Sulfonate-immobilized artificial cathode electrolyte interphases layer on Ni-rich cathode

Journal of Power Sources 360 (2017) 480e487

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Sulfonate-immobilized artificial cathode electrolyte interphases layer on Ni-rich cathode Bum-Jin Chae, Taeeun Yim* Department of Chemistry, Research Institute of Basic Sciences, College of Natural Science, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Amphiphilic sulfonate-based organic precursor is synthesized.
Organic precursor is immobilized on NCM electrode as an artificial CEI layer.
Sulfonate-based artificial CEI layer enhances surface stability for NCM electrode.
Artificial CEI layer effectively suppresses electrolyte decomposition.
NCM modified by artificial CEI layer exhibits improved cycling retention.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 March 2017 Received in revised form 26 May 2017 Accepted 11 June 2017

Although lithium nickel cobalt manganese layered oxides with a high nickel composition have gained great attention due to increased overall energy density for energy conversion/storage systems, poor interfacial stability is considered a critical bottleneck impeding its widespread adoption. We propose a new approach based on immobilizing the artificial cathode-electrolyte interphase layer, which effectively reduces undesired surface reactions, leading to high interfacial stability of cathode material. For installation of artificial cathode-electrolyte interphases, a sulfonate-based amphiphilic organic precursor, which effectively suppresses electrolyte decomposition, is synthesized and subjected to immobilization on cathode material via simple wet-coating, followed by heat treatment at low temperature. The sulfonate-based artificial cathode-electrolyte interphase layer is well-developed on the cathode surface, and the cell controlled by the sulfonate-immobilized cathode exhibits remarkable electrochemical performance, including a high average Coulombic efficiency (99.8%) and cycling retention (97.4%) compared with pristine cathode material. The spectroscopic analyses of the cycled cathode show that the sulfonatebased artificial cathode-electrolyte interphase layer effectively mitigates electrolyte decomposition on the cathode surface, resulting in decreased interfacial resistance between electrode and electrolyte. © 2017 Elsevier B.V. All rights reserved.

Keywords: Lithium ion battery Electrode Artificial cathode-electrolyte interphases Organic precursor Sulfonate

1. Introduction As demand increases for large-scale devices such as energy

* Corresponding author. E-mail address: [email protected] (T. Yim). http://dx.doi.org/10.1016/j.jpowsour.2017.06.037 0378-7753/© 2017 Elsevier B.V. All rights reserved.

storage systems (ESSs) and electric vehicles (EVs), high specific capacity-based electrode materials are gaining attention as candidates for increasing the energy density of lithium-ion batteries (LIBs), to allow for longer driving ranges and operating times in energy applications [1e5]. Among these materials, layered lithium nickel cobalt manganese oxide (Li(NixCoyMnz)O2, NCM) with Nirich composition (x  60%) is particularly notable because its

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high specific capacity (>180 mA h g1) provides a great opportunity to increase the overall energy density of the cell compared to commercial layered lithium cobalt oxide (LiCoO2, ~150 mA h g1) [6e8]. However, Ni-rich NCM materials still suffer from poor cycling performance [9e11]. This is attributed to the poor interfacial stability of Ni-rich NCM; electrolyte decomposition (via electrochemical oxidation of solvents) is significantly accelerated on the interface between electrode and electrolyte owing to high reactivity of Ni species, and the resulting decomposed adducts continuously accumulate on the electrode surface [12e15]. This increases interfacial resistance because accumulating adducts severely disturb Liþ migration between electrode and electrolyte, leading to rapid fading of cycling performance. Moreover, the electrochemical decomposition of electrolytes accompanies gaseous products, which causes severe swelling of the cell. Therefore, improving the surface stability of Ni-rich NCM materials is indispensable for ensuring the cycle-life and safety performance of these cells. One effective way to alleviate electrolyte decomposition on Nirich NCM electrodes is the use of functional additives in the electrolytes [16e20]. This approach is based on the formation of a stable cathode electrolyte interphase (CEI) layer on the electrode surface via the electrochemical reactions of the functional additives dissolved in the electrolytes. Once the functional additives are oxidized by electrochemical reactions, there are physically separated CEI layers on the electrode surface, allowing Liþ migration but preventing electron transfer between electrode and electrolyte, resulting in effective minimization of electrolyte decomposition [21e23]. This is an efficient and convenient approach to improve interfacial stability of Ni-rich NCM material. However, the use of functional additives in the cell is restricted if they irreversibly decompose on the anode surface due to the electrochemical reduction process. With these considerations, we propose an artificial CEIimmobilized Ni-rich cathode material with a high Ni composition (80%, Li(Ni0.8Co0.1Mn0.1)O2, NCM811) to improve electrochemical performance (Fig. 1). In this work, the sulfonate (SO3)-based organic CEI layers were first immobilized on NCM811 material by a simple wet process, followed by thermal treatment using an organic precursor. For embedding the artificial CEI layers, the SO3based organic CEI precursor was carefully designed and synthesized. It is well-known that the SO3 functional group plays a key role in suppressing electrolyte decomposition, resulting in improved interfacial stability and cycling performance [24e28]. In addition, synthesized SO3-based organic CEI precursor has an amphiphilic character, and we anticipated that this would allow it to be uniformly absorbed on the target substrate, resulting in uniform CEI coverage on the NCM811 electrode. To the best of our knowledge, this is the first attempt to develop an SO3-based, artificial CEI-immobilized Ni-rich cathode. We believe that these material strategies and a comprehensive understanding of the role of immobilized CEI layers, based on systematical analyses, will create further opportunities to build more-advanced LIBs.

evaporated under vacuum and the resulting crude products were purified by decantation several times, using ethyl acetate (Daejung) and diethyl ether (Daejung), to remove any remaining organic impurities. The purified solid product was dried in a vacuum oven for 24 h. To confirm the chemical structure of the N,N-dimethylpyrrolidinium methyl sulfonate, it was analyzed with nuclear magnetic resonance spectroscopy (NMR) using deuterated acetone as the NMR solvent. Typical procedure for wet-coating and heat treatment process. For wet-coating of NCM811, N,N-dimethyl methyl sulfate (0.5, 2.5, 5.0 g) was completely dissolved in 50 mL of N-methyl-2pyrrolidone (NMP, Aldrich) at room temperature. 5.0 g of NCM811 was then placed in the coating solution and stirred for 1 h. The precipitated solid was collected by filtration, and the wet-coated NCM811 was subjected to heat treatment under atmospheric pressure. The temperature was elevated from room temperature to 600  C at a rate of 1  C min1, maintained there for 3 h, then lowered to 25  C at a rate of 1  C min1. The surface morphology of the modified NCM811 was characterized with a field-emission scanning electron microscope (FESEM, JSM-7001F, JEOL) and the chemical compositions were measured with Fourier-transform infrared spectroscopy (FT-IR, VERTEX 70, Bruker) in the attenuated total reflectance (ATR) mode. A structure analysis of the modified NCM811 was carried out with X-ray diffraction (XRD, PANalytical) equipped with monochromatic Cu Ka radiation (l ¼ 1.54056 Å). Cathode preparation, evaluation of electrochemical performance, and analysis of cycled electrode. The cathodes were prepared as follows. A mixture of NCM811 or S-NCM811, poly(vinylidene fluoride) (PVDF) (KF3000, Kureha), and carbon black (Super P) at a ratio of 90:5:5 (weight %) was dispersed in the NMP and agitated for 3 h. The cathode slurry was coated onto aluminum foil and dried overnight in a vacuum oven at 120  C. The loading density of the cathode was approximately 9.50 mg cm2. To evaluate cycling performance, 2032 coin-cells were fabricated with a cathode (NCM811 or S-NCM811), a Li-metal anode, a poly(ethylene) (PE) separator (Celgard), and electrolytes (EC:EMC ¼ 1:2 þ 1 M LiPF6, PanaxEtec). The cells were charged to 4.3 V (vs. Li/Liþ) and discharged to 3.0 V (vs. Li/Liþ) with a 0.1 C current for two cycles (the formation step), then charged/discharged with a 1.0 C current (180 mA h g1) for 50 cycles at room temperature on a charge/ discharge unit (WBCS3000, Wonatech). After the cycling performance, the cells were disassembled in a glove box controlled under Ar atmosphere and the cycled cathodes were washed with dimethyl carbonate. The surface morphology of each electrode was measured with SEM, and the chemical components that developed on the cycled electrode surface were analyzed with X-ray photoelectron spectroscopy (XPS, K alpha, PHI 5000 versa Probe II) under N2 atmosphere. The electrochemical impedance spectroscopy (EIS) for each cathode was examined with an AC signal at an amplitude of 10 mV over a frequency range of 1 Me10 mHz using an electrochemical workstation (Zive MP1, Wonatech).

2. Experimental details

3. Results and discussion

Synthesis of sulfonate-based CEI precursor and characterization of chemical structure. The N,N-dimethylpyrrolidinium methyl sulfonate (organic CEI precursor) was synthesized as follows. A solution of 10.40 ml of N-methylpyrrolidine (100.0 mmol, Aldrich) with 50.00 ml of acetonitrile (Aldrich) was prepared and cooled to 0  C. Next, 10.43 ml of dimethyl sulfate (110.0 mmol, Aldrich) was slowly added to the solution for 1 h and the mixture was stirred at room temperature for 72 h for the N-methylation reaction (quaternization). After the reaction was complete, the solvents were

For installation of the artificial sulfonate-based CEI layer, amphiphilic N,N-dimethylpyrrolidinium sulfate was synthesized in a one-step quaternization process, as shown in Fig. 1 (details are provided in the experimental section), and its chemical structure was characterized by NMR spectroscopy (Fig. 2 and Fig. S1). The spectroscopic evidence for quaternization was easily confirmed by 1 H NMR spectroscopy showing that the 1H-peak for the methyl group (CH3) attached to the nitrogen atom (N) was remarkably shifted downfield (from 2.24 ppm for N-methylpyrrolidine to

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Fig. 1. (a) Overview of synthesis of S-NCM811, (b) synthetic scheme for sulfonate-based organic CEI precursor.

3.35 ppm), which is major proof of the quaternization of amine compounds [29,30]. In addition, the peak shift for methylsulfonate (from 3.99 ppm for dimethyl sulfate to 3.50 ppm) also supported the quaternization reaction. 13C NMR spectroscopy showed that 13 C-peaks directly connected to N were considerably shifted downfield, which was in agreement with the 1H NMR spectroscopy. This demonstrates that the artificial sulfonate-based CEI precursor was successfully synthesized with the simple one-step process. The sulfonate-immobilized NCM811 cathode material (SNCM811) was obtained by wet-coating of the CEI precursor, followed by heat treatment at a low temperature (Fig. 1a; details are provided in the experimental section). After the modification was complete, the surface morphology of the recovered S-NCM811 was characterized with SEM (Fig. 3a). The D50 value for secondary particles was approximately 5 mm regardless of surface modifications, but the surface morphology of S-NCM811 differed from that of the bare NCM811. The S-NCM811 surface was well-developed by the new coating layer, while the bare NCM811 only exhibited a clear surface coverage. The TEM image for 5% S-NCM811 indicated a CEI layer thickness of approximately 10 nm (Fig. S2). Note that the heating temperature seems to affect the degree of CEI layer distribution on the NCM811 surface (Fig. S3). A lower heating rate results in a more compact CEI layer on the NCM811 surface. The surface of 5% S-NCM811 synthesized at the slowest rates (1  C min1) was uniformly coated with SO3-based CEI layers, whereas the CEI layer on the surface of 5% S-NCM811 heated at 50  C min1 was difficult to observe. This implies that a low heating rate would be effective for developing SO3-based CEI layers on a NCM811 surface. The chemical composition of CEI coating layer was characterized by the FT-IR analysis (Fig. 3b). Absorbance signals corresponding to the sulfonate functional group were observed in the S-NCM811, with eS]Oe (1398 and 1165 cm1) and eSeOe (874 cm1) [31,32], whereas no significant peaks were observed in the NCM811. This means that artificial sulfonate-immobilized NCM811 was successfully achieved with the use of a sulfonate-based CEI precursor. Note that the XRD patterns for NCM811 and S-NCM811 were identical (Fig. S4) e the proposed approach did not affect the bulk properties but did modify the surface characteristics of the NCM811. Based on these material characterizations, we evaluated the electrochemical performance, as shown in Fig. 4. The cell with SNCM811 demonstrated less polarization behavior during the initial charge process compared to the cell with NCM811 (Fig. 4a). This implies that the sulfonate-based CEI layer might facilitate Liþ

migration because a partial negative charge that developed at the end of the sulfonate functional group could bind with Liþ, resulting in better kinetic behavior during the de-lithiation process [33e35]. Previous studies have reported that sulfonate-based functional groups would be effective for ion migration: the partial negative charge that exists on SO 3 functional groups attached to the Nafion or PEDOT polymer backbones aids cation migration in the cell by an ion hopping mechanism, resulting in higher ionic conductivity in the electrolytic cell [36e43]. In practice, we also observed a similar effect on the rate capability results, in which the cell cycled with 5% S-NCM811 exhibited an improved rate capability at a high C-rate (Fig. S5). The discharge specific capacity is well retained at 0.5 C (95.1%), 1.0 C (92.2%), and 5.0 C (82.4%), when compared with its discharge specific capacity of 0.1 C. On the contrary, a relatively low discharge specific capacity is observed in the cell cycled with only NCM811: 0.5 C (93.6%), 1.0 C (91.0%), and 5.0 C (79.3%). This indicates that embedding a SO3-functional group on the CEI layers is effective for facilitating Liþ migration in the cell. The S-NCM811 demonstrated a much-improved cycling performance compared to bare NCM811 (Fig. 4b). The initial discharge specific capacity of the cell controlled with S-NCM811 was lower (181.1 mA h g1 for 1%, 178.8 mA h g1 for 5%, and 171.6 mA h g1 for 10%) than that of the cell with NCM811 (184.3 mA h g1) as the surface components on the cathode surface increased, but the cycling retention of S-NCM811 was much improved compared to the cell with only NCM811. The 5% SO3-coated NCM811 exhibited optimized electrochemical performance in terms of cycling retention and average Coulombic efficiency; 97.4% of discharge specific capacity (174.0 mA h g1) and 99.8% of average Coulombic efficiency remained after 50 cycles, while the cell with only NCM811 showed drastic fading of cycling performance (86.5% of retention) with a low average Coulombic efficiency of 99.3%. Among the 5% SNCM811 cathodes, the 5% S-NCM811 synthesized at the lowest heating rate (1  C min1) showed an excellent cycling retention of 97.4% and the S-NCM811 prepared at 10  C mine1 also exhibited a moderate cycling retention of 97.1%. Otherwise, the cells with 5% SNCM811 prepared at a higher heating rate (50  C min1) displayed relatively low cycling retentions of 91.6% (Fig. S6). This seems to be attributed to the uniformity of the sulfonatebased artificial CEI layer, which is responsible for retarding electrolyte decomposition. After the evaluation of cycling performance was complete, surface analyses for the recovered electrodes were performed (Fig. 5). All of the surface behaviors appeared to be well-matched with the

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Fig. 2. NMR spectroscopy of sulfonate-based organic CEI precursors: (a) 1H and (b)

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13

C spectra.

Fig. 3. (a) SEM images for bare NCM811 (up) and S-NCM811 (down), (b) FT-IR spectra for NCM811 (black) and S-NCM811 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. (a) Potential profiles and (b) cycling performances at 1.0 C (black: NCM811, red: 1% S-NCM811, blue: 5% S-NCM811, orange: 10% S-NCM811). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. SEM images of electrodes: (a) cycled NCM811, and (b) cycled 5% S-NCM811.

Fig. 6. EIS results at (a) 1 cycle, and (b) 10 cycles (black: NCM811, blue: 5% S-NCM811). EIS was measured at 4.3 V (vs. Li/Liþ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

cycling results. The surface of the cycled NCM811 was heavily covered with thick layers, which is attributed to the decomposition adducts of the electrolytes. The EDS analysis revealed that the F composition was much higher in the recovered NCM811 electrode (6.15 wt%) than in the cycled 5% S-NCM811 electrode (3.77 wt%) (Fig. S7). Note that previous literatures indicate that the degree of chemical composition of the F element in an electrode is attributable to the degree of electrolyte decomposition [44e46]. The F element only exists in the lithium salt (LiPF6): the higher F composition on the recovered NCM811 electrode would be regarded as evidence for the occurrence of severe electrolyte decomposition during the electrochemical charging/discharging process. Indeed, this is a common behavior of NCM cathode materials, especially those with a high Ni composition. Irreversible electrolyte decompositions are greatly accelerated as Ni content increases, leading to rapid fading of the cell as surface resistance increases [47e49]. Otherwise, the recovered S-NCM811 exhibited a quite different surface morphology; the overall surface state seemed to be similar to its initial surface morphology, with relatively clean surface coverage. Additional EIS results for the cycled electrodes were well-harmonized with different surface morphologies (Fig. 6).

On the initial cycle, the resistance levels for the solid electrolyte interphase (SEI) layer (RSEI) and charge transfer (RCT) were different depending on the existence of artificial coating layer (RSEI: 20.5 U and RCT: 70.5 U for the NCM811 electrode; RSEI: 7.5 U and RCT: 17.9 U for the S-NCM811 electrode). In addition, there were remarkable differences in resistance with an increasing number of cycles: RSEI and RCT were well-maintained in the S-NCM811 electrode after 10 cycles (RSEI: 39.1 U and RCT: 70.0 U), while the NCM811 electrode exhibited significant increases in RSEI (94.7 U) and RCT (100.3 U). Note that increased RSEI and RCT are highly correlated to fading interfacial stability between electrode and electrolyte, which arises from electrolyte decomposition [50,51]. This strongly indicates that the sulfonate-immobilized artificial CEI layer is effective for retarding electrolyte decomposition, leading to enhanced surface stability of the NCM, even with a high-Ni composition. To understand the specific role of the sulfonate-based CEI layer, the recovered electrodes were further analyzed with XPS (Fig. 7). In the C1s spectra, common chemical functional groups are found at 285.0 and 285.7 eV for eCeCe, 286.7 eV for eCeOe, and 289.6 eV for eC]Oe (Fig. 7a) [52,53]. In addition, the cycled S-NCM811 indicated two distinguishable peaks at 287.5 and 290.8 eV, which

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Fig. 7. XPS results for cycled electrodes (a) C1s, and (b) F1s (up: NCM811, down: S-NCM811).

are attributed to the sulfonate functional group (eCeSe) and the PVDF binder, respectively [53e55]. The appearance of a PVDF peak in the cycled S-NCM811 is significant spectroscopic evidence for exploiting the role of the sulfonate-based CEI layer. Note that there was no PVDF peak in the cycled NCM811 because its surface was heavily covered with adducts triggered by electrolyte decomposition. In other words, the sulfonate-based CEI layer effectively suppressed electrolyte decomposition, resulting in a relatively clean surface that was responsible for a higher PVDF intensity. This explanation was well-supported by the F1s analyses (Fig. 7b). Although eCeF chemical moiety for the local chemical structure of PVDF has been observed at 687.3 eV [53,54], the intensity was much lower in the cycled S-NCM811 than in the cycled NCM811. This indicates that the sulfonate-based CEI layer plays a key role in suppressing undesired electrochemical reactions on the NCM811 surface and provides opportunities for ensuring remarkable longterm cycling performance based on the enhanced surface stability of NCM811. 4. Conclusions To enhance the interfacial stability of NCM materials with high Ni composition, a sulfonate-based organic CEI precursor was synthesized and subjected to immobilization on NCM811 cathode material by a simple wet-coating-based heat treatment. The sulfonate-based artificial CEI layer was well-developed on the surface of NCM811, and was found to be composed of sulfonate

functional groups, which retard electrolyte decomposition. In electrochemical performance testing, the 5% S-NCM811 exhibited a higher average Coulombic efficiency (99.8%) and cycling retention (97.4%, 174.0 mA h g1) for 50 cycles, compared to the NCM811 (86.5% retention and 99.3% average Coulombic efficiency). Systematic analyses by SEM, EIS, and XPS confirmed that the sulfonatebased artificial CEI layer that developed on the NCM811 surface contributed to the suppression of electrolyte decomposition and led to decreased interfacial resistance between electrode and electrolyte, resulting in remarkable cycling retention. We believe that the proposed approach, based on immobilization of an artificial CEI layer, will be effective for achieving a high level of interfacial stability on NCM materials, even with a Ni composition of >80%.

Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2016R1C1B1009452 and 2017R1A6A1A06015181).

Appendix A.. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.06.037.

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