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Article

NaBH4-Poly(Ethylene Oxide) Composite Electrolyte for All-Solid-State Na-Ion Batteries

by
Xiaoxuan Luo
1 and
Kondo-Francois Aguey-Zinsou
2,*
1
MERLin, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
2
MERLin, School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia
*
Author to whom correspondence should be addressed.
Batteries 2024, 10(9), 316; https://doi.org/10.3390/batteries10090316
Submission received: 28 July 2024 / Revised: 28 August 2024 / Accepted: 3 September 2024 / Published: 5 September 2024
(This article belongs to the Special Issue High-Performance Materials for Sodium-Ion Batteries)
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Abstract

:
A disordered sodium borohydride (NaBH4) environment to facilitate Na+ mobility was achieved by partially hydrolyzing NaBH4 and this significantly improved Na+ ionic conductivity to 10−3 S cm−1 at 75 °C. The reaction rate of NaBH4 self-hydrolysis, however, is determined by several parameters, including the reaction temperature, the molar ratio between NaBH4 and H2O, and the pH value; but these factors are hard to control. In this paper, poly(ethylene oxide) (PEO), capable of retaining H2O through hydrogen bonding, was used in an attempt to better control the amount of H2O available for NaBH4 self-hydrolysis. This strategy led to the ionic conductivity of 1.6 × 10−3 S cm−1 at 45 °C with a Na+ transference number of 0.54. The amorphous nature of the PEO matrix in hydrolyzed NaBH4 is also believed to provide a conduction path for fast Na+ conduction.

1. Introduction

The feasibility of solid-state sodium (Na) ion batteries as the next generation of storage systems for large-scale renewable farms has attracted significant attention. Unlike lithium, sodium mineral deposits are more abundant, and easier to extract [1,2,3]. Ideally, new generations of Na-based batteries should not rely on organic electrolytes to remove any risk associated with their flammability [4,5]. A range of solid-state electrolytes has been investigated to address these safety issues [6,7].
Inorganic solid-state electrolytes, including NASICON (Na1+xZr2SixP3−xO12) and sulfide (Na3PS4) electrolytes have acceptable ionic conductivities of 10−4–10−3 S cm−1 at room temperature [8]. However, in NASICON electrolytes, the rigidity of the electrode/NASICON interface leads to high interface resistance, and as such a gradual degradation of the battery performance [9]. Sulfide-based electrolytes are also limited as they are sensitive to moisture, which leads to their decomposition and release of toxic H2S gas [10].
Complex borohydrides are an emerging class of solid electrolytes for all-solid-state batteries as they have good electrochemical and thermal stability and ductility, which enables close contact between the electrode and electrolyte by simply cold pressing [11,12]. The advantages of sodium borohydride (NaBH4) as a solid-state electrolyte include its high thermal stability (up to 500 °C) and good stability against metal anode [13,14,15,16,17,18]. However, the ionic conductivity of NaBH4 is rather low (10−12 S cm−2 at 25 °C) [19].
To improve the ionic conductivity of NaBH4, the pseudo-binary system consisting of the NaBH4-NaI electrolyte was investigated. In this case, Na(BH4)1−xIx (x = 5) exhibited an ionic conductivity of 10−9 S cm−2 at 25 °C. Similarly, the formation of a solid solution between Na(BH4) and Na(NH2) led to an ionic conductivity of 2.0 × 10−6 S cm−1 at room temperature, and this improvement was attributed to the formation of the antiperovskite structure Na(BH4)0.5(NH2)0.5 with Na vacancies [13]. An alternative method to improve the ionic conductivity of NaBH4 is through NaBH4 surface oxidation [20]. Upon oxidation of NaBH4, Na vacancies can be created within the NaBH4 crystalline structure, leading to an ionic conductivity of 2.5 × 10−3 S cm−1 at 35 °C. However, the level of NaBH4 oxidation can be difficult to control [20].
PEO is often used to form composite electrolytes with sodium salts, such as NaBF4 and NaPF6 [21], and the ionic conductivity of the PEO-NaPF6 electrolyte is 6.3 × 10−4 S cm−1 at 80 °C [22]. In PEO composite electrolytes, PEOs have the tendency to coordinate with Na salts because the polar ether group has a lone pair of electrons. This is believed to lead to Na+ conduction paths whereby Na+ successively coordinates and dissociates from the polar ether group, and as such, a high portion of the amorphous region in PEO composite electrolytes tends to deliver high ionic conductivity [23]. In the PEO-H2O system, H2O also forms, on average, 1.2 hydrogen bonds per repeating unit of PEO [24]. As such, by controlling the amount of PEO, the amount of H2O hydrolyzing NaBH4 can be better controlled. Herein, we report on a hydrolyzed PEO-NaBH4-based composite electrolyte that achieved an ionic conductivity of 1.6 × 10−3 S cm−1 at 45 °C and demonstrated excellent electrochemical stability against Na metal anodes. This composite electrolyte also displayed a good Na+ transference number (tNa+) of 0.54 providing the possibility to build high-energy-density batteries [25].

2. Materials and Methods

All the operations were carried out under an inert atmosphere in an Argon-filled LC-Technology glove box (<1 ppm O2 and H2O). All chemicals were purchased from Sigma-Aldrich and stored in the glove box and this includes sodium borohydride (NaBH4, 99%), PEO (Mv = 6 × 105), and diglyme (anhydrous, 99.5%). PEO was dried under a high vacuum for 24 h at 55 °C before use.

2.1. Synthesis of the Polymer Electrolyte

First, 5.3, 10.5, 21.0, and 31.5 mg of PEO were suspended in 5 mL diglyme in four separate bottles, respectively. Then 1.6, 3.5, 7.0, and 10.5 µL of Mili-Q H2O was added dropwise to the PEO suspension containing 5.3, 10.5, 21.0, and 31.5 mg of PEO, respectively. The amount of H2O added was calculated based on the amount of theoretical hydrogen bonds in PEO to precisely control the level of NaBH4 hydrolysis. After stirring the mixtures overnight at 25 °C, 105 mg of NaBH4 was added to each PEO suspension and further stirred for 6 h at 60 °C. After this, the suspensions were dried on a Schlenk line under a high vacuum at 10−3 mbar for 24 h at room temperature to lead to a dry white powder. The resulting materials were named according to the mass ratio between NaBH4 and PEO, i.e., Hy-NaBH4-5PEO refers to the mass of NaBH4:PEO = 1:0.05. Other samples were named Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO, accordingly.

2.2. Material Characterizations

The detailed measurement conditions for X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and electrochemical impedance spectroscopy (EIS) were reported in Luo et al. [20,26].

2.3. Electrochemical Measurements

All electrochemical tests were performed in air-tight Swagelok cells on the SP-300 potentiostat at 45 °C. Linear sweep voltammetry (LSV) measurement was conducted with an asymmetric Na|Hy-NaBH4-20PEO|SS cell at a scan rate of 0.1 mV s−1 from open circuit voltage (OCV) to 5 V. Na|Hy-NaBH4-20PEO|Na cells were assembled and used for the galvanostatic and transference measurement. The symmetric cells were galvanostatically cycled at different C rates between −2 V and 2 V. The polarization voltage for Na+ transference measurement was set at 10 mV with cell configuration of Na|Hy-NaBH4-20PEO|Na.

3. Results

3.1. Characterization of Hy-NaBH4-PEO Composite Electrolyte

The crystalline nature of Hy-NaBH4-PEO was analyzed by XRD (Figure 1a). All the materials displayed diffraction peaks related to α-NaBH4. The diffraction patterns of Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, and Hy-NaBH4-20PEO do not show any peaks related to crystalline PEO. However, at 30 wt% PEO, a peak at 2θ = 23° corresponding to crystalline PEO appeared. At higher PEO concentrations, additional peaks corresponding to crystalline NaBO2 were also observed, including in Hy-NaBH4-20PEO and Hy-NaBH4-30PEO, and this confirmed that higher PEO amounts led to increased NaBH4 hydrolysis. Higher amounts of PEO can bond more water molecules, which in turn facilitates the hydrolysis of NaBH4 [26]. A broadening and shift of the NaBH4 (200) peak to a lower 2θ value was also observed across all the materials (Figure 1b), and this may be due to the partial hydrolysis of NaBH4 [27].
FTIR was used to identify the amorphous phases in Hy-NaBH4-PEO as well as any chemical bonding between PEO and NaBH4 (Figure 2). In Hy-NaBH4-PEO, the typical B–H stretching modes shifted to higher wavenumbers, suggesting a strengthening of the BH bonds after PEO incorporation [28]. The absorption bands at 1602–1352 cm−1 were assigned to the asymmetric stretching modes of the BO3 unit in NaBO2 [29,30,31], confirming the partial oxidation of NaBH4 in Hy-NaBH4-PEO. A new vibration located at 937 cm−1 appeared in Hy-NaBH4-PEO, and this was assigned to a lower symmetry of the tetrahedral BH4− anion as a result of the ion-pair interactions between PEO and the BH4− anion [32]. The stretching modes at 600–800 cm−1 appearing in Hy-NaBH4-20PEO and Hy-NaBH4-30PEO also suggested that higher amounts of NaBO2 in the materials increased the level of hydrolysis [33,34].
The interaction of NaBH4 with PEO was also evidenced by XPS analysis (Figure 3). B1s binding energy of 187.1 eV corresponding to NaBH4 shifted to a lower value compared to unmodified NaBH4 (187.7 eV). The same shift of NaBH4 to lower binding energies was observed in the Na1s spectrum from 1072.1 to 1071.1 eV, and this was assigned to a weakening of the interaction between Na+ and BH4− anions. This was interpreted as the result of the formation of the complex between NaBH4 and PEO through the coordination of the ether oxygen in PEO with Na. Such an interaction may be the cause of the observed improvement in ionic conductivity observed in the composite PEO-NaBH4 material.

3.2. Decrease in PEO Crystallinity in Hy-NaBH4-PEO

In the composite electrolytes, ionic conductivity has been reported to mainly occur in the amorphous regions through local segmental motion [37]. The level of amorphous phase can be evidenced by DSC [38]. As such, the melting transition temperatures of PEO in the Hy-NaBH4-PEO composites were thus analyzed by DSC (Figure 4). In PEO, the sharp endothermic peak located at 72.1 °C correlates with the melting of PEO [38,39]. For all Hy-NaBH4-PEO materials, the melting point of PEO reduced from 72.1 °C to 42.1, 43.5, 44.4, and 45.3 °C with 5, 10, 20, and 30 wt% PEO, respectively, and this reflected an overall decrease in crystallinity of PEO [40,41]. The degree of crystallinity (χc) of the PEO in the composite electrolyte was calculated by using Equation (1) [42]:
χ c = ∆ H m ∆ H P E O × 100 %
where ∆Hm is the melting enthalpy of Hy-NaBH4-PEO and ∆HPEO = 203 J g−1 [43] is the melting enthalpy of 100% crystalline PEO (Table A8.2). At the highest PEO loading (Hy-NaBH4-30PEO), PEO crystallinity was the highest at 21.6%. However, in Hy-NaBH4-5PEO, χc decreased to 10.7%, and this suggested a significant decrease in PEO crystallinity. The latter can be attributed to the steric interactions of Na+ in NaBH4 with the ether oxygen atoms in PEO, in agreement with previous observations in the NaBF4-PEO complex [44,45,46].

3.3. Electrochemical Properties of Hy-NaBH4-PEO

The ionic conductivity of the Hy-NaBH4-PEO composites was determined by EIS measurement and compared to the ionic conductivity of unmodified NaBH4 (Figure 5a). All the materials show a significant increase in the ionic conductivity at room temperature compared to the ionic conductivity of unmodified NaBH4 (1.4 × 10−10 S cm−1). The Hy-NaBH4-PEO materials also exhibited a jump in ionic conductivity at 45 °C, and this corresponds to the melting point of PEO as evidenced by DSC analysis (Figure 4). Hy-NaBH4-20PEO exhibits the highest ionic conductivity of 4 × 10−6 S cm−1 at 25 °C, which is four times higher than unmodified NaBH4. At 45 °C, the ionic conductivity in Hy-NaBH4-20PEO increased to 1.6 × 10−3 S cm−1. It should be noted that this value is comparable to the reported ionic conductivity of the widely investigated sulfide and NASICON electrolytes [2,47,48].
A correlation between the PEO amount and ionic conductivity of Hy-NaBH4-PEO at 25 °C is shown in Figure 5b. The ionic conductivity of unmodified NaBH4 is 10−10 S cm−1, however, a significant increase in Na+ conductivity to 8 × 10−7 S cm−1 is observed after the addition of 5% PEO. The conductivity increased by increasing the PEO amount and the maximum conductivity obtained was with 20 wt% PEO. However, by increasing the amount of PEO to 30 wt%, the ionic conductivity dropped to 1 × 10−7 S cm−1. Based on the XRD and FTIR analysis, this decrease in ionic conductivity may be due to the presence of the crystalline Na2B4O7 phase. In the case of Hy-NaBH4-5PEO, the slightly lower ionic conductivity (Figure 5b) may be due to some trapping of the mobile cation due to the BH4− anion and PEO ion-pair interaction. Similar effects have been observed in PEO-NaBH4/NaBF4 electrolytes [49,50].
Owing to the high ionic conductivity of Hy-NaBH4-20PEO at near room temperature, this material was further investigated. The transference number represents the effectiveness of Na+ diffusion. A high transference number leads to a reduced concentration polarization at the electrolyte/electrode interface and thus higher power density [51,52,53]. The Na transference number (tNa+) of Hy-NaBH4-20PEO as a Na+ electrolyte was measured using the DC polarization method. The current of the symmetrical cell under the polarization potential and the AC impedance spectra of the cell before/after polarization are shown in Figure S2. tNa+ calculated from the fitting results for Hy-PEO-20NaBH4 was 0.54, which is higher than the commonly used PEO-NaTFSI solid-state electrolyte (0.2–0.4) [54]. This means that the use of Hy-PEO-20NaBH4 as a solid-state electrolyte could potentially lead to a high-energy-density battery.
To demonstrate the feasibility of incorporating Hy-NaBH4-20PEO into a Na-ion battery, the electrochemical stability window of Hy-NaBH4-20PEO was measured by linear sweep voltammetry. An oxidative current corresponding to the electrolyte decomposition was observed around 3V (Figure 5c). This corresponds to an electrochemical stability window of 3 V, and, therefore, cathode materials such as NaFePO4 could be used with Hy-NaBH4-20PEO as the electrolyte [55].
To evaluate the long-term interface stability of Hy-NaBH4-20PEO against Na metal. The symmetric cell was tested by galvanostatic cycling with the current density of 41 μA cm−2 at 45 °C. In the first cycle, the cell showed Na+ plating/stripping with a voltage of 0.1 V/−0.1 V, and a gradual increase in overpotential is observed in the following Na plating/stripping. The cell shows a stable voltage (≈0.3 V) after the seventh plating. The gradual increase in overpotential during the initial cycling (Figure 6a) may be due to the partial decomposition of BH4− at the Na electrode interface, as reported in LiBH4 electrolytes [56,57]. Different current densities from 0.1 mA cm−2 to 0.2 mA cm−2 (20 min plating/20 min stripping) were applied to the galvanostatic cycling of the symmetric cell. The results (Figure 6b) show that the cell with the Hy-NaBH4-20PEO electrolyte had reversible Na plating/stripping with a low overpotential (0.01 and 0.04 V, respectively), indicating that the electrolyte is compatible with Na metal anodes at a current density of 0.2 mA cm−2.

4. Conclusions

In this paper, we reported a Hy-NaBH4-PEO composite electrolyte with an ionic conductivity of 1.6 × 10−3 S cm−2 at 45 °C, with a high transference number (0.54) and high electrochemical stability toward stable Na plating/stripping. By adding the PEO matrix, the level of NaBH4 self-hydrolysis could be controlled by adjusting the PEO amount. Higher amounts of PEO increased the level of H2O available to hydrolyze NaBH4. In this Hy-NaBH4-PEO composite electrolyte, the level of amorphous PEO is also believed to facilitate Na+ mobility. Understanding the conduction mechanism of Hy-NaBH4-PEO will help further increase the rate of Na+ ionic conductivity at room temperature.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/batteries10090316/s1. References [58,59,60,61,62,63] are cited in the Supplementary Materials.

Author Contributions

X.L.: investigation, writing—original draft preparation, K.-F.A.-Z.: writing—review and editing, and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) X-ray diffraction XRD patterns of Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO. Unmodified NaBH4 and PEO are shown as references. (b) Enlarged view of the (200) peak for Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO.
Figure 1. (a) X-ray diffraction XRD patterns of Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO. Unmodified NaBH4 and PEO are shown as references. (b) Enlarged view of the (200) peak for Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO.
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Figure 2. Fourier-transform infrared of Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO. Unmodified NaBH4 and PEO are shown as references.
Figure 2. Fourier-transform infrared of Hy-NaBH4-5PEO, Hy-NaBH4-10PEO, Hy-NaBH4-20PEO, and Hy-NaBH4-30PEO. Unmodified NaBH4 and PEO are shown as references.
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Figure 3. XPS of (a) B1s (b) Na1s of Hy-NaBH4-20PEO and unmodified NaBH4. The peak at 191.2 eV was assigned to the boron oxide phase [35,36].
Figure 3. XPS of (a) B1s (b) Na1s of Hy-NaBH4-20PEO and unmodified NaBH4. The peak at 191.2 eV was assigned to the boron oxide phase [35,36].
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Figure 4. DSC of the Hy-NaBH4-PEO composite electrolyte.
Figure 4. DSC of the Hy-NaBH4-PEO composite electrolyte.
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Figure 5. (a) Temperature dependence of the ionic conductivities of the Hy-NaBH4-PEO materials and unmodified NaBH4 shown as a reference; (b) correlation between the ionic conductivity of Hy-NaBH4-PEO with respect to the PEO amount; (c) linear sweep voltammetry (LSV) curves from open circuit voltage to 5 V with a scan rate of 0.1 mV s−1 for Hy-NaBH4-20PEO.
Figure 5. (a) Temperature dependence of the ionic conductivities of the Hy-NaBH4-PEO materials and unmodified NaBH4 shown as a reference; (b) correlation between the ionic conductivity of Hy-NaBH4-PEO with respect to the PEO amount; (c) linear sweep voltammetry (LSV) curves from open circuit voltage to 5 V with a scan rate of 0.1 mV s−1 for Hy-NaBH4-20PEO.
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Figure 6. Voltage profiles during Na plating/stripping processes in (a) Na|Na symmetric cell with Hy-NaBH4-20PEO as the electrolyte at a current density of 41 μA cm−2 at 45 °C, and (b) symmetric Na|Hy-NaBH4-20PEO|Na cell cycled at various current densities of 0.1 and 0.2 mA cm−2 at 45 °C.
Figure 6. Voltage profiles during Na plating/stripping processes in (a) Na|Na symmetric cell with Hy-NaBH4-20PEO as the electrolyte at a current density of 41 μA cm−2 at 45 °C, and (b) symmetric Na|Hy-NaBH4-20PEO|Na cell cycled at various current densities of 0.1 and 0.2 mA cm−2 at 45 °C.
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Luo, X.; Aguey-Zinsou, K.-F. NaBH4-Poly(Ethylene Oxide) Composite Electrolyte for All-Solid-State Na-Ion Batteries. Batteries 2024, 10, 316. https://doi.org/10.3390/batteries10090316

AMA Style

Luo X, Aguey-Zinsou K-F. NaBH4-Poly(Ethylene Oxide) Composite Electrolyte for All-Solid-State Na-Ion Batteries. Batteries. 2024; 10(9):316. https://doi.org/10.3390/batteries10090316

Chicago/Turabian Style

Luo, Xiaoxuan, and Kondo-Francois Aguey-Zinsou. 2024. "NaBH4-Poly(Ethylene Oxide) Composite Electrolyte for All-Solid-State Na-Ion Batteries" Batteries 10, no. 9: 316. https://doi.org/10.3390/batteries10090316

APA Style

Luo, X., & Aguey-Zinsou, K. -F. (2024). NaBH4-Poly(Ethylene Oxide) Composite Electrolyte for All-Solid-State Na-Ion Batteries. Batteries, 10(9), 316. https://doi.org/10.3390/batteries10090316

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