Electrospun membrane in batteries

Electrospun membrane has found potential usage in various parts of a battery. The key advantage for using electrospun membrane is its high surface area and porosity. In Lithium ion battery, it has been investigated for use as the anode material or the separator membrane. Apart from Li-ion battery, electrospun membrane may also be used in other types of rechargeable battery.

Li-ion battery is one of the most commonly used rechargeable battery in consumer electronic devices. With increasing demand for high performance rechargeable batteries, researchers are finding ways to increase the capacity of the battery and its charging cycles. An advantage of using electrospun material as an anode in Li-ion battery is its high porosity and interconnected pores that facilitate electrolyte penetration. This have the benefit of improving charge capacity and short recharge time. Self et al (2015) constructed an anode membrane by electrospinning a slurry of titania nanoparticles, conductive carbon black nanoparticles and poly (acrylic acid) as binder. The resultant anode material showed good charge/discharge cycling stability with a reduction of only 5% drop in capacity after 450 cycles at 0.5 C. At charge/discharge rate of 5 C, the capacity retention for electrospun composite was 31% which is much greater than 8% retention from conventional slurry cast electrode of the same composition. Highly compacted electrospun anode membrane with fiber volume fraction of 0.87 also demonstrated higher 2 C volumetric capacity of 21.2 mAh cm³ to slurry cast electrode (17.5 mAh cm³). To improve cyclability and capacity retention, Zhang et al (2018) constructed electrospun C-Mo₂C fiber membrane with fused junctions. The presence of MoO₂ during carbonization process caused a reaction between MoO₂ and C to form Mo₂C which improves mass transfer between fibers in contact with one another to form fused junctions. The fused junction potentially reduces charge transfer and sodium diffusion resistance. A capacity retention of 90% was recorded after 2000 cycles with a membrane capacity of 134 mA h g^-1. This compares well with pure C fibers membrane which exhibited a poor capacity retention of 60% and capacity of 80 mAhg^-1. Zeng et al (2021) attempts to harness the high theoretical capacity and low working potential of silicon as an anode material by constructing a core-shell electrospun fibers membrane.To address the low conductivity of Si, a core-shell fiber with carbon forming the shell as the conductive pathway encapsulating Si nanoparticles at the core. Having the carbon as the shell would also help to restrain volume change in Si which is detrimental to the structure of the anode membrane. A comparison of the core-shell Si@C fibers were made with fiber made of blended Si nanoparticles in a C matrix. In terms of physical characteristics between the two fibers, the Si@C fibers showed a smooth outer carbon layer with evenly dispersed Si nanoparticles at the core while the Si/C blended fibers had a rough surface from the nanoparticles sticking out of the fiber. Si@C fibers showed good cycle stability when tested to 50 cycles while Si/C blended fibers electrodes suffer severe capacity loss which can be attributed to volume expansion during the charge/discharge cycle. Overall, the Si@C anode showed a reversible capacity of 762.0 mAh g^-1 after 100 cycles and a capacity of 479.7 mAh g^-1 at a current density of 2 A g^-1. When assembled as a flexible battery, the battery was able to power a commercial LED in a bent position.

The SEM images of (a) SiSi@C NFs and (b) Si/C NFs; TEM images of (c) SiSi@C NFs and (d) Si/C NFs at low magnification; HMTEM image (e) and corresponding SAED (f) of SiSi@C NFs [Zeng et al 2021].

Li et al (2022) core-shell-structured silicon@silica (Si@SiO₂) nanoparticles loaded in carbon nanofibers as anode material for lithium batteries. To overcome the issue of Si nanoparticles (Si NPs) expansion, they incorporated a SiO₂ coating over the Si nanoparticles. This coating was able to inhibit expansion of SI NPs. These core-shell-structured silicon@silica nanoparticles (Si@SiO₂ NPs) were blended into polyacrylonitrile (PAN) solutions for electrospinning into nanofibers. Carbonization process was carried out to convert the PAN matrix into carbon fibers (CNFs) . The optimized Si@SiO2@CNF composite showed good reversible capacity of 477.4 mAh g^-1 after 100 cycles at 500 mA g^-1. After 300 cycles at 100 mA g^-1, the composite retained a rate capacity of 634.6 mAh g^-1.

With increasing use of Li-ion batteries, there is an interest in using clean and sustainable material derived from biomass as carbon anode while increasing the performance of the battery. Nitrogen (N)-doped carbons are known for their excellent conductivity and oxidation stability. Therefore to construct a bio-based nitrogen (N)-doped carbons anode membrane, Wang et al (2021) used a mixture of cellulose (CE) and chitosan (CS) for electrospinning into fibrous membrane followed by pyrolysis to form N-doped CE-based carbon. A small amount of polyacrylonitrile (PAN) was added to the CE/CS solution to facilitate electrospinning. A CE to CS ratio of 5:5 was found to exhibit the best best electrochemical performance, with a high specific capacity of 327 mAh g^-1 at 100 mA g^-1 and specific capacity of 399 mAh g^-1 at 30 mA g^-1 and excellent cycling stability after 300 cycles.

There are numerous research on using electrospun composite fibers to overcome the issues of capacity decay and reduced cycling performance. Tin oxide (SnO₂) is a potential electrode material for use in Li-ion batteries and like many other oxide materials, it suffers from significant volume change during charge-discharge cycles which leads to capacity decay and reduced cycling performance. Liang et al (2017) attempts to overcome this limitation by using hybrid structure of N-doped carbon fibers and SnO₂ nanoflowers (NC/SnO₂). NC nanofibers were fabricated by annealing of electrospun polyacrylonitrile (PAN) nanofibers. The hybrid structure of NC/SnO₂ were formed by hydrothermal growth of SnO₂ nanoflowers on the surface of NC nanofibers. The presence of NC nanofibers served as a framework to prevent agglomeration of SnO₂ nanoflowers and also as a conductive network to accelerate electronic transmission. Zhang et al (2019) also used hydrothermal post-treatment to grow NiO Nanosheets@ on TiO₂ nanofibers prepared by electrospinning. Transition metals especially nickel oxide (NiO) are attractive anode material because of its high theoretical capacity (718 mA h g^-1), high density (6.67 g cm^-3) and low cost. Construction of the NiO Nanosheets@ is by dipping the TiO₂ nanofibers sheet in NiSO₄·6H₂O and hexamethylenetetramine (HMT) followed by sintering. The resultant NiO/TiO₂ exhibited a high discharge capacity of 1388.9 mA h g^-1 at the rate of 0.05C and decreases to 509.5 mA h g^-1 at 0.5C. For charging cycle at 0.1C, the initial discharge capacity of NiO/TiO₂ electrode is 1285.6 mA h g^-1, and the capacity maintains at 487.1 mA h g^-1 after 50 cycles. This is much better than NiO electrode with initial discharge capacity is 655.2 mA h g^-1, decreasing to only 198.9 mA h g^-1 after 50 cycles. Qin et al (2020) constructed a gel-based electrode (GPE) made of electrospun poly(vinylidene-co-hexafluoropropylene) (P(VDF-HFP)) and polyamide 6 (PA6, nylon 6) for use in rechargeable lithium batteries. The sandwich-structured nylon-based composite GPE (gel-based electrode) was fabricated by first electrospinning PA6 fibers, followed by P(VDF-HFP) fibers and then another layer of PA6 fibers. PA6 exhibited high absorption of organic electrolyte and the P(VDF-HFP) layer restricted the dimensional shrinkage of wet PA6 membrane. The resultant GPE with LiPF6-based organic liquid demonstrated high ionic conductivity and inhibited formation and growth of lithium dendrites in Li metal batteries due to homogenous Li+ ion dissolution/deposition between the electrolyte and the Li anode. High porosity and interconnected pores in the electrospun membrane allows even distribution of Li+ ion flux and electrodeposition hence inhibiting lithium dendrites formation. The assembled Li/GPE/LPF cell demonstrated good cycle stability and delivered a capacity of 145.5 mAh g^-1 after 200 cycles.

Another widely investigated use of electrospun membrane in Li battery is as separator membrane between electrodes. For this function, the membrane need to exhibit thermal stability, high porosity for good electrolyte penetration and permits rapid Li ion transport. Park et al (2016) created a multicore-shell (MCS) electrospun separation membrane comprising of multiple cores of polyimide (PI) in the shell of polyvinylidene fluoride (PVdF) for enhanced thermal stability. The MCS structure was formed by phase-separated polymer blended solution. The electrospun MCS membrane was found to exhibit superior thermal and electrochemical stabilities up to 200 °C while commercial polyethylene membrane showed significant shrinkage. At charge/discharge rate of 1.0C/1.0C, 89.3% of the initial discharge capacity was maintained after 500 cycles MCS membrane while it was 83.6% for commercial membrane. Superior performance of MCS membrane could be attributed to the even distribution of multiple polyamide core which has higher melting temperature in the PVdF matrix. Kim et al (2018) constructed a separator made of electrospun cross-linked poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-co-HFP) fibers. Cross-linking was achieved by using a less volatile solvent, N-methyl-2-pyrrolidone (NMP) in the solution such that the deposited fibers were sufficiently wet as to cause fusion at contact points. The resultant separator showed higher amount of the liquid electrolyte impregnated in the pore networks of the PVDF-co-HFP separator compared to commercial membrane and this contributed to its greater ionic conductivity (2.3 × 10^-3 S/cm at room temperature). The amorphous HFP copolymer regions absorb the liquid electrolyte which allows Li ions to be transported through the cross-linked network. Yerkinbekova et al (2022) mixed polyethylene glycol diacrylate (PEGDA) and hydrolyzed 3-(Trimethoxysilyl)propyl methacrylate (HMEMO) as cross-linking agents into a solution of maleated lignin (ML) and poly(acrylonitrile) (PAN) for electrospinning into PAN/ML/HMEMO/PEGDA (PMHP) nanofibrous membrane to be used as lithium ion battery separator (LIB). During electrospinning, a UV source was used to initiate crosslinking between photosensitive acrylic groups of HMEMO and PEGDA and maleic groups of ML. Thermal treatment of the resultant electrospun membrane also encouraged further cross-linking by condensing the silanol groups of HMEMO to form siloxane bonds (Si-O-Si). The produced PAN/ML/HMEMO/PEGDA (PMHP) membrane with an average thickness of 25 µm showed high porosity and wettability, greater heat resistance, mechanical and electrochemical properties compared to commercial separators. The PMHP membrane with liquid electrolyte has an ionic conductivity value of 2.79x10^-3 S cm^-1 which was much greater than the same cell with commercial separator, Celgard 2400 (6.5x10^-4 S cm^-1). Assembled half-cell batteries with PMHP electrospun membrane and LiFePO₄ cathode showed a specific discharge capacity of 147 mAh g^-1 at 0.1 C and limited Li dendrites formation over 1000 h of continuous stripping and plating.

A simple way of constructing a composite membrane while enhancing separator performance is to use a polymer coating over inorganic nanofibers produced by electrospinning. Lee and Lee (2023) electrospun titanium isopropoxide/polyvinylpyrrolidone nanofibers followed by sintering to form titania inorganic nanofibers membrane. Pure titania membrane is brittle which makes it difficult to handle. Lee and Lee (2023) coated a layer of polyvinylidene fluoride-co-hexafluoroethylene (PVDF-co-HFP) over the titania membrane while retaining the interconnected structure and tortuous pores, Retention of the interconnected pores enabled high electrolyte uptake and the polymer coating increases the strength of the membrane, improved thermal stability and prevent the growth of dendritic formation during the charging and discharging process. When heated to temperature over 150 °, polypropylene membrane changed shape while the PVDF-co-HFP coated TiO₂ nanofiber membrane maintained its dimensional stability which may be attributed to the thermal stability of TiO₂ nanofibers. High electrolyte uptake in the porous membrane allows greater movement of lithium ions which leads to lower resistance. Lee and Lee (2023) evaluated the charge-discharge performance in Li/LiFePO₄ cells with liquid electrolyte-soaked PVDF-co-HFP coated TiO₂ nanofiber membrane. LiFePO₄ is usually used as a cathode material and has a theoretical capacity of 167 mAh?g^-1 and an operating potential of 3.4 V. The PVDF-co-HFP coated TiO₂ nanofiber membrane exhibited good retention of the discharge capacity at 150 mAh?g^-1 at the 50th cycle, which is 90% of the theoretical capacity of LiFePO4 (167 mAh·g^-1). This showed that the electrochemical reactions at the interface between the electrode and the electrolyte are excellent within the fibrous network and microporous structure.

It is common knowledge that a membrane with higher porosity would increase the amount of electrolyte uptake. Prasad et al (2020) was able to produce porous and smooth electrospun fibers by varying the composition of the solvent mixture and the collector medium. Using a solvent system of N,N-dimethylformamide (DMF)/acetone at 4:6 ratio and depositing in a water collector, the electrospun fiber showed the highest volume of porous fibers. Depositing the fibers on an aluminium substrate collector produces smooth fibers. The solvent uptake of the porous fibers at 540% were higher than other smooth fibers and commercial PE. Such high solvent uptake may be attributed to inter-fiber porosity and the porous morphology of the fibers. Discharge capacity of electrospun smooth PVDF fibers membrane has a stable discharge capacity of 98 mAhg^-1. Electrospun porous fiber membrane has an initial low discharge capacity before stabilising at 120 mAhg^-1 after 20 charge-discharge cycles. This higher discharge capacity may be attributed to the pores on the fibers. Cai et al (2019) constructed an electrospun membrane for Lithium-Ion battery separator made of side-by-side bicomponent thermoplastic polyurethanes (TPU) and polyimide (PI) fibers. This takes advantage of thermal stability of PI and elasticity, tensile strength and aging resistance of TPU. Compared to conventional polyethylene separation membrane, this electrospun TPU/PI membrane has much higher electrolyte uptake (10 times that of PE) due to its greater porosity and adsorption characteristics of TPU for the electrolyte. Electrochemical stability is also better with the TPU/PI membrane and it has excellent ionic conductivity (5.06 mS·cm^-1).

SEM images of the electrospun PI/PVdF MCS composite nanofibrous membrane, residual PI core fibrils after solvent extraction, and the schematic of the multicore-shell structure (a,d,g) MCS2, PI:PVdF=2:1wt. ratio, (b,e,h) MCS1, PI:PVdF=1:1wt. ratio, and (c,f,i) MCS0.5, PI:PVdF=1:2wt. ratio. The winkles along the electrospun fiber become severe, and the diameter of PI core fibrils tends to increase as the PI content is increased [Park et al 2016].

In the development of lithium sulfide battery, a limitation is the migration of polysulfide. To address this issue, one option is to introduce a functional separator which absorb the solubilised polysulfide and inhibit its migration. Huo et al (2019) proposed using electrospun polysulfonamide (PSA) membrane as the functional separator. With this separator, they were able to achieve initial specific capacities of 1506 mAh g^-1, 1398 mAh g^-1, 1265 mAh g^-1 and 1000 mAh g^-1 at the current densities of 0.05 C, 0.1 C, 0.2 C and 0.5 C, respectively which were slightly higher than commercial separator. With 100 cycles of charge-discharge at 0.2 C current density, the specific capacity of the PSA separator drops slightly to 1052 mAh g^-1. An additional benefit of using PSA as the separator is that the material itself is a fire retardant. The electrospun PSA membrane did not catch fire when placed in a burning flame for 60s. Yanilmaz et al (2020) proposed using TiO₂@porous carbon nanofibers (TiO₂@PCNFs) as interlayer between separator membrane and cathode for Lithium-sulfur (Li-S) batteries. A mixture of polyacrylonitrile (PAN) and polymethylmethacrylate (PMMA) solution was electrospun to form nanofibers for carbonization into porous carbon fibers. TiO₂ nanoparticles dispersion was electrosprayed simultaneously with electrospinning for uniform distribution of the nanoparticles with the nanofibers. Heat treatment was carried out to convert the PAN/PMMA nanofibers into porous carbon nanofibers decorated with TiO₂ nanoparticles. The presence of TiO₂ nanoparticles provide strong chemical adsorption for polysulfides while the carbon nanofibers provide excellent electron pathways. With this (TiO₂@PCNFs) interlayer, a high initial discharge capacity of 1510 mA h g^-1 was achieved. The cell containing this interlayer was able to deliver a reversible capacity of about 1100 mA h g^-1 compared to just 400 mA h g^-1 in the cell without.

Electrospun fibers may also be used with other chemical process to constructed a hierarchically organized anode membrane in lithium ion batteries. To overcome the limitation of low theoretical specific capacity of graphite and lithium insertion/extraction voltage in graphite anodes commonly used in commercial Lithium batteries, Wang et al (2019) constructed a membrane with dendritic hybrid architecture comprising of carbon nanotubes (CNTs) with nickel sulfide nanoparticles (NS) encapsulated inside, grown from the surface of porous electrospun N-doped carbon nanofibers (CNFs). The multi-stage process to construct this membrane starts with electrospinning of polyacrylonitrile (PAN)/nickel acetate to form a fibrous membrane. During annealing, PAN was converted into porous N-doped carbon fibers which served as a reducing agent to convert Ni salt into Ni nanoparticles. Thiophene introduced during annealing, decomposes into H₂S and gaseous hydrocarbons. The hydrocarbons served as the carbon source and the catalytic effect of Ni nanoparticles encouraged growth of carbon nanotubes (CNTs) epitaxially out of the CNFs. The Ni nanoparticles reacted with H₂S to form nickel sulfides nanoparticles inside the CNTs. As a lithium battery anode material, the resultant dendritic membrane showed high reversible capacity of 630 mA h g^-1 at 100 mA^-1 after 200 cycles and 277 mA h g^-1 at a high rate of 1000 mA g^-1.

For rechargeable batteries, electrospun fibers with fire retardant additives may be used to prevent fire in the event of catastrophic battery failure. Some cases of lithium ion battery catching fire is due to failure in the membrane separating combustible electrolytes in the battery which sets off an exothermic reaction when they come into contact with each other. Liu et al (2017) constructed a core-shell electrospun fibrous membrane with a fire retardant, triphenyl phosphate (TPP), a popular organophosphorus-based flame retardant, in its core. This thin membrane can be used as the separation layer in the lithium battery. The shell was made of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) due to its chemical inertness to the battery electrolytes and relatively low melting point of 160 °C. Their tests showed that the electrospun membrane was able to effectively extinguishes fire. Liang et al (2019) showed that core-shell electrospun fibers with flame retardant triphenyl phosphate (TPP) at its core and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) as the shell (TPP@PVDF-HFP) was able to extinguish the fire in an ignition test. This composite electrospun fibers may also be coated on commercial battery membrane (TPP@PVDF-HFP@commercial) separator to give it flame retardant property as shown by the same ignition experiment. Both TPP@PVDF-HFP and TPP@PVDF-HFP@commercial separator were tested as membrane separator in Li-batteries and it was found that both the cells showed good electrochemical cycling stability during the repeating charge and discharge although TPP@PVDF-HFP separator demonstrated higher specific capacity than that of TPP@PVDF-HFP@commercial separator. Both cells were also able to deliver a stable capacity of more than 120 mAh/g on average for over 100 cycles. C-rate test showed that both cells exhibited good rate capabilities. At higher discharge rate, the cell Coloumbic efficiency of each cycle gradually declined when using TPP@PVDF-HFP as separator.

Another method of preventing fire or explosion from Li batteries is to have a shutdown layer or separator. When there is a current leakage, this shutdown separator melts above a certain temperature and blocks further reaction in the battery by closing the pores in the membrane. Jeong et al (2020) examined the possibility of electrospinning poly-1-butene (PB) and derivatives, which are the members of poly(alpha-olefin) family as shutdown separators due to their electrochemical properties and melt temperature similarity to polypropylene (PP) and polyethylene (PE). However, unlike PP and PE, there are suitable solvents for dissolving PB for electrospinning into fibers. Electrospun PB deposited on a poly(ethylene terephthalate) (PET) membrane demonstrated a shutdown temperature of 120 °C with impedance rising sharply at the temperature which coincides with the melting point of PB. The impedance was greater when electrospun PB was deposited on both sides of the PET membrane instead of just one side.

In vanadium flow batteries (VFB), electrospun fibers may be used as a protective layer for its proton exchange membrane. Sulfonate poly(ether ether ketone) (SPEEK) membrane is a potential replacement for more costly Nafion membrane in VFB. However, it has poor chemical and mechanical stability which limits its use. Yu et al (2017) used electrospun polyacrylonitrile (PAN) nanofiber mat as a protective layer to improve the stability of SPEEK membrane. For this purpose, the SPEEK membrane was sandwiched between two electrospun PAN layers. The sandwiched membrane showed excellent rate performance and superior cycling stability over 1000 charge/discharge cycles at a current density of 120 mA cm^-2, much better than SPEEK membrane and Nafion membrane. Their result showed that electrospun PAN fibers were able to prevent damage to SPEEK membrane without affecting the VFB performance.

High catalytic performance of electrospun membrane with appropriate material has generated interest in using it for quasi-solid-state zinc-air batteries (ZABs). ZABs have high theoretical energy density (1370 W h kg^-1), use abundant low-cost starting materials and may be made flexible. This battery depends on air electrodes with excellent bifunctional electrocatalysis for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Pan et al (2019) fabricated CuCo₂O₄nanoparticles@N-carbon nanofiber (CuCo₂O₄NPs@N-CNFs) film by electrospinning of their precursors blend followed by carbonization/oxidation processes. The resultant CuCo₂O₄NPS@N-CNFs undergo a room-temperature in situ sulfurization by immersing into 2.0 m Na₂S solution for a couple of hours. The CuCo₂S₄ NSs@N-CNFs) films showed remarkable bifunctional catalytic performance (Ej= 10 (OER) - E1/2 (ORR) = 0.751 V) with excellent mechanical flexibility. As a cathode, it demonstrated a high open-circuit potential of 1.46 V, an outstanding specific capacity of 896 mA h g^-1, when assembled into a quasi-solid-state flexible zinc-air batteries (ZAB) with Zn NSs@carbon nanotubes (CNTs) film (electrodeposited Zn nanosheets on CNTs film) as the anode. The ZAB was able to retain 93.6% of its capacity after bending 1000 cycles thus showing excellent potential for use as a power source in wearable electronic devices.

In the development of environmentally friendly battery options, researchers have looked into the development of microbial fuel cells (MFC). In MFC, electroactive bacteria oxidizes organic matter and releases electrons to the anode surface which travels to the cathode via an external load as electricity. Amen et al (2022) constructed the MFC with electrospun membrane as the anode material. High specific area and porosities of electrospun carbon fiber membrane should allow high electron transfer capabilities and potentially addresses the issues of low power and current densities in most MFCs. The network of fibers may also encourage the formation of biofilm on the anode by the bacteria.

Schematic of µL-MFC configuration [Amen et al 2022].

In their setup, the carbon nanofibers were prepared by electrospinning and carbonizing polyacrylonitrile (PAN). Escherichia coli 0157 NCCP-14541 was used as the electroactive bacteria in the anode chamber. As MFC uses living bacteria to generate electricity, the power generation varies over incubation days. For their optimum setup, the power generated after the first 24 h was 2.24 Wm^-2 at 7.89 Am^-2. The maximum power output of 8.1 Wm^-2 at 19.34 Am^-2 was recorded after 6 days of operation while the maximum current output of 44.9 Am^-2 was recorded after 5 days of running time. However, the power and current was not sustained throughout the 8 days of measurement. The optimum thickness of the membrane was found to be 10 µm with less current production at 48 h for 5 µm and 10 µm thickness membrane. This could be due to the degree of penetration of the bacteria and biofilm into the anode membrane. For a thinner membrane, the density of carbon nanofibers was lower hence the maximum current is not realized. When the membrane is too thick, the inability of the bacteria to penetrate into the full thickness increases the distance which the electrons have to travel.

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