Investigation of the Inorganic Compounds NaMV2(PO4)3 (M = Fe, Co, Ni) as Anode Materials for Sodium-Ion Batteries.

The new compounds NaMV2(PO4)3 (M = Fe, Co, Ni) were synthesized via a sol-gel synthesis route, and their crystal structures were refined using the Rietveld method from X-ray powder diffraction data. NaCoV2(PO4)3 was also characterized by TGA, cyclic voltammetry, and galvanostatic cycling. The three phases crystallize with the orthorhombic symmetry and the space group Imma. The structures are isotypic to the stuffed α-CrPO4-type structure and contain two vacant sites in which two sodium atoms can be intercalated. When NaCoV2(PO4)3 is cycled at a 1C rate in the voltage ranges of 0.1-3 and 0.7-3 V vs Na+/Na, it delivers specific capacities of 190 and 75 mA h/g, respectively, with an average operation potential of ∼1.4 V. This attests to the electrochemical activity of this compound and indicates that the α-CrPO4-type compounds could be suitable for hosting other guest ions.

Introduction

Nowadays, lithium-ion batteries (LIBs) are the most commonly used batteries in portable consumer electronic devices and in electric vehicles (EVs). With the population growth and the global warming, investments in renewable energies (solar, wind, or tidal power) are rising and the demand for LIBs is predicted to increase significantly in the future. This may cause disruption in the raw-material supply chain; therefore, since sodium is one of the most abundant and cheap elements on Earth and has similar electrochemical properties to lithium, sodium-ion batteries (NIBs) could be alternatives to LIBs, especially for large-scale storage applications.

Researchers have already developed a wide range of positive electrode materials for NIBs, namely, the layered oxides NaMO2 and sulfides NaMS2; the polyanionic compounds NaMPO4, Na2MP2O7, Na3MPO4CO3, Na4M3(PO4)2P2O7, Na2M3(PO4)3, Na3M2(PO4)3, NaM2(SO4)2(PO4), Na2M2(SO4)3, NaMPO4F, Na2MPO4F, and Na3M2(PO4)2F3; the fluorides NaMF3; or the Prussian blue analogues Na2M2[M2(CN)6]·nH2O (M = divalent and/or trivalent transition metal).[1−17] Among these intercalation compounds, NaMO2 and Na3V2(PO4)2F3 are the best candidates for a practical application in NIBs.[18] Furthermore, a large number of negative electrode materials also were investigated. These materials were classified into six main categories, namely, carbonaceous (e.g., hard carbon, soft carbon, N-graphene foam), conversion (e.g., Fe2O3, Co3O4, CuO), alloying (e.g., Ge, Sb, Si, Sn, Ni3Sn2, SnSb, SnGeSb, Sn4P3, SnP3, red P), conversion + alloying (e.g., SnO2, SnO, SnS, SnS2, Sb2O4, Sb2O3, Sb2S3), organic (e.g., Na2C8H4O4, Na2C6H2O4, Na2C14H4O4N2, Schiff base polymers), and insertion materials (e.g., NaTiO2, P2-Na0.66[Li0.22Ti0.78]O2, Na2Ti3O7, TiO2, Li4Ti5O12) (refs (19, 20), and references therein). To our knowledge, only a few polyanionic compounds have been investigated as anode materials for NIBs [e.g., NaTi2(PO4)3 and Na3V2(PO4)3];[21,22] therefore, we decided to explore phosphates with multiple electron transfer to achieve high specific capacity with a relatively low potential. The Cr3+PO4 family of compounds[23] is particularly interesting since the β-CrPO4 structure (α-CrVO4-type) contains vacancies that may accommodate Li, Na, Ag, or Cu atoms.[24−33] The α-CrPO4 structure also contains large empty channels in which Li or Na atoms can be intercalated. Our research team was the first to report α-Na2Ni2Fe(PO4)3,[34] NaCoCr2(PO4)3, NaNiCr2(PO4)3, and Na2Ni2Cr(PO4)3,[35] crystallizing with the α-CrPO4-type structure, as anode materials for NIBs. Following our results, other researchers investigated vanadium-based compounds such as NaV3(PO4)3[36−40] and α-VPO4.[41]

In the present work, we describe the synthesis and crystal structures of NaMV2(PO4)3 (M = Fe, Co, Ni) determined by X-ray powder diffraction.[42] We then describe the thermal and electrochemical properties of NaCoV2(PO4)3 determined by TGA, cyclic voltammetry, and galvanostatic cycling measurements.

Results and Discussion

Structure Refinement

The crystal structure of NaCoV2(PO4)3 was solved using the structure of NaCoCr2(PO4)3 as a starting model.[35] A statistical disorder of Co/V was introduced, and constraints on their occupancies and atomic displacement parameters (ADPs) were applied. Since few ADP tensors displayed negative values, atoms of the same nature were restricted to have the same ADPs. The Rietveld analysis of the XRPD data collected at 293 K led to the reliability factors listed in Table [Rp = 0.9%, Rwp = 1.2%, RB = 6.7%, χ2 = 5.336]. The final atomic positions are given in Table . Figure shows good agreement between the experimental and calculated patterns. Only one unindexed peak was observed. It corresponds to the peak (022) of the phase NaVP2O7.[43] The elemental analysis confirmed the chemical composition of NaCoV2(PO4)3 (Figure b), and the TGA indicated that the phase is not stable at temperatures above 850 °C (Figure ). This may explain why all the samples prepared above 750 °C contained a larger amount of NaVP2O7.

Table 1

Crystallographic Data and Structure Refinements for NaMV2(PO4)3 (M = Fe, Co, Ni)

Crystal data
Chemical formula	NaFeV2P3O12	NaCoV2P3O12	NaNiV2P3O12
Mr	465.6	468.7	468.5
Space group	Imma	Imma	Imma
Temperature (K)	293	293	293
a, b, c (Å)	10.5067 (4)	10.4947 (10)	10.5091 (5)
	13.2437 (6)	13.1842 (12)	13.1212 (5)
	6.3946 (2)	6.3920 (6)	6.3540 (3)
V (Å3)	889.80 (6)	884.42 (14)	876.16 (6)
Z	4	4	4

(1): NaCoV2(PO4)3, (2): Ni12P5, (3): NaFeV2(PO4)3, and (4): NaVP2O7.

Table 2

Fractional Atom Coordinates and Isotropic Atomic Displacement Parameters (Å2) for NaMV2(PO4)3 (M = Fe, Co, Ni)

atom	Wyck.	site	occ.	x	y	z	Uiso (Å2)
NaFeV2P3O12
Na1	4e	mm2	1	0	1/4	0.405(2)	0.004(5)
V1	4a	2/m..	1	0	0	0	0.016(3)
Fe2/V2	8g	.2.	0.5/0.5	1/4	0.6338(4)	1/4	0.0205(19)
P1	4e	mm2	1	0	1/4	0.9199(16)	0.010(3)
P2	8g	.2.	1	1/4	0.4255(5)	1/4	0.010(3)
O1	8h	m..	1	0	0.1482 (6)	1.0328(17)	0.004(2)
O2	8i	.m.	1	0.1105 (11)	1/4	0.7642(15)	0.004(2)
O3	16j	1	1	0.2817 (8)	0.3593(5)	0.061(1)	0.004(2)
O4	16j	1	1	0.3596(7)	0.5002(5)	0.2816 (15)	0.004(2)
NaCoV2P3O12
Na1	4e	mm2	1	0	1/4	0.3934(19)	0.004(5)
V1	4a	2/m..	1	0	0	0	0.015(3)
Co2/V2	8g	.2.	0.5/0.5	1/4	0.6383(4)	1/4	0.024(2)
P1	4e	mm2	1	0	1/4	0.9077(17)	0.009(3)
P2	8g	.2.	1	1/4	0.4304(5)	1/4	0.009(3)
O1	8h	m..	1	0	0.1528 (7)	1.0384(16)	0.0105
O2	8i	.m.	1	0.1195 (11)	1/4	0.7707(15)	0.0105
O3	16j	1	1	0.2906 (7)	0.3689(5)	0.0581(10)	0.0105
O4	16j	1	1	0.1393 (7)	0.5014(4)	0.1963 (15)	0.0105
NaNiV2P3O12
Na1	4e	mm2	1	0	1/4	0.387(3)	0.054(10)
V1	4a	2/m..	1	0	0	0	0.000(2)
Ni2/V2	8g	.2.	0.5/0.5	1/4	0.6339(2)	1/4	0.0062(13)
P1	4e	mm2	1	0	1/4	0.9199(12)	0.008(2)
P2	8g	.2.	1	1/4	0.4263(4)	1/4	0.008(2)
O1	8h	m..	1	0	0.1497 (5)	1.0407(13)	0.0105
O2	8i	.m.	1	0.1170 (8)	1/4	0.7781(12)	0.0105
O3	16j	1	1	0.2838 (6)	0.3592(4)	0.0623(7)	0.0105
O4	16j	1	1	0.1384 (5)	0.5001(4)	0.2277 (12)	0.0105

Figure 1

Observed, calculated, and difference plots from the Rietveld refinement of the XRPD (Cu Kα radiation) pattern of NaCoV2(PO4)3. The asterisk corresponds to the peak (022) of the impurity phase NaVP2O7.

Figure 2

Images and EDX analyses for NaFeV2(PO4)3 (a), NaCoV2(PO4)3 (b), and NaNiV2(PO4)3 (c).

Figure 3

TGA thermal analysis for NaCoV2(PO4)3 after the first heat treatment at 500 °C.

For the NaNiV2(PO4)3 sample, the full-pattern matching performed using a single phase showed the presence of four unindexed peaks (see the inset of Figure ). Therefore, a search-match process within the ICDD database[44] was performed to identify the impurity phase that corresponds to Ni12P5.[45] Consequently, the atomic positions of NaCoV2(PO4)3 and Ni12P5 were used for the Rietveld refinement of NaNiV2(PO4)3 (1) and Ni12P5 (2) structures, respectively. The same constraints applied during the structural refinement of NaCoV2(PO4)3 were also applied to NaNiV2(PO4)3. The Rietveld analysis of the XRPD data collected at 293 K led to the reliability factors listed in Table [Rp = 1.9%, Rwp = 2.4%, RB ( = 8.9%, RB ( = 7.3%, χ2 = 3.028]. The final atomic positions are given in Table . Figure shows good agreement between the experimental and calculated patterns. The elemental analysis confirmed the chemical composition of NaNiV2(PO4)3 (Figure c) although in the presence of a large amount of impurity phase Ni12P5 [23.1 (3) wt %]. The formation of this phase is mainly due to the carbothermic reduction of nickel oxide at high temperatures. This phenomenon was studied in the past by several researchers.[46−54] It should be noted that the carbonaceous residues result from the decomposition of citric acid.

Figure 4

Observed, calculated, and difference plots from the Rietveld refinement of the XRPD (Cu Kα radiation) pattern of NaNiV2(PO4)3. The asterisk corresponds to the peaks of the impurity phase Ni12P5.

In the NaFeV2(PO4)3 sample, a large amount of impurity phase NaVP2O7 was observed [see the asterisk in Figure ]; therefore, the atomic positions of NaCoV2(PO4)3 and NaVP2O7 were used for the Rietveld refinement. The structure of NaFeV2(PO4)3 (3) was first refined and then the structure of NaVP2O7 (4). While the structure of NaFeV2(PO4)3 converged to a reasonable model, that of NaVP2O7 collapsed completely. Consequently, in the subsequent refinement cycles, the atomic positions of NaVP2O7 were constrained. The Rietveld analysis led to the reliability factors listed in Table [Rp = 0.024, Rwp = 0.035, RB (=0.097, RB (=0.190, χ2 = 6.350]. The amount of impurity phase NaVP2O7 was 28.0 (3) wt %. The final atomic positions are given in Table . Figure shows a good agreement between the experimental and calculated patterns. The elemental analysis confirmed the chemical composition of NaFeV2(PO4)3 (Figure a). A second Rietveld refinement also was performed by considering NaFeV2(PO4)3 as a single phase and the impurity peaks as excluded regions. Although the reliability factors were similar to those of the first refinement, the interatomic distances of Fe2–O were shorter than expected.

Figure 5

Observed, calculated, and difference plots from the Rietveld refinement of the XRPD (Cu Kα radiation) pattern of NaFeV2(PO4)3. The asterisk corresponds to the peaks of the impurity phase NaVP2O7 and the plus symbol corresponds to an unidentified impurity.

Crystal Structure

The NaMV2(PO4)3 (M = Fe, Co, Ni) compounds are isostructural to NaCoCr2(PO4)3, which crystallizes with the stuffed α-CrPO4-type structure.[35,55] The structure consists of layers of [(M2/V2)2O10] dimer units (M = Fe, Co, Ni) sharing corners and edges with the [P2O4] tetrahedra (Figure d). These layers are bridged by infinite chains of [V1O6] octahedra sharing corners with [P1O4] tetrahedra (Figure c). This gives rise to a three-dimensional framework with channels along the a and b axes, within which the sodium atoms are located (Figure a,b). Interatomic distances and bond valence sums (BVSs)[56,57] are listed in Table .

Figure 6

Projection views of the structure of NaMV2(PO4)3 (M = Fe, Co, Ni) along the a axis (a) and the b axis (b). View along the a axis of the infinite chains (c) and the layers (d). View of the coordination polyhedron of Na1 (e).

Table 3

Interatomic Distances (in Å) and Bond Valence Sums (BVSs) for NaMV2(PO4)3 (M = Fe, Co, Ni)a

	Distance (Å)
	NaFeV2(PO4)3	NaCoV2(PO4)3	NaNiV2(PO4)3
Na1–O1 (×2)	2.736(15)	2.606(15)	2.566(19)
Na1–O2 (×2)	2.574(15)	2.718(15)	2.772(19)
Na1–O3 (×4)	2.721(8)	2.717(7)	2.705(6)
	b<2.669>	<2.689>	<2.687>
	a0.76 b[8]	0.73 [8]	0.75 [8]
V1–O1 (×2)	1.974(8)	2.030(10)	1.981(7)
V1–O4 (×4)	2.031(8)	1.927(9)	2.052(6)
	<2.012>	<1.961>	<2.028>
	2.95 [6]	3.41 [6]	2.83 [6]
M2/V2–O2 (×2)	2.127(9)	2.016(9)	2.075(6)
M2/V2–O3 (×2)	2.019(6)	2.017(6)	2.018(5)
M2/V2–O4 (×2)	2.121(8)	2.174(8)	2.116(5)
	<2.089>	<2.069>	<2.069>
	2.38/2.42 [6]	2.21/2.58 [6]	1.96/2.54 [6]
P1–O1 (×2)	1.529(10)	1.530(11)	1.524(8)
P1–O2 (×2)	1.530(13)	1.529(12)	1.524(9)
	<1.5295>	<1.5295>	<1.524>
	5.07 [4]	5.07 [4]	5.14 [4]
P2–O3 (×2)	1.530(8)	1.530(7)	1.524(6)
P2–O4 (×2)	1.531(8)	1.531(8)	1.528(6)
	<1.5305>	<1.5305>	<1.526>
	5.05 [4]	5.05 [4]	5.12 [4]

Bond valence sum, BV = e( with the following parameters: b = 0.37, r0 (NaI–O) = 1.803, r0 (VIII–O) = 1.749, r0 (FeII–O) = 1.734, r0 (CoII–O) = 1.692, r0 (NiII–O) = 1.654, and r0 (PV–O) = 1.617 Å.

Average distances are given in <>, and coordination numbers are given in [].

Crystal Structure of NaCoV2(PO4)3

The vanadium atoms V1 and V2 occupy the atomic positions 4a (0,0,0) and 8g (1/4,0.6383,1/4), respectively, and are octahedral coordinated to oxygen atoms. The [V1O6] and [(Co2/V2)O6] octahedra are strongly distorted. In [V1O6] octahedra, the dV1–O distances range from 1.927 to 2.030 Å with an average distance of 1.961 Å, whereas in [(Co2/V2)O6] octahedra, the dV2–O distances are slightly larger. They range from 2.016 to 2.174 Å with an average value of 2.069 Å. This difference is mainly due to the presence of a statistical disorder of Co2+ and V3+ on the atomic position 8g (1/4,0.6383,1/4). Since the ionic radius of Co2+ (0.745 Å) is larger than that of V3+ (0.64 Å), dCo/V–O is larger than dV–O.[58] The [P1O4] and [P2O4] tetrahedra are regular in shape since the dP–O distances were constrained. The Na1+ ions are bonded to eight oxygen atoms belonging to six different [VO6] octahedra (Figure e). The dNa1–O distances range from 2.606 to 2.718 Å with an average value of 2.689 Å. The BVS values of 0.73 for Na1 indicate that Na1 is under bonded.

Comparison of NaMV2(PO4)3 (M = Fe, Co, Ni)

The quantitative comparison between the three isotypic structures of NaMV2(PO4)3 (M = Fe, Co, Ni) was performed using the program Compstru.[59−62] The numerical details of the comparison are given in Table . The crystal structures of the Ni and Co phases are highly similar with a measure of similarity Δ = 0.031. The positions of the cations are essentially coincident, whereas large displacements were observed for the atom pairs O3 and O4, which are coordinated to the P2 atom. Our careful examination using the Vesta program[63] indicates a slight tilt of the P2O4 tetrahedra (Figure S2). The crystal structures of the Fe and Co phases are also highly similar with a measure of similarity Δ = 0.019. Furthermore, the atom pairs O3 and O4 show the largest displacements as in the case of Ni vs Co.

Table 4

Numerical Details from the Comparison between the Structures of NaMV2(PO4)3 (M = Fe, Co, Ni) Using the Compstru Programa

			|u|/Å
atoms	Wyck.	(x,y,z)	Ni vs Fe	Co vs Ni	Co vs Fe
Na1	4e	(0,1/4,z)	0.1144	0.0407	0.0741
V1	4a	(0,0,0)	0.0000	0.0000	0.0000
V2/M2	8g	(1/4,y,1/4)	0.0013	0.0577	0.0593
P1	4e	(0,1/4,z)	0.0000	0.0775	0.0780
P2	8g	(1/4,y,1/4)	0.0105	0.0538	0.0646
O1	8h	(0,y,z)	0.0539	0.0432	0.0704
O2	8i	(x,1/4,z)	0.1117	0.0539	0.1032
O3	16j	(x,y,z)	0.0236	0.1484	0.1584
O4	16j	(x,y,z)	0.0627	0.2005	0.1426
degree of lattice distortion (S)			0.0037	0.0026	0.0015
maximum distance (dmax.)/Å			0.1144	0.2005	0.1584
arithmetic mean (dav)/Å			0.0429	0.1016	0.1027
measure of similarity (Δ)			0.025	0.031	0.019

|u| is the atomic displacement.

The crystal structures of the Ni and Fe phases were also compared (Δ = 0.025). In this case, the largest atomic displacements were observed for the atom pairs Na1 and O2. Although the dNa1–O2 distances were strongly shortened in the Fe phase (see Table ), no major change in the coordination polyhedron of Na1 was observed (Figure S3). The decrease of the cell volume [894.52, 889.8 (6), 884.42 (14), 876.16 (6) Å3] when replacing V2+ with Fe2+, Co2+, or Ni2+, respectively, is in good agreement with the decrease of the ionic radii of the octahedral coordinated atoms (IRV = 0.79, IRFe = 0.78, IRCo = 0.745, IRNi = 0.69 Å) and explains the shift of the XRPD patterns toward high θ angles (Figure S1). The formation of these four phases indicates that the α-CrPO4-type structure is very flexible and can accommodate small and large cations. It would be very interesting to test it as a host for magnesium and potassium.

Electrochemical Properties of NaCoV2(PO4)3

The electrochemical measurements were performed on the NaCoV2(PO4)3 sample since this phase was almost pure. Figure a shows the charge/discharge cycles of the [NaCoV2(PO4)3/electrolyte/Na] half-cell, between 0.1 and 3.0 V, at a 1C rate (1 e– transfer → 57 mA h/g). The charge/discharge profiles are nearly identical to those observed in NaV3(PO4)3. NaCo2+V2(PO4)3 and NaV2+V2(PO4)3 are isostructural, and in both compounds, the electrochemical process involves the redox couple V3+/V2+ as depicted in the equation below

Figure 7

Charge/discharge curves of the NaCoV2(PO4)3 sample, recorded at room temperature, in the CC mode, at a rate of 1C (1 e– transfer → 57 mA h/g), in the voltage range of 0.1–3.0 V vs Na+/Na (a), dQ/dV (b), and the capacity retention (c).

□2NaM2+(V3+)2(PO4)3 + 2 Na+ + 2 e–⇄ Na3M2+(V2+)2(PO4)3 (M2+ = V, Co; □ = vacancy)

NaCoV2(PO4)3 delivers an initial discharge and charge capacities of 290 and 180 mA h/g, respectively. An initial Coulombic efficiency of nearly 62% is due to the decomposition of the electrolyte and formation of the solid electrolyte interphase (SEI) layer [see the large irreversible peak around 0.5 V in the dQ/dV plot (Figure b)]. No conversion reaction took place since the ex situ XRPD pattern of the electrode after the first cycle was almost identical to that of the initial electrode (Figure S4). In the subsequent cycles, NaCoV2(PO4)3 delivers a reversible capacity of around 190 mA h/g that increases slightly during the first 40 cycles and then decreases to reach 75 mA h/g after 250 cycles (Figure c). This capacity fade is most probably due to the reduction of the electrolyte at low voltages.

It should be noted that a capacity of 190 mA h/g is much higher than 114 mA h/g expected for the intercalation/extraction of two sodium atoms. A similar phenomenon was observed in NaV3(PO4)3, and the authors concluded based on impedance spectroscopic analyses that the electrochemical process involves two different mechanisms.[36,37] Similarly, we conclude that in the voltage range of 3–0.7 V, NaCoV2(PO4)3 undergoes a multistep Na intercalation reaction with the insertion of two sodium atoms and the formation of Na3CoV2(PO4)3, and below 0.7 V, there is a pseudo-capacitive contribution. To confirm this, another cell was cycled between 0.7 and 3 V at a 1C rate (Figure a). After the formation of the SEI layer during the first discharge, the cell can be cycled reversibly for 250 cycles delivering an average specific capacity of 75 mA h/g (Figure b). This confirms that the capacitive contribution was eliminated and the electrolyte decomposition was avoided when the lower cutoff voltage was limited to 0.7 V.

Figure 8

Charge/discharge curves of the NaCoV2(PO4)3 sample, recorded at room temperature, in the CC mode, at a rate of 1C (1 e– transfer → 57 mA h/g), in the voltage range of 0.7–3.0 V vs Na+/Na (a) and the capacity retention (b).

The rate capability of NaCoV2(PO4)3 is depicted in Figure S5. In the voltage range of 0.1–3 V and at C/5 and 1C rates, the cell delivers ∼207 and 180 mA h/g, respectively. Meanwhile, at 5C, 10C, and 20C rates, no charge/discharge could take place. This slow charge discharge kinetic indicates that NaCoV2(PO4)3 most probably requires a carbon coating to increase its electronic conductivity. Hu et al. have demonstrated that the introduction of carbon is effective to improve the rate capability of NaV3(PO4)3.[37]

Conclusions

The new compounds NaMV2(PO4)3 (M = Fe, Co, Ni), synthesized via a sol–gel reaction route, crystallize with the stuffed α-CrPO4-type structure. Their structures, determined using XRPD data, are essentially coincident. These compounds are not stable at temperatures above 850 °C. Only the cobalt phase was almost pure when annealed at 750 °C for 24 h under argon. The structure of NaCoV2(PO4)3 contains two vacant sites, which enables the intercalation of two sodium atoms leading to the reduction of both vanadium atoms from V3+ to V2+ and the formation of the new phase Na3CoV2(PO4)3. This mechanism takes place between 3 and 0.7 V; however, between 0.7 and 0.1 V, a capacitive contribution is observed. After the first cycle in the voltage range of 0.1 to 3 V, the cell can be cycled reversibly and delivers ∼190 mA h/g. This capacity increases slightly during the first 40 cycles and then fades quickly to 75 mA h/g after 250 cycles. If the cell is cycled between 0.7 and 3 V, NaCoV2(PO4)3 exhibits good sodium uptake and removal behavior with a stable capacity of ∼75 mA h/g at a 1C rate and an operation potential around 1.4 V. These results indicate that the α-CrPO4-type structure is very flexible and can accommodate a wide range of cations and therefore can be used as a host for sodium and most probably for other guest ions such as magnesium or potassium.

Experimental Section

Synthesis

The compound NaCoV2(PO4)3 was prepared via a sol–gel reaction route, from a 1:1:2:3:2 molar ratio of sodium acetate CH3COONa (Aldrich, ≥99%), cobalt acetate tetrahydrate Co(CH3COO)2·4H2O (Aldrich, ≥99.995%), ammonium metavanadate NH4VO3 (Aldrich, ≥99%), ammonium dihydrogen phosphate NH4H2PO4 (Aldrich, ≥99.99%), and cirtic acid C3H5O(COOH)3 (Riedel-deHaën) (CA). First, NH4VO3 and CA with a 1:1 molar ratio were dissolved in 40 mL of water to form a clear blue solution, and then Co(CH3COO)2·4H2O was dissolved in 20 mL of water and added to the blue solution (solution A). The CH3COONa and NH4H2PO4 precursors were dissolved together in 40 mL of water (solution B). Solution B was then added dropwise to solution A under continuous stirring. The solution was finally left to dry at 90 °C overnight. The resulting powder was pelletized, put into an alumina crucible, and heated at 500 °C for 3 h under argon to decompose the precursors and release the H2O, NH3, and CO2 molecules. The pellet was then ground, pelletized, and annealed at 750 °C for 24 h under argon. The synthesis procedure mentioned above was also used to prepare NaFeV2(PO4)3 and NaNiV2(PO4)3. The progress of the reactions was followed by X-ray powder diffraction (XRPD). The three phases were formed since the XRPD patterns were very similar to the theoretical XRPD of NaV3(PO4)3; however, impurities were observed (Figure S1). Therefore, several other heat treatments were performed under different conditions. At temperatures below 750 °C, most of the samples were partly amorphous, whereas at temperatures above 750 °C, the amount of impurities increased and weight losses were noticed. It should be mentioned that the samples used in the current manuscript contain the least amount of impurities.

X-ray Powder Diffraction Measurement

To ensure the purity of our samples, XRPD measurements were performed. The data were collected at room temperature over the 2θ angle range of 5° ≤ 2θ ≤ 100° with a step size of 0.02° using a Bruker D8 advance diffractometer operating with Cu Kα radiation. Full-pattern matching refinements were performed with the Jana2006 program package.[42] The backgrounds were estimated by Legendre functions, and the peak shapes were described by a pseudo-Voigt function. Rietveld refinements were then performed.

Electron Microprobe Analysis

Semiquantitative energy-dispersive X-ray spectroscopy (EDX) analyses of the prepared samples were carried out with a 7610F (JEOL) scanning electron microscope (SEM). The experimentally observed compositions were close to the ideal ones of NaMV2(PO4)3 (M = Fe, Co, Ni).

Thermal Analysis

Thermal gravimetric analysis (TGA) was carried out on the NaCoV2(PO4)3 sample using a TA Instruments Discovery Thermogravimetric Analyzer (Discovery TGA). The measurement was conducted between 25 and 1250 °C at a heating rate of 10 °C/min. The experiment was performed in an alumina crucible under an argon atmosphere.

Electrochemical Cycling

Positive electrodes were made from mixtures of NaCoV2(PO4)3 powder, acetylene black (AB), and polyvinylidene fluoride (PVDF) in a weight ratio of 80:12:8. The resulting electrode films were cut into discs (Φ = 14 mm) and dried at 120 °C for 12 h under vacuum. The electrolyte was 1 M NaPF6 dissolved in ethylene carbonate (EC) and propylene carbonate (PC) (1 M NaPF6 in EC/PC). Coin-type cells (CR2032) embedding NaCoV2(PO4)3/NaPF6 + EC + PC/Na were assembled in an argon-filled glove box with a Whatman glass fiber separator (Grade GF/A−Φ = 20 mm). The areal mass loadings for cells 1, 2, and 3 were 0.291, 0.284, and 0.287 mg/cm2, respectively. Room-temperature galvanostatic cycling tests (constant current mode) for all the electrodes of NaCoV2(PO4)3 were performed using an Arbin cycler, and the cyclic voltammetry measurements were performed using a Solartron battery cycler (1470E).