Introduction

The demand for energy-storage batteries to realize a sustainable society is increasing annually1. Lithium-ion batteries are used in various portable devices because of their high working potentials (> 3 V)2. However, the low abundance of lithium and redox-center transition metals (such as Co and Ni)3,4 and their environmental impact, such as the soil contamination associated with their extraction, have become problematic5. Alternative solutions that have been proposed for these problems include the use of sodium-ion batteries, which are composed of ubiquitous elements, including sodium as the carrier ion, which has chemical properties similar to those of lithium, such as a high ionization tendency and stable monovalent ions6,7. In addition, commercially available lithium-ion batteries currently use flammable organic electrolytes, and their safety is a concern as their application in electric vehicles and large storage batteries continues to advance. In view of these concerns, all-solid-state batteries (ASSBs), which have inorganic solid electrolytes, are expected to be used as next-generation batteries because of their highly improved safety8,9,10. Furthermore, because ASSBs do not contain liquid electrolytes, new electrode materials like elemental sulfur and chloride, which would otherwise leach into the liquid electrolyte, can be used, thereby creating the possibility of increasing the energy density beyond those of conventional batteries11,12.

Chlorides are being extensively studied as solid electrolytes for ASSBs13,14. Compared to compounds with divalent anions, such as oxides and sulfides, which have received widespread attention, anionic compounds with a univalent anion, such as chlorides, undergo limited Coulombic interactions with carrier ions, which have high diffusivity15. Unlike oxides, chloride ions tend to exhibit high polarizability and deformability, similar to that of sulfides16,17,18. High deformability enables the use of a sinterless compaction process, which is advantageous for the fabrication of ASSBs19. The disadvantage of the use of sintering for densification is that it is inapplicable to certain materials owing to side reactions20 and the evaporation of elements21,22.

An additional benefit of chloride is that it is a promising electrolyte, particularly in cathodes, owing to its higher oxidation resistance (~ 4.0 V vs. Li/Li+) than those of sulfide (~ 2.5 V vs. Li/Li+) and oxide (~ 3.5 V vs. Li/Li+), which has its origins in the noble properties of chlorine23,24. This high resistance to oxidation is a feature of the high working potential required for high energy density when used in electrode materials. As mentioned previously, the high polarity of chloride electrolytes often causes dissolution of the constituent elements in the electrolyte solution when they are used as liquid electrolytes. This severely affects the cycle life of the battery because it lowers the number of reversible charge–discharge cycles. However, the dissolution reaction of chloride electrodes is suppressed when they are used in combination with solid electrolytes; thus, chloride electrodes are potentially suitable for use as high-voltage cathode materials for rechargeable ASSBs.

In this study, we focused on NaFeCl4 as an electrode material for ASSBs, because NaFeCl4 contains sodium chloride and the ubiquitous element Fe as the transition metal in the redox center. NaFeCl4 is registered on the ISCD (#16994)25 and has an orthorhombic crystal structure (S.G.: P212121), with Fe in the center of the tetrahedron formed by the chloride ions. In electrode materials based on the Fe2+/3+ redox reaction, Na2Fe2(SO4)326 exhibited a higher potential (3.8 V vs. Na/Na+) than conventional oxide electrodes (Fe2O327, 1.5 V vs. Na/Na+) because of the noble properties of the SO42− unit (which has an inductive effect). The same effect that was induced by the SO42− ion was expected for the lighter Cl− ion (molar mass per charge Cl−: 34.5 g mol−1, (SO42−)/2:48 g mol−1), and the charge–discharge properties of the Fe2+/3+ redox pair were evaluated at the NaFeCl4 electrode. The theoretical capacity of this material is 121 mAh g−1 at one Na per NaFeCl4 formula unit. We also assembled an ASSB using an electrode without electrolyte added to the electrode composite (hereafter referred to as an “electrolyte-free electrode”). Conventional oxide electrodes exhibit low deformability and can be fabricated by adding soft sulfide electrolytes or other materials. However, the addition of electrolytes has the effect of decreasing the theoretical energy density per electrode composite, in which case the reaction distribution becomes more complicated. For chloride electrodes, the active material in the electrode may have high deformability. Therefore, in ASSBs, the electrode composites that do not contain solid electrolyte powders may surpass conventional battery systems in terms of their energy density.

Results and discussion

Structure and deformability

The X-ray diffraction (XRD) pattern of the synthesized NaFeCl4 sample (Fig. 1a) indicates a single phase consisting of monoclinic NaFeCl4. The peaks of the raw material are no longer visible. The relative density of the uniaxially compressed pellet was calculated from the apparent density of the compact and crystal lattice density of the monoclinic NaFeCl4 (2.31 g cm−3). The value of 99.1% for NaFeCl4 (at 382 MPa) is higher than those for Li2FeCl4 (92% at 382 MPa)28 and Li3TiCl6 (86.1% at 350 MPa)29, which have recently been reported as highly deformable electrodes. The cross-sectional scanning electron microscopy (SEM) image of the NaFeCl4 powder compact (Fig. 1b) indicates that the grains were crushed by compaction, resulting in a dense structure with ill-defined grain boundaries. These results indicate that the NaFeCl4 powder has high deformability.

Figure 1
figure 1

Characterization of synthesized NaFeCl4. (a) XRD patterns of NaFeCl4 after milling and the raw material and the simulated pattern of orthorhombic NaFeCl4 from the ICSD database (#16994). (b) Cross-sectional SEM image of the pellet uniaxially compressed. (c) Conductivity diffusion coefficient of NaFeCl4 obtained from AC impedance measurements at 298 and 333 K and the chemical diffusion coefficients of typical oxide (NaxMnO330,NaTi2(PO4)331,and NaCoO232) and sulfide (NaxTiS233) cathode materials at room temperature.

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Electrochemical performance

An ASSB with a solid electrolyte was fabricated to evaluate this strongly ionic electrode-active material without it leaching into the electrolyte. In addition, taking advantage of this deformability, electrolyte-free electrodes were fabricated for application in ASSBs, and their charge–discharge characteristics were evaluated. Based on the conductivity diffusion coefficient (Fig. 1c) obtained from the impedance plot (Supplementary Fig. 1), the battery operating temperature was set at 333 K to ensure that the diffusion coefficient is higher than those of conventional electrode materials. The solid electrolyte (Na3PS4|Na2.25Y0.25Zr0.75Cl6) consists of two layers, one on the anode (Na10Sn4) side and the other on the cathode (NaFeCl4) side, to suppress the reactions between the electrodes and electrolytes (cell configuration: Na10Sn4 + acetylene black (AB)|Na3PS4|Na2.25Y0.25Zr0.75Cl6|NaFeCl4 + KB) based on an examination of reactions like the oxidation of Na3PS4, as shown in Supporting Section 1 (Supplementary Figs. S2 and S3). The resulting reversible capacity was 90.8 mAh (g-NaFeCl4)−1 (81.7 mAh (g-positive electrode)−1), and the average working potential was ~ 3.45 V (vs. Na/Na+), as is evident from the constant-current charge–discharge curves (Fig. 2a) and the dQ/dV curves (Figure S4).

Figure 2
figure 2

Characteristics of the ASSB (Na10Sn4 + AB|Na3PS4|Na2.25Y0.25Zr0.75Cl6|NaFeCl4 + KB) using a NaFeCl4 electrode without electrolyte (NaFeCl4:KB = 90:10 wt%) and with the bilayer electrolyte (Na3PS4|Na2.25Y0.25Zr0.75Cl6). (a) Constant-current charge–discharge curves. (b) Impedance plots before and after charging and discharging. (c) Cycle characteristics of discharge capacity (circle) and Coulombic efficiency (square).

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The results of the impedance measurements (Fig. 2b) indicate only a small semicircular resistance and no significant increase in the first charge–discharge process or after the cycle test. The battery also exhibits relatively stable cycling characteristics over 10 cycles (Fig. 2c). Based on the reversible capacity of this discharge capacity (~ 90 mAh g−1), the gravimetric energy density per positive electrode was calculated to be 281 Wh kg−1 at the reference potential of Na (~ 3.45 V). In Table 1, this value is compared with previously reported energy densities of bulk ASSBs with high-potential operation (> 3 V). This shows that the ASSB fabricated in this study via a simple process using only pressed powders, which does not require any coating or sintering process on the surface of the cathode active material, has higher energy density than other reported bulk ASSBs using inorganic and/or polymer electrolytes.

Table 1 Comparison of the energy density per positive electrode weight, Ema, of the ASSB in this study with that of other bulk ASSB with a high-potential cathode (> 3 V).
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The redox mechanism of the NaFeCl4 electrode was investigated by X-ray photoelectron spectroscopy (XPS) before and after the charge–discharge process. The Fe 2p XPS profile (Fig. 3) consists of two sets of doublet peaks (Fe 2p3/2 and Fe 2p1/2) and their satellite peaks. Deconvolution of each spectrum using the pseudo-Voigt function revealed that the Fe 2p2/3 peak is located near 711.0 eV before and after charge, whereas a high-intensity peak appears near 710.5 eV, and the intensity of the peak at 711.0 eV is lower after discharge. The Fe 2p2/3 peaks of FeCl2 and FeCl3 in the reference sample appear at 710.6 eV and 711.3 eV, respectively42,43, with the low- and high-energy peaks attributable to Fe2+ and Fe3+, respectively. The peak ratio after discharge was approximately 3:1, which is consistent with the fact that the discharge capacity was approximately 75% of the theoretical capacity (121.5 mAh g−1). This indicates that the charge–discharge process proceeded via the redox reaction of Fe2+/3+ in NaFeCl4. As mentioned previously, the redox reaction of Fe2+/3+ has been reported to have a low potential of approximately 1.5 V in conventional oxides (Fe2O3). In this material, the inductive effect of chlorine may be responsible for the higher potential (3.45 V vs. Na/Na+), which would be responsible for the high energy density listed in Table 1. The XPS profile after charging revealed a reversible return to the original Fe3+ state before the charge–discharge process. The XRD patterns before and after charging (Supplementary Fig. S4) also show a reversion to monoclinic NaFeCl4, indicating the occurrence of a reversible charge–discharge reaction involving a Fe2+/3+ redox reaction. The synthesis of Na2FeCl4, which can be charge-started, has not yet been reported; therefore, it is expected to be evaluated in future studies.

Figure 3
figure 3

Fe 2p XPS results of the NaFeCl4 electrode before and after the charge–discharge process. The spectra were fitted with 2p3/2 and 2p1/2 of Fe2+/3+ and satellite peaks.

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In summary, the NaFeCl4 electrode, composed of ubiquitous elements, was evaluated for application in a low-cost storage battery with high energy density and safety. An ASSB was operated at 333 K with an electrolyte-free electrode owing to the high deformability derived from chloride ions (relative density = 99% of the pellet uniaxially compressed at 298 K). In addition, owing to the inductive effect of chloride, high-potential operation (3.45 V vs. Na/Na+) was demonstrated with the most attractive Fe redox reaction (Fe2+/3+) in terms of the elemental strategy. Consequently, an outstanding energy density (281 Wh (kg-positive electrode)−1) was achieved for conventional bulk all-solid-state sodium-ion batteries without sintering or electrode coating treatment. This study demonstrates the potential of NaCl-based materials as high-energy-density electrode materials, which have previously been difficult to evaluate because of their elution into the electrolyte.

Methods

Preparation and evaluation of all-solid-state cells using NaFeCl4 electrodes

NaFeCl4 was synthesized from NaCl (Wako Pure Chemical Industries, Ltd., 99.5%) and FeCl3 (Sigma-Aldrich Japan LLC, 99.9%) powders by a mechanochemical method28. A stoichiometric mixture was placed in a 45 mL stainless steel pot with 74 ZrO2 balls (diameter = 5 mm) and milled using a planetary ball mill apparatus (Fritsch Japan Co., Ltd., P-7 classic-line, Japan) at a rotation speed of 300 rpm for 5 h. The as-produced yellow sample was probed by XRD (MiniFlex 600, Rigaku, Japan, CoKα line). The conductivity diffusion coefficient was measured at 298 and 333 K using the AC impedance method (VSP Potentiostat, BioLogic, France) with an AC voltage of 300 mV and a measurement frequency range of 102–106 Hz. Pellets (diameter = 10 mm, thickness =  ~ 0.50 mm) were prepared by sandwiching the powder between stainless steel plates and compressing under 382 MPa at 298 K. The pellets contain 10 wt% of Ketjen black (KB) as a conductivity aid. The cross-section of a pellet was polished with a #2000 file, and cross-sectional images were acquired using SEM (JSM-6360LV, JEOL, Japan) at a voltage of 10 kV. All the procedures were performed under dry Ar gas.

Electrolyte-free electrodes were prepared with NaFeCl4, and the charge–discharge characteristics of the all-solid-state sodium-ion batteries were evaluated. The NaFeCl4 electrode was mixed with KB (mixing ratio of NaFeCl4:KB = 90:10 wt%) as a conductive aid via ball milling at 300 rpm for 1 h. The solid electrolyte was the sulfide Na3PS444 and/or chloride Na2.25Y0.25Zr0.75Cl645, and the negative electrode (counter electrode) was Na10Sn4-AB (AB: acetylene black; mixing ratio of Na10Sn4:AB = 90:10 wt%)46. For cell fabrication, approximately 60 mg of the electrolyte was first placed in a polycarbonate pressure vessel with a cylindrical inner diameter of 10 mm, sandwiched between two pieces of stainless steel, and pressurized to 96 MPa. Subsequently, the cathode material was placed on one side and pressed at 96 MPa, whereas the anode material was placed on the other side and pressed at 382 MPa. The cells were then screwed in from the top and bottom and restrained. When two layers of the electrolyte were used, approximately 30 mg of the Na3PS4 electrolyte was used on the anode side and approximately 30 mg of the Na2.25Y0.25Zr0.75Cl6 electrolyte on the cathode side. Charging and discharging were evaluated using a potentiostat/galvanostat (VSP Potentiostat, BioLogic, France) at a temperature of 333 K, current density of 6.4 μA cm−2, and starting from the discharging process (Na storage). AC impedance measurements were performed in the frequency range of 1 − 106 Hz after charging and discharging. The charge–discharge mechanism of NaFeCl4 was investigated by conducting Fe 2p XPS measurements on the electrode before and after charging and discharging using a Cr Kα radiation source (PHI Quantes, ULVAC-PHI, Inc., USA) without surface etching treatment, which would be a concern in terms of alteration.