Poly-É¤-glutamate Binder to Enhance Electrode Performances of P2

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Poly-#-glutamate Binder to Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-ion Batteries Yusuke Yoda, Kei Kubota, Hayata Isozumi, Tatsuo Horiba, and Shinichi Komaba ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01362 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

Poly-ɤ-glutamate Binder to Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-ion Batteries Yusuke Yodaa, Kei Kubotaa,b, Hayata Isozumia, Tatsuo Horibaa,b, and Shinichi Komaba*a,b a

Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan.

b

Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyoku, Kyoto 615-8245, Japan

KEYWORDS: sodium ion batteries, layered materials, Na2/3Ni1/3Mn2/3O2, water-soluble binder, Poly-ɤ-glutamate. ABSTRACT: P2-Na2/3Ni1/3Mn2/3O2 (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage. Although by charging up to 4.5 V vs. Na+/Na, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of 23.2% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-ɤ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the 1st cycle and capacity retention of 89% after 50 cycles, while the PVdF electrode exhibits only 80% and 71%, respectively. AC impedance measurements reveal that the PGluNa electrode shows much lower resistance during cycles compared with the 1 ACS Paragon Plus Environment

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PVdF one. From surface analysis and peeling test of the electrodes, the PGluNa binder covers surface of the P2-NiMn particles and suppresses electrolyte decomposition and surface degradation. The PGluNa binder further enhances mechanical strength of the electrodes and suppresses electrical isolation of the P2-NiMn particles during sodium extraction. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance electrochemical performances of high-voltage positive electrode materials for Na-ion batteries.

1. INTRODUCTION Since lithium ion batteries were commercialized in 1991, the market of the batteries has been expanding rapidly and demands for them become diversified from electronic devices to electrified vehicles or stationary power storage systems. Such rapid expansion of the market naturally brings us anxiety about depletion and production restraint of natural resources. Therefore, environment-friendly batteries to contribute to sustainable society are recently focused, so that, research and development of rechargeable batteries without costly or toxic elements such as Li, Co, and Pb are attracting our attention.1 Sodium ion battery is one of the promising innovative batteries among many choices of proposed new batteries, because sodium is an abundant element and the resources are low-cost, which is preferable to large-scale application.2 As for a positive electrode material for the battery, our group has reported P2-NaxFe1/2Mn1/2O2 made from only abundant elements, which is very favorable in terms of practical perspective.3

P2-

NaxFe1/2Mn1/2O2 shows a high discharge capacity of 190 mAh g-1 in a voltage range of 1.5

2 ACS Paragon Plus Environment

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– 4.3 V to achieve high energy density comparable with lithium ion battery materials. However, its poor cyclability still remains to be a serious issue, for long-term cycling stability is required along with high energy density for practical rechargeable batteries. P2-Na2/3Ni1/3Mn2/3O2 (hereafter denoted as P2-NiMn) is another candidate layered material which shows a high operating potential and high capacity density.4

P2-

Na2/3Ni1/3Mn2/3O2 (hereafter denoted as P2-NiMn) is another candidate layered material which shows a high operating potential and high capacity density.4 P2-NiMn delivers large

rechargeable capacity of ca. 160 mAh g-1.

The charge/discharge mechanism was

investigated with operando XRD and DFT calculations.4-5 P2-NiMn transforms into O2type phase by Na extraction on charge to 4.5 V and reversibly changes back into P2-type one by Na insertion on discharge to 2.0 V. Ni2+/4+ redox mainly contributes into the reaction and partial redox of oxide ion is predicted.5 However, its rechargeable capacity decays rapidly during charge/discharge cycles because the P2-O2 transition on the flat voltage plateau at 4.2 V during charge results in large volume shrinkage of 23% accompanied by gliding of Ni1/3Mn2/3O2 slabs to avoid strong electrostatic repulsion between oxide ions in the interslab space (see schematic illustration of the phase transition in Supporting Information, Figure S1), leading to electric isolation of P2-NiMn particles as reported in previous reports.5-8 Standard electrode potential of Na is ca. 0.3 V higher than that of Li, and the average working voltages of NaxMeO2 (Me = transition metals) in Na cells are generally lower than those of LixMeO2 in Li cells. Therefore, larger capacity is required for the positive electrode materials of Na cells than that of Li cells in order to achieve comparable energy density as Li-ion batteries by compensating the lower cell voltages for Na cells. To 3 ACS Paragon Plus Environment


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achieve larger capacity in NaxMeO2, operation in a wider potential range is necessary. The wider potential range and larger capacity lead to large volume change triggering electric isolation among active materials and conductive agent, and detachment of the composite layer from aluminum current collector, resulting in increase of electrode resistance and poor cyclability9-10. One of breakthrough ideas for this drawback of P2NiMn is to modify the material composition to suppress the volume change during charge/discharge cycles.11-18

For example, we have succeeded in mitigating volume

shrinkage of P2-NiMn during charging by proper amount of Cu or Ti substitution for Ni or

Mn,

respectively6,

19

,

and

enhancing

the

cyclability.

Specifically,

P2-

Na2/3Cu1/12Ni1/4Mn2/3O2 as a copper-substituted product and P2-Na2/3Ni1/3Mn1/2Ti1/6O2 as a titanium-substituted one for P2-NiMn show smaller volume-shrinkage ratios, defined as a ratio of lattice volumes between 4.5 V charged and pristine samples, 11% and 12%, respectively, compare to 23% of P2-NiMn. Our finding of P2-NiMn showing the large volume change during charge/discharge motivated us to adapt new binders for P2-NiMn electrodes.

Our group previously

demonstrated that the reversibility and cycle life of high-capacity electrochemical lithiation of Si-based negative electrodes was improved by applying water-soluble polysaccharide20,

sodium

poly-acrylate

(PAANa)21,

or

lithium

poly-ɤ-glutamate

(PGluLi)22 as the functional binder instead of PVdF. PGluLi has three times higher adhesion strength and the Si negative electrodes shows better electrochemical performances than PVdF22. Moreover, Si negative electrodes with sodium polyglutamate (PGluNa) or potassium polyglutamate (PGluK) binder show excellent electrochemical performances similar to those with PGluLi one22. The polyglutamate binders are greatly


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effective in enabling to improve the electrode performance accompanied by large volume change such as Si with a volume expansion of ca. 300% from Si to Li3.75Si by lithiation. Hence, new binder enhancing adhesion strength of the composite electrode and good electrical contact among active material, conductive agent, and current collector is most likely to improve electrochemical performance of P2-NiMn. Another critical properties responsible for the poor cyclability of P2-NiMn is surface reactivity involving electrolyte decomposition, especially at high voltage region above 4.1 V vs. Na, which leads to continuous low Coulombic efficiency during cycles.7 Recently, we reported that uniform coverage of PGluLi binder on Si particles effectively suppresses the electrolyte decomposition and improves cyclability of the Si/graphite composite negative electrodes for lithium-ion batteries22 unlike conventional PVdF binder. Therefore, it is expected that sodium poly-ɤ-glutamate (PGluNa) binder enhances cycle performance of P2-NiMn. To the best of our knowledge, almost all previous studies on positive electrode materials of sodium ion batteries reported so far are demonstrated with the composite electrode only with PVdF binder. In few papers, the effects of different binders on the electrochemical properties of layered positive electrode materials are described.7, 23-24 Since P2-NiMn has water-resistance property even by being exposed to humid air4, we apply water-soluble binders to P2-NiMn. Nevertheless, we find many papers claiming that it is impossible to apply water-soluble binder to layered positive electrode materials because of the damage by Na+/H+ exchange during water-slurry processing.7, 25 In this manuscript, we show that P2-NiMn electrode with water-soluble PGluNa binder demonstrates superior electrochemical performance to that with PVdF. Mechanism of the enhanced electrochemical property is investigated in detail by



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mechanical property measurements of the electrodes with PGluNa binder and surface analyses of P2-NiMn electrodes after charge and discharge cycle.

2. EXPERIMENTAL 2.1 Synthesis of Na2/3Ni1/3Mn2/3O2 Na2/3Ni1/3Mn2/3O2 was prepared by a solid-state reaction.

A stoichiometric mixture of

reagent grade Na2CO3 (Nacalai Tesque, Inc., 99.8%), Ni(OH)2 (Wako Pure Chemical Industries, Ltd., 95 %), and Mn2O3 prepared by heating MnCO3 (Kishida Chem. Co., Ltd., 44% as Mn content) at 700 oC were ball-milled with acetone for 12 h at 600 rpm. The mixture was dried and pressed into pellets, followed by heating at 900 oC for 24 h in air according to the previous report.4

2.2 Phase identification Phase purity of P2-type Na2/3Ni1/3Mn2/3O2 was confirmed by using an X-ray diffractometer (MultiFlex, Rigaku Co., Ltd.) equipped with a high-speed position sensitive detector (D/teX Ultra, Rigaku Co., Ltd.).

Non-monochromatized Cu Kα

radiation was utilized as an X-ray source with a nickel filter. The samples were set in a homemade air-tight sample holder during XRD measurement to avoid air exposure.

2.3 Preparation of composite electrodes Binders used in this study are as follows: PVdF (Polysciences, Inc.), sodium carboxymethyl cellulose (CMCNa, substitution degree = 0.8 – 1.0, Daicel Chemical Industries, Ltd.), and 6 ACS Paragon Plus Environment

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sodium poly-ɤ-glutamate (PGluNa). PGluNa was prepared by neutralizing poly-ɤ-glutamic acid (Wako Pure Chemical Industries, Ltd., molecular weight: 1500000 – 2500000 mol g-1) with stoichiometric amount of sodium hydroxide dissolved in deionized water.22 Their molecular structures are shown in Figure 1. Na2/3Ni1/3Mn2/3O2 powder and acetylene black (AB, Strem Chemicals, Inc.) were mixed at a weight ratio of 8 : 1 by using a mortar and pestle. Obtained mixed powder was further mixed with PVdF binder and adequate amount of N-methyl-2-pyrrolidone (NMP, Kanto Chemical Co., Ltd., Japan) to prepare slurry of P2-NiMn: AB: PVdF (= 8: 1: 1 w/w). The slurry was cast on Al foil with a doctor-blade and dried at 80 oC under vacuum. All processes for preparing PVdF electrodes were carried out in an Ar-filled glove box not to expose the samples to air. As for CMCNa and PGluNa electrodes, P2-NiMn and AB were mixed with each binder and adequate amount of deionized water to prepare aqueous slurry containing P2-NiMn : AB : binder = 8 : 1 : 1 (w/w) in atmosphere. The slurry was cast on Al foil with a doctor-blade and dried at 80 oC in air followed by drying at 150 oC under vacuum overnight. It is noted that slight decrease in the mass of CMCNa and PGluNa powder was observed below 200

o

C in air in the

thermogravimetric measurement (see Figure S2), indicating no significant decomposition and damage of the binder polymer during drying the electrodes at 150 oC.

2.4 Electrochemical measurements The electrochemical measurement was carried out by using R2032-type coin cells consisting of the working electrode of P2-NiMn, glass fiber filter as separator (GB-100R, ADVANTEC, Co.), electrolyte solution of 1.0 mol dm-3 NaPF6 dissolved in propylene carbonate (PC) (Kishida Chemical Co., Ltd.), and sodium metal (Purity > 99 %, Kanto Chemical Co., Ltd., Japan) as a 7 ACS Paragon Plus Environment


counter electrode. Fabrication of the coin cells was carried out in an Ar-filled glove box. Galvanostatic charge-discharge test was conducted at ca. 25 oC in a voltage range of 2.0 – 4.5 V at C/20 (13.0 mA g-1) with a charge/discharge measurement system (TOSCAT-3100, TOYO System Co., Ltd.). Discharge-rate capability was evaluated from C/20 (13.0 mA g-1) to 2C (518 mA g-1). Self-discharging performance of the P2-NiMn composite electrodes was investigated by resting for 10 days after the 5th charging at C/20 up to 4.5 V.22 AC impedance measurement of the P2-NiMn composite electrodes were done with three electrode cells (TOYO System Co., Ltd.) after discharging to 3.5 V vs. Na+/Na in a frequency range from100 kHz to 10 mHz at an amplitude of 10 mV using a potentiostat/frequency response analyzer (VMP3 Multi-channel potentiostat, Bio-Logic Science Instruments Co., Ltd.).

2.5 Surface analysis of P2-NiMn electrodes X-ray photoelectron spectroscopy (XPS) was applied to analyze electrode surface with an Xray photoelectron spectrometer (JPS-9010MC, JEOL, Ltd.) with non-monochromatic Mg Kα used as an X-ray source. For the XPS, samples were prepared as follows. Coin cells after cycles were disassembled and the electrodes were rinsed moderately with diethyl carbonate (DEC) solvent. The rinsed electrodes were dried in Ar-filled glove box at room temperature overnight. Electrode surface morphology and particle distribution of P2-NiMn in the electrodes with PVdF and PGluNa binder were observed with a scanning electron microscope (SEM, JIB4500FE, JEOL Ltd.). Surface of the P2-NiMn particles in the tested electrodes was observed with a transmission electron microscope (TEM, JEM-2100F, JEOL Ltd.) at an accelerating voltage of 200 kV. For the TEM observation, samples were prepared as follows. Coin cells after


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cycles were disassembled and the composite layer was peeled off from aluminum collector and the composite layer was dispersed in dehydrated ethanol.

2.6 Evaluation of mechanical properties for P2-NiMn electrodes Adhesive strength of the P2-NiMn composite layer formed on aluminum foil was measured with a peel tester (STA-1150, A&D Co., Ltd.) and double-side tape (Scotch tape, SPS-12, 3M Japan, Ltd.) referring to the previous report.22

The measurement

conditions are as follows: 90 degrees for peeling angle, 10 mm min−1 for peeling speed, and 50 N for load range. A simplified peel test was also performed with an adhesive tape (Cellotape, CT-18, NICHIBAN Co., Ltd.).

And also scratch test of the PVdF and

PGluNa electrodes was performed with Surface And Interfacial Cutting Analysis System (SAICAS) (DN-GS, DAYPLA WINTES Co., Ltd.).26-27

The measurement was

performed in constant speed mode at horizontal and vertical velocities of 2 and 0.1µm s-1, respectively. Cutting depth was around 10 µm for both PVdF and PGluNa electrodes. The cutting blade was made of boron nitride with a cutting edge of 1 mm wide. The PVdF and PGluNa electrodes were immersed in propylene carbonate (PC) overnight at room temperature before the scratch test to simulate actual electrodes filled with electrolyte solution.

2.7 Evaluation of physical properties for PVdF, CMCNa, and PGluNa Thermal stability of PVdF, CMCNa, and PGluNa powder was examined by using a thermogravimetry (TG, DTG-60, Shimadzu Corporation). The TG analysis was performed in air at a heating speed of 5 °C min-1.



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Electrolyte absorption rate of PVdF, CMCNa, and PGluNa was examined by immersing the pure binder films in propylene carbonate for one day. The rate was quantified based on the weight gain of the films before and after immersion in propylene carbonate according to the following formula.28

/% =

/ − / × () /

3. RESULTS AND DISCUSSION 3.1 Binder dependency of electrochemical properties P2-NiMn is known to show no change of the XRD pattern when exposed to humid air.4 To survey compatibility of P2-NiMn with water as dispersion medium for water-soluble binder, structural stability of P2-NiMn was investigated by soaking the P2-NiMn in water. Figure 2 shows XRD patterns of P2-NiMn before and after immersing in water for one year. Before immersing the P2-NiMn sample synthesized, all diffraction lines can be indexed as P2-type structure with space group of P63/mmc expect for 1/6 2/3 1/3 and 1/6 2/3 2/3 superlattice peaks at 27.5° and 28.6° in 2 θ , respectively.

The lattice

parameters are coincident with those in the literatures4-5 (see Table S1).

Even after

immersing for one year, all diffraction lines can be indexed as P2-type structure, and no evidence of hydrated phase and decomposition is found unlike P2-Na2/3CoO229 or P2Na2/3Co1/3Mn2/3O2.30 The lattice parameters of a- and c-axis were almost unchanged after immersing (see Table S1), indicating that the original P2-type structure was maintained. Moreover, full width at half maximum (FWHM) of 002 and 004 peaks is also almost the 10 ACS Paragon Plus Environment

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same before and after immersion (see Table S1), suggesting no change in crystallite size and lattice distortion before and after immersing for one year.

Consequently, it is

explicitly demonstrated that P2-NiMn is enough stable against water and water-soluble binder is applicable to P2-NiMn. Herein, CMCNa, which is a well-known water-soluble binder for positive and negative electrodes in lithium-ion batteries,31-32 and PGluNa as a new water-soluble binder is chosen to evaluate the binder effect on the electrochemical performance of P2-NiMn electrodes comparing with PVdF binder. Figures 3(a-c) present galvanostatic charge and discharge curves of P2-NiMn electrodes with PVdF, CMCNa, and PGluNa binders tested in the voltage range of 2.0 4.5 V. The PVdF electrode in Figure 3(a) shows the largest discharge capacity of 165 mAh g-1 among the electrodes and obviously flat voltage plateaus are observed at 4.2 and 4.0 V on the first charge and discharge, respectively. However, the flat voltage plateaus gradually become sloping profiles with larger polarization during charge/discharge cycles and the capacity on the plateau region above 4.0 V simultaneously degraded. On the other hand, the CMCNa and PGluNa electrodes in Figures 3(b) and 3(c) show discharge capacity of 155 and 151 mAh g-1 with nearly flat voltage plateaus above 4.0 V at the same potential in the initial curves. The voltage plateaus become sloping profiles during cycles which is similar to the PVdF electrode.

Both the CMCNa and PGluNa electrodes,

however, show no significant increase of polarization on the plateau region during cycles, resulting in improved capacity retention rate of 86% and 89%, respectively, after 50 cycles compared to 71% of the PVdF electrode, as shown in Figure 3(d). Moreover, initial Coulombic efficiencies of the CMCNa and PGluNa electrodes are 92% and 95%, respectively, which is much higher than 80% of the PVdF electrode. This result suggests



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that electrolyte decomposition at high voltage region above 4.0 V and/or electrical isolation of the P2-NiMn particles could be to some degree prevented by utilizing the CMCNa or PGluNa binder compared to the PVdF electrode.

Serious capacity

degradation of P2-NiMn during cycles was previously reported for the electrode with a PVdF binder when the upper cut-off voltage was as high as 4.5 V in the Na cells 5, 7, 33 and similar results are obtained in this study. It should be noted that the better capacity retentions of the CMCNa and PGluNa electrodes are not due to their smaller capacities because their cycle tests with the charge capacity limited to 140 mAh g-1 in 2.0 – 4.5 V revealed faster decay in discharge capacity for the PVdF electrode than that of the PGluNa one although Coulombic efficiency is almost same (see Figure S3). PGluNa and CMCNa electrodes also achieves better rate capability than PVdF one as shown in Figure 3(f). Indeed, the CMCNa and PGluNa electrodes show discharge capacity of 150 mAh g1

slightly lower than 160 mAh g-1 of the PVdF one at a low rate of C/20 in Figure 3 and

volume change of P2-NiMn could be one of the influencing factors. The difference, however, decreases gradually with increase in discharge rate from C/20 to 1C. Discharge capacity of the PGluNa and CMCNa electrodes exceeded that of the PVdF one at 2C. The superior rate performance is thought to result from lower resistance and better mechanical durability of the composite layer with the PGluNa and CMCNa binder than those in PVdF one. Static electrochemical performance is also examined by self-discharge test. Figure 4 shows variation in Coulombic efficiency of the PVdF, CMCNa, and PGluNa electrodes during cycles through storage for 10 days in open circuit after 5th charge to 4.5 V at room temperature. The PGluNa electrode shows 82% of Coulombic efficiency at the 5th cycle,


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which is higher than 72% of the PVdF one. Order of the Coulombic efficiency in the self-discharge test among the PVdF, CMCNa, and PGluNa electrodes is consistent with 1st Coulombic efficiency mentioned in Figure 3(e). The PGluNa electrode shows the highest efficiencies, which implies that PGluNa binder suppresses electrolyte decomposition during charge/discharge cycles and storage at high potential. We believe that the suppression of electrolyte decomposition is attributed to improved passivation by covering the P2-NiMn particle surface with PGluNa thin coating as described in our paper.22 In all the electrochemical measurements, the PGluNa electrode clearly exhibits better electrochemical performances in terms of capacity retention, Coulombic efficiency, rate capability, and self-discharge. Not only the kinetics of Na (de)intercalation into P2-NiMn but also passivation of the composite electrode are superior in the PGluNa electrode. We further evaluated the PVdF and PGluNa electrodes by AC impedance measurement. Figure 5 shows nyquist plots of the PVdF and PGluNa electrodes discharged to 3.5 V.

It is difficult to clearly deconvolute the semicircles and assign into electrolyte,

surface, charge-transfer, and contact resistances by refining the Nyquist plot. Therefore, we discuss the results simply with assignment of the semicircles as total electrode resistance for a P2-NiMn electrode in the three-electrode cell. The PVdF electrode in Figure 5(a) shows gradual increase in diameter of the semicircle indicating that the total resistance gradually becomes large upon cycles. On the other hand, the PGluNa electrode in Figure 5(b) exhibits small and almost constant diameter of semicircles during cycles and no significant increase in the total resistance is observed for 30 cycles. These results mean that the PGluNa electrode shows much lower electrode resistance than that of the



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PVdF one during cycles. Two mechanisms can be proposed for the lower resistance of the PGluNa electrode. One is lower surface and charge-transfer resistance by suppression of electrolyte decomposition and less deposition of the decomposition products on surface of the P2-NiMn particles.

Surface structure of the P2-NiMn particles might be also

maintained as reported for Al2O3-coated P2-NiMn.34-35

The other is lower contact

resistance due to suppressed electrical isolation of the P2-NiMn particles in the composite electrode by high adhesion strength of the PGluNa binder. To prove the hypotheses, surface and mechanical properties of the PGluNa and PVdF electrodes and physical properties of the PGluNa and PVdF polymers were further investigated as described below.

3.2 Surface analysis of electrodes X-ray photoelectron spectroscopy (XPS) measurement was carried out to investigate coverage of the PGluNa binder on the P2-NiMn particles in the electrodes. Three pristine PGluNa electrodes were prepared at different PGluNa content, x = 2, 5, and 10 in P2NiMn: AB: PGluNa (= 80: 10: x w/w). O 1s spectra in Figure 6(a) show a peaks at 529 eV in binding energy assigned to lattice oxygen of Na2/3[Ni1/3Mn2/3]O236, and peaks at higher binding energy between 531 and 533 attributed to sodium carbonate on surface of P2-NiMn, and peptide bond (-O=C-N-) and sodium carboxylate (-COONa) in PGluNa.3637

Relative intensity of the former peak of lattice oxygen decreases while those of the

peaks of PGluNa increase with increase in the PGluNa content, which indicates that content of the lattice oxygen decreases and that of PGluNa increases at surface of the electrodes as PGluNa content increases in the surface layer within a few-nanometer depth.


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C 1s spectra in Figure 6(b) show decrease in intensity of the peaks at 284 eV attributed to sp2 hybridized carbon (-C-C-) in AB as PGluNa content increases, while intensity of peaks observed at 286, 288, and 291 eV, assigned to -C-N-, peptide bond (-O=C-N-)37, and sodium carboxylate (-COONa)22, respectively, becomes higher, which suggests decrease in AB content with increase in PGluNa at surface of the electrodes. From these results, it is most likely that surface of the P2-NiMn and AB particles is progressively covered with the increase in PGluNa binder content. As expected, better cycle stability is observed for the 80:10:10 electrode than that of the 80:10:2 one (as seen in Figure S4), which should be due to the improvement of surface passivation. On the basis of the XPS results for the PGluNa electrodes, we compare XPS spectra of the P2-NiMn electrodes with PVdF or PGluNa binder.

No significant difference of

particle distribution and morphology for P2-NiMn is confirmed in SEM images for the PVdF or PGluNa electrodes before and after 50cycles (see Figure S5). Figure 7(a) shows O 1s XPS spectra of pristine PVdF and PGluNa electrodes.

In the PGluNa

electrode, a peak of lattice oxygen at 529 eV is weak and peaks attributed to sodium carbonate and sodium carboxylate at 532 and 533 eV, respectively, are stronger as mentioned for the 80:10:10 electrode above, while the PVdF electrode shows a stronger peak of lattice oxygen at 529 eV. This difference suggests that surface of the P2-NiMn particles in the PGluNa electrode is more covered than that in the PVdF one. Figure 7(b) presents C 1s spectra of pristine PVdF and PGluNa electrodes. Although we find no difference in intensity of a peak at 284 eV corresponding to AB in the Figure, the peaks at 286, 288, and 291 eV attributed to PGluNa binder clearly appear compared with those of PVdF binder. Furthermore, Mn 2p1/2 and 2p3/2 peaks at 654 and 642 eV, respectively,



are very weak for the PGluNa electrode, while those for the PVdF electrode are quite clear and the intensity is much higher in Figure 7(c). The result of all the XPS spectra in Figure 7 clearly indicates that PGluNa binder thin coating is formed on P2-NiMn and AB particles more than PVdF one in the pristine electrodes, which is consistent of our observation in Si-graphite composite electrode with PGluNa and PVdF binders as already described. According to our previous observations,22 PGluNa should be amorphous after drying the slurry on Al foil considering our evidence of formation of amorphous lithium polyglutamate film, which is advantage of homogeneous coverage on the whole surface of P2-NiMn. Additionally, lone pair of electrons of not only oxygen but also nitrogen atoms of PGluNa are possibly capable to form coordinate bond with Li+ ions to induce desolvation of solvated Li+ ions.22, 38 Similar desolvation of solvated Na+ ions possibly occurs at the P2-NiMn electrode with the functional binder of PGluNa to enhance the Na+ diffusion across the interface of P2-NiMn and electrolyte, which should affect the much lower impedance of the PGluNa electrode as shown in Figure 5. We further investigate surface of the P2-NiMn electrodes with PVdF or PGluNa binder after charge/discharge. Figure 8 shows Mn 2p, O 1s, and C 1s XPS spectra of the PVdF and PGluNa electrodes after the 1st and the 10th cycle. In the PVdF electrodes (Figures 8 (a-c)), no significant difference is observed for Mn 2p1/2 and 2p3/2 peaks between the 1st and 10th cycle (Figure 8(a)), which evidences that the P2-NiMn particles remain to be exposed to electrolyte even after 10 cycles in the PVdF electrode. O 1s spectra in Figure 8(b) also supports no significant change of lattice-oxygen peak at 529 eV. However, stronger peaks are observed at 533 – 534 eV attributed to electrolyte decomposition products such as oxygenated and carbonaceous species (-COONa, -C=O-).36 Furthermore,


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C1s spectra in Figure 8(c) also show weaker AB peak at 284 eV and -CH2-CF2- one at 291 eV after the 10th cycle. These results for the PVdF electrodes suggest accumulation of decomposition products mainly on surface of AB, and surface of the P2-NiMn particles is not covered with the decomposition products. On the other hand, Mn 2p1/2 and 2p3/2, O1s, and C 1s spectra of the PGluNa electrode in Figure 8(d-f) show no significant change of peaks between the 1st and the 10th cycle, which suggests that coverage with PGluNa on the P2-NiMn particles is kept and PGluNa binder passivates the P2-NiMn particles during charge/discharge cycles. We believe that the PGluNa binder maintains stable surface of the composite electrode during charge/discharge cycles due to suppression of electrolyte decomposition. It is worth mentioning that the PGluNa binder covers surface of the P2-NiMn and AB particles and assists for formation of the stable passivation layer, which is consistent of the better cycle stability, better rate-performance, higher Coulombic efficiency, and lower electrode resistance during cycles as mentioned above. Furthermore, we investigated surface condition of the P2-NiMn particles with TEM. Figure 9 shows TEM bright field images of the P2-NiMn particles in the PVdF or PGluNa electrodes before and after 10 and 30 cycles. Pristine PVdF electrode shows smooth surface of P2-NiMn particles with clear lattice fringe. However, quite rough surface with some holes and pockets is observed after 10 cycles, and cleavage is also formed after 30 cycles. On the other hand, surface of P2-NiMn in the PGluNa electrode shown in Figure 8(b) presents no remarkable change even after 30 cycles as similar as reported previously for LiCoO2 (LCO)39, in which coverage of LCO particles with SBRCMCNa binder and deterioration of the LCO surface after high-voltage cycles is



sufficiently suppressed.

Similarly, PGluNa binder coverage would protect P2-NiMn

particles against high-voltage damage, such as electrolyte decomposition and corrosive dissolution of P2-NiMn, which is quite consistent with the XPS results described above. The positive effects of the surface coverage of P2-NiMn on the electrode performance were previously reported with Al2O3-coating34-35 and surface treatment such as binder coverage and ceramic coating, these methodology would be also applicable for highvoltage electrode materials in Na-ion batteries. In addition to the surface coverage, high adhesion strength of the PGluNa binder is expected to be synergic origin of the better performances such as better cycle stability, better rate-performance, higher Coulombic efficiency, and lower electrode resistance during cycles as mentioned above. Therefore, mechanical properties of the PGluNa electrodes and PGluNa polymer were examined and the results are discussed in the next section.

3.3 Mechanical properties of PGluNa electrode and PGluNa polymer Vertically peeling test was conducted based on our previous paper21 as shown in Figure 10(a), to measure the adhesion strength between composite layer and aluminum current collector. The results shown in Figure 10(b) confirms the obvious difference of adhesion strength between the PVdF and PGluNa electrodes. When they were peeled at speed of 10 mm min-1, the PGluNa electrode was peeled off with load intensity of 2.75 N cm-1 on average, corresponding to 180 times adhesion strength of the PVdF electrode of 0.0154 N cm-1. After peel test, most of the composite layer of the PVdF electrode was totally removed from the aluminum foil without any black residue, while the composite layer of the PGluNa electrode was not peeled off from the aluminum foil as shown in


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Figure 10(c), suggesting that the PGluNa binder contributes the considerably higher adhesion strength than that of the PVdF electrode, agreeing with our previous study.22 The results also indicate that adhesion strength measured for the PGluNa electrode by 90o peel test in Figure 10(b) is corresponding to the strength between the composite layer and double-side tape. Actual adhesion strength of PGluNa binding between composite layer and aluminum current collector is extremely high and could not measure in our experimental condition. In addition, preliminary bending test for the PVdF and PGluNa electrodes, of which details are shown in Figure S6, convinces higher adhesion strength and flexibility of the PGluNa electrode than that of the PVdF one. These adhesion tests were conducted in dry condition without immersing in electrolyte, but the adhesion strength of dried electrodes is not the same as wet electrode with organic electrolyte solution. Therefore, another peel test with an adhesive tape was carried out for the PVdF and PGluNa electrodes with or without immersion in 1 mol dm-3 NaPF6 / PC electrolyte solution as shown in Figure S7. The PVdF electrodes show obvious difference with and without soaking in the electrolyte, and complete and partial detachment of PVdF composite layer from Al foil, respectively. On the other hand, no detachment of PGluNa composite layer from Al current collector is observed with or without immersing in the electrolyte, convincing higher adhesion strength of the PGluNa electrodes than the PVdF one even after soaking in the electrolyte. Quantitative analysis of adhesion strength for the PVdF and PGluNa electrodes was further conducted in wet condition with electrolyte solvent. Wet PVdF and PGluNa electrodes with PC were used simulating actual wet electrodes in the cells. The adhesion strength is evaluated by using SAICAS27, 40 and horizontal force is detected during slicing



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the composite layer in the electrodes at the depth of 10 µm by using a boron nitride blade (width: 1 mm) (see photos of the electrode and blade during the operation in Figures 11(a-b)). Although the surface morphologies of the pristine PVdF and PGluNa electrodes were almost the same in appearance in the SEM images (as described above in Figure S5), the electrodes reveal different load intensities along horizontal direction in Figures 11(c-d). Average cutting load of the PVdF electrode is 0.026 N, while that of the PGluNa electrode is 1.27 N about 50 times higher than that of the PVdF one. The higher adhesion strength of the wet PGluNa electrode than the PVdF one is almost identical with the results of peel test with or without soaking in electrolyte.

The results suggest that

PGluNa binder has higher adhesion strength not only at the interface between the composite and Al foil but also of the inside of the composite, i.e. particle binding among P2-NiMn and AB particles, compared to those for the PVdF binder. It is reasonably believed that the high adhesion strength of the PGluNa binder is highly beneficial to suppress electrical isolation of the electrode materials during cycling accompanied with the large volume change of ca. 20%,4,

6, 19

resulting in the excellent electrochemical

performance of the P2-NiMn electrodes. Another factor which affects adhesion characteristics is electrolyte absorption of binder. Too much uptake of electrolyte solution inside the composite would lead to poor electronical connection among the P2-NiMn28 and AB particles and current collector. No electrolyte absorption, on the contrary, surely lead to poor electrode performance, because diffusion route of Na+ ions would be restricted and redox reaction of P2-NiMn would be deteriorated, resulting in poor ionic diffusion and high electrode resistance.

For the

electrolyte absorption test, pure polymer films of PVdF, CMCNa, and PGluNa prepared


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by casting and drying the polymer solution (see Figure S8) were immersed in PC for one day in an Ar-filled grove box. Weight gain of the films was compared before and after storage in PC and electrolyte absorption rate was calculated as compared in Figure 12. The PGluNa and CMCNa films show electrolyte absorption rate of 3.5% and 18%, respectively, lower than 25.5% of PVdF.

The much lower and moderate electrolyte

absorption rate of PGluNa binder is advantageous to realize optimum balance of the Na+ ion diffusion and electric solid-contact between P2-NiMn, AB, and Al foil for the superior performance of P2-NiMn. To sum up the mechanical properties of PGluNa polymer and the P2-NiMn electrode, the moderate electrolyte penetration and higher adhesion strength of PGluNa binder contribute to reversible sodium insertion / extraction and cycle stability of P2-NiMn with suppression of electrolyte decomposition on surface of the PGluNa coated P2-NiMn, and detachment and electrical isolation of the P2-NiMn particles, resulting in low initial Coulombic efficiency, better cycle and rate stability of the P2-NiMn composite electrodes in Na batteries compared to conventional PVdF binder. Considering the above discussion and results, what is elucidated in this study is illustrated in Figure 13.

In the PVdF electrode, electrolyte decomposition products

deposit on the P2-NiMn particles and surface structure deteriorates gradually during cycles. And also electrical isolation among the P2-NiMn particles, AB ones, and Al current collector due to the large volume change of P2-NiMn during charge/discharge increases the total electrode resistance and accelerates capacity decay during cycles. On the other hand, the PGluNa electrode in which PGluNa covers surface of P2-NiMn particles and the coverage film has less electrolyte swelling than PVdF, provides stable



surface structure and better electrochemical performance as exhibited in Figures 3, 4, and 5. Functional binders acting as a passivation layer with ionic conductivity and an highly adhesive binder such as CMCNa and PGluNa should be a key factor for not only negative electrode materials but also positive ones14, 41-46 to enhance the electrochemical properties of the electrode materials in Li-, Na-, and K-ion batteries, and thus battery performance would be upgraded by applying a proper functional binder.

4. CONCLUSION We applies water-soluble binders of CMCNa and PGluNa to P2-Na2/3Ni1/3Mn2/3O2 composite electrodes for high-voltage sodium-ion batteries. The PGluNa electrodes show superior electrochemical properties in terms of reversible capacity, Coulombic efficiency, capacity retention, and rate capability compared to the PVdF electrodes. The PGluNa binder is efficient to suppress self-discharge because of surface coverage of the P2-NiMn particles with PGluNa thin layer and suppression of both electrolyte decomposition and corrosive deterioration of the oxide.

Moreover, the PGluNa electrodes realize much

lower electrode resistance during cycles than the PVdF ones, which should be due to the higher adhesion strength and the moderate electrolyte penetration leading to suppressed electrical isolation of the P2-NiMn particles induced by the volume change of P2-NiMn. Finding of the efficient binder towards high-voltage layered sodium transition metal oxides paves a new way to improve electrode performance of high-voltage positive electrode materials for sodium-ion batteries instead of a conventional PVdF binder. However, all the results in this study are collected for simple P2-NiMn without modification of the composition. Therefore, combination of partial metal-substitution and 22 ACS Paragon Plus Environment

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PGluNa binder for the P2-NiMn will lead to further enhancement of electrochemical performance of the P2-NiMn positive electrodes for sodium-ion batteries.

ASSOCIATED CONTENT Supporting Information. Lattice parameters of Na2/3Ni1/3Mn2/3O2, TG curves of binder powder, charge–discharge curves of Na/P2-NiMn cells in limited charge capacity, capacity retention of PVdF (= 80:10:10 w/w), PGluNa (80:10:10 w/w), and PGluNa (80:10:2 w/w) electrodes, photos of the electrodes after bending test, photos of peel tests, SEM images, and photos of binder films.

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

ACKNOWLEDGEMENT This study was in part supported by Adaptable and Seamless Technology Transfer Program (A-STEP) of JST. The authors thank Dr. Ichihashi and Prof. Idemoto, TUS, for TEM measurements.

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Poly-ɤ-glutamate Binder to Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-ion Batteries

For Table of Contents Only


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CH2OR H CH2

CF2

n

O O

O

H OR

H

H

OR

NH

H

n

O

Na+

O

R=H, or CH2COONa PVdF

CMCNa Figure 1. Molecular structures of PVdF, CMCNa, and PGluNa.

ACS Paragon Plus Environment

PGluNa

n

104

116 110 112

(b)

103

1/6 2/3 1/3 1/6 2/3 2/3 004 100 101 102

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

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002


(a)

10

20

30

40

50

60

70

2deg.(CuK) Figure 2. (a) X-ray diffraction patterns of as-synthesized P2Na2/3Ni1/3Mn2/3O2 and (b) P2-Na2/3Ni1/3Mn2/3O2 immersed in water for one year.


20th10th

(b) 5.0 4.5 Voltage / V

4.0 3.5 3.0 2.5 2.0

th

th

20 10 0

(d)

1st

50

100

150

3.5 3.0 2.5

160 140 120 100 80

PVdF CMCNa PGluNa

60 40 20

50

100

150

Capacity / mAh g-1

20

30

Cycle number / -

40

50

20th 10th

3.5 3.0 2.5

200

1st

20th 10th 0

50

100

150

200

Capacity / mAh g-1

(f)

100

1st

4.0

1.5

98

200

C / 20 C / 10

C/5

C / 20

2C

1C

180 160

96 94 92 90

PVdF CMCNa PGluNa

88 86

140 120 100 80 60

PVdF CMCNa PGluNa

40

84

20

82 10

PGluNa

2.0

1st

20th10th 0

(e)

180

0

4.0

200

200

(c) 5.0 4.5

1st

20th 10th

2.0

1st

Capacity / mAh g-1

CMCNa

Voltage / V

PVdF

Coulombic efficiency / %

Voltage / V

(a)4.5

Capacity / mAh g-1

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


Capacity / mAh g-1

Page 33 of 43

0

10

20

30

40

Cycle number / -

50

0

5

10

15

20

25

Cycle number / -

Figure 3. Charge and discharge curves of (a) PVdF, (b) CMCNa, and (c) PGluNa electrodes in Na cells at a rate of C/20. (d) Capacity retention, (e) Coulombic efficiency, and (f) rate capability of PVdF, CMCNa, and PGluNa electrodes.


30


(a)

5.0

PVdF CMCNa PGluNa

4.5

Voltage / V

4.0 3.5 3.0 2.5 2.0 1.5

0

50

100

150

200

Capacity / mAh g-1

(b) 100 Coulombic efficiency / %

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

90

PGluNa

80 70

CMCNa

60

PVdF

50 40 30 20 10 2

3

4

5

6

7

8

9

Cycle number / -

Figure 4. Self-discharge test carried by 10-day storage of fully charged P2NiMN//Na cells up to 4.5 V of 5th charge. (a) Charge and discharge curves of PVdF, CMCNa, and PGluNa electrodes at 5th cycle, in which the cell was stored under open circuit condition for 10 days between 5th galvanostatic chare and discharge and (b) Coulombic efficiency variation of PVdF, CMCNa, and PGluNa electrodes inserting 10-day storage after 5th charge up to 4.5 V.


Page 34 of 43

Page 35 of 43

4000

(b)

PVdF

4000

3000

3000

2500

2500

2000

1st 5th 10th 20th 30th

1500

10th 1500 1000

500

500

0

500

1000 1500 2000 2500 3000 3500 4000

0

400

300

200

1st 5th 10th 20th 30th

100

2000

1000

0

PGluNa

3500

-Im(Z) / 

3500

-Im(Z) / 

(a)

-Im(Z) / 

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


5th 1st

0 0

100

20th

200

300

400

Re(Z) / 

30th

0

500

1000 1500 2000 2500 3000 3500 4000

Re(Z) / 

Re(Z) / 

Figure 5. Nyquist plots of P2-NiMn electrodes with (a) PVdF and (b) PGluNa binder in three electrode cells, measured at 3.5 V vs. Na+/Na on discharge.



(a)

O1s

-COONa

-N-C=O-

2wt.ratio 5wt.ratio 10wt.ratio

540

Na2CO3

Nax(Ni, Mn)O2

535 530 Binding energy / eV

(b) C1s

-CH2- CH2-

525

AB

-N-C-

2wt.ratio 5wt.ratio 10wt.ratio -N-C=O-

Na2CO3

NaOOC-

295

290 285 Binding energy / eV

280

Figure 6. XPS spectra of (a) O 1s and (b) C 1s of as-prepared P2-NiMn electrodes with PGluNa binder of which binder rations are x = 2, 5, and 10 in 8 : 1 : x = P2-NiMn : AB : PGluNa by weight.


Page 36 of 43

Page 37 of 43 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


(a) O 1s

-COONa

(b) C 1s

Na2CO3

-N-C=O-

-N-C=O-

(c)

-CH2-CH2-

-N-C-

Mn 2p3/2

NaOOC-CH2-CF2-

Mn 2p1/2

PVdF PGluNa

PVdF PGluNa

535

530

Binding eV Bindengenergy energy/ / eV

525

Mn 2p

AB

Na2CO3

Nax(Ni, Mn)O2

540

-CF2－CH2-

295

PVdF PGluNa

290

285


280

665

660

655

650

645

640

635


Figure 7. XPS spectra of (a) O 1s, (b) C 1s, and (C) Mn 2p of the pristine P2-NiMn electrodes with PVdF and PGluNa binder before cycles.



(a) Mn2p

(b) O1s Oxygenated and carbonaceous species

Mn 2p1/2

(c) C1s

Na2CO3

AB Na2CO3

Nax(Ni, Mn)O2

NaxPFyOz

Page 38 of 43

‐CH2‐CF2‐

10th cycle 1st cycle

Mn 2p3/2 10th cycle

‐CF2‐CH2‐ 10th cycle 1st cycle

st

1 cycle

660

655

650

645

640

635

630

540

Binding energy Bindeng energy/ /eV eV

(d) Mn2p

535

530

525


(e) O1s

290

(f) C1s

Na2CO3

285

Binding energy eV Bindeng energy // eV ‐CH2‐CH2－ ‐N‐C‐

280

AB

‐N‐C=O‐ Nax(Ni, Mn)O2

Mn 2p1/2

660

655

650

Mn 2p3/2

645

640

Bindengenergy energy/ / eV Binding eV



635

630

540

535

530

525

Binding eV Bindeng energy energy / eV


Na2CO3 ‐NaOOC‐

290

285

Bindingenergy energy/ eV / eV Binding

Figure 8. XPS spectra of (a) Mn 2p, (b) O 1s, and (c) C 1s of P2-NiMn electrodes with PVdF binder and (d) Mn 2p, (e) O 1s, and (f) C 1s of P2-NiMn electrodes with PGluNa binder after 1 (black) and 10 (green) cycles in Na cells.


280

Page 39 of 43 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


(a) PVdF

Pristine

10 cycles

30 cycles

(b) PGluNa

Pristine

10 cycles

30 cycles

Figure 9. TEM images of Na2/3Ni1/3Mn2/3O2 with (a) PVdF, and (b) PGluNa binder at pristine(upper) and after 10 (middle) and 30 cycles(bottom).



(a)

Al foil Composite layer Double-side tape Metallic plate

(b)

5 4

Load / N cm-1

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

Page 40 of 43

PGluNa

3 2 1

PVdF

0 0

50

100

Time / sec.

(c)

Al foil

PVdF

Double-side tape

PGluNa

Composite layer

Al foil

Double-side tape Figure 10. (a) Schematic illustration of 90o peel tests. (b) Results of 90o peel test of the P2-NiMn electrode with PVdF and PGluNa binder formed on Al foil. (c) Photos of PVdF and PGluNa electrodes after the peel test.


Page 41 of 43

(b)

1 mm

3

30

2

20

1

10

0

0

100

200 300 Time / sec.

400

0 500

(d) F(Horizontal) / N

(c)

PGluNa

Depth / m

PVdF

1 mm

3

30

2

20

1

10

0

0

100

200 300 Time / sec.

400

Depth / m

(a)

F(Horizontal) / N

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


0 500

Figure 11. Photos of cutting blades and the composite electrodes with (a) PVdF and (b) PGluNa binder during slicing the electrodes with SAICAS and the results of horizontal cutting load and corresponding depth for (c) PVdF and (d) PGluNa electrodes.



50

Electrolyte absorption rate / %

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

40

30

25.5 18

20

10

3.5 0

PVdF

CMCNa

PGluNa

Binder film

Figure 12. Electrolyte absorption rate of PVdF, CMCNa, and PGluNa films containing no P2-NiMn nor AB.


Page 42 of 43

Page 43 of 43 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


(a)

(b)

PVdF

P2‐NiMn particle AB PVdF

Decomposition products

PGluNa

PGluNa

After cycles

‐ Continuous electrolyte decomposition ‐ Surface deterioration of P2‐NiMn ‐ Electrical isolation

‐ Surface coverage by binder ‐ Stable SEI ‐ Good electrical contact

Figure 13. Schematic illustrations of P2-NiMn composite electrodes with (a) PVdF and (b) PGluNa binder before and after charge/discharge cycles.


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