Elastic Polyurethane as Stress‐Redistribution‐Adhesive‐Layer (SRAL) for Directly Integrated High‐Energy‐Density Flexible Batteries (2024)

Simultaneously realizing high electrochemical performance and mechanical reliability are crucial for flexible batteries. Herein, the authors propose a stress redistribution adhesive layer (SRAL) strategy that can align the distribution of generated stress and allowable stress, thus providing a general strategy to enhance mechanical reliability for directly integrated flexible battery.

2.1. Design Principles of Directly Integrated Flexible Battery Based on SRAL

For geometrically flexible batteries manufactured by directly integrating commercialized coin cells onto flexible substrates, mechanical reliability is the key for the device to work under complex deformation scenarios.^[16] Various patterned structures including spine‐like, kirigami‐inspired, and accordion‐like have been proposed to alleviate the stress in the inter‐cell spaces, while the adhesion at the cell/substrate using conventional glue is still vulnerable.^{[6, 14, 17]} Fundamentally, as shown in Figure1, the adhesion of coin cells onto flexible substrates using adhesives generates two interfaces (cell/adhesive and adhesive/substrate). When using conventional rigid glues (taking epoxy as an example) as the adhesive (Figure1a), the generated stress during deformation would maximize at the adhesive/substrate interface (Figure1b). This interface is usually weak because most flexible substrates expose inert surfaces and cannot form strong binding with the adhesive layer. Consequently, a mismatch between the generated stress and the maximum allowable stress would occur, thus leading to subsequent failure at the adhesive/substrate interface. Intuitively, the mechanical performance of the device can be improved if the distribution of the generated stress can align with the strength of the device with proper mechanical properties on each part (Figure1c). Instead of using rigid glue, we conceive that an elastic adhesive could perform as the SRAL, with which the maximum stress generated during deformation would be transferred out of the weak adhesive/substrate interface (Figure1d), thus improving the overall mechanical reliability for directly integrated flexible batteries.

To quantify the effectiveness of the SRAL strategy, we conducted the finite element analysis (FEA) to compare the stress distribution when using rigid adhesive or elastic SRAL during integration. The Young's modulus and Poisson's ratio values of rigid adhesive material were set as 1000MPa and 0.39, respectively. For PDMS, Young's modulus of 2.6692MPa and Poisson's ratio of 0.499 were used. For SRAL, the Young's modulus was set as 0.12MPa, and the Poisson's ratio was set as 0.48. For simulating the tensile deformation, the left side of the substrate was fixed while a displacement of 10mm was applied to the right side. The imprint method was employed to record the stress distribution on the contact surface including both the cell/adhesive and adhesive/substrate interfaces. We first compared the stress distribution at the two interfaces when using rigid glue or SRAL as the adhesive (Figure2a) when stretching a flexible substrate integrated with a single cell. For rigid glue, the stress on the upper contact surface (cell/rigid adhesive) was negligible (≈0.0013MPa). However, the stress distribution on the lower contact surface (rigid adhesive/substrate) was non‐uniform and maximized at the edges (8.7722MPa). In contrast, when using the SRAL for integration, the stress is distributed at the upper contact surface (cell/SRAL), and maximized at the edges (0.0505MPa). In this case, the stress on the lower contact surface (SRAL/substrate) was negligible (Figure2a). We further examined the stress distribution when the flexible device was integrated with multiple cells (Figure2b). The stress distribution at each interface corresponded well to the scenario when the device was integrated with a single cell, confirming the general effectiveness of the SRAL strategy. We further employed the cohesive zone method (CZM) to simulate the crack propagation process of a rigid battery undergoing upward displacement until detachment. First, we defined two types of CZM materials. For the upper contact interface (cell/adhesive) with stronger adhesion, we set a maximum normal contact stress of 1MPa, and a critical fracture energy for a normal separation of 1Jm⁻². For the lower contact interface (adhesive/substrate) with weaker adhesion, the maximum normal contact stress was set to 5KPa, and the critical fracture energy for normal separation was set to 5Jm⁻². For both CZMs, the artificial damping coefficient of 1ms was set for mathematical stability during simulation. Under the same condition, the flexible battery integrated using SRAL or the rigid adhesive layer showed a similar failure mechanism (cracks initiated from the edge of the adhesive/substrate interface and propagated inward) (Figure2c; Figure S2, Supporting Information). Therefore, redistributing the maximum generated stress out of the weaker adhesive/substrate interface during deformation should be effective for improving the robustness of such a directly integrated flexible battery when the device is deformed in other directions.

2.2. General Effectiveness When Using PU as the SRAL on Various Flexible Substrates

To verify the SRAL strategy, polyurethane (PU) was synthesized as the elastic adhesive. PU was a tunable block copolymer that could achieve different properties by adjusting the ratio of soft and hard segments.^[18] In this work, isophorone diisocyanate (IPDI) and BDO were selected as the hard segment in the polyurethane chain. It provided a large number of NCO groups to react with OH to form NHCOO‐ groups, which provided strong adhesion to various flexible rubbers, and metallic cells. The long‐chain macromolecule PTMG used provides excellent mechanical flexibility for polyurethane materials. Figure3a briefly illustrates the synthesis of polyurethane‐based SRAL. First, IPDI, poly(tetramethylene glycol) (PTMG), and 1,4‐butanediol (BDO) were reacted for chain extension.^[19] Then, triethanolamine (TEOA) was added. Before solidification, the semi‐polymerized polyurethane was dripped onto the desired regions of a semi‐cured flexible substrate. The cells were placed on the semi‐polymerized PU layer and gently tapped to remove excess bubbles. The system was then kept at 80°C for 24h for the PU to cure. As the curing reaction progresses, a cross‐linked molecular entanglement forms between PDMS and PU, thus significantly enhancing the adhesion between PDMS and PU. Meanwhile, FTIR indicates that no new chemical bonds are generated between PDMS and PU (Figure S3, Supporting Information). The structural composition, thermodynamic properties and mechanical properties of PU were characterized by FTIR (Figure S3, Supporting Information), TG‐DTG (Figure S4, Supporting Information) and tensile test (Figure S5, Supporting Information), and the results showed that PU remained stable below 275°C and had excellent mechanical properties, Young's modulus was ≈0.1217MPa.

To quantify the adhesion for the adhesive/substrate interface for such a directly integrated flexible battery (Figure3b), its flexible substrate was fixed at the bottom of a tensile testing machine, while the cell (integrated onto the flexible substrate) was clamped by the tensile mold. The specimen was then stretched upward until detachment. For systematic comparison, three common flexible substrates including PDMS, Ecoflex, and SEBS were coupled with our PU as well as other four commercial adhesives including Aradite, Teson T‐99, Teson 4‐min, and bb‐fox 502 (Figure3c–e). It's worth noting that all tested devices fail at the lower adhesive/substrate interface, confirming that redistributing the maximum stress out of this interface was useful to improve the robustness of the device under other deformation modes (lateral stretching, for example). Compared with other commercial adhesives, the adhesion of PU on various substrates showed first‐tier performance and was particularly better than epoxy on PDMS (301.0KPa vs 20–30KPa). Additionally, the maximum allowable stress for PU (or epoxy) on PDMS was similar to the results calculated using finite element analysis software (301.0 vs 268.26KPa), confirming the reliability of the CZM data used for simulation. These results demonstrate that the elastic PU binder could provide comparable or better adhesion than commercialized adhesives at the adhesive/substrate interface, while the elastic property of PU could redistribute the maximum generated stress out of this adhesive/substrate interface, thus functioning as SRAL for improving the overall mechanical reliability of the directly integrated flexible battery.

2.3. Electrochemical Demonstration of Flexible Batteries Based on SRAL

The stable output under complex working conditions is the key for the flexible battery to meet the demand for various flexible electronics.^[20] To evaluate the electrochemical performance of the directly integrated flexible battery using the SRAL strategy, we directly integrated multiple CR2032‐type NCM/LTO cells (average loading is 13.0±2.0mg NCM vs 12.0±2.0mg LTO) in series or parallel onto PDMS substrates (6.0×2.0×0.1cm³) using the prepared PU (Figure4a,b). The structures (Figures S6 and S7, Supporting Information) and half‐cell electrochemical performance (Figures S8–S11, Supporting Information) of the NCM and LTO raw materials were tested. The devices are stabilized at 0.2mA for five cycles and then cycled at 1.0mA under repeated bending/releasing. For each NCM/LTO full cell, the loading of NCM is 12.0±2.0mg and 10% excessive anode was used. The similar charge/discharge curves at the 8th and the 88th cycle (inset in Figure4b) indicate the robust electrochemical performance of the device upon bending. To further confirm the good electrical contact during deformation, we compared the charge–discharge curves of single cells, two cells in series as well as two cells in parallel during cycling with repeated deformation (Figure4c). The battery in series shows doubled output voltage and the battery in parallel shows doubled capacity compared to the single cell, indicating the reliable connection within our directly integrated flexible battery. To visualize the robustness of the flexible battery integrated using SRAL, we have used smaller LR41‐cells as units to be integrated on PDMS (6.0×5.0×0.1cm³) to form a directly integrated flexible battery to demonstrate the viability of fabricating directly integrated batteries with higher density of integration level (Figure4d). Compared with the use of rigid adhesives, this directly integrated battery fabricated with SRAL exhibits excellent environmental stability (Movie S1, Supporting Information). It can stably power LED displays even under harsh conditions, such as bending in water (Movie S2, Supporting Information) or violent shaking (Figure4d,e; Movie S3, Supporting Information), proving the potential of this SRAL integration strategy for practical applications.

Materials

Isophorone diisocyanate (IPDI, 98%), Triethanolamine (TEOA, 97%), and 1,4‐Butanediol (BDO,98%) were supplied by Sigma–Aldrich. Polytetramethylene glycol (PTMG, Mn=2000 gmol⁻¹) was provided by Shanghai Aladdin Biochemical Technology Co., Ltd. (China). The polydimethylsiloxane (PDMS) base and curing agent (Sylgard 184) purchased from Dow Corning were used for the preparation of PDMS films. Ecoflex 00–30 (Smooth‐On, Inc. PA.) was prepared by mixing equal volumes of prepolymers A and B, provided by the supplier. Polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) was purchased from Sigma–Aldrich. The percentage of polystyrene was 30wt% and the average molecular weight of SEBS was 118kgmol⁻¹. Commercial epoxy adhesive Aradite, and bb‐fox 502 were supplied by Araldite, and Teson T‐99 and Teson 4‐min were supplied by Teson.

Preparation of Stress‐Redistribution‐Adhesive‐Layer (SRAL)

PTMG2000, TEOA and BDO were dried under vacuum at 110°C for 6h before putting them into use. IPDI was kept in a low‐temperature (2−8°C) freezer until use.

The preparation process of SRAL involved two steps: pre‐polymerization and cross‐linking. Pre‐polymerization stage:PTMG2000, BDO (after vacuum drying), and IPDI with a given ratio (molar ratio 1:1:2.4) were added into a glass bottle with magnetic stirring. The mixture was continuously pumped with nitrogen gas, stirred at 300rmin⁻¹, and reacted at 60°C for 90min. Crosslinking stage: TEOA (vacuum‐dried) was added to the system after pre‐polymerization with stirring at 200rmin⁻¹ for 30min under a nitrogen atmosphere (molar ratio PTMG2000:BDO:IPDI:TEOA=1:1:2.4:0.27). After being mixed evenly, the mixture was taken out and dried (80°C, 24h).

Integration of the Cell Arrays

The electrodes were prepared by a conventional slurry coating‐drying process using NCM with the size of 8.0–12.0µm or LTO with the size of 4.0–7.0µm as the active materials. The binder was prepared by dissolving PVDF powder (Canrd) into N‐Methyl‐2‐pyrrolidone (NMP, Canrd) to form a 5% solution. For slurry preparation, the active material (NCM or LTO) was mixed with Acetylene black with a mass ratio of 90:5. Then, the calculated volume of 5% PVDF solution was added into the active material/conductor powders to form a slurry containing 5% binder. Slurries were formed by manual grinding and then coated on Al (cathode) or Cu (anode) substrates using four‐sided stainless steel notched rods with a certain gap height. The coated electrodes were dried in an oven at 60°C for 40min and then transferred to a vacuum oven and kept at 110°C for 10min. In general, the loading thickness of the cathode (excluding Al substrate) was 80.0±8.0µm. The thickness of the anode (excluding Cu substrate) was 100.0±10.0µm.

The electrodes were punched with a 12mm diameter disc punch and weighed with a microbalance with a readability of 0.001mg. CR2032‐type coin cells were assembled in an argon‐filled glove box using an electrolyte of EC/DMC containing 1.0molL⁻¹ LiPF₆.

For forming the cell arrays, the individual cell was interconnected using wires in series or parallel. The silver glue was used to ensure the conductivity and reliability of the contacts.

Fabrication of Flexible Batteries with SRAL

The flexible PDMS substrate was prepared by mixing the base and curing agent uniformly at a weight ratio of 10:1 followed by aging at 60°C for 20min to obtain semi‐cured PDMS. Drop the undried semi‐polymerized PU after chain extension on the desired area on the semi‐cured PDMS substrate. The LR41/CR2032 coin cells were respectively placed onto the semi‐cured PU and slightly tapped to expel trapped gas bubbles. The integrated battery was then kept at 80°C for 24h.

Electrochemical Tests

CV and EIS were tested using a CHI 660E electrochemical workstation. Before the EIS test, the cells were first run for three cycles, charged to the given voltage and then rested for 24h. The rate and cycling tests were performed on the LANHE CT3001A battery tester. Each individual NCM‐LTO coin cell was charged/discharged in the voltage range of 0–2.7V. For two cells connected in series, the tested voltage range was 0–4.95V. For two cells connected in parallel, the voltage range was 0–2.7V.

Mechanical Test

To quantify the mechanical property, the flexible substrate of the integrated battery was fixed to the bottom of the stretching machine. The stretching die was used to clamp the adhered coin cell. The coin cell was stretched upward until it was completely detached from the substrate.

Characterization of the Raw Materials

The phase and morphology of the active materials were characterized by XRD (PANalytical B. V. Empyrean) and SEM (HITACHI SU8010). The composition of the SRAL was analyzed using TG (NETZSCH STA 449F5) and FT‐IR (Thermo Fisher Scientific, Nicolet iS50).

Finite Element Analysis (FEA)

To analyze the stress distribution of flexible batteries with SRAL, modeling was run using the static structural module in the ANSYS workbench software. For the flexible substrate, the dimension of the flexible substrate was set as 24.0×56.0×0.3mm³. Two widely used flexible substrates are modeled: PDMS (Young's modulus was set as 2.6692MPa, Poisson's ratio was set to 0.499) and Ecoflex (Young's modulus was set as 0.43019MPa, Poisson's ratio was set as 0.48). A cylindrical model with a size of Φ8.9×0.5mm was used for both the SRAL and conventional rigid adhesive layer. The Young's modulus and Poisson's ratio of the SRAL were set as 0.12175MPa and 0.485. The Young's modulus and Poisson's ratio of the conventional rigid adhesive layer were set as 1000MPa and 0.39. For each integrated coin cell, it was represented by a cylindrical model with a size of Φ7.9×3.6mm. The battery was made of structural steel with Young's modulus of 20000MPa and a Poisson's ratio of 0.3. 3D nonlinear finite element analysis was used to simulate the lateral and vertical deformation of flexible batteries prepared using SRAL, and the stress transfer effect of SRAL, when subjected to deformation, was analyzed, which was realized in the commercial software ANSYS workbench. When simulating lateral deformation, the left side of the substrate was fixed, a 10mm displacement to the right was applied to the right side of the substrate, and the imprinted surface was applied to record each contact surface (SRAL/Battery, SRAL/substrate, Adhesive Layer /Battery, Adhesive Layer/Substrate) stress distribution. When simulating vertical deformation, fix the substrates on the left and right sides, apply an upward displacement of 10mm to the battery, and record and analyze the stress distribution of the contact surface.

Supporting Information

Supplemental Movie 1

Supplemental Movie 2

Supplemental Movie 3

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