Hyperbranched triphenylamine polymer for ultra-fast battery cathode

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Hyperbranched triphenylamine polymer for ultra-fast battery cathode Keiichi Yamamoto, Daichi Suemasa, Kana Masuda, Kazunari Aita, and Takeshi Endo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17943 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

Hyperbranched triphenylamine polymer for ultrafast battery cathode AUTHOR NAMES Keiichi Yamamoto*†, Daichi Suemasa†, Kana Masuda†, Kazunari Aita†, Takeshi Endo‡ † Advanced Materials Research Laboratories, JSR Co. Ltd., 100, Kawajiri-cho, Yokkaichi, Mie,510-8552, Japan ‡ Molecular Engineering Institute, Kindai University, 11-6, Kayanomori, Iizuka, Fukuoka, 8208555, Japan KEYWORDS Triphenylamine polymer, Hyperbranched polymer, Ultra-fast battery, Long cycle life, OrganicLFP hybrid cathode, Organic-inorganic charge transfer

ABSTRACT: A novel hyperbranched poly (triphenylamine) (PHTPA) was synthesized, and the electrochemical properties of this material were studied. PHTPA was synthesized by the facile method in one step reaction from affordable monomers. Despite the all aromatic structure, PHTPA showed good solubility in several organic solvents. The battery performance test of PHTPA showed the high discharge voltage, the ultra-fast charge-discharge performance of 100C-300C, and the long cycle life more than 5000 cycles. Moreover, the addition of the PHTPA

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to LiFePO4 (LFP) improved the charge transfer resistance (Rct) and Warburg coefficient, which is related to the diffusion of lithium ions in LFP, and consequently improved the chargedischarge performance of LFP itself at high C-rate (20C-100C). This behavior is understood to be the result of the organic-inorganic charge transfer. The superior cycle performance of PHTPA-LFP hybrid cathode was also found. PHTPA will serve as an additive for a high performance LIB.

1. INTRODUCTION Organic cathode materials for a Lithium ion battery (LIB) have recently captured attention1-4 because of their high rate performances5-10 and large capacities.11-14 However, there remains incompatibility issue between the high rate and the large capacity. To settle the issue, organicinorganic hybrid cathodes have been developed.15-23 Vlad et al. demonstrated that the addition of PTMA ultra-fast organic radical active material16 to LiFePO4 (LFP) enhanced the power capability, and investigated three configurations of the hybrid electrodes (h-3D, h-22P, h-22S). 16 Based on the results of the literature, it is reasonable to speculate that the contact area between PTMA and LFP will be important factor for the efficacy of the PTMA additive. A polymer coating on LFP will be more effective, and therefore a conducting polymer coated LFP electrodes have been investigated.17-23 However, their preparation, which is in situ coating polymerization on the inorganic active material particle, left some issues of the difficulty of the process, the limitation of organic-inorganic ratio, and the degradation of the inorganic materials in water.24 Therefore, a blending method which is easy to use in non-aqueous media with adjustable organic-inorganic active materials ratio was desired.


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To prepare polymer coated LFP by the blending method, an organic active material with a high rate performance and good solubility in an organic solvent has been required. PTMA was removed from the candidate list because it requires the oxidation process by m-CPBA after the polymerization.5 P(NDI2OD-T2), which is the latest ultra-fast organic active material, has been researched by Yao et al.8, but the material has the low discharge voltage because the active unit is n-type imide. As one of the high rate organic active materials having high voltage, poly triphenylamine (PTPA) has been researched.25-31 However, the frame work of the polymer should be changed to linear or branched to prepare the soluble polymer, because the typical PTPA is networked polymer, which is essentially insoluble. Hartwig et al.39 and Turner et al.40-41 have developed some soluble linear PTPA having aliphatic substituents, independently of each other. However, these polymers left some issues that the monomers are expensive, and these polymers have aliphatic substituents and biphenylene backbone, both of which reduce the concentration of redox active nitrogen atom in the polymer structure. Since the concentration of redox active unit relates to the capacity of the battery, the structure of inactive units should be minimized.28-29 Dendrimer has been known as a macromolecule having excellent solubility.32 However, the preparation of the dendrimer requires multi-step reaction; for example four steps reaction is required for the 3rd generation dendrimer42, therefore, it has been avoided in the industrial production. Hyperbranched polymer has been known as a polymer having similar property to the dendrimer.32 Tanaka et al. have produced a soluble substituent-free hyperbranched triphenylamine polymer by C-C coupling in one step reaction.43 However, this polymer has low molecular weight (4 x 103), which would be dissolved into an electrolyte solution in a battery if it was used as a battery material, and also has the biphenylene backbone, which is undesirable as mentioned above. Herein we report the first demonstration of the


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hyperbranched mono-phenylene backbone triphenylamine polymer (PHTPA), which is soluble in several organic solvents, and exhibits the high discharge voltage and ultra-fast charge-discharge performance.

2. EXPERIMENTAL SECTION 2.1. Synthesis of PHTPA and Typical PTPA The polymerization was carried out under inert atmosphere using standard Schlenk techniques. Bis(4-bromophenyl)amine of 3.27g (10mmol) , diphenylamine of 1.69g (10mmol) , sodium tertbutoxide of 2.88g (30mmol) and toluene of 10ml were added to 100ml flask. The mixture was stirred at r.t. for 30min, followed by addition of bis (tri-t-butylphosphine) palladium (0) of 5mg (0.01mmol) as the catalyst. The resulting mixture was allowed to stir at 100 °C for 6hrs. After reaction, the reaction mixture was washed twice with distilled water of 30ml to remove a byproduct NaBr. The organic phase was separated, and was poured into methanol of 50ml to remove the residual catalyst and monomers. The obtained pale-green powder was dried at 60 °C in vacuum for 24hrs. PHTPA dry powder was obtained (3.3g, 99% yield). Typical PTPA was synthesized by the same method described in the literature.30 2.2. Characterization of PHTPA 1

H-NMR spectrum was recorded on JEOL 400MHz spectrometer at room temperature using

CDCl3 as a solvent and TMS as the standard. FT-IR spectrum was recorded on Thermo Scientific Nicolet iS10 FT-IR with diamond ATR. The polymer weight vs polystyrene standard was determined on TOSOH HCL-8320GPC with TSKgelsuper HM-H as the column using THF as


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the eluent. The mass of repeat unit was determined on MALDI-TOFMS (Shimadzu, AXIMA Confidence) using 1,8,9-anthracenetriol and sodium trifluoroacetate as the matrix and THF as the solvent. 2.3. Cell Fabrication and Electrochemical Measurement. CR2032 coin cell composed of the components described in the Figure S5 of the Supporting Information was used for the measurement of the battery characteristics. The cathode was prepared by mixing PHTPA as the active material, acetylene black (AB) as the conductive additive, and polyvinylidene fluoride (PVDF) as the binder in a weight ratio of 40:50:10 unless otherwise noted. The area of the electrode was 1.33cm2, the loading mass of the active materials were around 0.7mg for the organic electrode, around 1.5mg for the LFP or PHTPA-LFP hybrid electrode, the thickness of the electrode was around 25μm. The current densities used at each C rate were adopted based on 1C=60mAh/g for PHTPA, 1C=150mAh/g for LFP and hybrid. The charge-discharge performances were measured by TOSCAT 3100 (TOYO-SYSTEM Co., Ltd.) and VMP3 (Bio-Logic Co., Ltd.) at 25 °C. For EIS measurement and Warburg coefficient, VMP3 (Bio-Logic Co., Ltd.) was used at 25 °C. The test cells were newly prepared and were measured after charging to SOC of 50%. XRD pattern of cathodes were recorded on Rigaku SmartLab with Cu Kα radiation. SEM images were recorded on JSM-6010LA (HITACHI).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PHTPA


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As shown in Scheme 1, PHTPA was synthesized by Buchwald-Hartwig reaction (C-N coupling)

35-38

in one step reaction in quantitative yield. The monomers used are affordable and

industrially available, which makes it possible for the polymer to be produced industrially. During polymerization, it was observed that the polymer has been dissolved in toluene which is used as the reaction solvent. After isolation of the polymer, it was revealed that the polymer powder can be dissolved easily in several organic solvents such as NMP, THF, chloroform, and toluene even though the polymer has the all aromatic structure. As shown in Figure S1-S4 of the Supporting Information, PHTPA obtained was characterized by 1H-NMR, FT-IR, GPC, and MALDI-TOFMS. The results of the characterizations were in accord with the structure of PHTPA shown in Scheme 1. The molecular weight of PHTPA was Mw=1.2 x 104. The feeding ratio of the monomers used in the polymerization (Bis (4-bromophenyl) amine / diphenylamine = 1.0) was successful to obtain the polymer with high solubility and high molecular weight. When high or low feeding ratio was used, an insoluble polymer or a low molecular weight polymer was obtained respectively. Fortunately, the polymer was insoluble in an electrolyte solution (1M LiPF6, EC:DEC=30:70). This result indicates that the polymer can be applied to LIB. 3.2. Cell Performance of PHTPA Figure 1a shows the charge–discharge curves and the rate capability of PHTPA. The charge cutoff voltage of 4.2V was suitable for PHTPA similar to typical PTPA.25 The mean discharge voltage of PHPTA was 3.4 (V vs. Li/Li+), which is slightly lower than that of the typical PTPA. Unfortunately, PHTPA exhibited the disappointing discharge capacity of 64 mAh/g at 0.5C rate, whereas the theoretical capacity of PHTPA is 160 mAh/g. However, high capacity retention was


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observed in high C-rate. The discharge capacity retention vs 0.5C at each C-rate were 86% (50C), 76% (100C), 51% (200C) and 26% (300C), respectively. This impressive rate capability is comparable with that of PTMA and P(NDI2OD-T2), both of which are top-class ultra-fast charging materials. Figure 1b shows the comparison of the performance of Typical PTPA. Considering that typical PTPA exhibited moderate rate capability, the hyperbranched frame work of PHTPA would enhance the rate capability of the polymer. DC-IR values calculated from the slope of a plot of the current I vs V delta (Figure S8) and Warburg coefficient (Figure S10) were listed in Table 1. PHTPA would be difficult to consider as high conducting, because DC-IR and charge transfer resistance (Rct) in EIS (Figure S9) of PHTPA is comparable with that of typical PTPA. However, Warburg coefficient for PHTPA is lower than that of typical PTPA significantly. It is reasonable that this Warburg coefficient contributes the high rate performance. We also found that an addition of SWCNT into a cathode composition is effective to reduce the

amount

of

the

conductive

additive

(Figure

1c).

The

cathode

consisted

of

PHTPA:AB:SWCNT:PVDF=80:9:1:10 shows similar performance shown in Figure 1a. This fact means that high capacity retention at high-rate region in Figure 1a is not attributed to the intrinsic capacity of acetylene black. The long life cycle performance of PHTPA at the cutoff voltage 4.0-2.8V was shown in Figure 2. No degradation was observed over 5,000 cycles even though it was tested at high C-rate (20C and 100C). As seen in previous works for nano-crystal of inorganic active materials44-51, the particle size of the PHTPA would contribute to the high rate performance and the long life cycle. The 5,000 cycled cell was opened and the appearance of the cathode was observed. No elution of PHTPA into the electrolyte was observed.


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The blend film of PHTPA and PVDF (the blend ratio is 4:1) was prepared by casting from NMP, and the SEM image was observed to clarify the reason why PHTPA exhibits the superior performance. The film without acetylene black was used in this observation because acetylene black reduced

the contrast of the SEM image. Figure 3 shows the SEM image of the film. An

unexpected phase separated structure was observed. It was assigned by SEM-EDS analysis that micronsized microspheres are PHTPA, fibers and films on the microspheres are PVDF and dark regions are vacancy. In the case of only PHTPA, no phase separated structure was observed (Figure S7 (b)). These results indicate that the formation of the microsphere of PHTPA would be attributed to the difference in polarity with PVDF, and the low viscosity of the hyperbranched polymer solution7a should reduce the microsphere size. From this morphology, it is likely that the microsphere and the microporous cathode structure including the microspheres of PHTPA would enhance the rate performance and the long life cycle of the cathode.10 Previous ultra-fast charging materials such as PTMA and P(NDI2OD-T2) might also show the characteristic morphology. 3.3. Cell Performance of Hybrid of LFP and PHTPA The coating test of a hybrid cathode consisted of PHTPA, inorganic active material LFP, and PVDF (weight ratio is 8:32:10) was carried out by the NMP-aided blending method. As shown in Figure 4b, the microspheres of PHTPA were observed in the coating film, while needle like pristine LFP as seen in Figure 4a was not observed. This result suggests that LFP particles are fully coated by PHTPA in the hybrid cathode, and the contact area between PHTPA and LFP surface is sufficiently large that they can interact with each other. If the typical PTPA was applied to the hybrid cathode, LFP would not be coated because of insolubility of the polymer.


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The cell performance of the hybrid cathode was measured. Figure 5 and Table 2 show the cell performance of the cathode of which the weight ratio is PHTPA:LFP:AB:PVDF=8:32:50:10. It is clear from Figure 5a that the addition of PHTPA improves the cathode performance. At 20Crate, the PHTPA-LFP hybrid cathode exhibited 50% capacity retention, whereas the cathode consisted of only LFP did not work well. Figure 5b shows the charge-discharge curves of the hybrid cathode cell. The plateau derived from LFP was observed even in high C-rate region. This result indicates that not only PHTPA but also LFP are allowed to charge at high C-rate in the hybrid cathode. A similar observation was also found in the hybrid cathode with PTMA and LFP, but PTMA 50 mol% content was needed, and the plateau was largely inclined.16 X-ray diffraction (XRD) of the PHTPA-LFP hybrid cathode was measured to estimate the state of charge (SOC) for LFP in the cathode after 100C charging. Figure S6 of the Supporting Information shows XRD pattern of the cathode for uncharged, 100C charged, and 1C charged respectively. The XRD pattern clearly showed the presence of FePO4 (FP), which is the charged state of LFP, appeared at 2θ=18.0, 30.1, 30.9 even though the charge was carried out at 100Crate. DC-IR for PHTPA-LFP hybrid is slightly low than that of only LFP (Table 2). Surprisingly, charge transfer resistance (Rct) in EIS for PHTPA-LFP hybrid is significantly low than that of bare LFP (Figure S9), and Warburg coefficient for PHTPA-LFP hybrid is one third as that of bare LFP (Table 2). This fact suggests that PHTPA enhances the diffusion of Lithium ions in LFP, which is the rate-determining step. Shi et al. reported a similar effect that the Warburg coefficient of PPy-Fe3O4 hybrid gel framework, which has PPy (polypyrrole) coated Fe3O4, was one tenth of that of LFP.22 Moreover, they reported that the diffusion coefficient of Lithium ions of PPy-LFP hybrid gel framework was about four times higher than that of control LFP.23 Wang


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et al. reported that the addition of polypyrrole (PPy) solid to LFP increases the diffusion of Lithium ions a little.21 We think that difference in

efficacy between hybrid gel framework of

PPy and solid PPy is attributable to differences of the contact area between the polymer and LFP particles. Therefore, it is understand that PHTPA has similar characteristics as PPy, and a significant improvement of Warburg coefficient for PHTPA-LFP hybrid in this work would be attributable to the contact area between the polymer and LFP, which is the result of the solubility of PHTPA. In general, the diffusion of Lithium ions is related to the shape or the particle size of the active materials52-55. However, consistent materials were used in this work, and there should be no change in the particle size or the shape of LFP during the addition of PHTPA into the LFP slurry. It is reasonable to suppose that some kind of effect of PHTPA beside LFP would affect the LFP surface, and then the Li ion inside LFP diffuses to compensate the Li ion on the LFP surface. Figure 6a shows the schematic process of the charge transfer step for high rate charging of the hybrid cathode of PHTPA and LFP. The charge-discharge curve of the hybrid cathode was compared with that of PHTPA and LFP (Figure 6b-d) to investigate the mechanism of the high rate performance for the hybrid cathode. Only the curve derived from PHTPA was observed at the low SOC in the hybrid cathode. At the middle SOC, the plateau derived from LFP appeared, and then PHTPA curve appeared again at high SOC. Since the discharge voltage of PHTPA is higher than 3.4V, which is sufficient to charge LFP, it is reasonable that the organic-inorganic charge transfer occurred at the middle SOC shown in Figure 6a. As mentioned above, the charge transfer resistance of PHTPA-LFP hybrid (Rct (hy)) is very low. The Rct (hy) can be presented as the equation 1 shown below, because there should be no charging mode except three modes shown in Figure S12a, b, c.


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Rct (hy)= xRct (PHTPA) + (1-x)( F Rct (LFP) + (1-F)Rct (CTS) )

(1)

Where, Rct (PHTPA) and Rct (LFP) are Rct for PHTPA and LFP shown in Figure S9 respectively. Rct (CTS) is Rct attributed to the charge transfer step. The Rct (CTS) should be zero, because the charge transfer step needs no external electron or ion (Figure S12c). The x (0.2) is the loading ratio of PHTPA in hybrid electrode. F is a contribution factor for a bare LFP which means the ratio of LFP direct charge (shown in Figure S12a) and the charge transfer step (Figure S12c). The F calculated from the equation 1 is 0.13, which means that 87% of LFP was charged through the charge transfer step (Figure S12c). This result is reasonable considering morphology of PHTPA-LFP hybrid shown in Figure 4b. We believe that this charge transfer step reduces Rct, and consequently attainment of the high rate performance of the hybrid cathode. Vlad et al. also proposed the similar theory in the research for PTMA-LFP hybrid cathode.16 Overall, we propose the mechanism for high rate charging of the hybrid cathode of PHTPA and LFP as presented in Figure 6 and Figure S12. At first, PHTPA was charged, and PHTPA+PF6- salt was formed at low SOC. Then, the PHTPA+PF6- salt charges neighboring LFP (charge transfer step, Figure S12c), in other words PHTPA+PF6- salt behaves like a catalyst which can charge LFP easily, finally the Li ion in LFP diffuses to compensate the Li ion on the LFP surface. However, information of the mechanism is not enough. Further studies will be needed to yield any findings about the mechanism. The correlation between PHTPA concentrations and their cell performance was investigated. As shown in Figure 7, high concentration of PHTPA increased the rate performance, while it decreased the capacity of the cathode. PHTPA 20wt% concentration was good balance, which has highest rate performance ever reported in the LFP-organic hybrid cathodes,16-20,23 and has


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little decline in the capacity. We also found that the cycle performance of PHTPA-LFP hybrid cathode is superior to that of LFP (Figure S11). It is likely that the coating of PHTPA on LFP can prevent degradation of LFP during the charge-discharge cycle. PHTPA will serve effectively as an additive for LIB in this concentration.

4. CONCLUSIONS In summary, the novel hyperbranched poly (triphenylamine) (PHTPA) was synthesized by the facile method in one step reaction from affordable monomers. Despite the all aromatic structure, PHTPA showed good solubility in several organic solvents. The molecular weight of PHTPA was sufficiently high to use as a battery component. PHTPA showed the superior electrochemical performance, including the high discharge voltage, ultra-fast charge-discharge rate of 100C-300C, and a long cycle life more than 5000 cycles. It is likely that the microporous cathode structure consisting of the microspheres of PHTPA contribute to the high rate performance and the long life cycle. Moreover, the addition of the PHTPA to LiFePO4 (LFP) improved the charge transfer resistance (Rct) and Warburg coefficient, which is related to the diffusion of lithium ions in LFP, and consequently improved the charge-discharge performance of LFP itself at high C-rate (20C-100C). This behavior is understood to be the result of the organic-inorganic charge transfer. The superior cycle performance of PHTPA-LFP hybrid cathode was also found. PHTPA will serve as an additive for a high performance LIB.

ASSOCIATED CONTENT


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The following files are available free of charge. Synthesis and characterization of PHTPA, cell fabrication and charge-discharge test method, XRD pattern and SEM images of cathodes, results of DC-IR measurement, EIS, Warburg coefficients, cycle performance of PHTPA-LFP hybrid cathode, schematic reaction equation of charge-discharge reaction on cathode. (PDF)

AUTHOR INFORMATION Corresponding Author E-mail address: [email protected]

ACKNOWLEDGMENT We thank Dr. Masaru Yao from National Institute of Advanced Industrial Science and Technology (AIST) for the technical support of the cell performance test.

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Figures, Scheme and Table Scheme 1. Synthesis of Poly Hyperbranched Triphenylamine (PHTPA).


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(a)

PHTPA:AB:PVDF=40:50:10

(b)

Typical PTPA:AB:PVDF=40:50:10

(c)

PHTPA:AB:SWCNT:PVDF=80:9:1:10

Figure 1. Charge-discharge curves and rate performance of PHTPA (a), Typical PTPA (b), SWCNT-aided PHTPA (c).


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Figure 2. Long life cycle performance of PHTPA for 20C (a) and 100C (b).

Figure 3. SEM image of the film made from PHTPA and PVDF (PHTPA:PVDF=40:10) (a) and SEM-EDS analyses for carbon and fluoride atom (b), (c).


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Figure 4. SEM images of LFP (a) and PHTPA coated LFP (PHTPA:LFP:PVDF=8:32:10) (b).

Figure 5. Rate capability (a) and charge-discharge curves (b) for PHTPA-LFP hybrid (weight ratio=20:80).


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Figure 6. a) The charge transfer step for high rate charging of hybrid cathode of PHTPA and LFP. b-d) Charge-discharge curves for hybrid of PHTPA and LFP (weight ratio=50:50), for PHTPA and for LFP respectively. All curves were tested at 1C-rate in this test. The blend ratio of 50:50 was utilized to facilitate understanding of changes of the curve in the hybrid cathode.


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Figure 7. Hybrid ratio dependence of rate performance with previous work for the organic-LFP hybrid cathode (a). In the case of absence of the data of 0.1C in previous work, the closer C-rate values were used. Hybrid ratio dependence of specific capacity (b) for the PHTPA-LFP hybrid cathode.


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Table 1. Rate performance, DC-IR and Warburg coefficient for PHTPA and Typical PTPA. Electrode Composition

Rate capability

DC-IR (Ω)

Warburg coefficient (Ω/s-1/2)

(100C,200C)

(Charge, Discharge)

(SOC=50%)

PHTPA:AB:PVDF=40:50:10

76%

51%

49, 39

8.4

Typical PTPA : AB:PVDF=40:50:10

75%

0%

46, 25

22.7

PHTPA:AB:SWCNT:PVDF=80:9:1:10

62%

46%

31, 26

13.9

Table 2. Rate performance, DC-IR, Rct and Warburg coefficient for LFP and PHTPA-LFP Hybrid. Active material

Rate capability

DC-IR (Ω)

Rct (Ω)

(10C,20C)

(Charge, Discharge)

(SOC=50%)

Warburg coefficient (Ω/s-1/2) (SOC=50%)

LFP

30%

0%

81, 59

45

32.7

PHTPA-LFP(20:80)

60%

50%

59, 49

15

11.8


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Table of contents (TOC) graphic


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Hyperbranched triphenylamine polymer for ultra-fast battery cathode

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