High-performance double-network ionogels enabled by electrostatic interaction

Production of highly conductive and mechanically robust ionogels is urgently needed for the development of diverse flexible electrical devices, but it remains challenging. Herein, we report a facile strategy to prepare high-performance ionogels (ionic conductivity of 1.9 S m−1, fracture strain of 170%) via electrostatic interaction between mechanically robust charged gel double networks and conductive ionic liquids. Ionogels based on charged polymer networks (with electrostatic interaction) exhibit obvious higher optical transmittance, ionic conductivity, and better mechanical properties compared with those based on neutral polymer networks (without electrostatic interaction). Ionic conductivity and mechanical properties of the ionogels can also be regulated by the double-network structure of the gels. We further develop an ionic skin sensor with the high-performance ionogels used as ionic conductors, which can exhibit excellent sensing performance even under harsh conditions. We envision that this new class of high-performance ionogels would be an attractive alternative to traditional hydrogels, and would extend the applications of ionic conductors to extreme environments.

. Fabrication processes of the PAMPS-based DN ionogel. The PAMPS-based DN ionogel was obtained through a three-step method, including constructing PAMPSbased hydrogel, exchanging water with IL and subsequent vacuum treatment at 80 °C.

Determining molar ratio of the second network to the first network
We obtained molar ratio of the second network to the first network by the following method. First, we achieved the mass fraction of the polymer network ( ) in a single-1 network hydrogel as follow: (1) where m sf is the weight of the dried framework, which was obtained by freeze drying the single network hydrogel, and m sh is the weight of the single network hydrogel before drying. Second, DN hydrogel was prepared by using a newly-fabricated single-network hydrogel (weight of m sh' ) with the same composition as the aforementioned singlenetwork hydrogel. In this case, the weight of the first dried framework can be calculated as m sh' ×η 1 . The as-prepared DN hydrogel was freeze dried and the weight of the dried framework (m df ) was obtained. The weight of the second dried framework should be m df -m sh' ×η 1 . So, molar ratio of second network to first network can be expressed as:

S3
IL content of the DN ionogels (C IL ) can be expressed as mass ratio of swollen IL to the DN ionogel and can be expressed as follow: where m i is the weight of the DN ionogel, m d is the weight of the DN hydrogel, and 2 is the framework mass ratio of the DN hydrogel. To obtain , we prepared a new DN 2 hydrogel with weight of m d1 and dried framework weight of m df1 by freeze drying. The framework mass ratio of the DN hydrogel was calculated as follow

Ionic conductivity and optical transmittance measurements
Ionic conductivity of the ionogels was measured using a four-probe resistance measuring instrument (RTS-9, Guangzhou Four-probe Tech Co., LTD). The four probes of the four-probe resistance meter are linearly distributed with equal spacing (s = 1 mm) and a S4 probe radius of 500 μm. The outer pair of probes are current carrying electrodes and the inner pair of probes are voltage sensing electrodes. The visible light transmittance of the ionogels was measured using a UV-vis spectrophotometer (UV-2600, SHIMADZU Co., LTD,) at 400 nm to 800 nm. As show in figure S2, the transmittance of the PAMPSbased DN ionogel can reach more than 90% in the visible light region. Figure S2. Transmittance of the PAMPS-based DN ionogel can reach more than 90% in the visible light region.

Conductivity stability of the PAMPS-based DN ionogel
We tested conductivity of the PAMPS-based DN ionogel in the temperature from -80 °C to 80 °C. As show in Figure S4a, the PAMPS-based DN ionogel can keep excellent S5 ionic conductivity (1.2-5.3 S m −1 ) in a wide temperatures range from −80 to 80 ℃. We also tested the conductivity change of PAMPS-based DN ionogel under ambient conditions for 28 days. As show in Figure S4b, our ionogel can keep long-term stability with ionic conductivity scarcely changed even for 28 days. Figure S4. (a) The PAMPS-based DN ionogel shows relatively high ionic conductivity over a wide temperature range from −80 to 80 °C. (b) Ionic conductivity of the ionogel is scarcely changed even for 28 d, indicating good long-term stability.

Morphologies of the ionogels
The morphologies of the ionogels were characterized by using a confocal laser scanning microscopy (CLSM). The CLSM was carried out using a Nikon laser confocal-   Figure S7. Ionic conductivity of PAMPS-based DN ionogels increases with increasing the IL content

Tensile Measurements:
Tensile measurements of the ionogels were performed on a tensile-compressive tester (Tensilon ESM303, Beijing Jeepin Times Technology Co., LTD,). In the tensile test, each testing sample was cut into dumbbell shape ( Figure S8). The thickness depends on the sample itself, and stretching speed is 5 mm/min. Figure S8. Image of the dumbbell-shaped testing sample.
S8 Figure S9. The capacitance changes of the ionic skin increase linearly with increasing pressure.

Applying the PAMPS-based DN ionogel to a sensor
We prepared the PAMPS-based DN ionogel into a capacitive sensor, and attached the sensor to an artificial joint to monitor mechanical motion in the temperature range of we placed the cyclic bending artificial joint in the oven and connected the sensor to handheld portable capacitance meter. When we press the ionic skin with a precise pressure and record the capacitance of the ionic skin with the simple portable capacitance meter, capacitance of the ionic skin increases linearly with increasing pressure ( Figure   S9).