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Nano-porous copper metal current collector for lithium ion batteries
Materials Letters 226 (2018) 8–12
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
Nano-porous copper metal current collector for lithium ion batteries Yuan Chen a, Huajun Feng a, Yihua Wang a,⇑, Zhe Tang b, Daniel Chua b a b
School of Engineering, Republic Polytechnic, 9 Woodlands Avenue 9, Singapore 738964, Singapore Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
a r t i c l e
i n f o
Article history: Received 2 August 2017 Received in revised form 27 April 2018 Accepted 1 May 2018 Available online 3 May 2018 Keywords: Nano-porous Current collector Lithium ion battery Sonochemical synthesis Energy storage and conversion Porous materials
a b s t r a c t A low-cost and scalable ultrasonic synthetic method to produce copper metal current collector with uniform nanopores on the surface is achieved for the first time. A layer of silicon active material was sputtered on the surface of as-prepared metal current collector (vs. metal lithium) as an innovative current collecting material for lithium ion battery. The assembled battery with this current collector showed an areal capacity of 2.2 mAh/cm2, comparable to that of commercially available lithium ion batteries. Due to the nano-porous structure of current collector, the silicon active material electrochemical capacity retention and cycling stability also improved significantly. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Safety and fast charging/discharging properties of lithium ion battery are highly desirable for purposes ranging from portable devices to electric vehicles. Metal films with dimensionally arrayed nanopores on the surface have been long sought-after in the quest for high-surface-area conductive current collectors [1–5] and catalytic materials [1,2]. Recently, it has been reported that electrodes with meso-structure produce high power batteries with ultra-fast charging/discharging property (within one minute [6]). In lithium ion battery field, porous structures can provide sufficient inner free space to absorb large volume expansions and thereby enhance the cycling stability, and are favourable for fast transport of Li+ to increase the rate performance. However, turning block metals into metal current collectors with dimensionally arrayed nanopores on the surface economically remains a significant challenge. There are several well-established approaches for preparing nano-porous metal films: notably through de-alloying, recently through deposition of metals on nanostructured templates and laser etching, etc. There has been an ongoing search over the past decades, but no commercial metal nano-porous network film is available yet. Ultrasound can generate oscillating bubbles and lead to extremely rapid heating or cooling rates, as well as pressures of up to several hundred mega-pascals in transient cavities while the bulk ⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.matlet.2018.05.019 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
conditions of the liquid solution remain at ambient temperature. In 2010, Ekaterina V. Skorb’s group reported that a metal aluminium film with nanopores on the interface was obtained in pure water through ultrasound treatment using ultrasound cell disrupting equipment [2]. However, it is difficult to make nanopores on a large piece of metal through ultrasound probe impacting due to the size limitations of the probe. In this paper, we report an ultrasonic synthetic method for producing metal current collector with uniform nanopores on every interface and their electrochemical performance as current collectors in lithium ion batteries. To our best knowledge, uniform formation of nanopores on metal surface through a combined ultrasonic and chemical etching method has not been reported. There are also no commercial lithium ion batteries adopting metal current collectors with uniform nanopores on the surface. 2. Experimental Rolled metal copper films (10 10 cm2, 10 mm in thickness) as raw materials were transferred to a glass beaker filled with dilute ammonia solution. Then the rolled copper films in ammonia solution were put into ultrasonic cleaner to stir for 30 min at room temperature. In this paper, the ultrasonic chemical treated copper film will be named as nano-porous copper film. After drying, morphology of copper films was observed using Scanning Electron Microscope (SEM). Energy Dispersive X-ray analyses (EDX) were obtained with an EDAX detector. X-ray diffraction (XRD) scans were acquired with a Bruker D8 Advance system. Rolled metal
Y. Chen et al. / Materials Letters 226 (2018) 8–12
copper films and nano-porous copper films were used as substrates. Silicon layers were sputtered to metal copper films with a Denton sputtering (Discovery system) machine using DC Mode at 200 W power in Ar gas atmosphere for 30 min. Electrochemical performance of metal copper films and nanoporous copper films (in an area of 1 1 cm2) as current collectors was investigated through a CR2032 coin cell (half-cell) with sputtering silicon as the active cathode material, metal lithium as the reference electrode and counter electrode, and Celgard film as separator. The electrolyte was 1.0 M LiPF6 in ethylene carbonate and diethyl carbonate with volume ratio of 1/1. To compare electrodes properties, electrochemical performance was evaluated using Neware battery tester at room temperature. 3. Results and discussion 3.1. Preparation of nano-porous copper metal current collector from metal copper film Usually defects on metal copper surface and subsurface can occur in various forms, e.g., folds, splashes, cracks, inclusions, and voids. The voids are caused by gas porosity, shrinkage porosity, and shrinkage cavities when metal copper is manufactured. In the atmosphere, the surface of copper oxidizes over the course of years to become a mixture of green basic salts, the patina, consisting mainly of the basic sulphate, with some basic carbonate. Fig. 1 shows the surface morphology as-received and after ultrasonic chemical treatment of rolled metal copper films. The as-received rolled metal copper surface (Fig. 1a and b) is smooth and flat visually. After ultrasonic chemical process treatment, uniform nano-holes appear on the surface of rolled metal copper film as shown in Fig. 1c and d. These nanopores on the surface are very
9
similar to dimensionally arrayed nanopores prepared by nanostructured polymer templates method [5]. The size of nano-holes might increase with processing time and ammonia solution concentration. The details of copper surface morphology reveal the sizes of nano-hole varying from 80 to 800 nm as shown in Fig. 1c and 1d. The compositions of these films are shown in Fig. 2a and b respectively. EDX analysis does not reveal any obvious compositional difference (with more than 97 at.% of copper and less than 3 at.% of oxygen) between the copper films as-received and after ultrasonic chemical treatment. Thus ultrasonic chemical treatment process does not change the composition of copper film. The nanoporous copper film looks darker than the metal copper film due to the induced nano-porous structure. As shown in Fig. 2c, Brunauer– Emmett–Teller (BET) tests show that adsorption cumulative surface area of nano-porous copper film is obviously larger than that of rolled metal copper film (0.499 m2/g vs 3.896 m2/g). Metal aluminium films with nanopores on parts of the interface as reported by the Ekaterina V. Skorb group are formed in purified water through ultrasound probe. Our method differs greatly from that of the Ekaterina V. Skorb group in reaction mechanisms [1,2] and the interface result. Through our method, nanopores were generated on every interface of copper film. They are not as the same as nanopores created using the probe cup-horn system from which ultrasound is attenuated rapidly, and only a few centimeters are sonochemically active [1,2]. The reaction mechanism of our method is as follows:
Cu þ 1=2O2 þ 4NH3 þ H2 O ! 2OH þ CuðNH3 Þ2þ 4 Because of low solubility of oxygen gas in water, the rate of dissolution is proportional to the surface area of the copper, and, at low oxygen pressures, is mainly determined by the transport step
Fig. 1. SEM images of as received rolled metal copper film at low magnification (1a) and high magnification (1b); SEM images of rolled metal nano-porous copper film with ultrasonic chemical treatment at low magnification (1c) and high magnification (1d).
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Y. Chen et al. / Materials Letters 226 (2018) 8–12
Fig. 2. EDX analyses of as-received copper current collector before ultrasonic chemical treatment (2a); after ultrasonic chemical treatment (2b); Pore size distribution of asreceived copper current collector before and after ultrasonic chemical treatment (2c).
of the dissolved oxygen to surface. With the oxygen concentration on the surface increasing, the chemical reaction on the surface will take place more quickly. Micro-streaming, shock waves, and liquid micro-jets in the vicinity of metal copper surfaces can also improve the oxygen gas transport to the copper surface and enhance dissolution rates of copper in ammonia solution so that ultrasound can change the copper film surface morphology effectively. This low-intensity ultrasonic cleaner with external or submersed plate is easily fitted to tanks of several cubic meters and allows a scale-up of nanopores preparation on several square meters of copper film. 3.2. Sputtering silicon on metal copper film or metal nano-porous copper film and their electrochemical performances The same sputtering conditions were used to form the silicon layer on rolled copper film and nano-porous copper film. Surface
morphology and particle size of the silicon films were investigated by SEM (see Fig. 3). More silicon cracking on rolled copper film can be seen in Fig. 3a. This sample is called silicon on smooth copper film. The adhesion between silicon layer and the nano-porous copper film is stronger due to a larger surface area. The absence of peaks corresponding to crystalline silicon in XRD analysis as shown in Fig. 3e, confirms the formation of amorphous structure of the silicon on these films. The electrochemical testing results of the silicon on copper film are shown in Fig. 4. The silicon active material on nanoporous metal copper film delivers an areal capacity of 2.2 mAh/cm2 and has excellent cycling stability as shown in Fig. 4b. This areal capacity is comparable with that of commercially available lithium ion batteries [7,8]. Compared with the capacity of the silicon on smooth copper film, the capacity of silicon active materials on nano-porous copper film is more stable in a coin battery case even after 100 cycles at a current
Y. Chen et al. / Materials Letters 226 (2018) 8–12
11
Fig. 3. SEM images of sputtered Si on metal copper film at low magnification (3a) and high magnification (3b); SEM images of sputtered Si on metal nano-porous copper film with ultrasonic chemical treatment at low magnification (3c) and high magnification (3d); XRD of silicon layer on copper current collector (3e).
rate of 0.2 C. This result indicates that using nano-porous copper film as current collector can improve electrochemical capacity retention of the silicon active material significantly. Nano-porous copper structure can provide sufficient adhesive force to avoid silicon active materials peeling off, and larger surface area to reduce surface tension during the lithiation
and delithiation processes on silicon anode, thereby enhancing the cycling stability. Moreover, the porous structure could be further improved by using microporous metal foams (larger surface area) as starting materials, and maintaining a higher volume of active materials to yield high-energy density and increase rate performance.
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Y. Chen et al. / Materials Letters 226 (2018) 8–12
Fig. 4. Electrochemical performances of (a) silicon on rolled smooth copper film and (b) silicon on nano-porous copper film.
4. Conclusions In summary, we have fabricated metal copper film with uniform nanopores on the surface for the first time using ultrasonic synthetic method. The process to do so is easy to commercialize at low-costs. The sizes of nano-holes vary from 80 to 800 nm and increase with processing time and ammonia concentration. This kind of metal copper film with uniform nanopores on the surface has the potential to be used as current collector to improve charge/discharge property of lithium ion batteries. Acknowledgements This work was supported by Ministry of Education of Singapore (Grant number MOE2015-TIF-1-G-013). References [1] E.V. Skorb, D.G. Shchukin, H. Möhwald, D.V. Andreeva, Ultrasound-driven design of metal surface nanofoams, Nanoscale 2 (2010) 722–727.
[2] E.V. Skorb, D. Fix, D.G. Shchukin, H. Möhwald, D.V. Sviridov, R. Mousa, N. Wanderka, J. Schäferhans, N. Pazos-Pérez, A. Fery, D.V. Andreeva, Sonochemical formation of metal sponges, Nanoscale 3 (2011) 985–993. [3] J.A. Martens, J. Jammaer, S. Bajpe, A. Aerts, Y. Lorgouilloux, C.E.A. Kirschhock, Simple synthesis recipes of porous materials, Microporous Mesoporous Mater. 140 (2011) 2–8. [4] J. Xiao, S. Yang, L. Wan, F. Xiao, S. Wang, Electrodeposition of manganese oxide nanosheets on a continuous three dimensional nickel porous scaffold for high performance electrochemical capacitors, J. Power Sources 245 (2014) 1027– 1034. [5] I. Vukovic, G. Brinke, K. Loos, Block copolymer template-directed synthesis of well-ordered metallic nanostructures, Polym. 54 (2013) 2591–2605. [6] J.H. Pikul, G.H. Zhang, J. Cho, P.V. Braun, W.P. King, High-power lithium ion microbatteries from interdigitated three-dimensional bicontinuous nanoporous electrodes, Nat. Commun. 4 (2013) 1732. [7] M. Ko, S. Chae, J. Ma, N. Kim, H.W. Lee, Y. Cui, J. Cho, Scalable synthesis of silicon-nanolayer-embedded graphite for high-energy lithium-ion batteries, Nat. Energy 1 (2016) 16113. [8] Z. Chen, C. Wang, J. Lopez, Z. Lu, Y. Cui, Z. Bao, High-areal-capacity silicon electrodes with low-cost silicon particles based on spatial control of selfhealing binder, Adv. Energy Mater. 5 (2015) 140826.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
Nano-porous copper metal current collector for lithium ion batteries Yuan Chen a, Huajun Feng a, Yihua Wang a,⇑, Zhe Tang b, Daniel Chua b a b
School of Engineering, Republic Polytechnic, 9 Woodlands Avenue 9, Singapore 738964, Singapore Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
a r t i c l e
i n f o
Article history: Received 2 August 2017 Received in revised form 27 April 2018 Accepted 1 May 2018 Available online 3 May 2018 Keywords: Nano-porous Current collector Lithium ion battery Sonochemical synthesis Energy storage and conversion Porous materials
a b s t r a c t A low-cost and scalable ultrasonic synthetic method to produce copper metal current collector with uniform nanopores on the surface is achieved for the first time. A layer of silicon active material was sputtered on the surface of as-prepared metal current collector (vs. metal lithium) as an innovative current collecting material for lithium ion battery. The assembled battery with this current collector showed an areal capacity of 2.2 mAh/cm2, comparable to that of commercially available lithium ion batteries. Due to the nano-porous structure of current collector, the silicon active material electrochemical capacity retention and cycling stability also improved significantly. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Safety and fast charging/discharging properties of lithium ion battery are highly desirable for purposes ranging from portable devices to electric vehicles. Metal films with dimensionally arrayed nanopores on the surface have been long sought-after in the quest for high-surface-area conductive current collectors [1–5] and catalytic materials [1,2]. Recently, it has been reported that electrodes with meso-structure produce high power batteries with ultra-fast charging/discharging property (within one minute [6]). In lithium ion battery field, porous structures can provide sufficient inner free space to absorb large volume expansions and thereby enhance the cycling stability, and are favourable for fast transport of Li+ to increase the rate performance. However, turning block metals into metal current collectors with dimensionally arrayed nanopores on the surface economically remains a significant challenge. There are several well-established approaches for preparing nano-porous metal films: notably through de-alloying, recently through deposition of metals on nanostructured templates and laser etching, etc. There has been an ongoing search over the past decades, but no commercial metal nano-porous network film is available yet. Ultrasound can generate oscillating bubbles and lead to extremely rapid heating or cooling rates, as well as pressures of up to several hundred mega-pascals in transient cavities while the bulk ⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.matlet.2018.05.019 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
conditions of the liquid solution remain at ambient temperature. In 2010, Ekaterina V. Skorb’s group reported that a metal aluminium film with nanopores on the interface was obtained in pure water through ultrasound treatment using ultrasound cell disrupting equipment [2]. However, it is difficult to make nanopores on a large piece of metal through ultrasound probe impacting due to the size limitations of the probe. In this paper, we report an ultrasonic synthetic method for producing metal current collector with uniform nanopores on every interface and their electrochemical performance as current collectors in lithium ion batteries. To our best knowledge, uniform formation of nanopores on metal surface through a combined ultrasonic and chemical etching method has not been reported. There are also no commercial lithium ion batteries adopting metal current collectors with uniform nanopores on the surface. 2. Experimental Rolled metal copper films (10 10 cm2, 10 mm in thickness) as raw materials were transferred to a glass beaker filled with dilute ammonia solution. Then the rolled copper films in ammonia solution were put into ultrasonic cleaner to stir for 30 min at room temperature. In this paper, the ultrasonic chemical treated copper film will be named as nano-porous copper film. After drying, morphology of copper films was observed using Scanning Electron Microscope (SEM). Energy Dispersive X-ray analyses (EDX) were obtained with an EDAX detector. X-ray diffraction (XRD) scans were acquired with a Bruker D8 Advance system. Rolled metal
Y. Chen et al. / Materials Letters 226 (2018) 8–12
copper films and nano-porous copper films were used as substrates. Silicon layers were sputtered to metal copper films with a Denton sputtering (Discovery system) machine using DC Mode at 200 W power in Ar gas atmosphere for 30 min. Electrochemical performance of metal copper films and nanoporous copper films (in an area of 1 1 cm2) as current collectors was investigated through a CR2032 coin cell (half-cell) with sputtering silicon as the active cathode material, metal lithium as the reference electrode and counter electrode, and Celgard film as separator. The electrolyte was 1.0 M LiPF6 in ethylene carbonate and diethyl carbonate with volume ratio of 1/1. To compare electrodes properties, electrochemical performance was evaluated using Neware battery tester at room temperature. 3. Results and discussion 3.1. Preparation of nano-porous copper metal current collector from metal copper film Usually defects on metal copper surface and subsurface can occur in various forms, e.g., folds, splashes, cracks, inclusions, and voids. The voids are caused by gas porosity, shrinkage porosity, and shrinkage cavities when metal copper is manufactured. In the atmosphere, the surface of copper oxidizes over the course of years to become a mixture of green basic salts, the patina, consisting mainly of the basic sulphate, with some basic carbonate. Fig. 1 shows the surface morphology as-received and after ultrasonic chemical treatment of rolled metal copper films. The as-received rolled metal copper surface (Fig. 1a and b) is smooth and flat visually. After ultrasonic chemical process treatment, uniform nano-holes appear on the surface of rolled metal copper film as shown in Fig. 1c and d. These nanopores on the surface are very
9
similar to dimensionally arrayed nanopores prepared by nanostructured polymer templates method [5]. The size of nano-holes might increase with processing time and ammonia solution concentration. The details of copper surface morphology reveal the sizes of nano-hole varying from 80 to 800 nm as shown in Fig. 1c and 1d. The compositions of these films are shown in Fig. 2a and b respectively. EDX analysis does not reveal any obvious compositional difference (with more than 97 at.% of copper and less than 3 at.% of oxygen) between the copper films as-received and after ultrasonic chemical treatment. Thus ultrasonic chemical treatment process does not change the composition of copper film. The nanoporous copper film looks darker than the metal copper film due to the induced nano-porous structure. As shown in Fig. 2c, Brunauer– Emmett–Teller (BET) tests show that adsorption cumulative surface area of nano-porous copper film is obviously larger than that of rolled metal copper film (0.499 m2/g vs 3.896 m2/g). Metal aluminium films with nanopores on parts of the interface as reported by the Ekaterina V. Skorb group are formed in purified water through ultrasound probe. Our method differs greatly from that of the Ekaterina V. Skorb group in reaction mechanisms [1,2] and the interface result. Through our method, nanopores were generated on every interface of copper film. They are not as the same as nanopores created using the probe cup-horn system from which ultrasound is attenuated rapidly, and only a few centimeters are sonochemically active [1,2]. The reaction mechanism of our method is as follows:
Cu þ 1=2O2 þ 4NH3 þ H2 O ! 2OH þ CuðNH3 Þ2þ 4 Because of low solubility of oxygen gas in water, the rate of dissolution is proportional to the surface area of the copper, and, at low oxygen pressures, is mainly determined by the transport step
Fig. 1. SEM images of as received rolled metal copper film at low magnification (1a) and high magnification (1b); SEM images of rolled metal nano-porous copper film with ultrasonic chemical treatment at low magnification (1c) and high magnification (1d).
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Y. Chen et al. / Materials Letters 226 (2018) 8–12
Fig. 2. EDX analyses of as-received copper current collector before ultrasonic chemical treatment (2a); after ultrasonic chemical treatment (2b); Pore size distribution of asreceived copper current collector before and after ultrasonic chemical treatment (2c).
of the dissolved oxygen to surface. With the oxygen concentration on the surface increasing, the chemical reaction on the surface will take place more quickly. Micro-streaming, shock waves, and liquid micro-jets in the vicinity of metal copper surfaces can also improve the oxygen gas transport to the copper surface and enhance dissolution rates of copper in ammonia solution so that ultrasound can change the copper film surface morphology effectively. This low-intensity ultrasonic cleaner with external or submersed plate is easily fitted to tanks of several cubic meters and allows a scale-up of nanopores preparation on several square meters of copper film. 3.2. Sputtering silicon on metal copper film or metal nano-porous copper film and their electrochemical performances The same sputtering conditions were used to form the silicon layer on rolled copper film and nano-porous copper film. Surface
morphology and particle size of the silicon films were investigated by SEM (see Fig. 3). More silicon cracking on rolled copper film can be seen in Fig. 3a. This sample is called silicon on smooth copper film. The adhesion between silicon layer and the nano-porous copper film is stronger due to a larger surface area. The absence of peaks corresponding to crystalline silicon in XRD analysis as shown in Fig. 3e, confirms the formation of amorphous structure of the silicon on these films. The electrochemical testing results of the silicon on copper film are shown in Fig. 4. The silicon active material on nanoporous metal copper film delivers an areal capacity of 2.2 mAh/cm2 and has excellent cycling stability as shown in Fig. 4b. This areal capacity is comparable with that of commercially available lithium ion batteries [7,8]. Compared with the capacity of the silicon on smooth copper film, the capacity of silicon active materials on nano-porous copper film is more stable in a coin battery case even after 100 cycles at a current
Y. Chen et al. / Materials Letters 226 (2018) 8–12
11
Fig. 3. SEM images of sputtered Si on metal copper film at low magnification (3a) and high magnification (3b); SEM images of sputtered Si on metal nano-porous copper film with ultrasonic chemical treatment at low magnification (3c) and high magnification (3d); XRD of silicon layer on copper current collector (3e).
rate of 0.2 C. This result indicates that using nano-porous copper film as current collector can improve electrochemical capacity retention of the silicon active material significantly. Nano-porous copper structure can provide sufficient adhesive force to avoid silicon active materials peeling off, and larger surface area to reduce surface tension during the lithiation
and delithiation processes on silicon anode, thereby enhancing the cycling stability. Moreover, the porous structure could be further improved by using microporous metal foams (larger surface area) as starting materials, and maintaining a higher volume of active materials to yield high-energy density and increase rate performance.
12
Y. Chen et al. / Materials Letters 226 (2018) 8–12
Fig. 4. Electrochemical performances of (a) silicon on rolled smooth copper film and (b) silicon on nano-porous copper film.
4. Conclusions In summary, we have fabricated metal copper film with uniform nanopores on the surface for the first time using ultrasonic synthetic method. The process to do so is easy to commercialize at low-costs. The sizes of nano-holes vary from 80 to 800 nm and increase with processing time and ammonia concentration. This kind of metal copper film with uniform nanopores on the surface has the potential to be used as current collector to improve charge/discharge property of lithium ion batteries. Acknowledgements This work was supported by Ministry of Education of Singapore (Grant number MOE2015-TIF-1-G-013). References [1] E.V. Skorb, D.G. Shchukin, H. Möhwald, D.V. Andreeva, Ultrasound-driven design of metal surface nanofoams, Nanoscale 2 (2010) 722–727.
[2] E.V. Skorb, D. Fix, D.G. Shchukin, H. Möhwald, D.V. Sviridov, R. Mousa, N. Wanderka, J. Schäferhans, N. Pazos-Pérez, A. Fery, D.V. Andreeva, Sonochemical formation of metal sponges, Nanoscale 3 (2011) 985–993. [3] J.A. Martens, J. Jammaer, S. Bajpe, A. Aerts, Y. Lorgouilloux, C.E.A. Kirschhock, Simple synthesis recipes of porous materials, Microporous Mesoporous Mater. 140 (2011) 2–8. [4] J. Xiao, S. Yang, L. Wan, F. Xiao, S. Wang, Electrodeposition of manganese oxide nanosheets on a continuous three dimensional nickel porous scaffold for high performance electrochemical capacitors, J. Power Sources 245 (2014) 1027– 1034. [5] I. Vukovic, G. Brinke, K. Loos, Block copolymer template-directed synthesis of well-ordered metallic nanostructures, Polym. 54 (2013) 2591–2605. [6] J.H. Pikul, G.H. Zhang, J. Cho, P.V. Braun, W.P. King, High-power lithium ion microbatteries from interdigitated three-dimensional bicontinuous nanoporous electrodes, Nat. Commun. 4 (2013) 1732. [7] M. Ko, S. Chae, J. Ma, N. Kim, H.W. Lee, Y. Cui, J. Cho, Scalable synthesis of silicon-nanolayer-embedded graphite for high-energy lithium-ion batteries, Nat. Energy 1 (2016) 16113. [8] Z. Chen, C. Wang, J. Lopez, Z. Lu, Y. Cui, Z. Bao, High-areal-capacity silicon electrodes with low-cost silicon particles based on spatial control of selfhealing binder, Adv. Energy Mater. 5 (2015) 140826.