Polyacrylonitrile-Polyvinyl Alcohol-Based Composite Gel-Polymer Electrolyte for All-Solid-State Lithium-Ion Batteries

The three-dimensional (3D) structure of batteries nowadays obtains a lot of attention because it provides the electrodes a vast surface area to accommodate and employ more active material, resulting in a notable increase in areal capacity. However, the integration of polymer electrolytes to complicated three-dimensional structures without defects is appealing. This paper presents the creation of a flawless conformal coating for a distinctive 3D-structured NiO/Ni anode using a simple thermal oxidation technique and a polymer electrolyte consisting of three layers of PAN-(PAN-PVA)-PVA with the addition of Al2O3 nanoparticles as nanofillers. Such a composition with a unique combination of polymers demonstrated superior electrode performance. PAN in the polymer matrix provides mechanical stability and corrosion resistance, while PVA contributes to excellent ionic conductivity. As a result, NiO/Ni@PAN-(PAN-PVA)-PVA with 0.5 wt% Al2O3 NPs configuration demonstrated enhanced cycling stability and superior electrochemical performance, reaching 546 mAh g−1 at a 0.1 C rate.


Introduction
With the development of energy storage applications, such as portable electronic devices, electric vehicles, and the growing need for storage of new green energy (solar, tide energy, wind, nuclear, biomass, etc.) in smart grids, the demand for lithium-ion batteries (LIBs) has recently been increasing rapidly [1][2][3]. To meet the performance requirements of some new advanced applications, many research works have shifted their attention to enhancing power density [4][5][6]. In order to obtain a high energy density for batteries, the electrode surface area may be increased to permit large mass loading of electrochemically active material per unit area and the development of new electrode materials. Traditional planar or two-dimensional (2D) electrode design places a limit on the active material mass loading inside a 2D surface, but it can be expanded by using thicker electrodes. The electrode thickness is nevertheless constrained by the following factors: risks of a thick electrode detaching from the current collector during repeated charge-discharge cycles; slow diffusion/charge and mass transfer of lithium-ion (Li + ) through a thick electrode layer; and, in the case of lithium (Li) metal-based batteries, the electrode's development of Li dendrites, which can short-circuit the battery [7,8]. The three-dimensional (3D) batteries concept is a result of the solution of these issues and the need to increase the areal capacity during the charging/discharging process in LIBs [45]. PVA layer was used to have stable interfacial contact with an electrode material. It demonstrates the excellent mechanical flexibility of the gel as well as the remarkable reversibility of Li + [46]. This paper reports a perfect conformal coating of a unique 3D structured NiO/Ni anode synthesized by a simple thermal oxidation process with three polymer electrolyte consisting of three layers of PAN-(PAN-PVA)-PVA with the addition of Al 2 O 3 nanoparticles (NPs) as nanofillers. A 3D anode may be coated uniformly and steadily with a polymer using the sol-gel dip-coating process. This coating could then be dried and loaded with a liquid electrolyte to create Li + conductive GPE. With a capacity retention of 96% and coulombic efficiency of 97%, the 3D NiO/Ni@PAN-(PAN-PVA)-PVA with 0.5 wt% Al 2 O 3 NPs configuration demonstrated stable cycleability up to 100 cycles.

Material Preparation
Anode material was synthesized from commercially available nickel (Ni) foam (Sigma Aldrich Chemie GmbH, Steinheim, Germany) (thickness 0.9 mm, bulk density 0.62 g cm −3 , porosity 93%). Ni foam with a thickness of 0.9 mm was cut into round shapes with 14 mm diameter. Then, they were rinsed with absolute ethanol and acetone (Sigma Aldrich Chemie GmbH, Riedstrasse 2, Steinheim, Germany) (1:1) in an ultrasonic bath (Elma Schmidbauer GmbH, Singen, Germany) for 1 h to remove the surface impurities, then dried in a furnace (Memmert GmbH, Schwabach, Germany) at 60 • C. After drying, NiO/Ni anode was fabricated by thermal oxidation at 700 • C for 5 min [47]. Then, three polymer layers PAN-(PAN-PVA)-PVA were coated on NiO/Ni anodes by a sol-gel dip-coating method. In a typical procedure, PAN and PVA were dissolved in N,N-Dimethylformamide (DMF) (Sigma Aldrich Chemie GmbH, Steinheim, Germany) and deionized water (DI water), respectively. The polymer solutions were then stirred with a magnetic stirrer at 600 r min −1 for 6 h. First, the cleaned NiO/Ni anode was dipped in the PAN (2 wt%) dissolved in DMF solution for 10 s and then removed and dried in a vacuum oven at 60 • C for 2 h. The second layer was PAN-PVA. The dried NiO/Ni anode was coated with a layer of PAN-PVA by dipping in PAN-PVA (1 wt%) dissolved in DMF solution for 10 s and then removed and dried in a vacuum oven at 60 • C for 2 h. The third layer of PVA was obtained by dipping in PVA (2 wt%) dissolved in DMF:DI water (7:3) solution for 10 s and then removed and dried in a vacuum oven at 60 • C for 2 h. Moreover, the different concentrations of Al 2 O 3 (0.25, 0.5 wt%) (Sigma Aldrich Chemie GmbH, Steinheim, Germany) were mixed with PAN, PAN-PVA, and PVA polymer solutions using a magnetic stirrer for 12 h to form a homogeneous solution. The experiment was repeated from the first step.

Materials' Characterization
The crystal structures of the obtained NiO/Ni anodes were analyzed using XRD (SmartLab, Rigaku Co., Takatsuki, Japan, Cu Kα radiation, λ = 0.154056 nm). The XRD data were obtained over a 2θ range from 20 to 80 • C at a scan rate of 6 deg. min −1 using 40 kV, 30 mA X-ray. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS, JSM-7500F, JEOL Ltd., Yamagata, Japan) were employed to investigate the morphology and homogeneity of the distribution of NiO/Ni anode components. Structural modifications in GPE were analyzed using Fourier-transform infrared spectroscopy (FTIR, Nicolet iS10 FT-IR Spectrometer, Thermo Fisher Scientific Inc., Ogden, UT, USA).

Electrochemical Investigation
The electrochemical performance of NiO/Ni@PAN-(PAN-PVA)-PVA anodes was investigated using the CR2032-type coin cells assembled in an argon-filled glovebox (M. Braun Inertgas-Systeme GmbH, Gerlingen, Germany). Metal Li was used as both counter and reference electrodes. A Celgard 2400 microporous polypropylene membrane was used as a separator. The electrolyte was 1 M LiPF 6 in a mixture of ethylene carbonate/ethyl- . The coin cells were tested galvanostatically on a multi-channel battery testing system (BT-2000, Arbin Inc., Hong Kong, China and Neware Battery tester, Neware Co., Hong Kong, China) at a current density of 0.1 C, between the cut-off potentials of 0.01 and 3.0 V. Cyclic voltammetry (CV) was performed using a VMP3 potentiostat/galvanostat (Bio-Logic Science Instrument Co., Seyssinet-Pariset, France) at the scan rate of 0.1 mV s −1 between 0.01 and 3.0 V.

Results and Discussions
To verify the purity of the phase, the as-prepared NiO/Ni anode was analyzed by XRD analysis. The XRD patterns for pristine Ni foam and thermally oxidized Ni foam at 700 • C for 5 min. are shown in Figure 1 bonate/ethyl-methyl carbonate/dimethyl carbonate (EC/EMC/DC, 1:1:1 vol mer film on the surface of the anode was activated by adding 4-5 drops electrolyte solution of EC, EMC and DC (1:1:1 (v/v) ratio). The coin cells w vanostatically on a multi-channel battery testing system (BT-2000, Arbin Inc China and Neware Battery tester, Neware Co., Hong Kong, China) at a curr 0.1 C, between the cut-off potentials of 0.01 and 3.0 V. Cyclic voltammetry formed using a VMP3 potentiostat/galvanostat (Bio-Logic Science Ins Seyssinet-Pariset, France) at the scan rate of 0.1 mV s −1 between 0.01 and 3.0

Results and Discussions
To verify the purity of the phase, the as-prepared NiO/Ni anode wa XRD analysis. The XRD patterns for pristine Ni foam and thermally oxidiz 700 °C for 5 min. are shown in Figure 1     A cross-sectional SEM analysis was also carried out to measure the thickness of the NiO and coated polymer layers. Figure 3b reveals that the surface of Ni foam was successfully covered with the NiO layer inside and outside with thicknesses in the range of ~1.605 and 1.957 μm. As shown in Figure 3b, the total thickness of the 3 layers is 0.0492 μm. The A cross-sectional SEM analysis was also carried out to measure the thickness of the NiO and coated polymer layers. Figure 3b reveals that the surface of Ni foam was successfully covered with the NiO layer inside and outside with thicknesses in the range of~1.605 and 1.957 µm. As shown in Figure 3b, the total thickness of the 3 layers is 0.0492 µm.   To confirm the coating of polymers and uniform distribution of Al2O3 NPs on the surface of NiO/Ni foam, the EDS analysis was acquired. The results are shown in Figure  4, where polymer coatings with Al2O3 reveal a uniform distribution of elements, as expected. addition of Al2O3 in the amount of 0.25 and 0.5 wt% in the coating further increased the layers' thickness by almost 1.5 and 5.1 times, 0.07498 μm and 0.253 μm, respectively (Figure 3c,d). To confirm the coating of polymers and uniform distribution of Al2O3 NPs on the surface of NiO/Ni foam, the EDS analysis was acquired. The results are shown in Figure  4, where polymer coatings with Al2O3 reveal a uniform distribution of elements, as expected.  FTIR results confirm the presence of polymers PAN and PVA layers after dip-coating. The resulting spectra have typical peaks between 400-3600 cm −1 attributed to PAN and PVA, as shown in Figure 5. The presence of peaks at 2326 cm −1 and 1447 cm −1 is characteristic of groups -C≡N, -O-CH3, while peaks at 1057-1394 cm −1 are a series of bands corresponding to the vibrations of esters for the chemical structure of PAN. The distinct groups for the PVA broadband are observed in ~2900-3500 cm −1 due to stretching-bound vibrations of -OH groups. Absorption in ~1600 and ~1454 cm −1 is due to scissor, pendulum,  Furthermore, CV measurements were conducted at a scanning rate of 0.1 mV s −1 within a potential window between 0.01 and 3.0 V to investigate the electrochemical activity of the produced electrodes. CV profiles of NiO/Ni foam and NiO/Ni@PAN-(PAN-PVA)-PVA electrodes with and without Al2O3 are shown in Figure 6a-d. CV for NiO/Ni foam electrodes shows that there is just one redox couple peak related to the oxidation of Ni 2+ [51]. The electrodes exhibit anodic and cathodic peaks at 2.18-2.29 V and 0.09-0.25 V, respectively. The formation of amorphous Li2O, the formation of a partially reversible solid electrolyte interphase (SEI) film, and the reduction of NiO to Ni are responsible for the first cycle's prominent cathodic peak at about 0.09-0.25 V [52]. The cathodic peak became broader in the subsequent cycles and shifted to about 1.2 V for all electrodes. The performance of NiO electrodes as reported in the literature [53] is in agreement with the anodic peaks, which have essentially not changed. It is important to note that the addition of 0.25 and 0.5% Al2O3 did not significantly change the peak potentials of any electrodes, but did cause the peak intensities of NiO/Ni@PAN-(PAN-PVA)-PVA and NiO/Ni@PAN-(PAN-PVA)-PVA to slightly rise. The peak intensities for NiO/Ni@PAN-(PAN-PVA)-PVA and NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.25 and 0.5% Al2O3 increased very marginally; however, it is important to note that the peak potentials of all electrodes remained similar. The most stable sample is defined as NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.5 wt% Al2O3. The overall lithiation and delithiation can be represented in Equations (1) and (2).
The NiO anode's electrochemical reactions during charge-discharge processes [54,55]:    [56]. During the charge cycle, a voltage plateau of 2.0-2.7 V can be observed, which corresponds to the reversible process of NiO production from Ni and Li2O. With the obtained CV profiles, the results are in good agreement.    The irreversible capacity increase during the first 10 cycles might be related to the influence of insoluble Al particles on the SEI formation (Figure 6k-l). Cycle performance curves for NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.25 and 0.5 wt% Al 2 O 3 are similar up to the 10th cycle with the subsequent decrease. However, NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.5 wt% Al 2 O 3 is more stable and reaches the minimum capacity of 395 mAh g −1 in the 100th cycle. The cell with the GPE and the addition of 0.5 wt% Al 2 O 3 has constant coulombic efficiency, which was close to 100% in the overall battery operation. Coating NiO/Ni with a dielectric polymer layers can efficiently act as a capsule, which mitigates the volume expansion of the electrode. Thus, the capacity retention of the NiO/Ni@PAN-(PAN-PVA)-PVA with 0.5 wt% Al 2 O 3 is improved compared to the Ni foam after the oxidation and polymer coating. Ni foam's 3D structure helps against the volume expansion. It is thought to be the primary factor behind the enhanced performance of NiO/Ni foam-based electrodes [41]. pores of the polymer. With the addition of Al2O3, only a gradual decrease in capacity was observed (Figure 6k-l). The irreversible capacity increase during the first 10 cycles might be related to the influence of insoluble Al particles on the SEI formation (Figure 6k-l). Cycle performance curves for NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.25 and 0.5 wt% Al2O3 are similar up to the 10th cycle with the subsequent decrease. However, NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.5 wt% Al2O3 is more stable and reaches the minimum capacity of 395 mAh g −1 in the 100th cycle. The cell with the GPE and the addition of 0.5 wt% Al2O3 has constant coulombic efficiency, which was close to 100% in the overall battery operation. Coating NiO/Ni with a dielectric polymer layers can efficiently act as a capsule, which mitigates the volume expansion of the electrode. Thus, the capacity retention of the NiO/Ni@PAN-(PAN-PVA)-PVA with 0.5 wt% Al2O3 is improved compared to the Ni foam after the oxidation and polymer coating. Ni foam's 3D structure helps against the volume expansion. It is thought to be the primary factor behind the enhanced performance of NiO/Ni foam-based electrodes [41]. Figure 7 shows the Nyquist plots of the electrochemical impedance spectra of the NiO/Ni@PAN-(PAN-PVA)-PVA electrode and NiO/Ni@PAN-(PAN-PVA)-PVA electrode with 0.5 wt% Al2O3 after 100 cycling. Both Nyquist plots share the same characteristics, including a semicircle in the middle frequency range that is often related with charge transfer and an inclined line in the low frequency range that is responsible for lithium ion diffusion in the majority of the electrodes. According to the results of the fitted equivalent circuit, the charge transfer resistances of NiO/Ni@PAN-(PAN-PVA)-PVA and NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of 0.5 wt% Al2O3 electrodes were measured to be 98.8 and 67.7, respectively. Consequently, the addition of Al2O3 nanoparticles in the GPE led to a decreased charge transfer resistance and activation energy compared to the GPE without Al2O3. In order to confirm the mechanical stability of Ni foam electrodes with various coating conditions after cycling coin-cells with NiO/Ni@PAN-(PAN-PVA)-PVA, NiO/Ni@PAN-(PAN-PVA)-PVA with the additions of 0.25, 0.5 wt% Al2O3 electrodes were disassembled and morphologies were investigated. Figure 8 shows the resulting images of the electrodes being retracted from the cells after 100 cycles. NiO/Ni@PAN-(PAN-PVA)-PVA electrodes retain a porous skeleton after cycling, and the polymer coating can be observed without any damage or cracks.

Conclusions
In summary, NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of Al2O3 by a facile dip-coating method was successfully synthesized. Changes in surface morphology of Ni foam after thermal treatment followed by coating were revealed by SEM. A homogeneous distribution of C and Al elements throughout the coating structure was also confirmed by the EDS-SEM study at the same time.
NiO/Ni had a perfect conformal coating with PAN-(PAN-PVA)-PVA layers and even retained a porous skeleton after 100 cycles without any damage or cracks. With a high specific capacity, high coulombic efficiency, and improved structural stability, the cell demonstrated stable cycling and rate capability.
A remarkable specific discharge capacity of 546 mAh g −1 after 10 cycles and gradual degrease to 383 mAh g −1 with constant coulombic efficiency over 100 cycles were as a consequence demonstrated by the coin-cells with the GPE and the addition of 0.5 wt% Al2O3. In addition, based on the obtained results, it can be concluded that the 3D electrode's high surface area, uniform Li + diffusion, provided by the polymer coating, and unique 3D structure all contribute significantly to its improved performance.

Conclusions
In summary, NiO/Ni@PAN-(PAN-PVA)-PVA with the addition of Al 2 O 3 by a facile dip-coating method was successfully synthesized. Changes in surface morphology of Ni foam after thermal treatment followed by coating were revealed by SEM. A homogeneous distribution of C and Al elements throughout the coating structure was also confirmed by the EDS-SEM study at the same time.
NiO/Ni had a perfect conformal coating with PAN-(PAN-PVA)-PVA layers and even retained a porous skeleton after 100 cycles without any damage or cracks. With a high specific capacity, high coulombic efficiency, and improved structural stability, the cell demonstrated stable cycling and rate capability.
A remarkable specific discharge capacity of 546 mAh g −1 after 10 cycles and gradual degrease to 383 mAh g −1 with constant coulombic efficiency over 100 cycles were as a consequence demonstrated by the coin-cells with the GPE and the addition of 0.5 wt% Al 2 O 3 . In addition, based on the obtained results, it can be concluded that the 3D electrode's high surface area, uniform Li + diffusion, provided by the polymer coating, and unique 3D structure all contribute significantly to its improved performance.