High-Performance Lithium Metal Rechargeable Battery using Ultrafine

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High-Performance Lithium Metal Rechargeable Battery using Ultrafine Porous Polyimide Separator with Three-Dimensionally Ordered Macroporous structure Motoko Nagasaki, and Kiyoshi Kanamura ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00537 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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ACS Applied Energy Materials

High-Performance Lithium Metal Rechargeable Battery using Ultrafine Porous Polyimide Separator with Three-Dimensionally Ordered Macroporous structure Motoko Nagasaki, Kiyoshi Kanamura* Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan KEYWORDS. Lithium metal; Lithium metal rechargeable battery; SEI; Polyimide separator; Electrolyte solution; Ethylene carbonate; LiPF6

ABSTRACT. It is very important to control the compositional and morphological changes of lithium metal anode in order to improve the cycle performance of lithium metal battery (LMB). In this work, we report that the combination of an ultrafine porous polyimide (PI) separator with three-dimensionally ordered macroporous (3DOM) structure and an electrolyte composed of ethylene carbonate (EC) solvent with high dielectric constant containing LiPF6 can improve the cycle performance of LMB using Li4Mn5O12 cathode. In LMBs in which depositions of lithium

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with high reactivity are repeated on the anode at every cycle, undesirable side reactions generating deteriorated products are promoted at interface between fresh deposited metallic lithium and electrolyte. The reaction of LiPF6 with water and subsequent undesirable side reactions hardly occur in EC solvent with high dielectric constant. In addition, the size and shape of the deposited lithium particles on the cycled anode in the cell using EC solution are uniform, so that EC solvent is suitable for LMBs. The 3DOM PI separator is superior to the conventional polypropylene separator in terms of the uniform current density distribution in addition to high permeability and reservation for electrolyte with low fluidity such as EC solution.

1. INTRODUCTION A lithium metal is an ideal anode material because it has low oxidation-reduction potential and high capacity density. Therefore, lithium metal batteries (LMBs) employing lithium metal anodes have been developed as a next generation battery with a high energy density. In LMB, metallic lithium deposits and dissolves during the charging/discharging processes and the morphological change of the anode occur at every cycle. The concentration of current density and promotes the lithium dendrite formation on the anode during the long-term charge/discharge


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cycles. Dendrites break away from the anode bulk and become an irreversible capacity (so-called dead lithium metal), resulting in the poor cycleability. Furthermore, dendrites penetrate into the separator, in the worst case it breaks through the separators, resulting in a short circuit of cell. It has been reported that a film so-called Solid Electrolyte Interphase (SEI) is formed at the interface between a lithium metal anode and an organic electrolyte, resulting from electrochemical decompositions of electrolyte in charging/discharging processes. It has been also reported that the chemical composition of the SEI film significantly affects the morphology of electrodeposited lithium. Therefore, it is very important to understand the chemical composition of SEI film and control the morphology of deposited lithium in order to suppress the dendrite formation and improve the cycle performance of LMB. Generally, polyolefine (e.g. polypropylene (PP)) membranes with one dimensional pore structure shown in Figure 1(a) are used as separators in conventional lithium ion batteries (LIBs). When such separators in which lithium ions hardly move in the horizontal direction are applied to LMBs, non-uniformly deposited lithium by non-uniform distribution of current density penetrates into the separator and easily causes a short circuit. In previous reports1-4, we have been developed an ultrafine porous polyimide (PI) membrane with three-dimensionally ordered macroporous (3DOM) structure showed in Figure 1(b), in which lithium ions can transfer in three dimensional directions. By using 3DOM PI membrane as a separator for LMBs, it is possible to provide the uniformly deposited lithium owing to the uniform distribution of current density. In addition, 3DOM PI membrane has the advantages of electrolyte permeability and preservation owing to a high porosity of ca. 70 % and affinity to organic electrolyte. Though an ethylene carbonate (EC), which is a cyclic carbonate ester, is desirable as solvent for electrolyte due to its high dielectric constant and salt solubility, it can hardly permeate into a commercially


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available polyolefin separator because of its high viscosity, low wettability and low affinity. 3DOM PI membrane can contain EC alone without mixing a linear carbonate ester, such as ethylmethyl carbonate (EMC) or dimethyl carbonate (DMC) whose wettability is high but dielectric constant are low. A spinel-type Li4Mn5O12 has been reported as an attractive cathode material due to its high theoretical capacity of 163 mAh g-1 in the 3 V region and good cycleability owing to its crystal structure which is stable against the insertion and extraction of lithium5,6. We have reported that a pristine Li4Mn5O12 can be inserted by lithium ions in its host structure and LMB using Li4Mn5O12 cathode can start from discharging process7. In this case, the cycleability of anode is improved due to the uniform anode surface (Figures S1 and S2 in Supporting Information). In this work, we investigated the effect of 3DOM PI separator on the cycle performance for the LMB starting from discharging process by using Li4Mn5O12 cathode, compared with a conventional PP separator. Both solutions of 1 mol dm-3 LiPF6 / EC and 1 mol dm-3 LiPF6 / EC:EMC (3:7 v/v %) were used as electrolytes to investigate the best combination of electrolyte and separator was evaluated in terms of the chemical composition change of the cycled anode, the surface morphology of the cycled anode and the resistance of the cell.

2. EXPERIMENTAL 2.1. Preparation of materials. The Li4Mn5O12 electrode was fabricated by coating the slurry of mixture consisting of 80 wt. % Li4Mn5O12 (synthesized by Toshima Manufacturing Co., Ltd.), 10 wt. % acetylene black and 10 wt. % polyvinylidene fluoride in N-methylpyrrolidone on Al foil current collector (thickness of 20 μm) using a doctor blade and dried at 120 °C under vacuum for 12 hours. The


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thickness and the density of the Li4Mn5O12 electrode were 24 μm and 1.8 g cm-3, respectively. 3DOM PI membrane was prepared by colloidal crystal templating method. A mixture of polyamic acid (supplied by JFE Chemical Corporation) and dispersed silica particles with 300 nm diameter (Seahostar® KE-P30, Nippon Shokubai Co., Ltd.) in dimethylacetamide was coated uniformly onto a glass plate. After drying at 60 ºC, obtained thin sheet was heated at 320 °C to convert the polyamic acid precursor to PI and immersed in a 10 wt. % HF aqueous solution for 5 hours to remove the silica particles. The characteristics of the obtained PI membrane having 3DOM structure were reported in the previous paper1. The thickness of the membrane was about 30 μm.

2.2. Assembly of cells. Laminate cells were fabricated in an argon-filled glove box. The Li4Mn5O12 electrode as cathode and a lithium foil (thickness of 20 μm) adhered to a copper foil (thickness of 10 μm) purchased from Honjo metal Co., Ltd. as anode were used. The areas of the cathode and the anode were 12.0 and 13.4 cm2, respectively. The ratio of capacity between the cathode and the anode was 1:10. 1 mol dm-3 LiPF6 in EC (Kishida Chemical Co., Ltd.) and 1 mol dm-3 LiPF6 in EC:EMC (3:7 v/v %) (Kishida Chemical Co., Ltd.) were used as electrolyte solutions. The water and HF contents in these electrolytes were less than 4 ppm and 40 ppm, respectively. The injection amount of the electrolyte was 600 μL. The 3DOM PI membrane (thickness of about 30 μm) or commercially available PP membranes (thickness of 25 μm) were used as a separator. PP membrane with a surfactant was used for 1 mol dm-3 LiPF6 / EC, because EC solution cannot permeate into a common use PP membrane without a surfactant.


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2.3. Electrochemical measurements. Electrochemical discharge/charge measurements were performed at 30 °C with a chargedischarge unit (HJ1001SM8A, Hokuto Denko Corp.) in the potential range of 2.0–3.7 V at 0.1 C rate for the first three formation cycles and then at 1 C rate for the subsequent cycles (1 C = 163 mA g−1). In this LMB system using the Li4Mn5O12 electrode, the electrochemical cycling was always started from discharging process. Electrochemical impedance spectroscopy was conducted by a potentiostat (1287, Solartron) and a frequency response analyzer (1255B, Solartron) system with an alternating current (AC) amplitude of 10 mV in the frequency range from 100 kHz to 10 mHz at 30 °C after charging process.

2.4 Surface analysis of lithium metal anodes. The lithium metal anodes were taken out from the cells and washed with dimethyl carbonate (DMC) after the discharge/charge cycles. These anode samples were sealed in a transfer vessel in a purified argon-filled glove box, and transferred to the analysis chamber of SEM or XPS equipment without air exposure. The surfaces of the anodes were observed by using a scanning electron microscope (SEM, JSM-6490A, JEOL Ltd.). XPS measurements were carried out with Versa Probe II (ULVAC-PHI, Inc.) using Al-Kα radiation (1486.6 eV). All spectra were recorded with a pass energy of 46.950 eV. Charge compensation was performed by an electron neutralizer and an ion gun, so that charge up of samples was successfully compensated in this study. Peaks were assigned based on the XPS data reported in the references shown in Table 1.

3. RESULTS AND DISCUSSION 3.1. Cycle performance of cells.


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Figures 2 (a) and (b) show discharge and charge curves of cells using 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm-3 LiPF6 / EC), 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7). Figures 3 (a) and (b) show the cycle performance and the coulombic efficiency of each cell, respectively. The coulombic efficiency is defined as the ratio of the discharge capacity to the charge capacity in the preceding charging process. Table 2 shows lithium consumption of cell per cycle about each cell calculated by cycle performance shown in Figure 3(a). Lithium consumption of cell per cycle was defined by following equation: Lithium consumption of cell per cycle (mAh/cycle) = {Pristine capacity of Li metal anode (mAh) ― Cell capacity at which the capacity retention becomes 10 % (mAh)} ÷ Number of cycles at which the capacity retention becomes 10 % (cycle) The cells using 1 mol dm-3 LiPF6 / EC had excellent cycle performance, resulting from high coulombic efficiency and low lithium consumption of cell per cycle. The cells using 1 mol dm-3 LiPF6 / EC:EMC=3:7 had very poor cycle performance owing to low coulombic efficiency and high lithium consumption of cell per cycle. 3DOM PI separator was much superior to PP separator in terms of the coulombic efficiency and lithium consumption of cell per cycle.

3.2 Chemical composition of the cycled anode. Figure 4 shows XPS spectra in the C 1s, O 1s, F 1s, and P 2p regions of the anodes taken from the 100 cycled cells using 3DOM PI (1 mol dm-3 LiPF6 / EC), PP with surfactant (1 mol dm-3 LiPF6 / EC), 3DOM PI (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP (1 mol dm-3 LiPF6 / EC:EMC=3:7), respectively. In the C 1s and O 1s regions, very large peaks were observed at the anodes using EC:EMC=3:7 solvent as compared with those using EC solvent, suggesting that many organic compounds generated by the decomposition reactions of the solvent exist on the


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surface of the 100 cycled anode in EC:EMC=3:7 solvent. For example, Li2CO3 (C 1s and O 1s, 290.2 eV and 532 eV), ROCO2Li (C 1s and O 1s, 288-292 eV and 533 eV), LiOH (O 1s, 531 eV), Li2O (O 1s, 528.5 eV), polyethylene oxide (PEO, C 1s and O 1s, 286 eV and 533 eV), and other polymeric species (C 1s, 285 eV) have been reported as film components containing C and O elements. In the F 1s and P 2p regions, very large peaks derived from LixPOyFz, LiF and phosphates were observed at the anodes using EC:EMC=3:7 solvent as compared with those using EC solvent. The mechanism of reaction between LiPF6 and water in LiPF6-based organic electrolyte solutions have been studied in the previous works and the following equations are reported8,9, LiPF6 ⇄ Li+ + PF6－ (1) LiPF6 ⇄ LiF + PF5

(2)

PF5 + H2O → POF3 + 2HF

(3)

It is also reported that the reaction of LiPF6 with water hardly occur in solvents with high dielectric constant10. In EC solvent with high dielectric constant (95.3 at 25 °C)11,12, LiPF6 easily ionizes to Li+ and PF6 － by the reaction (1). On the other hand, in EMC solvent with low dielectric constant (2.9 at 25 °C)12,13, non-ionized LiPF6 easily dissociates to LiF and PF5 by the reaction (2) due to high concentration of non-ionized LiPF6, resulting in the large peak derived from LiF. In addition, dissociated PF5 reacts with water contained slightly in electrolyte to generate POF3 and HF by the reaction (3) especially in EMC solvent. Chemical species assignable to LixPOyFz observed at the cycled anodes using EC:EMC=3:7 solvent are considered to be derived from PF5 and POF3. Furthermore, POF3 reacts with water to produce difluorophosphoric acid, monofluorophosphoric acid and phosphoric acid according to the following equations14:


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POF3

+ H 2O

H[PO2F2]

+ H 2O + HF

H2[PO3F]

+ H 2O + HF

H3[PO4] (4)

Organic compounds generated by trans-esterification reaction of these acids and carbonates have been reported as deteriorated products14,15. These reactions are considered to be promoted under the condition of higher concentration of phosphoric acid, resulting in the larger peak derived from phosphates in EC:EMC=3:7 solvent. In the case of especially LMBs, depositions of metallic lithium with high reactivity are repeated at every cycle on the anode, so that undesirable side reactions described above are promoted at the interface between fresh deposited metallic lithium and electrolyte. Therefore, it is considered that EC:EMC=3:7 solvent in which undesirable side reactions are promoted is not suitable for LMB. This trend is also confirmed in the case of a lithium metal symmetric cell in Figure S3 in Supporting Information.

3.3. Morphology of anode surface. Figure 5 shows the surface SEM images of anode taken from the 100 cycled cells using (a) 3DOM PI (1 mol dm-3 LiPF6 / EC), (b) PP with surfactant (1 mol dm-3 LiPF6 / EC), (c) 3DOM PI (1 mol dm-3 LiPF6 / EC:EMC=3:7) and (d) PP (1 mol dm-3 LiPF6 / EC:EMC=3:7), respectively. In the cells using 1 mol dm-3 LiPF6 / EC solution, the size and shape of the particles on the anode were relatively uniform. When 3DOM PI separator was used, particle size was slightly smaller and the grain boundary of particles was seemed to be unclear compared with PP with surfactant separator. This difference may be owing to 3DOM structure which can provide uniform current density distribution and control the particle growth of lithium. In the cells using 1 mol dm-3 LiPF6 / EC:EMC=3:7 solution, the size and shape of the particles on the anode were non-uniform regardless of the type of separator. Differences in the chemical composition of the anode surface observed in the XPS result may affect the morphology of the anode.


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3.4. Resistance of cell. Figures 6 (a), (b) and (c) show magnified Nyquist plots, Nyquist plots and bode plots at 100th cycle for the cells using 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm-3 LiPF6 / EC), 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7), respectively. The spectra were fitted using the equivalent circuit model shown in Scheme 1, which was designed by considering in previous papers16-19. The fitting data is shown as solid line in Figure 6. Table 3 shows the fitted values of resistance corresponding to each semi-circle. The value of resistance of electrolyte (RE) for the cell using PP with surfactant separator (1 mol dm-3 LiPF6 / EC) is larger than those of other cells, indicating that EC solvent with low wettability can hardly permeate into PP separator even though the surfactant is added. Moreover, even if electrolyte permeate, it easily leaks out due to poor electrolyte reservation of the PP separator, resulting in the lack of electrolyte in separator during discharge/charge cycles. The value of RE for the cell using 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) is lower than that of PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7). This result also indicates that 3DOM PI separator is superior to PP separator in terms of permeability and reservation for electrolyte. In the Nyquist plot of the cell using 3DOM PI (1 mol dm-3 LiPF6 / EC), the semi-circle corresponding to the charge-transfer resistance through the surface film on the cathode and the anode (R C and A) is observed in the high frequency range of > 200 Hz17,20. It is considered that the semi-circles of cathode and anode are overlapped due to the same time-constant. Appearance of


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a linear line in the low frequency range, which is associated with Li+ diffusion16, indicates that the Li+ diffusion smoothly occurs in the electrode bulk. In the Nyquist plot of the cell using PP with surfactant (1 mol dm-3 LiPF6 / EC), the first semi-circle in the high frequency range of > 200 Hz and a part of the second semi-circle in the range around 0.1 Hz are observed. A peak related to the second semi-circle is observed at around 0.1 Hz in the bode plot in Figure 6(c). As mentioned above, the amount of side reaction products of electrolyte is small on the surface of the anode extracted from this cell. The particle size is slightly larger and the grain boundary of lithium particles is clear. Therefore, the semi-circle in the range around 0.1 Hz is not derived from SEI film, but from the resistance due to the morphological change of lithium metal (RLi). For example, the grain boundary in lithium metal anode may be resistance component. In the Nyquist plot of the cell using 3DOM PI (1 mol dm-3 LiPF6 / EC:EMC=3:7), the first semi-circle in the high frequency range of > 200 Hz and a part of the second semi-circle in the range around 10 Hz are observed. A peak related to the second semi-circle is observed at around 10 Hz in the bode plot in Figure 6(c). Aurbach et al. have reported that the surface films covering the lithium metal electrode in solutions are comprised of an inner compact part and an outer porous part including reduction products from electrolyte19. The impedance spectrum of the porous part is observed in lower frequency range, rather than inner compact part. As mentioned above, the amount of side reaction products including organic compounds is large on the surface of the anode extracted from this cell, so that the second semi-circle in the range around 10 Hz is considered to be derived from resistance for Li+ migration in SEI film including resistive components (RSEI). Appearance of linear line in the low frequency range indicates that Li+ diffusion smoothly take place in the electrode bulk in this cell.


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In the Nyquist plot of the cell using PP (1 mol dm-3 LiPF6 / EC:EMC=3:7), the first semicircle in the range of > 200 Hz, the second semi-circle in the range around at 10 Hz are observed. The behavior in low frequency region is different from other cells. Such spectrum indicates that RC and A, RSEI, RLi and Li+ diffusion resistance are extremely large in this cell. Table 3 shows that all values of RE, RC and A, RSEI and RLi are small in the cell using 3DOM PI separator and 1 mol dm-3 LiPF6 / EC electrolyte solution, indicating that this combination is suitable for LMB.

5. CONCLUSION The reaction of LiPF6 with water and subsequent undesirable side reactions easily occur in EMC solvent with low dielectric constant, resulting in high lithium consumption of cell per cycle and low coulombic efficiency. Furthermore, the resistive side reaction products increase the resistance of cell, resulting in poor cycle performance. EC solvent with high dielectric constant is suitable for LMBs because side reactions hardly occur and deposited lithium is uniform. The 3DOM PI separator is much superior to the conventional PP separator in terms of the uniform current density distribution in addition to permeability and reservation for LiPF6 in EC solution. The combination of 3DOM PI separator and LiPF6 in EC solution can improve the cycle characteristics of LMBs.

Caption list of Figure, Table and Scheme.


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Figure 1. The SEM of pore structure and schematic images of lithium ion conduction for (a) PP membrane and (b) 3DOM PI membrane. Figure 2. Discharge and charge curves of cells using (a) 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm-3 LiPF6 / EC), (b) 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7). Figure 3. (a) The cycle performance and (b) the coulombic efficiency of cells at 1C rate. Figure 4. The XPS spectra in the C 1s, O 1s, F 1s, and P 2p regions of the anodes taken from the 100 cycled cells. Figure 5. Surface SEM images of the anode extracted from the 100 cycled cells using (a) 3DOM PI (1 mol dm-3 LiPF6 / EC), (b) PP with surfactant (1 mol dm-3 LiPF6 / EC), (c) 3DOM PI (1 mol dm-3 LiPF6 / EC:EMC=3:7) and (d) PP (1 mol dm-3 LiPF6 / EC:EMC=3:7). Figure 6 (a) Magnified Nyquist plots, (b) Nyquist plots and (c) bode plots at 100th cycle for the cells using 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm3

LiPF6 / EC), 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol

dm-3 LiPF6 / EC:EMC=3:7). Scheme 1. Equivalent circuit model and description of symbols for impedance analysis. Table 1. XPS data reported in the references. Table 2. Lithium consumption of cell per cycle. Table 3. The fitted values of resistance.


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FIGURES.

Figure 1. The SEM of pore structure and schematic images of lithium ion conduction for (a) PP membrane and (b) 3DOM PI membrane.


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Figure 2. Discharge and charge curves of cells using (a) 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm-3 LiPF6 / EC), (b) 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7).

Figure 3. (a) The cycle performance and (b) the coulombic efficiency of cells at 1C rate.


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Figure 4. The XPS spectra in the C 1s, O 1s, F 1s, and P 2p regions of the anodes taken from the 100 cycled cells.

Figure 5. Surface SEM images of the anode extracted from the 100 cycled cells using (a) 3DOM PI (1 mol dm-3 LiPF6 / EC), (b) PP with surfactant (1 mol dm-3 LiPF6 / EC), (c) 3DOM PI (1 mol dm-3 LiPF6 / EC:EMC=3:7) and (d) PP (1 mol dm-3 LiPF6 / EC:EMC=3:7).


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Figure 6 (a) Magnified Nyquist plots, (b) Nyquist plots and (c) bode plots at 100th cycle for the cells using 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm3

LiPF6 / EC), 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol

dm-3 LiPF6 / EC:EMC=3:7).


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SCHEMES. Scheme 1. Equivalent circuit model and description of symbols for impedance analysis.

TABLES. Table 1. XPS data reported in the references. Binding energy (eV) Components Li 1s

Metallic Lithium

52.321

Li2CO3

55.521

C 1s

O 1s

290.221

53221,22

F 1s

P 2p


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LiOH

Li2O

55.022

290.122

55.523

29023

53223

55.521

53221

54.622

531.622

53.721,22

528.721 528.822

ROCO2Li

54.524

288-29225

53325

288-29123,26 532.5-53426,26

RCH2OLi

288.226

532.526

PEO

286.523

53323

286.226

532.826

-CH2-CH2-

28521,23 285.526

Other polymeric species

28527

C-Li

283.528


19


282.421

CxLiy

LiF

LiPF6

Page 20 of 35

5622

685.522

55.526

68529

5726

68829

138.526 13829

688.526

LixPOyFz

13529

68729

LixPy

128.6-126.630

Phosphate

13331

Table 2. Lithium consumption of cell per cycle.

Solvent

Separator

Lithium consumption of cell per cycle (mAh/cycle)

EC

3DOM PI

0.18

EC

PP with surfactant

0.25

EC:EMC = 3:7

3DOM PI

0.82


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EC:EMC = 3:7

PP

1.17

Table 3. The fitted values of resistance. RE

Separator

Electrolyte

RC and A

RSEI

RLi

> 200 z

10 Hz

0.1 Hz

3DOM PI

1 mol dm-3 LiPF6 / EC

0.74

2.7

－

－

PP with surfactant

1 mol dm-3 LiPF6 / EC

1.3

2.9

－

13

3DOM PI

1 mol dm-3 LiPF6/EC:EMC=3:7

0.80

4.0

3.1

－

PP

1 mol dm-3 LiPF6/EC:EMC=3:7

0.92

4.2

48

22


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI #####. Li4Mn5O12 cathode material (Figures S1 and S2); Coulombic efficiency of lithium metal symmetric cell (Figure S3) (PDF).

AUTHOR INFORMATION Corresponding Author * Kiyoshi Kanamura Tel & fax: +81-42-677-2828 E-mail: [email protected]

ORCID Motoko Nagasaki : 0000-0003-4226-8323 Kiyoshi Kanamura : 0000-0002-0939-6748

Author Contributions All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This work was supported by Advanced Low Carbon Technology Research and Development Program – Specially Promoted Research for Innovative Next Generation Batteries – (ALCASPRING) from Japan Science and Technology Agency (JST). The XPS measurement was carried out at National Institute for Materials Science (NIMS) Battery Research Platform. We would like to thank T. Masuda for his help for the XPS measurement at NIMS Battery Research Platform.

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Figure 1. The SEM of pore structure and schematic images of lithium ion conduction for (a) PP membrane and (b) 3DOM PI membrane. 243x145mm (120 x 120 DPI)



Figure 2. Discharge and charge curves of cells using (a) 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm-3 LiPF6 / EC), (b) 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7). 243x137mm (120 x 120 DPI)


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Figure 3. (a) The cycle performance and (b) the coulombic efficiency of cells at 1C rate. 243x137mm (120 x 120 DPI)



Figure 4. The XPS spectra in the C 1s, O 1s, F 1s, and P 2p regions of the anodes taken from the 100 cycled cells. 243x137mm (120 x 120 DPI)


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Figure 5. Surface SEM images of the anode extracted from the 100 cycled cells using (a) 3DOM PI (1 mol dm-3 LiPF6 / EC), (b) PP with surfactant (1 mol dm-3 LiPF6 / EC), (c) 3DOM PI (1 mol dm-3 LiPF6 / EC:EMC=3:7) and (d) PP (1 mol dm-3 LiPF6 / EC:EMC=3:7). 243x137mm (120 x 120 DPI)



Figure 6 (a) Magnified Nyquist plots, (b) Nyquist plots and (c) bode plots at 100th cycle for the cells using 3DOM PI separator (1 mol dm-3 LiPF6 / EC), PP with surfactant separator (1 mol dm-3 LiPF6 / EC), 3DOM PI separator (1 mol dm-3 LiPF6 / EC:EMC=3:7) and PP separator (1 mol dm-3 LiPF6 / EC:EMC=3:7). 243x137mm (120 x 120 DPI)


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Scheme 1. Equivalent circuit model and description of symbols for impedance analysis. 243x137mm (120 x 120 DPI)


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