Synthesis and Electrochemical Properties of Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C Composite as Cathode Materials for Sodium-ion Batteries

Abstract

Due to the abundance and low cost of sodium, sodium-ion batteries have received renewed attention in large-scale applications. At present, the primary task is to explore a new generation of cathode materials with excellent performance. Herein, ascorbic acid was used as a carbon source, Na₂Fe_0.85Mg_0.15PO₄F/C (NFMPF/C) and Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C (NFMNPF/C) cathode materials were prepared by conventional solid-state reaction method on the basis of Na₂FePO₄F/C (NFPF/C). The electrochemical performance of the cathode material was improved by doping Mg and Ni on the Fe site, which was confirmed by the volt-ampere characteristic test and charge-discharge test. Compared with NFPF/C, the electrochemical performance of NFMPF/C has been greatly improved. After further doping of Ni, NFMNPF/C shows a stable discharge capacity, which can still maintain 91.3 % of the initial specific capacity after 50 cycles at 0.1 C-rate, while the rate capability shows a great improvement. The optimized NFMNPF/C sample can deliver a specific discharge capacity of 53 mAh/g at 5 C-rate. In general, the simple doping of NFPF/C optimizes the cycle life and rate capability at the same time. As a low-cost cathode material, this study provides more possibilities for the development of sodium-ion batteries.

1. Introduction

LIBs have always had great potential for energy storage. However, due to the high price and the lack of renewable resources, people have to choose Na-based materials as alternative resources. SIBs were discovered by researchers as early as 1980, but due to the disadvantage of weight and energy density of Na-based cathode materials, the research on Na-based cathode materials has stagnated again. In recent years, with the increasing development of large-scale equipment such as electric vehicles, researchers no longer pay too much attention to the volume of materials, so sodium-ion batteries have gained renewed attention. Currently, reported cathode materials for SIBs include transition metal oxides,^1,2 polyanionic compounds,^3–6 Prussian Blue,^7,8 and organic compounds and polymers. Among them, LiFePO₄ with olivine structure has always been the preferred cathode material for lithium-ion batteries. Through the study of its structure, Nazar’s team first proposed Na₂FePO₄F cathode material in 2007.⁹ The induction effect of PO₄ in Na₂FePO₄F makes the material obtain a higher redox potential. The existence of polyanion (PO₄)³⁻ with strong P-O covalent bond increases the potential. This is due to the strong polarization of the O ions toward the P ions, which reduces the covalency of the Fe-O bond. At the same time, the effective combination of F⁻ and PO4³⁻ also increases the potential of the active redox couple.¹⁰ The layered structure of Na₂FePO₄F provides a two-dimensional channel for the transport of Na⁺, with only 3.7 % reversible volume change, which can be used for both SIBs and LIBs.^9,11 However, because the electrons provided by the metal in the material are blocked by anionic groups, the material has low diffusion coefficient and poor conductivity,¹² resulting in unsatisfactory electrochemical performance, which hinders the application of Na₂FePO₄F cathode material.

At present, various measures have been taken to solve this problem, including surface coating,^13,14 Ion doping,^15–18 Material nanocrystallization.^19,20 For instance: Kawabe et al. prepared Na₂FePO₄F/C cathode material with ascorbic acid as carbon source by solid-state reaction method. After charge-discharge test, the discharge capacity reaches 110 mAh/g at 0.05 C-rate, and the discharge capacity can maintain 75 % of the initial capacity after 20 cycles; Jin et al. synthesize Na₂Fe_0.94Co_0.06PO₄F/C by sol-gel method. The discharge capacity of Na₂Fe_0.94Co_0.06PO₄F/C was 99.9 mAh/g at 0.2 C-rate after charge-discharge test, and it could maintain 45.6 mAh/g at 2 C-rate; Jianhua Yan et al. prepared Na₂FePO₄F/carbon nanotubes by nano-assembly of Na₂FePO₄F and carbon nanotubes. After charge-discharge test, the discharge capacity reached 100 mAh/g at 0.4 C-rate, and remained at 77.8 mAh/g after 400 cycles, while the microstructure was basically not destroyed. It can be seen from these studies that compared with Na₂FePO₄F, ascorbic acid as a carbon source for surface coating, equivalent cation doping and particle size reduction are effective methods to improve conductivity, thus achieving the effect of increasing cycle life and improving rate capability. However, it can be seen that in the process of Na₂FePO₄F optimization, the cycle life and rate capability cannot be improved at the same time, only one aspect can be taken into account, and the material still cannot meet the requirements. Therefore, using a simple method to achieve multiple aspects of optimization has become a problem.

In this paper, Na₂Fe_0.85Mg_0.15PO₄F/C and Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C cathode materials were prepared by conventional solid-state reaction method. After the electrochemical test of Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C, the cycle life and rate capability were optimized simultaneously. This shows that use of ascorbic acid not only increases the conductivity of the material, but also controls the growth of the particles. Not only carbon coating but also grain-size reduction upon Mg and Ni substitution may contribute to the enhanced interfacial ion transport kinetics. Meanwhile, the atomic valence of the raw material is not easy to be changed by doping the cations in the same valence state, and Mg, Ni co-doping can also play a role in stabilizing the structure of the material, expanding an ion channel and improving the conductivity, thereby effectively improving the electrochemical performance of the material. Solid-state reaction method is the most commonly used method in commercial production, which further confirms that Na₂FePO₄F cathode material can be commercialized.

2. Experimental

2.1 Synthesis of materials

The Na₂FePO₄F/C (NFPF/C) is synthesized by conventional solid state reaction method through a certain stoichiometric ratio of FeC₂O₄·2H₂O, NaHCO₃, NaF and NH₄H₂PO₄; Ascorbic acid was used as the carbon source. The precursor is prepared by ball milling, a sample obtained by ball milling is dried in a vacuum drying oven to be powdery, the dried sample is heated for 6 hours at 350 °C under Ar, and the pre-sintered sample is prepared by grinding and sintering for 8 hours at 650 °C under Ar. In order to obtain Na₂Fe_0.85Mg_0.15PO₄F/C (NFMPF/C) and Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C (NFMNPF/C), it is only necessary to add a certain stoichiometric ratio of MgO and NiO at the initial stage of precursor preparation, and the subsequent operations are the same.

2.2 Characterization of materials

The crystal structure of the NFPF, NFMPF and NFMNPF was analyzed by X-ray diffraction (Panalytical X’Pert-MPD type, CuKα-radiation source, 40 kV, 40 mA), and the crystal structure of the synthesized sample was characterized in the 2θ range of 10° to 70°. The microstructure of NFMNPF/C was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The contents of individual elements of NFMNPF/C were determined by inductively coupled plasma atomic emission spectrometry (ICP-OES). The carbon content of NFPF/C, NFMPF/C, and NFMNPF/C was determined by EDS.

2.3 Preparation of electrodes and electrochemical testing

Cathode material, Super P and PVDF were mixed uniformly in the ratio of 80 : 10 : 10 by NMP organic solvent. CR2032 coin cells was used with NaPF₆ as the electrolyte and Na sheet as the counter electrode. Assemble the battery in a glove box filled with Ar for testing. Galvanostatic charge-discharge tests were performed at 0.1 mV/s in the potential range of 2.0–3.8 V using a Land CT2001 battery tester. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed using an IviumSoft electrochemical workstation. The CV was performed in the voltage range of 2.0–4.0 V, the EIS was performed in the frequency range of 10 kHz–1 mHz while applying an amplitude of 5 mV. All electrochemical tests were performed at room temperature, respectively.

3. Results and Discussion

The crystal structure of NFPF is shown in Fig. 1a, which has an orthorhombic structure with Pbcn space group. NFPF consists of paired coplanar iron octahedra with two unique Na sites, and Fe₂O₆F₃ dioctahedra are connected along the a-axis. X-ray diffraction patterns of NFMPF, NFMPF and NFMNPF are shown in Fig. 1b. The XRD peaks belong to the Pbcn space group, a = 5.2200 (2) Å, B = 13.8540 (6) Å, and C = 11.7792 (5) Å. It can be clearly observed that all the diffraction peaks of NFPF are successfully indexed by PDF cards, and are similar to the PDF cards of Na₂MgPO₄F, Na₂FePO₄OH and Na₂CoPO₄F, so it can be judged that NFPF is isomorphic with Na₂FePO₄OH and Na₂CoPO₄F. The carbon material coated on the surface of the material exists in an amorphous form.²¹ In addition, the diffraction peaks of NFMPF and NFMNPF are the same as the main peaks of NFPF, and the impurity phase does not correspond to an obvious peak, so the doped sample contains little or no impurity phase. The XRD patterns of NFMPF, NFMPF and NFMNPF after 100 cycles is shown in the Fig. 1c. Compared with the original XRD patterns of NFMPF, NFMPF and NFMNPF, the main peaks of NFMPF, NFMPF and NFMNPF can still be indexed by PDF card, which indicates that the crystal structure of NFMPF, NFMPF and NFMNP is not changed by low cation doping. In order to verify the synthesis of NFMNPF/C, we studied the concentration of individual elements, and the results of ICP analysis are shown in Table 1. The element contents of Fe, Mg, and Ni samples were calculated to be basically consistent with the synthesized material, indicating that Mg and Ni were successfully doped at the Fe site and NFMNPF/C was effectively synthesized.

Figure 1.

Crystal structure of NFPF composites (a), XRD pattern (b) and XRD pattern after 100 cycles (c) of NFPF, NFMPF and NFMNPF composites.

Table 1. Element content of NFMNPF/C.

	Fe	Mg	Ni
Element content	29.34 %	2.51 %	1.65 %

In order to further characterize the microstructure of NFPF/C, NFMPF/C and NFMNPF/C, they were tested by SEM. It can be seen from Figs. 2a and 2b that the cathode materials of NFPF/C and NFMPF/C are fine spherical particles with a particle size of about 1–2 µm and a large degree of agglomeration. It can be seen from Fig. 2c that the NFMNPF/C particles are evenly distributed, with a particle size of about 500–700 nm. The small powder particles indicate that the amorphous carbon and substitution of Mg, Ni effectively suppresses the grain growth of NFPF/C during sintering.

Figure 2.

SEM image of NFPF/C (a), NFMPF/C (b) and NFMNPF/C (c) composites.

In order to further characterize the microstructure of NFMNPF/C, TEM tests were carried out. TEM image from Fig. 3a The spherical NFMNPF/C cathode material consists of extremely small nanoparticles. HRTEM material was used for further study, as shown in Fig. 3b, from which clear lattice fringes can be observed, with a spacing of about 0.196 nm from the (242) plane. Nanoparticles are coated with carbon, which means that the carbon material is uniformly coated outside the NFMNPF/C sample, and the carbon coating significantly improves the conductivity of the NFPF/C sample. Table 2 listed the carbon content of NFPF, NFMPF, and NFMNPF determined by EDS. The carbon content in the samples is around 10 wt%, and the doping of ions does not have a significant effect on the carbon content of the samples.

Figure 3.

TEM image (a) and HRTEM image (b) of NFMNPF/C composites.

Table 2. Carbon content of NFPF/C, NFMPF/C and NFMNPF/C.

	NFPF/C	NFMPF/C	NFMNPF/C
Carbon content	10.71 %	10.42 %	9.28 %

The initial charge-discharge curves of NFPF/C, NFMPF/C and NFMNPF/C in the charge-discharge test at low current density (0.1 C-rate) in the voltage range of 1.5–3.8 V are shown in Fig. 4a. The first charge capacity of NFMNPF/C is 126.4 mAh/g, the discharge capacity is 97.1 mAh/g, and the coulombic efficiency is 76.8 %. NFMPF/C and NFMNPF/C are consistent with NFPF/C. The discharge curve has two flat voltage plateaus due to the reversible phase transition of Na₂FePO₄F-Na_1.5FePO₄F and Na_1.5FePO₄F-NaFePO₄F,²¹ which are around 2.95 V and 3.05 V. The appearance of the two voltage plateaus is due to the different activation energies of Na⁺ migration in the two directions during Na⁺ deintercalation.²² In addition, due to the effective combination of F⁻ and PO4³⁻ increases the potential of the active redox couple, simultaneously the doping of Mg²⁺ and Ni²⁺, caused by the samples have higher Na⁺ diffusion coefficient and lower polarization between charge and discharge curves. The charge-discharge curve of the subsequent cycle of NFMNPF/C is shown in Fig. 4b, and we can see that the first coulombic efficiency is very low. The significantly lower coulombic efficiency of NFMNPF/C may be due to the decomposition of the electrolyte, resulting in a large loss of sodium ions; on the other hand, the poor reversibility may also lead to the deactivation of some sodium ions, and the SEI film produced during the reaction may also reduce the first cycle efficiency of the positive electrode.

Figure 4.

Charge/discharge curves in the first time (a) and subsequent cycles (b) of NFPF/C, NFMPF/C and NFMNPF/C composites at 0.1 C. Inset: CV curves of NFPF/C, NFMPF/C and NFMNPF/C composites at 0.1 mV/s.

The CV curve at a scan rate of 0.1 mV/s is shown in the inset of Fig. 4. A pair of distinct redox peaks can be seen in the figure, indicating that the electrode has good reversibility. NFMNPF/C exhibit two anodic peaks at about 2.9 V and 3.2 V, correspond to those cathodic peaks at 2.8 V and 3.1 V, respectively, which are consistent with the two voltage plateaus in the charge-discharge curves, and also correspond to the two reversible phase transition of NFPF/C. The CV curve of NFMNPF/C shows a small voltage hysteresis due to its low polarization, indicating that Na⁺ can be extracted smoothly with only a relatively low potential. The redox current of NFMNPF/C (0.85 mA and 0.89 mA) is higher than that of NFPF/C (0.65 mA and 0.53 mA) and NFMPF/C (0.74 mA and 0.75 mA). Compared with NFPF/C and NFMPF/C, the higher redox current of NFMNPF/C makes NFMNPF/C have good electronic conductivity and sodium ion diffusivity, thus showing excellent electrochemical performance.¹⁵

Figure 5a shows a comparison of the cycle life of NFPF/C, NFMPF/C, and NFMNPF/C samples at a current density of 0.1 C-rate. When the initial discharge capacity of NFPF/C, NFMPF/C and NFMNPF/C is about 100 mAh/g, the cycling stability of NFMPF/C and NFMNPF/C is better than that of NFPF/C. At 0.1 C-rate, the discharge capacity of the NFPF/C sample decreased from 99.1 mAh/g to 59 mAh/g after 50 cycles, and the capacity retention was only 59.5 %. Similarly, the discharge capacity of NFMPF/C sample decreased from 94.3 mAh/g to 81.6 mAh/g after 50 cycles, and the capacity retention rate reached 86.5 %. After 50 cycles at the same rate, the NFMNPF/C sample still maintains a capacity of 88.6 mAh/g, which is equivalent to 91.3 % of the initial discharge capacity, which further confirms its excellent cycle life. The results show that the appropriate doping of Mg²⁺ and Ni²⁺ improves the diffusion of Na⁺ and does not damage the crystal structure.

Figure 5.

Discharge curves at 0.1 C (a) and discharge curves at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C (b) of NFPF/C, NFMPF/C and NFMNPF/C composites.

The charge-discharge curves of NFPF/C, NFMPF/C and NFMNPF/C samples at different rates are shown in Fig. 5b. With the increase of current density, the discharge capacity decreases gradually due to the existence of polarization. Figure 5b shows that the discharge capacity of NFMNPF/C is 97.1, 90.1, 88, 81.6, 73.6, and 53 mAh/g at 0.1, 0.2, 0.5, 1, 2, and 5 of C-rate, respectively. When the current density is reduced to 0.1 C-rate, the capacity is almost equal to the initial value (92.6 mAh/g). The discharge capacity of NFMPF/C is 94.3, 88.9, 83.5, 79.3, 70.3 and 48.4 mAh/g at 0.1, 0.2, 0.5, 1, 2, and 5 of C-rate, respectively. When the current density is reduced to 0.1 C-rate, the capacity reaches 91.9 mAh/g. Under the same conditions, the discharge capacity of NFPF/C is 99.1, 89.1, 73.5, 60.9, 46.8 and 27.5 mAh/g, respectively. When the current density decreased to 0.1 C-rate, the remaining capacity was only 80.5 mAh/g. It is noteworthy that the discharge capacity of the NFMPF/C and NFMNPF/C samples at 5 C-rate far exceeds that of the NFPF/C sample at 2 C-rate. At the same time, the discharge capacity of NFMPF/C and NFMNPF/C samples is almost no loss compared with NFPF/C when the rate is reduced to 0.1 C-rate, which indicates that the crystal structure of NFMPF/C and NFMNPF/C samples is basically not damaged in the case of insertion and detachment. In this paper, on the premise of doping appropriate amount of magnesium to achieve the best rate capability, the material was further doped with nickel, which made the material show excellent cycle life and rate capability at the same time. The substitution of Mg²⁺ and Ni²⁺ improves the conductivity of the material,¹⁵ and also stabilizes the structure of the whole material.

EIS measurements are used to study the electrode dynamics of the sample. Figure 6a shows the Nyquist plot of the sample before cycling. The Nyquist plot has a semicircle in the high frequency region and a straight line in the low frequency region, corresponding to the charge transfer resistance (R_ct) and the Warburg impedance (W). The radius of the semicircle in the high frequency region on the Z real axis is attributed to R_ct. The slope of the slope line in the low frequency region represents W, which is related to the sodium ion diffusion in NFPF/C. R_ct represents the sodium ion charge transfer resistance at the interface between the electrode and the electrolyte. It can be seen from the figure that the NFMNPF/C sample exhibits a lower R_ct than NFPF/C and NFMPF/C, which means that the charge transfer reaction is easier. The inclined straight line in the low frequency region is related to ion diffusion, and the diffusion coefficient of sodium ions can be calculated from the graph in the low frequency region by using the following equation:²³   

\begin{equation} D = \frac{R^{2}T^{2}}{2A^{2}n^{4}F^{4}C^{2}\sigma^{2}} \end{equation} (1)

where R is the gas constant; T is the absolute temperature; A is the surface area of the cathode; n is the number of electrons per molecule during oxidation; F is Faraday’s constant (96486 C mol⁻¹); C is the sodium ion concentration; and σ is the War-burg factor, which follows the relationship:²⁴   

\begin{equation} Z_{\text{real}} = R_{\text{e}} + R_{\text{ct}} + \sigma\omega^{-1/2} \end{equation} (2)

where ω is the angular frequency. Figure 6b demonstrates a linear fit of Z_real with ω^−1/2, from which the slope σ can be obtained. The sodium diffusion coefficient of the sample can be calculated from the value of σ. D_Na⁺ of NFPF/C, NFMPF/C and NFMNPF/C were 1.53 × 10⁻¹¹, 7.81 × 10⁻¹² and 2.07 × 10⁻¹¹ cm² s⁻¹, respectively. Compared with NFPF/C and NFMPF/C, NFMNPF/C has smaller charge transfer resistance and higher Na⁺ diffusion coefficient. This indicates that the carbon coating and ion doping can effectively improve the electrochemical performance of NFPF/C cathode materials. At the same time, the above results also well explain that not only carbon coating, the particle size reduction after Mg and Ni substitution may contribute to the enhancement of interfacial ion transport kinetics.

Figure 6.

Nyquist plots (a) and linear fitting of the Z′ versus ω^−1/2 relationship (b) of NFPF/C, NFMPF/C and NFMNPF/C composites. Inset: equivalent circuit to fit the Nyquist plots.

4. Conclusion

In summary, the carbon-coated and Mg, Ni co-doped Na₂FePO₄F cathode materials were successfully prepared by the traditional solid-state method. After doping Mg and Ni on the surface of Na₂FePO₄F with carbon network, the conductivity of the material was significantly improved. Therefore, the cycle and rate performance of the material were significantly improved. Compared with Na₂FePO₄F/C and Na₂Fe_0.85Mg_0.15PO₄F/C, Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C still maintains 91.3 % of the initial discharge capacity after 50 cycles at 0.1 C-rate. At the same time, the discharge capacity of the material can still reach more than 50 % at a high current density of 5 C-rate. Therefore, the Na₂Fe_0.8Mg_0.15Ni_0.05PO₄F/C prepared in this paper has a stable initial discharge capacity, and the cycle life and rate capability have been significantly improved. As a high-performance cathode material prepared by traditional solid-state method, it provides unlimited possibilities for the commercialization of sodium-ion batteries in the future.

CRediT Authorship Contribution Statement

Feixiang Guo: Conceptualization (Lead), Data curation (Lead), Formal analysis (Lead), Funding acquisition (Lead), Investigation (Lead), Methodology (Lead), Project administration (Lead), Resources (Lead), Software (Lead), Supervision (Lead), Validation (Lead), Visualization (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Sen Wang: Conceptualization (Equal), Data curation (Equal), Formal analysis (Equal), Funding acquisition (Equal), Investigation (Equal), Methodology (Equal), Project administration (Equal), Resources (Equal), Software (Equal), Supervision (Equal), Validation (Equal), Visualization (Equal), Writing – original draft (Equal), Writing – review & editing (Equal)

Gaobin Liu: Conceptualization (Supporting), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Methodology (Supporting), Project administration (Supporting), Resources (Supporting), Software (Supporting), Supervision (Supporting), Validation (Supporting), Visualization (Supporting), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Tao Wang: Conceptualization (Supporting), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Methodology (Supporting), Project administration (Supporting), Resources (Supporting), Software (Supporting), Supervision (Supporting), Validation (Supporting), Visualization (Supporting), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Wencai Hu: Conceptualization (Supporting), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Methodology (Supporting), Project administration (Supporting), Resources (Supporting), Software (Supporting), Supervision (Supporting), Validation (Supporting), Visualization (Supporting), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Xueyan Han: Conceptualization (Supporting), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Methodology (Supporting), Project administration (Supporting), Resources (Supporting), Software (Supporting), Supervision (Supporting), Validation (Supporting), Visualization (Supporting), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Yongheng Fan: Conceptualization (Supporting), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Methodology (Supporting), Project administration (Supporting), Resources (Supporting), Software (Supporting), Supervision (Supporting), Validation (Supporting), Visualization (Supporting), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Qi Yuan: Conceptualization (Supporting), Data curation (Supporting), Formal analysis (Supporting), Funding acquisition (Supporting), Investigation (Supporting), Methodology (Supporting), Project administration (Supporting), Resources (Supporting), Software (Supporting), Supervision (Supporting), Validation (Supporting), Visualization (Supporting), Writing – original draft (Supporting), Writing – review & editing (Supporting)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

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