Flexible/shape-versatile, bipolar all-solid-state lithium-ion batteries prepared by multistage printing

Se-Hee Kim , Keun-Ho Choi , Sung-Ju Cho , JongTae Yoo , Seong-Sun Lee and Sang-Young Lee *
Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea. E-mail: syleek@unist.ac.kr; Tel: +82-52-217-2948

Received 12th June 2017 , Accepted 9th October 2017

First published on 9th October 2017


Bipolar all-solid-state lithium-ion batteries (LIBs) have attracted considerable attention as a promising approach to address the ever-increasing demand for high energy and safety. However, the use of (sulfide- or oxide-based) inorganic solid electrolytes, which have been the most extensively investigated electrolytes in LIBs, causes problems with respect to mechanical flexibility and form factors in addition to their longstanding issues such as chemical/electrochemical instability, interfacial contact resistance and manufacturing processability. Here, we develop a new class of flexible/shape-versatile bipolar all-solid-state LIBs via ultraviolet (UV) curing-assisted multistage printing, which does not require the high-pressure/high-temperature sintering processes adopted for typical inorganic electrolyte-based all-solid-state LIBs. Instead of inorganic electrolytes, a flexible/nonflammable gel electrolyte consisting of a sebaconitrile-based electrolyte and a semi-interpenetrating polymer network skeleton is used as a core element in the printed electrodes and gel composite electrolytes (GCEs, acting as an ion-conducting separator membrane). Rheology tuning (toward thixotropic fluid behavior) of the electrode and GCE pastes, in conjunction with solvent-drying-free multistage printing, enables the monolithic integration of in-series/in-plane bipolar-stacked cells onto complex-shaped objects. Because of the aforementioned material and process novelties, the printed bipolar LIBs show exceptional flexibility, form factors, charge/discharge behavior and abuse tolerance (nonflammability) that far exceed those achievable with inorganic-electrolyte-based conventional bipolar cell technologies.


Introduction

The forthcoming smart energy era, which will involve widespread use of flexible/wearable electronic devices, the Internet of Things (IoT), electric vehicles (EVs) and grid-scale energy storage systems (ESSs), has spurred the relentless pursuit of high-energy/safe rechargeable power sources.1–3 In particular, recent fire/explosion accidents involving lithium-ion batteries (LIBs) has motivated researchers to devote greater attention to safety failures. As a promising solution to simultaneously address the aforementioned issues of energy density and safety, bipolar all-solid-state LIBs have attracted extensive attention.4–9 The bipolar cell configuration enables minimal use of electrochemically inert components such as metallic foil current collectors and packaging substances, thereby contributing to a higher volumetric energy density beyond those achievable with a simple electrical connection of individual cells.

A key component in bipolar batteries is the solid-state electrolyte,5,10 which acts as an ion-conducting separator membrane and as an electrolyte in bulk electrodes. To prevent the flow of ions between the adjacent cells (i.e., to enable voltage build-up) inside bipolar batteries, nonfluidic solid-state electrolytes are essentially required. To date, (sulfide or oxide-based) inorganic solid electrolytes have been extensively investigated as attractive candidates;4,6,11,12 however, their longstanding problems, which include poor ionic conductivity, high grain boundary resistance, mechanical brittleness, chemical/electrochemical instability and complicated synthetic processes, pose a formidable challenge to their practical application.

Several approaches have been undertaken to resolve the aforementioned issues of inorganic electrolytes, which include the synthesis of new inorganic electrolytes,13–15 inorganic/polymer composite electrolytes7–9,15–17 and porous nonwoven-reinforced inorganic electrolytes.11 Some meaningful progress has been reported in previous studies; however, many problems (particularly related to chemical/electrochemical stability, mechanical flexibility and form factors) still remain unresolved. In addition to these material challenges, the high-pressure/high-temperature sintering-based fabrication processes, which may not be suitable for continuous mass production, should also be resolved for their facile commercialization.

Here, we demonstrate a new class of flexible/shape-versatile bipolar all-solid-state LIBs as an unprecedented material/process strategy to address the aforementioned problems associated with inorganic-electrolyte-based bipolar LIBs. The new bipolar LIBs are easily fabricated via solvent-drying-free, ultraviolet (UV)-curing-assisted multistage printing. Instead of using conventional inorganic solid electrolytes, we develop a new flexible/nonflammable gel electrolyte (i.e., sebaconitrile (SBN)-based electrolyte and a semi-interpenetrating polymer network (semi-IPN) skeleton). The new gel electrolyte is incorporated as a core element into the printed electrodes and into the printed solid-state gel composite electrolytes (GCEs, functioning as an ion-conducting separator membrane). The flexible/nonflammable gel electrolyte eliminates concerns about grain boundary resistance (which is often encountered in inorganic-electrolyte-based cells) while also substantially improving the mechanical deformability, form factors, and thermal stability of the resultant printed electrodes and GCE. In addition, the gel electrolyte in the printed electrode acts as a binder that holds electrode active materials and carbon conductive additives, indicating no inclusion of conventional polymeric binders used in LIBs. The rheological properties of the printable electrode and GCE pastes are elaborately tailored to exhibit non-Newtonian fluid (specifically, thixotropic fluid) characteristics, thus enabling the realization of seamlessly integrated (in-series and in-plane) bipolar-stacked cells onto complex-shaped objects (e.g., mini toy cars) via solvent-drying-free UV-assisted multistage printing. Notably, the multistage printing does not require the high-pressure/high-temperature sintering processes commonly used to fabricate inorganic-electrolyte-based bipolar all-solid-state LIBs.5,13 We previously reported the advantageous effects (i.e., short processing time, no use of processing solvents, no inclusion of conventional polymeric binders, no injection of liquid electrolytes) of the UV-curing-assisted printing process in cell fabrication elsewhere.18,19 The rationally designed electrodes and GCEs, in conjunction with the multistage printing process, enable the facile fabrication of bipolar all-solid-state LIB cells with exceptional mechanical flexibility, form factors, stable charge/discharge behavior and nonflammability.

Experimental

Preparation of the SWCNT-coated electrode active materials

The electrode active materials (here, LiCoO2 (KD10, Umicore) and LiTi4O12 (Süd Chemie) powders were chosen for proof of concept) were added and mixed into single-walled carbon nanotubes (SWCNT) suspension. After being subjected to filtering followed by rinsing and drying, the SWCNT-coated electrode active powders were obtained.

Fabrication of the printed bipolar LIBs

To fabricate the printed electrodes, the as-prepared SWCNT-coated electrode powders were mixed with carbon black conductive additives and gel electrolyte precursors, thus yielding the electrode pastes. The composition ratios of the electrode pastes (electrode active powder/carbon black/gel electrolyte) were 55/6/39 (w/w/w) for the LiCoO2 (LCO) cathode and 30/7/63 (w/w/w) for the LiTi5O12 (LTO) anode, respectively. The excessive increase in the powder content of the electrode pastes resulted in the poor printability due to loss of fluidic behavior, (i.e., serious agglomeration of the powders) (Fig. S1, ESI). Further optimization of the composition ratio will be implemented in our future studies. The gel electrolyte precursor was composed of the (1 M LiBF4 in SBN) electrolyte and semi-IPN skeleton precursor (UV-curable ethoxylated trimethylolpropane triacrylate (ETPTA, Aldrich) incorporating 1.0 wt% HMPP as a photoinitiator and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) (Arkema, HFP content = 6 mol%), (ETPTA/PVdF-HFP = 75/25 (w/w)). The composition ratio of the (1 M LiBF4 in SBN) electrolyte and semi-IPN skeleton precursor was 85/15 (w/w). The LTO anode paste was printed on an Al current collector using a stencil printing technique without any processing solvents and was then exposed to UV irradiation, resulting in the printed LTO anode. UV irradiation was performed using an Hg UV-lamp (Lichtzen) with an irradiation peak intensity of approximately 2000 mW cm−2. Subsequently, on top of the LTO anode, the GCE paste (gel electrolyte precursor/Al2O3 nanoparticles (average particle size ∼300 nm) = 60/40 (w/w)) was introduced via the same stencil printing and UV curing process, leading to a printed solid-state GCE layer on the LTO anode. The printed LCO cathode was then fabricated by printing the LCO cathode paste directly onto the GCE layer/LTO anode unit, followed by UV irradiation. After the Al current collector was placed on top of the printed LCO cathode/printed GCE layer/printed LTO anode assembly, a seamlessly integrated all-solid-state mono full cell was obtained. On top of the as-prepared mono full cell, the printing/UV curing processes were repeatedly conducted, enabling the fabrication of the printed bipolar cells (with in-series or in-plane configuration). Note that the LTO anode and LCO cathode shared the Al foil as a common current collector.

Structural/rheological/physical characterization of the printed GCE and electrodes

The UV curing reaction of the printed GCE and electrodes was examined using an FT-IR spectrometer (Alpha Platinum ATR, Bruker) with a spectral resolution of 4 cm−1. The glass transition temperatures (Tgs) of the semi-IPN were examined by differential scanning calorimetry (Q200, TA Instruments, heating rate = 10 °C min−1). The weight loss of the printed GCE and electrodes was estimated as a function of temperature using a thermogravimetric analyzer (Q500, TA Instruments, heating rate = 10 °C min−1) with the sample under a nitrogen atmosphere. The ionic conductivity of the printed GCE was measured with an impedance analyzer (VSP classic, Bio-Logic) over a frequency range from 1 to 106 Hz. The electronic conductivities of the printed electrodes were estimated using a four-point probe technique (CMTSR1000N, Advanced Instrument Technology). The interfacial exothermic reaction between the delithiated LCO and the electrolyte was measured by differential scanning calorimetry (Q200, TA Instruments, heating rate = 10 °C min−1). The morphologies of the printed GCE and electrodes were characterized using field-emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (S4800, Hitachi). The mechanical flexibility of the printed GCE and electrodes was investigated under various deformation modes such as bending and folding. The rheological properties of the printable GCE and electrode pastes were investigated using a rheometer (Haake MARS 3, Thermo Electron GmbH).

Fabrication and electrochemical performance of the printed bipolar LIBs

The printed LIBs were fabricated using 2032-type coin cells (for half cells) and Al pouch cells (for mono and bipolar-stacked full cells, width and length of 10 mm × 10 mm). The assembly of the cells was performed in an argon-filled glove box. A control cell was prepared by assembling an LCO cathode, LTO anode and polyethylene (PE) separator (thickness = 20 μm, Tonen), followed by filling with the carbonate-based liquid electrolyte (1 M LiPF6 in EC/DMC = 1/1 v/v). The electrochemical performance of the cells was investigated under various charge/discharge conditions using a cycle tester (PNE Solution).

Results and discussion

A schematic of the flexible/shape-versatile bipolar cells prepared via the UV-assisted multistage printing process is shown in Scheme 1. Details on the cell fabrication are discussed in the following section. The solid-state GCE was composed of the gel electrolyte (1 M LiBF4 in SBN/semi-IPN skeleton = 85/15 (w/w))) and Al2O3 nanoparticles (average particle size ≈ 300 nm), wherein the composition ratio of the gel electrolyte/Al2O3 nanoparticles was 60/40 (w/w). The Al2O3 nanoparticles were incorporated as a mechanical spacer to prevent electrical contact between the electrodes and as a rheology-tuning agent for the printing process.18,19 The composition ratio of gel electrolyte/Al2O3 nanoparticles = 60/40 (w/w) was determined by considering both rheological behavior (related to printing processability) of the GCE pastes and ionic conductivity of the resulting GCE films (Fig. S2, ESI). The SBN, because of its superb thermal stability and reliable electrochemical performance,20–22 was chosen as a new Li-ion-conducting medium that outperforms conventional carbonate-based ones.
image file: c7ee01630a-s1.tif
Scheme 1 Schematic of the flexible/shape-versatile bipolar cells prepared via the UV-assisted multistage printing process.

The fabrication procedure of the printed GCE, along with its photographs and chemical structure of the major components, is depicted in Fig. 1a. The GCE pastes (i.e., prior to UV curing) were stencil-printed onto a substrate and then solidified upon exposure to UV irradiation for a short time (<30 s) without using any processing solvents. Cross-sectional SEM and EDS images of the resulting GCE film show that the Al2O3 nanoparticles were uniformly and compactly dispersed in the through-thickness direction (Fig. 1b and Fig. S3, ESI). In addition, highly percolated interstitial voids formed between the Al2O3 nanoparticles were observed in the GCE film; these voids were originally occupied by the SBN-based electrolyte that was etched using dimethyl carbonate prior to the SEM analysis.


image file: c7ee01630a-f1.tif
Fig. 1 Synthesis and characterization of the printed GCE. (a) Schematic of the procedure used to fabricate the stencil-printed GCE, along with photographs of the GCE and chemical structure of its major components. The GCE consisted of the gel electrolyte ((1 M LiBF4 in SBN) electrolyte and semi-IPN skeleton) and Al2O3 nanoparticles. (b) Cross-sectional SEM and EDS images showing the dispersion state of Al2O3 nanoparticles and the presence of interstitial voids (originally occupied by the SBN-based electrolyte) in the printed GCE. (c) DSC thermograms showing two different Tgs of the ETPTA polymer network and PVdF-HFP in the semi-IPN. The insets show the (Tgs) of the pristine ETPTA polymer network and PVdF-HFP, respectively. (d) The mechanical flexibility of the printed GCE upon the bending cycles (bending radius = 5 mm, deformation rate = 10 mm min−1). (e) The ionic conductivity of the printed GCE as a function of temperature. (f) TGA profiles of the GCE and carbonate-based control electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)) chosen as the control electrolyte. (g) The isothermal (80 °C) ionic conductivity of the GCE and carbonate-based control electrolyte as a function of time. (h) The nonflammability test of the GCE (top photographs) and the carbonate-based control electrolyte (bottom photographs).

Another important component of the GCE film is the semi-IPN skeleton, which consists of a UV-crosslinked ETPTA network and PVdF-HFP, wherein the composition ratio of ETPTA/PVdF-HFP is 75/25 (w/w). We previously reported23 that the semi-IPN skeleton in gel polymer electrolytes effectively improves both their mechanical flexibility and their ionic conductivity. The (ETPTA/PVdF-HFP) semi-IPN structure was characterized by measuring the Tgs of the ETPTA polymer network and the PVdF-HFP (Fig. 1c). Two different Tgs (71 °C for the UV-cured ETPTA polymer network and −24 °C for the PVdF-HFP), which are similar to those of the pristine ETPTA network and PVdF-HFP (insets of Fig. 1c), were observed for the semi-IPN, indicating phase separation of the ETPTA network and the PVdF-HFP. To confirm this phase-separated structure, a semi-IPN film without Al2O3 nanoparticles was prepared as a model system, and its SEM image was analyzed after selective removal of the PVdF-HFP. A large number of pores originating from the PVdF-HFP were uniformly dispersed in the ETPTA matrix (Fig. S4, ESI), verifying the well-developed phase separation between the ETPTA matrix and PVdF-HFP in the semi-IPN. In addition, the photocuring of the ETPTA matrix was verified by observing the change in the FT-IR peaks (Fig. S5, ESI) assigned to the acrylic C[double bond, length as m-dash]C bonds (1610–1625 cm−1) of the ETPTA and the insoluble polymer fraction (>99%)19,24 after exposure to UV irradiation. The semi-IPN skeleton plays a viable role in the mechanical flexibility of the GCE film (Fig. 1d). No structural disruption was observed in the GCE film even after 10[thin space (1/6-em)]000 bending cycles (bending radius = 5 mm, deformation rate = 10 mm min−1).

The ionic conductivity of the GCE was measured as a function of temperature (Fig. 1e). Over a wide range of temperature, the GCE delivered satisfactory ionic conductivity, demonstrating the formation of well-developed ion channels. A salient feature of the GCE is its thermal stability. The thermogravimetric analysis (TGA) profiles (Fig. 1f) show that the GCE is negligibly volatile until 150 °C because of the high boiling point (200 °C) of SBN, whereas a conventional carbonate-based liquid electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)) exhibited substantial weight loss even below 100 °C. To further verify this superior thermal stability, the isothermal ionic conductivity of the GCE was compared with that of the carbonate-based electrolyte at a high temperature (80 °C). The GCE exhibited stable ionic conductivity for longer than 100 h, whereas the carbonate-based electrolyte rapidly lost ionic conductivity after 30 h (Fig. 1g). In addition, the GCE exhibits nonflammable behavior (Fig. 1h), unlike the flammable carbonate-based electrolyte. This nonflammability of the GCE demonstrates its promising potential as a new alternative to inorganic solid electrolytes in terms of battery safety.

The GCE described above was compared with the previously reported sulfide (or oxide)-based electrolytes in order to underline its unique characteristics, with special attention to synthesis, ionic conductivity, thickness as a separator membrane, mechanical flexibility, safety and shape versatility (Table S1, ESI).

The printed electrodes consisted of the gel electrolyte (1 M LiBF4 in SBN/semi-IPN skeleton), carbon black conductive additives and electrode active materials. Here, as a proof-of-concept, LCO and LTO powders were chosen for the printed cathodes and anodes, respectively. The fabrication procedure of the printed electrode, along with its photograph and major components, are schematically depicted in Fig. 2a. The formation of the UV-cured ETPTA polymer network in the semi-IPN skeleton of the printed electrodes was verified by the changes in the FT-IR peaks assigned to the acrylic C[double bond, length as m-dash]C bonds (Fig. S6, ESI). The composition ratios of the printed electrodes were electrode active powder/carbon black additive/gel electrolyte = 55/6/39 (w/w/w) for the LCO cathode and 30/7/63 (w/w/w) for the LTO anode. Note that the printed electrodes already included the gel electrolyte as a major element, indicating that no additional incorporation of electrolytes into the electrodes was needed. This result was similar to those of previously reported inorganic-electrolyte-embedded bulk electrodes.25,26 In addition to this role as an ion conductor, the gel electrolyte in the printed electrode also acted as a binder that holds electrode active materials and carbon black additives, revealing that traditional polymeric binders such as polyvinylidene fluoride (PVdF) and carboxymethyl cellulose (CMC)/styrene-butadiene rubber (SBR) were not included. Fig. 2b shows that the LCO powders and carbon black additives are uniformly dispersed in the through-thickness direction of the printed LCO cathode. In addition, highly reticulated interstitial voids (originally occupied by the SBN-based gel electrolytes) were formed between the powders.


image file: c7ee01630a-f2.tif
Fig. 2 Fabrication and characterization of the printed electrodes. (a) Schematic of the procedure used to fabricate the stencil-printed electrode, along with photographs of the electrode. The printed electrodes consisted of the gel electrolyte ((1 M LiBF4 in SBN) electrolyte and semi-IPN skeleton), carbon black additive and an electrode active material (LCO and LTO powders were chosen for proof of concept). (b) Cross-sectional SEM and EDS images showing the dispersion state of the LCO powders and the presence of interstitial voids (originally occupied by the SBN-based electrolyte) in the printed LCO cathode. (c) SEM image of the SWCNT-coated LCO powders. The inset shows the morphology of the pristine LCO powders. (d) Comparison of the electronic conductivity of the SWCNT-coated LCO and pristine LCO. (e) Comparison of the discharge rate capability between the SWCNT-coated LCO and pristine LCO, where coin-type half cells (Li metal anode/PE separator/LCO cathode) were examined at a constant charge current density of 0.1C. (f) Change in the electronic resistance of the printed LCO cathode (vs. control LCO cathode (LCO/carbon black/PVdF binder = 95/2/3 (w/w/w)) prepared by slurry casting) as a function of the longitudinal compression cycle (bending radius = 5 mm, deformation rate = 10 mm min−1). (g) Photographs showing the multi-folding of the printed LCO and control LCO cathodes. (h–j) Comparison of the thermal stability of the printed LCO and control LCO cathodes incorporating the carbonate-based liquid electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)): (h) TGA profiles; (i) DSC thermograms showing interfacial exothermic reactions between delithiated LCO and electrolytes; (j) nonflammability tests of the printed LCO cathode (left photograph) and the control LCO cathode (right photograph).

Prior to fabricating the printed electrodes, the LCO and LTO powders were coated with SWCNTs to improve their electronic conductivity. Similar approaches based on CNT-mediated surface modification have been reported in previous publications.27–29 Details of the coating procedure are described in the experimental section. Fig. 2c shows that the LCO particles were coated with highly reticulated SWCNT networks. The solid content in the LCO/SWCNT suspension solution was 4.6 wt%. From the TGA profiles (Fig. S7, ESI), the compositional ratios of the SWCNT-coated electrode active powders were estimated to be LCO/SWCNT = 99.75/0.25 (w/w) and LTO/SWCNT = 99.35/0.65 (w/w), respectively. The introduction of the electrically conductive SWCNT layers on the LCO (and LTO) surface led to an increase in the electronic conductivity of the resulting electrodes (Fig. 2d and Fig. S8, ESI). As a consequence, the SWCNT-coated LCO showed remarkable improvement in discharge rate capability (Fig. 2e), where coin-type half cells (Li metal anode/PE separator/LCO cathode) were examined at a constant charging current density of 0.1C.

To quantitatively analyze the mechanical flexibility of the electrodes, their electronic resistance was monitored as a function of the longitudinal compression cycle (bending radius = 5 mm, deformation rate = 10 mm min−1). The printed LCO cathode showed no substantial change in electronic resistance after 600 bending cycles (Fig. 2f). By contrast, the electronic resistance of the control LCO cathode (prepared by slurry casting, LCO/carbon black/PVdF binder = 95/2/3 (w/w/w)) fluctuated with the bending cycle and eventually mechanically broke after 66 cycles. To further visualize this superior flexibility, the printed LCO cathode was subjected to multi-folding (Fig. 2g). Neither detectable cracks nor defects were observed at the printed LCO cathode, whereas the control LCO cathode showed structural rupture at the folded edges. These results demonstrate that the semi-IPN skeleton plays an important role in the enhancement of the mechanical flexibility of the printed LCO cathode.

The thermal stability of the printed electrodes was investigated. Fig. 2h compares the TGA profiles of the printed LCO cathode (the SBN/semi-IPN gel electrolyte was already included) and the control LCO cathode incorporating the carbonate-based liquid electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)). Negligible weight loss was observed at the printed LCO cathode up to 150 °C, whereas the control LCO cathode showed rapid weight loss due to volatilization of the carbonate liquid electrolyte. In addition, the interfacial exothermic reaction between the delithiated LCO materials and electrolytes was examined (Fig. 2i). The thermogram of the control LCO cathode shows two characteristic exothermic peaks (Tpeak = 220 and 250 °C), indicating vigorous interfacial exothermic reactions.30,31 By contrast, the exothermic peaks were shifted to higher temperatures in the printed LCO cathode. To highlight this superb thermal stability of the printed LCO cathode, a flammability test of the delithiated LCO cathode was conducted. The printed LCO cathode showed no flame even upon exposure to an ignition lighter, whereas the control LCO cathode instantly caught fire (Fig. 2j). The aforementioned results demonstrate that the (SBN/semi-IPN) gel-electrolyte-embedded printed electrodes could be used to prevent fire or explosion accidents in LIBs.

Compared with the sintering processes commonly used to prepare inorganic-electrolyte-based bipolar all-solid-state LIBs, UV-curing-assisted printing provides a noteworthy advantage of eliminating the need for high-pressure/high-temperature sintering, thus enabling the facile fabrication of bipolar LIBs with various form factors. A prerequisite for the printing process is control of the rheological properties of the printable GCE and electrode pastes. A basic understanding of the relationship between the rheological properties of the electrode/electrolyte pastes and the printing processability was reported in our previous studies.18,19,32

Fig. 3a shows that the viscosities of the GCE and electrode pastes tend to decrease with increasing shear rate, revealing their shear-thinning behaviors. The viscoelasticity analysis (Fig. 3b) reveals that the storage modulus (G′) is higher than the loss modulus (G′′) in the low-shear-stress region and that the opposite trend occurs in the high-shear-stress region, thereby demonstrating typical thixotropic fluid behavior19,33 that is suitable for the stencil printing process. Such thixotropic fluid behavior was further verified by the hysteresis loops34 in the rheograms (i.e., flow curves) (Fig. 3c). These well-tuned rheological properties enabled the realization of a (letter-shaped) printed GCE and LCO cathode on miniature toy cars with curvilinear surfaces (Fig. 3d).


image file: c7ee01630a-f3.tif
Fig. 3 Rheology control of the GCE and electrode pastes for multistage printing. (a) Viscosities of the GCE and electrode pastes as a function of shear rate. (b) Viscoelastic properties (G′ and G′′) as a function of shear stress. (c) Hysteresis loops in the rheograms. (d) Photographs showing the fabrication of the (letter-shaped) printed GCE (top photographs) and LCO cathode (bottom photographs) on miniature toy cars with curvilinear surfaces. (e) Cross-sectional SEM image of the printed bipolar cells (here, the three cells were connected in series) and schematic of their structures, wherein the LTO anode and LCO cathode shared the Al foil as a common current collector. (f) Cross-sectional SEM image of the printed bipolar cells (here, the three cells were connected in parallel) and schematic of their structures.

On the basis of our understanding of the above-described physicochemical/rheological characteristics of the GCE and electrode pastes, we fabricated bipolar-stacked LIBs through stencil-based multistage printing followed by UV curing. As depicted in the Scheme 1, the LTO anode, GCE and LCO cathode pastes were sequentially stencil-printed onto Al foil current collectors and then solidified by UV curing, yielding a printed mono full cell. In the printed mono full cell, the LTO anode, GCE layer and LCO cathode were seamlessly integrated (Fig. S9a, ESI). On top of the as-prepared mono full cell, the UV-curing-assisted printing process was repeatedly conducted, eventually leading to the fabrication of bipolar-stacked cells. Here, the LTO anode and LCO cathode shared the Al foil as a common current collector. Fig. 3e shows a cross-sectional SEM image of the printed bipolar 3-stacked cells in which the three cells were connected in series. Neither delamination nor cracking was observed between the mono full cells in the bipolar 3-stacked cells.

Another advantage of the multistage printing process is the facile realization of printed bipolar cells with an in-plane configuration. On top of an Al foil current collector, the LCO cathode and LTO anode pastes were printed side-by-side and subjected to UV curing. Subsequently, the GCE pastes were printed/UV-cured onto the as-prepared LCO cathode and LTO anode. The electrode pastes were then again printed side-by-side onto the GCE layers and UV-cured. After an Al foil current collector was placed on top of the final electrode layers, the fabrication of the bipolar-stacked cells with in-plane configuration were completed. This stepwise fabrication process, along with the corresponding photographs, is illustrated in Fig. 3f. We again emphasize that processing solvents (and solvent drying steps), porous separator membranes and liquid-electrolyte injection were not needed in the UV-assisted multi-stage printing, underscoring the simplicity and scalability of the process presented herein.

The electrochemical performance of the printed bipolar cells was characterized at room temperature. As a first step, the charge/discharge behaviors of the printed LCO cathode and LTO anode were investigated, wherein a coin-type half cell (printed LCO cathode (or printed LTO anode)/(1 M LiBF4 in SBN)-soaked PE separator/lithium metal) was cycled at a fixed charge/discharge current density of 0.1C/0.1C in the voltage ranges from 3.0 to 4.2 V and from 1.0 to 2.5 V, respectively. Fig. 4a shows that the printed LCO cathode and LTO anode presented the stable charge/discharge profiles during cycling. This result appeared similar to those (Fig. S10, ESI) of conventional slurry-cast LCO cathode and LTO anode, wherein the composition ratios of these control electrodes were LCO powder/PVdF binder/carbon black additive = 95/2/3 (w/w/w) and LTO powder/PVdF binder/carbon black additive = 88/2/10 (w/w/w), respectively. The control electrodes were soaked with carbonate-based liquid electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)) in order to be electrochemically activated. Next, the printed mono full cell (LCO cathode/GCE/LTO anode, Al pouch-type cell) was cycled between 1.5 and 2.7 V at a constant charge/discharge current density (0.05C/0.05C). The mono full cell showed normal charge/discharge behaviour without any internal short-circuit failure and also reasonable capacity retention (= 95% after the 50th cycle) during cycling (Fig. 4b). In addition, no structural disruption in the cell was observed after the cycle test (Fig. S9b, ESI). These results demonstrate the electrochemical viability of the printed full cell as a new solid-state LIB.


image file: c7ee01630a-f4.tif
Fig. 4 Electrochemical characterization of the printed bipolar cells at 25 °C. (a) Charge–discharge profiles of the printed LCO cathode and LTO anode, where a coin-type half cell (printed LCO cathode (or printed LTO anode)/(1 M LiBF4 in SBN)-soaked PE separator/lithium metal) was cycled at a fixed charge/discharge current density of 0.1C/0.1C in the voltage range from 3.0 to 4.2 V and from 1.0 to 2.5 V, respectively. (b) Cycling performance of the printed mono full cell (LCO cathode/GCE/LTO anode), where the cell was cycled between 1.5 and 2.7 V at a constant charge/discharge current density (0.05C/0.05C). (c) The charge/discharge profiles of the printed bipolar cells connected in series as a function of cell number (1 → 3 cells). (d) Comparison of the charge/discharge profiles of the printed bipolar cells (in-series vs. in-plane).

The charge/discharge profiles of the printed bipolar cells with an in-series configuration were compared as a function of cell number (Fig. 4c). As the cell number increased, the working voltages of the resulting cells increased from 2.4 V (mono cell) to 5.4 V (two cells) to 7.2 V (three cells). The cycling stability of the printed bipolar cells was investigated as a function of cell number under a constant charge/discharge current density (0.1C/0.1C). The bipolar 2-/3-stacked cells as well as the mono full cell showed reliable capacity retention with cycling (Fig. S11, ESI). All the cells exhibited stable charge/discharge profiles. Most of the (inorganic electrolyte-based) all-solid-state LIBs reported to date were evaluated at high temperatures (above 30 °C) in order to alleviate the problem of slow kinetics observed for the inorganic electrolytes.35,36 By contrast, this study exhibited the satisfactory level of electrochemical results at room temperature (i.e., 25 °C), demonstrating the performance viability of the printed bipolar cells.

The charge/discharge profiles of the printed bipolar cells were examined in detail using two mono cells connected in-series or in-plane. In the voltage range from 3.0 to 5.4 V, normal charge/discharge behavior and good coulombic efficiency (∼98%) were observed for both the in-series and in-plane configurations (Fig. 4d). Moreover, no substantial difference in the charge/discharge profiles was observed between the two configurations, underlining the versatility and effectiveness of the multistage printing process.

A critical challenge facing traditional bipolar cells based on inorganic electrolytes is the mechanical stiffness (i.e., poor flexibility). In this study, the printed bipolar 2-stacked cell (here, the two cells were connected in series) with a charge voltage of 5.4 V was subjected to a longitudinal compression cycle (bending radius = 10 mm, deformation rate = 10 mm min −1). No appreciable change in the charge/discharge behavior was observed before/after 100 bending cycles (Fig. 5a). This result shows that the printed GCEs and electrodes, both of which contained flexible semi-IPN skeletons, resulted in dramatic improvement in the mechanical flexibility of the resulting bipolar cells.


image file: c7ee01630a-f5.tif
Fig. 5 Mechanical flexibility and thermal stability of the printed bipolar cells. (a) Charge/discharge profiles of the printed bipolar 2-stacked cell (i.e., the two cells were connected in series) before/after 100 bending cycles (bending radius = 20 mm, deformation rate = 200 mm min−1). (b) Photographs showing the safety robustness of the printed bipolar 2-stacked cell. The cell continued to power an LED lamp even after being horizontally cut in half. (c) Charge/discharge profiles of the printed bipolar 2-stacked cell before/after exposure to thermal shock (130 °C/0.5 h). The photograph of the cell after the thermal shock test, along with that of the control cell (consisting of LCO cathode, LTO anode, carbonate-based electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)) and PE separator) is also shown. (d) Photographs showing the stepwise fabrication of the printed bipolar 2-stacked cell on the curved roof of a miniature toy car. (e) Charge/discharge profiles of the printed bipolar 2-stacked cell on a miniature toy car. (f) Nonflammability test of the printed bipolar 2-stacked cell and control cell; the Al pouch-based packaging substances were removed prior to this test.

In addition to this mechanical flexibility, another advantage of the printed bipolar cells is their safety under harsh operating conditions. The printed bipolar 2-stacked cell continued to power an LED lamp even after being horizontally cut in half (Fig. 5b and Fig. S12, ESI), exhibiting that both the solid-state GCEs and the gel electrolytes of the printed electrodes were free from leakage problems and prevented electrical contact between the LCO cathode and the LTO anode even after being cut in half. To further demonstrate the solid-state feature of the electrolytes, the printed bipolar 2-stacked cell was exposed to thermal shock (130 °C/0.5 h)37 conditions. No substantial change in the cell dimensions or in the charge/discharge profiles was observed for the printed bipolar cell. By contrast, a control LCO/LTO cell incorporating the carbonate-based electrolyte (1 M LiPF6 in EC/DMC = 1/1 (v/v)) and a PE separator became severely swollen and eventually lost its electrochemical activity (Fig. 5c).

To highlight the superior safety of the printed bipolar cell, a flammability test was conducted. A bipolar 2-stacked cell was fabricated directly on the curved roof of a miniature toy car using a multi-stage printing process (Fig. 5d). The prepared bipolar cell showed normal charge/discharge profiles (Fig. 5e). After the Al pouch-based packaging substance, which is vulnerable to fire, was removed, the fully charged, printed bipolar 2-stacked cell (in-plane configuration) was exposed to a flame. Notably, the printed cell operated an LED lamp without explosion or structural disruption, whereas the control cell instantly caught fire (Fig. 5f and Fig. S13, ESI). The remarkable abuse tolerance of the printed cell is attributed to the presence of the nonflammable GCE and electrodes. These safety results demonstrate that the printed bipolar cell is a promising alternative to traditional inorganic-electrolyte-based bipolar cells.

The printed bipolar all-solid-state cells described above were compared with the previously reported bipolar LIBs in order to highlight their salient features, with a focus on the electrochemical performance, safety (nonflammability), manufacturing process, mechanical flexibility and form factors (Table S2, ESI).

Conclusions

In summary, we demonstrated a printed bipolar all-solid-state LIB with exceptional flexibility, shape versatility, charge/discharge behavior, nonflammability and manufacturing simplicity that far exceed those achievable with the currently widespread (sulfide- or oxide-based) inorganic electrolytes. The printed bipolar cells were composed of GCEs (gel electrolytes and Al2O3 nanoparticles) and bulk electrodes (LCO or LTO active powder, carbon black additive and gel electrolyte). The gel electrolyte, which consisted of the nonflammable SBN electrolyte and flexible semi-IPN skeleton, was used instead of traditional inorganic electrolytes. The LCO and LTO powders were coated with SWCNTs prior to electrode fabrication to improve their electronic conductivity. The flexible/nonflammable GCEs and electrodes were rheologically tuned and combined with a UV-assisted, stencil-based multistage printing process, eventually enabling facile realization of printed bipolar cells with various form factors (including in-series/in-plane configurations). Notably, the multistage printing did not involve high-pressure/high-temperature sintering processes. Future work will be devoted to replacing the Al2O3 nanoparticles with inorganic electrolytes to enhance ionic transport in the printed bipolar cells. The multistage printing-based bipolar cell strategy described herein holds great promise as an effective and scalable platform technology to move bipolar all-solid-state batteries one step closer to commercialization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning and Wearable Platform Materials Technology Center. (2015R1A2A1A01003474 and 2016R1A5A1009926). This research was also supported by Korea Forest Research Institute (Grant No. FP 0400-2016-01).

References

  1. M. Armand and J.-M. Tarascon, Nature, 2008, 451, 652–657 CrossRef CAS PubMed.
  2. B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928–935 CrossRef CAS PubMed.
  3. B. Scrosati, J. Hassoun and Y.-K. Sun, Energy Environ. Sci., 2011, 4, 3287–3295 CAS.
  4. K. Yoshima, Y. Harada and N. Takami, J. Power Sources, 2016, 302, 283–290 CrossRef CAS.
  5. Y.-S. Hu, Nat. Energy, 2016, 1, 16042–16043 CrossRef CAS.
  6. Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba and R. Kanno, Nat. Energy, 2016, 1, 16030–16036 CrossRef CAS.
  7. T. Sato, T. Morinaga, S. Marukane, T. Narutomi, T. Igarashi, Y. Kawano, K. Ohno, T. Fukuda and Y. Tsujii, Adv. Mater., 2011, 23, 4868–4872 CrossRef CAS PubMed.
  8. Y. Gambe, Y. Sun and I. Honma, Sci. Rep., 2015, 5, 8869–8873 CrossRef CAS PubMed.
  9. T. Matsuo, Y. Gambe, Y. Sun and I. Honma, Sci. Rep., 2014, 4, 6084–6088 CrossRef CAS PubMed.
  10. J. Janek and W. G. Zeier, Nat. Energy, 2016, 1, 16141–16144 CrossRef.
  11. Y. J. Nam, S.-J. Cho, D. Y. Oh, J.-M. Lim, S. Y. Kim, J. H. Song, Y.-G. Lee, S.-Y. Lee and Y. S. Jung, Nano Lett., 2015, 15, 3317–3323 CrossRef CAS PubMed.
  12. N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama and K. Kawamoto, Nat. Mater., 2011, 10, 682–686 CrossRef CAS PubMed.
  13. J. van den Broek, S. Afyon and J. L. Rupp, Adv. Energy Mater., 2016, 6, 1600736 CrossRef.
  14. H. Wakayama, H. Yonekura and Y. Kawai, Chem. Mater., 2016, 28, 4453–4459 CrossRef CAS.
  15. W. Liu, N. Liu, J. Sun, P.-C. Hsu, Y. Li, H.-W. Lee and Y. Cui, Nano Lett., 2015, 15, 2740–2745 CrossRef CAS PubMed.
  16. E. Peled, D. Golodnitsky, G. Ardel, J. Lang and Y. Lavi, J. Power Sources, 1995, 54, 496–500 CrossRef CAS.
  17. K. K. Fu, Y. Gong, J. Dai, A. Gong, X. Han, Y. Yao, C. Wang, Y. Wang, Y. Chen and C. Yan, Proc. Natl. Acad. Sci. U. S. A., 2016, 113, 7094–7099 CrossRef CAS PubMed.
  18. S. H. Kim, K. H. Choi, S. J. Cho, S. Choi, S. Park and S. Y. Lee, Nano Lett., 2015, 15, 5168–5177 CrossRef CAS PubMed.
  19. E. H. Kil, K. H. Choi, H. J. Ha, S. Xu, J. A. Rogers, M. R. Kim, Y. G. Lee, K. M. Kim, K. Y. Cho and S. Y. Lee, Adv. Mater., 2013, 25, 1395–1400 CrossRef CAS PubMed.
  20. Q. Wang, P. Pechy, S. M. Zakeeruddin, I. Exnar and M. Grätzel, J. Power Sources, 2005, 146, 813–816 CrossRef CAS.
  21. K. Xu, Chem. Rev., 2014, 114, 11503–11618 CrossRef CAS PubMed.
  22. D. Zhou, Y.-B. He, R. Liu, M. Liu, H. Du, B. Li, Q. Cai, Q.-H. Yang and F. Kang, Adv. Energy Mater., 2015, 5, 1500353 CrossRef.
  23. H.-J. Ha, Y. H. Kwon, J. Y. Kim and S.-Y. Lee, Electrochim. Acta, 2011, 57, 40–45 CrossRef CAS.
  24. H.-J. Ha, E.-H. Kil, Y. H. Kwon, J. Y. Kim, C. K. Lee and S.-Y. Lee, Energy Environ. Sci., 2012, 5, 6491–6499 CAS.
  25. B. R. Shin and Y. S. Jung, J. Electrochem. Soc., 2014, 161, A154–A159 CrossRef CAS.
  26. Y. Li, W. Zhou, X. Chen, X. Lü, Z. Cui, S. Xin, L. Xue, Q. Jia and J. B. Goodenough, Proc. Natl. Acad. Sci. U. S. A., 2016, 113, 13313–13317 CrossRef CAS PubMed.
  27. M. Sano, A. Kamino, J. Okamura and S. Shinkai, Nano Lett., 2002, 2, 531–533 CrossRef CAS.
  28. E. Menna, F. Della Negra, M. Prato, N. Tagmatarchis, A. Ciogli, F. Gasparrini, D. Misiti and C. Villani, Carbon, 2006, 44, 1609–1613 CrossRef CAS.
  29. T. Fujigaya, J. Yoo and N. Nakashima, Carbon, 2011, 49, 468–476 CrossRef CAS.
  30. J.-M. Kim, J.-H. Park, C. K. Lee and S.-Y. Lee, Sci. Rep., 2014, 4, 4602–4608 CrossRef PubMed.
  31. J.-H. Park, J.-M. Kim, C. K. Lee and S.-Y. Lee, J. Power Sources, 2014, 263, 209–216 CrossRef CAS.
  32. K.-H. Choi, J. Yoo, C. K. Lee and S.-Y. Lee, Energy Environ. Sci., 2016, 9, 2812–2821 CAS.
  33. F. Pignon, A. Magnin and J.-M. Piau, J. Rheol., 1998, 42, 1349–1373 CrossRef CAS.
  34. S. R. Raghavan, M. W. Riley, P. S. Fedkiw and S. A. Khan, Chem. Mater., 1998, 10, 244–251 CrossRef CAS.
  35. Y. Zhao, C. Wu, G. Peng, X. Chen, X. Yao, Y. Bai, F. Wu, S. Chen and X. Xu, J. Power Sources, 2016, 301, 47–53 CrossRef CAS.
  36. Y. Li, W. Zhou, S. Xin, S. Li, J. Zhu, X. Lü, Z. Cui, Q. Jia, J. Zhou and Y. Zhao, Angew. Chem., Int. Ed., 2016, 55, 9965–9968 CrossRef CAS PubMed.
  37. S.-H. Kim, K.-H. Choi, S.-J. Cho, J.-S. Park, K. Y. Cho, C. K. Lee, S. B. Lee, J. K. Shim and S.-Y. Lee, J. Mater. Chem. A, 2014, 2, 10854–10861 CAS.

Footnote

Electronic supplementary information (ESI) available: Fig. S1–S13, Tables S1 and S2, Movies S1 and S2. See DOI: 10.1039/c7ee01630a

This journal is © The Royal Society of Chemistry 2018