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Regulating the composition distribution of layered SEI film on Li-ion battery anode by LiDFBOP
Journal Pre-proof Regulating the Composition Distribution of Layered SEI film on Li-ion Battery Anode by LiDFBOP
Dongni Zhao, Jie Wang, Peng Wang, Haining Liu, Shiyou Li PII:
S0013-4686(20)30137-7
DOI:
https://doi.org/10.1016/j.electacta.2020.135745
Reference:
EA 135745
To appear in:
Electrochimica Acta
Received Date:
19 November 2019
Accepted Date:
19 January 2020
Please cite this article as: Dongni Zhao, Jie Wang, Peng Wang, Haining Liu, Shiyou Li, Regulating the Composition Distribution of Layered SEI film on Li-ion Battery Anode by LiDFBOP, Electrochimica Acta (2020), https://doi.org/10.1016/j.electacta.2020.135745
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Journal Pre-proof Regulating the Composition Distribution of Layered SEI film on Li-ion Battery Anode by LiDFBOP
Dongni Zhaoa, Jie Wanga, Peng Wanga, Haining Liuc, Shiyou Li*a,b aCollege
bGansu cCAS
of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, P.R. China
Engineering Laboratory of Electrolyte Material for Lithium-ion Battery, Lanzhou 730050, China
Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai
Institute of Salt Lakes Chinese Academy of Sciences, Chinese Academy of Sciences, Xining, 810008, P.R. China
Abstract The so-called solid-electrolyte interface (SEI) derived from the interaction between electrolyte and electrode plays a decisive role in Li-ion battery performance. Lithium difluoro(bisoxalato) phosphate (LiDFBOP) as the derivative of LiPF6 has caused great attention among the researchers. However, the mechanism of effect for LiDFBOP salt has not yet revealed, which limits the findings for the fundamental cause of performance improvement. In this work, the effect of LiDFBOP on the mesocarbon microbead (MCMB) anode electrode has been investigated with aid of surface analysis technique, such as scanning electron microscope (SEM), X-ray photoelectron spectrum (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). On the basis of the modification with LiDFBOP, the long-term cycling and degradation resistance of Li/MCMB half cells have been achieved, which results from the preferential decomposition of LiDFBOP regulating the decomposition law of the electrolyte system. The SEI film with layer structure has different composition distribution under the obstruction of LiDFBOP of the decomposition of LiPF6. The more organic ingredients are distributed in the SEI film on the electrolyte side, which leads to low impedance and fast transportation of lithium ions. The special ingredient distribution is ascribed to the obstruction of LiDFBOP on the decomposition of LiPF6. This work signifies the research of the effect of SEI film
Journal Pre-proof distribution on the battery performance. Key words Lithium difluoro(bisoxalato) phosphate; SEI film; MCMB; Interface modification Introduction Lithium-ion battery (LIB) development has experienced many changes so far. In addition to the high specific energy and high energy density requirements of the battery, it also places high demands on its safety [1-4]. Lithium metal as a negative electrode has a lower electrode potential and a theoretical specific capacity of up to 3860 mAh g-1, which can be used as a key material for high energy density batteries. However, lithium metal still presents significant challenges during electrochemical deposition/circulation. These include: 1) the growth of lithium dendrites; the growth of lithium dendrites not only causes the degradation of battery performance but when the dendrites pierce the separator and contact with the positive electrode, it will cause an internal short circuit of the battery, causing safety problems. 2) Formation of the "dead lithium" layer; during the cycle, dendritic or mossy lithium will fall off the lithium sheet, forming a "dead lithium" layer, which not only reduces the coulombic efficiency, but also greatly increases the internal resistance of battery affecting cycle performance. 3) Due to the "no host" nature of metallic lithium, there is an unrestricted volume effect during the cycle [5, 6]. Therefore, lithium metal anodes are greatly limited in application. In comparison, graphite-based anode materials have attracted much attention due to their excellent layered structure. Among them, mesocarbon microbead (MCMB) with a brilliant mass specific capacity (about 300 mAh g-1) and a low irreversible mass ratio capacity (about 20 mAh g-1) have been studied [7], while low-cost graphite has a high mass specific capacity (350 mAh g-1), but its irreversible specific capacity (about 50 mAh g-1) is higher than MCMB carbon [8, 9]. Graphite shows a higher capacity decay rate, which is not suitable for power batteries that require long cycle and high volume
Journal Pre-proof specific energy. Moreover, artificial graphite and natural graphite have more chemical side reactions than mesophase products, thermal stability and chemical stability are not as good as mesophase products. MCMB makes a contribution once again on the development of lithium ion batteries [10]. The thermodynamically favourable chemical reaction at the electrode and electrolyte interface is the key to determining performance and even research [11-13]. There are many factors in the polarization of the negative electrode of the battery, in which the composition of the solid electrolyte interface (SEI) on the negative electrode has a profound effect on the polarization or impedance of the battery. Regulating the properties of SEI film (including composition, thickness and roughness) is important for the good performance of the battery [11, 14]. Therefore, it is important to find the suitable electrolyte formula and study the interface reaction mechanism and the modification of the interface to improve the performance. The use of electrolyte additives as a cost-effective means for optimizing electrolyte formulations has yielded fruitful results [15]. The additive includes film-forming additives [16], such as vinylene carbonate (VC), 3-propane sultone (PS) and so on, have been proven to improve the performance of MCMB anode electrode. However, these additives have inferior compatibility with some of the high-voltage cathode material. Based on this problem, we introduced lithium difluoro(bisoxalato) phosphate (LiDFBOP) [17-19] as a functional additive which has been proven to have excellent properties on the side of cathode electrode, to investigate the effect mechanism on the MCMB anode electrode. In this work, LiDFBOP was added in the electrolyte system to investigate the compatibility with Li/MCMB half cell. We use the high precision instrument secondary time-of-flight ion mass spectrometry (TOF-SIMS) to detect the difference of SEI film composition on the electrode surface with different electrolyte systems. By analyzing the SEI film on the surface of the negative electrode,
Journal Pre-proof the regulation of the structure and composition of the SEI film derived by LiDFBOP can be obtained and can provide some ideas for the future research of electrolyte formula. Experimental Section Material preparation Electrolytes were divided into two parts of reference and experiment samples. LiPF6- dimethyl carbonate (DMC)/ ethylene carbonate (EC) (1:1, by volume) (marked as Baseline) was purchased from Chaoyang Yongheng Chemical Co., Ltd. And LiDFBOP-EMC solvent synthesized by our laboratory was added after screening at the ratio of 1% in the Baseline system (marked as 1% LiDFBOP). Anode sheet was made up of 80 wt. % MCMB, 10 wt. % polyvinylidenefluoride (PVDF) and 10 wt. % carbon black, which was coated on the copper foil. CR2025 coin cells were assembled in the glove box with argon atmosphere using above mentioned electrolytes. Moreover, lithium foil was served as a counter electrode and the reference electrode, and Celgard porous polypropylene separator (2400) was used to separate the positive and negative electrodes to avoid short circuit. Electrochemical performance test Electrochemical properties of half-cells were analyzed by Land cell tester CT2001A (Wuhan, China) in the voltage of 0.01-2.0 V at different rate. And electrochemical impedance spectra for Li/MCMB half-cells with different electrolytes were measured on the electrochemical workstation (CHI660E, Shanghai, China). All cells were tested at initial lithiated state at 0.01 V at room temperature. A sinusoidal AC perturbation of 5.0 mV was applied to the electrode over the frequency range of 100 kHz to 10 mHz. Surface characterization test Scanning Electron Microscope (SEM, JSM-5600, Japan) and Energy Dispersive Spectrometer
Journal Pre-proof (EDS, JSM-5600, Japan) were adopted for characterization of surface morphology and element distribution of SEI films. And X-ray Photoelectron Spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical Ltd.) as well as TOF-SIMS, (IonTOF5, Germany) were used as surface material composition and in-depth analysis. Theoretical calculations Quantum chemical calculations were performed by Gaussian 09 package [22-23]. The computing environment used in this study was provided by the Chinese National Grid (CNGrid) of the Chinese Academy of Sciences and the Chinese Academy of Sciences Supercomputing Environment (ScGrid) based on SCE software (http://www.cngrid.org, http://www. Scgrid.cn) [24]. DFT calculation was the method used for geometry optimization with B3LYP along with the basis set of 6-31++G (d, p). Frequency analyses with the same basis set were done to check imaginary frequency for optimized structure. In order to investigate the effect of solvents, the structures were optimized by using the polarized continuum mode (PCM), with a dielectric constant of 46.44 (EC: DMC, by volume of 1:1). Energy was taken into consideration in 6-311++G (2d, 2p) basis set, which contained polarization functions and could better describe atomic orbital. Under the above method and the basis group, compounds were optimized to obtain their stable structures. Also, and vertical electron affinity (VEA) were obtained by energy calculation. Also, we perform the electrostatic potential (ESP) analysis. Results and Discussions Initial discharge process for Li/MCMB half cell with additive-optimized electrolyte The first discharge process of the anode electrode is a process of active kinetic reaction between the main electrolyte and the interface. Therefore, it is of great significance to conduct an in-depth study of the first discharge curve. Fig. 1 shows the first discharge curve for Li/MCMB half-cell with two
Journal Pre-proof different electrolyte systems. It can be found that the electrolyte system containing LiDFBOP releases more capacity, which is about 390.1 mAh g-1, while the Baseline electrolyte system is only 277.2 mAh g-1. This indicates that the 1% LiDFBOP system has more active lithium ion intercalation during the first lithium intercalation process, and it can be inferred that a strong activation reaction occurs at the interface, and part of the lithium ions participate in the formation of the SEI film on the negative surface. And from Fig. 1a, it can be found that the 1% LiDFBOP system clearly shows a platform between 1.4 and 1.8 V, which releases about 11.2 mAh g-1 capacity, accounting for 2.8% of the total discharge capacity. It is derived from the ring opening reaction of the oxalic acid ring in the LiDFBOP additive. To further analyze the reasons for the platform generated here, we have a corresponding differential capacity curve for the discharge curve (Fig. 1b). We can find that the Baseline system does not show any peaks above 1.0 V, but the 1% LiDFBOP has two small peaks at 1.67 V and 1.40 V, respectively. It is inferred that there may be a decomposition ring opening reaction as presented in Scheme 1, the oxalic acid ring is subjected to an electron-opening reaction on the surface of the electron-rich electrode, corresponding to a peak position of 1.67 V, and the further complexation reaction corresponds to 1.40 V. Electrochemical performance of Li/MCMB half cells modified by LiDFBOP additive Further, in order to evaluate the improvement of the electrochemical performance of the additive system, we tested the cycle performance of the Li/MCMB half-cell system. Fig. 2 shows the cycling performance tests of the two systems and analyzes the changes in impedance during the process. From the cycle performance of the two systems in Fig. 2a, it can be found that the cycle performance of the Li/MCMB half-cell after the addition of the LiDFBOP additive is significantly improved, the capacity retention rate is increased from 71.2% to 88.1%, and the capacity is also obvious. The improvement
Journal Pre-proof of Coulomb's efficiency is also relatively stable, which will become the key to the long life of the battery. It can be analyzed from the Fig. 1 that LiDFBOP preferentially decomposes to form films on the electrode surface in the electrolyte system, and the oxalic acid ring opens to form a decarboxylated dimer from LiDFBOP attached to the electrode surface, which hinders the contact between electrolyte and the electrons on the electrode surface, thus decomposition has a certain degree of inhibition. Therefore, lithium ions will not effectively participate in forming repeated film, and the loss of irreversible capacity is reduced. Impedance can be used as a good indicator to evaluate the properties of the interface between the electrolyte and the electrode. Fig. 2b tests the change in impedance after the first discharge in both systems, and Fig. 2c shows the corresponding resistances evaluated by fitting impedance measurements with the shown equivalent circuit. It can be analyzed by the analog circuit that the impedance values of the two systems are composed of the following five parts. The intercept of the spectra and the abscissa in the high frequency region is Rs, which is defined as the ohmic impedance Rs., which is mainly derived from the bulk impedance of the battery case itself and the electrolyte. The semicircle in the high frequency region of Rf and CPE1 can be ascribed to diffusion of lithium ions through the SEI film on the surface of active material particles. And the mid-high frequency region corresponding to Re in parallel with CPE2 is a semicircle related to the transport process of electrons inside the active material particles. The medium frequency region is actually composed of two semicircles, mainly the interface film resistance the charge transformation resistance Rct in parallel CPE3. Finally, the diagonal line of the low frequency region mainly represents the Warburg impedance W of the diffusion. It can be observed from our tests that the W impedance is not particularly significant in the Li/MCMB half cells system, so it will not be discussed too much here. From the
Journal Pre-proof simulation values in Fig. 2c, we can find that the Rf of the system containing LiDFBOP additive decreases, which indicates that LiDFBOP can regulate the properties of the interface film, so that the resistance value is reduced, which is beneficial to the transmission of lithium ions and improve the transmission power. It also can be observed that there is obvious difference in the Re in the two electrolyte systems. The system containing the additive reduces the damage of the electrode surface due to the decomposition of the solvent by optimizing the surface of the negative electrode electrode, so that the resistance of electron transport in the material is lowered. This also explains from another perspective that an optimized electrolyte system helps protect the interface from erosion and damage. Also, we found that the Rct value has increased to some extent, which indicates that the LiDFBOPderived interface has no beneficial effect on surface charge transport, which may affect the rate performance of Li/MCMB. However, it is further proved that the formed SEI film effectively hinders the charge transfer, and the surface charge transfer is slow, which can reduce the large contact between the electrolyte and the electron, thereby further decomposing.
Analysis of surface properties of MCMB negative electrode In order to explain the mechanism of action of the additive LiDFBOP, it is necessary to analyze the improvement of the surface properties of the negative electrode MCMB by LiDFBOP. The interface is the “conversation window” for the transport of lithium ions from the liquid electrolyte to the solid phase electrode material. The study of interface properties can have a preliminary judgment on the dynamic properties of different systems. First, we analyzed the morphology changes of the interface film under the two systems after the first discharge, and the SEM images obtained at different resolutions are shown in Fig. 3. By comparison, it can be clearly found that there are significant
Journal Pre-proof differences in the MCMB anode surface with and without interface films formed. The interface film of the Baseline electrolyte system is not very uniform, and there are obvious cracks, which will be detrimental to the SEI film protecting the MCMB electrode surface from further corrosion of the electrolyte. In comparison, the film formed by the 1% LiDFBOP system uniformly grows on the surface of the electrode, and the morphology of the film is uniformly distributed on the surface of the electrode like “scales”, so that the uniform SEI film morphology can hinder ion transport better and inhibit the thickening of SEI. The rapid transportation protects the surface of the electrode without causing further decomposition of the electrolyte, thereby reducing the continuous decomposition and re-generation of the SEI film and reducing the loss of active lithium ions. The distribution of surface elements, and the surface element content and distribution analysis of the two films after the first electrode discharge were further analyzed by EDS. It is found that in the Baseline system, the content of C was significantly higher, and also the P content is significantly higher by two orders of magnitude, which indicates that more LiPF6 was decomposed, resulting in the parasitic reaction of solvents to thicken the SEI and impede the smooth transportation of lithium ions. It can be inferred that P content is very low in the presence of LiDFBOP which may be due to the effective hindering of the decomposition of electrolyte species by the outermost organic layer that contains the reduction product of the additive and hence, the growth of the innermost inorganic layer is suppressed. It may speculatively be assumed that the inner inorganic layer in the case of LiDFBOP is thinner. Observing the distribution of elements, we found that the F element distribution on the surface of the additive system is relatively more uniform, so that the uniform SEI film formed will effectively protect the electrode, and the SEI film will not be further decomposed and regenerated due to the tip effect of the electron caused by local unevenness. Thus the formed thin SEI film may explain
Journal Pre-proof the relatively lower film resistance Rf in the presence of the additive (see Fig. 2 (c)), which is advantageous for the improvement of electrochemical performance. For the composition of surface chemicals, we further tested by XPS. The composition of the SEI film was analyzed by fitting the peak shape of the two systems on the surface after the end of the initial discharge. It can be found by analysis that the SEI film derived by 1% LiDFBOP is thin, which can be gained by C 1s [20-21]. The peak position of 284.74 eV in the C 1s spectrum represents the characteristic valence of the surface of MCMB, and 287.23 eV corresponds to the peak position of PVDF in the F 1s spectrum. It can be seen that these characteristic peaks are higher in the additive system than those in the Baseline system. This result is consistent with the results analyzed in the impedance in Fig. 2b-c. In addition, by analyzing the composition of the surface film with the two systems, the composition of the surface film system is analyzed as follows. In the C 1s spectrum, 289.24 eV corresponds to Li2CO3 and 532.74 eV in the O 1s spectrum. LiF is also one of the inorganic components of the interface film, which corresponds to 687.99 eV in the F 1s spectrum, and the content of LiF in the additive system is higher than that in the Baseline system, indicating that the inorganic system accounts for a higher proportion of the film formed by the additive. In addition, analysis of the organic components in the SEI film, we can find that in the Baseline electrolyte system, the composition of organic matter is relatively higher, which is also the reason why the film structure is loose and thick. Moreover, the organic SEI film is deposited on the outer layer of the inorganic film, and its structure is not that stable, and easy to repeatedly decompose and form a film. Further analysis of the P 2p spectrum, 134.11 eV corresponds to the possible decomposition product LixPyOzF of the main salt LiPF6, and 136.74 eV corresponds to PF5 or PF6- which is not decomposed on the surface. In summary, the interfacial film derived from LiDFBOP additive is thinner, in which the proportion of
Journal Pre-proof organic substances is lower than that of inorganic substances, so the inorganic content of LiDFBOP system is lower than that of Baseline system. From this analysis, it is apparent that the intensity of the two characteristic peaks of the system containing the additive is not very high, which confirms the above-mentioned speculative mechanism, because the preferential decomposition of the additive LiDFBOP hinders the large-scale decomposition of the main salt LiPF6. Further, we will take a more sophisticated instrument TOF-SIMS for deeper research and explanation. First, we analyzed the distribution of surface elements and found that the difference in surface materials between the two systems was relatively large (Fig. 6). First, we analyzed lithium-containing compounds. We found that 6 Li- is significantly more abundant in the 1% LiDFBOP system, which means more surface-active lithium. In addition, the distribution of LiO- on the surface of the electrode is also obvious, which indicates that Li2O has a relatively large distribution on the surface of the negative electrode of the additive system. Through the distribution analysis of these lithium-containing compounds before surface sputtering, we can preliminarily judge that LiDFBOP has good lithium ion transport properties, because more active lithium is actively detected on the electrode surface. The carbon and phosphorus-containing materials on the surface of the two systems are further analyzed to analyze the decomposition of the solvent and lithium salt. CO2- may be a characteristic fragment of Li2CO3, and CHO2- may represent fragment ions of some organic carbonates. It is found that there are not too many inorganic carbon-containing decomposition products detected on the surface of the two systems, but there are obvious organic carbon-containing substances distributed on the surface. Moreover, in the Baseline system, the amount of such organic carbonaceous materials is more than that of the additive system, which indicates that a larger amount of solvent undergoes single electron reduction to form an organic substance attached to the electrode surface. This is consistent with the
Journal Pre-proof results of the analysis in the above XPS, but the inorganic carbonaceous material is not clearly detected. It can be speculated after analysis that the inorganic carbonaceous material is generally attached to the electrode side, and the organic substance is attached thereto, thus causing inorganic substances to be more difficult to detect. Similarly, PO2- and POF2-, as LiPF6 and LiDFBOP decomposition products, are mostly inorganic, which are covered on the electrode side and thus the signal is weaker before sputtering. However, it can be analyzed from PO2- that the decomposition of the main salt LiPF6 in the Baseline electrolyte system is significantly more than that of the additive system. This also proves the previous conclusion that the preferential decomposition of LiDFBOP hinders the decomposition of a part of LiPF6, optimizing the surface film properties and improves the overall electrochemical performance. Then we started the sputtering analysis to further determine the distribution structure of the surface SEI film under the two systems (Fig. 7). From Fig. 7b, we found that the surface distribution of the organic carbon-containing compound CHO2- is mainly on the electrolyte side, the distribution is the closer to the electrode, the less of the content. However, the inorganic carbonate (such as CO2-) and the inorganic phosphorus-containing compound are distributed more near the electrode side. We performed a 3D distribution analysis of surface fragment ions at a depth of about 1500 s (about 50 nm). It mainly analyzed that the decomposition of the phosphorus-containing inorganic compounds and organic carbonate derived by the main salt and solvents are analyzed and shown in Fig. 7d. In consistence with the above analysis, organic matter is mainly distributed on the surface, and more amounts of phosphorus-containing inorganic compounds exist in the Baseline system. Based on the above analysis, it can be concluded that the SEI film tailored by the LiDFBOP has special structure, whose organic ingredients are mainly distributed on the SEI film near the electrolyte
Journal Pre-proof side. And for the Baseline system, SEI film structure mainly exhibits a mosaic-like distribution. It’s well known that the inorganic substances are able to exert the mechanical strength of the stable film and organic substance can increase the transfer rate of lithium ions. But high interface impendence is caused by more inorganic ingredients near the electrode side and organic part may be so unstable that constantly decomposes. Therefore, the SEI film biased toward the layered structure can better exert the properties of the decomposition products of different components than the mosaic structure. A schematic representation of the specific film composition distribution is shown in Scheme 2. Theoretical analysis of LiDFBOP with density functional theory calculation In order to verify some of the above calculations regarding the preferential reduction mechanism of LiDFBOP, we analyzed the key components in the electrolyte by electrostatic potential distribution and vertical electron affinity. Through the electrostatic potential (ESP) distribution of Fig. 8a, we can analyze the sites where the solvent and lithium salt in the electrolyte are easily adsorbed to the surface of the electrode, which can be used as the analysis of electronegativity of molecule. In the contour map, the red and blue colors represent the most electronegative and electropositive ESP values, respectively. This can be used to preliminarily determine which substances in the electrolyte system are more likely to preferentially reach the electrode surface. From the analysis in the figure, we found that the lithium salt is more likely to be attracted to the electrode surface, especially LiDFBOP additive, while the solvents are relatively delayed. Further we compare the priority reduction order of these components by calculating the vertical electron affinity of the electrolyte components. Vertical electron affinity (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 Fig. 8b. It can be concluded from the Fig. 8b that LiDFBOP has a higher electronic
Journal Pre-proof affinity, which shows that it is easier to get electrons on the surface of the electrode to cause reductive decomposition. This confirms some of our previous experimental results on a theoretical level. Therefore, the results of the combination of electrostatic potential distribution indicate that LiDFBOP may be dragged to the surface of the electrode faster as the solvent is “solved” by the solvation, and the anion of LiDFBOP is reductively decomposed during the desolvation process. The reduction process of solvent is suppressed by the decomposition product of LiDFBOP to some extent. Therefore, a large amount of inorganic substances is detected in the system containing LiDFBOP, which is consistent with the conclusions obtained in the previous TOF-SIMS. Conclusions By comparing the improved electrolyte system with the blank system, the capacity of Li/MCMB half cells with 1% LiDFBOP electrolyte system has gotten obvious elevation. The capacity retention of Li/MCMB half cells increase from 71.2% to 88.1%, which can be contributed to the low film impedance of SEI film derived by LiDFBOP. It can be proven from our testing that LiDFBOP can reduce the invalid decomposition of main salt LiPF6, which can increase the content of organic ingredients and reduce the inorganic substance with high impedance. The mechanism of effect of LiDFBOP involves its initial reduction on the electrode surface to hinder the more decomposition of LiPF6. Besides, it is displayed that the more organic substances in the resulting SEI film can promote the rapid transfer of lithium ions and reduce the loss of irreversible capacity. The results of ESP and VEA show that LiDFBOP can preferentially reduce and decompose on the surface of the electrode to change the distribution of the SEI film composition, so that the organic component increases but does not cause the interface impedance to increase and improve the performance. The research gives a detailed reference to the analysis of surface materials by TOF-SIMS, which laid the foundation for
Journal Pre-proof further analysis in the future. Acknowledgements This work was supported by the Natural Science Foundation of China (no. 51962019), the Gansu Science and Technology Plan Project (18JR3RA160) and Science and Technology Plan Project of Baiyin City (2017-2-11G).
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Scheme 1 Possible reduction reaction of LiDFBOP on the anode surface.
Scheme 2 Different SEI film structure distribution derived from two electrolyte systems.
O 2Li O
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Value 0 0 0 1 0 0 1 0 0 600 1 800 Z' 00/ ohm 0.5
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DataCycling File: Fig. 2 (a) performance at ambient temperature; (b) EIS spectra with Li/MCMB half cell after Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting:
Run Simulation / Freq. Range (0.001 - 1000000) 100 0 Complex Calc-Modulus
the first discharge at fully lithiated state; (c) corresponding analog impedance value of two electrolytes
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Fig. 3 SEM of MCMB surface with different electrolytes after initial discharge: (a-c) for fresh MCMB; (d-f) for Baseline system; (g-i) for 1% LiDFBOP system.
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Fig. 4 EDS analysis for MCMB surface with different electrolyte systems after initial discharge: (a) Baseline electrolyte; (b) 1% LiDFBOP electrolyte.
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Fig. 5 XPS analysis for MCMB surface with different electrolytes
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Fig. 6 TOF-SIMS chemical maps of different fragment ions after Cs+ sputtering acquired on graphite harvested from 50-cycle half cells with baseline electrolyte and 1% LiDFBOP
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Fig. 7 TOF-SIMS depth profiles of fragments of interest acquired on MCMB anode retrieved from 50cycle with different electrolyte systems, including (a) lithium-containing compound; (b) carbonaceous-containing compounds (representing solvent decomposition); (c) PO2- (representing inorganic SEI species related to LiPF6 decomposition); (d) 3D distribution of special fragment particle with different electrolyte systems, including CHO2- and PO2-.
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LiPF6
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Li ...DFBOP
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Fig. 8 (a) Electrostatic potential maps and (b) VEA of DMC, EC, LiDFBOP and LiPF6
Journal Pre-proof Dongni Zhao: Conceptualization, Methodology, Software, Validation, Writing - Original Draft Jie Wang: Investigation, Resources Peng Wang: Formal analysis Haining Liu: Formal analysis Shiyou Li: Writing - Review & Editing, Funding acquisition, Project administration
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Dear editor A Achuthan, We are sorry for our fault not to provide correct figure, we have revised and added the figure below. We apologize for the loss of your valuable time, and hope that our amendments will facilitate the early publication of the article.
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Fig. 2 (a) Cycling performance at ambient temperature; (b) EIS spectra with Li/MCMB Data File: Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting:
Run Simulation / Freq. Range (0.001 - 1000000) half cell after the first discharge at fully lithiated state; (c) corresponding analog 100 0 Complex Calc-Modulus
impedance value of two electrolytes
Coulombic efficiency / %
Discharge capacity / mAh g-1
Yours, Shiyou Li
Dongni Zhao, Jie Wang, Peng Wang, Haining Liu, Shiyou Li PII:
S0013-4686(20)30137-7
DOI:
https://doi.org/10.1016/j.electacta.2020.135745
Reference:
EA 135745
To appear in:
Electrochimica Acta
Received Date:
19 November 2019
Accepted Date:
19 January 2020
Please cite this article as: Dongni Zhao, Jie Wang, Peng Wang, Haining Liu, Shiyou Li, Regulating the Composition Distribution of Layered SEI film on Li-ion Battery Anode by LiDFBOP, Electrochimica Acta (2020), https://doi.org/10.1016/j.electacta.2020.135745
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Regulating the Composition Distribution of Layered SEI film on Li-ion Battery Anode by LiDFBOP
Dongni Zhaoa, Jie Wanga, Peng Wanga, Haining Liuc, Shiyou Li*a,b aCollege
bGansu cCAS
of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050, P.R. China
Engineering Laboratory of Electrolyte Material for Lithium-ion Battery, Lanzhou 730050, China
Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai
Institute of Salt Lakes Chinese Academy of Sciences, Chinese Academy of Sciences, Xining, 810008, P.R. China
Abstract The so-called solid-electrolyte interface (SEI) derived from the interaction between electrolyte and electrode plays a decisive role in Li-ion battery performance. Lithium difluoro(bisoxalato) phosphate (LiDFBOP) as the derivative of LiPF6 has caused great attention among the researchers. However, the mechanism of effect for LiDFBOP salt has not yet revealed, which limits the findings for the fundamental cause of performance improvement. In this work, the effect of LiDFBOP on the mesocarbon microbead (MCMB) anode electrode has been investigated with aid of surface analysis technique, such as scanning electron microscope (SEM), X-ray photoelectron spectrum (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). On the basis of the modification with LiDFBOP, the long-term cycling and degradation resistance of Li/MCMB half cells have been achieved, which results from the preferential decomposition of LiDFBOP regulating the decomposition law of the electrolyte system. The SEI film with layer structure has different composition distribution under the obstruction of LiDFBOP of the decomposition of LiPF6. The more organic ingredients are distributed in the SEI film on the electrolyte side, which leads to low impedance and fast transportation of lithium ions. The special ingredient distribution is ascribed to the obstruction of LiDFBOP on the decomposition of LiPF6. This work signifies the research of the effect of SEI film
Journal Pre-proof distribution on the battery performance. Key words Lithium difluoro(bisoxalato) phosphate; SEI film; MCMB; Interface modification Introduction Lithium-ion battery (LIB) development has experienced many changes so far. In addition to the high specific energy and high energy density requirements of the battery, it also places high demands on its safety [1-4]. Lithium metal as a negative electrode has a lower electrode potential and a theoretical specific capacity of up to 3860 mAh g-1, which can be used as a key material for high energy density batteries. However, lithium metal still presents significant challenges during electrochemical deposition/circulation. These include: 1) the growth of lithium dendrites; the growth of lithium dendrites not only causes the degradation of battery performance but when the dendrites pierce the separator and contact with the positive electrode, it will cause an internal short circuit of the battery, causing safety problems. 2) Formation of the "dead lithium" layer; during the cycle, dendritic or mossy lithium will fall off the lithium sheet, forming a "dead lithium" layer, which not only reduces the coulombic efficiency, but also greatly increases the internal resistance of battery affecting cycle performance. 3) Due to the "no host" nature of metallic lithium, there is an unrestricted volume effect during the cycle [5, 6]. Therefore, lithium metal anodes are greatly limited in application. In comparison, graphite-based anode materials have attracted much attention due to their excellent layered structure. Among them, mesocarbon microbead (MCMB) with a brilliant mass specific capacity (about 300 mAh g-1) and a low irreversible mass ratio capacity (about 20 mAh g-1) have been studied [7], while low-cost graphite has a high mass specific capacity (350 mAh g-1), but its irreversible specific capacity (about 50 mAh g-1) is higher than MCMB carbon [8, 9]. Graphite shows a higher capacity decay rate, which is not suitable for power batteries that require long cycle and high volume
Journal Pre-proof specific energy. Moreover, artificial graphite and natural graphite have more chemical side reactions than mesophase products, thermal stability and chemical stability are not as good as mesophase products. MCMB makes a contribution once again on the development of lithium ion batteries [10]. The thermodynamically favourable chemical reaction at the electrode and electrolyte interface is the key to determining performance and even research [11-13]. There are many factors in the polarization of the negative electrode of the battery, in which the composition of the solid electrolyte interface (SEI) on the negative electrode has a profound effect on the polarization or impedance of the battery. Regulating the properties of SEI film (including composition, thickness and roughness) is important for the good performance of the battery [11, 14]. Therefore, it is important to find the suitable electrolyte formula and study the interface reaction mechanism and the modification of the interface to improve the performance. The use of electrolyte additives as a cost-effective means for optimizing electrolyte formulations has yielded fruitful results [15]. The additive includes film-forming additives [16], such as vinylene carbonate (VC), 3-propane sultone (PS) and so on, have been proven to improve the performance of MCMB anode electrode. However, these additives have inferior compatibility with some of the high-voltage cathode material. Based on this problem, we introduced lithium difluoro(bisoxalato) phosphate (LiDFBOP) [17-19] as a functional additive which has been proven to have excellent properties on the side of cathode electrode, to investigate the effect mechanism on the MCMB anode electrode. In this work, LiDFBOP was added in the electrolyte system to investigate the compatibility with Li/MCMB half cell. We use the high precision instrument secondary time-of-flight ion mass spectrometry (TOF-SIMS) to detect the difference of SEI film composition on the electrode surface with different electrolyte systems. By analyzing the SEI film on the surface of the negative electrode,
Journal Pre-proof the regulation of the structure and composition of the SEI film derived by LiDFBOP can be obtained and can provide some ideas for the future research of electrolyte formula. Experimental Section Material preparation Electrolytes were divided into two parts of reference and experiment samples. LiPF6- dimethyl carbonate (DMC)/ ethylene carbonate (EC) (1:1, by volume) (marked as Baseline) was purchased from Chaoyang Yongheng Chemical Co., Ltd. And LiDFBOP-EMC solvent synthesized by our laboratory was added after screening at the ratio of 1% in the Baseline system (marked as 1% LiDFBOP). Anode sheet was made up of 80 wt. % MCMB, 10 wt. % polyvinylidenefluoride (PVDF) and 10 wt. % carbon black, which was coated on the copper foil. CR2025 coin cells were assembled in the glove box with argon atmosphere using above mentioned electrolytes. Moreover, lithium foil was served as a counter electrode and the reference electrode, and Celgard porous polypropylene separator (2400) was used to separate the positive and negative electrodes to avoid short circuit. Electrochemical performance test Electrochemical properties of half-cells were analyzed by Land cell tester CT2001A (Wuhan, China) in the voltage of 0.01-2.0 V at different rate. And electrochemical impedance spectra for Li/MCMB half-cells with different electrolytes were measured on the electrochemical workstation (CHI660E, Shanghai, China). All cells were tested at initial lithiated state at 0.01 V at room temperature. A sinusoidal AC perturbation of 5.0 mV was applied to the electrode over the frequency range of 100 kHz to 10 mHz. Surface characterization test Scanning Electron Microscope (SEM, JSM-5600, Japan) and Energy Dispersive Spectrometer
Journal Pre-proof (EDS, JSM-5600, Japan) were adopted for characterization of surface morphology and element distribution of SEI films. And X-ray Photoelectron Spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical Ltd.) as well as TOF-SIMS, (IonTOF5, Germany) were used as surface material composition and in-depth analysis. Theoretical calculations Quantum chemical calculations were performed by Gaussian 09 package [22-23]. The computing environment used in this study was provided by the Chinese National Grid (CNGrid) of the Chinese Academy of Sciences and the Chinese Academy of Sciences Supercomputing Environment (ScGrid) based on SCE software (http://www.cngrid.org, http://www. Scgrid.cn) [24]. DFT calculation was the method used for geometry optimization with B3LYP along with the basis set of 6-31++G (d, p). Frequency analyses with the same basis set were done to check imaginary frequency for optimized structure. In order to investigate the effect of solvents, the structures were optimized by using the polarized continuum mode (PCM), with a dielectric constant of 46.44 (EC: DMC, by volume of 1:1). Energy was taken into consideration in 6-311++G (2d, 2p) basis set, which contained polarization functions and could better describe atomic orbital. Under the above method and the basis group, compounds were optimized to obtain their stable structures. Also, and vertical electron affinity (VEA) were obtained by energy calculation. Also, we perform the electrostatic potential (ESP) analysis. Results and Discussions Initial discharge process for Li/MCMB half cell with additive-optimized electrolyte The first discharge process of the anode electrode is a process of active kinetic reaction between the main electrolyte and the interface. Therefore, it is of great significance to conduct an in-depth study of the first discharge curve. Fig. 1 shows the first discharge curve for Li/MCMB half-cell with two
Journal Pre-proof different electrolyte systems. It can be found that the electrolyte system containing LiDFBOP releases more capacity, which is about 390.1 mAh g-1, while the Baseline electrolyte system is only 277.2 mAh g-1. This indicates that the 1% LiDFBOP system has more active lithium ion intercalation during the first lithium intercalation process, and it can be inferred that a strong activation reaction occurs at the interface, and part of the lithium ions participate in the formation of the SEI film on the negative surface. And from Fig. 1a, it can be found that the 1% LiDFBOP system clearly shows a platform between 1.4 and 1.8 V, which releases about 11.2 mAh g-1 capacity, accounting for 2.8% of the total discharge capacity. It is derived from the ring opening reaction of the oxalic acid ring in the LiDFBOP additive. To further analyze the reasons for the platform generated here, we have a corresponding differential capacity curve for the discharge curve (Fig. 1b). We can find that the Baseline system does not show any peaks above 1.0 V, but the 1% LiDFBOP has two small peaks at 1.67 V and 1.40 V, respectively. It is inferred that there may be a decomposition ring opening reaction as presented in Scheme 1, the oxalic acid ring is subjected to an electron-opening reaction on the surface of the electron-rich electrode, corresponding to a peak position of 1.67 V, and the further complexation reaction corresponds to 1.40 V. Electrochemical performance of Li/MCMB half cells modified by LiDFBOP additive Further, in order to evaluate the improvement of the electrochemical performance of the additive system, we tested the cycle performance of the Li/MCMB half-cell system. Fig. 2 shows the cycling performance tests of the two systems and analyzes the changes in impedance during the process. From the cycle performance of the two systems in Fig. 2a, it can be found that the cycle performance of the Li/MCMB half-cell after the addition of the LiDFBOP additive is significantly improved, the capacity retention rate is increased from 71.2% to 88.1%, and the capacity is also obvious. The improvement
Journal Pre-proof of Coulomb's efficiency is also relatively stable, which will become the key to the long life of the battery. It can be analyzed from the Fig. 1 that LiDFBOP preferentially decomposes to form films on the electrode surface in the electrolyte system, and the oxalic acid ring opens to form a decarboxylated dimer from LiDFBOP attached to the electrode surface, which hinders the contact between electrolyte and the electrons on the electrode surface, thus decomposition has a certain degree of inhibition. Therefore, lithium ions will not effectively participate in forming repeated film, and the loss of irreversible capacity is reduced. Impedance can be used as a good indicator to evaluate the properties of the interface between the electrolyte and the electrode. Fig. 2b tests the change in impedance after the first discharge in both systems, and Fig. 2c shows the corresponding resistances evaluated by fitting impedance measurements with the shown equivalent circuit. It can be analyzed by the analog circuit that the impedance values of the two systems are composed of the following five parts. The intercept of the spectra and the abscissa in the high frequency region is Rs, which is defined as the ohmic impedance Rs., which is mainly derived from the bulk impedance of the battery case itself and the electrolyte. The semicircle in the high frequency region of Rf and CPE1 can be ascribed to diffusion of lithium ions through the SEI film on the surface of active material particles. And the mid-high frequency region corresponding to Re in parallel with CPE2 is a semicircle related to the transport process of electrons inside the active material particles. The medium frequency region is actually composed of two semicircles, mainly the interface film resistance the charge transformation resistance Rct in parallel CPE3. Finally, the diagonal line of the low frequency region mainly represents the Warburg impedance W of the diffusion. It can be observed from our tests that the W impedance is not particularly significant in the Li/MCMB half cells system, so it will not be discussed too much here. From the
Journal Pre-proof simulation values in Fig. 2c, we can find that the Rf of the system containing LiDFBOP additive decreases, which indicates that LiDFBOP can regulate the properties of the interface film, so that the resistance value is reduced, which is beneficial to the transmission of lithium ions and improve the transmission power. It also can be observed that there is obvious difference in the Re in the two electrolyte systems. The system containing the additive reduces the damage of the electrode surface due to the decomposition of the solvent by optimizing the surface of the negative electrode electrode, so that the resistance of electron transport in the material is lowered. This also explains from another perspective that an optimized electrolyte system helps protect the interface from erosion and damage. Also, we found that the Rct value has increased to some extent, which indicates that the LiDFBOPderived interface has no beneficial effect on surface charge transport, which may affect the rate performance of Li/MCMB. However, it is further proved that the formed SEI film effectively hinders the charge transfer, and the surface charge transfer is slow, which can reduce the large contact between the electrolyte and the electron, thereby further decomposing.
Analysis of surface properties of MCMB negative electrode In order to explain the mechanism of action of the additive LiDFBOP, it is necessary to analyze the improvement of the surface properties of the negative electrode MCMB by LiDFBOP. The interface is the “conversation window” for the transport of lithium ions from the liquid electrolyte to the solid phase electrode material. The study of interface properties can have a preliminary judgment on the dynamic properties of different systems. First, we analyzed the morphology changes of the interface film under the two systems after the first discharge, and the SEM images obtained at different resolutions are shown in Fig. 3. By comparison, it can be clearly found that there are significant
Journal Pre-proof differences in the MCMB anode surface with and without interface films formed. The interface film of the Baseline electrolyte system is not very uniform, and there are obvious cracks, which will be detrimental to the SEI film protecting the MCMB electrode surface from further corrosion of the electrolyte. In comparison, the film formed by the 1% LiDFBOP system uniformly grows on the surface of the electrode, and the morphology of the film is uniformly distributed on the surface of the electrode like “scales”, so that the uniform SEI film morphology can hinder ion transport better and inhibit the thickening of SEI. The rapid transportation protects the surface of the electrode without causing further decomposition of the electrolyte, thereby reducing the continuous decomposition and re-generation of the SEI film and reducing the loss of active lithium ions. The distribution of surface elements, and the surface element content and distribution analysis of the two films after the first electrode discharge were further analyzed by EDS. It is found that in the Baseline system, the content of C was significantly higher, and also the P content is significantly higher by two orders of magnitude, which indicates that more LiPF6 was decomposed, resulting in the parasitic reaction of solvents to thicken the SEI and impede the smooth transportation of lithium ions. It can be inferred that P content is very low in the presence of LiDFBOP which may be due to the effective hindering of the decomposition of electrolyte species by the outermost organic layer that contains the reduction product of the additive and hence, the growth of the innermost inorganic layer is suppressed. It may speculatively be assumed that the inner inorganic layer in the case of LiDFBOP is thinner. Observing the distribution of elements, we found that the F element distribution on the surface of the additive system is relatively more uniform, so that the uniform SEI film formed will effectively protect the electrode, and the SEI film will not be further decomposed and regenerated due to the tip effect of the electron caused by local unevenness. Thus the formed thin SEI film may explain
Journal Pre-proof the relatively lower film resistance Rf in the presence of the additive (see Fig. 2 (c)), which is advantageous for the improvement of electrochemical performance. For the composition of surface chemicals, we further tested by XPS. The composition of the SEI film was analyzed by fitting the peak shape of the two systems on the surface after the end of the initial discharge. It can be found by analysis that the SEI film derived by 1% LiDFBOP is thin, which can be gained by C 1s [20-21]. The peak position of 284.74 eV in the C 1s spectrum represents the characteristic valence of the surface of MCMB, and 287.23 eV corresponds to the peak position of PVDF in the F 1s spectrum. It can be seen that these characteristic peaks are higher in the additive system than those in the Baseline system. This result is consistent with the results analyzed in the impedance in Fig. 2b-c. In addition, by analyzing the composition of the surface film with the two systems, the composition of the surface film system is analyzed as follows. In the C 1s spectrum, 289.24 eV corresponds to Li2CO3 and 532.74 eV in the O 1s spectrum. LiF is also one of the inorganic components of the interface film, which corresponds to 687.99 eV in the F 1s spectrum, and the content of LiF in the additive system is higher than that in the Baseline system, indicating that the inorganic system accounts for a higher proportion of the film formed by the additive. In addition, analysis of the organic components in the SEI film, we can find that in the Baseline electrolyte system, the composition of organic matter is relatively higher, which is also the reason why the film structure is loose and thick. Moreover, the organic SEI film is deposited on the outer layer of the inorganic film, and its structure is not that stable, and easy to repeatedly decompose and form a film. Further analysis of the P 2p spectrum, 134.11 eV corresponds to the possible decomposition product LixPyOzF of the main salt LiPF6, and 136.74 eV corresponds to PF5 or PF6- which is not decomposed on the surface. In summary, the interfacial film derived from LiDFBOP additive is thinner, in which the proportion of
Journal Pre-proof organic substances is lower than that of inorganic substances, so the inorganic content of LiDFBOP system is lower than that of Baseline system. From this analysis, it is apparent that the intensity of the two characteristic peaks of the system containing the additive is not very high, which confirms the above-mentioned speculative mechanism, because the preferential decomposition of the additive LiDFBOP hinders the large-scale decomposition of the main salt LiPF6. Further, we will take a more sophisticated instrument TOF-SIMS for deeper research and explanation. First, we analyzed the distribution of surface elements and found that the difference in surface materials between the two systems was relatively large (Fig. 6). First, we analyzed lithium-containing compounds. We found that 6 Li- is significantly more abundant in the 1% LiDFBOP system, which means more surface-active lithium. In addition, the distribution of LiO- on the surface of the electrode is also obvious, which indicates that Li2O has a relatively large distribution on the surface of the negative electrode of the additive system. Through the distribution analysis of these lithium-containing compounds before surface sputtering, we can preliminarily judge that LiDFBOP has good lithium ion transport properties, because more active lithium is actively detected on the electrode surface. The carbon and phosphorus-containing materials on the surface of the two systems are further analyzed to analyze the decomposition of the solvent and lithium salt. CO2- may be a characteristic fragment of Li2CO3, and CHO2- may represent fragment ions of some organic carbonates. It is found that there are not too many inorganic carbon-containing decomposition products detected on the surface of the two systems, but there are obvious organic carbon-containing substances distributed on the surface. Moreover, in the Baseline system, the amount of such organic carbonaceous materials is more than that of the additive system, which indicates that a larger amount of solvent undergoes single electron reduction to form an organic substance attached to the electrode surface. This is consistent with the
Journal Pre-proof results of the analysis in the above XPS, but the inorganic carbonaceous material is not clearly detected. It can be speculated after analysis that the inorganic carbonaceous material is generally attached to the electrode side, and the organic substance is attached thereto, thus causing inorganic substances to be more difficult to detect. Similarly, PO2- and POF2-, as LiPF6 and LiDFBOP decomposition products, are mostly inorganic, which are covered on the electrode side and thus the signal is weaker before sputtering. However, it can be analyzed from PO2- that the decomposition of the main salt LiPF6 in the Baseline electrolyte system is significantly more than that of the additive system. This also proves the previous conclusion that the preferential decomposition of LiDFBOP hinders the decomposition of a part of LiPF6, optimizing the surface film properties and improves the overall electrochemical performance. Then we started the sputtering analysis to further determine the distribution structure of the surface SEI film under the two systems (Fig. 7). From Fig. 7b, we found that the surface distribution of the organic carbon-containing compound CHO2- is mainly on the electrolyte side, the distribution is the closer to the electrode, the less of the content. However, the inorganic carbonate (such as CO2-) and the inorganic phosphorus-containing compound are distributed more near the electrode side. We performed a 3D distribution analysis of surface fragment ions at a depth of about 1500 s (about 50 nm). It mainly analyzed that the decomposition of the phosphorus-containing inorganic compounds and organic carbonate derived by the main salt and solvents are analyzed and shown in Fig. 7d. In consistence with the above analysis, organic matter is mainly distributed on the surface, and more amounts of phosphorus-containing inorganic compounds exist in the Baseline system. Based on the above analysis, it can be concluded that the SEI film tailored by the LiDFBOP has special structure, whose organic ingredients are mainly distributed on the SEI film near the electrolyte
Journal Pre-proof side. And for the Baseline system, SEI film structure mainly exhibits a mosaic-like distribution. It’s well known that the inorganic substances are able to exert the mechanical strength of the stable film and organic substance can increase the transfer rate of lithium ions. But high interface impendence is caused by more inorganic ingredients near the electrode side and organic part may be so unstable that constantly decomposes. Therefore, the SEI film biased toward the layered structure can better exert the properties of the decomposition products of different components than the mosaic structure. A schematic representation of the specific film composition distribution is shown in Scheme 2. Theoretical analysis of LiDFBOP with density functional theory calculation In order to verify some of the above calculations regarding the preferential reduction mechanism of LiDFBOP, we analyzed the key components in the electrolyte by electrostatic potential distribution and vertical electron affinity. Through the electrostatic potential (ESP) distribution of Fig. 8a, we can analyze the sites where the solvent and lithium salt in the electrolyte are easily adsorbed to the surface of the electrode, which can be used as the analysis of electronegativity of molecule. In the contour map, the red and blue colors represent the most electronegative and electropositive ESP values, respectively. This can be used to preliminarily determine which substances in the electrolyte system are more likely to preferentially reach the electrode surface. From the analysis in the figure, we found that the lithium salt is more likely to be attracted to the electrode surface, especially LiDFBOP additive, while the solvents are relatively delayed. Further we compare the priority reduction order of these components by calculating the vertical electron affinity of the electrolyte components. Vertical electron affinity (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 Fig. 8b. It can be concluded from the Fig. 8b that LiDFBOP has a higher electronic
Journal Pre-proof affinity, which shows that it is easier to get electrons on the surface of the electrode to cause reductive decomposition. This confirms some of our previous experimental results on a theoretical level. Therefore, the results of the combination of electrostatic potential distribution indicate that LiDFBOP may be dragged to the surface of the electrode faster as the solvent is “solved” by the solvation, and the anion of LiDFBOP is reductively decomposed during the desolvation process. The reduction process of solvent is suppressed by the decomposition product of LiDFBOP to some extent. Therefore, a large amount of inorganic substances is detected in the system containing LiDFBOP, which is consistent with the conclusions obtained in the previous TOF-SIMS. Conclusions By comparing the improved electrolyte system with the blank system, the capacity of Li/MCMB half cells with 1% LiDFBOP electrolyte system has gotten obvious elevation. The capacity retention of Li/MCMB half cells increase from 71.2% to 88.1%, which can be contributed to the low film impedance of SEI film derived by LiDFBOP. It can be proven from our testing that LiDFBOP can reduce the invalid decomposition of main salt LiPF6, which can increase the content of organic ingredients and reduce the inorganic substance with high impedance. The mechanism of effect of LiDFBOP involves its initial reduction on the electrode surface to hinder the more decomposition of LiPF6. Besides, it is displayed that the more organic substances in the resulting SEI film can promote the rapid transfer of lithium ions and reduce the loss of irreversible capacity. The results of ESP and VEA show that LiDFBOP can preferentially reduce and decompose on the surface of the electrode to change the distribution of the SEI film composition, so that the organic component increases but does not cause the interface impedance to increase and improve the performance. The research gives a detailed reference to the analysis of surface materials by TOF-SIMS, which laid the foundation for
Journal Pre-proof further analysis in the future. Acknowledgements This work was supported by the Natural Science Foundation of China (no. 51962019), the Gansu Science and Technology Plan Project (18JR3RA160) and Science and Technology Plan Project of Baiyin City (2017-2-11G).
Journal Pre-proof References [1] X. Zeng, M. Li, D. Abd El‐ Hady, W. Alshitari, A.S. Al‐ Bogami, J. Lu, K. Amine, Commercialization of Lithium Battery Technologies for Electric Vehicles, Adv. Energy Mater. (2019) 1900161. [2] X. Zeng, C. Zhan, J. Lu, K. Amine, Stabilization of a High-Capacity and High-Power Nickel-Based Cathode for Li-Ion Batteries, Chem 4 (2018) 690-704. [3] Y. Ding, Z.P. Cano, A. Yu, J. Lu, Z. Chen, Automotive Li-Ion Batteries: Current Status and Future Perspectives, Electrochem. Energy Rev. 2 (2019) 1-28. [4] K. Bourzac, 4 Big questions, Nature 526 (2015) S105-S105. [5] Z.A. Ghazi, Z. Sun, C. Sun, F. Qi, B. An, F. Li, H.M. Cheng, Key Aspects of Lithium Metal Anodes for Lithium Metal Batteries, Small (2019) 1900687. [6] Y. Chen, Y. Luo, H. Zhang, C. Qu, H. Zhang, X. Li, The Challenge of Lithium Metal Anodes for Practical Applications, Small Methods (2019) 1800551. [7] Y. Li, Y. Lu, P. Adelhelm, M.-M. Titirici, Y.-S. Hu, Intercalation chemistry of graphite: alkali metal ions and beyond, Chem. Soc. Rev. 48 (2019) 4655-4687. [8] S. Huang, H. Guo, X. Li, Z. Wang, L. Gan, J. Wang, W. Xiao, Carbonization and graphitization of pitch applied for anode materials of high power lithium ion batteries, J. Solid State Electrochem. 17 (2013) 1401-1408. [9] M.-L. Lee, S.-C. Liao, J.-M. Chen, J.-W. Yeh, H.C. Shih, Core-Shelled MCMB-Li4Ti5O12 Anode Material for Lithium-ion Batteries, J. Chin. Chem. Soc. 59 (2012) 1206-1210. [10] P. Ridgway, H. Zheng, A.F. Bello, X. Song, S. Xun, J. Chong, V. Battaglia, Comparison of Cycling Performance of Lithium Ion Cell Anode Graphites, J. Electrochem. Soc. 159 (2012) A520-
Journal Pre-proof A524. [11] E. Peled, The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems-The Solid Electrolyte Interphase Model, J. Electrochem. Soc. 126 (1979) 2047-2051. [12] R. Fong, U. von Sacken, J.R. Dahn, Studies of Lithium Intercalation into Carbons Using Nonaqueous Electrochemical Cells, J. Electrochem. Soc. 137 (1990) 2009-2013. [13] M. Nie, D. Chalasani, D.P. Abraham, Y. Chen, A. Bose, B.L. Lucht, Lithium Ion Battery Graphite Solid Electrolyte Interphase Revealed by Microscopy and Spectroscopy, J. Phys. Chem. C 117 (2013) 1257-1267. [14] P. Verma, P. Maire, P. Novák, A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries, Electrochim. Acta 55 (2010) 6332-6341. [15] S. Zhang, A review on electrolyte additives for lithium-ion batteries, J. Power Sources 162 (2006) 1379-1394. [16] J.-P. Jones, M.C. Smart, F.C. Krause, B.V. Ratnakumar, E.J. Brandon, The Effect of Electrolyte Composition on Lithium Plating During Low Temperature Charging of Li-Ion Cells, ECS Trans. 75 (2017) 1-11. [17] B. Liao, H. Li, M. Xu, L. Xing, Y. Liao, X. Ren, W. Fan, L. Yu, K. Xu, W. Li, Designing Low Impedance Interface Films Simultaneously on Anode and Cathode for High Energy Batteries, Adv. Energy Mater. 8 (2018) 1800802. [18] J.-G. Han, I. Park, J. Cha, S. Park, S. Park, S. Myeong, W. Cho, S.-S. Kim, S.Y. Hong, J. Cho, N.-S. Choi, Interfacial Architectures Derived by Lithium Difluoro(bisoxalato) Phosphate for LithiumRich Cathodes with Superior Cycling Stability and Rate Capability, ChemElectroChem 4 (2017) 5665.
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Scheme 1 Possible reduction reaction of LiDFBOP on the anode surface.
Scheme 2 Different SEI film structure distribution derived from two electrolyte systems.
O 2Li O
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Value 0 0 0 1 0 0 1 0 0 600 1 800 Z' 00/ ohm 0.5
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DataCycling File: Fig. 2 (a) performance at ambient temperature; (b) EIS spectra with Li/MCMB half cell after Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting:
Run Simulation / Freq. Range (0.001 - 1000000) 100 0 Complex Calc-Modulus
the first discharge at fully lithiated state; (c) corresponding analog impedance value of two electrolytes
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Fig. 3 SEM of MCMB surface with different electrolytes after initial discharge: (a-c) for fresh MCMB; (d-f) for Baseline system; (g-i) for 1% LiDFBOP system.
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Fig. 4 EDS analysis for MCMB surface with different electrolyte systems after initial discharge: (a) Baseline electrolyte; (b) 1% LiDFBOP electrolyte.
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Fig. 5 XPS analysis for MCMB surface with different electrolytes
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Fig. 6 TOF-SIMS chemical maps of different fragment ions after Cs+ sputtering acquired on graphite harvested from 50-cycle half cells with baseline electrolyte and 1% LiDFBOP
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Fig. 7 TOF-SIMS depth profiles of fragments of interest acquired on MCMB anode retrieved from 50cycle with different electrolyte systems, including (a) lithium-containing compound; (b) carbonaceous-containing compounds (representing solvent decomposition); (c) PO2- (representing inorganic SEI species related to LiPF6 decomposition); (d) 3D distribution of special fragment particle with different electrolyte systems, including CHO2- and PO2-.
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Fig. 8 (a) Electrostatic potential maps and (b) VEA of DMC, EC, LiDFBOP and LiPF6
Journal Pre-proof Dongni Zhao: Conceptualization, Methodology, Software, Validation, Writing - Original Draft Jie Wang: Investigation, Resources Peng Wang: Formal analysis Haining Liu: Formal analysis Shiyou Li: Writing - Review & Editing, Funding acquisition, Project administration
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Journal Pre-proof Dear editor A Achuthan, We are sorry for our fault not to provide correct figure, we have revised and added the figure below. We apologize for the loss of your valuable time, and hope that our amendments will facilitate the early publication of the article.
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Fig. 2 (a) Cycling performance at ambient temperature; (b) EIS spectra with Li/MCMB Data File: Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting:
Run Simulation / Freq. Range (0.001 - 1000000) half cell after the first discharge at fully lithiated state; (c) corresponding analog 100 0 Complex Calc-Modulus
impedance value of two electrolytes
Coulombic efficiency / %
Discharge capacity / mAh g-1
Yours, Shiyou Li