Structure and Deformability. The X-ray diffraction (XRD) pattern of the synthesized NaFeCl4 sample shown in 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 of Li2FeCl4 (92% at 382 MPa)26 and Li3TiCl6 (86.1% at 350 MPa)27, which have recently been reported as highly deformable electrodes. The cross-sectional scanning electron microscopy (SEM) image of the NaFeCl4 powder compact shown in 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.
Electrochemical Performance. Taking advantage of this deformability, electrolyte-free electrodes were fabricated in the aforementioned electrode composite layer, and their charge–discharge characteristics were evaluated. Based on the value of 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 high compared to that of conventional electrode materials. In addition, the potential window and chemical stability of the electrolyte were examined, as shown in Supporting Section 1 (Supplementary Figure S3). The solid electrolyte (Na3PS4 | Na2.25Y0.25Zr0.75Cl6) consists of two layers, one on the side of the anode (Na10Sn4) and the other on the side of the cathode (NaFeCl4), to suppress the reactions between the electrodes and electrolytes (cell configuration: Na10Sn4 + acetylene black (AB) | Na3PS4 | Na2.25Y0.25Zr0.75Cl6 | NaFeCl4 + KB). 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 shown in Fig. 2a.
The results of the impedance measurements, shown in Fig. 2b, indicate only a small semicircular resistance and no significant increase in the 1st charge–discharge process or after the cycle test. The battery also exhibits relatively stable cycling characteristics over 10 cycles, as shown in 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 311 Wh kg-1 at Na reference potential (~ 3.45 V). In Table 1, this value is compared with previously reported energy densities of bulk ASSBs with high-potential operation (> 3 V).
Table 1
Comparison of the energy density per positive electrode weight, Ema, of the ASSB in this study with that of other bulk all-solid-state sodium-ion batteries with a high-potential cathode (> 3 V). 33–42
Report | Em (Wh kg− 1) | Cathode active material (CAM) | Coating for CAM | Solid electrolyte (SE) | Weight ratio of cathode (CAM : SE : others) | Preparation | Operating temperature (K) |
This experimental | 311 | NaFeCl4 | Non. | Na3PS4 | Na2.25Y0.25Zr0.75Cl6 | 90 : 0 : 10 | Power Compress | 333 |
Edward Matios et al. 2019 | 326 | Na3V2P3O12 | Graphene | NZSP | 80 : 0 : 10 | Sintering | 353 |
Zhizhen Zhang et al. 2016 | 278 | Na3V2P3O12 | Carbon | La-substituted NZSP | 57 : 15 : 13 | Sintering | 353 |
Hideo Yamauchi et al. 2019 | 173 | Na2O-Fe2O3- P2O5 glass | Non. | β”—Al2O3 | 72 : 25 : 3 | Sintering | 303 |
Akitoshi Hayashi et al. 2014 | 91.6 | NaCrO2 | Non. | Na3PS4 | 36 : 55 : 9 | Power Compress | 303 |
L. Duchêne et al. 2017 | 185 | NaCrO2 | Non. | Na4(B12H12)(B10H10) | 70 : 20 : 10 | Power Compress | 303 |
Wei Niu et al. 2020 | 169 | Na0.67Ni0.33Mn0.67O2 | C2H4(CN)2 -NaClO4 | PEO-NZSP-NaClO4 | 70 : 0 : 30 | Power Compress | 328 |
Chenglong Zhao et al. 2019 | 220 | NVP | Non. | PEO-NaFSI | 60 : 0 : 40 | Power Compress | 353 |
Yu Yao et al. 2020 | 288 | NVP | Non. | PEGDMA-NaFSI | 70 : 0 : 30 | Power Compress | 333 |
Xingwen Yu et al. 2019 | 237 | Na2MnFe(CN)6 | Non. | PEO-NaClO4- Na3Zr2Si2PO12 | 60 : 20 : 30 | Power Compress | 333 |
a E m is the energy density per positive electrode weight assuming a Na metal anode. If not stated otherwise, it was calculated from the initial discharge curve. Other information, such as the materials used, their mixing ratios, fabrication conditions, and working temperatures, is also shown.
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 superior energy density compared to other reported bulk ASSBs.
The redox mechanism of the NaFeCl4 electrode was investigated with the aid of X-ray photoelectron spectroscopy (XPS) before and after the charge–discharge process. The Fe 2p XPS profile shown in 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, respectively,42,43 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 Figure S4) also show a reversion to monoclinic NaFeCl4, indicating the occurrence of a reversible charge–discharge reaction involving a Fe2+/3+ redox reaction.
In summary, the NaFeCl4 electrode composed of ubiquitous elements was evaluated for use 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 (99% relative density 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 (311 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.