Active Mechanism of the Interphase Film-Forming Process for an

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Active Mechanism of the Interphase Film-Forming Process for an Electrolyte Based on a Sulfolane Solvent and a Chelato-Borate Complexe Chunlei Li, Peng Wang, Shiyou Li, Dongni Zhao, Qiuping Zhao, Haining Liu, and Xiao-Ling Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05125 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Applied Materials & Interfaces

Active Mechanism of the Interphase Film-Forming Process for an Electrolyte Based on a Sulfolane Solvent and a Chelato-Borate Complexe Chunlei Lia,b, Peng Wanga, Shiyou Lia,b, Dongni Zhaoa, Qiuping Zhaoa,b, Haining Liuc, Xiaoling Cuia,b,* a

College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050,

China b

Gansu Engineering Laboratory of Electrolyte Material for Lithium-ion Battery,

Lanzhou 730050, China c

Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese

Academy of Sciences, Xining 810008, China * Corresponding author: E-mail: [email protected]

ABSTRACT: Electrolytes based on sulfolane (SL) solvents and lithium bis(oxalato)borate (LiBOB) chelato-borate complexes have been reported many times for use in advanced lithium-ion batteries due to their many advantages. This study aims to clarify the active mechanism of the interphase film-forming process to optimize the properties of these batteries by experimental analysis and theoretical calculations. The results indicate that the self-repairing film-forming process during the first cycle is divided into three stages: the initial film formation with an electric field force of ~1.80 V, the further growth of the preformation solid electrolyte interface (SEI) film at ~1.73 V, and the final formation of a complete SEI film at a potential below 0.7 V. Additionally, we can deduce that the decomposition of LiBOB and SL occurs throughout nearly the entire process of the formation of the SEI film.

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The decomposition product of BOB- anions tends to form films with an irregular structure, while the decomposition product of SL is in favor of the formation of a uniform SEI film. KEYWORDS: Lithium bis(oxalato)borate, Sulfolane, Electric double layer, Solid electrolyte interface, Density functional theory

1. Introduction The high power and energy density of lithium-ion batteries (LIBs) have made them widely successful in a number of fields, such as transportation, communication and the aerospace industry 1-4. The electrolyte is one of the most vital components of a LIB and greatly affects the performance of cells. Most importantly, it can be decomposed to form a solid electrolyte interphase (SEI) film on the anode surface that effectively prevents any further contact between electrode and electrolyte 5-7. Generally, an ideal SEI layer should be thick enough to prevent electron tunneling; thus, an effective SEI film on the graphite anode surface might mainly form during the first cycle, consuming approximately 10% to 20% of the initial cell capacity 8. The cell performance is changed by the electrolyte with variations in the SEI film. There is a close relationship among the forming process of the SEI layer, the morphology of the active material, the chemical properties of the electrode surface, and the composition of the electrolyte 9. According to the results of Lu et al., for graphite, one of the most common anode materials, there is a distinct difference between the SEI layer formed at the basal plane and at the edge plane 10,11. Commonly,


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the SEI layer at the edge plane is thought to be several times thicker than that at the basal plane, which indicates a higher reaction current at the edge plane than at the basal plane

12,13

. Thus, researchers developed graphitized mesocarbon microbeads

(MCMB) for use as an anode material. Their high crystallization reduces the defect concentration at the crystal surface 14. In addition, the spherical and lamellar stacking structure allows for Li+ ions to easily intercalate and deintercalate in any direction. More importantly, it avoids the deformation of particles resulting from inhomogeneous intercalation, which greatly degrades the performance of the battery. Furthermore, the SEI layer should be smooth, with a one-piece structure. However, many researchers have found that the SEI layer is irregular. In recent research, the SEI has been considered to have a bilayer type structure 5. For example, island-like and gel-like films have been observed at the bottom and on the surface of the graphite anode, respectively, for LiBOB-based electrolytes

16,17

. To describe this

process, a complex model of the SEI was created by hybrid Monte Carlo (MC) / molecular dynamics (MD) reaction simulations

15

. In addition, in recent decades,

many theories about SEI films have been reported to explain their formation process and structure-function relationship two-electron reduction

21-23

18

. Paths of one-electron reduction

6,19,20

and

have been presented to verify the formation of SEI films

with different components, based on the uneven current distribution. It should be noted that the electron tunneling effect will not be possible when the film grows thicker than 1 nm, and radicals produced in the initial film-forming process are able to transfer charges and form the outer layer


24

. However, such a


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film-forming process has not been fully experimentally established. Another unsolved problem is that there are no precise descriptions of the formation details of the SEI film, which is considered to be the key factor influencing the properties of LIBs. In our previous work 25, a ternary electrolyte consisting of 1 M LiBOB-sulfolane (SL) / γ-butyrolactone (GBL) / diethyl carbonate (DMC) (1:1:1, by volume) was developed for 5-V-class lithium-ion chemistry, due to its wide electrochemical oxidation window of nearly 5.5 V vs Li/Li+. This novel system has been shown to be compatible with both intercalation hosts, such as LiNi0.5Mn1.5O4 cathodes, and carbon anodes. In addition, it combines the properties of sulfur-containing solvents with LiBOB to increase the conductivity of the SEI layer

26, 27

. As a result, it is attracting

widespread attention in the high-voltage electrolyte field

5, 28-30

. The outstanding

properties are attributed to the particular SEI film formed by the synergistic effect of SL and LiBOB. Herein, we present an active mechanism of the SEI layer formation process for an electrolyte based on SL as a solvent and the LiBOB chelato-borate complex. We use X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to study the SEI-forming process and report the relationships between chemical and morphological properties as a function of their formation potential. Furthermore, quantum chemistry calculations have been increasingly applied when estimating molecular properties (bond length, dipole moment, and molecular energy), and in this work, we utilize this powerful tool to support our findings at a


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molecular scale. Vertical electron affinity (VEA), electric double layer (EDL), theoretical dipole, and electron density are calculated to corroborate the results of electrochemical measurements. 2. Experimental 2.1. Theoretical Calculation Quantum chemistry calculations were completed using the Gaussian 09 package through density functional theory (DFT) 31. Full geometry optimization was used with Becke’s three-parameter (B3) exchange functional, along with the Lee-Yang-Parr (LYP) nonlocal correlation function (B3LYP), at the 6-311G(d,p) level. Vibration frequency was calculated with the same basis set to validate whether the optimized structure was at the lowest-energy state. VEA and theoretical dipole were taken into consideration for the aug-cc-pvtz basis set that contains diffuse functions, to describe molecular orbit more clearly. A polarized continuum model (PCM) with a dielectric constant of 28.5 was used to investigate the effect of solvents containing DMC/GBL/SL with a volume ratio of 1:1:1. In addition, the quantum theory of atoms in molecules (QTAIM) was used to quantitatively evaluate the Mulliken bond order and electron density of research molecules using Multiwfn 32. 2.2. Preparation of Electrolytes To prepare the electrolytes for testing, DMC (Chaoyang Yongheng Chemical CO., Ltd), GBL (Aladdin Industrial Corporation) and SL (Aladdin Industrial Corporation) were dried with 0.4 nm molecular sieve and alkali metal for at least two days until the water content was typically less than 20 ppm, as determined by Karl Fischer titration.



Electrolytes were prepared by dissolving 1.0 M LiBOB in two kinds of solvent mixtures: 1) GBL and DMC were mixed according to the volume ratio of 1:1 (named as Sample 1), and 2) SL/GBL/DMC with the volume ratio of 1:1:1 (named as Sample 2). 2.3. Measurements CR2032 coin cells were assembled and used as the experimental devices. The negative electrode sheet was based on copper foil coated with a mixture of 84 wt.% mesosphere carbon microbeads (MCMB), 8 wt.% carbon black and 8 wt.% poly(vinylidene fluoride) (PVDF). The loading for the MCMB electrode is 1.184 g cm-1, and the electrode was roll pressed to 0.02 mm to ensure that the study electrodes were nearly identical. The electrolytes mentioned above acted as the electrolyte material and Celgard (2400) porous polypropylene was used as the separator material. All the battery pack assembling processes were completed inside an argon atmosphere glove box. All the electrochemical property tests on the experimental cells of Li/MCMB were carried out on a Land cell tester CT2001A (Wuhan, China) in a voltage range of 2.00 V to 0.01 V at room temperature. Surface element analysis was performed by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical Ltd.). The scanning electron microscope (SEM, JSM-5600, Japan) was employed to observe the morphologies and sections of SEI layers on the anode material surface. Meanwhile, energy-dispersive X-ray spectroscopy (EDS, JSM-5600, Japan) elemental maps were combined with the results from SEM to study the element distribution of the different structures of SEI.


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3. Results and discussion First, the basic properties of Sample 1 and Sample 2 were studied. Figure S1 shows the cycling performance of cells with the two different electrolytes. From the 3rd to the 50th cycle, the discharge capacity of the cell with Sample 1 declines from 280.75 mAh g-1 to 240.30 mAh g-1, with a capacity loss of approximately 14.28%. On the other hand, the capacity of the Sample 2 cell increases slightly due to the interphase activation process (from 268.03 mAh g-1 to 283.10 mAh g-1). Compared to Sample 1, the cell with Sample 2 exhibits better capacity retention at 0.2 C. In addition, EIS studies of Li/MCMB half cells were conducted to clarify the reason for the capacity change. The Nyquist plot is shown in Figure S2, and the element parameters of R(SEI) and R(ct) for cells using these two different electrolytes are listed in Table S1. The R(SEI) values of the cells after the 50th cycle are 215.2 Ω and 154.2 Ω, for Sample 1 and Sample 2, respectively, indicating that the conductive property of the SEI film is improved by the presence of SL. Meanwhile, the R(ct) value of cells with Sample 2 decreases by almost half (from 69.6 Ω to 37.81 Ω). As a result, the impedance of the SEI layer and charge transfer are both lowered by adding SL. That is, the addition of SL enhances the battery performance.



Figure 1. (a) The initial charge and discharge curves of Li/MCMB cells with Sample 1 (orange) and Sample 2 (black) electrolytes, (b) the differential capacity plots of Li/MCMB cells with Sample 1 (orange) and Sample 2 (black) in the initial cycle. The orange, blue and purple shaded regions represent the first, second and third stages, respectively, which aids in the study of the film forming process at different stages.

The charge and discharge curves of Li/MCMB cells with Sample 1 and Sample 2 in the initial cycle are illustrated in Figure 1a. There is an obvious difference between the lengths of the plateau at ~1.75 V for the two charging processes, where Sample 1 is much longer than Sample 2. Thereby, it can be inferred that the battery with Sample 1 suffers from a more time-consuming process at ~1.75 V. Generally, this plateau near 1.75 V corresponds to the decomposition of LiBOB, which is accompanied by the generation of cross-linked borate radicals and the subsequent formation of a primary passivating surface ﬁlm 33. This suggests that less LiBOB salt has been involved in the


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reduction reaction for Sample 2, compared to Sample 1. It has been recognized that the formation of a primary SEI film including cross-linked borate radicals is resistive for the transportation of Li+ ions, though it is protective for both intercalation hosts, cathode and anode. This is consistent with the EIS testing result that the impedance can be reduced by employing SL as a cosolvent. Additionally, in light of some previous works, during the discharging process, the potential range from ~1.75 V to ~0.07 V is associated with the irreversible reduction of both the electrolyte components and the surface chemical groups of graphite, after which a structurally compact and effective SEI film results. Moreover, the discharging-charging curve of Sample 2 is almost symmetric below 0.07 V, at which point the well-known process of lithiation-delithiation of Li+ ions will take place. This indicates that most of the Li+ ions insertions are reversible for Sample 2, but a portion of the movement of Li+ ions might have been impeded for Sample 1 during the graphite-lithiation cycle. That is, the transportation of Li+ ions can happen more smoothly in the presence of SL, mainly due to the reduction of migration resistance for the SEI layer. Furthermore, the electrode reaction process was explored by differential capacity plots, as shown in Figure 1b. There are two peaks (at approximately 1.73 V and 1.80 V) near 1.75 V shown for Sample 2, as opposed to the curve of Sample 1, showing only one strong peak at 1.80 V. This indicates that two or more different reactions have taken place at reductive potentials close to ~1.75 V for Sample 2. The peak area at 1.80 V is smaller than that at 1.73 V, suggesting that the reaction at 1.80 V is terminated after a short electron-consuming process. Therefore, the primary



passivating surface film is mainly subjected to reductive reactions taking place at 1.73 V. That will be discussed in detail later. The peaks have an obvious difference in intensity at potentials lower than 0.5 V, which corresponds to the strength of decomposition reactions of solvents (especially when the solvent has a high reduction potential, such as SL), the inevitable slight decomposition of LiBOB salt, and the deintercalation & intercalation processes of Li+ ions. The differential capacity plots of the first three cycles for Sample 2 are presented in Figure 3S. All peaks above 0.07 V almost disappear after 3 cycles, which is regarded as the completion point of solvent/salt reduction to continuously repair the SEI layer

34

. This result demonstrates that the intercalation potential of MCMB is

below ~0.07 V. It can be further deduced that the intercalation amount of Li+ ions for Sample 2 is higher than that for Sample 1, as the peak area of Sample 2 below ~0.07 V is larger than that of Sample 1. This means that the SEI layer of cell in Sample 1 is resistive for Li+ ion transfer due to the lack of the SL solvent. To study the active mechanism of the SEI layer formation process for the electrolyte based on the SL solvent and LiBOB chelato-borate complex, we divided the discharge process into three steps, named Stages 1, 2 and 3, in a decreasing manner of potential, as shown in Figure 1b. 3.1 The effect of electric field force on the film formation process (Stage 1)


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Figure 2. The VEA of five components in Sample 2 (calculated at aug-cc-pVTZ level), and the schematic diagram of VEA, shown as a graph in the top right corner.

VEA is a parameter used to measure the electron accepting ability of the molecule, which is the energy difference of a neutral molecule before and after gaining one electron, as shown in the inset graph of Figure 2. Evidently, molecules with low VEA are less chemically active

39

. Compared with frontier orbital energy

(such as LUMO), VEA can give a better reflection of the electron accepting ability, especially for large molecular deformation or bond breakage during solvent reduction 40

. Here, it has been employed to study the reduction sequence of electrolyte

components and determine the corresponding reactions of the peaks in differential capacity plots. The following trend is observed in decreasing VEA: Li+…BOB- (the BOB- anions coexist with Li+ ions) > SL > GBL > DMC > BOB-. That is, the



Li+…BOB- compound has the highest ability to gain electrons. Thus, the first reaction peak at ~1.80 V in Figure 1b is mainly attributed to the coreduction of BOB- anions and Li+ ions. As mentioned above, the reaction at ~1.80 V is weakened after a short electron-consuming process. To explain this phenomenon, the theory of EDL is introduced. It has been well accepted that EDL is a monolayer structure that appears on the surface of an object upon exposure to a fluid. Usually, the thickness of EDL is less than 1 nm, making the electric intensity very strong. Organic molecules become reactive in EDL and electrochemical reactions take place mainly in this high-intensity field. That is, EDL structure has a direct influence on the film-forming reaction course. Since strongly polar molecules will be pulled to the surface of the electrode by the electric force field, the structure of EDL will be affected by the dipole moment 34. Thus, we should first investigate the dipole moment of molecules in solution.

Figure 3 (a) The electrostatic potential maps and dipole moments of DMC, GBL, TMS and BOB-. (b) The sketch map of the motion curve for DMC, GBL, SL and BOB- in an electrostatic field.


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DFT calculations were performed to quantitatively evaluate the theoretical polarity of the solvents in this electrolyte system, and the results are shown in Figure 3a. The polarity values of research solvents followed the trend SL > GBL > DMC, and all of these are much more polar than BOB- anions. As reported by Jiang et al., the relative number density of counter-ions (the ion with the opposite charge of the electrode) in the surface layer near the positive electrodes decreases as the dipole moment increases 35. In this electrolyte, SL and GBL molecules have more advantages over BOB- anions in migrating to the electrode surface. Only small amounts of BOBanions appear near the graphite anode surface because of the so-called shielding efficiency that originates from the interaction between EDL and the electrode to maintain the electrode’s surface electroneutrality

35-38

. In addition, the concentration

of BOB- anions will further decrease due to the force of the electric field that drives the BOB- anion away from the electrode surface, as displayed in Figure 3b. Then, the small amount of remaining BOB- anions will be reduced successively during the discharge process. It should be noted that in addition to its low concentration, the molecular volume of BOB- anions is incredibly large, which hinders the supplementary migration from the bulk electrolyte system. Subsequently, BOBanions near the electrode surface are almost completely consumed in a short time, corresponding to the small peak approximately 1.80 V. 3.2 The further growth of the preformation SEI film at high potential (Stage 2) As shown in Figure 2, the VEA of SL is close to that of Li+…BOB-, which indicates that SL and Li+…BOB- have similar reduction potentials. It can be deduced



that the second reaction peak at ~1.73 V in Figure 1b is partly caused by the reduction of SL. Furthermore, we speculate that BOB- anions will be involved in this reduction stage due to the decrease of potential weakening the shielding efficiency for strongly polar substances. That process will promote diffusion from the bulk electrolyte to the electrode surface for BOB- anions. This is consistent with the observation of the differential capacity plots, which show two different reaction processes in a narrow voltage range, corresponding to the single reduction of Li+…BOB- at ~1.80 V and the multiple reductions of Li+…BOB- and SL at ~1.73 V. As has been reported, the SEI layer formed from the coexistence of S-containing solvents and chelato-borate complexes seems to function synergistically to improve the properties of LIBs 25,41.


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Figure 4. XPS spectrum of (a) B (a) 1s, (b) C 1s, (c) O 1s and (d) S 2p for MCMB electrode surface at 1.80 V (above) and 1.73 V (below) with Sample 2. In addition, (e) the rose diagram of the atomic concentration for the SEI layer at 1.80 V (gray) and 1.73 V (yellow).

To further study the electrode reactions during the first two stages, XPS measurements of the electrode surface were conducted, as depicted in Figure 4. The peak at 283.04 eV in the C 1s spectra, which is assigned to the C of MCMB

42

,

decreases with the reduction of potential due to the surface coverage of MCMB. Furthermore, as expected, B is observed at 1.80 V, but no S element is found during that phase. This indicates that only Li+…BOB- (and no SL) is reduced at ~1.80 V.



However, both B and S (at approximately 166.79 eV and 168.83 eV in the S 2p spectra, attributed to Li2SO3 and lithium alkyl oxy-sulfite (ROSO2Li), respectively 25) elements are observed at 1.73 V, implying the multiple reductions of Li+…BOB- and SL in stage 2. Moreover, an obvious peak shift is observed for the B 1s spectra, in which peaks are captured at 192.36 eV and 191.11 eV for the potentials of 1.80 V and 1.73 V, respectively. Clearly, nucleophilic reactions will take place due to the electron deficient structure of B atoms, and the corresponding electron density difference plot is depicted in Figure S4. Therefore, the reductive decomposition of LiBOB will take place, yielding a BO3-based symmetrical compound, by one or more electron mechanisms, as has been reported by Xu et al. 41 Because enough electrons can be provided from the fresh electrode surface at 1.80 V, we believe that a multiple electron mechanism probably generates oligomeric borate during the initial decomposition process, corresponding to the peak at 192.36 eV. Then, the electrode surface will be partly covered by an initial SEI layer with irregular bulges, which will hinder electron transportation to a certain extent. Therefore, we believe that a single electron-induced, multistep decomposition mechanism for the BOB- anion plays the leading role at 1.73 V, which may generate a series of B-containing semicarbonate-like species, corresponding to the peak of 191.11 eV 41. We admit that there is some doubt about our deduction, and we will find direct evidence in due course. 3.3 The formation of a complete SEI film at low potential (Stage 3)


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Figure 5. SEM micrographs of MCMB particle at (a) initial conditions, (b)1.80 V, (c) 1.73 V, (d) 0.7 V, (e) 0.07 V, (f) 0.01 V and (g) after 20 cycles; and (h) the diagram of structural change process.

A successive film-forming process can be seen in the MCMB microstructure (Figure 5). The growth of the SEI film reveals a “crackle forming-spot priming” self-repairing process. At 1.80 V, an initial film, which is rough and contains many cracks, has been formed along with a few reductions of LiBOB. Then, the rough film will be rapidly filled in by the multiple reductions of LiBOB and SL at ~1.73 V. Subsequently, the surface topography of MCMB will be completely covered, associated with the irreversible reduction of both the electrolyte components and the surface chemical groups of graphite. It should be noted that the interface layer is more



similar to a series of irregularly directed bulges than a film-like structure, as shown in Figures 5d-e. In the next stage, a complete SEI film will be formed (Figure 5f) and repaired through some whole cycle processes (Figure 5g). That is, the following pattern is observed in the structural change processes of the SEI film: rough film with many cracks, smooth film without cracks, uneven film and finally uniform film, as sketched in Figure 5h.

Figure 6. (a-c) The synthetic mechanism of MCMB; (d) the vertical and (e) cross-sections of the used MCMB, in which scrap is painted blue and the split MCMB pellet is painted gray; (f) the sketch of MCMB with SEI layer and (g) the section view of the MCMB with SEI layer.


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As mentioned above, we have observed a series of irregularly directed bulges during the most important film formation stage—Stage 3. This disordered arrangement can be seen to be similar to the bulges of fresh MCMB when comparing Figure 5e with 5a, so we suspect that the morphology is affected by the unique inner shape of graphite anode materials. According to Mochida et al., the formation process could be described as the directed self-assembly of aromatic planar molecules shown in Figures 6a-c, which is called the microdomain building theory 17. Generally, the MCMB could be classified into four types: tellurian, onion, concentric circle and other types. As shown in Figures 6d and 6e, parallel lamellas and planes have been observed in the vertical and cross-sections, respectively. This suggests that the MCMB used in this study are of the tellurian type, as shown in the diagram at the bottom of Figures 6d-e. Thus, we should consider the surface to be the edge of highly oriented pyrolytic graphite (HOPG). In general, the growth rate of the SEI layer linearly depends on the product of current density and the square root of time 44. Electrons tend to collect at the edge of lamella (similar to the edge of HOPG) rather than the gaps between lamellas, resulting in the uneven distribution of current density. As a result, the SEI layer is apt to grow along the edge of lamella and form the irregularly directed bulges as shown in Figure 6g.



Figure 7. (a) SEM micrograph and (b) element area profile of SEI at 0.7 V; SEM micrographs of (c) MCMB surface and (d) section of electrode after 20 cycles, and the element area profile of the section: (e) the green dots stand for S, and (f) red dots stand for B.

Figures 7a and 7b give the SEM micrograph and element distribution of SEI film at 0.7 V after 20 cycles. As seen, the smooth area, as circled by the red dotted line, is mainly assembled with sulfur-containing species; the irregular bulges, as circled by the white dotted line, are mainly composed of boron-containing species. This indicates that the decomposition product of BOB- anions tends to form an irregular structure, and the decomposition product of SL is in favor of the formation of a uniform SEI film. That corresponds with the result in Figure 5. It should be noted that SEI layers generally have poor conductivity, so SEM observation usually takes place after gold spraying. However, the attached gold particles will affect the detector precision for B and S elements, so an unprocessed


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electrode was selected in this study. As a direct consequence, it is difficult to observe the fine SEI structure in three-dimensions. Thus, it is necessary to analyze the SEI structure by the section of SEI element area profile. After 20 cycles, MCMB particles are covered by a smooth and dense film, as shown in Figure 7c. However, when the electrode is sliced vertically (as shown in the top left corner of Figure 7d), a different morphology is found under surface layer. The region circled by a white dotted line is part of the surface area of an MCMB sphere, and the outside denotes the so-called SEI layer. It is obvious that the SEI film is irregular in the vertical direction. The uniform dense layer and uneven fluffy layer are observed at the bottom and on the surface of MCMB anode, respectively. By comparing Figure 7e with Figure 7f, there are obvious differences between the number of green dots (sulfur) and red dots (boron) inside and outside the circle. Inside the circle, the number of green dots is smaller than that of red dots, which suggests that boron-containing substances are the major components of the SEI inlayer. However, green dots occupy the major space outside the circle, which means that the decomposition products of SL solvent will cover the decomposition products of BOB- anions. Meanwhile, the XPS spectra proved of negative electrode surface at 0.01 V shows that the ratio of B:S decreased with the voltage reducing as shown in Figure S5 and Table S2. That is, in accordance with the deduction that the decomposition of the SL solvent takes place after BOB- anions. Additionally, we can deduce that the decomposition of LiBOB and SL accompanies the entire process of



the formation of SEI film because both boron and sulfur elements are observed at different depths. Moreover, the results from the EDS measurement suggest that the sulfur-containing substances suppress the generation of boron-containing substances. While the reduction of LiBOB can lead to the formation of a stable and effective SEI layer and subsequently protect anode materials from exfoliation, this process is accompanied by the increase of Li+ ion transfer resistance. In addition, as has been reported, the appropriate introduction of conductive sulfurous compounds in SEI layers can effectively compensate for the drawbacks of LiBOB by decreasing the SEI layer resistance on the anode. Thus, the as-formed SEI film is good for the performance improvement of the cell. 4. Conclusions In this work, the active mechanism of the SEI layer formation process for electrolytes based on SL solvent and LiBOB chelato-borate complexes has been studied. We have deduced that the decomposition of LiBOB and SL accompanies nearly the entire process of the formation of SEI film. The preformed BO3-based symmetrical compounds contribute to the improvement of compatibility between electrolyte and electrode, and the resulting conductive sulfurous compounds compensate for the drawbacks of LiBOB by decreasing resistance on the electrode. Additionally, the results indicate that the film-forming process during the first cycle could be divided into three stages:


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(1) An initial film formation process with the effect of an electric field force at ~1.80 V. At this stage, a few LiBOB complexes will be reduced to form a rough and crack-containing film, and this reaction will become weaker because of the so-called shielding efficiency. (2) The further growth of the preformation SEI film at ~1.73 V. At this stage, the multiple reductions of LiBOB and SL will take place, and the decomposition products of BOB- anions tend to form an irregular structure, while the decomposition products of SL are in favor of the formation of a uniform SEI film. (3) The formation of complete SEI film at a potential below 0.7 V. At first the SEI layer is apt to grow along the edge of lamella and forms the irregularly directed bulges. Then, a uniform and dense film will be formed, after a self-repairing process.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx/xxxxxx. The cycle performances, electrochemical impedance spectroscopy of MCMB/Li cells with two different electrolytes; The differential capacity plots of Li/MCMB cell with Sample 2 in the first three cycles; The charge density difference plot before and after getting one electron of LiBOB in x-y plane. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interest.



Acknowledgements This work was supported by the Natural Science Foundation of China (No. 51502124 and 21766017) and the Science and Technology Project of Baiyin City (No. 2017-2-11G). It acknowledges the computing resources and time of the Super-Computing Center of Cold and Arid Region Environment and Engineering Research Institute of Chinese Academy of Sciences and Supercomputing Environment of Chinese Academy of Sciences.

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TOC Figure:


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Active Mechanism of the Interphase Film-Forming Process for an

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