% TTE additive, although the reversible capacity is not obviously enhanced, an increase of the first coulombic efficiency to 91.0% is obtained. It reveals that the introduction of the TTE additive in the electrolyte


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contributes to an improvement of the first coulombic efficiency. Fig.1b shows the cyclic voltammograms (CVs) for the graphite in the electrolyte without and with 3wt% TTE additive. The onset of the reductive decomposition potential for the electrolyte with the 3wt% TTE additive is enhanced from ca. 0.75V to 1.25V.

The

elevation of the reduction potential is obviously related to the sacrificial decomposition reaction of the TTE additive. The cycling behavior of the graphite anode in the electrolyte with different contents of TTE additive is depicted in Fig.2a. In the control electrolyte, a slight capacity decay with increasing cycle number is observed. After 200 cycles, capacity retention of the cell is obtained to be a little more than 90%. In presence of 3 wt% TTE additive, the cell capacity after 200 cycles is not decreased at all. Of course, high content of TTE additive brings about a decrease of the reversible capacity in the initial cycles, which could be attributed to the high impedance of the electrode as it will be discussed later. The rate capability of the graphite anode in the electrolyte with different amounts of TTE additive are presented in Fig.2b. At discharge rates below 5C, the rate performance of the graphite anode in the electrolyte with TTE additive is slightly worse than that in the control electrolyte. However, after a dozen cycles, the delivered capacity of the graphite anode in the electrolyte with TTE additive at the current from 5C to 50C is significantly higher than that in the control electrolyte. At 50C rate, the capacity retention of the graphite in the electrolyte with 3wt% TTE is ca. 90.5%, remarkably higher than that of ca. 61.6% in the control electrolyte. Fig.2c and Fig.2d compares the discharge curves of the graphite anode in the electrolyte without



and with 3 wt% TTE additive.

It can be seen that not only the ohmic resistance is

decreased, but also the Li diffusion resistance is reduced by the addition of 3wt% TTE additive. Of course, the activation process of the graphite in the electrolyte with TTE additive may be related to the arrangement of the SEI film in the initial several cycles. The impedance evolution of the graphite anode in the electrolyte without and with different contents of TTE additive at different electrochemical stages are shown in Fig.3. The high-frequency and medium-frequency semicircles represent the resistance arising from the SEI (RSEI) and charge-transfer resistance (Rct) at the electrodeelectrolyte interface, respectively. The low-frequency tail is assigned to the Li+ migration within the graphite particles.49-50 The fitting results with equivalent circuit are also presented in this figure. It is interesting that the high-frequency and medium-frequency semicircle (RSEI + Rct) of the graphite anode in the electrolyte without and with TTE additive shows to be very different after the cell formation and after the rate test. After the cell formation (Fig.3a), the electrode impedance increases with higher content of TTE additive. This explains the low reversible capacity of the graphite with high content of TTE additives in initial cycles. After the rate test (Fig.3b), however, an obvious impedance decrease for the cell with TTE additive is obtained. This should be attributed to the electrochemical activation of the graphite anode. At this electrochemical stage, graphite anode exhibits the lowest impedance in the electrolyte containing 3 wt% TTE additive. After 200 cycles (Fig.3c), the graphite anode in the control electrolyte exhibits a significant impedance growth. However, the impedance rise is effectively suppressed by addition of TTE additive, especially with


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3 wt% TTE in the electrolyte. Morphologies of the graphite anodes without and with 3wt% TTE before and after 200 cycles are provided in Fig.4. After the cell formation, no considerable morphological difference is observed for the graphite particle in the electrolyte without and with 3wt% TTE additive. The graphite particle shows quite smooth with clean surface. After 200 cycling, a thick surface layer is developed on the graphite surface in the electrolyte without TTE additive. This is ascribed to the recurrent deposition of the electrolyte reduction reactions due to the rearrangement of the SEI film. In the electrolyte with 3 wt% TTE additive, the graphite morphology closely reassemble that after the cell formation, indicating TTE additive minimized the subsequent side reactions at the electrode/ electrolyte interface. As seen in Fig. 4e,f, the thickness of the surface layer is obtained to be more than 100nm and the layer is loosely deposited on the graphite after 200 cycles in the control electrolyte. By contrast, in presence of 3 wt% TTE additive, the surface film is seen compact, dense and continuous. Its thickness is seen to be less than 60 nm. This evidences the high quality and good stability of the SEI film in presence of the TTE additive. To compare the mechanical property of the SEI film developed in the electrolyte without and with 3wt% TTE additive, AFM was adopted to get the micromorphology and Young, modulus of SEI films developed on the graphite after 200 cycles. Fig. 5a,b shows the plane image of the graphite surface cycled in the electrolyte without and with 3wt% TTE additive. More particle-like precipitates can be seen deposited on the graphite surface in the control electrolyte, which is attributed to the decompositon



of the electrolyte components. Fig. 5c, d describe the 3D images of the graphite surface. Obviously, the graphite surface is smoother after cycling in the electrolyte with 3 wt% TTE additive. Fig.5e,f show the SEI modulus distribution of the SEI film developed in the electrolyte without and with 3% TTE additive after 200 cycles. A wide modulus distribution is obtained for the cycled graphite in the control electrolyte with an average modulus of ca. 117.3 Mpa. By contrast, the modulus distribution of the SEI film for the cycled graphite in the electrolyte containing TTE additive is narrow and concentrated. The average modulus is obtained to be of ca. 146.1 MPa. The wide modulus distribution for the SEI developed in the control electrolyte is related to its complicate chemical components. The low average modulus, on the other hand, implies the loose structure of the surface film. Undoubtedly, the remarkable mechanical difference of the SEI film developed in the electrolyte without and with TTE additive is related to its quality and stability on the graphite surface. XPS is utilized to identify the chemical composition of the SEI film on the graphite. Fig.6a-d show the F1s spectra for the SEI layer developed in the electrolyte without and with 3wt% TTE additive before and after 200 cycles. The two pronounced peaks are assigned to LiF (≈684.5 eV) and P-F (≈687 eV),51-52 respectively. By comparison between the signals obtained from the graphite after the cell formation (see Fig.6a and b), intensity of the LiF peak is obviously enhanced by addition of TTE additive, indicating TTE participates in the interfacial decomposition reaction which produces high content of LiF species. As seen in Fig.6c and d, it is interesting to see that the LiF peak on the graphite cycled in the electrolyte with TTE


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additive is not quite strong compared to that developed in the control electrolyte. This is believed to be attributed to the good stability of the SEI film formed in the electrolyte with TTE additive. The ever-lasting side reactions on the graphite anode in the control electrolyte contribute to the continuous increase of the LiF species during the long-term cycles. Fig. 6e-h show the C1s spectra for the graphite surface. There are five pronounced peaks, i.e. 283.5 eV (C-Hx bond of the SP), 284.8 eV (C-C related to the graphite structure), 286.6eV (C-O), 288.4eV (C=O) and 289.8 (Li2CO3).53-54 The latter three typical peaks are associated with the SEI species. Obviously, the relative intensity of these peaks on the graphite anode in the electrolyte with 3% TTE additive is considerably lower than those obtained in the control electrolyte. This agrees well with the higher coulombic efficiency of the graphite anode in the first formation. After 200 cycles, the intensity of these typical peaks is also quite weak, implying the side reactions of the electrolyte on the graphite surface is effectively suppressed by the presence of TTE additive. For the graphite cycled in the control electrolyte, the peak intensity (C=O, and Li2CO3) grows remarkably after 200 cycles. This explains the thick SEI layer observed by SEM. It is known the most important approach to extend the life-span of lithium ion full cell is to stabilize the SEI on the graphite anode. To further confirm the improved SEI stability by addition of the TTE additive, graphite/NCM523 full cells with electrolyte containing different contents of the TTE additive are investigated. Fig. 7a shows the initial charge-discharge profiles of the full cell formed between 2.8 V and 4.3 V at 0.1 C rate.

It should be noted that the first charge plateau of the cell at around 3.25 V



becomes less unapparent in presence of TTE additive in the electrolyte. Besides, the potential separation between the charge and discharge plateau profiles gets narrow, indicating less polarization of the cell. The addition of TTE additive contributes to an increase of the reversible capacity and the first coulombic efficiency of the cell. With 3 wt% TTE, the first colombic efficiency is improved from 76.21% to 85.56% and the reversible capacity is enhanced from 142.2 mAh g-1 to 155.5 mAh g-1. However, further increasing the TTE content to 5 wt% brings about a decrease of the first coulombic efficiency and the reversible capacity. Table S1 lists the charge-discharge capacity and the first coulombic efficiency for the full cells with the electrolyte containing different amounts of TTE additive. Fig. S1 shows the differential capacity plots (dQ/dV versus electrode potential) of the full cell in the first cycle. Obviously, the intensity of the irreversible reactions during the first charge is decreased by the addition of TTE additive. This agrees well with the increase of the first coulombic efficiency of the full cell. Rate capability of the graphite/NCM523 full cells with electrolyte containing different contents of TTE additive is also compared in Fig.S2 At 5C rate (800 mAg−1), the capacity retention of the cell with the electrolyte containing 3 wt% TTE additive is obtained to be 67.6%, significantly higher than the 23.7% for the cell with the control electrolyte. Fig.7b exhibits the long-term cycling performance for the full cells at 0.5C charge-discharge rate. Compared with the reference cell, the cycling stability of the full cells with 3 wt% TTE additive is remarkably enhanced. After 200 cycles, capacity retention of the cell is 92.1%, significantly higher than the 72.8% obtained


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with the control electrolyte. Further increasing the TTE content to 5 wt% leads to a capacity decline, which should be related to the high internal resistance resulted from the thick SEI as discussed above. Fig.S3 displays the discharge profiles for the full cells at different cycle number in the electrolyte without and with 3 wt% TTE additive. For the reference cell, the rapid shift of the discharge profile end to the left is related to the deficiency of the transferable lithium related to the SEI instability on the graphite anode. In presence of TTE additive, the continuous side reactions at the electrode/electrolyte interface are greatly decreased. The cycling performance of the cells at 60°C is also presented in Fig. S4. An electrochemical enhancement is also obtained under this condition. Fig. 8 shows the SEM and cross-sectional images of the NCM523 cathode in the full cell without and with 3% TTE additive before and after 200 cycles. Here, it is hard to observe any morphological difference for the NCM cathode after the cell formation. TEM images of the NCM523 particle after the cell formation in the electrolyte with different contents of TTE additive was shown in Fig.S5 It is seen the CEI film on the NCM523 surface in presence of TTE is complete, thin and smooth. After 200 charge-discharge cycles, a distinct morphological difference is observed. In the electrolyte without TTE, the cross-sectional image shows thick film on the NCM523 particles. Meanwhile, NCM particles are densely stacked and most of the pores within the electrode are jammed. This phenomenon is attributed to the accumulated electrolyte oxidation. By contrast, in presence of 3 wt% TTE additive, the morphology the NCM cathode shows to be quite smooth and clean, very similar to



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that after the cell formation, implying the CEI is stabilized by the TTE additive used in this study.

The graphite morphologies in the full cell without and with 3% TTE

additive before and after 200 cycles are presented in Fig.S6 Similar to that obtained in half cells, a significant morphological difference is observed, especially after 200 cycles. A lot of precipitated alien particles on the graphite in the reference cell are observed while the graphite surface is quite clean with 3 wt% TTE additive. The X-ray diffraction (XRD) patterns for both the NCM 523 cathode and the graphite anode after 200 charge-discharge cycles are depicted in Fig.9.

From Fig.9a,

the intensity of the typical (003) peak for the NCM523 cathode cycled in the reference cell is significantly decreased compared to that cycled in the electrolyte with 3 wt% TTE additive. The reduction of the typical XRD peak implies a structural distortion or degradation of the NCM523 after long-term cycles. From this, the presence of TTE additive is able to stabilize the crystal structure of the NCM523 cathode during long period of time. This is believed to be attributed to the stable and compact CEI film on its surface in presence of TTE additive.

Fig.9b shows the intensity of the typical

(002) for the graphite anode after 200 cycles is also effectively enhanced by TTE additive. The well preserved graphitic structure is benefited from the compact and stable SEI film as discussed above. To further elucidate the interfacial properties for both the NCM523 cathode and the graphite anode in the full cell without and with 3wt% TTE additive, XPS analyses were carried out and the results are shown in Fig.10. For the C1s and F1s spectra of the NCM523 cathode (see Fig.10a-d), the intensity of C=O, LiCO3 detected on the


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NCM523 cathode cycled in the electrolyte with 3% TTE additive is considerably lower than that cycled in the control electrolyte. Meanwhile, the relative intensity of C-O and C-F2 on the NCM523 cycled in the electrolyte with 3% TTE additive appears to be stronger. It reveals that TTE additive participates the CEI formation on the NCM523 surface. Besides, the signal of LiF on the NCM523 cathode is attenuated by addition of 3wt% TTE. This reveals a thin and stable CEI aroused from TTE decomposition. The Ni 2p3/2 and Mn 2p3/2s spectra obtained from the cycled graphite in the electrolyte without and with 3wt% TTE additive after 200 cycles are depicted in Fig, 10e,f. It is interesting that each of the peak intensity is remarkably decreased by addition of 3 wt% TTE additive. This explains the disappearance of the alien particles on the graphite in presence of TTE additive. This result shows that TTE additive is effective to alleviate the transition metal dissolution-migration-deposition within the full cell.

4. Conclusion In summary, the bi-functional interfacial role of TTE as an electrolyte additive with graphite/NCM523 full cell is investigated. Cell performance and the post-test analyses clearly show that TTE additive contributes to a thinner, complete and compact passivation layer on both graphite anode and NCM cathode. The continuous decomposition of the control electrolyte at the electrode/electrolyte interphase is greatly suppressed. As the result, the well-known lithium inventory loss during long-term cycles is significantly reduced. Meanwhile, the dissolution of the transition metal from the NCM cathode is also greatly reduced. In presence of 3 wt% TTE



additive, the first coulombic efficiency, rate capability and cycling stability of the graphite anode are simultaneously improved. Life-span of the graphite/NCM523 full cell is also greatly extended. XPS studies show that the TTE-derived F-species are directly developed on the graphite surface, which favors the compact, uniform and stable SEI developed on the graphite surface. Moreover, the CEI developed on NCM523 cathode also exhibits good stability. The bi-functional interphase role of TTE additive is of great promise for NCM-based LIBs of high performance and long durability.

ASSOCIATED CONTENT Supporting Information Available: [The list of charge-discharge capacities and the first coulombic efficiencies for the full cell with the electrolyte containing different amounts of TTE additive; the corresponding differential plots (dQ/dV vs. electrode potential) and rate capability of the full cell in the electrolyte with different contents of TTE additive; The discharge profiles for the full cells at different cycle number in the electrolyte without and with 3wt% TTE additive. The cycling performance of the graphite/NCM523 full cell in the electrolyte containing different contents of TTE additive at 60°C; TEM images of the NCM523 after the cell formation in the electrolyte. The morphologies of graphite in the full cell without and with 3% TTE additive before and after 200 cycles.]

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Qunting Qu)


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Page 19 of 40 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


*E-mail: [email protected](Honghe Zheng) ORCID Qunting Qu: 0000-0003-2590-2695 Honghe Zheng: 0000-0001-9115-0669 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are greatly indebted to the Natural Science Foundation of China (NSFC, contract no. 21875154 and 21473120) for high performance lithium ion batteries.

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Page 27 of 40


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 28 of 40

Table 1. The HOMO/LUMO energies of EC, EMC and TTE adopted in this work. Molecule

HOMO

LUMO

EC

-0.3101au

-0.0107au

EMC

-0.2991au

0.0025au

TTE

-0.3543au

-0.0036au

Table 2. The first charge-discharge capacity and coulombic efficiencies of the graphite anode in the 1 mol L-1 LiPF6 EC+DEC electrolyte without and with TTE additive.

Samples

Charge capacity

Discharge capacity

Irreversible capacity

First coulombic

(mAh g-1)

(mAh g-1)

(mAh g-1)

efficiency (%)

BE

413.1

362.3

52.8

87.7

1 wt%

403.8

363.7

40.1

90.1

3 wt%

403.2

366.9

36.3

91.0

5 wt%

404

365.5

38.5

90.5


Page 29 of 40

BE TTE-1% TTE-3% TTE-5%

2.0

(a)

Voltage (V)

1.5

1.0

0.5

0.0

0

100

200

300

400

Specific Capacity (mAh/g) 0.3

BE TTE-3%

(b)

0.2

Current (mA)

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


0.1

0.0

-0.1

-0.2 1.0

1.2

1.4

1.6

-0.3 0.0

0.5

1.0

1.5

2.0

Voltage (V)

Fig. 1 (a) The initial charge–discharge profiles of the graphite anode in the electrolyte containing different contents of TTE additive. (b) The cyclic voltammogram (CV) curves of the graphite anode in the electrolyte without and with 3wt% TTE at 0.5 mV/s scan rate.



0.1C

100

300

(a)

200

100


0

0

50

100

150

Capacity Retention (%)

Specific Capacity (mAh/g)

400

0.1C 0.2C 0.5C 1C 2C

5C 10C 20C

30C 50C

90

(b) 80

70


60

200

0

5

10

Cycle Number

15

20

25

30

35

Cycle Number

50C,30C,20C,10C,5C,3C,2C,1C,0.5C,0.2C,0.1C

50C,30C,20C,10C,5C,3C,2C,1C,0.5C,0.2C,0.1C 2.0

2.0

(c)

(d)

1.5

Voltage (V)

Voltage (V)

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

Page 30 of 40

1.0

1.5

1.0

0.5

0.5

0.0

0.0 0

50

100

150

200

250

300

350

400

0

50


100

150

200

250

300

350


Fig. 2 (a) Long-term cycling behavior and (b) Rate capability of the graphite anode in the electrolyte containing different contents of TTE additive, (c) discharge curves of the graphite anode in the control electrolyte, (d) discharge curves of the graphite anode in the electrolyte with 3 wt% TTE additive.


400

Page 31 of 40

120

(a) Rs

80

-ZIm(Ohm)

Cdl

CSEI


100

W RSEI

60

40

20

0 0

50

100

150

Rct

R(Ω cm2)

BE

TTE-1%

TTE-3%

TTE-5%

RS

2.977

3.004

3.166

3.289

RSEI

6.711

8.718

8.488

8.87

RCT

31.1

36.05

44.78

55.79

R(Ω cm2)

BE

TTE-1%

TTE-3%

TTE-5%

RS

3.974

3.559

3.139

3.257

RSEI

3.84

2.699

2.069

2.473

RCT

13.545

12.7

9.095

12.215

200

ZRe(Ohm) 50


-ZIm(Ohm)

40

(b)

30

20

10

0 0

10

20

30

40

50

60

70

ZRe(Ohm) BE TTE-1% TTE-3% TTE-5%

100

80

-ZIm(Ohm)

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)

60

R(Ω cm2)

BE

TTE-1%

TTE-3%

TTE-5%

RS

4.205

3.954

3.905

3.928

20

RSEI

6.809

5.742

5.229

5.578

0

RCT

55.206

38.41

13.69

22.71

40

0

20

40

60

80

100

120

140

ZRe(Ohm)

Fig. 3 Nyquist plots of the graphite anode in the electrolyte containing different contents of TTE additive measured at different electrochemical stages. (a) After the cell formation; (b) after rate test; (c) after 200 cycles.



(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4 SEM images for the graphite anode in the electrolyte without and with 3% TTE additive, (a, b) after formation and (c, d) after 200 cycles, and (e,f) the corresponding TEM images after 200 cycles.


Page 32 of 40

Page 33 of 40

(b)

(a)

(c)

(d)

14

6

BE

(e)

(f)

TTE-3%

12 10

counts (%)

4

counts (%)

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


2

8 6 4 2 0

0 108

110

112

114

116

118

120

122

136

Young,s modulus (MPa)

138

140

142

144

146

148

Young,s modulus (MPa)

Fig. 5 AFM height images of the graphite anode in the electrolyte (a,c) without and (b,d) with 3% TTE additive after 200 cycles,

(e,f) the corresponding SEI modulus

distribution.



Experiment Fitting P-F LiF


(b)

Intensity / a.u.

Intensity / a.u.

(a)

678

680

682

684

686

688

690

692

678

694

680

682

684

686

688

690

692

694

Binding energy / eV

Binding energy / eV



(d)

Intensity / a.u.

Intensity / a.u.

(c)

678

680

682

684

686

688

690

692

678

694

680

682

Experiment Fitting C-C CO3

(e)

C=O C-O

283

284

285

286

287

288

289

290

291

(f)

286

287

288

694

292

C=O C-O

282

283

284

285

286

287

288

289

289

290

291

292


(h)

C-HX

Intensity / a.u.

C=O C-O

285

692

C-HX

(g)

C-HX

284

690

Binding energy / eV


283

688


Binding energy / eV

282

686

Intensity / a.u.

Intensity / a.u.

C-HX

282

684

Binding energy / eV

Binding energy / eV

Intensity / 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

Page 34 of 40

290

291

292

C=O C-O

282

283

284

285

Binding energy / eV

286

287

288

289

290

291

292

Binding energy / eV

Fig. 6 XPS spectra of the graphite anodes without and with 3wt % TTE additive: (a, b) F 1s spectra after the cell formation, (c, d) F 1s spectra after 200 electrochemical cycles, (e, f) C 1s spectra after the cell formation, and (g, h) C 1s spectra after 200


Page 35 of 40

cycles. 4.5

(a) Voltage (V)

4.0

3.5 3.6

3.0

3.4

3.2

2.5

3.0

0

5

10

15

20

25

30


35

2.0 0

50

100

150

200


(b)

100

Capacity Retention(%)

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


90

80


70

60 0

50

100

150

200

Cycle Number

Fig. 7 (a) The first charge-discharge profiles of the graphite/NCM523 full cells and (b) the long-term cycling behavior of these full cells in the presence of different contents of TTE additive.



(a)

(b)

(c)

(d)

(e)

(f)

Fig. 8 SEM images of the retrieved NCM523 electrode after cycled in the electrolyte without and with 3% TTE additive (a,b) after formation and (c, d) after 200 cycles., and (e,f) the corresponding cross-sectional image after 200 cycles.


Page 36 of 40

(018/110)

(113)

(107)

(104)

(105)

(101) (006/102)

TTE-5% TTE-3% TTE-1% NCM

Intensity / a.u.

(a)

10

20

30

40

50

60

70

80

（ 110（

（ 104（

（ 103（

（ 004（

（ 102（

(b) （ 100（（ 101（

TTE-5% TTE-3% TTE-1% NG

（ 002（

2θ/degree

Intensity / 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


(003)

Page 37 of 40

10

20

30

40

50

60

70

80

2θ/degree

Fig. 9 XRD patterns of (a) the NCM523 cathode and (b) the graphite anode after 200 cycles in the full cells with electrolyte containing different contents of TTE additive.



Experiment Fitting C-H C-O C=O LiCO3 C-F2

282

284

286

288

290

292

Experiment Fitting C-H C-O C=O LiCO3

(b) Intensity / a.u.

Intensity / a.u.

(a)

C-F2

282

294

284

286

288

290

292

294

Binding energy / eV

Binding energy / eV

(d)



Intensity / a.u.

Intensity / a.u.

(c)

678

680

682

684

686

688

690

692

678

694

680

(e)

Ni 2p3/2

NI2+

682

684

686

688

690

692

694

Binding energy / eV

Binding energy / eV

(f)

BE TTE-3% Ni4+

BE TTE-3%

Mn 2p3/2

Mn 2p1/2

Intensity / a.u.

Intensity / 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

Page 38 of 40

846

848

850

852

854

856

858

860

862

Binding energy / eV

Fig 10.

864

866

868

870

635

640

645

650

655

660

Binding energy / eV

(a,b) The C1s and (c,d) the F1s spectra of the XPS spectra for the NCM523

cathode and (e,f) graphite anode after 200 cycles in the electrolyte without and with 3 wt% TTE additive.


Page 39 of 40

TOC/Abstract Graphic For Table of Contents Only

Capacity Retention(%)

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


100

90

80


70

60 0

Carbon

Oxygen

50

100

Cycle Number

Hydrogen


150

200

Fluorine


The possible functions of TTE electrolyte additive on the full cell 218x254mm (72 x 72 DPI)


Page 40 of 40

Fluoro-Ether as a Bifunctional Interphase Electrolyte Additive with

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