In recent years, with the continuous development of wearable devices, the market demand for flexible lithium-ion batteries is also constantly expanding. However, due to the structural characteristics of lithium-ion batteries, the energy density of the lithium- However, wearable devices often have high demands on battery life in order to meet the demand for convenience of use, so people are paying more attention to the energy density of Li-O 2Battery, Li-O 2The theoretical specific energy of the battery up to 3500Wh / kg, but because of Li-O 2The battery needs O in the air 2As a cathode, it is therefore necessary to design a porous cathode structure, Li-O 2This semi-open structure of the battery determines its sensitivity to moisture in the air. Li-O 2Another challenge that batteries face when it comes to wearables is the flexible design that traditional Li-O 2The battery is generally rigid structure design, not suitable for direct use in flexible wearable devices, so in recent years people in Li-O 2The flexibility of the battery also did a lot of work, for example, Changchun Institute of Qingchao Liu bamboo structure in the ancient inspired by the design of a similar structure of soft Li-O 2Battery, not only has good flexibility, but also both waterproof function, and can even be used underwater.
Yan-Bin Yin of Jilin University recently also developed a high-security, flexible and foldable Li-O 2Battery structure as shown below, the battery uses a tubular hollow structure design to ensure good battery flexibility, and the use of polyimide PI and poly (vinylidene fluoride - hexafluoropropylene copolymer) PVDF-HFP composition Of the composite membrane (PIPV), the membrane has good waterproof and thermal stability, and can have a high ionic conductivity, with the battery's other structural design makes this soft Li-O 2The battery not only has excellent cycle life and excellent rate performance, more waterproof and fireproof features, the battery can even be used underwater.
As a key technology of the invention, the preparation process of the PIPV separator is as shown in the above c, first, a layer of polyimide PI film is coated on the surface of the Li metal foil, and then the surface of the PI film is uniformly coated with a layer of poly Vinylidene fluoride - hexafluoropropylene copolymer) PVDF-HFP slurry, and finally dried under an argon atmosphere.
The microstructure of the PI film is shown in the following figure a, it can be seen that the PI film has a very large number of micropores, the surface becomes very smooth after PVDF-HFP coating (FIG. B), almost all the micropores have disappeared, The diaphragm becomes very smooth.From the contact angle measurement of Figure c, the contact angle of the water drop on the PIPV diaphragm is 96, indicating the hydrophobic nature of the diaphragm.The two properties described above make the diaphragm have very good waterproofing properties , To ensure the safety of the metal Li in a water environment (Figure g).
Thermal stability is also in Li-O 2Important factors to consider in battery design, Yan-Bin Yin PP, GF and PIPV films heated to 200 ° C (Figure h below), you can see the PP membrane has completely contracted after heating, melting, but PIPV did not occur significantly The results show that PIPV still has good thermal stability under high temperature of 200 ℃.
As an electronic device that requires close contact with the human body, security is a priority for wearable devices, especially Li-O 2The battery is very lively metal Li, the security we need to take more seriously.To this Yan-Bin Yin of this Li-O 2Battery in water, high temperature, bending and other conditions under the safety of the test validation, the test results shown below, you can see this one Li-O 2Battery is not only not afraid of water, and even under normal work in the water, not to mention bending more, bent 180 degrees underwater, still able to discharge normally.To verify the stability of the battery at high temperatures, Yan-Bin Yin use high temperature flame burning the battery, you can see the battery did not fire the phenomenon of burning, indicating that the battery also has good thermal stability at high temperatures.This feature makes the battery already have as a wearable electronics Equipment, chemical power the most basic qualities, then you need to see its electrochemical properties to meet the needs of wearable devices.
The picture below shows the Li-O 2Battery electrochemical performance test results, can be seen from Figure a, the battery at 0.2mA current discharge capacity can reach 115mAh (4259mAh / g), when the current increased to 1.5mA, still able to release 52mAh (1926mAh / g ) Capacity of the battery in the better rate performance premise, but also has a very good cycle performance, can be seen from d below, after more than 100 cycles, the battery discharge curve almost no change.
Yan-Bin Yin targets Li-O 2Battery-sensitive moisture vulnerability, developed this composite membrane PIPV, not only very effective solution to the Li-O 2Battery performance degradation in water environment characteristics, but also through the electrolytic structure design makes the battery even under water to work, which greatly improves the wearable device safety and convenience. Thermal stability is also the battery Advantages, at 200 ℃ PIPV diaphragm is still able to maintain a good mechanical structure, to ensure the safety of the battery at high temperatures.
