Toward Solid-State 3D-Microbatteries Using Functionalized

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2407−2413

Toward Solid-State 3D-Microbatteries Using Functionalized Polycarbonate-Based Polymer Electrolytes Bing Sun,‡ Habtom Desta Asfaw,§ David Rehnlund, Jonas Mindemark, Leif Nyholm, Kristina Edström, and Daniel Brandell* Department of Chemistry - Ångström Laboratory, Uppsala University, 75121 Uppsala, Sweden S Supporting Information *

ABSTRACT: 3D microbatteries (3D-MBs) impose new demands for the selection, fabrication, and compatibility of the different battery components. Herein, solid polymer electrolytes (SPEs) based on poly(trimethylene carbonate) (PTMC) have been implemented in 3DMB systems. 3D electrodes of two different architectures, LiFePO4-coated carbon foams and Cu2O-coated Cu nanopillars, have been coated with SPEs and used in Li cells. Functionalized PTMC with hydroxyl end groups was found to enable uniform and well-covering coatings on LiFePO4coated carbon foams, which was difficult to achieve for nonfunctionalized polymers, but the cell cycling performance was limited. By employing a SPE prepared from a copolymer of TMC and caprolactone (CL), with higher ionic conductivity, Li cells composed of Cu2O-coated Cu nanopillars were constructed and tested both at ambient temperature and 60 °C. The footprint areal capacity of the cells was ca. 0.02 mAh cm−2 for an area gain factor (AF) of 2.5, and 0.2 mAh cm−2 for a relatively dense nanopillar-array (AF = 25) at a current density of 0.008 mA cm−2 under ambient temperature (22 ± 1 °C). These results provide new routes toward the realization of all-solid-state 3D-MBs. KEYWORDS: Li battery, 3D microbattery, polymer electrolyte, nanopillars, carbon foam

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INTRODUCTION Three-dimensional microbatteries (3D-MBs) are energy storage devices intended for a large range of microelectronic applications including medical implants, autonomous sensors and telecommunication devices.1 The limited volume available in these devices (1−10 mm3) requires the battery to be made with a minimum volume while still being able to provide adequate energy and power densities. These requirements impose enormous challenges regarding the electrode design, choice of active materials, integration of suitable electrolytes as well as the development of facile cell assembly steps for 3DMBs.2 Although a large variety of 3D electrodes in the microor nanoscale regime have been developed, including nanotubes, nanopillars and porous foams,2−4 a corresponding development of suitable electrolyte systems for 3D-MBs is still to be realized. All-solid-state 3D-MBs in this context constitute a desirable battery concept in terms of volume minimization (as thin electrolytes can be used) and battery safety (as there is no risk of electrolyte leakage).5,6 The limited advancement within the solid electrolyte field has, however, hindered the development of all-solid-state 3D-MBs. Liquid electrolytes are still used in most 3D-MB prototypes due to their high ionic conductivities, yielding good cell performances at room temperature.7−11 However, the safety and leakage concerns of these cells, as well as their relatively low volumetric energy densities, may significantly limit their practical use.12,13 In this context, solid polymer electrolytes © 2017 American Chemical Society

(SPEs) offer advantages in terms of safety, mechanical integrity toward volume changes, chemical stability, versatility with respect to molecular modifications, and ease of processing.13−17 SPEs have therefore been considered desirable electrolyte systems for all-solid-state 3D batteries,3−6,16 even though the major challenge to attain sufficiently high room-temperature ionic conductivities remains to be overcome. In comparison with ceramic solid electrolytes,16 SPEs facilitate the attainment of thin, flexible and adhesive coatings on microscopic surfaces. As this enables use as versatile separators together with Li-metal electrodes to yield less problems with lithium dendrite growth than with most liquid electrolytes,17 SPEs are very attractive for the development of 3D-MBs. A few research groups have presented possible fabrication routes to integrate gel or hybrid polymer electrolytes into 3DMBs, primarily by soaking a polymer matrix with conventional liquid electrolytes7,9−11,18,19 or by electrodepositing electronically insulating polymer layers directly on electrode surfaces.20−25 Recent advances using atomic layer deposition (ALD) techniques showed that controlled deposition of inorganic electrolytes,26 such as LiPON separating LiCoO2 and Si electrodes for 3D-MB applications, can be achieved.27 Alternatively, the integration of polyether-derived polymers Received: September 11, 2017 Accepted: December 4, 2017 Published: December 4, 2017 2407

DOI: 10.1021/acsami.7b13788 ACS Appl. Mater. Interfaces 2018, 10, 2407−2413

Research Article

ACS Applied Materials & Interfaces

(100 g L−1, Merck), (NH4)2SO4 (20 g L−1, Merck), and diethyltriamine (DETA, 80 mL L−1, Fluka) as previously described.46,47 An initial potential pulse to −1.5 V vs Cu was followed by a pulsed current program consisting of cathodic pulses of 2 mA/cm2 for 0.25 s, 30 mA/ cm2 for 0.05 s, and 0 mA/cm2 for 0.5 s. The pulsed-current program was repeated 4000 times to obtain Cu nanopillars with an average length of 2−8 μm. The diameters of the nanopillars were controlled by template-assisted electrodeposition using polycarbonate (PC) membranes (Cyclopore, Whatman) containing pores with a diameter of 1 or 2 μm. After the deposition, the PC template was dissolved in dichloromethane (Sigma-Aldrich) to yield a 3D current collector with an inherent Cu2O surface oxide layer.46,47 Polymer Electrolyte Fabrication on 3D Electrodes. Two polycarbonate-based host polymers were synthesized for use as SPEs: PTMC with 10% of the monomers functionalized by OH-groups (P(TMC−OH)) and a 20% TMC and 80% caprolactone (CL) random copolymer (P(TMC-CL)). Detailed descriptions of the polymer synthesis and characterization procedures are provided elsewhere.32,33 The ionic conductivities and transport number data for these different PTMC-based SPEs have also been reported previously32,34,35 and a comparison of the ionic conductivities is provided in the Figure S1. Diluted homogeneous mixtures with controlled mass loadings of high-molecular-weight polymers (457 000 g mol−1 for P(TMC−OH) and 246 000 to 338 000 g mol−1 for the copolymer32,34) doped with LiTFSI (Purolyte, dried at 120 °C for 48 h) in anhydrous acetonitrile (Sigma-Aldrich) were prepared to form polymer coatings with a thickness of a few micrometers. A molar [TMC−OH]:[Li+] ratio of 8:1 and a [TMC20CL80]:[Li+] ratio of 4.6:1 were chosen based on previous results regarding their optimal ionic conductivities.32,34 The P(TMC−OH) system was coated on LiFePO4-carbon foams by dipping the electrode into the polymer solution for up to 6 h. The obtained electrolyte thicknesses were estimated to be less than 5 μm using SEM. For comparison, the PTMC analogue without the OH functionalization was also subjected to the same treatments. For the Cu2O-nanopillar electrodes, diluted P(TMC-CL)-LiTFSI in acetonitrile was dipped onto the substrate and left to slowly infiltrate the volume between the nanopillars at RT in a glovebox overnight. The solvent was then removed by drying under vacuum (down to 1 mbar) at 60 °C for 20 h. Surface Analysis. The morphology of the polymer coated electrodes was examined with a ZEISS Merlin high-resolution scanning electron microscope (HR-SEM) equipped with an Aztec energydispersive X-ray spectroscopy (EDX) system. The accelerating voltage used for the EDX analysis on the SPE-coated samples was 10 to 12 kV. Electrochemical Cycling. The electrochemical performance of the electrodes was examined by galvanostatic cycling at different current densities between 0.05 and 3 V vs Li+/Li at ambient temperature (22 ± 1 °C) and 60 °C for the Cu nanopillar electrodes with the native Cu2O thin film serving as the active material. The cycling study on LiFePO4-carbon foam electrode coated with SPE was performed between 2.7 and 4.2 V vs Li+/Li using a current density of 0.005 mA cm−2.

on nonplanar electrode surfaces has been demonstrated, for instance, the self-assembly of polyethylene amine-based oligomers21,28 and poly(propylene oxide)-derived oligomers electropolymerized on 3D Cu2O-nanopillars.23 Other recent examples also include initiated chemical vapor deposition polymerization (iCVD) of vaporized monomers on Si nanowires to produce nanosized polymer coatings at ambient temperatures.29 The dependence on liquid electrolyte components to provide sufficient ion transport, or complex deposition conditions, however, leaves issues associated with their integration in practical cells unsolved. In this study, we focus on the integration of SPEs in 3D-MBs and for the first time evaluate their practical performance in these types of cells. We particularly exploit electrolytes based on poly(trimethylene carbonate) (PTMC) hosts, which provide good design flexibility as promising alternatives to conventional SPEs based on poly(ethylene oxide) (PEO).30−37 The conventional PEO-based SPEs commonly exhibit limited ion transport rates, e.g., room-temperature ionic conductivities from 1 × 10−5 to 1 × 10−8 S cm−1 and cationic transference numbers t+ of ca. 0.2 to 0.3 for PEOxLiTFSI.38−40 In comparison, the present PTMC-based SPEs are modified either with functional side groups that can provide different surface functionalities (i.e., hydroxyl groups)32 or increased flexibility by copolymerization with ester units (i.e., caprolactone).33,34 High cation transference numbers (t+ > 0.6) and ionic conductivities of the order of 1 × 10−5 S cm−1 at room temperature, have previously been obtained for PTMC-based SPEs.33,34,41 Experimental and computational investigations on the origin of these favorable electrolyte properties point toward weaker ion binding in polycarbonates and polyesters compared to PEO and PEO-like segments.41−43 The use of PTMC-based SPEs in 3D-MBs which can operate at ambient temperatures constitutes an important step toward the realization of all-solidstate 3D-MBs. All these advantageous functionalities enable the use of thin SPE coatings directly integrated on 3D electrode surfaces which facilitates the design of all-solid 3D-MB prototypes. Two different 3D electrode substrates based on porous carbon foams (representing an aperiodic sponge architecture) and Cu nanopillars (representing an interdigitated architecture) are employed and the sequential deposition of the corresponding active materials, LiFePO4 on carbon foam and Cu2O on Cu nanopillars, is described and evaluated in SPEbased 3D-MB cells at ambient temperature (22 ± 1 °C) and 60 °C.

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EXPERIMENTAL METHODS

LiFePO4-Coating of Emulsion-Templated Carbon Foams. Emulsion-templated carbon foams were manufactured from high internal phase emulsion polymers (polyHIPE) using previously described procedures.44,45 Thin pieces of the carbon foams were immersed in a sol−gel LiFePO4 precursor, composed of 0.5 M FeSO4· 7H2O (Sigma-Aldrich, ≥ 99.0%), 0.5 M NH4H2PO4 (Merck, 99.0%), 0.5 M LiOOCCH3 (Merck), and 1 M citric acid (Merck, ≥ 99.5%) dissolved in a 9:1 mixture of distilled water and methanol (SigmaAldrich, ≥ 99.6%), for 48 h. The gel-coated foams were then taken out of the solution and dried overnight at 70 °C in air and subsequently in vacuum at 120 °C to eliminate traces of water. The foams were then subjected to thermal pyrolysis at 700 °C in an argon atmosphere to yield a layer of LiFePO4 with a thickness of around 200 nm on the surfaces of the foams. Electrodeposition of 3D Cu2O−Cu Nanopillars. 3D Cu nanopillar current collectors were manufactured by template assisted electrodeposition in an aqueous solution containing CuSO4 5H2O

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RESULTS AND DISCUSSION Two fabrication routes were investigated to build all-solid-state Li-polymer 3D-MBs based on an aperiodic foam and a nanopillar electrode architecture, respectively. Schematic illustrations of the electrode fabrication and cell assembly are shown in Figure 1. The first scheme in Figure 1a shows the sequential deposition of a porous layer of LiFePO4 onto a carbon foam substrate via a sol−gel approach and the formation of a micrometer thick layer of the SPE on the LiFePO4. In the second scheme (Figure 1b), a 3D-microbattery was manufactured by infiltrating the SPE into the empty spaces between the Cu2O-coated Cu nanopillars; here, the Cu2O and the Cu nanopillars act as the active material and current collector, 2408


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many well-known bulk polymers (resistivity values of 1 × 1015 to 1 × 1020 Ω cm are typical), it is far from obvious that the necessary resistance could be reliably obtained in nanometerthin films of polymer electrolyte; hence, it is clear that the electronic conductance of ultrathin SPEs will constitute a challenge. The thickness of the SPE layer must consequently be increased sufficiently to minimize the leakage current between the electrodes. Another practical criterion that must not be overlooked while applying SPEs for high-surface-area electrodes, especially those with aperiodic architecture, is complete and even surface coverage to provide sufficient electronic blocking. Because of these concerns, the experiments with the porous carbon foam electrodes were first carried out in favor of microsized SPE layers with surfactant properties. Solvent-free electrolytes based on PTMC, functionalized P(TMC−OH) and P(TMC-CL) doped with LiTFSI were compared during our first evaluation to integrate them in 3D foam electrodes. These SPEs had different functionalities depending on their side groups and backbone structures, which influenced both their transport properties and the physical properties of the electrode/SPE interface. When the conventional PTMC-based electrolyte was used (see Figure 2a,

Figure 1. Schematic illustrations showing the step-by-step fabrication of the all-solid-state Li-polymer 3D-microbatteries based on (a) a LiFePO4-coated carbon foam electrode and (b) a Cu2O-coated Cu nanopillar electrode.

respectively. With both electrode designs Li foils were used as combined counter and reference electrodes. It is known that the fabrication of thin and completely covering solid electrolytes onto 3D electrodes is a challenging task,5,12,21,23,48,49 which becomes increasingly difficult with increasing geometrical complexity of the electrode. The surface area of the electrode is usually expressed using the area gain factor (AF), which is a geometrical figure-of-merit used to estimate the increase in the energy density per footprint surface area of a 3D-MB electrode.2,12 The AF is defined as the ratio between the total electrochemically active surface area and its footprint area for a given 3D electrode and the AF value for the carbon foam was previously reportedly to exceed 90.44 For the Cu2O/Cu nanopillar architecture, electrodes with estimated AFs of ca. 2.5 and 25 were prepared using PC membranes with different pore sizes (i.e., 2 and 1 μm).10 The use of the membrane with 1 μm pores resulted in less space between the nanopillars and thus a denser nanopillar arrangement, i.e., AF = 25. The SEM images of the bare substrates seen in Figure S2 were used to estimate the AFs. Previous studies have featured uniform nanosized SPE coatings on 3D nanopillar electrodes obtained by electropolymerization of acrylic oligomers.6 The latter electropolymerization approach, however, proved to be less successful for the highly porous and complex LiFePO4-coated carbon foam electrode architectures which feature an aperiodic arrangement of interconnected macropores (see Figure S2). The reason for this was that short-circuiting occurred during the cell assembly in all cases. The short-circuiting problem indicated above may pose a general problem when nanometerscale thin SPE layers are employed for 3D-MBs since polymer materials possess both ionic and electronic conductivity. This means that the electronic conductivity of the SPE also needs to be taken into consideration as a too thin SPE layer can give rise to a significant leakage current between the electrodes. This can perhaps be illustrated using the following example. If we assume a potential difference of 3 V between the electrodes and an SPE thickness of 10 nm, an electric field strength in the SPE of 3 × 109 V m−1 would clearly be obtained. Moreover, if a maximum leakage current density of 0.02 μA cm−2 is allowed (which incidentally is a rather high current for a 3D-MB), the electronic resistivity of the SPE layer would have to be at least 1.5 × 1014 Ω cm (corresponding to an electronic conductivity of about 6.7 × 10−15 S cm−1). While this is well in the range of

Figure 2. SEM images depicting the LiFePO4-carbon foam coated with (a, b) nonfunctionalized PTMC8LiTFSI, (c, d) functionalized PTMC−OH8LiTFSI, and (e, f) Cu2O−Cu nanopillars coated with PTMC−OH8LiTFSI.

b, Figure S4a), clear heterogeneities were observed in the polymer coating. This is particularly visible in the shadowed regions for the foam substrate. Moreover, the EDX results in Figures S3 and S4e clearly show substrate signals for the samples coated with the PTMC or PTMC-CL copolymer based SPEs, indicating that the coverage of these polymer coatings was incomplete. These cells also short-circuited immediately after the assembly, which could be a result of the incomplete SPE layer between the two electrodes. In contrast, significantly better results were obtained with the OH-functionalized PTMC, which can be attributed to the favorable adsorption and strong adhesion through hydrogen bonding between the 2409


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Figure 3. Cycling performance of two Li/P(TMC-CL)-LiTFSI/Cu2O/Cu-nanopillar cells with area gain factors of (a) 2.5 and (b) 25, respectively, at different cycling rates and temperatures (a, b) 22 ± 1 °C and (c) 60 °C; (d) the cycling voltage profiles of the cells. The solid symbols in a−c refer to the discharge capacities while the open ones represent the charge capacities. The footprint areal capacities (left y axis) were calculated by normalizing the total capacity with respect to the substrate area, whereas the estimated areal capacities (right y axis) were calculated on the basis of the estimated electrochemically active surface area. The current density was calculated using the footprint area.

good surface attachments to particle surfaces. The limited interfacial contacts between the SPE layer and the Li surface might additionally affect the electrochemical cell performance. Overall, the practical cell performance observed for the LiFePO4-carbon foam cell can be considered unsatisfactory due to a possible combination of limitations in the bulk ionic conductivity, interfacial contacts with the counter electrode and the lack of electronic wiring, although the surfactant property obviously resulted in an improvement of coverage and uniformity−and device performance−of the SPE coatings as compared to those without surfactant groups. When the OH-functionalized SPE was coated on Cu2O−Cu nanopillars with a length of 2 to 8 μm (as illustrated in Figure 2e, f), a relatively complete polymer coating was obtained. The length of the nanopillars made it more challenging to produce uniform coatings along the nanopillar surfaces as compared to the foam geometry. This problem can, however, be solved by using polymer coatings locally formed through in situ techniques (e.g., pulsed-current electropolymerization and iCVD23,52) or simply employing infiltrated polymer electrolytes with enhanced ionic conductivities and Li+ transport properties. The latter cell construction using direct P(TMC-CL) SPE infiltration into the voids between the Cu2O coated Cu nanopillars (Figure 1b) constitutes a more straightforward 3DMB fabrication technique than that used for the carbon foam electrode. The polymer electrolyte solution can be expected to infiltrate the space between the nanopillars. Since cutting the 3D Cu substrate without damaging the coated SPE layer proved technically difficult, thereby preventing visualization of the cross sections, the correlation between the areal capacity and the area gain factor of the electrode can provide information regarding the success of the SPE coverage process. When varying the distance between the nanopillars, the electrochemical areal capacity should vary accordingly if the

surface oxygen atoms and OH functional groups. Previous comparative studies using scratch-tests on P(TMC−OH) and PTMC-coated Cu2O and TiO2 flat substrates have demonstrated an improved adhesion strength for P(TMC−OH) as compared to PTMC.32 Here, in the case of the porous substrates, the positive effect of introducing functional groups was also found for the LiFePO4 layer on the carbon foams. As seen in Figure 2c, d and Figure S4b, a much more even polymer coating was achieved compared to that for the original PTMC. The suppressed Fe and P signals from the LiFePO4 coating in the EDX maps (Figure S3) indicate that a relatively complete and even coating of P(TMC−OH) on the order of a few μm was achieved on the substrate. As a result, the electrochemical cell could be fabricated without any short-circuiting. These results highlight the importance of using polymer functionalization to promote the surface attachment and the coverage of the sophisticated foam architectures. The complexity of the porous geometry of the carbon foams clearly increases the risk of short-circuiting the SPE-based cells. Short-circuiting was observed for several other cells containing PTMC-based SPEs as well as cross-linked PPO-based SPEs derived from our previous study.23,50 Nonetheless, the Lipolymer 3DMB cell containing P(TMC−OH)-coated foam was found to be the only system that did not fail in this sense. The voltage profile for the galvanostatic cycling of 3D LiFePO4carbon foam cells is shown in Figure S5. It can be seen that an open circuit voltage of around 3.2 V vs Li+/Li was obtained at 60 °C. This potential value is somewhat lower than the 3.4 V obtained when using liquid electrolytes.51 Considering that the LiFePO4-coated carbon foam electrode is a binder-free system without additional electronic conducting additives, the polymer coating can serve as a combined electrolyte, separator and binder. It might therefore block the electronic conduction between the LiFePO4 particles while at the same time offering 2410



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CONCLUSIONS This work describes the application of solvent-free polymer electrolytes to two types of 3D electrodes based on a porous carbon foam and an array of copper nanopillars, respectively. The employed SPEs were derived from functional polycarbonates which have major advantages, either by surfactant properties or via enhanced ionic conductivity, as compared to conventional PEO-based SPEs. The functionalized P(TMC− OH) polymer electrolyte provided good coverage of the SPE coatings on the highly porous LiFePO4 carbon foam electrode. For the cells consisting of PTMC-based SPEs without such surfactant functionalization, short-circuits were observed, which might be resulting from incomplete coverage and/or poor attachments of the SPE. The cell cycling performance of LiFePO4 carbon-foam-coated by P(TMC−OH) was, however, still unsatisfactory. Furthermore, the copolymer P(TMC-CL)based SPEs with relatively high ionic conductivity showed promising electrochemical performance both at room temperature and at 60 °C when using a 3D electrode based on Cu2Ocoated Cu nanopillars infiltrated with the copolymer SPE. Stable cycling with capacities of ca. 0.2 mAh cm−2 footprint area at AF = 25 could be achieved at ambient temperature 22 ± 1 °C. An alternative and effective approach in electrolyte development has thus been demonstrated in conjunction with state-of-the-art 3D electrode designs.

SPE coating involves the interpillar spaces. As shown in Figure 3, the footprint area capacity of the denser electrodes (AF = 25) was found to be around 10 times higher than that for the electrode with the less dense structure (AF = 2.5), indicating that the electrochemical areal capacities were almost equivalent. This suggests an effective wetting of the electrode surfaces by the polymer coating even for the denser nanopillar arrangement. A footprint area capacity of around 0.2 mAh cm−2 was achieved at a current density of 0.008 mA cm−2 (per footprint area). When taking the AF values into account, comparable areal capacities per electrochemical surface area were obtained for both cells: i.e., around 0.008 mAh cm−2 (as seen on the right y-axis in Figure 3a, b) at a current density of 0.008 mA cm−2, suggesting that the capacity gain was indeed proportional to the surface area gain factor (i.e., AF) for these cells. This confirms that the influence from void spaces in the coated Cu2O electrodes was negligible and that the polymer electrolyte coatings were sufficiently uniform throughout the 3D electrode assembly to access to full areal capacity regardless of the density of the nanopillar arrangement. In Figure 3c, d, it is seen that a capacity enhancement could be obtained by cycling at 60 °C where the ionic conductivity is significantly increased (as shown in Figure S1). This facilitates faster ion conduction than that at RT and also improves the interfacial contacts as the polymer layer softens at higher temperatures. The relatively slow ion transport in the electrolyte at RT could explain the relatively low first cycle capacity in Figure 3a. As seen in Figure 3c, there was a distinct enhancement of the first cycle capacity which could be explained by the enhanced mass transport rate in the electrode and the improved Li-ion transport in the electrolyte. The mass transport rate within the electroactive layer of the Cu2O electrode thus also increases with increasing temperature. The footprint area capacity at 60 °C was found to be around 1.5 times larger than the RT value for the less dense substrate (AF = 2.5) at a current density of 0.008 mA cm−2. Our previous XPS studies of the interfaces of SPE-based cells, in which the surface species of the electrode/SPE interfaces where studied after cycling at elevated temperatures (e.g., 50 °C),36,53 indicate that there was some degradation of the salt and the polymer matrix at the anode surfaces. In the present case, the capacities for each current density did not show any obvious fading at 60 °C, which suggests that the buildup of surface species did not have any detrimental effect during cycling up to one month. The cycling profile in Figure 3d confirms that the voltage plateaus observed at 1.4 to 1.0 V and at potentials higher than 2.5 V vs Li+/Li during the first discharge/charge were consistent with the reduction of Cu2O and the oxidation of Cu, respectively.46,54 In liquid electrolyte based cells, pulverization of Cu2O due to electrochemical milling can lead to an increase in the active surfaces of the particles, which can contribute to an enhancement in capacity during successive cycling.46 When using the present solid electrolytes such an effect was, however, not observed. This is likely due to the fact that the freshly created electrode surfaces, as a result of electrochemical milling, are not accessible by the less diffusive SPE, at least not at ambient temperature. Stable cycling could hence be achieved at 60 °C and ambient temperature (i.e., 22 ± 1 °C). The latter is notable considering traditional PEO-based SPEs cannot be cycled at RT, in contrast to the important advances of polycarbonate-based SPEs which operate well from high to ambient temperatures.34,55

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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/acsami.7b13788. Results of the ionic conductivities of the different PTMC-based SPEs, SEM images of the bare 3D electrodes, EDX results of SPE-coated LiFePO4 carbon foam electrodes, as well as the first electrochemical cycling of the SPE-coated LiFePO4 carbon foam Li cell (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bing Sun: 0000-0003-2314-6945 Habtom Desta Asfaw: 0000-0001-5861-4281 David Rehnlund: 0000-0003-2394-287X Jonas Mindemark: 0000-0002-9862-7375 Leif Nyholm: 0000-0001-9292-016X Kristina Edström: 0000-0003-4440-2952 Daniel Brandell: 0000-0002-8019-2801 Present Addresses

‡ B.S. is currently at Electrochemical Energy Storage Section, Electrochemistry Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland § H.D.A. is currently at Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by STandUP for Energy. Dr. Mario Valvo (Uppsala University) and Dr. Matthew Roberts 2411


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(University of Oxford) are acknowledged for fruitful discussions.

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Toward Solid-State 3D-Microbatteries Using Functionalized

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