Electro-Spun PAN/Silica-Li Composite Gel Electrolytes for Advanced Lithium-Ion Batteries

TL;DR

This paper introduces a new gel electrolyte for lithium-ion batteries that improves safety and performance through a polymer-ceramic composite.

Contribution

The novel contribution is the development of a PAN/SiO2-Li composite gel electrolyte with high ionic conductivity and dendrite suppression.

Findings

The composite gel electrolyte shows high thermal stability and a wide electrochemical window.

It achieves a room-temperature ionic conductivity of ~4.4 × 10−3 S cm−1 and a lithium-ion transference number of 0.72.

The electrolyte effectively suppresses lithium dendrite growth in symmetric Li||Li cells.

Abstract

Gel polymer electrolytes (GPEs), which combine the safety of solid electrolytes with the high ionic conductivity of liquid electrolytes, have long been regarded as ideal electrolyte materials. This study proposes a polymer/ceramics composite gel electrolyte aimed at improving the performance of lithium-ion batteries. A nanofiber membrane was fabricated by electrospinning a mixture of polyacrylonitrile and lithium-salt-grafted helical mesoporous silica nanoparticles, followed by plasticizer absorption to obtain the composite gel electrolyte film (PAN/SiO2-Li). Experimental results indicate that this gel electrolyte membrane possesses high thermal stability, a wide electrochemical window (>5.3 V vs. Li/Li+), high room-temperature ionic conductivity (~4.4 × 10−3 S cm−1), and a good lithium-ion transference number (0.72). In symmetric Li||Li cells, this electrolyte suppresses lithium dendrite growth and maintains stable lithium deposition/stripping. This work presents a practical electrolyte design strategy for developing GPEs with enhanced safety and performance.

Figures

Full text

1. Introduction

With the rapid acceleration of global industrialization, environmental pollution and energy shortages have become increasingly critical. The progressive depletion of fossil fuels has stimulated an urgent demand for highly efficient and clean technologies for energy conversion and storage; lithium-ion batteries (LIBs) have emerged as one of the most promising energy carriers owing to their light weight, extended cycle life, superior energy density, a minimal self-discharge rate and minimal pollution [1,2]. Currently, they are broadly employed in electric vehicles, large-scale stationary storage systems and portable electronics [3,4,5,6], imposing strict requirements on energy density, safety, cost and lifetime, and forcing the battery community to re-examine every component of the cell, especially the electrolyte that governs ion transport and interfacial stability. Presently, commercial LIBs still rely on liquid electrolytes because these media provide outstanding ionic conductivity and can form stable interfaces with a wide range of electrode materials. Unfortunately, the liquid electrolytes are commonly highly volatile and flammable, posing serious safety risks such as leakage, fire, and thermal runaway. Furthermore, the uncontrolled proliferation of lithium dendrites—caused by irregular lithium buildup on the anode surface—can pierce the separator, leading to internal short circuits, continuous consumption of electrolyte and rapid capacity fade [7,8,9]. Replacing liquid electrolytes with solid analogues is therefore regarded as a crucial strategy for next-generation safe batteries.

Among the reported solid electrolytes, inorganic ceramic electrolytes exhibit high ionic conductivities, but their inherent brittleness and poor interfacial contact with electrodes result in large interfacial resistance, hindering practical applications [10]. Polymer electrolytes primarily classified as solid polymer electrolytes (SPEs) and GPEs. In contrast to their inorganic solid counterparts, SPEs are inexpensive, light and easily processed; their excellent flexibility can accommodate the volume changes of electrodes during cycling, thus reducing interfacial resistance and improving interfacial stability. At ambient temperature, the ionic conductivity of SPEs falls well short of the level necessary for practical applications because ion transport in polymers relies on segmental motion of chains. In addition, SPEs usually suffer from narrow electrochemical windows and poor thermal stability [11,12].

GPEs are formed by a polymer matrix, an organic plasticizer, and a lithium salt. This composition enables them to merge the advantageous properties of liquid and solid electrolytes. By constructing a three-dimensional network swollen with liquid plasticizer, GPEs maintain high ionic conductivity while simultaneously offering superior interfacial compatibility and thermal safety. Consequently, GPEs are currently considered the most promising electrolyte candidates for future LIBs [13]. Recent research efforts have focused on building continuous ion-conducting pathways, minimizing interfacial impedance and imparting non-flammability [14,15].

Polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) and its copolymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)), polypropylene carbonate (PPC), and polyethylene oxide (PEO) are the most frequently employed polymer hosts for GPEs [16,17,18]. Among them, PAN possesses a relatively low lowest unoccupied molecular orbital level, excellent electrochemical stability and a wide electrochemical window, which permits it to align with high-voltage cathodes and deliver high energy density with improved safety. PAN-based GPEs therefore show great potential in lithium-metal batteries [19,20,21]. For example, Ren et al. [21] fabricated a high-performance GPE by incorporating green natural lignin into a PAN matrix. Exhibiting a high lithium-ion transference number (tLi+) of 0.82 and being cost-effective, the membrane adheres to sustainable development concepts. Electrospinning has also become a popular technique to produce PAN-based GPEs because it can readily generate porous membranes with high porosity and excellent chemical stability [22,23]. Wu et al. [22] fabricated a novel fibrous GPE by electrospinning PAN as the host matrix. The resulting three-dimensional, highly interconnected nanofiber network not only provides a mechanically robust scaffold but also creates an exceptionally large internal surface area. This unique architecture markedly enhances liquid-electrolyte uptake and retention, while the continuous, tortuous pathways facilitate intimate contact between the electrolyte solution and the polar nitrile groups of the PAN chains. Consequently, strong and uniform Li^+^-dipole interactions are established throughout the entire matrix, which effectively dissociate lithium salts, reduce ion-pair formation, and lower the energy barrier for ion hopping. As a result, the bulk ionic conductivity and tLi+ are simultaneously increased, leading to significantly improved ion-transport efficiency compared with conventional gel polymer electrolytes.

Nevertheless, the mechanical strength of polymer membranes is still insufficient for long-term cycling. Researchers found that incorporating inorganic fillers into GPEs can both enhance lithium-ion transport and mechanical robustness. The polymer phase can provide continuous ion-conducting pathways, and the inorganic phase suppresses polymer crystallinity and improves segmental mobility, thereby promoting ion migration [24]. Common fillers include SiO_2_, TiO_2_ and ZrO_2_ [25,26,27,28,29,30,31]. For example, Xie et al. [29] prepared a PAN-based GPE by electrospinning with SiO_2_ nanofluids as nanofillers. The membrane showed low crystallinity, high electrolyte absorption, excellent flexibility and large porosity, delivering an ionic conductivity of 3.44 × 10^−3^ S cm^−1^ and a tLi+ of 0.73 while effectively reducing electrode polarization and lithium dendrite formation. However, nano-sized inorganic fillers tend to agglomerate, leading to poor deteriorated performance.

Moreover, most LIB systems are dual-ion conductors in which both cations and anions are mobile. Since the conductivity is due to the dissociation of lithium salts within the polymer matrix, concentration polarization inevitably occurs during repeated charge/discharge processes, resulting in increased internal resistance and voltage loss [32]. In single-ion-conducting polymer electrolytes, anions are bonded to the polymer structure, allowing solely lithium ions to migrate, which can markedly alleviate concentration polarization and raise the lithium-ion transference number, thus enhancing cycling stability [33].

Considering the afore-mentioned intrinsic limitations of GPEs, a single modification strategy is insufficient to achieve comprehensive performance enhancement. Herein, we propose a facile approach that integrates electro-spun polymer nanofiber scaffolds and ceramic composite strategies. Firstly, lithium salt-grafted SiO_2_ nanoparticles were synthesized and blended with PAN. The mixture was then electro-spun into a nanofiber membrane. After absorbing plasticizer, a composite gel electrolyte membrane (denoted as PAN/SiO_2_-Li) was obtained. Electrochemical characterization demonstrates that the as-prepared gel electrolyte simultaneously possesses outstanding thermal stability, a wide range of electrochemical window and high ionic conductivity, offering great promise for safe, high-energy rechargeable LIBs.

2. Materials and Methods

2.1. Materials

Anhydrous ethanol (>99.7%), aqueous ammonia (25 wt%), tetraethyl orthosilicate (TEOS), and 3-aminopropyl-trimethoxysilane (APTMS) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). PAN (M_w_ = 150,000), LiFePO_4_, N,N-dimethylformamide (DMF, >99.9%) were provided by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Super-P and Li foil were obtained from Tianjin Zhongneng Lithium Industry Co., Ltd. (Tianjing, China). Liquid electrolyte (1 M LiPF_6_ in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 v/v)) was supplied by Zhangjiagang Guotai-Huarong Chemical New Material Co., Ltd. (Zhangjiagang, China). Cetyltrimethylammonium bromide (CTAB, 99%) was supplied by Shanghai Jingchun Biochemical Technology Co., Ltd. (Shanghai, China). 1-hexanol (>99.5%) was bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China). Fmoc-L-Ala (97%) and cetylpyridinium chloride (CPC) were purchased from Shanghai Meryer Biochemical Technology Co., Ltd. (Shanghai, China).

2.2. Preparation of Composite Gel Electrolyte Membrane

2.2.1. Helical Mesoporous SiO2 Nanofibers

SiO_2_ was synthesized according to the literature [34].

First, CPC (200 mg) and Fmoc-L-Ala (18.4 mg) were dissolved in 500 mL of deionized water. After dissolution, 3 mL of ammonia aqueous solution (NH_3_·H_2_O) was introduced, and the mixture was stirred at 40 °C for 30 min. Thereafter, 148 μL of 1-hexanol was added into the solution and 6 mL of TEOS was added after 5 min. After stirring for about ten seconds, the mixture was then left static for 36 h. The resulting white flocculent product was dried at 70 °C for 6 h, and to remove the template, the material was subjected to a final calcination at 700 °C for 6 h.

2.2.2. Li-Grafted Helical SiO2 Single-Ion Conductor (SiO2-Li)

Lithium acrylate (100 mg) and APTMS (1 mL) were suspended in 5 mL of anhydrous ethanol and maintained at 80 °C under continuous stirring for 24 h. Thereafter, 200 mg of helical SiO_2_ and 1 mL of 0.1 M HCl were added. The mixture was stirred for an additional 24 h. The product was filtered, washed and dried to obtain white SiO_2_-Li powder.

2.2.3. Electro-Spun PAN/SiO2-Li Nanofibrous Membrane and Gel Electrolyte

The fabrication process was shown in Scheme 1: PAN (400 mg), DMF (3.6 mL) and SiO_2_-LiA (60 mg) were mixed and stirred at 70 °C for 1.5 h. The homogeneous solution was electro-spun at −5 kV (collector)/−10 kV (needle), 15 cm working distance and 0.1 mm min^−1^ feed rate. The as-collected membrane was dried overnight at 70 °C to yield PAN/SiO_2_-Li nanofibrous film. Discs (Ø 16 mm) were punched, dripped in EC/DMC (1:1 v/v) containing 1 M LiPF_6_ until saturation, and composite gel electrolyte membrane was finally obtained.

2.3. Methods

Field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) (Tokyo, Japan) was employed to examine the product’s morphology and microstructure at an acceleration voltage of 10.0 kV. Thermogravimetric analysis (TGA) was conducted on a TG/DTA 6300 (Hitachi, Tokyo, Japan) instrument under an air atmosphere, with the temperature ramped from 25 to 800 °C at a heating rate of 10 °C min^−1^. XPS analysis (EXCALAB 250 XI, Thermo Fisher Scientific) (Waltham, MA, USA) was conducted using a spectrometer (Al Kα source) with a pass energy of 100.0 eV. Atomic Force Microscope (AFM) was performed on Bruker’s Dimension Icon (Billerica, MA, USA).

2.4. Cell Assembly and Electrochemical Measurements

We performed the electrochemical measurements with a CHI660E potentiostat/galvanostat (CH Instruments) (Shanghai, China) and carried out the galvanostatic charge/discharge cycling using a LAND CT3002A test system. Electrochemical impedance spectroscopy (EIS) was recorded over an exceptionally wide temperature window with an a.c. perturbation amplitude of 5 mV in the frequency domain 100 kHz–10 MHz. Linear-sweep voltammetry (LSV) was conducted from the open-circuit potential up to 7.0 V versus Li/Li^+^ at a scan rate of 5 mV s^−1^. Symmetrical Li//Li coin-type cells were cycled at ambient temperature on a Land CT2001A (Wuhan, China) instrument.

We assembled the CR2016 coin cells inside a glove box maintained under an inert argon atmosphere. The cathode slurry was prepared by dispersing LiFePO_4_ (LFP), acetylene black and PVDF binder for N-methyl-2-pyrrolidone (NMP) at 8:1:1. The homogeneous slurry was doctor-bladed onto aluminum foil current collectors, vacuum-dried at 120 °C for 12 h, and punched into discs with 14 mm diameter. Metallic lithium foil served as the combined counter and reference electrode, and a gel polymer electrolyte membrane punched into 16 mm-Diameter discs acted as separator and ion-electrochemical material. The liquid electrolyte consisted of 1 M LiPF_6_ dissolved in a 1:1 v/v mixture of EC and DMC.

The voltage window is fixed between 2.5 V and 4.2 V for Li/Li^+^.

2.5. Calculations

The ionic conductivity (σ) was studied through the EIS of the stainless-steel (SS) symmetrical battery:

$$[eqn]$$

where l, Rb and S represent the electrolyte film thickness, the bulk resistance obtained from EIS, and the electrode contact area, respectively.

The activation energy of lithium ion conduction (Ea) was derived from the Arrhenius equation:

$$[eqn]$$

where A, T and R denotes the pre-exponential factor, the absolute temperature, and the Boltzmann constant.

We employed combined chronoamperometry and EIS measurements with a symmetrical Li/Li cell to determine the tLi^+^ at ambient temperature.

$$[eqn]$$

where I0 and Is denote the initial current and the steady-state current, respectively, ΔV corresponds to a 10 mV polarization voltage, and R0 and Rs represents the interface resistance before and after polarization, respectively (unit: Ω).

3. Results and Discussion

The obtained PAN/SiO_2_-Li is shown in Figure 1. The obtained SiO_2_ are intertwined, spiral-like nanofibers with tens of microns in length and roughly 500 nm in diameter. When 15 wt% of these SiO_2_-Li nanofibers were dispersed in PAN-spinning dope, the resulting electro-spinning process yielded a flexible, self-standing PAN/SiO_2_-Li composite nanofiber membrane. The two constituents are therefore mechanically inter-locked and stack layer-by-layer into a three-dimensional structure with high porosity. Energy-dispersive X-ray spectroscopy (EDS) mapping images (Figure 1d) verify that the silicon (Si), and Oxygen (O) signals are perfectly super-imposed over the carbon (C) skeleton, proving that the SiO_2_-Li moiety is homogeneously distributed throughout the whole mat at the sub-micron scale.

The TGA curve of PAN/SiO_2_-Li (Figure 1e) reveals that the PAN/SiO_2_-Li membrane is thermally stable up to ~200 °C. The minor mass loss (~10%) observed between 200 °C and 320 °C is ascribed to the gradual decomposition of the residual organic coupling agent that was grafted onto the SiO_2_ surface during the LiA functionalization step, rather than to any degradation of the PAN backbone itself.

To guarantee the intrinsic safety of high-energy LIBs, the dimensional stability of three different separators was evaluated by placing circular discs (d = 16 mm) in a convection oven for 1 h at 200 °C and 250 °C, respectively (Figure 1f). The commercial Celgard2325 membrane served as the reference. Digital photographs taken immediately after heat treatment show that the Celgard separator suffers from severe global shrinkage (>35% at 150 °C, total meltdown at 250 °C), whereas the neat PAN electro-spun mat already exhibits an obvious reduction in diameter (~5% at 250 °C). In striking contrast, the PAN/SiO_2_-Li composite film retains its original geometry almost perfectly; the calculated dimensional change is below 3% even after 60 min at 250 °C. This extraordinary thermal robustness is reasonably attributed to the hard ceramic SiO_2_ nanofibers.

The as-spun PAN/SiO_2_-Li nanofibrous mat was converted into a gel electrolyte membrane by a simple post-treatment: circular discs (d = 16 mm) were dripped in a 1:1 v/v mixture of EC/DMC that had been pre-doped with 1 M LiPF_6_. Subsequently, to investigate its electrochemical properties, the gel electrolyte membrane was sandwiched between two electrodes (e.g., SS plate, lithium metal foil, or LFP electrode) and assembled into a series of coin cells within an argon-filled glove box. The results were shown in Figure 2 and the data were collected in Table 1.

Ionic conductivity (σ) serves as a key parameter for evaluating electrochemical performance. Hence, EIS was recorded over a temperature span of 25 to 95 °C for the evaluation of its temperature-dependent ionic conductivity (Figure 2a). Typical Nyquist plots (Figure 2b) collected every 10 °C display vertical spikes at high frequency. The resulting Arrhenius plot (log σ vs. 1000/T) (Figure 2c) is perfectly linear, indicating that ion migration in the PAN/SiO_2_-Li gel follows a thermally activated hopping mechanism without any abrupt jump that would signal a phase change in the polymeric matrix. Consequently, the ionic conductivity increases monotonically with temperature, rising from 4.4 mS cm^−1^ at 25 °C to 6.5 mS cm^−1^ at 95 °C.

To evaluate its practical application potential, LSV was then employed to determine its electrochemical stability window. The current–voltage curve exhibits a flat, noise-less baseline until a sharp anodic wave sets in at 5.4 V (Figure 2d), which is because of the irreversible oxidative decomposition of the carbonate plasticizers and/or the polymer matrix. Because the onset potential is well beyond the upper cut-off voltage required by commercial high-energy cathodes (4.2 V vs. Li^+^/Li), the PAN/SiO_2_-Li gel electrolyte is expected to withstand aggressive charging protocols without significant side reactions, thereby providing an extra safety margin for next-generation lithium batteries.

To determine tLi+ for the PAN/SiO_2_-Li gel membrane, a symmetrical Li|PAN/SiO_2_-Li |Li cell was polarized at 25 °C with a small DC bias of 10 mV until the current reached a steady state, while the initial and final impedances were recorded to correct for interfacial resistance evolution. The calculation, performed according to the classical Bruce–Vincent equation [35], yields a transference number of 0.72 (Figure 2e). The pronounced increase is ascribed to the SiO_2_ nanofiber, which immobilizes the anions through hydrogen bond interaction and creates a continuous, three-dimensional network of lithium-rich hopping sites. By tethering the anions and simultaneously providing additional Li^+^ donors, the SiO_2_-LiA modification effectively funnels the ionic current through the lithium species, thereby mitigating the internal concentration gradient that otherwise develops during high-rate charge/discharge cycles. As Table 1 showed, compared with other gel electrolytes reported in the literature, PAN/SiO_2_-Li demonstrates benign comprehensive performance: a high level of ionic conductivity, wide electrochemical stability window and an excellent tLi+.

We performed the cycling stability test on the Li symmetric cell by applying a constant current density of 0.1 mA cm^−2^ for 200 h, and subsequently increasing it to 1 mA cm^−2^ for a further 100 h (as shown in Figure 2f). The insets present the resulting voltage–time profile over 10 h (10 consecutive cycles). The curve exhibits a perfectly flat plateau at ±100 mV with no detectable increase in over-potential, corroborating that concentration polarization is efficiently suppressed inside the three-dimensional, single-ionic conducting network. The absence of voltage spikes or random oscillations further implies that dendritic lithium propagation is effectively mitigated, since any filament penetration would immediately produce a local short and a sudden drop in cell polarization.

Rate-capability tests were subsequently performed at 25 °C by discharging the cell at progressively higher C-rates (1 C = 170 mA g^−1^) (Figure 2g). The initial discharge capacities are 123.6, 114.8, 104.3 and 88.3 mAh g^−1^ at 0.1, 0.2, 0.5 and 1 C. When the current is abruptly reset to 0.1 C after 50 cycles, the capacity immediately rebounds to 115.4 mAh g^−1^, demonstrating excellent reversibility and minimal polarization-induced loss.

The assembled LFP|PAN/SiO_2_-Li|Li cell was then subjected to a long-term cycling test at a current density of 0.1 C (Figure 2h). The battery exhibited excellent cycling stability, as evidenced by a stable discharge specific capacity over 50 cycles and a coulombic efficiency approaching 100%. Furthermore, the LFP|PAN/SiO_2_-Li|Li cell successfully powered a white LED in Figure 2h, indicating that this battery represents a suitable candidate for practical energy storage applications.

XPS analysis (Figure 3a–h) was applied to characterize the chemical composition of both the solid electrolyte interface (SEI) and the space charge layer for comparative purposes. In the C 1s spectra (Figure 3a,e), the characteristic peak corresponds to the C-C and C≡N bond in PAN. Peaks of CO_3_^2−^ appearing after cycling stem from the electrochemical decomposition of the carbonate-based electrolyte. The N 1s and Si 2p spectrum (Figure 3f,g) reveal the emergence of N-O and SiO_4_^−^ signatures after cycling, confirming the participation of both nitrogen and silicon in the redox reactions. In the Li 1s XPS spectrum (Figure 3d), LiF originates from the decomposition of the lithium salt LiPF_6_. Compared with Li_2_CO_3_, LiF possesses higher rigidity and stronger stability, forming a more robust space-charge layer that effectively resists lithium dendrite penetration. LiF generated from LiPF_6_ decomposition was also identified in the F 1s spectrum (Figure 3h). The above XPS results fully demonstrate the feasibility and superior performance of this electrolyte design.

AFM images were also taken to characterize the electro-deposition process of lithium on the Li-metal surface. It can be observed from the Figure 3i,j that, compared with the pristine lithium foil, the cycled lithium foil exhibits minimal lithium deposition. The hydrogen bonding that forms between the silanol (–OH) groups of the SiO_2_ surface and F^−^ ions accounts for the observed phenomenon., which inhibits the ionization of the lithium salt and facilitates the migration of abundant anions, thereby encouraging uniform lithium deposition [36].

In this work, the precisely designed composite gel electrolyte film exhibited superior electrochemical properties, which could be explained by the reasons below. As is displayed in Figure 3f, SiO_2_ nanofibers grafted with lithium salt act as near single-ion conductors. The PAN-derived three-dimensional network serves a dual function: it imparts superior mechanical robustness and thermal stability to the electrolyte, while simultaneously ensuring the establishment of continuous pathways for lithium-ion conduction. Doping with silica offers additional mechanical reinforcement. In particular, the surface-grafted acrylic acid groups facilitate the dissociation of the LiPF_6_ electrolyte. Concurrently, hydrogen bonding interactions between the silica surface and fluorine atoms lead to substantial immobilization of anions [36]. This synergistic mechanism promotes Li^+^ migration, increases the Li^+^ transference number, reduces concentration polarization, and ultimately ensures uniform lithium deposition with minimal dendrite formation. These combined optimizations enable the PAN/SiO_2_-Li electrolyte to deliver outstanding performance in LIBs.

4. Conclusions

In conclusion, this study proposes a straightforward strategy to synthesize cost-effective, safe, and high-performance quasi-solid-state lithium-ion batteries. To increase the lithium-ion transference number and inhibit lithium dendrite growth alongside the preservation of high ionic conductivity, an electrospinning technique was employed to construct a three-dimensional network that provides continuous ion-conduction pathways. Furthermore, lithium salt-grafted mesoporous helical silica nanofibers were incorporated, which not only promotes electrolyte dissociation but also supplies mobile Li^+^ while immobilizing anions by hydrogen bonding interactions. Consequently, the lithium-ion transference number was successfully increased to 0.72, accompanied by a high ionic conductivity of 4.4 × 10^−3^ S cm^−1^. The polyacrylonitrile framework combined with silica doping also ensures excellent thermal stability (>250 °C). This approach can be readily extended to other polymer matrices or single-ion conductors, offering a promising pathway for developing safer and higher-performance LIBs.