Adiponitrile as Lithium-ion battery electrolyte additive: a positive and

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Adiponitrile as Lithium-ion battery electrolyte additive: a positive and peculiar effect on high-voltage system Xingyu Wang, Wen-Dong Xue, Kai Hu, Yan Li, Yong Li, and Rong-Yi Huang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00968 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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ACS Applied Energy Materials

Adiponitrile as Lithium-Ion Battery Electrolyte Additive: A Positive and Peculiar Effect on HighVoltage System

AUTHOR NAMES. Xingyu Wang,† Wendong Xue,*† Kai Hu, † Yan Li, † Yong Li, † Rongyi Huang‡

AUTHOR ADDRESS. †

School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing 100083, China. ‡

Anhui Key Laboratory of Functional Coordination Compounds and College of Chemistry and

Chemical Engineering, Anqing Normal University, Anqing 246011, PR China. KEYWORDS. adiponitrile, LiNi0.5Mn1.5O4 half cells, carbonate electrolyte, high voltage, oxidative decomposition.

1 Environment ACS Paragon Plus

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

ABSTRACT.

Due to its novel property and unique utility, nitriles are attractive as an additive to lithium-ion battery electrolytes. However, when it is applied to high-voltage batteries, the effects and mechanisms are not clearly explained. In particular, we need to for explore its mechanism. In this work, adiponitrile (ADN) has been employed as the additive in electrolyte of 1 M LiPF6EC/DMC/EMC (1:1:1 by weight). The cycling tests for LiNi0.5Mn1.5O4 half cells after 150 cycles at 1 C (1 C=147 mA/g) from 3.5 to 5.0 V show that adding 1 wt% ADN into electrolyte can improve the capacity retention of the battery from 69.9% to 84.4%. Moreover, the rate performance can also be significantly improved. Based on the EIS measurement, it shows that a little of ADN can stabilize the interfacial impedance avoiding the possible increase during the cycling. To further clarify its mechanism, XRD, SEM, XPS measurements and DFT calculation have been conducted, which display that when adding it into the liquid electrolyte, the cathode particles maintain good spinel shape and the molecules group of ADN-S tend to be oxidized primarily to form a very thin film on cathode surface. All these results indicate that ADN has potential applications in high performance electrolyte for storage system.


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TEXT 1. Introduction Since the lithium-ion batteries (LIBs) were successfully developed by SONY, it has been applied as an energy-storage instrument in many fields such as mobile phone, laptop and even the automobile, due to its advantages of high working voltage, high energy density and long cycling life1-3. Especially the electric vehicle (EV) and hybrid electric vehicle (HEV) industry develop rapidly now, where the LIBs are considered as the main energy storage source4. Clearly, it can predict that the demand for LIBs of higher energy density will continue to grow. To improve the energy density, efforts have been made to develop cathode materials with the high specific capacity density or higher working voltage up to 5 volts. However, there is a serious problem that the commercial electrolyte commonly used in the LIBs cannot match the high voltage cathode materials well, that is to say, as the working voltage of LIBs is above 4.5V, the EC-based binary or ternary-solvent electrolyte begins to decompose in the surface of cathode material particles5-6. Especially in the high voltage situation,

, a

strong Lewis acid produced by LiPF6, is considered as the initiator to induce the cyclic carbonate EC to cleave its ring and polymerize. Besides, it is believed that the reaction of solvent-solvent molecular group causes the passivated layers on cathode surface7-8. Nowadays, the researchers have found two ways to solve the above problems: one is to use the more stable co-solvent which can perform better under high working potential; the other is to put less film-forming additives into the electrolyte. The former solution is to add some oxidationresistant solvents whose value of HOMO is lower such as sulfones9-12 nitriles13-16 and fluoro solvents17-21. In this way, the potential windows of these solvents could be broadened. However,



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when adding these materials as co-solvents, develops some new problems such as increase of viscosity, decrease of conductivity and poor compatibility with the graphite negative electrode22. Thus, recently researchers are more willing to develop suitable additive of which the quantity is less 5%. Many functional additives, such as tris (pentafluorophenyl) phosphine (TPFPP)

23

,

tris(trimethylsilyl)phosphite (TMSP)24, dimethyl phenylphosphonite (DMPP)25, phenyl trifluoromethyl sulfide (PTS)26 and lithium bis(2-methyl-2-fluoromalonato)borate (LiBMFMB)27 have been developed which ensure the better performance of LIBs under high working voltage. On the same condition, there exists a similar mechanism on the electrode in which these additives are preferentially oxidized forming a protective film on the surface of the cathode material particles to reduce the oxidation of electrolyte through avoiding the contact of carbonate-based solvent with the cathode material particles. The nitrile as additive for the high voltage electrolyte have been investigated 28-31. Because of the existence of –CN, researchers propose that it could eliminate trace water in the electrolyte and complex the metal ion to stabilize interface characteristic. In some reports, the cathode particles were covered with a film when adding the nitriles into electrolyte. However, there is no clearly explanation for this result, because theoretically the HOMO of ADN is lower than other carbonate solvents, leading to the higher oxidation potential. How does it work actually in this situation? For further understanding, we put 1 wt % adiponitrile (ADN) into the EC-based carbonate solvents to explore the impact of ADN on the LiNi0.5Mn1.5O4 half cells through various measurements and try to explain its mechanism with the help of Density functional theory (DFT) calculations. 2.Experimental


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2.1 Electrolyte preparation and cells assembly The ethylene carbonate (EC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC) and Lithium hexafluorophosphate (LiPF6) were purchased from Xianghe Kunlun Chemicals Co., Ltd., China. The adiponitrile (ADN) of which purity is above 99% was obtained from Rhawn, the reagent brand of Shanghai Rin Technology Development Co., Ltd, and LiNi0.5Mn1.5O4 was provided by the Shenzhen Kejing Star Technology Co., Ltd. 80 wt% LiNi0.5Mn1.5O4 particles, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) were dissolved into N-Methyl pyrrolidone (NMP) to prepare the LiNi0.5Mn1.5O4 electrode. Then two kinds of electrolyte (experimental group and control group) were prepared by adding 1 M LiPF6 into the mixed solvents of EC/DMC/EMC (1:1:1, by weight) with and without 1 wt% ADN. Finally, microporous membrane (Celgard 2400) was used as separator to assemble the 2025-type LiNi0.5Mn1.5O4/Li coin cells. The two processes above were done in the argon atmosphere glove box where both the content of O2 or H2O is below 0.1 PPm. 2.2 Electrochemical measurements All the electrochemical performances were measured by using the CR2025 coin cells. After assembling the cells well, they were placed in the argon atmosphere glove box for 12 h, and taken out to perform the first charge-discharge and cycle test with 0.1 C, 0.2 C, 0.5 C and 1 C (1 C=147 mA/g) on a LAND CT2001A test system (Wuhan, China) at room temperature and the voltage of all tests was from 3.5 V to 5 V. The electrochemical impedance spectroscopy (EIS) of the coin cell (CR2025) before and after cycles in which lithium foil was used as anode and reference electrode with stainless shell



as working electrode was performed through a CHI660E (Shanghai, China) from 0.1 HZ to 100 kHZ with the amplitude of 5 mV. 2.3 Physical characterization The morphological research of cathode before and after discharge was made on the Qutanta FEG 450 Field-Emission Environmental Scanning Electron Microscope (FEI, American) which can reach an accelerating voltage of 20 kV. And the crystal structure of LNMO electrodes before and after cycles was measured by X-ray diffraction (XRD, BRUKER D8 ADVANCE, Germany) operated at 60 KV and 80 mA using Cu Kα radiation in the 2θ range of 10-90°. Finally, the surface composition analysis of the cathode electrode was carried on the X-ray Photoelectron Spectroscopy (AXIS Ultra DLD, Japan) with a focused monochromatized Al Kα radiation (hυ = 1486.6 eV) under ultra-high vacuum. Its results were analyzed by the software XPSPEAK41. The cathode electrodes of all tests above were disassembled from the coin cells and washed with DMC three times to clean out the residual electrolyte in the argon atmosphere glove box. Before physical characterization the electrode was kept in the vacuum condition to prevent its oxidation. 2.4 Theoretical calculations In our work, all of the calculations were performed by Gaussian 09 package 32, with Density functional theory (DFT) as method. The equilibriums were optimized through B3LYP method along with the 6-311++G (d) basis set. In order to confirm each optimized stationary point, frequency analyses with the same basis set were done. The charge distribution was analyzed through Chelpg theory. Furthermore, to investigate the effects of solvent, polarized continuum


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models (PCM) was used to optimized structures with acetone as the solvent whose value of dielectric constant is 20.5. 3. Results and discussion 3.1 Electrochemical performance of LNMO with and without ADN Figure 1 shows the cyclic performance of half cells with and without 1% ADN at room temperature. From Figure 1a it can be seen that the LNMO with 1% ADN (experimental group) performs a higher initial discharged capacity (136 mAh/g) than the control group (without ADN, 128 mAh/g) at 1C. After 150 cycles, a similar result occurs in which the discharge capacity retention of experimental group is 84.4% (114.8 mAh/g) better than the control group (69.9%, 89.5 mAh/g). As to the coulomb efficiency, LNMO cells with ADN always keep a high value of nearly 95%, while the value of control group in initial cycles is only about 70%, then increases to 90% with the more cycling which is shown in Figure 1b. Especially, the initial charge capacity of control group reaches 187 mAh/g which is higher than the theoretical reversible capacity (146.7 mAh/g). It demonstrates that there exits continuing side reaction (forming-film react, possible) in the LNMO electrode without ADN. Therefore, it can be initially proved that less ADN has a positive effect on the LNMO electrode. To explore the side reaction of two systems in the initial cycles, their first charge-discharge and correspond differential capacity curves (dQ/dV) are shown in Figure 1c and 1d. It can be seen that in the first charge process above 4.8V there exists an increment of 40 mAh/g which can lead to the loss of irreversible capacity. In contrast, the first charge and discharge capacity of the experimental group is approximate to be 150 and 133 mAh/g, and the charge capacity do not increase obviously from 4.8 to 5.0 V. Figure 1d shows the dQ/dV curves of two systems



corresponding to the firs cycle in which two peaks of charge process of two systems both appear around 4.7 V and 4.8 V, corresponding to the oxidation reaction of Ni2+/Ni3+ and Ni3+/Ni4+. However, in the control group, several peaks occur in the area of higher voltage, demonstrating that there still exists the sustained side reaction above 4.8 V. As for the reduction, peaks of two systems occur around 4.7 V. Unlike the control group, the difference value of experimental group is smaller which shows the reversibility of LNMO with ADN is better than LNMO without ADN. Meanwhile, the rate capability performance of two systems in different rates (0.1C,0.2 C 0.5 C and 1 C) at room temperature is shown in Figure 1e. The capacity of both two systems decreases with the increase of current density, which is a normal circumstance of LIBs. While in all rates, the capacity of experimental group is clearly higher than the control group. Finally, Figure 1f shows the discharge voltage platform of two systems after 20 cycles at 0.1 C and room temperature. It could be seen that the voltage of experiment group is higher than the control group and the value of control group drops down with cycling. In contrast, the voltage of experiment group is very stable. Therefore, the above analyses of cyclic performance demonstrate that adding slight ADN into the commercial carbon-based electrolyte can depress the side reaction of electrolyte on the cathode surface, restrain the polarization in charging process and improve the rate capacity of LNMO half cells at room temperature. The EIS trends and equivalent circuit of two systems before and after cycles (fully discharged) are shown in Figure 2, from which it is observed that the curves of two systems are similar both consisting of a semicircle and slope line. The R1 of two systems is approximate to be about 5 Ω and it stands for the impedance of electrolyte. The semicircle at medium to high frequencies can be assigned to Li+ migration through surface film of the articles and charge


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transfer reaction33. Thus, R2 represents the interfacial impedance containing SEI and charge transfer resistances. The slope line at low frequencies is ascribed to the Li+ diffusion in the bulk of LNMO phase, relating to the Warburg impedance. Similarly, after activation of the two systems, the interface impedance decreased slightly, indicating that the SEI film is formed on the positive electrode interface of the two systems after 5 cycles. As the number of cycles increases, the interface impedance of the system without ADN increases continuously, and the impedance increases to 1300 Ω after 50 cycles, which we think is related to the continuous oxidation film formation. However, the impedance of the system containing ADN maintains around 380 Ω, which is related to the stable film formed on the surface of particles. So, the EIS measurement results demonstrate that adding less ADN into the EC-based electrolyte have a positive influence on its resistance and it can inhibit the oxidative decomposition reaction of carbonate solvents on cathode particles surface and keeps the impedance stabilized. 3.2 Morphology and composition analysis of LNMO with and without ADN To investigate the effect of ADN on the particles-electrolyte interface reaction, scanning electron microscope (SEM) and X-ray Photoelectron Spectroscopy (XPS) measurements were applied in the fresh, before and after cycling LNMO electrode. Figure 3 shows the SEM images of three kinds of LNMO electrode. Figure 3a exhibits the morphology of fresh LNMO electrode, and as can be seen that the particles distribute evenly on the surface of fresh electrode. The LNMO particles, whose surface is smooth and clean, present a typical octahedral spinel shape, no membrane forming. Then the same measurement to LNMO electrode without ADN after cycles in Figure 3b shows that on electrode surface there exists a thick film which is formed during the process of side reaction. The formation of film causes the loss of electrode capacity and increment of resistance which is consistent with the analysis results of cycling and EIS



measurements. The spinel shape of some particles is destroyed possibly because the HF obtained from decomposition of LiPF6 can dissolve the metal ions. A bit differently, a similar result is not observed in surface of LNMO electrode with ADN as shown in Figure 3c. Instead the surface of the particle is covered with a thin film and the particles maintain the spinel structure, which may be caused by the ADN-involved reaction. In conclusion, the SEM result demonstrates that the common carbonate solvents are easy to decompose in surface of cathode particles when the working voltage is above 4.5 V. However, less ADN can suppress the decomposition of carbonate solvents to keep its stability and is likely to participate in the film-forming reaction. In order to verify the destruction of cathode particles after cycling we carried out the measurement of XRD whose result is shown in Figure 4. In this result we get the conclusion that there exits two main peaks in the spectra of aluminum current collector, also appearing in fresh and cycled electrode. In addition to this, there are there main peaks in 18.78°, 36.44° and 44.31°, respectively corresponding to (111), (311) and (400) of LNMO cathode particles. Nevertheless, the peak intensity of electrode after cycling without ADN is lower than the other two, especially for (111). Actually, the dissolution of LNMO particles appears on (111) surface easier34. So, this results reveals the LNMO particles after cycling without ADN suffer destruction however the other two keep more stable structure. Then we explored the surface components of electrode, and the interfacial component of three kinds of LNMO electrode before and after cycles was measured through XPS, which is shown in Figure 5. First, the N1s spectra of LNMO electrode with ADN contains two main peaks responding to the –CN (399.7 eV) and C-N=C (400.6 eV) respectively and no obvious peaks


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appear in the fresh and control electrode. The result confirms the less ADN participates in the film-forming indeed and the composition of thin film in the surface contains –CN and C-N=C. Then for the C1s and O1s spectra several main peaks are detected in three systems. The C=O (289.4 eV,533.7 eV) and C-O (286.3 eV,532.2 eV) bond in C1s and O1s spectra respectively correspond to the ROCO2Li or ROCO2R’35 which are the decomposition product of electrolyte. The peak of C=O and C-O bond in electrode without ADN is much weaker than that with ADN which demonstrates less ADN can suppress the decomposition of EC-based electrolyte consisting with the above electrochemical analysis results. Besides, in the spectra of electrode without ADN the C-F bond (PVDF), C-C bond (acetylene black) in C1s and Metal-O bond in O1s is also weaker than that with ADN, due to a thick membrane covering on the surface of particles. As for the F1s spectra, except for the C-F bond (PVDF) there are two peaks around 684.6 eV and 686.9 eV which correspond to the LiF and LixPOyFz both obtaining from the hydrolysis of LiPF6. The difference in peak area explains that less ADN can depress the hydrolysis of LiPF6. 3.3 DFT calculations and the explanation of the mechanism Density function theory was performed in previous researches to explore the reaction mechanism of solvents molecules, such as verifying the priority oxidation of various additives near the interface. So, in our work we also apply it to the analysis of mechanism. Many researchers have explained nitriles’ effect of stabilizing electrolyte/electrode interface in former reports,29,30,36 which made good results. Unlike the oxidation analysis of a single molecule, we analyzed the interaction between adiponitriles and other solvent molecules which is necessary and needs to calculate the oxidation potential (EOX) of related substances. The equation is given below:



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(1) Where G(M) and G(M+) is the free energy of solvated complex M and its oxidation at 298.15 K respectively, F is the Faraday constant and the subtraction of 1.4 V is used to convert from the absolute electrochemical scale used in calculations to the experimentally measured potential vs. Li+ /Li. First, we got the optimized structure and charge distribution of several solvent molecules as shown in Figure 6. For the ADN molecule, the C in the -CN group shows a strong positive charge, resulting in a high activity of the H atom on the C atom adjacent to the -CN group. Further, the C=O group in the carbonate solvent exhibits strong electronegativity. Therefore, in the mixed solution the ADN and the carbonate-based solvent easily form the complex molecule ADN-S, which is different from the case of a pure carbonate solvent. To estimate its interaction with other molecules, we obtain the oxidation potential and optimized structure of different groups ( seen in Figure 7. From it we conclude that

-S, S=EC, EMC and DMC) and the result can be could lower the oxidation potential of the system

causing that the complex is easier to be oxidized. Meanwhile during the reaction, carbonate molecules react with the

to produce HF which can destroy the spinel structure of LNMO

causing the loss of capacity. It is consistent with the above analysis. On the other hand, the impact of solvent molecules on each other is also significant. So, we have estimated the oxidation potential and optimized structure of ADN-S groups (S=EC, EMC and DMC). As shown in Figure 8, when considering the interaction of ADN and carbonate molecules, the different charge regions of the two molecules will attract each other, causing the alkyl group adjacent to -CN in ADN to be close to the C=O group of the carbonate. And the


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distance between H on the alkyl group and O on the C=O is about 2.4 Å. In contrast to

-S,

when the ADN-S complex group undergoes an oxidation reaction, H near the C=O group is captured by the C=O group. No HF is produced in the process and the oxidation potential of this group is relatively low, indicating that the carbonate solvents are more easily oxidized after the addition of ADN. So, in this situation ADN-S groups tend to be oxidized primarily and further polymerized on the surface of cathode. Finally summarizing above achievements, when adding ADN the decomposition of LiPF6 is suppressed and the complex groups are easier to be oxidized and participate in the film formation reaction. The thin film on cathode surface containing the -CN-R group prevents continuous polymerization reactions which always occur in the system without suitable additives.

Conclusions The spinel LiNi0.5Mn1.5O4 is considered as the promising high voltage cathode. However, the commercial electrolyte does not suit with it, due to the instability of electrode/electrolyte interface. In this work, 1 wt % ADN is added into the 1M LiPF6-EC/DMC/EMC (1:1:1 by weight) electrolyte. The charge-discharge measurement showed that less ADN can improve the capacity retention from 69.9% to 84.4% after 150 cycles at 1 C and strengthen its rate performance. Especially, the EIS result demonstrates that it can stabilize the interfacial impedance possibly through suppressing the oxidative decomposition of electrolyte. Then, it is confirmed by the result of SEM and XPS measurements where a thick film is observed and much oxidative decomposition product is detected in the LNMO electrode without ADN. However, the similar



result does not occur in surface of electrode with ADN. Instead, the system containing ADN has a more stable impedance and forms a thinner film on the surface of the particles. Besides, XRD result demonstrates that ADN can suppress the generation of HF, thus preventing dissolution of metal ion and damage of cathode structure. Finally, through the DFT calculations we conclude that the complex of ADN-S can be oxidized on the cathode particles surface primarily, forming a very thin layer on surface and preventing the continuous contact with electrolyte. Corresponding Author E-mail address: [email protected] (Wendong Xue*). ORCID Wendong Xue: 0000-0002-7539-7001 Xingyu Wang: 0000-0002-5408-601X ACKNOWLEDGMENT The authors acknowledge that this work was supported by National Key Research and Development Plan (2016YFE0111500), National Natural Science Foundation of China (Grant no. 21674011) and Beijing Natural Science Foundation (2172040)

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FIGURES

Fig.1. The electrochemical performance of two kinds of LNMO/Li half cells at room temperature: cyclic performance at 1C (a and b), first charge-discharge and dQ/dV curves at 0.1C (c and d), rate performance (e) and discharge voltage platform at 0.1C (f)

Fig.2. The electrochemical impedance spectroscopy of two kinds of LNMO/Li half cells at room temperature: with ADN(a); without ADN(b)


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Fig.3. The SEM result of three kinds of LNMO electrode at room temperature: fresh LNMO electrode (a), LNMO electrode after 20 cycles at 0.1C without ADN (b) and LNMO electrode after 20 cycles at 0.1C with ADN (c)



Fig.4. The XRD patterns of Al current collector,fresh and cycled LNMO electrodes with and without ADN.

Fig.5. The XPS spectra of fresh and cycled LNMO electrodes with and without ADN.


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Fig.6. The optimized structure and Chelpg charge distribution of isolate ADN, EMC, DMC,EC and

Fig.7. The optimized structure of

-S before and after oxidation and its correspond oxidation potential.


molecule.


Fig.8. The optimized structure of ADN-S before and after oxidation, together with its correspond oxidation potential and Chelpg charge distribution.

Graphical Abstract In this work, we applied common carbonate liquid electrolyte with and without 1 wt % ADN in LiNi0.5Mn1.5O4 half cells and conducted a series of electrochemical tests whose results reveal that the half cells with ADN show a better performance. To explain the above, we analyzed the property of cathode surface under the help of SEM, XRD and XPS


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measurements, finding that cathode surface without ADN is covered by a thick film and its spinel structure is destroyed by HF generated in side reaction, however, there exists a very thin film on the surface and particles maintain good structure in system with ADN. A highlight in our experiment is that we try to explore the interaction between ADN and other solvent molecules and demonstrate the possibility of preferential oxidation of them with the help of DFT calculation and the analytical result is acceptable.


Adiponitrile as Lithium-ion battery electrolyte additive: a positive and

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