Bidirectional Planar Flexible Snake‐Origami Batteries

Abstract With the rapid development of commercial flexible/wearable devices, flexible batteries have attracted great attention as optimal power sources. However, a combination of high energy density and excellent arbitrary deformation ability is still a critical challenge to satisfy practical applications. Inspired by rigid and soft features of chemical molecular structures, novel bidirectional flexible snake‐origami lithium‐ion batteries (LIBs) with both high energy density and favorable flexibility are designed and fabricated. The flexible snake‐origami battery consists of rigid and soft segments, where the former is designed as the energy unit and the latter served as the deformation unit. With the unique features from such design, the as‐fabricated battery with calculating all the components exhibits a record‐setting energy density of 357 Wh L−1 (133 Wh kg−1), compared with the cell‐scale flexible LIBs achieved from both academic and industry. Additionally, a design principle is established to verify the validity of utilizing rigid‐soft‐coupled structure for enduring various deformations, and the intrinsic relationship between battery structure, energy density, and flexibility can be confirmed. The results suggest that the design principle and performance of bidirectional flexible snake‐origami batteries will provide a new reliable strategy for achieving high energy flexible batteries for wearable devices.


Battery assembly
Commercial LiCoO 2 electrodes and graphite electrodes were provided by Jiangsu HaiTao new energy technology Company, used as positive electrode and negative electrode, respecitively. First, all electrodes and ceglard separators were cut into comb-like shape with designed geometric size, as shown in Figure S1(a). Then the electrodes and separators were wound with comb spine in Figure S1(a) and folded into bi-directional shape battery in Figure S1(c). The batteries were dried in a vacuum oven at 80 °C for 12h. After drying, the batteries were injected electrolyte in Ar 2 filled glovebox (O 2 <0.1 ppm, H 2 O<0.1ppm). The electrolyte was 1mol/L LiPF 6 in the EC (ethylene carbonate)/DMC(dimethyl carbonate) with volume ratio of 1:1(DoDoChem).
After 4h infiltration, the batteries were sealed by aluminized plastic film under vaccum environment.
In this work, considering the operability of the assembly process and performance of battery, snake-origami batteries with 3*3 array were assembled and tested. In the Figure S4, When any structure parameters changed the relative energy density also changed and the d x /l and d y /ꞷ exhibited the greatest impact on E R , thence3 3*3 array was choosed. And we set the d x /l= d y /ꞷ=0.2. This design of snake-origami batteries have better relative energy density and flexibility form the Figure 2, when The actual geometric size of above assemblied battery was shown in Figure S2.
The snake-origami batteries with 3*3 array were assembled in Ar 2 filled glovebox. All the structure parameters of snake-origami batteries were idential that Nx=Ny=3 and the winding layer numbers was about 6-7. The width of the gap in X-and Y-direction are both 2 mm. ꞷ was the width of comb spine in the battery, which is 10mm. The width of the comb tooth is 10 mm，which l=10 mm. Bending radius and shift can describe the bending state more reasonably. Different bending tests were run sequentially on the snake-origami batteries, which were closed to practical working envrionment of flexible battery. The snake-origami batteries were cycled 15 cycle, about four days, after every bending test. The test steps were as follows. The batteries rest 2h after cycling, then bending test. After bending test, the battery cycled after resting 2h.

Electromchemical test
The snake-origami batteries used LiCoO 2 and graphite electrode were tested under the voltage between 2.5V and 4.2V by charge/discharge battery testing of Neware instruments. The assembled batteries were tested indoor to maintain relatively constant temperature and humidity. The assembled snake-origami batteries were measured at room temperature, about 25°C. And the relative humidity of the air in the room is about 30%. The batteries were chareged to 4.2V at constant current and held at 4.2V until current was reduced to 0.025C. Then, the batteries were discharged to 2.5V. The electrochemical impedance spectroscopy (EIS) measurements were tested by a BioLogic VMP3 instruments. The requency range of EIS was from 10 6 Hz to 10 −1 Hz.
While commerical lithium-ion batteries are mainly measured by battery capacity (mAh) and energy density (Wh). This flexible battery device used commerical LiCoO 2 electrodes and graphite electrodes, which were provided by Jiangsu HaiTao new energy technology Company. The uniformity of commerical electrodes is relatively good, which have same areal density and specific capacity. This flexible battery chose capacity as the evaluation criterion to show the consistency of battery capacity.
Specific capacity can be obtained by calculating the quotient of battery capacity (mAh) and the quality of the electrodes.

Numercial simulation
The mechanical deformation of snake-origami batteries was analyzed by threedimensional standard nonlinear finite element method in the commercial software ABAQUS. In all cases, four-node quadrilateral stress/displacement elements with reduced integration were used. For simplicity, linear isotropic elasticity was adopted for the battery structure with effective modulus and Poisson ratio based on experimental parameters. The pressure of 1 atm was applied to both sides of deformable layers to simulate the vacuum conditions inside the aluminized plastic film. The simply supported boundaries were adopted at the ends of structures and the cylinder was fixed.

Calculated the theoretical energy density of snake-origami batteries
The area specific capacity of LiCoO 2 positive electrode was 2.90mAh/cm 2 and mass loading of it was 21.2mg/cm 2 (double coated). The thickness of positive was 80µm. The area specific capacity and mass loading of graphite negative electrode were 3.40mAh/cm 2 and 10.52mg/cm 2 (double coated), respectively. The thickness of negative was 86µm.The ratio of negative/positive was 1.17 and the negative was excessive. The weight of snake-origami batteries was 15.2g, including electrode, electrolyte and aluminized plastic film.
From the Figure S1, the geometric size of the snake-origami batteries was shown.

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The width L: The length L R: The thickness of separator was 15µm and the thickness of aluminized plastic film (single layer) was 113µm.
So, the capacity of battery was 532.4mAh.
The width of the sealed edge is 2 mm and 20mm. The thickness of the edge part is 0.25 mm. After assembled, the length of snake-origami batteries without extra edge is 55mm and with extra edge is 75mm. The width of snake-origami batteries without extra edge is 44.5mm and with extra edge is 46.5mm.
The total surface area of snake-origami batteries was 34.87cm 2 .
The total volume of snake-origami batteries was 5.66cm 3 .
The theory energy density of snake-origami batteries was calculated as follows.
3.8V is the average discharge voltage of the snake-origami batteries, which is similar to the nominal voltage of LiCoO 2 . The average discharge voltage was calculated by the ratio of energy and capacity, which can be obtained from the charge-discharge of Gravimetric Energy Density:

Relative energy density
As shown in Figure S1(a-b), the width and length of electrode is L and L R, respectively. ꞷ was the width of comb spine in the battery. The gaps of different direction were defined as d x (X-direction) and d y (Y-direction) . In the Figure S1b, the wide gap was described as a, which was used to folded insipred by origami in the Y-direction of snake-origami batteries. For the snake-origami battery, N x and N y were the number of rigid segment in X-and Y-direction . The E represents the energy density of the snake-origami batteries in Figure S1(a). The E total is descirbed as the energy density of conventional battery assembled by the complete electrode in Figure S1(b). As shown in Figure S1(c), k was the winding layer numbers of the snake-origami batteries and conventional batteries and e 0 was the area capacity of positive electrode. S and V were the total surface area and volume of the snake-origami batteries, respectvely. Relative energy density can be calculated by the ratio of E/ E total .
The length of electrode in the battery L R : The wide gap of conventional battery: The thickness of single layer battery: The thickness of battery in Figure S1(a-b): ( ) The energy density of the snake-origami batteries: The energy density of the conventional batteries: The relative energy density was defined as

Effective Flexibility
In order to describe the flexibility of battery, effective flexibility was defined. The effective flexibility was a function of geometric structure parameters, under different mechanical deformation.
The equation of effective flexibility was exhibited as followed.             Figure S13. Optical images of the snake-origami batteries under twisted state.   Video S1. The video of snake-origami batteries powering LED display screen under dynamic loading in X-direction.