Isophorone Diisocyanate: An Effective Additive to Form Cathode

Mar 16, 2018 - In this work, we propose a novel electrolyte additive, isophorone diisocyanate (IPDI), to construct the surface protective interlayer. ...
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Surfaces, Interfaces, and Applications

Isophorone diisocyanate: an effective additive to form cathode-protectiveinterlayer and its influence on LiNi0.5Co0.2Mn0.3O2 at high potential Yang Liu, Dandan Sun, Jingjing Zhou, Yingping Qin, Deyu Wang, and Bingkun Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00011 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 17, 2018

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Isophorone diisocyanate: an effective additive to form cathode-protective-interlayer and its influence on LiNi0.5Co0.2Mn0.3O2 at high potential Yang Liu†, Dandan Sun†, Jingjing Zhou†, Yinping Qin*‡, Deyu Wang*‡, Bingkun Guo† †Materials Genome Institute, Shanghai University, Shanghai, 200444, China. ‡Ningbo Institute of Materials Technology and Engineering, Chinese Academy of China, Ningbo, 315201, China

KEYWORDS: isocyanate, freeradical-onium ion, nucleophilic addition, surface layer, lithium ion electrolyte

ABSTRACT

In this work, we propose a novel electrolyte additive, isophorone diisocyanate(IPDI), to construct the surface protective interlayer. This membrane is produced via nucleophilic addition between the IPDI’s diisocyanate groups and the freeradical-onium ion oxidative intermediate of propylene carbonate(PC). In the electrolyte with IPDI added between 10 ~ 20 mM, LiNi0.5Co0.2Mn0.3O2 presents the excellent performance, demonstrating the relative wide operational window to form the optimal protective membrane. This protective membrane ameliorates the cyclic stability. Although all systems deliver ~ 185 mAh g-1 under 1 C between 2.5 - 4.6 V (vs. Li+/Li), the cells in the suitable electrolyte maintain 90.4 % in the 50 cycles and

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83.2 % in the 200 cycles, whereas the control cells are seriously dropped to 73.4 % and 69.8 %. The cells in the electrolyte with the appropriate IPDI also present the good rate capability, attaining ~ 143 mAh g-1 under 5 C, much higher than the cells in the control electrolyte(92.4 mAh g-1). The additive proposed in this work is helpful to augment the energy density of lithium ion battery and prolong the one-drive distance of electric vehicles.

Introduction Lithium ion batteries are the most promising energy storage technology, which has been widely utilized in portable electronics, vehicles and grids.1 Besides novel materials, the high-voltage electrolytes play the crucial role on meeting the ever-increasing demands of energy density.2 The widened working windows of electrolytes are capable to utilize cathodes at high redox potentials directly, such as Li-Rich & LiNi0.5Mn1.5O43, 4, and enhance the practical capacity of the conventional cathodes including LiCoO2 and LiNixCoyMn1-x-yO25, 6. Therefore, developing novel electrolytes attracts the tremendous attentions from academic community and industry. To positively broaden the working potential, the anti-oxidative solvents, such as sulfones and fluorine-contained solvents7-10, have been introduced into the electrolyte system firstly.11-13 Their incorporation obviously widens the electrolytes’ working window ~ 0.2 - 0.3 V. Generally, the adsorption principle is the working mechanism of these solvents. Namely, the solvents or decomposed fragments accumulate on cathode particles to form the dielectric layer and stabilize the solvents in the electrolyte side. However, the absorbed film would be affected by the working temperature or other inside environments easily.14

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Inspired by the role of SEI membrane, the cathode protective interlayer should be the effective remedy to protect the solvents from decomposition. Since most of the decomposed products from solvents are instable,15 the appropriate materials are intensively explored to form the surface protective layer. Till date, the ceramics, such as oxides and phosphates,16, 17 are widely used to inert the cathodes’ sensitive surface and improve their stability. Also aromatic derivatives, which are used as the anti-overcharge additive by in-circuit short, could form a dense thin layer on cathode with the appropriate concentration.18, 19 Other additives such as borates20, 21,

phosphates22, phosphites23, and nitriles24 have been investigated in recent years too. But the building of a stable surface film on the cathodes is still a challenge at high potential. In our previous work, a novel strategy was developed to construct a stable interface film by hexamethylene diisocyanate(HDI) and PC via an onium ionization-polymerization.25, 26 The amide-based polymer layer, formed by the freeradical-onium ion oxidative intermediate of PC and HDI, could be stable at 5.2 V(vs. Li+/Li) and suppress the electrochemical process on the surface of LiNi1/3Ni1/3Mn1/3O2 in cyclic voltammetry. Compared with other systems containing larger concentrations of the additives, the cell’s performance is obviously improved with 1 mM HDI added, namely the characteristics of polymer membrane is very sensitive to HDI amount. Therefore we screen the diisocyanate chemicals to expand the concentration region for the better handling in practical application. In this work, we shall propose the performance of isophorone diisocyanate, IPDI, which is a more suitable additive to form cathodic protective membrane in the base electrolyte (PC + DMC, v:v, 1:1, 1 M LiPF6 added). Considering the bridging methyls near N atom could lower the nucleophilic ability of -NCO and the freeradical-onium ion’s half-life of oxided-PC,32, 33 the

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thickness of the IPDI-formed-film would not as sensitive to isocyanate’s concentration as that of HDI. Probably due to the aforementioned reason, LiNi0.5Co0.2Mn0.3O2 presents the excellent cyclic and rate capability between 10 ~ 20 mM with the working potential up to 4.6 V(vs. Li+/Li).

Results and discussion The cathode protective membrane formed IPDI in PC based electrolyte was produced by charging the electrolyte with 500 mM IPDI at 4.6 V (vs. Li+/Li) for 3 hours with LiNi0.5Co0.2Mn0.3O2 as cathode. Observed with TEM, an interfacial film is formed on the cathode particles, as shown in Fig.1b&c. This film is only ~ 0.9 nm, which is much thinner than the same electrolyte with HDI additive operated in the same condition (7 nm). While only 100 mM IPDI is added, there is much less stuff could be detected on LiNi0.5Co0.2Mn0.3O2(Fig.S1a). When the concentrations of IPDI decrease to 20 and 10 mM, the surfaces of the cathodes keep almost as smooth as the raw ones(Fig.S1b&c). If the sediments is uniformly deposited on the electrode, the thickness of the surface layer should be ~ 0.18 nm, 0.36 Å and 0.18 Å in the systems containing 100, 20, 10 mM IPDI. X-ray photon spectroscopy(XPS) was utilized to investigate the composition of the surface layer. As shown in Fig.1d, the nitrogen signal is detected at 400.6 eV, corresponding to the amide group, instead of isocyanate. The Fourier-transform infrared spectroscopy(FtIR) has also been taken to identify the groups formed during polymerization(Fig.S2). While IPDI is added, a new peak is found at 1771 cm-1 which should be attributed to the amide group too. Both

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of XPS & FtIR suggest the -N=C=O should have been changed to -NH-C=O during polymerization.

Fig.1. TEM(a, b, c) and XPS(N1s, d) measurements of LiNi0.5Co0.2Mn0.3O2 electrodes raw and charged at 4.6 V(vs. Li+/Li) for 3 hours in different electrolytes.

According to the spectrum mentioned above and literatures25, 26, the reaction mechanism is illustrated in Scheme S1. A PC molecule would be oxidized to be a an oxygen freeradical -

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onium ion on the surface of the cathode during charging, then the polymer A would be formed by the peroxide groups and amide groups which are generated from the dimerization between freeradicals and the nucleophilic addition between the charged methines and isocyanate groups of HDI. Because of the instability of peroxide group, we conjecture the amide-based polymer B as the final product which is transferred from polymer A via decomposition-reaggregation. Moreover, our results demonstrate that the kinetics of this reaction could be tailored by the bridge groups. In IPDI molecule, the carbon atom neighboring isocyanate group connects more methylenes than that in HDI, which could augment the outer electron cloud density of N atom and reduce the nucleophilic activity of –NCO. Hence, the improved uniformity of sediments and reduced yield should be attitude to the decreased polymerization rate of IPDI & PC, which affected by the lowed react activity of isocyanate group.

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Fig.2. SEM images of LiNi0.5Co0.2Mn0.3O2 electrodes charged at 4.6 V(vs. Li+/Li) for 3 hours in base(a) and IPDI added(b) electrolytes.

Furthermore, we also noticed that this membrane may play a large role to keep secondary particles from breaking. As shown in Fig.2b, the secondary particles of cathode materials keep a smooth surface likes to the raw LiNi0.5Co0.2Mn0.3O2(Fig.S3) without obvious defects in the investigated electrolyte, whereas the particles clearly cracked after polarization in the base electrolyte(Fig.2a). This phenomenon should be attributed to the protection of the IPDI-film and indicates that the particles’ cracking of ternary oxide should be related to the electrolyte corrosion.29, 30 The particles’ entirety may be another reason to prolong the operational life of cathodes, as well as the suppression of electrolyte decomposition.

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Fig.3. The resistances of cathodes after being charged at 4.6 V(vs. Li+/Li) for 3 hours in different concentrations of IPDI (a), comparison of the initial charge and discharge curves at 0.2 C(b), cycled at 1 C(c), rate performances(d) of LiNi0.5Co0.2Mn0.3O2 in base electrolytes with and without IPDI added between 2.5 - 4.6 V(vs. Li+/Li).

The electrochemical properties of the interfacial film were further studied in Fig.3. Fig.3a displays the effects of the surface films on the resistance of cathodes eliminating the influences of electrolytes. Testing by a semiconductor parameter analyzer with Ф 50 nm probes, the resistance of LiNi0.5Co0.2Mn0.3O2 polarized in base electrolyte is 9.4*108 kΩ, which is the normal value for the Nickel cobalt manganese ternary layer oxides.27 In the systems with 1 ~ 20 mM IPDI, the resistances of particles are lowered to ~ 2 % of the control, confirming the existence the surface membrane and improvement on surface conductivity of this membrane. As IPDI concentration further augmenting, the particle’s resistances are suddenly jumped to 1.8*108 kΩ in 50 mM electrolyte and 5.8*108 kΩ in 100 mM electrolyte, indicating that the membrane formed in both systems loses the conducting capability. The EIS(Fig.S4) and CV(Fig.S5) spectrum also show the similar results. While 10 ~ 20 mM IPDI is added, simples display lower polarization than the cell using base electrolyte. The electrochemical spectrum suggest the IPDIfilm would observably reduce the cathode’s resistances while the additive is using in the range between 10 ~ 20 mM, which is much broader than that of HDI.25 Considering the relationship between the thickness of the amide-based film with the isocyanate’s concentration and the electrochemical performances of the cells, we could speculate the thickness of the IPDI-formedfilm would not as sensitive to as that of HDI in the work.

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The electrochemical performances of LiNi0.5Co0.2Mn0.3O2 cells using electrolytes with and without IPDI are evaluated between 2.5 - 4.6 V(vs. Li+/Li). As shown in Fig.3b, the initial discharge capacities in all systems are ~ 190 mAh g-1 at 0.2 C, indicating the significant attractions of cathodes working at high potential. Furthermore, their initial charge and discharge curves almost overlapped, confirming that IPDI plays the chemical reaction to polymerize in the first cycle. The tiny additional capacities which are proportional to the concentrations of IPDI may be attributed to the wettability enhanced by isocyanate.31 The cyclic stability of LiNi0.5Co0.2Mn0.3O2 could be significantly improved by IPDI. Using the additive 10 & 20 mM, LiNi0.5Co0.2Mn0.3O2 delivers an initial capacity of ~ 188 mAh g1

and maintains ~ 170 mAh g-1 in 50 cycles, 90.4 % capacity retained and 157.4 mAh g-1 in 200

cycles with a capacity retention of 83.2 %, compared to the 73.4 % and 69.8 % of that using base electrolyte at 1 C(Fig.3c, Table S1). In the rollout charge-discharge process, the capacity of cell in base electrolyte fails much faster than the simples with additive added, which should be interpreted as the cracking of particles as shown in SEM images(Fig.2). It is suggested the appropriate concentration of IPDI is 10 ~ 20 mM for the cycling of LiNi0.5Co0.2Mn0.3O2 cells as shown in Fig.3c. While the additive < 10 mM or > 20mM is added, the cells fail much faster than the simples with IPDI 10 ~ 20 mM. The former may be presumed as a stable surface layer is not constructed for tiny amounts of additive,26 and the latter should be considered as too much precipitate is formed which increases the interface resistance of particles and decrease the opportunity for lithium ions to intercalate back into the host structure.25 The rate capabilities of LiNi0.5Co0.2Mn0.3O2 in the base and the electrolyte containing IPDI are provided in Fig.3d. The cell in the 10 mM IPDI-containing electrolyte shows excellent rate performance. When the current rate increases from 0.2 C to 5 C, the discharge capacity of

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the cell using the base electrolyte rapidly decreases from 188.3 mAh g-1 to 92.4 mAh g-1, but that of the cell with 10 ~ 20 mM remains at ~ 143 mAh g-1. When the low current rate of 0.2 C is used again, the discharge capacities of the cell with the additives can be restored better.

Fig.4. The TEM(a, b) and SEM(c, d) images of the LiNi0.5Co0.2Mn0.3O2 cathodes after 200 cycles at 1 C in different electrolytes between 2.5 - 4.6 V(vs. Li+/Li). The surface morphologies of the LiNi0.5Co0.2Mn0.3O2 cathodes after cycling are investigated in Fig.4. TEM(Fig.4a) and SEM(Fig.4c) images show the amorphous sediments on

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the cathode in base electrolyte. Fig.4c also shows the cracking of the LiNi0.5Co0.2Mn0.3O2 particles. As shown in Fig.4b&d, the surface of electrode is much smoother while 10 mM IPDI is added. Combined with evidences mentioned above and the literatures,29, 30 it could be confirmed that the IPDI-built- film would suppress the electrolyte decomposition and the cracking of ternary layer oxide particles at high potential, both of which improve the electrochemical performances of LiNi0.5Co0.2Mn0.3O2 between 2.5 - 4.6 V(vs. Li+/Li). Conclusions We have investigated the characteristics of cathode protective interlayer produced from isophorone diisocyanate, IPDI, in PC based electrolyte. This membrane not only improved the surface conductivity of cathodes, but also kept the particles from cracking. Owing to the more methylenes neighbouring the nitrogen of isocyanate groups in IPDI, the LiNi0.5Co0.2Mn0.3O2 cells presented the obviously improved performance in the electrolytes with 10 ~ 20 mM IPDI contained, which is suitable to be handled in practical application. Our approach demonstrated that diisocyanate chemicals with suitable bridge group are the effective additives to widen the operation windows of lithium ion batteries, which are helpful to prolong the one-drive distance of electric vehicles and per-charge operational time of consumer electronics.

EXPERIMENTAL SECTION LiNi0.5Co0.2Mn0.3O2 was purchased from Contemporary Amperex Technology Co., Limited, Carbonic esters and LiPF6 were purchased from Zhangjiagang Guotai-Huarong New Chemical

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Materials Co. Ltd., battery grade. Other chemicals such as isophorone diisocyanate(IPDI, 99 %) and polyvinylidene fluoride(PVdF, 99 %) were purchased from Aldrich. IPDI and carbonate esters were mixed in various volume ratios, and a concentration of 1 M LiPF6 was added in an argon-filled glovebox with the H2O and O2 content below 1 ppm to prepare the electrolytes. The base electrolyte was PC + DMC (v:v, 1:1) with 1 M LiPF6 added and used as control. The cathode was prepared by 80 wt% LiNi0.5Co0.2Mn0.3O2, 10 wt% Super P and 10 wt% PVdF, and the cell was assembled with a LiNi0.5Co0.2Mn0.3O2 cathode, a separator(Celgard 2400), a lithium foil anode and the electrolyte (50 µL) in the same argon-filled glovebox. Electrochemical impedance spectroscopy(EIS) and cyclic voltammogram(CV) were measured on an electrochemical workstation (Solartron, FRA 1455A). The cells in different electrolytes were cycled on a Land cell tester (CT2001A, Wuhan Jinnuo Company) between 2.5 - 4.6 V(vs. Li+/Li). The cycled cathodes were disassembled carefully from the cells, washed with DMC, wiped and vacuum dried in the glove box. The surface of samples were examined by scanning electron microscopy(SEM, HitachiS-4800) and transmission electron microscopy (TEM, FEI Tecnai F20), measured by X-ray Photoelectron Spectroscopy (XPS, PHI 3056) and analyzed with GASA. The powders of electrodes were pressed to pellets with KBr and tested on the Fourier-transform infrared spectroscopy (FtIR, Nicolet 6700) with a resolution of 0.02 cm-1. The i-v measurements were carried on a semiconductor parameter analyzer (Keithley 4200 SCS) at room temperature with Ф 50 nm tungsten probes.

ACKNOWLEDGMENT

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This work was supported by the National Natural Science Foundation of China (Grant No. 21503247, 51602191, 51602190), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the 111 Project (D16002) and Program of Shanghai Subject Chief Scientist(16XD1401100).

AUTHOR INFORMATION Corresponding Author These authors contributed equally: Yinping Qin, E-mail: [email protected] Deyu Wang, E-mail: [email protected]

Supporting Information Available: Table of the discharge capacities of simples in different electrolytes during cycling; speculation of the formation mechanism of the amide-based surface film; TEM images of the LiNi0.5Co0.2Mn0.3O2 electrodes in different electrolytes and SEM image of the raw electrode; FtIR spectra of the raw and charged cathodes; EIS and CV spectra of the charged simples in different electrolytes.

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(24) P. B. Hong; M. Q. Xu; D. R. Chen; X. Q. Chen; L. D. Xing; Q. M. Huang; W. S. Li, Enhancing Electrochemical Performance of High Voltage (4.5 V) Graphite/LiNi0.5Co0.2Mn0.3O2 Cell by Tailoring Cathode Interface. J. Electrochem. Soc. 2017,164, A137-A144. (25) Liu, Y.; Qin, Y.; Peng, Z.; Zhou, J.; Wan, C.; Wang, D., Hexamethylene diisocyanate as an electrolyte additive for high-energy density lithium ion batteries. J. Mater. Chem. A 2015, 16, 8246-8249. (26) Xu, X.; Qin, Y.; Yang, W.; Sun, D.; Liu, Y.; Guo, B.; Wang, D., Influence of HDI as a cathode film-forming additive on the performance of LiFe0.2Mn0.8PO4/C cathode. RSC Adv. 2017, 7, 41970-41972. (27) Matsuta, S.; Kato, Y.; Ota, T.; Kurokawa, H.; Yoshimura, S.; Fujitani, S., Electron-SpinResonance Study of the Reaction of Electrolytic Solutions on the Positive Electrode for Lithium-Ion Secondary Batteries. J. Electrochem. Soc. 2001, 148, A7-A10. (28) Kanamura, K.; Toriyama,S.; Shiraishi, S.; Takehara, Z-i., Studies on Electrochemical Oxidation of Nonaqueous Electrolytes Using In Situ FTIR Spectroscopy. J. Electrochem. Soc. 1995, 142, 1383-1389. (29) Shen C.-H.; Wang Q.; Chen H.-J.; Shi C.-G.; Zhang H.-Y.; Huang L.; Li J.-T.; Sun S.-G., In Situ Multitechnical Investigation into Capacity Fading of High-Voltage LiNi0.5Co0.2Mn0.3O2. ACS Appl. Mater. Interfaces 2016, 8, 35323-35335.

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(30) Lee Y.-M.; Nam K.-M.; Hwang E.-H.; Kwon Y.-G.; Kang D.-H.; Kim S.-S.; Song S.-W., Interfacial Origin of Performance Improvement and Fade for 4.6 V LiNi0.5Co0.2Mn0.3O2 Battery Cathodes. J. Phys. Chem. C 2014, 118, 10631-10639. (31) Wu, F.; Zhu, Q.; Li, L.; Chen, R.; Chen, S., A diisocyanate/sulfone binary electrolyte based on lithium difluoro(oxalate)borate for lithium batteries. J. Mater. Chem. A 2013, 1, 36593666. (32) Samarasingha, P.; Tran-Nguyen, D.-H.; Behm, M.; Wijayasinghe, A., LiNi1/3Mn1/3Co1/3O2 synthesized by the Pechini method for the positive electrode in Li-ion batteries: Material characteristics and electrochemical behaviour Electrochim. Acta. 2008, 53, 7995-8000.

BRIEFS: With multiple methylenes connect the carbon atom neighboring –NCO, the bridge group would tailor the kinetics of the nucleophilic addition between the isocyanate group and the freeradical-onium ion oxidative intermediate of propylene carbonate, presents relative wide operational window(10 ~ 20 M IPDI) to form the optimal protective membrane, which could protect

cathode materials from the corrosion in electrolyte and considerably improve the electrochemical performances of ternary layer oxide at high potential(~23.2 % in 50 cycles and ~ 35.4 % under 5 C).

SYNOPSIS (Word Style “SN_Synopsis_TOC”).

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