Insulative Microfiber 3D Matrix as a Host Material Minimizing Volume

Mar 30, 2017 - Batteries using metallic lithium (Li) as an anode have attracted a great deal of attention because they have the potential to achieve h...
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Insulative Microfiber 3D Matrix as a Host Material Minimizing Volume Change of the Anode of Li Metal Batteries Shoichi Matsuda,† Yoshimi Kubo,† Kohei Uosaki,† and Shuji Nakanishi*,‡ †

Global Research Center for Environment and Energy based on Nanomaterials Science, National Institute of Material Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan S Supporting Information *

ABSTRACT: Batteries using metallic lithium (Li) as an anode have attracted a great deal of attention because they have the potential to achieve high energy density over Li-ion batteries. In order to use Li metal as a practical anode of a secondary battery, there are many problems to be overcome. A large volume change of the anode accompanying repetitive deposition and dissolution of Li is one such problem. Here we report that a 3D matrix consisting of insulative microfibers on the Li anode functions as a layer absorbing the volume change associated with the deposition/dissolution of Li as high as 10 mAh/cm2. This result suggests that the use of an insulative 3D matrix layer is an effective way to minimize anode volume change under practical operating conditions.

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serious problem related to the extremely large volume change of the Li metal anode remains unsolved. To achieve an improvement of energy density more than two times that of Liion batteries in the fully packaged state, it is necessary to utilize at least a 20 μm thickness of Li metal for reversible deposition/ dissolution.3 Under such deep discharge/charge operating conditions, the large volume change of the Li metal anode causes macroscale distortion of the battery components and fluctuations of the SEI. Thus, a novel strategy to minimize the volume change of the Li metal anode is essential for implementation of Li metal-based rechargeable batteries. The use of an interconnected 3D matrix as a substrate material for the Li metal anode is an effective approach to minimize the large volume change. A 3D matrix with a suitable internal pore structure could store the Li metal deposits and decrease the possibility of undesired Li metal dendrite growth. Several materials, including graphene foam and copper nanowires, have been reported as promising candidates for such a 3D matrix, whereby improvement of the Li metal anode performance has been achieved.15−22 These studies have motivated us to further explore an ideal 3D matrix material for the deposition of large amounts of Li metal.

here is growing demand for rechargeable batteries with energy densities as high as 500 Wh/kg in the fullpackaged state, especially for use in electric vehicles. Although the market share for Li-ion batteries has continuously expanded since their commercialization in 1991 by Sony, the limited theoretical energy density of conventional Li-ion batteries will no longer meet the advanced energy storage requirements. Therefore, to achieve energy storage devices with much higher energy densities, it is necessary to adopt a new operating principle that differs from that of Li-ion batteries. Potential candidates for next-generation batteries, such as lithium−oxygen and lithium−sulfur batteries, adopt Li metal as an anode material due to attractive features such as a low redox potential of −3.04 V (vs SHE) and a high theoretical energy capacity of 3860 mAh/g. Although recent intense studies have made significant progress with oxygen- and sulfur-based cathodes,1−4 serious problems related to the Li metal anode still remain to be solved.5−8 The fundamental problems related to the use of a Li metal anode can be classified into the following three issues: (i) an optimal electrolyte composition is required to achieve a high Coulombic efficiency by the formation of an ideal solid electrolyte interphase (SEI), (ii) safety concerns related with Li dendrite growth must be addressed, and (iii) a solution to the extremely large volume change of the Li metal anode is required. Numerous studies aimed at solutions to the two former problems have been conducted in recent decades and have demonstrated promising achievements.9−14 However, the © XXXX American Chemical Society

Received: February 23, 2017 Accepted: March 29, 2017

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DOI: 10.1021/acsenergylett.7b00149 ACS Energy Lett. 2017, 2, 924−929

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters

matrix was evaluated using an electrochemical cell with Ni foil as a working electrode and Li foil as a counter electrode. The IMF matrix was placed between the Ni foil current collector and a standard polypropylene separator (Figure 1c). Figure 2

In the present study, we focused on an insulative microfiber (IMF) matrix as a potential candidate for the 3D matrix due to the following reasons: (i) the insulative properties ensure the prevention of undesired side reactions with the electrolyte and the prevention of undesired Li metal formation of the top part of the 3D matrix, which is expected to decrease the possibility of Li metal dendrite growth, and (ii) large internal spaces can be formed by stacking the micrometer-sized fiber, which is advantageous for the storage of a large amount of Li metal deposits. To fabricate the IMF matrix, the electrospinning method was adopted because the diameter and thickness of the fiber matrix can be easily controlled according to the electrospinning time and the precursor concentration. The high controllability of the diameter and thickness of the fiber matrix is advantageous for clarifying the pore size suitable for suppressing the volume change associated with the Li metal deposition/dissolution. The free-standing polyacrylonitrile (PAN)-based IMF matrix was fabricated by the electrospinning method (Figure 1a). The

Figure 2. Voltage profiles of the electrochemical cell with the IMF matrix at a current density of 1.0 mA/cm2 with a capacity of 10 mAh/cm2. At points a, b, c, and d, the SEM observations were conducted.

shows a representative voltage profile for the cell obtained at a current density of 1.0 mA/cm2 and with a capacity of 10 mAh/ cm2, which corresponds to the utilization of a ∼50 μm thickness of Li metal. A stable plateau was observed at around −50 mV/50 mV during the cathodic/anodic processes, and a sharp increase was found at the end of the anodic process. This voltage profile can be assigned to the reversible Li metal deposition/dissolution process. Next, the details of the Li metal deposition behavior with the IMF matrix were clarified. SEM analyses were conducted on the IMF matrixes removed from electrochemical cells after Li metal deposition/dissolution processes with specific energy capacities. The top- and side-view SEM images after Li deposition at a capacity of 2.0 mAh/cm2 (point a in Figure 2) revealed that Li metal deposits were formed in the internal space of the IMF matrix (Figure 3a,e). At this point, the estimated amount of Li metal is on average 10 μm thick, but already, Li metal deposits were partially seen above the IMF matrix, indicating that Li deposition occurred unevenly in the internal space of the IMF matrix, and it may be derived from the nonuniform Li+ flux in the matrix. For the sample obtained after Li deposition at a capacity of 10 mAh/cm2 (corresponding to an average 50 μm thickness of Li metal, point b in Figure 2), most of the internal spaces in the IMF matrix were occupied with Li metal deposits and little space remained vacant (Figure 3b,f). These results clearly reveal that Li metal deposition proceeded at the internal spaces of the IMF matrix in a bottom-up manner associated with the cathodic process. SEM analysis after Li dissolution with a capacity of 6.0 mAh/cm2 (corresponding to an average 20 μm thickness of Li metal, point c in Figure 2) revealed that the Li metal deposits formed during the cathodic process mostly disappeared (Figure 3c,g). Finally, the internal spaces of the IMF matrix returned to a vacant state when the voltage reached the cutoff voltage of 1.0 V (Figure 3d,h). Importantly, the thickness of the IMF matrix was almost constant during the Li metal deposition/dissolution process, suggesting that the volume change of the anode was largely suppressed by the use of the IMF matrix as a host material for Li metal deposition/ dissolution (Figure 3i).

Figure 1. (a,b) Photograph and SEM image of the IMF matrix obtained from a precursor solution of DMF with 18 wt % PAN. The scale bar is 10 mm in (a) and 100 μm in (b). (c) Schematic illustration of the electrochemical cell configuration used in this study.

details of the fabrication procedure are described in the Experimental Section. Figure 1b shows a SEM image of the IMF matrix, where the IMF forms internal spaces with sizes of hundreds of micrometers, which allows for the storage of large amounts of Li metal deposits. The electrochemical performance for Li metal deposition/dissolution in the system with the IMF 925

DOI: 10.1021/acsenergylett.7b00149 ACS Energy Lett. 2017, 2, 924−929

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Figure 3. (a−d) Top-view and (e−h) side-view SEM images of the IMF matrixes removed from electrochemical cells after Li metal deposition with capacities of (a,e) 2.0 and (b,f) 10 mAh/cm2 and after Li metal dissolution processes with capacities of (c,g) 6.0 and (d,h) 9.8 mAh/cm2 (fully stripped). The scale bar is 50 μm in the side view and 100 μm in the top view. (i) Schematic illustration of reversible deposition/ dissolution of Li metal in the internal spaces of the IMF matrix.

of the IMF matrix are ∼85% and 100 μm, respectively, storage of Li metal as high as 20 mAh/cm2 is overcapacity for the internal spaces of the IMF matrix. In such an overcapacity condition, the undesired volume expansion of the anode proceeded with the formation of further Li metal deposits. For the practical operation of Li metal-based batteries, it is necessary to operate the cells at higher current densities (>3.0 mA/cm2) and to utilize Li foil as a substrate material instead of Ni foil. Therefore, we investigated whether Li metal

Under conditions where Li metal formation occurs with an overcapacity of the internal space volume in the IMF matrix, the Li metal is no longer stored at the internal spaces but is deposited over the top surface of the IMF matrix. Figure S1 shows SEM images obtained after Li metal deposition with a capacity of 20 mAh/cm2, which reveals that the internal spaces of the IMF matrix were fully occupied with Li metal deposits and the top surface of the IMF matrix was covered with Li metal deposits. Considering that the porosity and the thickness 926

DOI: 10.1021/acsenergylett.7b00149 ACS Energy Lett. 2017, 2, 924−929

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ACS Energy Letters deposition/dissolution occurs in the internal spaces of the IMF matrix under such practical conditions. Figure S2 shows SEM images of an anode that was removed from an electrochemical cell after Li metal deposition at a current density of 5.0 mA/ cm2. Even at this higher current density, the IMF matrix stored the Li metal deposits with a capacity of 10 mAh/cm2. In addition, when the Ni foil was replaced with Li foil, essentially the same phenomenon was observed (Figure S3). These results clearly revealed the usefulness of the IMF matrix as a 3D matrix for the practical operation of Li metal-based rechargeable batteries. It should be noted that the concept of the IMF matrix demonstrated in the present study is essentially different than that of a separator with a three-dimensionally ordered pore structure (3-DOM separator).23 Although it has been reported that a 3-DOM separator is used for Li metal-based rechargeable batteries, the Li metal does not deposit/dissolve in the interior space of the 3-DOM separator. The cyclability of Li metal deposition/dissolution in the internal spaces of the IMF matrix was also investigated. Figure S4 shows the voltage profiles of electrochemical cells with and without the IMF matrix at the 10th and 30th cycles, which were obtained at a current density of 1.0 mA/cm2 and with a capacity of 1.0 mAh/cm2. No clear difference in the voltage profiles was observed, and the Coulombic efficiency was maintained at over 95% during the 30th cycle for the both cases (Figure S5). These results suggest that the introduction of the IMF matrix has almost no negative effect on the electrochemical performance for Li metal deposition/dissolution. The relationship between the Li metal deposition behavior and the size of the internal spaces in the IMF matrix was further investigated by changing the fiber diameter of the IMF matrix. IMF matrixes with thinner fiber diameters can be obtained with the electrospinning method using precursor solutions with lower polymer concentrations.24 IMF matrixes were prepared with fiber diameters of about 5.0 μm (IMF-5) or 1.0 μm (IMF1). SEM observations revealed that the size of the internal space of the IMF-5 was in the range of 5−10 μm and that of IMF-1 was in the range of 0.1−1.0 μm (Figure S6). Electrochemical cells were assembled with Ni foil as a working electrode and Li foil as a counter electrode, and the fabricated IMF matrix was placed between the Ni foil current collector and the separator. Li metal deposition was performed under galvanostatic conditions. Figure 4a shows a side-view SEM image of IMF-5 removed from the electrochemical cell after Li metal deposition with a capacity of 10 mAh/cm2. The internal spaces were fully occupied with Li metal deposits. The Li metal formation behavior obtained in the system with IMF-5 was consistent with that obtained with IMF-10 (IMF matrix with a ∼10 μm fiber diameter, Figure 3g). In contrast, different Li metal formation behavior was observed in the system with IMF-1, where Li metal formed at the interface between the Ni foil and IMF matrix but not in the internal spaces of the IMF matrix (Figure 4b). It should be noted that no clear morphological difference was observed between the Li metal deposits obtained with and without the IMF-1 matrix (Figure S7). Considering that the particle size of the Li metal deposits obtained under the experimental conditions was in the range of 2.0−5.0 μm, these results suggest that the relative size of the internal spaces with respect to the size of Li metal deposits is a crucial factor to determine whether Li metal deposition proceeds in the internal spaces of the IMF matrix or at the interface between the Ni foil and IMF matrix.

Figure 4. Side-view SEM images of (a) IMF-5 and (b) IMF-1 removed from electrochemical cells after Li metal deposition with a capacity of 10 mAh/cm2. Scale bars are 50 μm.

In summary, reversible Li metal deposition/dissolution was demonstrated to proceed in the internal spaces of the IMF matrix, whereby a minimum volume change of the anode was realized. This result is significant because suppression of the volume change of the Li metal anode is crucial for implementation of Li metal-based rechargeable batteries. Considering that the oxidized PAN fiber has high shape stability and thermal stability under battery-operating conditions,25−28 the IMF matrix presented in this study is expected to be applicable to practical Li metal-based rechargeable batteries. However, for practical utilization of the IMF matrix, several issues remain to be solved, such as improvement of the utilization ratio of internal spaces in the IMF matrix and improvement of the Coulombic efficiency. We consider that such optimization can be realized in combination with technologies such as appropriate surface modification of the IMF matrix to improve wettability with the electrolyte, the use of novel electrolyte additives to form an ideal SEI layer, the use of a separator with a controlled porous structure to achieve homogeneous current distribution,24 and proper control of micro/nanostructures of Li metal.29,30 Although it is also necessary to consider the inflow/outflow of the electrolyte accompanying the Li dissolution/deposition, countermeasures for dealing with this problem need to be taken depending on the type of batteries in the future.

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EXPERIMENTAL METHODS Fabrication of the IMF Matrix. The electrospinning method was used for fabrication of the IMF matrix. Solutions of 18, 16, or 14 wt % polyacrylonitrile (PAN; Mw: 150 000, Aldrich) dissolved in N,N-dimethylformamide (DMF; Wako) were used as precursors. The prepared solutions were loaded into 10 mL syringes with an 18 gauge needle. The solution was then electrospun using an electrospinning system (Kato Tech Co., Ltd., Japan) with an applied voltage of 20 kV. The electrospinning time was controlled to produce a fiber thickness of ∼100 μm. The obtained IMF matrix was calcined at 230° using a temperature ramp rate of 10°/h. The obtained IMF 927

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bridging mechanistic understanding and battery performance. Energy Environ. Sci. 2013, 6, 750−768. (3) Gallagher, K. G.; Goebel, S.; Greszler, T.; Mathias, M.; Oelerich, W.; Eroglu, D.; Srinivasan, V. Quantifying the promise of lithium−air batteries for electric vehicles. Energy Environ. Sci. 2014, 7, 1555−1563. (4) Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances in lithium−sulfur batteries based on multifunctional cathodes and electrolytes. Nature Energy 2016, 1, 16132. (5) Kim, H.; Jeong, G.; Kim, Y. U.; Kim, J. H.; Park, C. M.; Sohn, H. J. Metallic anodes for next generation secondary batteries. Chem. Soc. Rev. 2013, 42, 9011−9034. (6) Xu, W.; Wang, J.; Ding, F.; Chen, X.; Nasybulin, E.; Zhang, Y.; Zhang, J. G. Lithium metal anodes for rechargeable batteries. Energy Environ. Sci. 2014, 7, 513−537. (7) Cheng, X. B.; Zang, R.; Zhao, C. Z.; Wei, F.; Zhang, J. G.; Zhang, Q. A Review of solid electrolyte interphases on lithium metal anode. Adv. Sci. 2016, 3, 1500213−500232. (8) Tikekar, M. D.; Choudhury, S.; Tu, Z.; Archer, L. A. Design principles for electrolytes and interfaces for stable lithium-metal batteries. Nat. Energy 2016, 1, 16114−16120. (9) Shiraishi, S.; Kanamura, K.; Takehara, Z. I. Surface condition changes in lithium metal deposited in nonaqueous electrolyte containing HF by dissolution-deposition cycles. J. Electrochem. Soc. 1999, 146, 1633−1639. (10) Stark, J. K.; Ding, Y.; Kohl, P. A. Dendrite-free electrodeposition and reoxidation of lithium-sodium alloy for metal-anode battery. J. Electrochem. Soc. 2011, 158, A1100−A1105. (11) Ding, F.; Xu, W.; Graff, G. L.; Zhang, J.; Sushko, M. L.; Chen, X.; Shao, Y.; Engelhard, M. H.; Nie, Z.; Xiao, J.; et al. Dendrite-free lithium deposition via self-healing electrostatic shield mechanism. J. Am. Chem. Soc. 2013, 135, 4450−4456. (12) Qian, J.; Henderson, W. A.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J. G. High rate and stable cycling of lithium metal anode. Nat. Commun. 2015, 6, 6362−6370. (13) Kang, H. K.; Woo, S. G.; Kim, J. H.; Yu, J. S.; Lee, S. R.; Kim, Y. J. Few-layer graphene island seeding for dendrite-free Li metal electrodes. ACS Appl. Mater. Interfaces 2016, 8, 26895−26901. (14) Cheng, X. B.; Hou, T. Z.; Zhang, R.; Peng, H. J.; Zhao, X. Z.; Huang, J. Q.; Zhang, Q. Dendrite-free lithium deposition induced by uniformly distributed lithium ions for efficient lithium metal batteries. Adv. Mater. 2016, 28, 2888−2895. (15) Mukherjee, R.; Thomas, A. V.; Datta, D.; Singh, E.; Eksik, O.; Shenoy, V. B.; Koratkar, N.; Li, J. Defect-induced plating of lithium metal within porous graphene networks. Nat. Commun. 2014, 5, 3710−3719. (16) Cheng, X. B.; Peng, H. J.; Huang, J. Q.; Wei, F.; Zhang, Q. Dendrite-free nanostructured anode: Entrapment of lithium in a 3d fibrous matrix for ultra-stable lithium−sulfur batteries. Small 2014, 10, 4257−4263. (17) Cheng, X.-B.; Peng, H.-J.; Zhang, R.; Zhao, C.-Z.; Zhang, Q.; Huang, J.-Q. Dual-phase lithium metal anode containing a polysulfideinduced solid electrolyte interphase and nanostructured graphene framework for lithium−sulfur batteries. ACS Nano 2015, 9, 6373− 6382. (18) Yang, C. P.; Yin, Y. Y.; Zhang, S. Z.; Li, N. W.; Guo, Y. G. Accommodating lithium into 3D current collectors with a submicron skeleton towards long-life lithium metal anodes. Nat. Commun. 2015, 6, 8058−8066. (19) Lu, L. L.; Ge, J.; Yang, J. N.; Chen, S. M.; Yao, H. B.; Zhou, F.; Yu, S. H. Free-standing copper nanowire network current collector for improving lithium anode performance. Nano Lett. 2016, 16, 4431− 4437. (20) Liu, Y.; Lin, D.; Liang, Z.; Zhao, J.; Yan, K.; Cui, Y. Lithiumcoated polymeric matrix as a minimum volume-change and dendritefree lithium metal anode. Nat. Commun. 2016, 7, 10992−11000. (21) Lin, D.; Liu, Y.; Liang, Z.; Lee, H. W.; Sun, J.; Wang, H.; Yan, K.; Xie, J.; Cui, Y. Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes. Nat. Nanotechnol. 2016, 11, 626−632.

matrix was fourced into a diameter of 16 Mm. The porosity of the IMF matrixes investigated in the present study (IMF-10, IMF-5 and IMF-1) was ∼85%. Electrochemical Characterization of Lithium Metal Deposition/ Dissolution. Electrochemical cells were assembled in an Ar-filled glovebox with a water content of less than 1 ppm. The cells consisted of Ni foil (30 μm thick, the Nilaco Corporation) as a working electrode and metallic lithium foil (200 μm thick; Honjo Metal Co., Ltd.) as a counter electrode. A standard polypropylene separator was used. A solution of 1 M lithium bis(fluorosulfonyl)imide (LiFSI) in dimethyl ether (DME) and 1,3-dioxolane (DOL) (DME/DOL = 1:1 by volume, Kishida Chemical Co., Ltd.) was used as the electrolyte. Galvanostatic experiments were conducted using a multichannel potentiostat (VMP3, Bio-Logic Science Instruments). SEM Analyses. Scanning electron microscopy energydispersive X-ray spectroscopy electron backscatter diffraction (SEM-EDS-EBSD; JSM-7800F, Jeol) was used for characterization of the electrodes. Argon ion etching (acceleration voltage, 2 keV; ion beam current, 20 mA) was applied to obtain depth profiles. Prior to the analyses, the electrodes were removed from the electrochemical cells, washed three times with DME, and dried under vacuum. The samples were not exposed to the ambient atmosphere during the preparation procedures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00149. Supplemental Figures S1−S7, showing SEM images, voltage profiles, and a CE versus cycle number plot (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuji Nakanishi: 0000-0002-3313-2689 Author Contributions

S.M. proposed the concept and performed the experiments. S.M. and S.N. wrote the manuscript. All of the authors discussed the results and reviewed the final version of the manuscript before submission. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the ALCA-SPRING and Ministry of Education, Culture, Sports, Science and Technology (MEXT) Program for Development of Environmental Technology using Nanotechnology. This work was conducted with support from the National Institute for Materials Science (NIMS) Battery Research Platform.

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