Bi₂O₃-Assisted Sintering of Na₃Zr₂Si₂PO₁₂ Electrolyte for Solid-State Sodium Metal Batteries

Abstract

Solid-state sodium metal batteries using non-flammable solid-state electrolytes are recognized as next-generation energy storage technology in view of their merits of high safety and low cost. However, the lower ion conductivity (below the application requirements of 10⁻³ S cm⁻¹) and interface issues that exist in electrolytes/electrodes for most solid-state electrolytes hinder their practical application. In this paper, NASICON-type Na₃Zr₂Si₂PO₁₂ (NZSP) electrolytes with enhanced ion conductivity are synthesized by the Bi₂O₃-assisted sintering method. The influence of the Bi₂O₃ sintering agent content on the crystalline phase, microstructure, density and ion conductivity as well as the electrochemical performances applied in batteries for the obtained NZSP electrolytes are investigated in detail. With the presence of Bi₂O₃, the formed Na₃Bi(PO₄)₂ impurity increased the Si/P ratio in the NASICON structure with higher Na⁺ occupancy, then enhanced the ionic conductivity to a level of 1.27 × 10⁻³ S cm⁻¹. Unfortunately, the Bi₂O₃-assisted sintered NZSP shows a degradation in the cycling stability when applied to solid-state sodium batteries because of the decreased interfacial stability with Na anodes. The formation of a Bi-Na alloy during cycling might be conducive to Na dendrite growth in electrolytes, degrading the cycling performance. This work presents a facial method to improve the ion conductivity of NASICON-type electrolytes and gives insight into the interface issues of solid-state sodium metal batteries.

1. Introduction

Sodium-ion batteries are recognized as promising next-generation energy storage technology due to their merits of low cost and an abundance of sodium resources compared to traditional lithium ion batteries [1,2]. Similar to conventional lithium-ion batteries, sodium-ion batteries use organic liquid electrolytes to show potential safety hazards such as combustion and explosion. Alternatively, solid-state sodium metal batteries employing solid-state electrolytes and sodium metal anodes can effectively solute the safety issues originating from organic liquid electrolytes and highly improve the energy density of devices [3,4]. In view of their excellent structural and chemical stability, high ionic conductivity (>10⁻⁴ S cm⁻¹) and wide electrochemical window (>5 V), NASICON-type solid electrolytes have received abundant research attention in recent years [5,6]. As a typical NASICON structure, Na₃Zr₂Si₂PO₁₂ (NZSP) was firstly exploited in 1976 by Hong and Goodenough and reported widely owing to its relative higher ionic conductivity [7,8]. Nevertheless, the ionic conductivity of NZSP electrolyte is still lower than that of the liquid electrolyte organic (10⁻³–10⁻² S cm⁻¹) [9]. In addition, the poor contact between rigid NZSP electrolytes and electrodes results in large interfacial impedance and the growth of dendrites as well as other interfacial issues. Therefore, improving the ionic conductivity of NZSP electrolytes and effectively solving the interface problem are the top priorities for the practical application of NZSP in solid-state metal sodium batteries.

The ionic conductivity of a NASICON electrolyte is determined by several factors, such as the number and distribution of Na ions, phase structure as well as the relative density. The most commonly used strategy to improve the ion conductivity of a NZSP electrolyte is the partial substitution of Zr⁴⁺ in a Na₃Zr₂Si₂PO₁₂ structure by variable heterogeneous ions [10]. For example, Lu et al. reported that the mobility of Na⁺ at the Na3 site could be promoted by doping 5 at% Ca²⁺, leading to an enhanced ionic conductivity of 1.67 × 10⁻³ S cm⁻¹ [11]. Yang et al. employed Mg²⁺ as the substitution ion for Zr⁴⁺ by the means of non-equivalent substitution to generate a deficiency in positive charge and then to increase the concentration of Na⁺, resulting in higher ion conductivity at 1.33 × 10⁻³ S cm⁻¹ [12]. Zhang et al. introduced La³⁺ into a Zr⁴⁺ site to form a new Na₃La(PO₄)₂ phase, which not only improved the Na⁺ concentration in the Na₃Zr₂Si₂PO₁₂ (3.4 × 10⁻³ S cm⁻¹) but also increased the relative density [13]. Miao R et al. reported the influence of a Bi₂O₃ additive on the electrochemical performance of Na_3.1Y_0.1Zr_1.9Si₂PO₁₂ inorganic solid electrolytes [14]. We summarize the ionic conductivities of NASICON electrolytes with different ionic dopants in

Table 1. The ionic conductivities of previously reported NASICON electrolytes with different dopants at room temperature.

Components	Ionic Conductivity (S cm−1)	Reference
Na3Zr1.9Ca0.1Si2PO12	1.67 × 10−3	[11]
Na3Zr1.7La0.3Si2PO12	3.4 × 10−3	[13]
Na3.1Y0.1Zr1.9Si2PO12 + 1 wt.% Bi2O3	1.21 × 10−3	[14]
Na3Zr1.9Nb0.1Si2PO12	2.11 × 10−4	[15]
Na3Zr1.9Ti0.1Si2PO12	5.97 × 10−4	[15]
Na3Zr1.8Zn0.2Si2PO12	2.50 × 10−3	[15]
Na3Zr1.9Yb0.1Si2PO12	1.7 × 10−4	[16]
Na3Zr1.9Gd0.1Si2PO12	6.0 × 10−4	[16]
Na3Zr1.9Ce0.1Si2PO12	9.0 × 10−4	[16]
Na3Zr1.9Al0.1Si2PO12	4.39 × 10−4	[17]
Na3Zr1.9Fe0.1Si2PO12	7.53 × 10−4	[17]
Na3Zr1.9Co0.1Si2PO12	1.55 × 10−4	[17]
Na3Zr1.9Ni0.1Si2PO12	6.18 × 10−4	[17]
Na3Zr1.9Ga0.1Si2PO12	1.06 × 10−3	[18]
Na3.4Zr1.6Sc0.4Si2PO12	4.0 × 10−3	[19]
Na3.1Zr1.95Mg0.05Si2PO12	3.5 × 10−3	[20]
Na3.2Zr1.8Sm0.2Si2PO12	1.36 × 10−3	[21]
Na3.3Zr1.7Ho0.3Si2PO12	1.07 × 10−3	[21]
Na3.33Ce0.02Sc0.33Zr1.65Si2PO12	2.44 × 10−3	[22]
Na2.9K0.1Zr2Si2PO12	7.734 × 10−4	[23]
Na3.4Zr2Si2.4P0.6O12	5.5 × 10−3	[24]
Na3.4Zr1.9Mg0.1Si2.2P0.8O12	3.6 × 10−3	[25]
Na3Zr2Si2PO12 + 5 wt.% Na2SiO3	1.45 × 10−3	[26]
Na3.2Zr2Si2.2P0.8O12 + 0.5 NaF	3.6 × 10−3	[27]

.

In this study, the Bi₂O₃ with a low melting point (825 °C) was introduced into the sintering process of a NASICON (Na₃Zr₂Si₂PO₁₂) solid electrolyte so as to create liquid flux at high temperatures and improve the relative density of the electrolyte pellet. After the introduction of Bi₂O₃, the new phase of Na₃Bi(PO₄)₂ was generated at the grain boundary, which altered the Si/P ratio of Na₃Zr₂Si₂PO₁₂ phase and further enhanced the Na⁺ occupancy. The optimized ionic conductivity of Na₃Zr₂Si₂PO₁₂-Bi is 1.27 × 10⁻³ S cm⁻¹, which is much higher than that of pure Na₃Zr₂Si₂PO₁₂ (2.84 × 10⁻⁴ S cm⁻¹). However, the direct contact of Na with Na₃Zr₂Si₂PO₁₂-Bi triggers the formation of a Na_xBi alloy at the metal/electrolyte interface, which accelerates the dendrite growth of Na inside the electrolyte pellets. Such Na penetration into the electrolyte prompts the decreased electrochemical performance of the symmetrical battery and the solid sodium metal full battery. We believe the electrochemical performance of Na₃Zr₂Si₂PO₁₂-Bi-based solid sodium metal batteries can be further enhanced by a physical obstruction between Na metal and Na₃Zr₂Si₂PO₁₂-Bi electrolyte.

2. Materials and Methods

The Na₃Zr₂Si₂PO₁₂-xBi (Na_3+xZr_2-xM_xSi₂PO₁₂, x = 0, 0.1, 0.2, 0.3 and 0.4) was synthesized by conventional high-temperature solid-state reaction. Stoichiometric amounts of Na₂CO₃, NH₄H₂PO₄, ZrO₂, SiO₂ and Bi₂O₃ were mixed by high-energy ball milling for 1 h at a rotating speed of 1400 rpm and then calcinated at 1100 °C for 12 h. The 15%-excessive Na₂CO₃ and NH₄H₂PO₄ were added to compensate for the volatilization of Na and P. After ball milling, the obtained mixture was further ball-milled for 1 h. The resultant dry powder was sieved through a 200-mesh sieve, then mixed with PVA binder and pressed into green pellets with a diameter of 16 mm and a thickness of 1 mm. The green pellets were pre-calcinated at 650 °C for 2 h to remove the PVA binder and finally calcinated at 1200 °C for 12 h to obtain dense electrolyte pellets.

The phase composition of Na₃Zr₂Si₂PO₁₂-xBi was confirmed by X-ray diffractometer (ARL Equinox 3000, Thermo Fisher, MA, USA). The microstructure was observed by scanning electron microscope (Quanta FEG, FEI, NY, USA) equipped with energy dispersive X-ray spectroscopy. The relative density of electrolyte pellets was measured by electronic densitometer (YD-100ES, YiEnDa, China) based on the Archimedes principle. The ionic and electronic conductivities were tested on electrochemical station (CHI 760E). The blocking electrode was constructed by sputtering Au on two sides of electrolyte pellets. The ionic conductivity was determined by electrochemical impedance spectroscopy (EIS, CHI 760E, ChenHua, Shanghai, China) technique ranging from 100 kHz to 0.1 Hz with an amplitude of 5 mV under open circuit potential. The electronic conductivity was determined by the DC polarization method at a constant voltage of 0.1–1.8 V for 1800 s at room temperature.

Sodium foils with a diameter of 8 mm were attached to both sides of the polished electrolyte sheet and then repetitively rolled to modify the interfacial contact. The Na/Na₃Zr₂Si₂PO₁₂/Na and Na/Na₃Zr₂Si₂PO₁₂-xBi/Na symmetric batteries were assembled into CR2032 coin cell to investigate the Na plating/stripping behaviors in an argon-filled glove box at room temperature. Critical current measurements were performed by the galvanostatic cycling at current densities stepping from 0.1–0.6 mA cm⁻² (step: 0.05 mA cm⁻², time: 0.5 h) at room temperature. The CR2032 full battery was assembled using sodium foil as the anode, NZSP pellet as the electrolyte and NVP as the cathode material in an argon-filled glove box at room temperature. Commercial Na₃V₂(PO₄)₃ (Hubei Energy Technology Co., Ltd.), acetylene black and poly(vinyldifluoride) (PVDF) with a weight ratio of 8:1:1 were mixed in N-methyl pyrolidone (NMP) to form a homogeneous slurry, ergo coated on aluminum foil by doctor blade and finally dried at 120 °C for 24 h in vacuum oven. The mass loading of NVP was ~1 mg cm⁻². Ten μL of liquid electrolytes (1 M NaClO₄ in EC/DMC (1:1) + 5% FEC) was added between cathode and electrolyte pellet to establish wetted interface.

3. Results and Discussion

Figure 1a displays the XRD pattern of solid electrolytes of Na₃Zr₂Si₂PO₁₂, which is sintered at 1200 °C. The diffraction peaks of Na₃Zr₂Si₂PO₁₂ can be owned as NASICON structures (PDF #84-1200), as shown in Figure 1b. A small amount of ZrO₂ impurity can be found because of the excessive volatilization of Na and P during the high-temperature sintering process [13], while for Na₃Zr₂Si₂PO₁₂-xBi (x = 0, 0.1, 0.2, 0.3, and 0.4) samples, it is obvious that the Bi₂O₃ addition can facilitate the crystallization of Na₃Zr₂Si₂PO₁₂ and decrease the formation of ZrO₂ impurities. With enhanced Bi₂O₃ contents, especially for x > 0.3, the intensity of the ZrO₂ impurities gradually enhances. Moreover, the introduction of Bi₂O₃ induces the generation of the Na₃Bi(PO₄)₂ phase, as confirmed by the characteristic peaks located at 21.5°, 25.5°, 32.2° and 33.6° (PDF #81-0336). Because the formation of Na₃Bi(PO₄)₂ phase is cause by the solid-state reaction of Bi₂O₃, Na₂CO₃ and NH₄H₂PO₄, the generation of the Na₃Bi(PO₄)₂ phase can increase the Si/P ratio of the NASICON structure and enhance the Na⁺ occupancy, which may contribute to enhanced ionic conductivity. To investigate the origin of Na₃Bi(PO₄)₂ formation, the solid-state reaction of Na₂CO₃, NH₄H₂PO₄ and Bi₂O₃ was proceeded at 800 °C. As shown in Figure 1c, the XRD pattern can be indexed as the pure Na₃Bi(PO₄)₂ phase (PDF #81-0036) [28]. Therefore, before the phase formation of Na₃Zr₂Si₂PO₁₂, parts of Bi₂O₃ were converted into Na₃Bi(PO₄)₂, and the remaining Bi₂O₃ melted at a high temperature and was substituted for the Zr⁴⁺ site of Na₃Zr₂Si₂PO₁₂.

According to the Archimedes principle, the relative densities of Na₃Zr₂Si₂PO₁₂-xBi (x = 0, 0.1, 0.2, 0.3, 0.4) are 2.89, 3.07, 3.10, 3.08 and 3.02 g cm⁻³, corresponding to the relative densities of 89.2, 94.8, 95.7, 95.1 and 93.2 %, respectively (Table 1). To investigate the effects of the Bi₂O₃ addition on the morphology of Na₃Zr₂Si₂PO₁₂ electrolytes, scanning electron microscopy (SEM) has been performed on the Na₃Zr₂Si₂PO₁₂-xBi (x = 0, 0.1, 0.2, 0.3, 0.4) samples, as shown in Figure 2. The grain size of the initial Na₃Zr₂Si₂PO₁₂ sample is large and variable, and there are plenty of holes between the grains. The relative density of a Na₃Zr₂Si₂PO₁₂ pellet is 89.2 %. When x = 0.1 and 0.2, the grain size of Na₃Zr₂Si₂PO₁₂ particles becomes smaller and uniform, and there are no obvious gaps or holes observed. The highest relative density for a Na₃Zr₂Si₂PO₁₂-0.2Bi sample is 95.7%. As is known, Bi₂O₃ is a typical low melting point oxide (825 °C), which forms liquid flux at high temperatures and bonds the grain together, enabling enhanced crystallization and densification rates during sintering [28]. When x = 0.3 and 0.4, the grain size of Na₃Zr₂Si₂PO₁₂ particles further decreases, and thereupon, a small number of gaps and holes reappear between the grains, which weakens the density.

Figure 3a displays the EIS profiles of Na₃Zr₂Si₂PO₁₂-xBi (x = 0, 0.1, 0.2, 0.3, 0.4) at room temperature. The EIS curve consists of a semicircle in the high-frequency region and an inclined line in the low-frequency region [28]. The intersection of the semicircle’s destination and the inclined line (Z’-intercept) is the total resistance of an electrolyte pellet. After Bi₂O₃ introduction, the total resistance and the radius of the semicircle in the high-frequency region decrease obviously, demonstrating a decreased total resistance. Moreover, the total resistance is composed of grain resistance (R_g) and grain boundary resistance (R_gb) [29]. The R_g and R_gb of Na₃Zr₂Si₂PO₁₂-xBi samples are obtained by fitting the EIS curve (Figure 3b), and the corresponding results have been summarized in

Table 2. Density and conductivity of Na₃Zr₂Si₂PO₁₂-xBi samples (x = 0, 0.1, 0.2, 0.3 and 0.4) at room temperature.

NZSP-xBi	ρ (g cm−3)	Rg (Ω)	Rgb (Ω)	σb (S cm−1)	σgb (S cm−1)	σtotal (S cm−1)
x = 0	2.89	95.0	146.0	7.20 × 10−4	4.69 × 10−4	2.84 × 10−4
x = 0.1	3.07	40.0	46.0	1.95 × 10−3	1.69 × 10−3	9.04 × 10−4
x = 0.2	3.10	38.4	23.7	2.05 × 10−3	3.32 × 10−3	1.27 × 10−3
x = 0.3	3.08	42.5	44.5	2.03 × 10−3	1.94 × 10−3	9.92 × 10−4
x = 0.4	3.02	50.3	54.7	1.45 × 10−3	1.33 × 10−3	6.92 × 10−4

.

The total ionic conductivities of Na₃Zr₂Si₂PO₁₂-xBi (x = 0, 0.1, 0.2, 0.3, 0.4) are 2.84 × 10⁻⁴, 9.04 × 10⁻⁴, 1.27 × 10⁻³, 9.92 × 10⁻⁴ and 6.92 × 10⁻⁴, respectively. When x ≤ 0.2, the grain boundary conductivity grows significantly faster than the grain conductivity. When x = 0.2, the grain boundary conductivity (σ_gb = 3.32 × 10⁻³ S cm⁻¹) exceeds the grain conductivity (σ_b = 2.05 × 10⁻³ S cm⁻¹). The increasing of the grain conductivity originates from the enhanced density and the generation of Na₃Bi(PO₄)₂ phase, which increases the Si/P ratio and Na⁺ occupancy of Na₃Zr₂Si₂PO₁₂ [30,31]. The increasing of the grain boundary conductivity is attributed to the reduction of gaps and pores between grains. when x ≥ 0.2, σ_g and σ_gb simultaneously decrease, and the σ_gb is smaller than σ_g. The reason why σ_gb is smaller than σ_g is the formation of insulating ZrO₂ impurity, which hinders the Na⁺ transport between Na₃Zr₂Si₂PO₁₂ grains [32]. Figure 3c shows the i-t curves of Na₃Zr₂Si₂PO₁₂ and Na₃Zr₂Si₂PO₁₂-0.2Bi pellets under a fixed voltage of 1 V. The measured steady-state currents are 8.48 × 10⁻⁷ A for Na₃Zr₂Si₂PO₁₂ and 7.35 × 10⁻⁷ A for Na₃Zr₂Si₂PO₁₂-0.2Bi. Correspondingly, the electronic conductivities at room temperature are calculated to be 6.67 × 10⁻⁸ S cm⁻¹ for Na₃Zr₂Si₂PO₁₂ and 5.23 × 10⁻⁸ S cm⁻¹ for Na₃Zr₂Si₂PO₁₂-0.2Bi.

To investigate the effects of the Bi₂O₃ addition on the Na⁺ transport carrier of Na₃Zr₂Si₂PO₁₂ electrolytes, the EIS plots of Na₃Zr₂Si₂PO₁₂ and Na₃Zr₂Si₂PO₁₂-0.2Bi at different temperatures have been measured, as shown in Figure 4a,b,d,e. Figure 4c,f depicts the Arrhenius curve of Na₃Zr₂Si₂PO₁₂ and Na₃Zr₂Si₂PO₁₂-0.2Bi, respectively. The activation energy E_a of the Na₃Zr₂Si₂PO₁₂-0.2Bi sample at 228–373 K is 0.28 eV, which is much smaller than that of Na₃Zr₂Si₂PO₁₂ (0.31 eV), which is in line with the previously reported E_a value (0.26–0.40 eV) [32]. The reduced activation energy of the Na₃Zr₂Si₂PO₁₂-0.2Bi samples suggests that the addition of Bi₂O₃ is feasible for Na+ transport [30].

To investigate the Na plating/stripping behavior, the Na/Na₃Zr₂Si₂PO₁₂/Na and Na/Na₃Zr₂Si₂PO₁₂-0.2Bi/Na symmetrical batteries were assembled. Critical current density (CCD) is a critical parameter to evaluate the tolerance of Na dendrite growth [33]. As shown in Figure 5a,b, the CCD of the Na/Na₃Zr₂Si₂PO₁₂/Na symmetric cell is 0.35 mA cm⁻², which is slightly higher than that of Na/Na₃Zr₂Si₂PO₁₂-0.2Bi/Na (0.30 mA cm⁻²). The decrease of CCD after Bi₂O₃ introduction originates from the spontaneous reaction of Na with Bi³⁺ of Na₃Zr₂Si₂PO₁₂-0.2Bi, which would induce the formation of a Na-Bi alloy and thereupon accelerate the dendrite growth [34,35,36]. After CCD tests, the symmetrical batteries were disassembled (Figure 5c). Conspicuously, the large area of the black region can be observed. The Na/Na₃Zr₂Si₂PO₁₂-0.2Bi/Na pellet was then ground into powder, and the corresponding XRD pattern can further confirm the existence of trace amount of Bi metal (Figure 5d).

Finally, we assembled solid sodium full batteries, using Na₃Zr₂Si₂PO₁₂ and Na₃Zr₂Si₂PO₁₂-0.2Bi as an electrolyte and commercial Na₃V₂(PO₄)₃ as a cathode. For Na₃V₂(PO₄)₃/Na₃Zr₂Si₂PO₁₂/Na and Na₃V₂(PO₄)₃/Na₃Zr₂Si₂PO₁₂-0.2Bi/Na, the initial discharging capacities were 96.8 mA h g⁻¹ and 96.1 mA h g⁻¹, with initial coulombic efficiencies of 91.1% and 94.1%, respectively (Figure 5e). After 100 cycles, Na₃V₂(PO₄)₃/Na₃Zr₂Si₂PO₁₂/Na and Na₃V₂(PO₄)₃/Na₃Zr₂Si₂PO₁₂-0.2Bi/Na maintain reversible capacities of 95.5 mA h g⁻¹ and 92.4 mA h g⁻¹ with capacity retentions of 98.66% and 96.15%, respectively. Although the addition of Bi₂O₃ into an Na₃Zr₂Si₂PO₁₂ electrolyte can increase the relative density/ionic conductivity and decrease the electronic conductivity, it does not manifest a positive effect on the electrochemical performance of symmetrical and full batteries due to the formation of Na-Bi alloys at the Na/electrolyte interface. Such Na-Bi alloys would promote the Na dendrite growth and further penetrate through the electrolyte. However, due to its benefits of enhanced ionic conductivity, decreased electronic conductivity and bulk density, we believe the extra interfacial modifying layer on a Na₃Zr₂Si₂PO₁₂-0.2Bi surface, which acts as a physical shield so as to avoid the direct contact of Na with Bi³⁺, can improve the electrochemical performance of Na₃Zr₂Si₂PO₁₂-0.2Bi-based solid sodium metal batteries.

4. Conclusions

In this work, the Na₃Zr₂Si₂PO₁₂ electrolyte is prepared by a Bi₂O₃-assisted high-temperature solid-state reaction. At high temperatures, liquid Bi₂O₃ flux can prompt a relative dense microstructure of Na₃Zr₂Si₂PO₁₂ by bonding the grain. Meanwhile, the presence of Na₃Bi(PO₄)₂ impurities can enhance the Si/P ratio of Na₃Zr₂Si₂PO₁₂ and the Na⁺ occupancy, thus increasing the ionic conductivity of the Na₃Zr₂Si₂PO₁₂ electrolyte. The optimized Na₃Zr₂Si₂PO₁₂-0.2Bi electrolyte manifests an ionic conductivity of 1.27 × 10⁻³ S cm⁻¹ at room temperature and a low activation energy of 0.28 eV. Despite these, due to the spontaneous reaction of Na and Bi³⁺ when assembling solid-state symmetrical and full batteries, the Bi₂O₃ introduction cannot further enhance the electrochemical performance.