Separators have the critical function of preventing electronic conduction (short circuiting) while permitting ionic conduction through the electrolyte which is soaked into the porous structure. Conventional separators of microporous polyolefins, used in lithium-ion batteries, have good mechanical strength and chemical stability, but poor thermal stability and low electrolyte wettability. The development of entirely new classes of inexpensive insulating materials that optimize both thermal stability and electrolyte wettability are therefore needed and separators where a functionality is included can even enhance the cycling stability of a battery.

In this presentation a survey of different strategies to develop modified or new cellulose-based separators or to utilize conducting polymers with the goal to tailor structural and mechanical properties such as porosity, surface wettability, thermal stability, and at the same time make separators possible to produce. We will show the following examples:

• How mesoporous Cladophora cellulose (CC) separators constitute very promising alternatives to existing ones based on their high crystallinity, good thermal stability and straightforward manufacturing. [1]
• How overoxidized polypyrrole, with high thermal and ionic conductivity, is deposited as a layer on a traditional separator, will enhance the rate capability of Li-ion batteries. [2]
• How a low-cost, renewable, mechanical flexible nanocellulose can be used as both, anode and cathode host, as well as a mesoporous separator in a lithium metal battery design. Nanocellulose with flexible building blocks can produce a 3D porous conducting cellulose paper (CCP) current collector in combination with carbon nanofibers. After electrochemically plating Li into the interconnected nanocellulose/carbon fibers architectures a stable Li metal anode can be obtained. [3]
• A bilayered cellulose‐based separator design will be presented that can enhance the electrochemical performance of Li‐ion batteries via the inclusion of a porous redox‐active layer. The proposed flexible redox‐active separator consists of a mesoporous, insulating nanocellulose fiber layer that provides the necessary insulation between the electrodes and a porous, conductive, and redox‐active polypyrrole‐nanocellulose layer. The latter layer provides mechanical support to the nanocellulose layer and adds extra capacity to the Li-ion batteries s. [4]

The advantes and disadvantages of the different systems will be discussed based on careful electrochemical and material science characterisation of the materials as such and their function in a battery.

References:

[1] Ruijun Pan, et al., J. Power Sources 321 (2016) 185

[2] Zhaohui Wang, et al., Energy Technology 3 (2015) 563

[3] Zhaohui Wang, et al., Accepted by ACS Applied Energy Materials 2018

[4] Zhaohui Wang, et al., Advanced Science 5 (2018) 1700663

Acknowledgements

The TriLi project supported by the Swedish Energy Agency and the battery fond is recognaised as well as StandUp for Energy.