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Photo-Cross-Linkable Polymeric Binder for Si Anodes in Lithium Ion Batteries
Sueun Lee
Energy Conversion and Storage Program Interdisciplinary Green Energy
Graduate School of UNIST
2013
Photo-Cross-Linkable Polymeric Binder for Si Anodes in Lithium Ion Batteries
Sueun Lee
Energy Conversion and Storage Program Interdisciplinary School of Green Energy
Graduate School of UNIST
1 Contents
Abstract...2
List of Figures...3
I. INTRODUCTION...6
1.1 Lithium Ion Batteries…...6
1.2 Si-based Anode Materials for Lithium Ion Batteries...10
1.2.1 Introduction of Si based anode materials...10
1.2.2 The challenges of Si based anode materials.…...11
1.2.3 Nanostructured Si based anode materials.…...11
1.3 Polymeric Binders in Lithium Ion Batteries...13
1.3.1 Introduction of polymeric binders...13
1.3.2 The development of new polymeric binders...14
II. EXPERIMENT...24
III. RESULTS AND DISCUSSIONS...26
IV. CONCLUSION...33
V. REFERENCES...37
VI. ACKNOWLEDGEMENTS...44
2 Abstract
Silicon has received much interest as an attractive anode material for lithium ion batteries due to its superior theoretical specific capacity of ca. 4200 mAh g-1, which is an order of magnitude higher than those of the conventional graphite anodes. However, Si-based anodes suffer from severe volume expansion during the lithiation and delithiation, which cause high stress and pulverization, resulting in degradation of cycle life. Recent studies have demonstrated that the development of polymeric binders plays an important role in controlling the electrochemical performance of batteries by inhibiting mechanical fracture of Si anodes during cycling. According to the previous reports, poly (acrylic acid) (PAA) binders effectively accommodate volume expansion of Si particle due to reversible cross-linking nature between carboxylic groups. However, this reversible cross-linking is often susceptible to the environmental changes.
Herein, we report for the first time that PAA functionalized with photo-cross-linkable benzophenone moiety (PAA-BP) with an aim to enhance the cycle life of batteries. PAA-BP binders form irreversible cross-linked structure by covalent bond upon UV-irradiation, leading to the enhanced mechanical and chemical stability that endure the environmental changes. We demonstrate PAA-BP binders effectively bind Si particles together and suppress the large volume change with superior cycle stability and high capacity.
3
List of Figures
Figure 1. Comparison of the different battery technologies in terms of volumetric and gravimetric energy density. The share of worldwide sales for Ni–Cd, Ni–MH and Li-ion portable batteries is 23, 14 and 63%, respectively.
Figure 2. A representative scheme of lithium ion batteries. The anode is a graphitic carbon that holds Li in its layers and the cathode is a Li-intercalation compound—usually an oxide because of its higher potential—that often is characterized by a layered structure. Both electrodes are able to reversibly insert and remove Li ions from their respective structures. On charging, Li ions are deintercalated from the layered oxide compound and intercalated into the graphite layers. The process is reversed on discharge. The electrodes are separated by a non-aqueous electrolyte that transports Li ions between the electrodes.
Figure 3. Increasing demands for lithium-ion batteries in electric-powered applications. Owing to high capacity , energy density and cycle durability, the demands of lithium ion batteries increase consistently.
Figure 4. (a) Specific capacity as a function of cycle numbers of Si films with different thickness of 1 um and 250 nm. (b) The image of craking induced by volume expansion in Si film. (c) Delamination and peeling of the Si film from the collector electrode after cycling.
Figure 5. Swelling of (a) PAA and CMC binder (b) PVdF binder in diethyl carbonate (DEC) solution.
A change of thickness in PAA and CMC is neglible compared to PVdF, indicating the stable mechanical properties upon electrolyte. (c) Stiffness of PAA, CMC and PVdF in dry and wet state in DEC.
Figure 6. Cycle performance of Si anode with different binder (a) PAA, (b) Na-CMC and (c) PVdF.
Square symbols correspond to anodes with Si particles coated by carbon.
Figure 7. (a) Scheme of the crosslinking mechanism between PAA and CMC binder. The condensation reaction of the carboxylic groups of PAA and the hydroxyl groups of CMC forms the cross-linked structure. (b) The chemical bonding between Si particle and cross-linked PAA-CMC binder via hydrogen bonding. (c) The Li deintercalation capacity of Si anode with PVdF, PAA, CMC, and c-PAA-CMC binder. (d) The specific capacity as a function of cycle number and Coulombic efficiency of Si anode prepared with c-PAA-CMC binder.
4
Figure 8. (a) Photograph of Giant kelp forest (Macrocystis pyrifera algae) in the Pacific Ocean. The inset image shows the structure of alginate of (left) mannuronic and (right) guluronic acid structure.
(b-e) The Young’s modulus of alginate and PVdF binder in dry and wet state by AFM measurement.
Figure 9. XPS spectra (a) C1s (b) Si2p mode of algingate, Si powders, Si electrode and Si powders extracted from the electrode after extensive purification. (c) The reversible deintercalation capacity versus cycle number of Si anode with alginate binder. (d) The comparison of reversible deintercalation capacity of Si anode with different binder of PVdF, CMC and alginate
Figure 10. (a) Traditional approaches to accommodate volume expasion of Si anode, using acetylene black (AB) as the conductive additive and PVdF as the binder. (b) New concept of conductive polymer with dual functionality, as a conductive additive and binder to retain electric connectivity and mechanical integrity of Si anode during lithiation and delithiation. (c) The structure of the PF-type conductive polyer used in this study.
Figure 11. FT-IR spectra of (PAA/PAAm) multilayer film (a) as-prepared (b) after heating at 175 °C for 3h and (c) after heating and 1-day immersion in aqueous solution adjusted pH. Downward arrow(b) indicate the anhydride peak by dehydration reaction, forming the cross-linked structure between polymer chains. However, (c) the anhydride peak disappers after upon soaking aqueous solution, suggesting the cross-linked sturcture is reversible and is susceptible to environmental changes.
Figure 12. (a) Schematic illustration of irreversible cross-linked structure of PAA-BP binder to accommodate the volume expansion of Si anode. (b) Synthetic approach for preparation of PAA-BP binder
Figure 13. FT-IR spectra of PAA-BP binder of (a) reversible thermal cross-linking (b) irreversible photo-cross-linking. In Figure a, the peak corresponding to the anhydride at 1760 and 1800 cm-1 indicated by grey region disappeared under humidity condition, resulting in reversible cross-linking.
In contrast, no changes were observed in Figure b, indicating the irreversible cross-linking can endure the environment changes. The PAA-BP film was firstly dried at 150 °C for 2 h under vacuum and then placed in a humidity chamber at a relative humidity of 80% for 24 h.
Figure 14. 1H NMR spectra of PAA-BP in DMSO-d6. The degree of substitution of benzophenone was calculated to be around 3.7%.
5
Figure 15. (a) UV/vis spectra of PAA-BP binder at different UV-irradiation time. (Inset) Decrease in absorbance of BP moiety at 294 nm upon UV-irradiation. (b) FT-IR spectra of PAA-BP binder before and after UV-irradiation for 30 min. The characteristic BP peaks at 1600 cm-1 (aromatic ring C=C) and 1641 cm-1 (carbonyl group C=O) decreased upon UV-irradiation. For IR measurement, the PAA substituted with 16.7% BP was employed to measure the peak of PAA-BP clearly.
Figure 16. (a) Cycle performance of (circle) bare PAA-BP electrode and (triangle) cross-linked PAA- BP electrode under UV-irradiation. (b, c) Voltage profiles of (b) cross-linked PAA-BP electrode and (c) bare PAA-BP electrode at various cycle numbers. (d) Rate performance of (red) cross-linked and (blue) bare PAA-BP electrode at different current rates.
Figure 17. (a) Ex situ measurement for thickness trace of the PAA-BP electrodes at full lithiation, delithiation, and after 60 cycles. Inset image shows the (left) bare and (right) cross-linked electrode after 60th cycles, indicating disintegration of bare PAA-BP electrode. The thickness of bare PAA-BP electrode after 60th cycles was measured with the film residue. (b) Swelling of PAA and PAA-BP binder in electrolyte solution. Change in PAA-BP binder is lower than PAA, resulting in stable mechanical properties upon wetting in the aprotic electrolyte solution.
Figure 18. (a) XRD pattern, (b) TEM images of carbon-coated silicon active-material used in the study.
6 I. INTRODUCTION
1.1 Introduction of Lithium Ion Batteries.
The Li-metal was introduced as anode materials in battery technology in 1970s because Li is the most electropositive(-3.04 V versus standard hydrogen electrode), the lightest metal with high energy density. Due to its high capacity and energy density, it was commercialized in various electronic field.
However, lithium is precipitated on the negative electrode during charging, forming of dendrites which easily cause short circuiting. Also, because of the high chemical reactivity of metallic lithium resulting in poor battery characteristics, unstable cycle durability in terms of safety issues, the new approaches were explored by the prominent groups. In 1985, Akira Yoshino groups assembled a prototype cell using carbonaceous material as a negative electrode and transition-metal oxide as a positive electrode to provide lithium ion to negative electrode such as LiCoO2. Although polyacetylene was used as negative electrode, there were some limitations such as low density and chemical stability. Thus, new carbonaceous materials were explored by Yoshino groups and graphite was has been used as negative electrode. The crystalline structural carbonaceous material such as graphite showed high capacity without decomposition of electrolyte solvent (propylene carbonate).
This is a new concept of secondary lithium ion batteries by Yosino groups. In 1991, the secondary lithium ion batteries were commercialized by Sony Corporation.1-5
Li-ion batteries provide high energy density and capacity compared to the conventional nickel- cadmium and nickel-metal hydride batteries (Figure 1).1 The operating voltage of Li-ion batteries is around 3.7 V, which is three times of that of nikel-based batteries. In specific, Li-ion battery system is based on reversible electrochemical redox reaction (Figure 2).2 When electrochemical redox reactions occur at the electrodes, ions are transferred between the anode and the cathode through the electrolyte.
At the same time, electron transfer occurs between the two electrodes. These electrons move through the external wire connecting the two electrodes, thus forming a closed circuit. At anode, electrochemical oxidation of the electrode (oxidation, A → A+ + e-) takes place, converting chemical energy into electric energy. At cathode, the electrons transferred from negative electrode participate in electrochemical reduction (reduction, B+ + e- → B).6-10
As shown in Figure 3,3 the demands for Li-ion batteries have increased significantly in the recent years. The consumers use including cell phones, mobile computers, and various electronics has prompted the development of Li-ion batteries with high capacity and long cycle life. Especially, Li- ion batteries will be used primarily in hybrid and electric vehicles in the near future.11, 12
Figure energy d 14 and 6
1. Comparis density. The 63%, respect
son of the di share of wo ively.1
ifferent batte orldwide sale
7
ery technolo es for Ni–Cdogies in term d, Ni–MH an
ms of volume nd Li-ion por
etric and gra rtable batteri
avimetric ies is 23,
Figure 2 Li in its potentia and rem layered discharg the elect
2. A represen layers and t l-that often i move Li ions oxide comp ge. The elect
trodes.2
ntative schem the cathode i is characteriz from their re pound and trodes are se
me of lithium is a Li-interc zed by a laye espective str intercalated parated by a
8
m ion batteriecalation com ered structure ructures. On
into the gr a non-aqueou
es. The anod mpound-usual e. Both elect charging, Li raphite laye us electrolyt
de is a graphi lly an oxide
rodes are abl i ions are de
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hat holds its higher bly insert from the ersed on
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Figure high cap consisten
3. Increasing pacity, energ ntly.3
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9
ion batteries urability, thes in electric- e demands o
-powered app of lithium io
plications. O on batteries
Owing to increase
10
Recently, many research groups have shown the advanced technology of Li-ion batteries in various fields of anode and cathode materials and electrolyte to enhance energy density and capacity of Li-ion batteries. 13-21
1.2 Si-based Anode Materials for Lithium Ion Batteries
1.2.1 Introduction of Si anode materials
Although the Li-metal has a high energy density and capacity of 3860 mA h g-1, the unstability and high reactivity of Li in electrolyte have disrupted the commecialization of Li-metal batteries.22 This has stimulated intense development of various materials as alternatives. The carbonaceous materials and alloys have been widely studied. Among these materials, graphite is conventionally used as negative anode material for lithium ion batteries owing to its exceptionally low volumetric expasion, leading to high reversibility and cycle stability during lithiation and delithiation.23-27
Commercialized Li-ion battery system composing LiCoO2 as the cathode material and graphite as the anode material is based on following reaction. 6
LiCoO2 ↔ Li1-nCoO2 + nLi+ + ne- (cathode) nLi+ + ne- + C ↔ LiyC6 (anode)
Both electrodes reversibly insert (intercalation) and remove (deintercalation) Li-ions from their respective structures. On charging, Li-ions are deintercalated from the LiCoO2 compound and intercalated into the graphite layers. The process is reversed on discharge. The electrodes are separated by a non-aqueous electrolyte that transports Li ions between the electrodes as shown in Figure2.2, 6
However, despite many advantages, various approches to develop the new anode materials have been explored in recent years, because of the low theoretical capacity of graphite (372 mA h g-1). The carbon-based materials including carbon nanotubes and graphene have received more attention for the anode materials to overcome the limitations of graphite.26-32 For example, Ruoff and co-workers show that reduced graphene oxide/Fe2O3 composite (R-GO/Fe2O3) improve the electrochemical performance with high discharge and charge capacities of 1693 and 1227 mA h g-1, respectively.33
Recently, silicon has received much interest as an attractive anode material for LIBs due to its superior theoretical specific capacity of ca. 4200 mA h g-1, which is an order of magnitude higher than those of the conventional graphite anodes. It is abundant in nature and non-toxic, and can be alloyed with up to 4.4 lithium atoms per silicon atom to create an lithiated alloy of Li4.4Si with high
11
electrochemical capacity. Additionally, silicon shows a long plateau over much of its discharge curve, providing a stable voltage during cycling, and does not suffer from solvent co-intercalation.12, 34-39
1.2.2 The challenges of Si-based anode materials
However, silicon-based anodes suffer from large volume changes (ca. 300%) during lithiation and delithiation. Especially, the volume change causes high stress and pulverization, and eventually breaks electrical contact of electrode, and this results in degradation of electrode and an unstable cycle life.12, 40, 41 As an alternative to bulk Si-based anodes, nanostructured Si-anode materials42-50 such as Si nanotubes,51, 52 nanowires53, 54 and film structures55, 56 have been extensively developed to imrprove the stability of cycle life. Nanostructured electrodes provide the necessary volume of free space to accommodate volume expansion during alloying, thus minimize material stress and electrode pulverization to enhance reversibility and cycling stability. Figure 4a shows the specific capacity of amorphous Si film anodes of thickness 1μm and 250 nm. The Si films significant decrease of theoretical capacity after cycle. Figure 4b and c shows the stress-induced crack of the film, which is induced by these volume changes of Si anode during cycling. These cracks and pulverization of the Si anode, lead to loss of electrical contact and fading of capacity, which shortens cycle life and eventually results in cell failure.12, 38
1.2.3 Nanostructured Si anode materials.
These nanostructures provide the free volume to accommodate volume expasion during lithiation, alleviating material stress and electrode pulverization for stable cycle life. For example, Cui and co- workers have reported successful development of Si nanowires, accommadating large volume expasion without pulverization and showing short Li insertion distance. In this study, nanowire Si anode exhibit stable cycle life with discharge capacity close to 75% of maximum of theoretical capacity. Nanowire Si system provides free space between nanowires accommodating volume expasion during lithiation and delithiation and individual nanowire is directly connected to the current collector enhancing the electrical contact.57 As another approach, Cho and co-wokers have demonstrated the high reversible charge capacity of 3247 mA h g-1 with superior capacity retension using Si nanotubes to increase the surface area accessible to the electrolyte. The high contact with electrolyte allows the lithium ion to intercalate Si nanotubes. Additionally, Si films show the high capacity and long cycle life owing to the small amount of active materials minimizing total volume expansion. Thus, the thin films show the improved performance.58
Figure 4 μm and and peel
4. (a) Specifi 250 nm. (b) ling of the Si
ic capacity a The image o i film from th
s a function of craking ind
he collector e
12
of cycle num duced by vol electrode aftmbers of Si fi lume expans ter cycling.38
ilms with dif ion in Si film
fferent thickn m. (c) Delam
ness of 1 mination
13
1.3 Polymeric Binders in Lithium Ion Batteries1.3.1 Introduction of polymeric binders
Although remarkable improvements in the electrochemical performance of Si-based anodes have been achieved, relatively less attention has been paid to the development of the inactive, yet still important component of battery electrode, such as polymer binders.59-67 Poly(vinylidene) difluoride (PVdF) has been conventionally used as binder for anode due to its good electrochemical properties.
However, PVdF binder is connected with Si anode via weak van der Waals forces only and is inefficient to accomodate huge volume changes of Si particles during the repeated cycles due to degradation of mechnical strength. Dahn and co-workers suggest poor performance of PVdF binder for batteries because PVdF can be easily broken by internal strain during cycle. Elastomer PVdF- TFE-P system was developed to enhance the mechanical properties of PVdF binder.68, 69
Additionally some research groups have focused on the development and modification of CMC (carboxymethylcellulose) binder. CMC is a polymeric derivative of cellulose with various degree of substitution of hydroxyl groups (-OH) and carboxy-methyl groups (CH3COO-).70 Li et al. reported sodium carboxymethyl cellulose (Na-CMC) as potential binder for Si anodes. In this study, Si anode with CMC binder shows good capacity retention and Coulombic efficiency due to strong stiffness and a small elongation at break of CMC, which endure the volume expansion of Si particles upon lithiation.71 Liu et al. studied a mixture of commercial butadiene block copolymer (SBR) latex and Na-CMC (1:1) as a binder for Si anodes. Fifty charge–discharge cycles of 1000 mA h g-1 were obtained for the carbon-coated Si using a SBR–CMC binder, while only a few were obtained for the same material using a PVDF binder.59 Although CMC binder shows good performance as polymeric binder with Si anode, it has been reported some limitations of CMC. The use of CMC with organic solvent is limited due to low solubility of CMC despite its eco-friendly property. The use of water affects long-term stability as well as Coulombic efficiency of Si anode and causes surface oxidation of Si anode. Next, the modification of the molecular weight of CMC is difficult because the CMC is derived from natural cellulose.68
Until now, many studies on improvement of cycle stability of Si anode using polymeric binder have focused on the development of PVdF and CMC binders. However, recent studies have demonstrated that the development of new polymeric binders and they play an important role in controlling the electrochemical performance of batteries by inhibiting mechanical fracture of Si anodes during cycling.68, 72-78
14
1.3.2 The development of new polymeric bindersPoly(acrylic acid) (PAA) has received interest as a new polymeric binder for Si anode in lithium ion batteries. Magasinski et al. demonstrated for the first time that PAA binder improves the cycle stability of Si-anode In this study, they suggest the some factors affect the performance of a polymeric binder such as.68
(1) adhesion strength of binder between the electrode and the current collect foil.
(2) the mechanical properties of binder.
(3) the interaction of binder with the electrolyte.
First, the strong adhesion is important to integrate the anode. Because the CMC and PAA have many functional hydroxy groups to interact with SiO2 (surface of Si particles) and CuO (copper surface) via hydrogen bonding, they show better adhesion than PVdF binder. In a separate report, Konno and co-workers have also investigated that adhesion strength of the SiO composite with various polymeric binders by the peel test. As expeted, PAA has the highest adhesion strength of 2.3 N cm-1, which is 10 times larger than that of PVdF (0.3 N cm-1). Moreover, in terms of mechanical properties, PAA shows the highest stress at break of 90 MPa compared to that of other binders such as Na-CMC (30 MPa) and PVdF (37 MPa). Finally, the binder needs to interact with electrolyte weakly for the stable cycle life of Si anode. If an electrolyte solvent reaches the surface of Si anode, it causes the solvent decomposition products that disturb the interaction between binder and Si anode, resulting rapid anode degradation and unstable cycle stability. The swellability of PAA and CMC in DEC solutioun is negligible, indicating the very weak interaction with electrolyte, but PVdF shows high swelling in diethyl carbonate solution (DEC) as shown in Figure 5. These results indicate the mechanical properties of PVdF can be degraded in electrolyte. As expected, the mechanical properties of PAA and CMC are not affected by DEC solution because the stiffness of them does not change from a dry state to a wet state. In contrast, the stiffness of PVdF dramically decreases upon DEC solution, suggesting the weak interaction with Si anode and failure of holding Si particles. Figure 6 shows the first twenty cycles of capacities during lithiation and delithiation with PAA, Na-CMC and PVdF binder. The anode with PAA binder shows a stable cycle life. Interestingly, carbon coated Si anode shows good performance, leading to capacity retension of 94% after 20 cycles. However, the Si anode with Na-CMC and PVdF binder shows rapid degredation and unstable cycle life. Although the mechanical properties of PAA and CMC are similar, the carboxylic groups of PAA play an important role via strong hydrogen bonding with SiO2.68, 72
Figure 5 Change upon ele
5. Swelling o in PAA and ectrolyte. (c)
of (a) PAA a d CMC is neg
Stiffness of
and CMC bin glible compa PAA, CMC
15
nder (b) PVdared to PVdF and PVdF in
dF binder in d F, indicating
n dry and we
diethyl carbo the stable m et state in DE
onate (DEC) mechanical p
EC.68
solution.
properties
Figure 6 Square s
6. Cycle per symbols corr
formance of respond to an
f Si anode wi nodes with S
16
ith different Si particles cobinder (a) P oated by carb
PAA, (b) Na- bon.68
-CMC and (cc) PVdF.
17
Additionally, carboxylic acid groups in PAA form anhydride groups after heating process. The hydrogen bonding and anhydride groups bind together tightly, forming the cross-linked structure.
These physical and cross-linked structure improve the mechanical properties of PAA, accommodating the large volume expansion of Si-based anode.
As another approach, Choi and co-workers showed that the thermally cross-linked PAA-CMC binder improved the cycle performance of Si anode due to the improved mechanical property of the three- dimensionally interconnected network. Figure 7 shows PAA and CMC form the three-dimensional cross-linked structure via condensation reaction (dehydration). In the reaction, the hydroxyl groups (- OH) of CMC react with carboxylic groups (-COOH) of PAA to form the covalent ester bond. This covalent bond hold Si particles together and retain the electrical network under large movement of Si particles. As a result, Si anode with cross-linked PAA-CMC binder exhibit the improved cycle stability and the high coulombic efficiency (Figure 7).74
Yusin and co-workers have recently employed the new type of binder like sodium alginate, a high- modulus natural polysaccharide extracted from brown algae, to yield a remarkably stable battery anode compared with conventional polymeric binders such as PVdF, PAA, and CMC. Figure 8 shows alginate binder have 6.7 times higher stiffness than that of PVdF binder in dry state. Moreover, the stiffness of alginate binder does not change in electrolyte solvent, indicating little-to-no-interaction between alginate and electrolyte. However, the stiffness of PVdF binder decrease nearly 50 times, suggesting a unstability in electrolyte. In this study, they proposed the strong interaction between the carboxylic acid groups of binder and the hydrolyzed thin surfaces of SiO2 covering the Si particles prevented the anode from disintegration. As shown in Figure 9, the bare Si anode shows a strong bulk Si peak at ~99.2 eV and hydroxyl groups peak on the surface of Si at ~103 eV. After mixing with alginate binder, SiO(O)-R peak appear at 103.9 eV. Interestingly, the Si anode retains substantial content of alginate (Si2p spectra) after the extensive purification. These result indicate the strong hydrogen bond between the SiO2 (-OH groups) and carboxylic groups of alginate. The Si anode with alginate binder exhibit the superior electrochemical performane. The charge and discharge profile showed the high stable anode performance for more than 1300 cycles (Figure 9). Low cost and environmentally friendly alginate binders can be replaced with the conventional binders.75
Liu et al. have reported the conductive binder, which has dual functionality of both binder and conductive additive to solving the electric connectivity of the conventional binders. In this study, the n-type doped polymeric binder (PFFOMB) is employed because the p-type doped polymer is not stable below potential 1 V (Li/Li+) as shown in Figure 10. The Si anode with PFFOMB conductive binders shows good performace with 2100 mA h g-1 after 650 cycles due to high electric connectivity of PFFOMB binder. This approach provides potentials and oppurtunities of new materials by tailoring the key electronic states through combined techniques.76
Figure condens cross-lin binder v and c-PA efficienc
7. (a) Sch sation reactio nked structur via hydrogen AA-CMC b cy of Si anod
eme of the on of the car re. (b) The bonding. (c) inder. (d) T de prepared w
e crosslinkin rboxylic grou
chemical bo ) The Li dein he specific with c-PAA-
18
ng mechanisups of PAA onding betw
ntercalation c capacity as -CMC binder
sm between and the hyd een Si parti capacity of S
a function o r.74
n PAA and droxyl group
cle and cros Si anode with of cycle num
d CMC bind ps of CMC f
ss-linked PA h PVdF, PAA mber and Co
der. The forms the AA-CMC A, CMC, oulombic
Figure 8 inset im (b-e) Th
8. (a) Photog mage shows t
he Young’s m
graph of Gia he structure modulus of al
ant kelp fore of alginate o lginate and P
19
st (Macrocy of (left) man PVdF binderstis pyrifera nnuronic and in dry and w
algae) in the d (right) gulu wet state by A
e Pacific Oc uronic acid s AFM measur
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rement.75
Figure 9 extracted versus deinterc
9. XPS spec d from the e
cycle numb alation capac
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(b) Si2p mod ter extensive
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20
de of alginga e purificationalginate b ferent binder
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ders, Si elect eversible dei The compa MC and algi
trode and Si intercalation arison of r inate.75
powders capacity reversible
Figure 1 black (A polymer mechani conducti
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21
commodate vd PVdF as tive additive hiation and d
volume expa the binder.
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sion of Si an (b) New co to retain elec (c) The stru
node, using a oncept of co ctric connect ucture of the
acetylene onductive tivity and e PF-type
22
Even though this PAA binder assists in building a cross-linked structure, the chemical interactions between the carboxylic acid groups are still reversible in its chemical nature due to its susceptibility to the environmental changes such as humidity and temperature. As shown in Figure 11, the anhydride bond corresponds to 1804, 1780 and 1042 cm-1 in FT-IR spectra after heating of PAA, making cross- linked structure between polymer chains. However, these peaks disappear upon moisture condition, indicating the formed cross-linked sturucture is reversible and unstable to enviromental changes.79 Thus, the reversible chemical de-cross-linking of PAA binders can occur during the processing of cell assembly in a practical situation, which in turn leads to rapid degradation of the Si anodes and increased cell impedance during cycling.
Therefore, we report herein the use of robust PAA polymers which are functionalized with photo- cross-linkable benzophenone (PAA-BP) binder with an aim to enhance the cycle life of Si-based anodes. Upon UV-irradiation, PAA-BP binder successfully forms irreversible cross-linked structure by covalent C-C bond, leading to the mechanically and chemically enhanced stability that endure the environmental changes. Si anode with PAA-BP binder demonstrate the improved cycle performance due to the irreversible cross-linked structure which endure large volume expasion of Si particles during lithiation and delithiation.
Figure 1 for 3h an indicate polymer suggesti
11. FT-IR sp nd (c) after h the anhydr r chains. Ho ing the cross-
pectra of (PA heating and 1 ride peak by owever, (c) t -linked sturc
AA/PAAm) m 1-day immer y dehydratio the anhydrid cture is revers
23
multilayer fil rsion in aqueo on reaction,de peak disa sible and is s
lm (a) as-pre ous solution forming the appers after susceptible to
epared (b) af adjusted pH e cross-linke upon soakin o environme
fter heating a H. Downward ed structure ng aqueous ental changes
at 175 °C d arrow(b)
between solution, s.79
24 II. EXPERIMENT
Materials
Poly(acrylic acid) (Mw of 50000 g mol-1) was purchased from Polysciences, N,N’-dimethyformamide, 4-hydroxybenzophenone, 6-bromo-1-hexanol, 4,4’-(dimethylamino) pyridine and 1-[3- (dimethylamino)propyl]-3-ethylcarbodiimide methiodide were purchased from Sigma-Aldrich.
Carbon coating process was progressed to provide additional conductivity for silicon powder. For the synthesis of carbon-coated Si nanoparticles, Si nanopowder (< 100 nm, 0.4 g, Aldrich) was mixed with phthalocyanine (0.1 g, Aldrich) using a mortar, and then the mixture was heated at 900 °C for 2 h under Ar atmosphere. This carbon-coated silicon powders were used as an active material to evaluate the performance of binders.
Synthesis of 4-(6-hydroxyhexyloxy)benzophenone.80
4-hydroxybenzophenone (2.01 g, 10 mmol) and K2CO3 (2.07 g, 15 mmol) were dissolved in 20 ml of DMF, and the solution was stirred for 30 min at 100 °C. To this solution, 6-bromo-1-hexanol (1.99 g, 11 mmol) in 10 ml of DMF was added dropwise and the final solution mixture was stirred overnight at 100 °C. The mixture was cooled to room temperature and the solvent was evaporated under reduced pressure. The excess water was added to the solution to precipitate the white powder, and the product was dried. The product was recrystallized from ethanol.
Synthesis of PAA substituted with photo-cross-linkable benzophenone (PAA-BP).80
Poly(acrylic acid) (1 g, 14 mmol), N,N’-(dimethylamino)pyridine (DMAP) (0.34 g, 2.8 mmol) and 4- (6-hydroxyhexyloxy) benzophenone (0.84 g, 2.8 mmol) were dissolved in 50 ml of DMF at 0 °C. 1- [3-(Dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) (1.67 g, 5.6 mmol) was added to the solution. After 30 min, the mixture was warmed to room temperature and stirred overnight. The mixture was then added dropwise to acetone to form a precipitate. The solid product was additionally washed twice with acetone. The pure product was prepared by dialysis against deionized water adjusted to pH 3 for 3 days to remove any residual byproduct. The degree of substitution was determined to be around 3.7%, on the basis of 1H NMR spectrum.
25
Measurement and chracterizationElectrochemical characterization.
Galvanostatic charge and discharge cycling (WonATech WBCS 3000 battery measurement system) was performed in the potential window from 0.0 to 2.0 V vs. Li/Li+ with a two-electrode 2016 coin- type half cell, where Li metal foil was used as the counter electrode. Si active materials were mixed with carbon black (Super P) and PAA-BP binder in a 8:1:2 weight ratio to fabricate working electrodes. The first lithium insertion and extraction capacities were measured at a current density of 100 mA g-1, and the cycling performance of the half cells was monitored at a current density of 200 mA g-1 at 30 °C. To investigate the rate performance of the half cells, lithium deinsertion capacities were measured at the current densities of 0.2 - 20 A g-1 at 30 °C. The electrolyte comprised 1.3 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (30:70, v/v) with 5 wt. % fluoroethylene carbonate (FEC, Soulbrain Co. Ltd.). Microporous polyethylene film was used as a separator. Cells were assembled in an Ar-filled glove box with less than 1 ppm of both oxygen and moisture. After cycling, cells were carefully opened in a glove box to retrieve their electrodes, and electrodes were subsequently rinsed in DMC to remove a residual LiPF6-based electrolyte, and dried at room temperature. For the ex situ measurement of thickness change, these dried electrodes were utilized.
Photo-Cross-Linking.
Electrodes with PAA-BP binder were irradiated for 30 min in air by UV lamps (UVP, 100 W) at a distance of 15 cm. Electrodes were placed at the center of the box having a dimensions of 15.5 × 16.5 × 15 cm3. The UV intensity was 391 mW cm-2.
Swelling test of PAA and PAA-BP binder.
PAA solution and PAA-BP solution were deposited on Si wafers using spin-coating method.
Thickness of every film was measured independently before soaking the film in diethyl carbonate (DEC) solution and average thickness of was around 20 nm. PAABP films were irradiated for 30 min by UV lamp. PAA and PAA-BP films were placed in a container filled with DEC solution. Film thickness was measured at predetermined time points (5, 15, and 30 min) by ellipsometry.
Characterizations.
Powder X-Ray diffraction (XRD) data were collected on a Rigaku D/MAX2500V/PC powder diffractometer using Cu-Kα radiation (λ=1.5405 Å) operated from 2θ = 10 – 80°. The carbon coating morphology was examined using a field-emission transmission electron microscope (TEM; JEOL JEM-2100F) operating at 200 kV. 1H NMR spectroscopy (VNMRS 600 spectrometer 600 MHz,
26
Varian, USA) was used with CDCl3 and DMSO-d6 as the solvent. The photo-cross-linking process was investigated by ultraviolet-visible (UV-vis) spectroscopy (VARIAN, Cary 5000) and Fourier- transform infrared (FT-IR) (Varian). The change of thickness of the PAA-BP thin film on SiO2 substrates was measured by ellipsometry (EC-400 and M-2000 V, J. A. Woollam Co., Inc.).
III. RESULTS AND DISCUSSION
The schematic diagram of Figure 12 shows our strategy how PAA-BP binder can accommodate the huge volume change of Si anode during lithiation and delithiation. The role of three-dimensionally interconnected binder structure is the efficient accommodation of the repetitive volume changes upon cyclings. Without cross-linked binders, the composite electrode is gradually deformed on cyclings due to the linear polymeric structure of binders, and finally loses the electrical contact between electroactive powders. However, the cross-linked structure enables to limit the deformation of the composite electrode due to the strong irreversible covalent chemical bonding formed between linear polymers. It is known that the PAA binder forms the cross-linked anhydride structure by dehydration reaction between carboxylic groups at high temperature (150 °C), which results in the improvement of cycle performance due to the effectively suppressed volume expansion of Si particle.73, 74 However, this thermal cross-linking of carboxylic acid groups is still reversible and susceptible to the environmental changes such as humidity and temperature, as proved by FT-IR measurement as shown in Figure 13 . Figure 13a shows the FT-IR spectra of PAA-BP heated at 150 °C for 2 h. After heating, the anhydride peaks appeared at 1760 and 1800 cm-1. However, these anhydride peaks disappeared under humidity condition, suggesting these peaks are readily hydrolyzed and susceptible to the moisture.79 Thus, the reversible chemical de-cross-linking of PAA binders can occur during the processing of cell assembly in a practical situation, which in turn leads to rapid degradation of the Si anodes and increased cell impedance during cycling. In case of irreversible crosslinking, Figure 13b the new covalent bond can endure the enviroment changes. Accordingly, the irreversible cross-linking regardless of environmental changes is required for the long-term storage of electrodes under practical conditions. Therefore, we report herein the use of robust PAA polymers which are functionalized with photo-cross-linkable benzophenone (PAA-BP) as a binder for Si-based anodes and demonstrate the improved cycle performance of Si anode due to the irreversible cross-linked structure of benzophenone moiety under UV-irradiation as shown in Figure 12.
Figure accomm binder.
12. (a) Sch modate the vo
hematic illus olume expan
tration of ir nsion of Si an
27
rreversible cnode. (b) Sy
cross-linked ynthetic appr
structure of roach for pre
f PAA-BP b eparation of
binder to PAA-BP
Figure photo-cr indicated In contra the envi then plac
13. FT-IR sp ross-linking.
d by grey re ast, no chang ironment cha ced in a hum
pectra of PA In Figure a egion disappe ges were obs anges. The P midity chamb
AA-BP binde a, the peak
eared under served in Fig PAA-BP film ber at a relativ
28
er of (a) revcorrespondin humidity co gure b, indica m was firstly ve humidity
versible therm ng to the an ondition, resu ating the irre y dried at 15
of 80% for 2
mal cross-lin nhydride at
ulting in rev versible cros 50 °C for 2 h
24 h.
nking (b) irr 1760 and 18 versible cross
ss-linking ca h under vac
reversible 800 cm-1 s-linking.
an endure uum and
29
Benzophenone (BP) moiety has been traditionally employed in chemical and industrial application as a photo-responsive unit. 80-85 BP moiety of the PAA-BP polymer provides an irreversible cross- linked structure by the formation of covalent bond between BP and backbone of PAA upon UV- irradiation, leading to the enhanced mechanical and chemical stability that endures the environmental changes. More specifically, excited BP creates biradicals under UV irradiation (ex = 365 nm) that can abstract hydrogen from neighboring PAA backbone (C-H) to form cross-linked structure by a new C- C bond.82
In this study, PAA-BP was prepared according to the known protocol as shown in Figure 12b. The successful conjugation of BP along the backbone of PAA was characterized with 1H NMR spectra with a degree of substitution determined to be around 3.7% (Figure 14). Although the degree of functionalization can be easily tuned up to 16%, we limit the degree of functionalization to 3.7% in the current study in order to keep the solubility of the functionalized polymer while still achieving the required cross-linking network structure. According to report by Yusin and co-workers, the sufficient pore volume is needed for Si expansion because the Li-ion transport site significantly decrease among the fully expanded Si particles, thus resulting the failure of electrochemical performance during cycle.
Based on this reason, the high degree of cross-linking may cause the low permeablility of Li-ions and thus indicate the degradation of cycle performance.75, 81
The photo-cross-linking process of PAA-BP was then investigated by UV/vis spectroscopy as shown in Figure 15a. The absorbance of BP moiety at 294 nm decreased with UV-irradiation time due to increase of n-π* transition in the carbonyl group, resulting in the loss of carbonyl group.The formed biradicals abstract hydrogen from C-H group of surrounding polymer backbone, forming cross-linking structure by new C-C covalent bond. To crosscheck the process of photo-cross-linking in film state, FT-IR spectroscopy measurement was performed (Figure 15b). Before UV-irradiation, the characteristic BP peaks at 1600 cm-1 (aromatic ring C=C), 1641 cm-1 (carbonyl group C=O), and PAA peak at 1710 cm-1 (carboxylic acid group) were observed, respectively. Upon UV-irradiation, however, peaks corresponding to BP moiety significantly decreased, indicating the successful formation of cross-linking. UV-irradiation have no effect on the rest of polymer chain because the carboxylic acid peak at 1710 cm-1 remain constant upon UV-irradiation.80, 81
Figure was calc
14. 1H NMR culated to be
R spectra of around 3.7%
PAA-BP in
%.
30
n DMSO-d6. The degree of substitutiion of benzoophenone
Figure 1 absorban before a C=C) an PAA sub
15. (a) UV/v nce of BP m and after UV nd 1641 cm-1 bstituted wit
vis spectra of moiety at 29 V-irradiation
1 (carbonyl g th 16.7% BP
f PAA-BP bi 94 nm upon
for 30 min.
group C=O) was employ
31
inder at diffeUV-irradiat The charact decreased up yed to measur
erent UV-irra tion. (b) FT teristic BP p pon UV-irra re the peak o
adiation time -IR spectra eaks at 1600 diation. For of PAA-BP c
e. (Inset) De of PAA-B 0 cm-1 (arom
IR measurem clearly.81
ecrease in BP binder matic ring
ment, the
32
The electrochemical performance of carbon-coated Si nanopowders with PAA-BP binders is evaluated (Figure 16). The Si composite electrodes were first prepared with PAA-BP binders, and then the photo-cross-linking reaction of BP groups were progressed via the exposure of electrodes under UV-irradiation. As expected, the cross-linked PAA-BP electrode via UV- irradiation shows negligible capacity fading over 100 cycles, while gradual capacity fading was observed in the case of the bare PAA-BP binder electrode without UV-irradiation (Figure 16a). Moreover, the cross-linked PAA-BP electrode shows the better rate performance than the bare PAA-BP electrode, showing that ca. 1420 mA h g-1 is delivered even at a specific current density of 20 A g-1 (ca. 12.5 C rate) (Figure 16d). This improvement of cycle stability and rate performance realized through UV-irradiation is attributed to the three-dimensionally interconnected binder structure, resulting in the less deformation of the composite electrode on volume change. This interconnected structure allows to sustain better electrical contact between Si and conducting agent of carbon. The low degree of deformation is also supported by negligible thickness change of the cross-linked PAA-BP electrodes at the first cycle. For that purpose, the thickness change of electrodes after full lithiation and delithiation was measured via ex situ thickness measurement (Figure 17). The cells were disassembled after cycles of full lithiation/delithiation and the electrode thickness was measured manually. As shown in Figure 17a, after full lithiation until the redox potential of the working electrodes reached 0 V vs. Li/Li+, the thickness of the cross-linked PAA-BP electrode increased by 38%, whereas the bare PAA-BP electrode was expanded by 63%.
To our surprise, the thickness of the cross-linked PAA-BP electrode is fully recovered into the original state after full delithiation until the redox potential of the working electrodes reached 2 V vs.
Li/Li+, but the bare PAA-BP electrode remained as the expanded state even after full delithation although the volume expansion decreased from 63 to 38%. In spite of the cross-linked PAA-BP electrode showed a negligible volume change during the first cycle, it was found that the electrode was expanded by 230% after 60 cycles. This observation can be attributed to the swelling effect of gas evolution originating from electrolyte decomposition.86 In the case of the bare PAA-BP electrode, however, because the electrode was too deformed and eventually detached from the surface of the Cu foil. In contrast, cross-linked PAA-BP electrode conformally retains onto the Cu foil as shown in the inset of Figure 17a. This feature also indicates that the cross-linked PAA-BP binder exhibited the smaller volume change during cyclings, compared to the bare PAA-BP binder. Therefore, it is also required to alleviate the swelling effect for the commercialization of Si-based anodes, although the cross-linked PAA-BP binder is efficient to accommodate the volume change of Si-based electrodes.
This work will be reported in our further work.
Also, the three-dimensionally interconnected structure of PAA-BP binders was further confirmed through the swelling behavior of binder films in the diethyl carbonate (DEC) solution. (Figure 17b).
The conventional PAA and cross-linked PAA-BP polymer films were soaked in the DEC solution,
33
and the thickness change was measured at various soaking time. According to report by Kovalenko and co-workers, little-to-no interaction between polymeric binder and electrolyte is required to achive the stable mechanical properties in electrolyte. In accord with that report, our PAA-BP binder showed little swelling (101.8%) in DEC solution, whereas the conventional PAA films displayed relatively larger swelling (104.9%), suggesting good mechanical stability in electrolyte solution.68, 75
IV. CONCLUSION
In conclusion, we have successfully synthesized photo-cross-linkable polymeric binder PAA-BP and investigated the process of cross-linking of PAA-BP by UV/Vis and FT-IR spectra upon UV- irradiation. PAA-BP binder shows a better stability than conventional PAA binder in DEC solution, indicating PAA-BP binder retains the stable mechanical properties. We have demonstrated PAA-BP binder accommodate the volume expansion of Si-based electrode via irreversible cross-linked structure under UV-irradiation. Without UV-irradiation, bare PAA-BP binder shows the significant deformation of electrode, resulting unstable cycle stability. Upon UV-irradiation, however, irreversible cross-linked PAA-BP binder exhibits improvement of electrochemical performance with a high reversible capacity of 1600 mA h g-1 over 100 cycles. We believe this study will offer not only potentials for further development of polymeric materials in energy field, but also opportunities for advances in lithium ion batteries.
Figure 1 BP elect (c) bare (blue) ba
16. (a) Cycle trode under U
PAA-BP ele are PAA-BP
e performanc UV-irradiati ectrode at va P electrode at
ce of (circle) on. (b, c) Vo arious cycle t different cu
34
bare PAA-B oltage profilnumbers. (d urrent rates.
BP electrode es of (b) cro d) Rate perfo
and (triangle oss-linked PA ormance of (r
e) cross-link AA-BP elect red) cross-lin
ked PAA- trode and
nked and
Figure delithiat after 60t electrod binder i mechani
17. (a) Ex s tion, and afte
th cycles, ind e after 60th in electrolyte ical propertie
itu measurem er 60th cycles dicating disin cycles was m e solution. C es upon wett
ment for thic s. Inset imag ntegration of
measured w Change in P ting in the ap
35
ckness trace ge shows the f bare PAA-B with the filmPAA-BP bin protic electro
e of the PAA e (left) bare a BP electrode residue. (b) nder is lowe lyte solution
A-BP electro and (right) c e. The thickn Swelling of er than PAA n.
odes at full l cross-linked e
ness of bare f PAA and A, resulting
ithiation, electrode PAA-BP PAA-BP in stable
Figure study.
18. (a) XRDD pattern, (bb) TEM ima
36
ages of carboon-coated sillicon active--material useed in the
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