LiNi0.5Mn1.5O4-Hybridized Gel Polymer Cathode and Gel Polymer Electrolyte Containing a Sulfolane-Based Highly Concentrated Electrolyte for the Fabrication of a 5 V Class of Flexible Lithium Batteries.

The design and fabrication of lithium secondary batteries with a high energy density and shape flexibility are essential for flexible and wearable electronics. In this study, we fabricated a high-voltage (5 V class) flexible lithium polymer battery using a lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode. A LiNi0.5Mn1.5O4-hybridized gel polymer cathode (GPC) and a gel polymer electrolyte (GPE) membrane, both containing a sulfolane (SL)-based highly concentrated electrolyte (HCE), enabled the fabrication of a polymer battery by simple lamination with a metallic lithium anode, where the injection of the electrolyte solution was not required. GPC with high flexibility has a hierarchically continuous three-dimensional porous architecture, which is advantageous for forming continuous ion-conduction paths. The GPE membrane has significant ionic conductivity enough for reliable capacity delivery. Therefore, the fabricated lithium polymer pouch cells demonstrated excellent capacity retention under continuous deformation conditions. This study provides a promising strategy for the fabrication of scalable and flexible 5 V class batteries using GPC and GPE containing SL-based HCE.

Introduction

Rapid technological innovation and growing market demand for flexible electronic devices, such as roll-up displays, wearable electronic products, and smart electronics, require the development of reliable and highly effective power sources.[1,2] Lithium-ion batteries (LIBs) have been established as state-of-the-art power sources for portable electronics. In recent decades, significant progress has been achieved in the fabrication of flexible LIBs based on custom-designed materials.[3−5] In particular, electrodes based on advanced carbon-based materials, such as carbon nanotubes,[6,7] nanofibers,[8,9] and graphene,[10−12] are used for flexibility. However, for practical applications, viable production processes using procurable and economical materials are of great importance. Concomitantly, the attainable energy density of the fabricated flexible LIBs determines their application prospects.

Cobalt-free spinel structured lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO) is a potential cathode owing to its ultrahigh working voltage of 4.7 V versus Li/Li+, a large theoretical capacity of 147 mAh g–1, and a relatively low cost.[13−17] Based on the mass of the active material, LNMO can provide a high energy density of 690 Wh kg–1, which is 20% and 30% higher than that of conventional cathode materials, LiCoO2 and LiFePO4, respectively.[18,19] However, the application of this cathode material for practical energy storage is challenging and stymied by the decomposition of conventional carbonate-based electrolyte.[20,21] In particular, the carbonate-based electrolytes suffer from oxidative decomposition at operating voltages above 4.4 V, which leads to fading cyclic performance and safety concerns of LIBs. Furthermore, LiPF6 easily decomposes to form HF in the presence of water at a high voltage. Trace amounts of HF may accelerate the dissolution of the transition metal, causing a structural collapse of the LNMO electrode.[22] Therefore, selecting a suitable electrolyte is crucial for the practical application of LNMO-based batteries.

Ionic liquids (ILs) are defined as molten salts having melting points lower than 100 °C. Due to their characteristic properties such as nonvolatility, high thermal stability, and high ionic conductivity, well-designed ILs could meet the rigorous demands/criteria of electrolyte applications in Li/Na-ion batteries, Li-sulfur batteries, Li-oxygen batteries, and so on.[23] We previously reported that equimolar mixtures of Li salts and certain glymes form room temperature liquids with high thermal and electrochemical stabilities, low flammability, and a negligible vapor pressure similar to typical ILs, classified as solvate ionic liquids (SILs).[24−27] The glyme-based SILs exhibit several remarkable properties, in addition to IL-resembling properties, such as oxidation stability enhancement of the solvent (glymes) by coordinating with Li+,[25] low solubility toward ionic materials owing to the negligible activity of free solvent,[26] and unique electrochemical reactions due to the instability of the solvate ions, as observed reversible electrochemical intercalation of graphite in the absence of solid electrolyte interphase (SEI) forms such as ethylene carbonate.[27] Subsequently, the concept of SILs has been extended to highly concentrated electrolytes (HCEs) with unique physicochemical properties using Li salts and various solvents.[28−30] In certain HCEs, significant electrochemical phenomena are reported, including high reversibility of lithium deposition/stripping without lithium dendrite formation[31] and fast electrochemical reactions of electrode active materials.[32] Among HCEs, sulfolane (SL)-based HCEs exhibit unconventionally high Li+-transference number owing to the highest diffusivity of Li+ among the chemical species in the electrolytes (Li+, anion, and SL), which indicates a significant contribution of the Li+ hopping conduction by the ligand (anion and SL) exchange reactions.[33,34] The SL-based HCEs have been successfully applied in Li-LiCoO2[33] and Li-sulfur cells.[34−36] The high oxidative stability of SL effectuates the SL-based HCEs suitable for electrolytes of high-voltage cathode materials.[37−40] Further, the main capacity fading mechanism of LNMO-based LIBs is the oxidative decomposition of the electrolytes; however, for LIBs using high-nickel LiNiMnCoO2, another high-voltage cathode material, solvation-driven degradation mechanism by the dissolution of the transition metals is predominant.[41]

In this study, we fabricated a gel polymer cathode (GPC) by incorporating an SL-based HCE and a fluorinated polymer into a LNMO cathode. The electrochemical performance of GPC demonstrated favorable cyclability and rate capability. Furthermore, a gel polymer electrolyte (GPE) membrane was successfully prepared by combining a SL-based HCE and a fluorinated polymer, which exhibited favorable mechanical stability. Considering the advantages of GPC and GPE, a high output voltage pouch cell (∼4.7 V) was fabricated by laminating them using a lithium metal anode. The cell delivered stable discharge capacities under continuous bending, which demonstrated its high flexibility. The flexible battery utilizing GPC and GPE may provide a new perspective for future wearable electronics.

Experimental Section

Materials

LNMO powder was synthesized according to the previous studies.[42] It is widely accepted that the synthesis procedures for disordered LNMO (Fd3̅m) and ordered LNMO (P4332) are different. Our proposed method is similar to the procedures to synthesize the disordered LNMO, that is, calcination in the air followed by annealing.[42] The prepared LNMO was characterized by X-ray diffraction (XRD, Figure S1), revealing its pure phase ascribed to the cubic spinel structure with space group of Fd3̅m (JCPDF #80-2183).[43] Acetylene black (AB) was obtained from Denka Co., Ltd. (Japan). N-Methyl-2-pyrrolidone (NMP) was purchased from Wako Pure Chemical Corp. (Japan). Poly(vinylidene fluoride) (PVDF, Solef 5130) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Kynar flex 2801) were provided by Solvay (Belgium) and Arkema (France), respectively. Battery-grade lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) was obtained from Solvay, Japan. Battery-grade SL was purchased from Kishida Chemical (Japan). HCE, [Li(SL)2][TFSA], was prepared by mixing Li[TFSA] and SL at a molar ratio of 1:2 at 60 °C, overnight in an Ar-filled glovebox to obtain a homogeneous liquid.

Preparation of GPC Containing LNMO

To make LNMO-GPCs, a slurry comprising 67.2 wt % LNMO, 6.4 wt % AB, 6.4 wt % PVDF, and 20 wt % [Li(SL)2][TFSA] electrolyte in NMP was prepared. The viscous slurry was coated onto a carbon-coated aluminum foil current collector using the doctor blade technique. After solvent drying, LNMO-GPC was cut into disks with diameters of 16 mm and rectangles with a length of 45 mm and a width of 37 mm to assemble 2032-type coin cells and flexible pouch cells, respectively, and dried under vacuum at 70 °C for 12 h. The thickness of LNMO-GPC was ∼30 μm, and the density of LNMO-GPC was ∼1.5 g cm–3. The mass-loading of the active material in the LNMO-GPC was ∼3.0 mg cm–2.

Electrochemical Tests for Coin Cell

The electrolyte, [Li(SL)2][TFSA] (120 μL), was soaked in a glass filter separator (GA-55, Advantec, Japan) with a diameter of 17 mm. A Li metal foil (Honjo Metal, Japan) with a thickness of 200 μm and a diameter of 16 mm was used as the anode. The coin cells were assembled inside an argon-filled glovebox, where both the moisture and the oxygen content levels were maintained below 0.3 ppm. Galvanostatic charge and discharge tests of the cells were performed at 30 °C using a Nagano BTS-2004 battery testing system (Japan). The voltage range was 3.4–4.9 V, and the 1 C rate was set at 147 mA g–1.

Characterization

X-ray diffraction (XRD) analyses were performed on a Rigaku RINT-2000 diffractometer with Cu Kα radiation (λ = 1.5418 Å). The morphology and elemental distribution of the samples were observed by field-emission scanning electron microscopy (FE-SEM, SU8000, Hitachi High-Technologies, Japan) and energy-dispersive X-ray spectroscopy (EDX), respectively. Tensile tests were performed on a Shimadzu EZ-LX (Shimadzu, Japan) equipped with a 10 N load cell for rectangular samples at a strain rate of 1 cm min–1. Linear sweep voltammetry (LSV) experiments were performed using an electrochemical analyzer (VMP3, Biologic, France) at a scan rate of 1 mV s–1 at 30 °C, based on a three-electrode cell (stainless steel as the working electrode and Li metal as the counter and reference electrodes). Electrochemical impedance spectroscopy (EIS) measurements were performed using a ModuLab XM (Solartron Analytical, U.K.) in the frequency range of 1 MHz to 0.01 Hz with a voltage amplitude of 10 mV.

Preparation of GPE Membrane

The chemicals for the preparation of the GPE membrane, 70 wt % [Li(SL)2][TFSA] (0.250 g) and 30 wt % PVDF-HFP (0.107 g), were dissolved in acetone (3.6 mL) and stirred in a vial for 1 h at room temperature. The solution was then transferred to a flat Petri dish with a diameter of 80 mm, placed in a dry chamber (dew point < −60 °C), and dried at room temperature overnight. Thereafter, the GPE membrane was obtained by drying overnight at 70 °C under vacuum. Thickness of the GPE was measured using a thickness gauge (PG-01, TECLOCK, Japan).

Fabrication of Flexible LNMO-Li Polymer Pouch Cell

For a pouch cell, two layers of GPC, two layers of GPE membranes, and one layer of anode coated with Li metal on both sides of the Cu foil (Li thickness = 20 μm, Honjo Metal) was laminated using a layer-by-layer process in a dry chamber (dew point < −60 °C). The mass loading of the active LNMO was similar to that of the coin cell. The dimensions (length [mm] × width [mm]) of the GPC, GPE membrane, and anode were 45 × 37, 49 × 43, and 49 × 41, respectively. A few drops of [Li(SL)2][TFSA] were added between the GPC/GPE membrane and the anode/GPE membrane to slightly wet the electrodes to ensure firm contact. Aluminum and nickel strips were attached to the sides of the cathode and anode, respectively, as electrode tabs. Finally, the cell core was packaged in a flexible aluminum plastic film bag using an edge-bonding machine.

Computational Methods

Gaussian 16 program[44] was used for the DFT calculations. The HOMO energy levels of SL and the SL-(Li[TFSA])2 complex were calculated at the B3LYP/6-311G** level using optimized geometries at the same level. The geometry of the complex in the crystal structure[34] was used for the initial geometry of the geometry optimization.

Results and Discussion

HCE-Based LNMO-GPC

LNMO-GPC was fabricated using a slurry comprising LNMO, AB, PVDF, and a highly concentrated [Li(SL)2][TFSA] electrolyte in NMP (Figure a). The viscous slurry was bladed onto a carbon-coated aluminum foil current collector and dried to form a uniform film over a large area. This solution-based preparation method and its compatibility with roll-to-roll coating render this system readily scalable for large-area electrode films.[45] The morphology of LNMO-GPC was examined by SEM. As shown in Figure b, GPC showed a relatively smooth surface consisting of a hierarchically continuous three-dimensional (3D) porous foam-like network, and the diameters of the holes in the GPC were approximately 2 μm. Furthermore, the SEM images indicated the coexistence of nanosized pores on the surface, which were generated by the packing of conductive carbon (Figure c). This 3D interconnected structure was advantageous for electrolyte penetration when a liquid electrolyte was used with GPC, promoting the transportation of ions for LNMO-GPC. In addition, uniform coverage of GPE on LNMO particles was observed without exposure to the typical crystallization edge observed in the newly prepared sample (Figure S2). Elemental mapping was conducted to evaluate the distribution of the components in GPC. As shown in Figure d–k, the homogeneous distribution of Ni, Mn, O, C, N, S, and F was confirmed by SEM-EDX. This indicates that GPC did not induce agglomeration of the components. In particular, the uniform distribution of N, S, and F illustrated that the LNMO particles were surrounded by GPE containing the highly concentrated [Li(SL)2][TFSA] electrolyte.

Figure 1

(a) Schematic of the fabrication process, (b–d) SEM images, (e) nickel, (f) manganese, (g) oxygen, (h) carbon, (i) nitrogen, (j) sulfur, and (k) fluorine elemental mappings of LNMO-GPC.

Coin-type LNMO-Li half-cells were assembled using [Li(SL)2][TFSA] as the liquid electrolyte and GA-55 as the separator to evaluate the electrochemical stability of LNMO-GPC. The cell displayed two characteristic voltage plateaus: a long and significant voltage plateau at ∼4.7 V originating from the Ni2+/4+ redox processes, and a short voltage plateau at ∼4.0 V attributed to the Mn3+/4+ redox reaction (Figure a).[18] The charging curve of our material (Figure a) coincides with that of the disordered LNMO materials.[14] Moreover, the XRD pattern of disordered (Fd3̅m) LNMO agrees with that of our material (Figure S1).[14,43] These results suggest that our LNMO material can be assigned as a disordered (Fd3̅m) LNMO. The initial specific discharge capacity at 0.1 C was 128 mAh g–1. With an increase in the current density to 0.2 and 0.5 C, a decrease in plateau voltage was observed with discharge capacities of 120 and 90 mAh g–1, respectively. However, the capacity contributions of the Ni2+/4+ plateaus were high, and the well-defined Mn3+/4+ plateaus under a high current density represented the stable structure of the GPC. Figure b displays the rate capability of the cell, indicating the stable cycling performance of GPC at different charge/discharge rates (the charge and discharge rates are the same). Remarkably, when the current density returned to 0.1 C, 99% of the original capacity was recovered, indicating the resilience of the GPC. In addition, the high stability and robustness of for long-term cycling are crucial for GPC. Galvanostatic charge/discharge measurements at 0.2 C were conducted (Figure c), and after 100 cycles, the capacity retention relative to the initial capacity was 86.1%, which corresponded to an average capacity decay of 0.14% per cycle.

Figure 2

(a) Initial charge–discharge curves, (b) rate performance, and (c) cycling performance at 0.2 C rate for LNMO-GPC in a half-cell (LNMO-GPC∥Li; 1C corresponds to 147 mAh g–1).

The high durability of the cycling test mainly comes from the oxidative stability of [Li(SL)2][TFSA]. As described in the Introduction, most solvents are coordinated to Li+, and trace amounts of free (uncoordinated) solvents remain in HCEs. The HOMO level of the Li+-coordinated solvents tends to decrease because of the strong positive electric potential of the Li+ ion, leading to the enhancement of the oxidation stability of the coordinated solvent.[25] Meanwhile, SL-based electrolytes have relatively high oxidation stabilities.[37−40] We previously reported the liquid structures of [Li(SL)2][TFSA],[34] where the two oxygen atoms of the SL sulfonyl group coordinated to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions, in other words, the SL- and anion-bridged, chainlike Li-ion coordination structure. Therefore, the Li+-coordinated SL should exhibit higher oxidation stability than the free SL that is known to be antioxidative. To confirm the change in the electronic state of SL owing to the coordination with Li+, the optimized structures of SL and SL-(Li[TFSA])2 complex were calculated. The initial geometries of the complexes used for geometry optimization were prepared based on their crystal structures.[34] As shown in Figure S3, the two oxygen atoms of SL are coordinated to different Li+ ions in the optimized structure (B3LYP/6-311G** level). The optimized structure in Figure S3 is similar to that of the crystalline SL-Li[TFSA] complex.[34] The HOMO energy levels of SL were calculated for the isolated SL molecule and the SL-(Li[TFSA])2 complex at the B3LYP/6-311G** level using the optimized structures at the same level, and the results are summarized in Table S1. As was the case in the glyme–LiTFSA complexes,[25] a distinct decrease was observed in the HOMO energy levels of SL by the coordination to Li+ (compared with the uncoordinated free solvent). As Li+ has a positive charge, the electric potential around the SL coordinated to Li+ is high. The high electric potential stabilizes the electrons of SL, resulting in a low HOMO energy level. The lowering of the HOMO energy level leads to a favorable oxidation stability.

However, the SL-based HCEs are reductively unstable in contact with Li metal, as revealed by first-principles molecular dynamics simulations.[46] We demonstrated that SL was reductively decomposed to generate tetrahydrothiophene and concurrently formed Li2O on the anode surface.[47] However, the Coulombic efficiency approached nearly 100% when the cycle number increased (Figure c), suggesting that an effective SEI was formed during the initial cycles, preventing further reductive decomposition of the electrolyte in the extended cycles. In this experiment, thick GA55 separators were used, where a large amount of electrolyte was preserved. A large amount of electrolyte in GA55 may contribute to the formation of the SEI and to the high cycling performance shown in Figure c.

HCE-Based GPE Membrane

Currently, most of the reported flexible LIBs employ carbonate-based liquid electrolytes. GPEs are promising replacements for liquid electrolytes, with the advantages of suppressed internal shorting, no leakage of liquid electrolytes, and less flammability.[11] An applicable method to obtain mechanically reliable GPEs should be explored. Figure a shows a schematic of the fabrication process of the GPE membrane. The precursors comprising HCE, [Li(SL)2][TFSA], and the PVDF-HFP polymer were dissolved in acetone. After evaporation of the solvent, the GPE membrane was obtained because of the partial crystallinity of PVDF-HFP, which served as cross-linking points for the membranes. The photograph shown in Figure b demonstrates that the membrane was a transparent solid and flexible enough for direct handling. Furthermore, the membrane could be controlled to have a uniform thickness of 35 μm. Burning test results showed that the GPE membrane had considerable flame retardancy (Figure S4). The obtained membrane had a smooth surface morphology, as shown in Figure S5a,b, which contained a few parts of the rumples. The rumples appeared to originate from the partially crystalline nature of PVDF-HFP. Thereafter, the distribution of [Li(SL)2][TFSA] and PVDF-HFP in the GPE membrane was examined using SEM-EDX elemental mapping (Figure S5c–h). The distribution of carbon, nitrogen, oxygen, sulfur, and fluorine elements was uniform on the scale of 10–100 μm, which was consistent with the morphology of the LNMO gel polymer electrode.

Figure 3

(a) Schematic of the fabrication process, (b) photograph of [Li(SL)2][TFSA]/PVDF-HFP GPE membrane, (c) tensile stress–strain curves for [Li(SL)2][TFSA]/PVDF-HFP GPE.

We have already reported the electrochemical properties of GPE comprising [Li(SL)2][TFSA] and PVDF-HFP.[48,49] The ionic conductivity and Li+-transference number, estimated by the Bruce–Vincent method,[50] at 30 °C is 1.5 × 10–4 S cm–1 and 0.65, respectively.[48] To evaluate the mechanical properties of the obtained GPE membrane, stress–strain measurements were performed. The tensile stress–strain curves are shown in Figure c, and the tensile strength and maximal strain are summarized in the inset. Remarkably, the obtained GPE membrane can sustain a fracture stress of 3.1 MPa at a maximal strain of 186%, which was higher than that of some previously reported ion gel electrolytes.[51−54]

Previous studies on SL-based electrolytes demonstrated their oxidative decomposition threshold >5 V versus Li/Li+.[37−39] The oxidative stability of the SL-containing GPE membrane was experimentally evaluated by LSV. Figure S6 demonstrates the high oxidation stability of the [Li(SL)2][TFSA]/PVDF-HFP GPE membrane approaching 5 V, which reflects the HOMO level calculation (vide supra) and confirms the high antioxidation capability of GPE.

Flexible LNMO-Li Polymer Pouch Cell

After studying the basic performance of LNMO-GPC and the fundamental characterization of the GPE membrane, pouch-type cells were assembled to verify their practical utility. Figure a depicts the inner structure of the prepared pouch cell, which comprises two sheets of a single coated LNMO-GPC, two sheets of [Li(SL)2][TFSA]/PVDF-HFP GPE membranes with a thickness of 35 μm, and one sheet of both-side-covered lithium anode on a Cu foil. After five initial charge–discharge cycles at the flat state to stabilize the pouch cell, the typical charge–discharge curves showed two well-defined plateaus of Ni2+/4+ and Mn3+/4+, with a discharge capacity of 116 mAh g–1 (Figure b). Then, the pouch cell underwent repeated 90° bending and immediate recovery to a flat state to simulate its application to flexible devices. The measurement itself in Figure b was conducted under flat state. The external deformation resulted in a minimal increase in the capacities of 121, 124, and 125 mAh g–1 after 50, 100, and 500 bending cycles, respectively. The increased capacities may be due to the firmer contact between the LNMO-GPC and GPE membranes by external bending.

Figure 4

(a) Schematic of the flexible pouch cell, (b) charge and discharge curves of the flexible cell at 0.05 C after activation of five charge–discharge cycles at flat state and then after 50, 100, and 500 bending cycles between 0–90° (1 C corresponds to 147 mAh g–1), (c) SEM image of surface, and (d) cross-sectional SEM image of LNMO-GPC after 500 bending cycles.

To explore the effect of bending on the mechanical stability, the LNMO-GPC and GPE membranes were characterized by SEM by observing the structural changes after 500 bending cycles. As shown in Figure c, the 3D porous structures were retained for LNMO-GPC, and no obvious collapse was observed. The cross-sectional SEM image shows the firm contact between the GPC layer and the carbon-coated aluminum current collector after prolonged bending (Figure d). No change in the elemental distribution of LNMO-GPC after 500 bending cycles was observed (Figure S7), suggesting its good mechanical stability. In addition, SEM characterization of the GPE membrane after 500 bending cycles was conducted to observe the morphological changes (Figure S8). Although the membrane exhibited more rumples and wrinkles after extended bending, the homogeneous elemental distribution remained unchanged.

Furthermore, EIS measurements were performed under different bending states (flat, 90°, and 180° bending and rolling). Nyquist plots of the flexible pouch cells with different bending states measured in the fully charged state (4.9 V vs Li anode) are shown in Figure a. The Nyquist plots exhibit two overlapped semicircles in all spectra under different bending conditions. The equivalent circuit used to interpret the plots is shown in Figure b,[11] where R1 is the electrolyte resistance. The first semicircle from high frequency to midfrequency, denoted as the parallel combination of R2 and CPE1 (100 Hz to 100 kHz), originates from the interfacial impedance at the Li metal anode|GPE. Similar semicircles at a similar characteristic frequency were observed for a Li|GPE|Li symmetrical cell and a Li|GPE|LiCoO2 cell.[48] The second semicircle from midfrequency to low frequency, denoted as the parallel combination of R3 and CPE2 (1–100 Hz), could be attributed to the charge transfer processes between the LNMO active particle and the electrolyte, as seen at the GPE|LiCoO2 interface.[48] The low-frequency spur can be attributed to the diffusion-limiting impedance, designated by Ws1. A comparison of the EIS spectra of the cells under different bending conditions showed that the interfacial resistances (R1 and R2) were almost constant, verifying the strong adhesion between the current collector/GPC coating layer/GPE/Li anode interfaces. In addition, the electrolyte resistance (R1) remained unchanged, which also supported stable interfaces. These results support that the intimate contact between the electrolyte, LNMO particles, and conductive carbon through the PVDF binder may have been formed by this simple solution-based preparation method, which ensures a stable ionic/electronic conductivity for the electrode.

Figure 5

(a) Nyquist plots of the flexible cell under different bending states at the cell voltage of 4.9 V, (b) equivalent circuit used for simulating the experimental impedance data, (c) demonstration of the flexible cell connected in series with a red LED, and (d) cycle performance of the flexible cell cycled at 0.05 C under flat and bending states (1 C corresponds to 147 mAh g–1).

To demonstrate the flexibility of the cell, bending tests were performed via a connection in series with a light-emitting diode (LED). As shown in Figure c, after the initial charging to 100% SOC, the fabricated cell lit an LED continuously under a flat state and continued to operate even under a 90° bending state. When the cell was further subjected to 180° bending and rolling, the brightness of the LED remained unchanged. The cycling performance under various bending conditions was monitored (Figure d). The discharge capacity and Coulombic efficiency remained almost unchanged. After the stress was removed (flat state), a discharge capacity of 115 mAh g–1 was maintained. The Coulombic efficiency of the pouch cell during the cycling test was stabilized above 90%, which was slightly lower than the corresponding value of approximately 100% for the coin cell with the liquid electrolyte (Figure c). Although the same areal LNMO loading (∼3 mg cm–2) cathode was used in the pouch cell and the coin cell, the electrolytes in the coil and pouch cells are different, that is, liquid electrolyte and GPE, respectively. The ionic conductivity is reduced from 4.2 × 10–4 S cm–1 for the liquid electrolyte to 1.5 × 10–4 S cm–1 for the GPE owing to the gelation.[48] Further, the actual current enhanced from 0.18 to 0.73 mA owing to the increase in the electrode area for the pouch cell. These variations may lead to a large overpotential and undesired side reactions on the anode side, which causes the difference in the Coulombic efficiency between the pouch and coin cells.[55] We believe that lithium anode protection in our future study would circumvent this limitation for the 5 V class flexible lithium polymer battery.

Conclusion

The present study demonstrated a flexible lithium polymer battery comprising GPC and GPE. A SL-based HCE was incorporated in both the LNMO-GPC and GPE membranes because of its high oxidative stability. The lithium polymer battery fabricated by simple lamination of GPC, GPE, and lithium metal anode could be bent for more than 500 bending cycles between 0 and 90° without any apparent capacity decay, as implied by the lack of morphological change and because of the strong adhesion between the current collector/GPC coating layer/GPE/Li anode interfaces. The fabricated 5 V class lithium polymer battery exhibited stable electrochemical behavior during continuous bending, which was confirmed by EIS measurements. The present study provides a new perspective on flexible batteries for future wearable electronics.