Preparation and characterization of Na₃BO₃–Na₂SO₄ glass electrolytes with Na⁺ ion conductivity prepared by a mechanical milling technique

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

Abstract

The (100 − x)Na₃BO₃·xNa₂SO₄ (0 ≤ x (mol%) ≤ 50) glasses were prepared by mechanical milling. Halo patterns were observed in the compositions 0 ≤ x ≤ 50 by XRD measurements. The Raman spectra indicated that all the glasses were composed of BO₃³⁻ anions, SO₄²⁻ anions and Na⁺ cations. The (100 − x)Na₃BO₃·xNa₂SO₄ glasses showed good deformation properties and a dense pellet was prepared by cold-press. The conductivities of the glasses increased with increasing Na₂SO₄ content, and the 50Na₃BO₃·50Na₂SO₄ glass showed the highest conductivity of 5.9 × 10⁻⁸ S cm⁻¹ at 25 °C.

Keywords:

• Glass
• Solid electrolyte
• Sodium ion conductor
• Mechanical milling

1 Introduction

Recently, the diversification of portable electronics requires rechargeable energy sources that are safe, economical and environmental friendly. Lithium-ion batteries are known as one of the types of rechargeable batteries with the highest energy density [1] for portable electronic device applications. However, a growing number of fire accidents involving cell phones and electric vehicles are raising questions regarding the safety of lithium-ion batteries using flammable organic liquid electrolytes. Furthermore, the cost of lithium-based batteries is high because lithium is a rare material. All-solid-state sodium secondary batteries are one of the solutions to these issues. The batteries are expected to remove safety hazards because of nonflammable inorganic solid electrolytes [1–3]. Sodium secondary batteries can be low-cost since the sodium element is abundant and ubiquitous [4]. As one example of sodium batteries, sodium-sulfur batteries, which employ β-alumina as the solid electrolyte, have been used for large-scale energy storage because of their high theoretical specific energy densities of over 760 Wh kg⁻¹ [5]. Oxide-based solid electrolytes represented by the β-alumina and NASICON show high conductivities of over 10⁻³ S cm⁻¹ at room temperature [6]. The preparation process of these electrolytes includes high-temperature sintering to reduce the resistance of grain boundaries. However, this process sometimes causes chemical reactions between electrolytes and electrodes, which generate large interfacial resistance [7]. In order to solve this issue, the preparation process for electrode-electrolyte interfaces without high-temperature sintering is desirable. Materials with low-melting temperatures, possessing a small cohesive energy, have excellent formability achieving good solid–solid contacts by pressing at room temperature. Good formability is important to make appropriate conduction paths of carriers between electrodes and electrolytes as well as to reduce the resistance of grain boundaries [8]. It is considered that the conductivity of glassy materials is usually higher than that of corresponding crystalline ones [9,10]. Therefore, glassy materials with low-melting temperatures are very attractive. Moreover, the conductivity of Li⁺ ion conducting glasses is known to increase with increasing Li⁺ ion content [11,12]. The glasses are thus required to contain a large amount of Li⁺ ions [13] and thus ortho-oxosalt compositions are expected to show high conductivities. It is difficult to fabricate ortho-oxosalt compositions by melt quenching. In the systems Li₂O–M_xO_y (M = B, Si, P, Ge or Al), the glass forming region by mechanical milling is wider than that by rapid quenching [14]. Glasses at ortho-oxosalt compositions such as Li₃BO₃ and Li₄SiO₄ can be prepared by mechanical milling. In addition, pseudobinary systems show “mixed-anion effect” by which the conductivity shows the highest value at a certain composition [15]. We discovered a series of Li⁺ ion conducting glasses in lithium ortho-oxosalt pseudobinary systems with high Li⁺ ion concentration and “mixed-anion effect” such as Li₄SiO₄–Li₃BO₃ and Li₃BO₃–Li₂SO₄ glasses [7,16]. The conductivity of the Li₃BO₃–Li₂SO₄ glasses is nearly 10⁻⁶ S cm⁻¹ at room temperature, which is relatively high among lithium oxide glasses. Moreover, the glasses show good formability only by cold-press. However, sodium ortho-oxosalt pseudobinary glasses have not been studied.

In this study, glasses in the system Na₃BO₃–Na₂SO₄ were prepared by mechanical milling. The effects of the addition of Na₂SO₄ to Na₃BO₃ on conductivity and structure of the glasses were examined.

2 Experimental

Starting materials were NaOH (99.99%; Aldrich Chem.), H₃BO₃ (99.5%; Wako Chem.), and Na₂SO₄ (99.9%; Wako Chem.). A mixture of NaOH and H₃BO₃ was heated in an Al₂O₃ crucible at 400 °C for 4 h and at 450 °C for 2 h. After each heat treatment, the sintered samples were ground and Na₃BO₃ crystal powders were prepared. The Na₃BO₃–Na₂SO₄ glasses were prepared by mechanical milling (MM) using a planetary ball mill apparatus (Fritsch Pulverisette 7). A mixture of the crystals of Na₃BO₃ and Na₂SO₄ was put into a 45 ml ZrO₂ pot with 160 ZrO₂ balls (5 mm in diameter) and milled at the rotating speed of 370 rpm with the milling period of 15–125 h. All processes were conducted in a dry Ar atmosphere.

X-ray diffraction (XRD) measurements (Cu Kα) were conducted using a diffractometer (Ultima IV; Rigaku) to identify crystal phases of the milled samples. Raman spectra of the samples were measured using a Raman spectrometer (LabRAM HR-800; Horiba) with 325 nm He–Cd laser. Differential thermal analyses (DTA) were performed using a thermal analyzer (Thermo-plus 8120; Rigaku) to determine their glass transition and crystallization temperatures. The milled samples were sealed in Al pans in an Ar-filled glove box and heated at 10 °C min⁻¹ under N₂ gas flow up to 550 °C. Ionic conductivities were measured for the pelletized samples pressed under 720 MPa at room temperature. The diameter and thickness of the pellets were 10 mm and about 0.8 mm, respectively. Gold electrodes were formed by sputtering with a quick coater (SC-701; Sanyu Electron) on both surfaces of the pellets. Then stainless steel disks were attached to the pellets as current collectors. The prepared cells were sealed in a silica glass tube in an Ar-filled globe box. Then AC impedance measurements were conducted under dry Ar gas flow using an impedance analyzer (SI-1260; Solartron). The frequency range and applied voltage were respectively 0.1 Hz to 8 MHz and 100 mV. The density of the powder-compressed pellets (d₁) was calculated from the weight and volume of the pellets and the density of the powders (d₂) was measured using a gas pycnometer (AccuPyc II 1340; Shimadzu). The relative density was defined as d₁/d₂. The micro-structure of a cross-section of the pellets was observed by scanning electron microscope (SEM) (JSM-6610A; JEOL).

3 Results and discussion

The XRD patterns of the mixture of starting materials and milled samples at the composition 50Na₃BO₃·50Na₂SO₄ (mol%) are shown in Fig. 1. The pattern of the mixture showed the peaks attributed to Na₃BO₃ and Na₂SO₄ crystals. After the sample was milled for 20 h, a halo pattern was mainly observed and weak peaks attributable to Na₂SO₄ crystal remained. The pattern of the sample milled for 85 h showed halo pattern, indicating that amorphous sample was prepared.

Fig. 1 XRD patterns of 50Na₃BO₃·50Na₂SO₄ samples milled for 0, 20 and 85 h.

The XRD patterns of the (100 − x)Na₃BO₃·xNa₂SO₄ samples after mechanical milling are shown in Fig. 2. Halo patterns were observed in the range from x = 0 to 50. Small peaks attributed to Na₂SO₄ crystal remained in the pattern of the sample of x = 67. Amorphous samples were prepared at the compositions 0 ≤ x ≤ 50.

Fig. 2 XRD patterns of the (100 − x)Na₃BO₃·xNa₂SO₄ samples prepared by mechanical milling.

DTA curves of the amorphous samples are shown in Fig. 3(a). An enlarged DTA curve for the x = 10 sample is also shown in Fig. 3(b). An endothermic behavior attributable to glass transition was observed at 200–240 °C for the samples of x = 0 and x = 10, suggesting that the obtained amorphous samples are glasses. For the samples of x = 20, 33, and 50, glass transition behaviors were not clearly observed although all the samples showed exothermic peaks attributable to crystallization. The crystallization temperatures (T_c) shifted to the lower temperature side with increasing Na₂SO₄ content. Glass transition temperatures (T_g) probably shifted to the lower temperature side along with the shift of T_c, suggesting that Na₂SO₄ crystal was incorporated into the glass matrix. Endothermic peaks were observed at 520–550 °C for the glasses of x = 20, 33 and 50. All these samples were found to melt after the DTA measurement, indicating that these peaks are attributed to the temperatures at which melting began. It is noted that the initial melting temperatures decreased by the addition of Na₂SO₄.

Fig. 3 (a) DTA curves of the (100 − x)Na₃BO₃·xNa₂SO₄ samples prepared by mechanical milling and (b) an enlarged DTA curve of the 90Na₃BO₃·10Na₂SO₄ sample.

The Raman spectra of the (100 − x)Na₃BO₃·xNa₂SO₄ glasses are shown in Fig. 4. The bands at about 890 cm⁻¹ and 990 cm⁻¹ are attributable to BO₃³⁻ and SO₄²⁻ units, respectively [17,18]. No other peaks were observed except for those of the BO₃³⁻ and SO₄²⁻ units. Therefore, ionic glasses, which are composed of isolated BO₃³⁻ and SO₄²⁻ anions and Na⁺ cation, were prepared by mechanical milling.

Fig. 4 Raman spectra of the (100 − x)Na₃BO₃·xNa₂SO₄ glasses.

Cross-sectional SEM images of powder-compressed pellets are shown in Fig. 5. Densities of powder-compressed pellets (d₁), powders (d₂), and relative density (d₁/d₂) of the (100 − x)Na₃BO₃·xNa₂SO₄ glasses are listed in Table 1. Grain boundaries were clearly observed in the SEM image of the glass of x = 0. On the other hand, the SEM image of the glass of x = 50 showed less grain boundaries, suggesting that formability of the pellets improved with increasing Na₂SO₄ content. As shown in Fig. 3(a), melting temperatures decreased with an addition of Na₂SO₄. A material with lower melting temperature tends to have less cohesive energy [7], and thus the glasses showed better formability with the Na₂SO₄ content.

Table 1 Densities of pellets (d₁), powders (d₂) and relative density (d₁/d₂) of (100 − x)Na₃BO₃·xNa₂SO₄ glasses.

	d1/g cm^−3	d2/g cm^−3	d1/d2/%
x = 0	2.12	2.41	88
x = 10	2.27	2.41	94
x = 20	2.28	2.42	94
x = 33	2.41	2.43	99
x = 50	2.42	2.45	99

d₁: calculated from the volume measured by dimension of the pellet.

d₂: calculated from the volume measured by a gas pycnometer.

Fig. 5 Cross-sectional SEM images of the (100 − x)Na₃BO₃·xNa₂SO₄ glasses.

Nyquist plots for the pelletized glass of x = 50 at temperatures of 53, 67, and 80 °C are shown in Fig. 6. Semicircles and spikes were observed in Nyquist plots, and similar plots were obtained for all the compositions and temperatures. With an increase in the measuring temperature, the arcs diminished. The conductivities were calculated from the resistance of semicircle including bulk and grain boundary components.

Fig. 6 Nyquist plots of the 50Na₃BO₃·50Na₂SO₄ glass at 53, 67 and 80 °C.

Temperature dependence of conductivity for the pelletized glasses of x = 0 and x = 50 is shown in Fig. 7. The conductivity for the glasses increased with increasing temperature and its temperature dependence obeyed the Arrhenius relation, σ = σ₀ exp(−E_a/kT), where σ₀ and E_a are respectively pre-exponential factor and activation energy for conduction. The conductivity at room temperature and the activation energy for all the glasses are summarized in Table 2. The glass of x = 50 showed the highest conductivity of 5.9 × 10⁻⁸ S cm⁻¹ at room temperature and the lowest activation energy of 47 kJ mol⁻¹. As shown in Table 2, the conductivity (σ₂₅) increased and the activation energy decreased monotonously with increasing Na₂SO₄ content. There are two reasons why the conductivity of the glasses increased and activation energy decreased with increasing Na₂SO₄ content. First, the resistance of grain boundary decreased because of better formability of the glasses with increasing Na₂SO₄ content. Second, the bulk resistance decreased by so-called “mixed-anion effect.” Glasses in the Na₃BO₃–Na₂SO₄, pseudobinary system showed good formability with the conductivity of over 10⁻⁸ S cm⁻¹.

Table 2 Conductivity and activation energy of (100 − x)Na₃BO₃·xNa₂SO₄ (mol%) glasses.

	σ25 (10^−8 S cm^−1)	Ea (kJ mol^−1)
x = 0	1.5	61
x = 10	2.7	59
x = 20	3.1	52
x = 33	3.1	51
x = 50	5.9	47

Fig. 7 Temperature dependence of conductivities for the glasses of Na₃BO₃ and 50Na₃BO₃·50Na₂SO₄.

4 Conclusions

The (100 − x)Na₃BO₃·xNa₂SO₄ glasses were prepared by mechanical milling. In the XRD patterns, halo patterns were observed in the range of 0 ≤ x ≤ 50. The glasses added with Na₂SO₄ exhibited higher conductivities than the mother Na₃BO₃ glass and the conductivities of the glasses increased with increasing Na₂SO₄ content. The glass at the composition of x = 50 showed the highest conductivity of 5.9 × 10⁻⁸ S cm⁻¹ at room temperature. Good deformation character was also achieved in the glass electrolyte.

Notes

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.