Ionic Conductivity Analysis of NASICON Solid Electrolyte Coated with Polyvinyl-Based Polymers

by

Tiago Afonso Salgueiro

Abstract The global environmental crisis necessitates reliable, sustainable, and safe energy storage solutions. The current systems are nearing their capacity limits due to the reliance on conventional liquid electrolytes, which are fraught with stability and safety concerns, prompting the exploration of solid-state electrolytes, which enable the integration of metal electrodes. Solid-state sodium-ion batteries emerge as an appealing option by leveraging the abundance, low cost, and sustainability of sodium. However, low ionic conductivity and high interfacial resistance currently prevent their widespread adoption. This study explores polyvinyl-based polymers as wetting agents for the NASICON-type NZSP (Na3Zr2Si2PO12) solid electrolyte, resulting in a combined system with enhanced ionic conductivity suitable for Na-ion solid-state full cells. Electrochemical impedance spectroscopy (EIS) performed on symmetric cells employing NZSP paired with different wetting agent compositions demonstrates a significant reduction in interfacial resistance with the use of poly(vinyl acetate)—(PVAc-) based polymers, achieving an impressive ionic conductivity of 1.31 mS cm−1 at room temperature, 63.8% higher than the pristine material, notably reaching 7.36 mS cm−1 at 90 °C. These results offer valuable insights into the potential of PVAc-based polymers for advancing high-performance solid-state sodium-ion batteries by reducing their total internal resistance.solid-state sodium-ion batteries; NASICON; NZSP; symmetric cells; ionic conductivity; electrochemical impedance spectroscopy; polyvinyl-based polymers; wetting agentsKeywords:

1. Introduction The urgent need to meet the ever-increasing global energy demands with efficient and sustainable energy storage solutions has driven the quest for alternatives to present lithium-ion batteries. While these batteries have been instrumental in various applications, they face challenges such as limited energy density, finite cycle life, and environmental concerns related to lithium extraction and processing [1,2]. In response, solid-state batteries, particularly sodium-ion systems, have emerged as promising alternatives. Solid-state batteries employ solid electrolytes that allow for the use of metallic anodes, resulting in higher energy densities and prolonged cycle lives [3]. Beyond performance, these devices offer other advantages such as eliminating the risk of electrolyte leakage, allowing for the creation of simpler designs that can more easily be adapted to each individual application [4]. Solid-state electrolytes are also more chemically and mechanically stable, offering greater resistance to dendrite formation and penetration [5].Solid-state sodium-ion batteries (SSIBs) combine the advantages of solid-state technology with the abundant and widely distributed nature of sodium, making them more sustainable and cost-effective compared to their lithium-ion counterparts [2]. However, SSIBs encounter performance-related challenges that need to be overcome in order to make them a true alternative to lithium-based systems. The lack of performance arises mainly from the larger sodium ionic radius, which hinders ionic mobility and adsorption and causes larger volumetric variations during the charge/discharge process, ultimately leading to lower capacity and cycling stability [4,6]. Another major hurdle is the poor quality of the electrode/electrolyte interface. Since both the electrode and the electrolyte are solid materials, the effective contact area is greatly reduced when compared to liquid electrolyte systems, resulting in a reduction in the overall ionic conductivity of the cell.The sodium-ion solid-state electrolyte Na3Zr2Si2PO12 (NZSP) is a NASICON (Na super ionic conductor-) type material, widely recognized as one of the most promising in SSIB research due to its 3D paths for the movement of sodium ions and strong covalent framework [7]. NZSP has an initial reported high ionic conductivity of 0.67 mS cm−1 at room temperature, which has since been increased to as high as ≈2 mS cm−1 through novel manufacturing techniques, such as solution-assisted solid-state reaction, spark plasma sintering, sol–gel reduction, and an additive-assisted solid-state process, although the traditional solid-state method remains the simpler procedure [8,9,10,11,12]. The stable NASICON structure offers mechanical strength and some resistance to dendrite formation [5,13]. The chemical and thermodynamic stability and resistance to air and humidity of NZSP also have the added benefit of making it compatible with a wide range of electrode materials [14,15]. The main challenge currently hindering the application of NZSP as a solid electrolyte in SSIBs is the high interfacial resistance at the electrode/electrolyte interface, resulting in a decrease in ionic mobility and total ionic conductivity of the cell [5]. Considerable efforts have been made to overcome this challenge, namely, by doping elements in bulk and adjusting the chemical composition of the grain boundary [7,11]. Leng et al. successfully decreased the grain-boundary resistance and achieved a high ionic conductivity of 1.36 mS cm−1 for a Mg2+-doped NZSP [16]. Introducing additional interphases, such as ionic liquids, liquid electrolytes, or polymers as wetting agents, is another established efficient strategy used to modify the interface of NZSP and achieve more intimate electrode contact and stability [14,17]. Fu et al. employed a coating of epoxy resin to F−-assisted NZSPFx and obtained an ionic conductivity of 0.67 mS cm−1 at ambient temperature, paired with an improvement in dendrite growth inhibition [18].In this work, three wetting agents: Poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), and commercial PVAc-based wood glue are analyzed in symmetric cells with gold blocking electrodes, with the aim of finding the configuration that results in the lowest total resistance when applied to the NZSP solid electrolyte.Ion conduction in polymer electrolytes is generally understood to occur mainly in the amorphous region of the host polymer matrix, where the molecular chains move more freely than in the crystallized region and oscillate above the glass transition temperature, enabling ion conduction [19]. Polymer electrolytes have good flexibility for compensating electrode volume changes during battery operation and low interfacial resistance, although they typically suffer from low ionic conductivity at ambient temperature and less stability, compared to ceramic electrolytes [5].Polyvinyl-based polymers have been shown in various studies to have good performance as dielectrics in supercapacitors [20] and as a host polymer in solid polymer electrolytes [21]. Bhargav et al. synthesized an NaBr:PVA polymer electrolyte with an ionic conductivity of 1.36 × 10−4 S cm−1, while Sasikumar et al. successfully implemented PVAc in a P(VdF-HFP)-LiTFSI-EC complex, resulting in a blend solid polymer electrolyte with a higher ionic conductivity of 1.1 mS cm−1 [22,23]. These limited but promising results regarding polyvinyl-based electrolytes provide the impetus for the investigation of these polymers in the role of wetting agents for solid-state electrolytes, such as NZSP, integrating the advantages of ceramic and polymer. The use of glycerin as a plasticizer for these polymers is also considered, due to its successful implementation in sodium-ion polymer electrolytes, as a means to provide increased mobility for sodium ions by reducing the crystallinity of the polymer electrolytes and increasing the prevalence of the amorphous region [5,20,24].

2. Materials and Methods 2.1. Material Preparation The NZSP ceramic electrolyte was used as-purchased from 4 to One Co., Ltd., Ulju-gun Ulsan, Republic of Korea. This pellet is fabricated in mass-production facilities via the solid-state reaction route, which includes consecutive processing steps of Na3PO4·12H2O, SiO2 and ZrO2 precursor mixing, calcination at 400 °C and 1000 °C, compaction, and sintering at 1230 °C, resulting in a circular-shaped pellet with 20 mm diameter and 1 mm thickness with a bulk density of 3.03 g cm−3 (92% of the theoretical value) and a high ionic conductivity of ≈0.8 mS cm−1 [10,25,26].The SUPERTITE® (Ideias adesivas Unecol S.L., Valencia, Spain) PVAc-based wood glue and commercial 95% glycerol-content glycerin were used as-obtained from general stores. Poly(vinyl alcohol) ([-CH2CHOH-]n; MW = 89,000–98,000, 99+% hydrolyzed) was acquired from Sigma-aldri