In Situ Crosslinked Carboxymethyl Cellulose-Polyethylene Glycol

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Materials and Interfaces

In Situ Crosslinked Carboxymethyl Cellulose-Polyethylene Glycol Binder for Improving the Long-Term Cycle Life of Silicon Anodes in Li Ion Batteries Dongsoo Lee, Hyunjung Park, Alan Goliaszewski, Yun-ki Byeun, Taeseup Song, and Ungyu Paik Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00870 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Industrial & Engineering Chemistry Research

In Situ Crosslinked Carboxymethyl Cellulose-Polyethylene Glycol Binder for Improving the Long-Term Cycle Life of Silicon Anodes in Li Ion Batteries

Dongsoo Leea, Hyunjung Parka,b, Alan Goliaszewskic, Yun-ki Byeund, and Taeseup Songa*, Ungyu Paika

a

Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea.

b

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62

Nanyang Drive, Singapore 637459, Singapore. c

Ashland Specialty Ingredients, 500 Hercules Road Wilmington, DE 19808, USA.

d Steelmaking research group, Technical Research Laboratory of POSCO, Pohang, Gyeongbuk,

37859, Korea.

*Corresponding author E-mail: [email protected]

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Abstract To increase the energy density of Li-ion batteries (LIBs), silicon has been widely studied due to its relative abundance and high theoretical specific capacity (~3572 mAh g-1). However, silicon experiences drastic volume changes up to 300% associated with Li. Here, we report in situ crosslinked carboxymethyl cellulose-polyethylene glycol (CMC-PEG) binder and its application to the silicon anode to improve cycle life. Through in situ crosslinking during the electrode drying process, the crosslinked CMC-PEG binder is simply prepared without an additional process. In particular, the crosslinked CMC-PEG binder is effective in enhancing cohesion between active materials and adhesion between active materials and a current collector. The silicon anode with the crosslinked CMC-PEG binder shows stable cycling performance with a capacity of ~ 2000 mAh g-1 up to 350 cycles at 0.5 C. In terms of simplicity, this binder has potential to be used for silicon anodes and other electrodes experiencing volume expansion during cycling. Keywords: Lithium ion batteries, Silicon Anode, In situ Crosslinking, CMC-PEG binder


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1. introduction Lithium ion batteries (LIBs) are used in most electronic devices today; however, there are still concerns about cost, energy density, cycle life, and safety issues, among which the development of LIBs with high energy density and long cycle life is important for practical applications.(1-4) Among the possible candidates, silicon has been widely studied due to its abundance and high theoretical specific capacity (~ 3572 mAhg-1) with a relatively low working voltage (~ 0.5 V vs Li/Li+).(5, 6) However silicon experiences drastic volume changes up to 300% during continuous charging and discharging processes, which leads to the loss of electrical contact between active materials and the continuous formation of a SEI (solidelectrolyte interphase) layer.(7, 8) Because of the inherent disadvantages with silicon anodes, many researchers have introduced several concepts to prevent pulverization of the active materials during cycling to improve cycle life and lithium ion conductivity of the silicon anodes.(9) Nano-porous materials, nanowires, and nanotubes are effective in improving cycle retention by reducing the stress of silicon materials attributed to the volume changes during charging and discharging.(10-13) Another approach is to modify the polymer binders, which could provide reversible strong bonds with the silicon materials through higher binding strength, and enhance the lithium ion conductivity with conducting functional groups.(14, 15) However, in terms of the cost and cycle life, there remain limitations to their practical use. Binder is one of the major electrode components, despite low content in the electrode, which has a great effect on the mechanical and electrochemical properties of LIBs.(16) To improve the cycle performance of the silicon anode, binders should have robust bonding with active materials and current collectors, which is related to the cohesion and adhesion properties of the electrode.(17) Conducting functional groups are also important for stable cycling behavior in the silicon anode.(18) In this respect, many studies on robust bonding and crosslinking concepts of polymeric binders have been reported.(19) Among various candidates,



carboxymethyl cellulose (CMC) has been widely used in practice for anodes up to the present because of its environmental friendliness, good adsorption property with active materials, excellent dispersion properties in a slurry state with carboxyl functional groups (COO-) in water, and low cost. However, CMC has limitations in terms of the mechanical property to be applied to silicon anodes. Nevertheless, with an appropriate crosslinking agent, CMC could improve mechanical strength to withstand the volume expansion of silicon. In this respect, we studied the crosslinking binder of CMC with PEG. Polyethylene glycol (PEG) is an environmentally friendly synthetic polymer that is conducting, amphiphilic, and water soluble. Therefore, a novel crosslinked binder from CMC with polyethylene glycol diglycidyl ether (PEGDE) as a crosslinking agent could be used as an effective binder to improve the mechanical and electrochemical properties of the electrodes.(20, 21) Especially, a small amount of PEGDE could provide a large effect on a variety of properties in the silicon anode. Here, we report an in situ crosslinked CMC-PEG binder and its application to the silicon anode for the long-term cycle performance in LIBs. The crosslinked CMC-PEG binder is simply prepared through the crosslinking reaction between CMC and PEGDE under the electrode drying process of the coated slurry onto the current collector without additional process. The crosslinked CMC-PEG binder is effective in enhancing the cohesion between active materials and adhesion between active materials and a current collector with multidimensional bonding sites inducing hydrogen bonds with the silicon. With the crosslinked CMC-PEG binder, the electrode peeling strength including adhesion and cohesion was quite enhanced to ~2 gf/mm. Furthermore, the silicon anode with the crosslinked CMC-PEG binder exhibited stable cycling performance, showing a capacity of ~2000 mAh g-1 with high Coulombic efficiencies up to 350 cycles at 0.5 C. This in situ crosslinked CMC-PEG binder is prepared by a facile approach and has a potential to be applied to the industry level of LIBs with its simplicity.


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2. Experimental methods 2.1 Preparation of silicon electrodes Silicon powder (APS≤50nm, laser synthesized from the vapor phase) was purchased from Alfa Aesar. Sodium carboxymethyl cellulose (CMC, Mw ~250,000, DS = 1.2) and polyethylene glycol diglycidyl ether (PEGDE, Mw ~526) were purchased from Sigma-Aldrich. For slurry preparation of the reference electrode with CMC, silicon powder, conducting agent (Ketjen black), CMC, and SBR were placed into deionized water in a weight ratio of 85 : 5 : 8 : 2. For slurry preparation of the electrode prepared with the crosslinked CMC-PEG binder, silicon powder, conducting agent (Ketjen black), CMC, PEGDE, and SBR were placed into deionized water in a weight ratio of 85 : 5 : 6.4 : 1.6 : 2. The slurry was mixed at 2000 rpm for 5 min by Thinky mixer. After 30 min aging, the prepared slurry was coated onto a copper current collector and then dried at 110 °C for 10 min. Afterward, the prepared electrodes were dried in a vacuum oven at 120 °C for 4 h. The areal mass loading of silicon in the electrode was 0.4 mg cm-2. 2.2 Material characterization To investigate the crosslinking reaction between CMC and PEGDE, Fourier-transform infrared spectrometry (Nicolet 5700, ThermoElectron, US) was used. For the analysis, two different binder films were prepared with 2 wt% binder solutions in water after drying at 25 °C; 1) CMC binder film, 2) crosslinked CMC-PEG binder films. Prior to analysis, the prepared binder films were dried in a vacuum oven at 120 °C for 4 h. The mechanical properties of CMC and crosslinked CMC-PEG binder films were measured with a universal testing machine, UTM (AGS-J, Shimadzu, Kyoto, Japan) and the specimens were prepared using an ASTM D638Type 5 cutter. Samples were elongated to obtain a stress-strain curve within a standard deviation of less than ± 10%. To identify the adhesion and cohesion strength of the silicon



electrodes, the 180-degree peeling test was conducted using the UTM. For nano-indentation analysis, a nano-indenter (Nano Instruments, MTS Systems Corp., Eden Prairie, USA) with a Berkovich tip (radius of tip < 100 nm) was used. To minimize data deviation, the indentation position was changed, and the test was conducted at least five times. For analysis of the electrodes before and after cycle testing, field-emission scanning electron microscopy (FESEM, JEOL JSM07600F) was used. X-ray photoelectron spectrometry (XPS, VG microtech ESCA2000) was used for analysis of the binding effect between the crosslinked CMC-PEG binder and silicon and of electrode stability after cycling. 2.3 Evaluation of electrochemical performances Before evaluation, all electrodes and binder films were dried at 120 oC for 4 h in a vacuum oven, during which CMC and PEGDE were crosslinked by heat. For the half cell test, 2032 round type coin cells were fabricated using the silicon electrode as a working electrode, lithium foil with a thickness of ~ 0.2 μm as a counter electrode, 1.3 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC (3/7 vol%) with 5 wt% FEC, PANAX StarLyte) as electrolyte, and a polypropylene (PP) separator. All of the coin half cells were tested using a battery cycle tester (TOSCAT 3000, Toyo System, Tokyo, Japan) in the voltage window of 0.01 – 1.5 V vs. Li/Li+. EIS was performed at an overpotential of 2.85 V with a frequency from 1 to 100,000 Hz and an amplitude of 2 mV using a potentiostat (Autolab, PGSTAT – 72637) before the cell test.

3. Results and discussion Carboxylate group of CMC and epoxide group of PEGDE can have a crosslinking reaction under alkaline conditions or upon heating. A crosslinked CMC-PEG binder from CMC with PEGDE polymers, was confirmed using FTIR analysis, as shown in Figure 1. These spectra show normal adsorption bands at 3323, 2916, 2875, 1417, 1323, and 893 cm-1, which are


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assigned to CH2 stretching, CH2 scissoring bending, CH bending, and vibrations of the α(1 → 4) glucopyranose ring, respectively.(20) The intensities of the bands at 2916, 2875, 1417, and 1323 cm-1 of crosslinked CMC-PEG were noticeably increased compared to those of CMC, which indicated that CMC had a crosslinking reaction with PEGDE.(22) This result substantiates that the CMC and PEGDE in a slurry state became in situ crosslinked CMC-PEG under the drying conditions above 120 °C, which is described in Scheme. 1. Generally, crosslinked polymers have poor solubilities in solvents, so it is difficult to use in practice.(23) However, this in situ CMC-PEG binder would have advantages in processability, such as a slurry stability and electrode preparation, because the crosslinking reaction takes place under the electrode drying process. Furthermore, the abundant functional groups containing carboxyl, ether, and hydroxyl groups of the crosslinked CMC-PEG binder can provide multi-dimensional bonding sites, inducing hydrogen bonds with the surface of the silicon, which is one of the most pivotal factors for improving the mechanical and electrochemical properties of the silicon anode.(19, 24) The conductive functional groups of the PEG can also help improve the lithium ion conductivity in the silicon anode.(25, 26) We expect that the above positive effects of the crosslinked CMC-PEG binder can help to make the silicon anode more mechanically and electrochemically stable during cycling. To evaluate the mechanical property of the binder films prepared with CMC and crosslinked CMC-PEG binders, UTM has been employed. Figure 2(a) shows that the mechanical property of the crosslinked CMC-PEG binder film was enhanced after crosslinking. The stress and strain of the binder film prepared with the crosslinked CMC-PEG binder, persisted until breakdown, increased from 15.8 MPa, 4.6%, to 32.2 MPa, 6.1 %, which would lead to improved electrode integrity and enhanced electrochemical properties including Coulombic efficiency and a cycling performance. Considering the mechanical strength of the binder and the slurry stability (Figure S1, Supporting Information), we optimized the ratio of CMC to



PEG to 8:2 for the further evaluation. The PEGDE improved mechanical properties of the binder films despite a small amount of 25% compared to the CMC content.(27, 28) Figure S2 (Supporting Information)shows the nanoindentation test result of a 2 mN loading using a Berkovich tip. The

crosslinked CMC-PEG binder shows improved elastic recovery compared to that of the CMC binder. The enhanced elastic recovery of the crosslinked CMC-PEG binder would increase the mechanical stability of the silicon anode upon continuous volume changes during cycling. Figure S3 (Supporting Information) shows the swelling ratio of the binder films after soaking in electrolyte for 24 h. Swelling ratio of the crosslinked CMC-PEG binder is reduced to the 2.5 % compared to that of the CMC binder with 3.2 %, which demonstrates that the crosslinked CMCPEG binder has high stability in the electrolyte soaking state. Figure 2(b) reveals the electrode peeling strength including cohesion between the active materials and adhesion between the active materials and copper current collector through the conventional 180-degree peeling test by UTM. The cohesion and adhesion strengths are extremely important parameters for reducing the stress caused by the volume expansion of the silicon anode during cycling. The peeling strength of the electrode prepared with crosslinked CMC-PEG binder was quite enhanced to ~2 gf/mm compared to that of the electrode prepared with CMC binder (~1.1 gf/mm), which reveals that the in situ crosslinked CMC-PEG binder effectively increased the adhesion and cohesion strength of the electrode.(19, 20, 29) The optical images after the electrode peeling test exhibited the clear differences between the electrodes, as shown in Figure S4 (Supporting Information). These results provide strong evidence that the in situ crosslinked CMC-PEG

binder imparts the silicon electrodes with higher mechanical stability with multi-dimensional functional groups compared to the silicon anode prepared with CMC binder.(30-33) Silicon electrodes were evaluated to demonstrate the beneficial effect of the crosslinked CMC-PEG binder on the electrochemical properties through the coin half-cell test. In this study, we used 50 nm silicon nanoparticles, which had high purity with a native oxide layer of ~ 2 nm


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(Figure S5, Supporting Information). Figure 3(a) shows charge-discharge profiles of silicon anodes with CMC and crosslinked CMC-PEG binders at first cycle (0.1 C, 357.2 mA g-1) in the voltage window of 0.01 – 1.5 V vs. Li/Li+. The charge(delithiation) / discharge(lithiation) capacity and Coulombic efficiency at first cycle were 2860.4 / 3604.5 mAhg-1, 79% and 3089 / 3816.6 mAhg-1, 81% for the electrodes prepared with CMC and CMC-PEG binders, respectively, which demonstrates that the electrode prepared with the crosslinked CMC-PEG binder had higher initial charge/discharge capacity and Coulombic efficiency compared with the electrode prepared with CMC binder. Figure 3(b) exhibits the cycle performance up to 300 cycles at 0.2 C. The electrode prepared with CMC binder showed continuous capacity fading, and after 37 cycles, the capacities of the CMC electrode were below 1000 mAhg-1 due to the continuous SEI layer formation and the loss of electrical contact of the active materials by volume changes of the silicon.(34) Conversely, the electrode prepared with the crosslinked CMC-PEG binder exhibited highly improved cycle retention up to 300 cycles with capacities of 2095.6, 1928.6, and 1835 mAhg-1 at 100, 200, and 300 cycles, respectively. The capacity fluctuation in the initial cycles was due to the volume changes of Si associated with Li, which was stabilized in further cycling. The improved physicochemical properties of adhesion and cohesion strength with robust bonding between the binder and silicon surface in the electrode prepared with the crosslinked CMC-PEG binder reduce the stress and prevent the delamination of active materials from the electrode caused by volume changes during repeated charging and discharging.(35) Figure 3(c) shows voltage profiles of the electrode prepared with the crosslinked CMC-PEG binder during cycling at 0.5 C. The electrode shows nearly 2000 mAhg1

up to 200 cycles and steadily stabilized polarization curves during cycling, which

demonstrates that the crosslinked CMC-PEG binder provides the silicon electrode with the enhanced electrochemical stability with improved lithium ion conductivity and low overpotentials. Figure 3(d) verifies the improved cycle stability of the silicon electrode



prepared with the crosslinked CMC-PEG binder at 0.5 C, showing stable capacities up to 350 cycles. The capacity retention of the electrode prepared with the crosslinked CMC-PEG binder is ~ 78% from the third cycle to the 350 cycles. By contrast, the electrode prepared with CMC shows abrupt capacity fading at 0.5 C because of poor mechanical and electrochemical stability of the electrode. These results derive from the enhanced physicochemical properties of the crosslinked CMC-PEG binder.(36) Figure 4(a) shows the rate capability of the electrodes prepared with CMC and crosslinked CMC-PEG binders. The electrode with CMC showed very poor rate capability. However, the electrode prepared with the crosslinked CMC-PEG binder exhibited higher capacities and more stable performances. After the 4C rate test, the electrode prepared with the crosslinked CMC-PEG binder shows restored capacities, which substantiates the electrochemical reversibility of the electrode with the CMC-PEG binder within different C-rate conditions. Figure 4(b) shows the EIS spectra of the electrodes prepared with CMC and crosslinked CMC-PEG binders before cycling, which indicates that the charge transfer resistance (Rct) of the electrode prepared with the crosslinked CMC-PEG binder was much smaller than that of the electrode prepared with CMC. The electrical conductivity of the electrode prepared with the crosslinker CMC-PEG binder was also smaller than that of the electrode prepared with CMC (Figure S6, Supporting Information). In addition to improved electrochemical performance, Figure 5 shows that the electrode prepared with the crosslinked CMC-PEG binder has a high stability during cycling. After 30 cycles, the CMC electrode had a collapsed morphology due to the irreversible reactions such as the continuous SEI layer formation and pulverization of the active materials caused by volume changes of the silicon, while the CMC-PEG electrode had the similar morphology with its initial state; an increased particle size may be due to SEI (solid electrolyte interface) layer formation and inner pore generation at the initial cycle.(37, 38) Optical images of the lithium metal provide clear evidence of irreversible reactions where the color of the lithium metal in


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the electrode with CMC binder changed completely to black, but not in the electrode with crosslinked CMC-PEG binder. Figure 6 substantiates the stable SEI formation of the electrode prepared with the crosslinked CMC-PEG binder before cycle, after first cycle, and after 30 cycles through XPS analysis. Before analysis, the electrodes were washed thoroughly with acetonitrile at least two times to eliminate residual electrolyte so there was no LIPF6 peak in the F 1s spectrum.(39-41) The Si 2p peaks disappeared, and a LIF peak emerged from electrolyte decomposition after 1 cycle, i.e., formation of the stable SEI layer on the silicon surface. The O 1s peak shift from 532.4 eV to 531.5 eV indicates that functional groups of CMC-PEG binder produce a stable SEI layer. The formation of a stable SEI layer is very important for maintaining a reversible capacity and Coulombic efficiency in the silicon anode.

4. Conclusions In summary, an in situ crosslinked CMC-PEG binder was explored as an effective binder for silicon anodes to overcome capacity fading during cycling resulting from the volume changes of silicon. The pivotal effects of using the crosslinked CMC-PEG binder were confirmed via physicochemical studies on the binder and the electrode, and the electrochemical properties. In particular, the improvement in mechanical properties with the CMC-PEG binder demonstrates that the mechanical stability of the binder can help to enhance electrochemical stability during cycling and the abundance of functional groups of the binder can stabilize the electrode. According to the electrochemical evaluations, the initial capacity and Coulombic efficiency were slightly increased with the crosslinked CMC-PEG binder, and the noteworthy improvement was observed in the cycle performance. The silicon anode with the crosslinked CMC-PEG binder exhibited stable cycling behavior with ~ 2000 mAhg-1 up to 350 cycles at 0.5 C. XPS analysis also indicated stable SEI layer formation on the silicon surface. All of these results substantiate that the mechanical and electrochemical properties were greatly



improved by adding a small amount of in situ crosslinker (PEGDE). The in situ crosslinked CMC-PEG is a promising binder that could be applied to the silicon anode to increase the energy density of the batteries.

Associated content Supporting Information Stress-strain behaviors of the binder films by UTM; Typical force-distance loading curves of binder films; Swelling ratio of the binder films after soaking in the electrolyte; Optical image of the electrode and the corresponding glass substrate after 180-degree electrode peeling test; SEM image and HR-TEM image of the pure silicon particles; Sheet resistance of the electrodes by the 4 prove station; Summary of capacitive parameters of previously reported works

Acknowledgments This work was supported by funds from Ashland Specialty Ingredients, and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No.20194010201890).

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Figures and captions

Scheme 1. Polymer structures of (a) carboxymethyl cellulose (CMC) and polyethylene glycol diglycidyl ether (PEGDE) and (b) cross-linked CMC-PEGCMC-PEG binder. (c) Schematic illustration of in situ crosslinking in the silicon electrode during the drying process.


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Figure 1. FT-IR spectra of CMC and CMC-PEG binder films in the range of (a) 500 – 4000 cm-1, (b) 2800 – 2950 cm-1, and (c) 1300 – 1450 cm-1.



Figure 2. (a) Stress-strain behavior of CMC and CMC-PEG binder films measured by UTM. (b) Adhesion and cohesion strength of silicon electrodes from the 180° peeling test through UTM.


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Figure 3. (a) Initial charge-discharge profiles of silicon electrodes with CMC binder and CMCPEG binder at 0.1 C rate between 0.01 and 1.5 V versus Li/Li+ (b) Cycling performance of the silicon electrodes at 0.2 C rate. (c) Charge-discharge profiles of silicon electrode with CMCPEG binder during cycling. (d) Cycling performance of the silicon electrodes at 0.5 C rate.



Figure 4. (a) Rate capability test for the silicon electrodes at 0.2 C to 4.0 C rate. (b) Nyquist plot of CMC and CMC-PEG electrodes before cycling measured by potentiostat.


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Figure 5. Scanning electron microscopy (SEM) images of the electrodes: (a) pristine CMC electrode, (b) pristine CMC-PEG electrode, (c) CMC electrode and (d) CMC-PEG electrode after discharging for 30 cycles and optical images of the Li metal.



Figure 6. High resolution XPS spectra of the silicon electrode surface with CMC-PEG binder for (a) C 1s, (b) F 1s, (c) C 1s, and (d) O 1s peaks before cycle, after 1 cycle, and after 30 cycles between 0.01 and 1.5 V versus Li/Li+.


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Page 25 of 25 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


Graphical abstract


In Situ Crosslinked Carboxymethyl Cellulose-Polyethylene Glycol

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