WO2011038499A1 - Sodium metal phosphate olivines for sodium-ion batteries - Google Patents

Abstract

Novel olivine phase compounds for use as cathode material for sodium-ion batteries, having a high energy density and corresponding high rate performance, and a process for their synthesis by a low temperature solid state method are described herein. In particular, olivine phase compounds having the general formula Na[Mn_I-xM_x]PO₄, where M is selected from the group consisting of Fe, Ca and Mg and where preferably 0 ≤ x ≤ 0.5, and Na_xFePO₄, where O ≤ x ≤1, are synthesized by a simple solid state reaction at low temperature. These compounds exhibit preferential properties for use as cathode materials in sodium-ion batteries, including reversible electrochemical de/intercalation.

Description

SODIUM METAL PHOSPHATE OLIVINES FOR SODIUM-ION BATTERIES

This application claims priority from United States Provisional Patent application No. 61/272,517 filed October 2, 2009.

TECHNICAL FIELD The present invention relates to batteries. In particular, the present invention relates to improved sodium-ion battery cathode materials.

BACKGROUND OF THE INVENTION

Lithium ion rechargeable batteries play a vital role as the prominent power source for cell phones, laptop computers, digital cameras, power tools, electrical assist bicycles, and many other consumer products. In recent years, lithium ion battery technology has been harnessed for use in automotive and stationary applications. Indeed, in the near future, numerous electric vehicles and extended range electric vehicles powered with large- scale lithium-ion storage batteries, will be made available to the public.

At the present time, lithium resources are confirmed to be unevenly distributed in South America. The cost of lithium raw materials has roughly doubled from the first practical application in 1991 to the present, and such cost may drastically increase as the demand of lithium increases in response to commercialization of the large-scale lithium-ion batteries, particularly for automotive applications. In contrast to lithium resources, however, there is no doubt that sodium deposits are inexhaustible and unlimited around the world. Next to lithium, the electrochemical equivalent and standard potential of sodium are the most advantageous for aprotic battery applications.

Notwithstanding the merits of using sodium compositions in aprotic battery systems, only a very limited number of successful reports are found in the field of sodium insertion of metal oxide systems, such as layered NaMe0₂ (where Me = transition element) materials compared to LiMe0₂. For example, Doeff et al. (Doeff, M. M., Peng, M. Y., Ma, Y. & De Jonghe, L. C, J. Electrochem. Soc. 141 , L145 (1994)) and Braconier et al. (Braconnier, J. J.; Delmas, C; Hagenmuller, P. Materials Research Bulletin 1982, 17, 993) reported the reversible de(intercalation) of Na from Nao.₄₄Mn0₂ and Na_xM0₂ (M=Co, Cr and Ni), respectively at high potential and Bruce et al. (C. Zhang, S. Gamble, D. Ainsworth, A. M. Z. Slawin, Y. G. Andreev, P. G. Bruce, Nature Materials 8, 580 (2009)) reported the same for Na_0.44MnO₂ using a sodium polymer electrolyte. Tarascon et al. (Tarascon, J. M; Guyomard, D. G.; Wilkens, B.; McKinnon, W. R.; Barboux, P. Solid State Ionics 1992, 57, 1 13; and Tarascon, J. M.; Hull, G. W. Solid State Ionics 1986, 22, 85) demonstrated that it is possible to (de)intercalate Na ions in λ-Μη0₂ and Na_xMo204.

Regarding low potential materials, it has been demonstrated that Na ions can be electrochemically de/inserted in hard carbons; and recently, the concept of using metal oxide materials has been introduced. In the case of phosphate -based cathodes, similarly, only a handful of materials have been developed: namely NaVP0₄F (Barker, J., Gover, R. K. B., Burns, P. & Bryan, A. J. Hybrid-ion. A lithium-ion cell based on a sodium insertion material. Electrochemical and Solid-State Letters, 9, A 190 (2006)) reported to have a tavorite structure; Na3V2(P0₄)3F₃ (Le Meins, J-M., Crosnier-Lopez, M. P., Hemon- Ribaud, A., & Courbion, G. Phase transitions in the Na₃M₂(P0₄)₂F₃ family: synthesis, thermal, structural, and magnetic studies. J. Solid. State Chem. 148, 260 (1999)), and Na₂FeP0₄F (Ellis, B.; Makahnouk, W. R. M; Makimura, Y.; Toghill, K.; Nazar, L. F. Nature Mater. 6, 749 (2007)). The latter compound, Na₂FeP0₄F, has numerous advantages over the first, NaVP0₄F, including a layered or two dimensional structure that is particularly suitable for ion mobility and an inexpensive nontoxic metal. However, owing to the 3.4V redox couple, Na₂FeP0₄F suffers from the viewpoint of a lower gravimetric capacity and lower energy density. For this reason, formation of a novel, metastable mixed metal olivine-phase of a sodium metal phosphate is desired, especially for a low-cost nontoxic metal such as iron, or manganese which has a higher voltage redox couple and hence exhibits a higher energy density. Notably, both NaFeP0₄ and NaMnP0₄ form as an electrochemically inactive maricite phase, not the olivine phase, under conventional synthetic conditions at high temperature.

SUMMARY OF THE INVENTION Novel olivine phase compounds for use as cathode material for sodium-ion batteries, having a high energy density and corresponding high rate performance, and a process for their synthesis by a low temperature solid state method are described herein.

In particular, the present invention teaches metastable olivine phases of Na[Mnj. _xM_x]P0₄ (where M is selected from the group consisting of Fe, Ca and Mg) nanorods formed by a topatactic reaction using simple solid state synthesis at low temperatures. This is in contrast to the high temperature synthesis of NaMnP0₄, for example, which forms the electrochemically inactive maricite phase. Na[Mni_-xM_x]P0₄ exhibits reversible electrochemical de/intercalation, making this composition practical for use as cathode materials in sodium-ion batteries.

According to one aspect of the present invention, there is provided a compound in the olivine phase for use as a cathode having the general formula Na[Mni_-xM_x]P0₄, wherein M is selected from the group consisting of Fe, Ca and Mg, and wherein 0 < x < 0.6, and preferably 0 < x < 0.5. Even more preferably, where M is Ca or Mg, 0 < x < 0.2, and where M is Fe, 0 < x < 0.5.

In another aspect of the present invention, a process for producing Na[Mni._xM_x]P0₄ is disclosed, the process characterized by the steps of: (i) synthesizing ammonium metal phosphates ΝΗ₄ΜΡ0₄·Η₂0, where M is selected from the group consisting of Mn, Mno.₅Feo.5, Mno_.8Cao.₂ and Mn_0.8Mg₀.₂; (ii) ball-milling, or grinding, the resultant product from step (i) with excess sodium acetate trihydrate (CH₃C0₂Na'3H₂0); and (iii) heating the mixture from step (ii) to form Na[Mni_-xM_x]P0₄, where M is selected from the group consisting of Fe, Ca and Mg. At the heating step (step (iii)), the temperature may be in the range of 65 to 100°C. Un-reacted sodium acetate trihydrate is removed by ethanol wash. The oxidation of NaMP0₄ where M = Mno.₅Feo 5 is accomplished using a slight excess of NOBF₄ or N0₂BF_4> in acetonitrile, or other suitable solvent, to obtain Na₀ sMPO₄ and Nao.₂MP0₄, respectively.

In another aspect of the present invention, the olivine phase compound Na_xFeP0₄ is described and a process for synthesizing same is taught, via oxidation of LiFePC with a slight excess of NOBF or other conventional oxidizing agent, in acetonitrile, or other suitable solvent, to obtain FeP0₄, which is reduced with excess Nal in acetonitrile to obtain Na_xFeP0₄, where 0 < x < 1.

In yet another aspect of the present invention, use of Na[Mni-_xM_x]P0₄ in the olivine phase as a cathode active material for a sodium-ion battery, wherein M is selected from the group consisting of Fe, Ca and Mg, and wherein 0 < x < 0.6, and preferably 0 < x < 0.5. In another of its aspects, it may be characterized by the use of Na_xFeP0₄, as the cathode active material, where 0 < x < 1.

In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of the preferred embodiments is provided below by way of example only and with reference to the following drawings, in which:

Figure 1 illustrates the structures of (a) Olivine and (b) Maricite; Figure 2 shows the XRD patterns of NaMP0₄ (M=Mn₀.5Fe_{0 5}, Mn, Fe, Mn_0.8Cao_.2, Mn₀.₈Mgo.₂);

Figure 3 shows the Rietveld refinements of NaMP0₄ for M as indicated: (a) Mn; (b) Fe; (c) Fe₀.₅Mn_0.5; and (d) Mgo_.2Mn₀.₈;

Figure 4 shows a schematic diagram for the topochemical synthesis of NaMP0₄ nanorods; Figure 5 illustrates SEM images of (a) ΝΗ₄ΜΡ0₄·Η₂0 and (b) NaMP0₄; Figure 6 illustrates TEM images of (a) NaMnP0₄ (inset: ED pattern); (b) NaMgo_.2MnO.8PO₄ (inset: EDS profile) and (c) NaCao.₂Mn0.8P0₄ (inset: EDS profile);

Figure 7 A shows the XRD patterns of NaMP0₄ for M = Fe at various temperatures;

Figure 7B shows the XRD patterns of NaMP0₄ for M - Mno_.5Feo 5 at various temperatures; Figure 7C shows the XRD patterns of NaMP0₄ for M = Mn at various temperatures;

Figure 8 shows the XRD patterns of Na_xFeo_.5Mno_.5P0₄ for varying degrees of x;

Figure 9 shows the Rietveld refinement of Nao.₅Fe_0.5Mn₀.₅P0₄;

Figure 10 shows the XRD patterns of Nao.5Feo.5Mno.5PCU: (a) as prepared; and (b) after two (2) months; Figure 1 1 illustrates a discharge profile of NaMno_.5Feo_.5P0₄ over three cycles; Figure 12 shows the XRD patterns of Na_xFeP0₄; and

In the drawings, preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the invention.

DETAILED DESCRIPTION OF THE INVENTION

All terms used herein are used in accordance with their ordinary meanings unless the context or definition clearly indicates otherwise. Also, unless indicated otherwise except within the claims the use of "or" includes "and" and vice-versa. Non-limiting terms are not to be construed as limiting unless expressly stated or the context clearly indicates otherwise (for example, "including", "having", "characterized by" and "comprising" typically indicate "including without limitation"). Singular forms included in the claims such as "a", "an" and "the" include the plural reference unless expressly stated or the context clearly indicates otherwise. Referring to Figure 1 , which illustrates the structures of (a) olivine and (b) maricite, it is well known that the thermodynamically stable form of NaFeP0₄ is the mineral maricite. NaFeP0₄ is similar to the well known LiFeP0₄ olivine structure in terms of its phosphate framework, but the Ml and M2 sites are occupied by Fe and Na respectively, which is exactly the reverse of LiFeP0₄. This gives rise to a different connectivity of the Fe and Na octahedra which blocks Na-ion migration pathways, and hence results in a structure that is not amenable to Na⁺ (de)insertion. NaMnP0₄ adopts the maricite structure in its thermodynamically stable form, and has also been reported as the mineral natrophilite, where the Ml and M2 sites are both half occupied by Mn and Na . It is similarly electrochemically inactive.

The crystallization of Na[Fe,Mn]P0₄ as a metastable olivine phase thus entails the use of low temperature synthesis methods. This approach employs a low temperature molten salt synthesis, based on the topotactic transformation of NH₄FeP0₄*H₂0 to LiFeP0₄ in hydrothermal media. The transformation of ΝΗ₄ΜΡ0₄·Η₂0 to NaMP0₄ follows a similar pathway by direct ion exchange between NH₄ ⁺ and Na⁺ using molten CH₃C0₂Na-3H₂0.

As described herein, new metastable olivine phases of sodium metal phosphates, Na[Mni_-xM_x]P0₄ (where M is selected from Fe, Ca and Mg) nanorods are synthesized by a simple solid state reaction at low temperature (< 100°C), which is attributed to a topotactic reaction that converts ΝΗ₄[Μη_]-χΜ_χ]Ρ0₄·Η₂0 (M= Fe; Ca; Mg) to Na[Mni_-xM_x]P0₄. The crystal structures of these olivine phase compounds were characterized via XRD Rietveld refinement and electron diffraction. In contrast to that of LiMP0₄ (M=Fe, Mn), a full range of solid solution behavior was observed for olivine Nai._xMno_.5Feo_.5P0₄ and is ascribed to ion size effects. The solid solution behavior of NaMno_.5Feo_.5P0₄ was confirmed by electrochemical characterization.

The reaction of ammonium metal phosphates ΝΗ₄ΜΡ0₄·Η₂0, where M is selected from the group consisting of Mn, Mno.₅Feo.₅, Mn₀.8Ca₀.₂ and Mno.gMgo_.2, with sodium acetate trihydrate (CH₃C0₂Na^»3H₂0) between 65 and 100°C readily forms the desired olivine Na[Mni_-xM_x]P0₄ phases where M is selected from the group consisting of Fe, Ca and Mg), and wherein 0 < x < 0.6, and preferably 0 < x < 0.5. The Ca or Mg substituted NaMnP0₄ olivine phases are prepared with 10 - 20% substitution of alkaline earth ion on the Mn²⁺ site, in order to adjust the framework slightly to favour Na⁺ mobility. Un-reacted sodium acetate trihydrate is removed by ethanol wash. The oxidation of NaMP0 where M = Mn₀ 5Fe_{0 5} is accomplished using a slight excess of NOBF₄ or N0₂BF_4i in acetonitrile, or other suitable solvent, to obtain Na_0.5MPO₄ and Nao.₂MP0₄, respectively. Note that other conventional oxidizing agents, including K₂S₂0₈ can be utilized at this stage. Further note that the materials can be post-heat treated to increase their crystallinity. Synthesis of Na[Mni_-xM_x]P0₄ as the olivine phase preferably takes place under low temperatures, that is less than 350°C, and preferably less than 200°C.

Regarding the synthesis of NaFeP0₄ in the olivine phase, it is noted that the olivine phase of the pure iron compound NaFeP0₄ cannot be prepared by the direct method described above, which produced maricite instead, because NH₄FeP0₄ ^»H₂0 decomposes about 100 degrees lower than the temperature at which ion exchange occurs. Therefore, in accordance with the present invention, the NaFeP0₄ olivine phase is prepared by a chimie douce method, wherein olivine LiFeP0₄ is delithiated at room temperature with NOBF₄ to form the orthorhombic phase of FeP0₄ following the standard procedure, which is then sodiated with excess Nal.

The process for synthesizing a compound in the olivine phase having the general formula general formula Na_xFeP0₄, where 0 < x < 1 , is thereby characterized by the steps of oxidizing LiFeP0₄ with excess of a nitronium salt in acetonitrile to produce FeP0₄; and reducing FeP0₄ with excess Nal in acetonitrile. Again, any suitable conventional oxidizing agent may be used, including but not limited to NOBF₄, N0₂ BF₄ and K₂S₂0₈.

Referring next to Figure 2, the corresponding X-ray diffraction (XRD) patterns of the materials NaMP0₄, where M = Mn_{0 5}Fe₀.5, Mn, Fe, Mn_{0 8}Ca_{0 2}, or Mn_{0 8}Mg_{0 2} are in good accord with the predicted patterns based on crystallization of the target olivine phases. Figure 3 shows Rietveld refinement of the XRD patterns from Figure 2 for NaMnP0₄, NaFeP0₄, Na[Mgo.₂Mn₀.8]P0₄, and NaFe₀.5Mno_.5P0₄, for the purpose of confirming their structures. The lattice parameters of Na_xMP0 (M=Fe, Mn, Fe_0.5Mn₀.5, Mgo_.2Mno.8, Ca_0.2Mn_0.8) compounds obtained from Rietveld refinement are summarized in Table 1 below. a(A) b(A) c(A) Vol me(A³ ) wRp (%)

NaFeP0₄ 10.4206(4) 6.2166(2) 4.9529(2) 320.86 5.72

NaMnP0₄ 10.5577(2) 6.3359(1 ) 4.9966( 1 ) 334.24 6.93

NaFeo_.5Mno.5PO₄ 10.4954(3) 6.2966(2) 4.9808(2) 329.16 6.71

Nao.5Feo 5Mno.5PO4 10.2678(5) 6.0905(2) 4.9608(3) 3 10.23 6.30

NaMg_{0 2}Mno.₈P0₄ 10.5166(2) 6.3089(1) 4.9844(1) 330.71 6.41

NaCao.2Mno.gPO₄* 10.62 6.38 5.02 340 -

The lattice parameters of NaFeP0₄ synthesized here by chemical insertion of Na are in agreement with those of electrochemically derived NaFeP0₄. The new mixed metal phases have lattice parameters in accord with their end members, suggesting they are solid solution phases. The unit cell volume of NaFeo.5Mno.5PO4 lies about half-way between

2+

that of NaFeP0₄ and NaMnP0₄. Mg2+ substitution on the Mn site decreases the unit cell volume in accord with its smaller cation size, whereas Ca²⁺ substitution results in an increase. Importantly, the occupancies of both Ml and M2 sites are allowed to vary during the final stages of the refinement to allow for anti-site mixing, with almost no cation disorder between the sites. This is another advantage of the proposed topatactic reaction sequence, which is displayed in Fig. 4, a schematic diagram for the topochemical synthesis of NaMP0₄ nanorods. Comparison of the structures of olivine NaMP0₄ (4a) and ΝΗ₄ΜΡθ4·Η₂0 (4b), where M = Fe, Mn or Mg, indicates how these compositions could be related by a simple transformation. The connectivity of the iron and phosphate polyhedra in the (100) plane of NaMP0₄ is identical to that in the corresponding (101) plane of ΝΗ4ΜΡΟ4Ή₂Ο. Note the difference in their space groups switches the a and b axes, where the repeating polyhedral motif is the same as shown in Figure 4. Upon rapid ion exchange of NH₄ ⁺ for Na⁺, the adjacent sheets are "knitted" together by condensation of the Na0₆ octahedra to crystallize olivine NaMP0₄ (4c). The structure of the ΝΗ₄ΜΡ0₄·Η₂0 precursors is perfectly ordered due to the bulky NH₄ ions, and thus this order is faithfully replicated in the structure of the resulting products. The temperature is too low to induce atomic position rearrangements such as switching of the Fe/Na sites.

As illustrated in Figure 5, SEM images reveal the transformation of the flat plate morphology of the: (a) layered ΝΗ₄ΜΡ0₄·Η₂0 crystals to the (b) nanorod morphology of the NaMP0₄ olivine structure. The formation of nanorods is caused by a high interface strain at the grain boundaries between ΝΗ₄ΜΡ0₄·Η₂0 and NaMP0₄. The d-spacing corresponding to the (010) reflection in ΝΗ₄ΜΡ0₄·Η₂0 is quite different from the equivalent (100) reflection for NaMP0₄ owing to the size of the bulk NH₄ ⁺ ion in the interlayer space, for example, in the case of M = Mn, doio ⁼ 8.8 A (ΝΗ₄ΜηΡ0₄·Η₂0) and dioo =4.7 A (NaMnP0₄). Although the replacement of NH₄ ⁺ for Na⁺ is topotactic, it is not instantaneous within the interlayer gallery, and hence this large mismatch induces cleavage during the ion exchange process that creates nanorods. Based on the topatactic transformation of ΝΗ₄ΜΡ0₄·Η₂0 (Pmn2i space group) to NaMP0₄ (Pnma space group) the long axis of the NaMP0₄ nanorod must be either the b or c axis.

TEM and electron diffraction study results, displayed in Figure 6A reveal that the long axis of all the NaMP0₄ nanorods is the b axis, which is the facile direction of ion transport for olivine materials. However, the nanorod morphology can be readily altered by ball-milling the material, either with - or ideally, with carbon to enhance the conductivity of the material for use as a sodium-ion electrode. Elemental analysis carried out in the TEM using EDS also identified that Mg and Ca were substituted into the corresponding Na[Mgo_.2Mno.8]P0₄ lattices (shown in Figure 6B) and Na[Cao_.2Mno_.8]P0₄ lattices (shown in Figure 6C).

Importantly, although NaFeP0₄ cannot be prepared directly as the olivine phase, this is easily accomplished for all of the NaMnP0₄ compounds (including substitution with 1/2 Fe and/or Mg). However, the olivine phase does convert to the more stable maricite phases at about 450°C for NaFeP0₄ at a higher temperature, ca. 500°C for Na[Fe₀.₅Mno.₅]P0₄, and at the highest temperature, ca. 550°C for NaMnP0 , as shown by XRD patterns in Figure 7. Thus, the olivine NaMP0₄ materials are metastable phases, but are perfectly stable under normal operating conditions of an electrochemical cell containing a non-aqueous electrolyte.

As shown in Figure 8, desodiation of olivine a(Feo.5 no.5)P0₄ to form Nao_.5(Fe_0.5Mno ₅)P0₄ and Na₀.2(Feo.5Mno.₅)P0₄ forms a single phase composition as shown by the XRD patterns. Notably, the XRD peaks gradually shift to a higher 2Θ angle as the degree of Na extraction increases, and no evidence for a two phase mixture of the end member phases is observed. Referring next to Figure 9, formation of the single phase of Nao.5(Feo_.sMno.5)P0₄ is confirmed by Rietveld refinement, and produces good agreement factors. The occupancy of Na was refined to be 0.50(1), that is, the target value. In Figure 10, the phase stability of Nao.5Feo 5Mno.5PO4 is demonstrated by XRD patterns of Nao.5Feo.5Mno.5PO4: (A) as prepared and (B) after 2 months. As shown in Figure 10, the Nao.5(Feo.5Mno.5)P0₄ phase is stable under air at room temperature for more than at least 2 months.

The above is in strong contrast to the distinct two phase behavior on delithiation observed for LiFeP0₄, but similar to that observed for delithiation of Li(Fe_xMn_y)P0₄ (i.e, x = 0.6, y = 0.4) to form Li_{0 5}(Fe_xMn_y)P0₄. This has been experimentally confirmed by for a composition very close to Lio.5(Feo_.5Mn₀.5)P0₄. However, the latter shows limited solid solution behavior, but Na(Fe₀ 5Mno_.s)P0₄ exhibits solid solution behavior over the full region. The volume difference between two phases is a critical factor to determine whether a solid solution is formed. Based on the Hume-Rothery rules for metals, the formation of a solid solution is unfavorable when the atomic radii of the solute and solvent atoms differ by more than 15%. In the case of Na(Feo_.sMno.5)P0₄, the volume difference is 21%, which is much larger than the corresponding volume difference for LiFeP0₄ (6.7 %). Therefore, this phenomenon is not explained by the previous simple model, rather the driving force to make the solid solution appears to be the interface stress caused by significant strain at the grain boundaries between Na-rich and Na-poor phases of Nai. x(Feo.5Mno.5)P0₄. The interface effect on the formation of solid solutions in intercalation compounds shows that the miscibility gap decreases as the size of Li_xTi0₂ nanoparticles decreases. This result is attributable to the energy penalty due to interface energy: (x₂ - x)G₁ - (x ~ x₁)G₂ Α(^χ)γ

^ΔΟπ>ιχ(^χ) Δ0(χ)

Where A(x), γ, VLi, V are the area of the interface between the two phases 1 and 2, the interface energy, the (inserted) Li-ion molar volume and the volume of the particle, respectively. The last term is related to the interface energy, and its effect becomes more dominant as particle size decreases, because A(x) and V scale with r² and r³, respectively (where r is the particle radius). Therefore, the energy gain due to phase separation, represented by the free energy of mixing AG_mj_x, decreases for smaller particle sizes, resulting in the decrease of miscibility gap for Li_xTi0₂ nanoparticles. In the case of Na(Feo_.5Mn_{0 5})P0₄, the particle size is small. Conversely, the interface energy, γ is very high due to the high interface strain owing to the large size of the Na ion. Therefore, a single phase regime is formed. Such solid solution behavior was also confirmed by the appearance of sloping voltage profiles in the electrochemical analysis. The charge- discharge curves of NaFeo_.sMno.sPCX} over three cycles are shown in Figure 1 1. Cells were constructed using an electrolyte comprised of 1M NaC10₄ in PC at room temperature. As FeP0₄ is partially sodiated, the XRD reflections for Nai_-xFeP0₄ are noticeably shifted compared to NaFeP0₄, but not continuously. In the XRD patterns shown in Figure 12, the formation of a single phase on desodiation of NaFeP0₄ is illustrated, however the peak broadening did not permit refinement of the structure. Its formation may be attributed to subtle factors that drive interface strain between NaFeP0₄ and FeP0₄ and force cation ordering, forces which are clearly different than dictate single phase formation in Nai_-xMno_.5Feo_.5P0₄.

In summary, metastable olivine phases of Na[Mni_-xM_x]P0₄, where M is selected from Fe, Ca and Mg, nanorods are formed by a topatactic reaction using simple solid state synthesis at low (< 100°C) temperatures, and optimally in the range of 65 to 100°C. This is in contrast to the high temperature synthesis of NaMnP0₄ for example, which forms the electrochemically inactive maricite phase. Na(Feo_.5Mno_.5)P0₄ exhibits a single phase reaction on desodiation, not a two phase reaction as in the case of LiFeP0₄. Both nanorod formation and the solid solution behavior of Na[Mni_-xM_x]P0₄ are closely related to the large interface strain between the Na-rich and Na-poor phases due to the large size of the Na ion. Na[Mn_1-xM_x]P0₄ exhibits reversible electrochemical de/intercalation, making these materials practical as cathode materials in sodium-ion batteries.

It will be appreciated by those skilled in the art that other variations of the preferred embodiments may also be practiced without departing from the scope of the invention.

Claims

CLAIMS What is claimed is:

1. A compound in the olivine phase having the general formula Na[Mni._xM_x]P0₄, characterized in that M is selected from the group consisting of Fe, Ca and Mg, and 0 < x < 0.6.

2. The compound of claim 1 characterized in that 0 < x < 0.5.

3. The compound of claim 1 characterized in that 0 < x < 0.2 where M is Ca or Mg.

4. The compound of claim 1 characterized in that 0 < x < 0.5 where M is Fe.

5. A process for synthesizing a compound in the olivine phase having the general formula Na[Mni_-xM_x]P0₄, characterized in that M is selected from the group consisting of Fe, Ca and Mg and 0 < x < 0.5, the process characterized by the steps of:

(i) synthesizing ammonium metal phosphates ΝΗ₄ΜΡ0₄·Η₂0, characterized in that M is selected from the group consisting of Mn, Mn₀.₅Fe₀.5, Mn₀.₈Cao.2 and Mn₀.8Mg₀.₂;

(ii) milling the resultant product from step (i) with excess sodium acetate trihydrate (CH₃C0₂Na^»3H₂0); and

(iii) heating the mixture from step (ii) to a temperature sufficient to convert the mixture to Na[Mni_-xM_x]P0 , characterized in that M is selected from the group consisting of Fe, Ca and Mg.

6. The process of claim 5, characterized in that the temperature is in the range of 65 to 100°C.

7. The process of claim 5, characterized by the additional step of removing un-reacted sodium acetate trihydrate by ethanol wash.

8. A cathode material for a sodium-ion battery having the general formula Na[Mni. xM_x]P0₄, characterized in that M is selected from the group consisting of Fe, Ca and Mg, and 0 < x < 0.5.

9. Use of the cathode material of claim 8 in a sodium-ion battery.

10. A compound in the olivine phase having the general formula Na_xFeP0₄, characterized in that 0 < x < 1.

11. A process for synthesizing a compound in the olivine phase having the general formula general formula Na_xFeP0₄, characterized in that 0 < x < 1 , the process characterized by the steps of:

(i) oxidizing LiFeP0₄ with excess of an oxidizing agent in acetonitrile to produce FeP0₄; and

(ii) reducing FeP0₄ with excess Nal in acetonitrile.

12. The process of claim 1 1 characterized in that the oxidizing agent selected from the group consisting of NOBF₄, N0₂ BF₄ and K^Og.

13. A cathode material for a sodium-ion battery having the general formula Na_xFeP0₄, where 0 < x < 1.

14. Use of the cathode material of claim 13 in a sodium-ion battery.