23.08.2021

Functionalized separator for next-generation batteries

Advancements in battery technology have dramatically increased demand for improvements in liquid solid separation design, as the separator plays a critical role in ensuring the safety and electrochemical performance of the cells. Current separators, either in commercial usage or under investigation, have yet to meet the high stability and lifespan performance standards necessary to prevent deterioration in the efficiency and reliability of the battery technologies. Recently, considerable effort has been devoted to developing functionalized separators, ranging from designing a variety of new materials and modification methods, and increasingly, to optimizing advanced preparation processes. In order to understand how the mechanisms of vibrating separator performance are affected by different properties, we will first summarize recent research progress and then have in-depth discussions regarding the separator’s significant contribution to enhancing the safety and performance of the cell. We then provide our design strategy for future separators, which not only meets the requirements of different type of batteries, but also aims for multifunctionality. We hope such a perspective could provide new inspiration in the development of liquid solid separator research for future battery technologies.

The global demand for data storage and processing is increasing exponentially. To deal with this challenge, massive efforts have been devoted to the development of advanced memory and computing technologies. Chalcogenide phase-change materials (PCMs) are currently at the forefront of this endeavor. In this Review, we focus on the mechanisms of the spontaneous structural relaxation – aging – of amorphous PCMs, which causes the well-known resistance drift issue that significantly reduces the device accuracy needed for phase-change memory and computing applications. We review the recent breakthroughs in uncovering the structural origin, achieved through state-of-the-art experiments and ab initio atomistic simulations. Emphasis will be placed on the evolving atomic-level details during the relaxation of the complex amorphous structure. We also highlight emerging strategies to control aging, inspired by the in-depth structural understanding, from both materials science and device engineering standpoints, that offer effective solutions to reduce the resistance drift. In addition, we discuss an important new paradigm – machine learning – and the potential power it brings in interrogating amorphous PCMs as well as other disordered alloy systems. Finally, we present an outlook to comment on future research opportunities in amorphous PCMs, as well as on their reduced aging tendency in other advanced applications such as non-volatile photonics.

Air filter paper with a high filtration efficiency that can remove small-size pollutant particles and toxic gases is vital for human health and the environment. We report a nanofiltration paper that is based on wood fiber filter paper with good mechanical properties and a three-dimensional network structure. The filter paper was prepared by impregnation with multi-walled carbon nanotubes (MWCNTs) and phenol-formaldehyde (PF). The results showed that MWCNTs were present on the surfaces of the fibers and between the pores, which increased the specific surface area of the fibers and enhanced the effective interception of the particles. The optimum impregnation concentration of the MWCNT was 0.1%. Compared with the cellulose fibers (CFs), the average pore diameter of the 0.1% MWCNT–CF filter paper was reduced by 8.05%, the filtration efficiency was increased by 0.64%, and the physical properties were slightly enhanced. After impregnation with PF, the mechanical properties of the air filter paper were significantly enhanced. The PF on the fiber surfaces and at the junction of the fibers covered the MWCNTs. Based on the change in the filter paper properties after impregnation, the optimal filter paper strength index and filtration performance were observed at a solid PF content of 8.4%.

Particles with different sizes and components in the air are in contact with and absorbed by the human body.1 Epidemiological studies have shown a strong correlation between airborne particulate exposure and respiratory disease, cardiovascular disease, and mortality.2,3 Inhalable particulate matter (PM10, aerodynamic diameter < 10 μm) and fine particulate matter (PM2.5, aerodynamic diameter < 2.5 μm) are more likely to carry harmful substances, such as heavy metals or gaseous pollutants, into the human respiratory tract and even the alveolus, causing health hazards.4 Common pollutant control methods include source control, ventilation, and purification.5 Filter paper is an effective material to reduce the concentration of particulate matter in air purification.


Filter paper usually removes particles based on five physical effects: gravity, collision, screening, diffusion, and static electricity.6 The removal efficiency of filter paper is closely related to the relative size of the particle diameter and the paper pore size. Smaller pore sizes of the paper correspond to smaller sizes of the particles that it can intercept under the same filtration efficiency.7,8 Adjusting the structure of the filter paper to improve the air flow resistance can increase the residence time of the pollutant particles in the filter paper, resulting in a higher removal efficiency.9,10 However, the filter filtration resistance directly affects the energy consumption of the pressure leaf filter, such that extremely high filtration resistances are not recommended.11 The filtration efficiency has exhibited dependence on the fiber coarseness. Specifically, finer fibers have exhibited higher filtration efficiencies at a constant pressure drop.12 However, air filter paper must maintain a certain porosity to allow air flow. Nanofibers can increase the specific surface area of the filter paper to generate filter papers with small pore sizes, high filtration efficiencies, and high porosities.


Traditional air filter paper is mainly composed of micron-grade fibers with high air permeability, small airflow resistance, and a poor filtration effect for small particles, which do not meet the requirements of many modern industries for high filtration precision air filter paper. In our work, multi-walled carbon nanotube (MWCNT) air filter paper with a high filtration efficiency and antibacterial activity for extremely small particle pollutants was prepared.13 To reduce the pore size of the filter paper, MWCNTs with large aspect ratios and high specific surface areas were introduced to cellulose fiber (CF) filter paper, which has a low filtration resistance and high mechanical strength due to its porous nanofiber structure.14 The MWCNT–CF filter paper exhibited high air permeability following the incorporation of the MWCNTs. In addition, the MWCNTs exhibited a fine antibacterial ability, which is suitable for industries that require antibacterial, high-efficiency air filter paper.

First, the performance of the CF filter paper was studied after loading MWCNTs with different contents. The MWCNT dispersion (10–15 nm in diameter, 10% concentration) was purchased from Beijing Carbon Yang Technology Co., Ltd., China. CF filter paper (quantitative 90 g m−2) was purchased from Shandong Longde Technology Co., Ltd., China. The MWCNTs were diluted with deionized water to 0.01%, 0.05%, 0.1%, 0.5%, and 1%. The CF filter paper was then dipped into the diluted MWCNT dispersing solution for 30 s and dried in an oven (electric blast drying oven, DGG-101-1, Tianjin Tianyu Experimental Instrument Co., Ltd., China.), with a drying time and temperature of 15 min and 105 °C, respectively.15,16


2.2 Preparation of the PF–MWCNT–CF air vertical pressure leaf filter paper

In this study, phenol-formaldehyde (PF) resin was used to impregnate the MWCNT–CF air filter paper to improve the physical strength of the paper. PF resin (solid content 58%) was purchased from Shanghai Kain Chemical, China. The CF filter paper loaded with 0.5% MWCNTs was impregnated for the second time with PF (dissolved in 99.5% anhydrous methanol). The experiments followed an impregnation time of 30 s, a drying temperature of 105 °C, and a drying time of 15 min. By heating the PF, the gelatinous resin formed a polymer chain resin,17 which gradually hardened from a viscous flow state and appropriately improved the strength of the paper.

2.3 Characterization

The prepared samples were characterized by field-emission scanning electron microscopy (SEM; GeminiSEM 500, Zeiss, Germany), a computer-controlled tensile testing machine (CP-KZ300, Sichuan Changjiang Paper Instrument Co., Ltd., China), and a burst strength tester (969920, L&W, Sweden). The air permeability and pore size distribution of the samples were measured by a digital air permeability meter (YG461E, Ningbo Textile Mill, China) and a capillary pore size analyzer (Porolux 100, Porometer NV, Belgium).


2.4 Filtration performance

The Palas MFP 3000 filter material test system (MFP 3000, Palas Company, Germany) and test dust ISO A2 fine ash were used for testing. The following test conditions were implemented: an end pressure of 2000 Pa, a dust concentration of 1000 mg m−3, a rated gas flow rate of 66 L min−1, a sample area of 100 cm2, and a sample surface velocity of 11.1 cm s−1. The filtration efficiency, filtration resistance, and dust retention of the filter paper were tested.

After the CF filter paper was impregnated with an MWCNT dispersion, MWCNTs adhered to the surface of the vibrating filter paper, thus reducing the pore size and resulting in a higher filter paper surface content compared with that inside the paper. When the load of MWCNTs was greater than 4.56%, the pores readily became blocked, which was not conducive to filtration. At a loading rate of 3.68%, the resistance increased by 26.56% and the dust-holding capacity only increased by 8.33%. At this time, the filtration efficiency was 99.69% (2000 Pa), and the paper strength was also enhanced.


The filtration performance of the MWCNT–CF filter paper impregnated with PF showed little change. The burst index of PF–MWCNT–CF filter paper increased by 110% compared with MWCNT–CF filter paper. The tensile index increased constantly, but the dust retention was too low when the PF solid content was 10%. Thus, the optimal impregnation concentration was 8.4%. At this time, the longitudinal tensile index and transverse tensile index of the filter paper increased by 77.01% and 80.41% respectively, compared with MWCNT–CF filter paper.

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