CA3004687C - Olivine-type compounds: method for their preparation and use in cathode materials for sodium-ion batteries - Google Patents

Abstract

This disclosure provides for Olivine-type compounds, their preparation and use in cathode materials for sodium-ion batteries. The olivine-type compounds of the invention are obtained by a direct synthesis embodying a hydrothermal method.

Description

CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 1 OLIVINE-TYPE COMPOUNDS: METHOD FOR THEIR PREPARATION AND USE IN CATHODE MATERIALS FOR SODIUM-ION BATTERIES FIELD OF THE INVENTION [0001] The present invention relates generally to cathode materials for sodium- ion batteries. More specifically, the invention relates to olivine-type compounds, their preparation and use in cathode materials for sodium-ion batteries. The olivine-type compounds of the invention are obtained by a direct synthesis embodying a hydrothermal method. BACKGROUND OF THE INVENTION [0002] During the past two decades, the demand for Electrical Energy Storage (EES) systems has increased for portable applications and for static applications, predominantly in the form of batteries [1]. The most attractive path to store the energy in order to restore it as electricity is to convert chemical energy into electrical energy. Batteries provide stored chemical energy with the ability to deliver it as electrical energy with high conversion efficiency. Currently, Lithium ion (Li-ion) technology is the most used for portable applications and it is spreading to the automotive industry [2, 3]. [0003] Presently, the most important cathode materials for commercially available Li-ion batteries are the 3.5 ¨ 4V (versus Li) LiCo02 (layered), LiMn20.4 (spine!), LiFeP0.4 (olivine) and LiMnii3Liii3C01/302 (layered), which deliver capacities of 150, 120, 155 and 160 mAh g-1, respectively [4, 5, 6]. The triphylite (olivine) lithium iron and manganese phosphates, LiFePO4 (LFP) and LiMnPO4 (LMnP), have been identified as suitable materials for Li- ion batteries [7, 8]; in particular, LFP is now a commercial success since it delivers the highest capacity (.=-- 170 mAh g-1) in a safe voltage window [9]. LFP presents several features such as low cost, non-toxicity, high thermal stability, which are determinant for large-scaled applications such as hybrid electric vehicles (H EV) [10]. [0004] However, the future availability and the price of lithium led the research community to find suitable alternatives [11, 12]. In contrast to lithium, sodium is one of the most abundant elements on earth and its resources are virtually unlimited (for example, in the ocean). Moreover, sodium is the second lightest alkali metal next to the lithium ion. On the basis of element abundance, sodium ion batteries (NIBs) are an ideal alternative to lithium ion batteries CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 2 (LIBs). Moreover, the sodium technology is based on the same fundamental processes of ion intercalation as Li-ion [13, 14, 15, 16, 17]. Its redox potential is very suitable (Eo(Na+/Na) -2.71 V versus the standard hydrogen electrode), which is only 0.3 V above that of lithium leading to only a small energy penalty. On the basis of these findings, rechargeable sodium ion batteries are promising systems for electrochemical energy storage (EES) applications. [0005] Sodium ion (Na-ion) batteries, structures, components, and charge storage mechanisms are similar to Li-ion batteries except that sodium ions are replaced instead of lithium ions [15]. Basically, a sodium-ion battery is constituted by two sodium insertion materials (one positive electrode and one negative electrode), which are soaked in the electrolyte as an ionic conductor. The final performance of the battery depends on the selected components. The first studies of sodium ion batteries were reported in 1980, when Newman et al. showed the highly reversible electrochemical sodium insertion into TiS2 at room temperature [18]. Many of the materials suitable for Na-ion batteries are similar to their lithium counterparts, which have been exhaustively studied for the past 20 years for Li-ion batteries such as layered transition metal oxides, olivine, and NASICON frameworks. Moreover, the research into Na-ion batteries is exploring new polyanionic frameworks such as fluorophosphates, pyrophosphates, fluorosulfates, sulfates, and nitridophosphates, as well as Prussian-blue analogues as alternative cathode materials to the layered oxides and polyanion structures. FIG. 1 shows voltage versus capacity for reported materials for Na-ion batteries. One example that delivers one of the highest theoretical capacities is olivine NaFePO4 (154 mAh g-1) which in turn is a low- cost and environmentally compatible material. For this reason, it is important to analyze the capability of sodium phospho-olivines as electrode materials in order to take advantage of them with respect to their features and previous studies of their Li counterpart (LiFePO4) [19]. [0006] Contrary to LiFePO4 (LFP), NaFePO4 (NFP) can exist in two different structures; olivine and maricite [20, 21], both of which crystallize in the orthorhombic system with Pnma as a space group. Both structures display a similar framework composed of phosphate groups. The difference comes from the occupancy of the two octahedral sites by Na + and Fe2+ ions. In the olivine structure Na + and Fe2+ ions occupy the M1 and M2 positions, respectively (FIG. 2A), while in the maricite structure, the occupancy corresponds to the opposite site (FIG. 2B). Both of them are made of distorted PO4 tetrahedra and distorted Fe06 octahedra. In the olivine structure, Fe06 octahedra are corner-shared forming 2D layers parallel to the bc plane, which are linked by PO4 tetrahedra leading to 1D channels for Na + diffusion along the b direction, CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 3 making it suitable for cathode material. In contrast, in the maricite structure, edge-sharing Fe06 octahedra form chains along the b direction, which are linked by PO4 tetrahedra building a 3D crystal structure. The connectivity of Na and Fe octahedral was thought to block Na + ion diffusion channels, limiting Na ion insertion and extraction. [0007] Olivine NaFePO4 is metastable with respect to maricite structured NaFePO4, which was considered electrochemically inactive for a long time [19, 22]. Very recently and for the first time, Kim et al. showed that maricite NaFePO4 can function as an excellent cathode material for rechargeable Na batteries [23]. They reported that Na ions can be inserted and deinserted from nano-sized maricite NaFePO4 with simultaneous transformation into amorphous FePO4. However, the high voltage achieved for the first desodiation could be an inconvenience for future commercial applications due to the electrolyte stability. [0008] Olivine NaFePO4 is the analog material of LiFePO4, one of the most commercialized and suitable materials for Li-ion batteries (as discussed above). Nevertheless, a big challenge is to achieve the direct synthesis of the electrochemically active olivine NaFePO4. Synthesis of NaFePO4 using conventional methods such as hydrothermal or solid state routes leads to the thermodynamical product maricite [24, 25]. Currently, the chemical or electrochemical cation exchange from LiFePO4 is the most effective way for obtaining the electrochemically active NaFePO4 with an olivine phase [21, 26, 27]. However, olivine structure can also be obtained using other indirect methods such as precursor-based methods for low temperature [28], topotactic reaction [29] and hollow amorphous NaFePO4 via template method [30]. [0009] There is a need for novel olivine-type compounds for use in cathode materials for sodium-ion batteries. Also, there is a need for more efficient and cost- effective methods for preparing such compounds. SUMMARY OF THE INVENTION [0010] The inventors have designed and prepared novel olivine-type compounds for use in cathode materials for sodium-ion batteries. The compounds of the invention are sodium phospho-o/ivines. They are prepared by a direct synthesis which embodies a hydrothermal method. The preparation method of the invention is efficient and cost- effective. CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 4 [0011] The invention thus provides the following in accordance with aspects thereof: (1) A compound of general formula NahMiM'JM"kPO4, wherein: 0 < h 1, 0 i 1, 0 5j 5 1 and 0 5 k 5 1, and wherein M, M' and M" are each independently a metal. (2) A compound of general formula Nai..MxM'Pat, wherein: 0 5. x < 1, preferably, x 0 or x 0.1; and wherein M and M' are each independently a metal. (3) A compound of general formula Na1..xMxM'1_yM"yPO4, wherein: 0 x < 1 and 0 y 5 1, preferably x 0 or x 0.1 and 0 y 5 0.25; and wherein M, M' and M" are each independently a metal. (4) A compound of general formula NaM1.õM'xPO4, wherein: 0 5 x 1, and wherein M and M' are each independently a metal. (5) A compound of general formula NaM1_x_yM'xM"yPO4, wherein: 0 5 x < 1 and 0 5 y < 1 and x+y < 1; and wherein M, M' and M" are each independently a metal. (6) A compound according to any one of (1) to (5) above, wherein the metal is selected from the group consisting of: Li, Mg, Ca, Mn, Fe, Co, Ni and Cu. (7) A compound according to any one of (1) to (5) above, wherein the metal is selected from the group consisting of: Li, Mg, Ca, Mn and Fe. (8) A compound according to any one of (1) to (5) above, wherein the metal is selected from the group consisting of: Li, Mn and Fe. (9) A compound according to any one of (1) to (5) above, wherein the metal is selected from the group consisting of: Li and Mn. (10) A compound according to any one of (1) to (9) above, which is a sodium phospho-olivine compound. (11) A compound of general formula Na1_xLixMnPO4, wherein: 0 5 x < 1; preferably, x 0 or x 0.1. (12) A compound having the formula Na0 966Li034MnPO4. CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 (13) A compound of general formula Na1.xLixFe1_yMnyPO4 or Na1_xLiõFeyMn1_yPO4, wherein: 0 x < 1 and 0 5 y 1, preferably x 0 and 0 5 y 5 0.25. (14) A compound having the formula Nao oLioiFeo22Mno.78PO4 or Nao 9Lio iFeo 25M no 75PO4. (15) A compound of general formula NaMn1_xMgxPO4, wherein: 0 5. x 1. (16) A compound of general formula Na1_xMgxMnPO4, wherein: 0 5 x 1. (17) A compound of general formula NaFe1Mn.LiyPO4, wherein: 0 x < 1 and 05.y<1andx+y<1. (18) A method of preparing a sodium phospho-olivine compound, the method comprising a hydrothermal method. (19) A method of preparing a compound of general formula NahM,M',M"kPO4, wherein: 0< h 5 1,0 5 i 1,0 5J 1 and 0 5 k 5 1, and wherein M, M' and M" are each independently a metal, the method comprising the steps of: (a) preparing an aqueous mixture comprising a M-containing compound, a M'-containing compound and a M"-containing compound to obtain a M-M'-M" mixture; (b) adding a P-containing compound to the mixture M-M'-M" mixture to obtain a M-M'-M"-P mixture; (c) adding a Na-containing compound to the M-M'-M"-P mixture to obtain a Na-M-M'-M"-P mixture; and (d) introducing the Na-M-M'-M"-P mixture into an autoclave to perform crystal growth and obtain the compound of general formula NahM,M'JM"kPO4. (20) The method of (19) above, further comprising the steps of: (e) fast-cooling the crystals; and (f) drying the cooled crystals. (21) The method of (20) above, wherein step (f) is performed at a temperature of about 50-85 C, preferably 60-75 C; for a period of about 6-12 hours, preferably about 8-10 hours. (22) The method of (19) above, wherein step (a) comprises preparing separate CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 6 aqueous solutions of the M-containing compound, the M'-containing compound and the M"-containing compound, first mixing two of the three solutions and then adding the third solution to obtain the M-M'-M" mixture. (23) The method of (19) above, wherein each of steps (a)-(c) is performed under stirring. (24) The method of (19) above, wherein step (b) comprises preparing an aqueous solution of the Na-containing compound and adding the solution to the M-M'-M"- P mixture. (25) The method of (19) above, wherein step (d) is performed under an atmosphere of 02, N2 or a combination thereof. (26) The method of (19) above, wherein step (d) is performed at a temperature of about 150-250 C, preferably about 200 C; for a period of about 2-6 hours, preferably about 4 hours; under a pressure of about 1.5-2.5 MPa, preferably about 2 MPa. (27) The method of (19) above, wherein water used is deionized water bubbled under N2. (28) The method of (19) above, wherein the M-containing compound, the M'-containing compound and the M"-containing compound are each independently selected from the group consisting of MnSO4, Li0H, FeSO4 and MgSO4; preferably, the compounds are each independently a hydrated compound. (29) The method of (19) above, wherein the P-containing compound is H3PO4. (30) The method of (19) above, wherein the Na-containing compound is Na0H. (31) The method of (19) above, wherein the Na-containing compound is used in excess amount. (32) The method of (19) above, wherein, in the general formula NahM,M'JM"kPO4, h = 1 and 0 i :5 0.2 and 0 j 5. 0.8 and 0.85 5 i+j+k 5. 1. CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 7 (33) A cathode material for sodium-ion batteries, comprising a compound as defined in any one of (1) to (18) above. (34) A cathode material for sodium-ion batteries, comprising a compound as defined in any one of (1) to (18) above and a carbon material. (35) A cathode material for sodium-ion batteries, comprising a compound prepared by the method as defined in any one of (19) to (32) above. (36) A cathode material for sodium-ion batteries, comprising a compound prepared by the method as defined in any one of (19) to (32) above and a carbon material. (37) A cathode for sodium-ion batteries, comprising a cathode material as defined in any one of (33) to (36) above. (38) A sodium-ion battery, comprising a cathode as defined in (37) above. [0012] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0013] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. [0014] In the appended drawings: [0015] FIG. 1: Schematic illustration of voltage versus capacity of intercalation materials for sodium ion batteries, wherein theoretical capacities and achieved ones are shown with blue bars and green ovals, respectively. [0016] FIG. 2A: Schematic view of ofivine-NaFePO4; FIG. 2B: Schematic view of maricite- NaFePO4. [0017] FIG. 3: Rietveld refinement of the XRD of Na09Li01MnPO4, with Yobs (red line), Ycalc (black CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 8 line), Yobs-Ycalc (blue line) and Bragg position (vertical blue lines). [0018] FIG. 4A and FIG. 4B: SEM images of Na0,9Li0,1MnPO4. [0019] FIG. 5: Rietveld refinement of neutron powder diffraction of Na0,966Li0034MnPO4. Yobs (red line), Ycale (black line), Yobs Y " calc (blue line) and Bragg position (vertical blue lines), wherein A = 1.544402. [0020] FIG. 6A and FIG. 6B: Schemes of a crystal structure for Na0,966Li0.034PO4 along [010] and [001] directions, where sodium and lithium are located in the same position (blue-orange balls). [0021] FIG. 7A and FIG. 7B: SEM images of Na0.966Li MnPOD -0 034¨ ¨ - 4- [0022] FIG. 8A and FIG. 8B: Rietveld refinements of XRD of Na0.9Li0,1Fe0.22Mn078PO4. FIG. 8A shows peaks that correspond to maricite; and FIG. 8B shows refinements of both olivine and maricite structure, with Yobs (red line), Y = cate (black line), Y Y alas- calc (blue line) and Bragg position (vertical blue lines). [0023] FIG. 9A and FIG. 9B: SEM images of Na0.9Li0 iFe0.22MnomPO4. [0024] FIG. 10: Rietveld refinement of Na0.9Li0 Feo 25M no 75PO4. [0025] FIG. 11A and FIG. 11B: Schemes of a crystal structure for Na09Li0.1Fe025Mn0.75PO4 along [010] and [001] directions, wherein sodium and lithium are located in the same position (blue- orange balls), and Mn and Fe share the same position (green-red). [0026] FIG. 12: SEM image of Na0,9Li01Fe0.25Mn0.75PO4. [0027] FIG. 13: Lebail profile matching of NaMnPO4 olivine phase. [0028] FIG. 14: Rietveld refinement matching of NaMnPO4 maricite phase. CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 9 [0029] FIG. 15: XRD diffraction pattern matching of Na1_xMgõMnPO4. [0030] FIG. 16: LeBail profile matching for Na1_xLixFeyMn1_yPO4 (x = 0.1, y = 0.5), with Yobs (red line), )(colo (black line), Yobs-Yealc (blue line) and Bragg position (vertical blue lines-maricite and vertical red lines-olivine). [0031] FIG. 17: LeBail profile matching for Na1_xLixFeyMn1_yP0.4 (x = 0.1, y = 0.75), with Yobs (red line), Yõ1, (black line), Y Y obs- caic (blue line) and Bragg position (vertical blue lines-maricite and vertical red lines-olivine). [0032] FIG. 18: Rietveld refinement of neutron diffraction for maficite Na1_xLi,FePO4, with Yobs (red line), Y calc (black line), obs- - Y Y - calc (blue line) and Bragg position (vertical blue lines), wherein A = 1.544402. [0033] FIG. 19: SEM image of a sample of Na1_xLixFeP0.4. [0034] FIG. 20: X-Ray diffraction patterns of NaFe05Mn05PO4 and NaFe075Mn025PO4 obtained after hydrothermal method in 02 atmosphere. [0035] FIG. 21: SEM image of mixture active material:carbon (80:20) after ball milling. [0036] FIG. 22A: Galvanostatic charge and discharge curves of Na1_xLixMnPO4; FIG. 22B: Capacity retention of Na1_xLixMnPO4 (from Example 1). DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS [0037] The present invention relates to novel Ovine-type compounds which may be used in cathode materials for sodium-ion batteries. The compounds of the invention are sodium phospho-o/ivines. They are prepared by a direct synthesis which embodies a hydrothermal method. The invention is illustrated in further details by the non-limiting examples outlined below. [0038] Generally, direct synthesis of olivine Nai_xLixFei_yMnyPO4 was carried out by CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 hydrothermal method. Sodium hydroxide (NaOH, Sigma Aldrich, 97%), iron sulfate (FeS047H20, Sigma Aldrich, 99%), manganese sulfate (MnSO4-1-120, Sigma Aldrich, 99%), lithium hydroxide (Li0H-H20, Sigma Aldrich, 98%) and orthophosphoric acid (H3PO4, Alfa Aesar, 85%) were used as a precursors according to the following reaction: 3-NaOH + (1-x)=FeSO4-7H20 + x- MnSO4- H20 + y-Li0H-1-120 + H3PO4 --> Na1..Li.Fe1_yMnyP0.4 [0039] The following Table 1 outlines some detailed examples performed: Table 1. Moles of reactants used for each hydrothermal synthesis Compound NaOH FeSO4-7H20 MnSO4.H20 LiOH.H20 MgSO4 H3PO4 Ex. 1 Na0.9Li01MnPO4 0.36 0.108 0.012 - 0.12 Ex. 2 Na0.9Li01MnPO4 0.36 - 0.089 0.012 - 0.12 Ex. 3 Na0,9Li01Fe022Mn0,751304 0.36 0.03 0.078 0.012 - 0.12 Ex. 4 Na0,9Li01Fe022Mn0.75PO4 0.036 0.03 0.089 0.0012 - 0.012 Ex. 5b NaMnPO4 0.036 0.012 0.012 Ex. 5a NaMnPO4 0.36 - 0.12 0.12 Ex. 6 NaMnMgPO4 0.036 - 0.0108 - 0.0012 0.012 Ex. 7 Na0,9Li01Fe050Mn0.50PO4 0.036 0.0054 0.0054 0.0012 - 0.012 Ex. 8 Nao 9Li01Fe075Mn025PO4 0.036 0.0078 0.003 0.0012 - 0.012 Ex. 9 Nao oLioiFePO4 0.36 0.108 - 0.012 - 0.12 Ex. 10a Na0Fe05Mn05PO4 0.036 0.006 0.006 - - 0.012 Ex. 10b Na0Fe0 75Mn0.25PO4 0.036 0.009 0.003 0.012 Example 1: Preparation of Nai..õLixMnPO4 (x 0.1, see Table 1) [0040] Excess NaOH (0.36 moles) was dissolved in 100 mL of deionized water. 0.108 moles of MnSO4-1-120 and 0.012 moles of Li0H-1-120 were then dissolved in 90 mL and 10 mL of deionized water, respectively. LiOH solution was added to MnSO4 solution and mixed under stirring until a homogeneous solution was obtained. 0.12 moles of H3PO4 solution were added in a graduated cylinder, which was then filled to 100 mL with deionized water. This last solution was added to the Mn-Li and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Mn-Li-P and was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 300 mL) was 10. A glass liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 11 Powders crystallized in the olivine structure were grown in the autoclave under stirring at 200 C for 4 hours (P = 2 MPa). [0041] It should be noted that the deionized water was previously bubbled under N2 for 4-5 hours. Also, the solutions, the mixtures and the autoclave sealing were carried out in a glove box under N2 atmosphere. The obtained solution was the filtered and the product was washed with water (3-4 times) and ethanol (1 time). Subsequently, it was dried at 60- 75 C overnight. [0042] X-Ray diffraction measurement was used for the structural characterization. FIG. 3 shows the Rietveld refinement for the obtained compound, where impurities (maricite phase or unknown) are highlighted (*). After refinement, the unit cell parameters obtained were a = 10.5391(6) A, b = 6.3307(4) A and c = 4.9906(3) A, which are slightly lower than those reported for the olivine NaMnPO4 with Pnma space group (see Table 2 below). This slight difference could be attributed to the small amount of Li in the structure. Electronic microscopy was used to study the degree of homogeneity and the particle size as well. As it can be seen from FIG. 4A and FIG. 4B, two different morphologies can be observed, one of which is attributed to the olivine iron manganese phosphate (big crystals) and the other one to impurities (small crystals). Compositional studies of the final product were carried out by EDAX analysis. These measurements demonstrated that the ratio for Na:Mn:P was close to 1:1:1 (Li ions cannot be detected by EDAX measurements, Table 3 below). [0043] In this example, direct synthesis of olivine Na1_xLixMnPO4 has been demonstrated, with the presence of some unknown impurities and impurities attributable to the maricite phase. Table 2. Refined unit cell parameters of olivine Na1.xLi.Fe1_yMnyPO4and comparison with reported values for NaFePO4 and NaMnPO4 after a Rietveld refinement a Ex. 1 Na0,9Li01MnPO4 10.5391(6) 6.3307(4) 4.9906(3) Ex. 2 Nao gLio., MnPO4 10.555(1) 6.3383(9) 4.9965(7) Ex. 3 Na09Li0.1 Fe0.22Mn0.781D04 10.516(1) 6.3060(7) 4.9795(7) Ex. 4 Na09Li01Fe0.25Mn0.75PO4 10.5249(9) 6.3067(5) 4.9745(5) Ex. 5a NaMnPat 10.5446(5) 6.3294(3) 4.9920(2) Database NaFePO4 [21] 10.4063(6) 6.2187(3) 4.9469(3) CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 12 Database NaMnPO4 [29] 10.5578(3) 6.3359(2) 4.9966(2) Table 3. Summary of the composition of every compound determined by EDAX %At Compound Na Fe Mn P 0 Ex. 1 Na0,9Li01MnPO4 17.06 11.33 15.91 55.70 Ex. 2 Na09Li01 MnPO4 16.30 15.13 18.53 50.04 Ex. 3 Na09Li0.1 Feo 22Mno.78PO4 16.00 3.62 9.86 16.72 53.80 Ex. 5b Na09Li0 Fe025Mno.75PO4 14.65 4.75 12.64 17.52 50.42 Ex. 4 Nao gLioiFePO4 17.90 13.40 17.20 51.50 Example 2: Preparation of Nai_xLixMnPO4 (x 0.1, see Table 1) [0044] In this case a lower amount of MnSO4.1-120 was used with respect to Example 1 (see Table 1). Excess NaOH (0.36 moles) was dissolved in 100 mL of deionized water. Then 0.089 moles of MnSO4-1-120 and 0.012 moles of Li01-1-1-120 were dissolved in 90 mL and 10 mL of deionized water, respectively. LiOH solution was added to MnSO4 solution and mixed under stirring until a homogeneous solution was obtained. 0.12 moles of solution of H3PO4 were added in a graduated cylinder, which was then filled to 100 mL with deionized water. This last solution was added to the Mn-Li solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Mn-Li-P solution and was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 300 mL) was 10. A glass liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. Powders crystallized in the olivine structure were grown in the autoclave under stirring at 200 C for 4 hours (P 2 MPa). [0045] It should be noted that the deionized water was previously bubbled under N2 for 4-5 hours, and that the solutions, the mixtures and the autoclave sealing were carried out in a glove box under N2 atmosphere. However, a test was done using no bubbled water and sealing the autoclave under air, which also resulted in olivine phase. [0046] After that the obtained solution was filtered and the product was washed with water (3-4 CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 13 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. [0047] In this synthesis, a high degree of purity has been achieved. For this reason, the structural characterization was carried out using neutron powder diffraction. The aim of the use of neutron diffraction was to refine the occupancy and the site position of lithium ions. For the Rietveld refinement the olivine NaMnPat structure was used as a starting structural model. FIG. reveals that the compound crystallized with the characteristic orthorhombic olivine phase (Pnma). The unit cell parameters determined were a = 10.555(1) A, b = 6.3383(7) A and c = 4.9965(7) A (Table 2). Thanks to the neutron diffraction, it was possible to refine the occupancies and to determine in which site lithium was located, and also the final composition of the compound. Table 4 below summarizes the atomic positions and the occupancies for each atom in the structure. Taking into account the results obtained in the Rietveld refinement, it was found that the Li is located in the sodium site with a final composition of Na0.966Li -0 034M nPat. The polyhedral representations of the final crystal structure along [010] and [001] directions are shown in FIG. 6A and FIG. 6B. Table 4. Final atomic coordinates, and occupancy of olivine Na09661- i0.034MnPO4 atom Atomic position 13;80 Occ. Na 0.00000 0.00000 0.0000 1.092(7) 0.483(1) Li 0.00000 0.00000 0.0000 1.092(7) 0.017(1) Mn 0.2841(1) 0.75000 0.9922(3) 0.543(3) 0.5000 0.1065(1) 0.75000 0.4385(3) 0.568(2) 0.5000 01 0.1132(1) 0.75000 0.7450(2) 0.734(1) 0.5000 02 0.4676(1) 0.75000 0.1610(2) 0.734(1) 0.5000 03 0.17524(7) 0.9433(1) 0.3233(1) 0.734(1) 1.0000 [0048] Moreover, the high degree of homogeneity can be seen from scanning electron microscopy images (FIG. 7A and FIG. 7B), where it shows that the particles have a rhombohedral-shape from 5 pm to 20 pm in length. [0049] In this example, direct synthesis of olivine Na0,966Li0034MnPO4 has been demonstrated. In addition, the addition of the non-stoichiometric amount of MnSO4-1-120 leads to a more pure compound than the use of stoichiometric amount of MnSO4-1-120. Finally, neutron diffraction CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 14 allowed for determining the occupancy and the site in the crystal structure for the lithium ion. It is confirmed that Li and Na are located in the same crystallographic position and that LiOH= H20 is used in excess. However, the possibility of the presence of anitisites could not be ruled out. Example 3: Preparation of a Nai_xLixFeyMni_yPO4 (x gr, 0.1, y .=-.4 0.22, see Table 1) [0050] Excess NaOH (0.36 moles) was dissolved in 100 mL of deionized water. Then, 0.03 moles of FeSO4-7H20, 078 moles of MnS0.4=H20, and 0.012 moles of Li01-1-1-120 were dissolved in 25 mL, 65 mL, and 10 mL of deionized water, respectively. First, FeSO4 solution was added to MnS0.4 and was stirred until a homogeneous solution was obtained. Then, LiOH solution was added to Fe-Mn solution and mixed under stirring until a homogeneous solution was obtained. 0.12 moles of H3PO4 solution were added in a graduated cylinder, which was filled to100 mL with deionized water. This last solution was added to the Fe-Mn-Li solution and it was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the solution of Fe-Mn-Li-P, which was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 300 mL) was 10. A glass liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. Powders crystallized in the olivine structure were grown in the autoclave under stirring at 200 C for 4 hours (P = 2 MPa). [0051] It should be noted that the deionized water was previously bubbled under N2 for 4-5 hours. In addition, the solutions, the mixtures and the autoclave sealing were carried out in a glove box under N2 atmosphere. Afterwards, the obtained solution was filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. [0052] The structural characterization was carried out using X-Ray diffraction. The Rietveld refinement revealed that the compound crystallized with the orthorhombic olivine phase (Pnma), however, there are some peaks that correspond to maricite phase as impurities (*) (FIG. 8A). A second Rietveld refinement was carried out using both olivine and mar/cite structure which determined that the unit cell parameters for the olivine phase are a = 10.516(1) A, b = 6.3060(7) A and c = 4.9795(7) A (FIG. 8B and Table 2). Moreover, Rietveld refinement allows us to do a phase quantification, which determined that 75% corresponds to olivine and 25% to mar/cite. CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 [0053] The scanning electronic images show the presence of two different morphologies; one of them corresponds to the olivine compound (rectangular-shaped) and the other one to the maricite as impurities (FIG. 9A and FIG. 9B). Moreover, EDAX measurements determined that the ratio for Na:Fe:Mn:P was close to 1:0.25:0.75:1. Li ions cannot be detected by EDAX detector, as in the previous example. [0054] Neutron diffraction has been performed to be able to determine the atomic coordinates, the atomic occupancy, and to elucidate the amount of lithium as well as its site in the crystal structure. Due to the presence of impurities, obtaining a stable refinement for determining the final composition presented some challenges. Nevertheless, it has been used to confirm that the structure of Nao oLio.iFe0.22Mno.78PO4 corresponds to the olivine phase. [0055] In this example, the direct synthesis of olivine Nao.oLio iFe0.22MnomPO4 has been demonstrated, with the presence of maricite phase as an impurity. Example 4: Preparation of a NaiõLixFeyMni_yPO4 (x 0.1, y 0.25, see Table 1) [0056] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. 0.003 moles of FeSO4.7H20, 0.0078 moles of MnSO4 120, and 0.0012 moles of Li0H-1-120 were then dissolved in 2.5 mL, 6.5 mL, and 10 mL of deionized water, respectively. First, FeSO4 solution was added to MnS0.4 and was stirred until a homogeneous solution was obtained. Then, LiOH solution was added to Fe-Mn solution and mixed under stirring until a homogeneous solution was obtained. After that, 0.012 moles of H3PO4 solution were added in graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Fe- Mn-Li solution and it was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the solution of Fe-Mn-Li-P, which was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 30 mL) was 10 or higher. Powders crystallized in the olivine structure were grown in the autoclave at 200 C for 4 hours. In order to avoid any transformation from olivine to maricite, a fast cooling (quenching) of the autoclave was carried out. [0057] After that, the obtained solution was filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. It should be noted that the CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 16 use of bubbled or non-bubbled deionized water does not affect to the final product. In both cases olivine phase has been obtained. Also, in this example, solutions, mixtures and the autoclave sealing were carried out under 02 atmosphere. [0058] It has been observed that when kept under non-02 atmosphere (more than 1 hour purge under N2 in a glovebox) the reaction is not successful. It seems that 02 is necessary to obtain the olivine phase. In addition, fast cooling (quenching) has been used to avoid the phase transformation from olivine to maricite. [0059] This methodology has also been used in Example 1 and Example 2, which, in both cases, results in pure olivine phase (not shown in this disclosure). [0060] The structural characterization was carried out using X-Ray diffraction and the Rietveld refinement confirmed that the compound crystallized with the orthorhombic olivine phase (Pnma). The use of a quenching during the cooling process helps avoid the presence of maricite impurities. However, there is a small peak (*) which is attributable to an unknown impurity (FIG. 10). In the Rietveld refinement, the occupancy of Na:Fe:Mn:P was fixed according to the ICP measurement (1:0.25:0.75:1); the unit cell parameters were determined to be a = 10.5249 (9) A, b = 6.3067 (5) A and c = 4.9745 (5) A (Table 2). The polyhedral representations of the final crystal structure along [010] and [001] directions are shown in FIG. 11A and FIG. 11B. The scanning electronic images show the presence of rectangular-shaped single particles with a width of 1.5-2 pm and from 6 to12 pm long (FIG. 12). [0061] In this example, direct synthesis of olivine Na09Li0 iFe025Mn075PO4 has been demonstrated, with an absence of maricite phase as impurity thanks to fast cooling (quenching). This reaction was carried out in 02 and using no-bubbled deionized water. Examples 5a and 5b: Preparation of NaMnPO4 (x 0.1, see Table 1) Example 5a: Reaction under Oa atmosphere [0062] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. Then 0.012 CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 17 moles of MnSO4.1-120 were dissolved in 10 mL of deionized water. 0.012 moles of solution of H3PO4 were added in a graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Mn solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Mn-P solution and was also stirred until a homogeneous dense solution was obtained (total volume 30 mL). A Teflon liner was filled with the final solution/mixture and the autoclave was sealed in 02 atmosphere. Powders crystallized in the olivine structure were grown in the autoclave under stirring at 200 C for 4 hours. [0063] In this embodiment of the invention, the deionized water was not bubbled previously. Further, the solutions, the mixtures, and the autoclave sealing were carried out in air atmosphere. Afterwards, the obtained solution was filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. [0064] X-Ray diffraction was used for the phase identification and the structural characterization. FIG. 13 corresponds to a LeBail profile matching which reveals that the compound crystallized with the characteristic orthorhombic olivine phase (Prima). The unit cell parameters were determined to be a = 10.5446(5) A, b = 6.3294(3) A and c = 4.9920(2) A (Table 2). In this case, the impurities are not attributed to maricite phase. Example 5b: Reaction under Nz atmosphere [0065] Excess NaOH (0.36 moles) was dissolved in 100 mL of deionized water. Then 0.108 moles of MnSO4.H20 were dissolved in 100 mL. Then, 0.12 moles of H3PO4 solution were added in a graduated cylinder, which was then filled to 100 mL with deionized water. This last solution was added to the Mn-solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Mn-P and was also stirred until a homogeneous dense solution was obtained (total volume 300 mL). A glass liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. Powders crystallized in the maricite structure were grown in the autoclave under stirring at 200 C for 4 hours (P 2 MPa). [0066] In this embodiment of the invention, the deionized water was previously bubbled under N2 for 4-5 hours. In addition, the solutions, the mixtures, and the autoclave sealing were carried CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 18 out in a glove box under N2 atmosphere. After that, the obtained solution was filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. [0067] X-Ray diffraction was used for the phase identification and structural characterization. FIG. 14 corresponds to a Rietveld profile matching which reveals that the compound crystallized with the characteristic orthorhombic maricite phase (Pnma). However, there are some peaks that correspond to olivine phase as impurities (*). The determined unit cell parameters were a = 9.0990(4) A, b = 6.9010(2) A and c = 5.1176(2) A for the maricite phase and a = 10.540(1) A, b = 6.3314(6) A and c = 4.9926(6) A for the olivine phase. Moreover, Rietveld refinement allows for phase quantification, in which it was determined that 90% corresponds to mar/cite and 10% to olivine. [0068] If the reaction is carried under air atmosphere, an olivine phase is obtained, whereas maricite phase is obtained under N2 and using bubbled deionized water. Example 6: Preparation of NaMn1_xMgxPO4 (x 0.1, see Table 1) [0069] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. 0.0108 moles of MnSO4-1-120 and 0.0012 moles of MgSO4 were then dissolved in 9 mL and 1 mL of deionized water, respectively. First, MgSO4 solution was added to MnS0.4 and was stirred until a homogeneous solution was obtained. 0.012 moles of H3PO4 solution were subsequently added to a graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Mn-Mg solution and it was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the solution of Mn-Mg-P, which was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 30 mL) was higher than 10. Powders crystallized in the olivine structure were grown in the autoclave at 200 C for 4 hours. In order to avoid any transformation from olivine to maricite, a fast cooling (quenching) of the autoclave was carried out. [0070] FIG. 15 illustrates the X-Ray diffraction pattern of the compound obtained by hydrothermal method. The pattern has been compared with the reported one in Example 2 (Nal_ xLixMnPO4). The XRD reveals that the compound crystallized with the characteristic CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 19 orthorhombic olivine phase (Pnma). Nonetheless, there are some peaks attributed to unknown impurities (*). The Lebail profile matching is ongoing in order to determine the unit cell parameters. However, when comparing both compositions, the peaks of Na1_xMgxMnPO4 are shifted at higher 20 leading to a decrease of the unit cell parameters. Taking into account the higher ionic radii of Mg than Li, peaks shifted at lower 20 were expected. [0071] In this example, the method was performed under 02 atmosphere. Example 7: Preparation of a Na1_xLixFeyfian1_yPO4 (x 0.1, y$=,- 0.5, see Table 1) [0072] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. Then 0.0054 moles of FeS047H20, 0.0054 moles of MnSO4.H20, and 0.012 moles of Li0H.H20 were dissolved in 4.5 mL, 4.5 mL and 1 mL of deionized water, respectively. First, Mn-solution was added to FeSO4 solution and was mixed. Then LiOH solution was added to the Fe- Mn solution and mixed under stirring until a homogeneous solution was obtained. After that 0.012 moles of solution of H3PO4 were added in a graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Fe-Mn-Li solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Fe- Mn-Li-P solution and it was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 30 mL) was higher than 10. A Teflon liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. Powders crystallized in the maricite structure were grown in the autoclave under stirring at 200 C for 4 hours. [0073] In this embodiment of the invention, the deionized water was previously bubbled under N2 for 4-5 hours. The solutions, the mixtures, and the autoclave sealing were carried out in a glove box under N2 atmosphere. The obtained solution was then filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60- 75 C overnight. [0074] X-Ray diffraction was used for the structural characterization and phase identification. FIG. 16 corresponds to a XRD LeBail profile matching of the compound obtained after hydrothermal synthesis, which reveals that the compound crystallized with the characteristic orthorhombic maricite phase (vertical blue lines). However, there are some peaks that correspond to olivine phase as impurities (FIG. 16, vertical red lines). The determined unit cell CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 parameters were a = 9.0563(5) A, b = 6.8781(5) A and c = 5.0861(3) A for the maricite phase and a = 10.522(2) A, b = 6.321(2) A and c = 4.9827(8) A for the olivine phase (estimated composition NaFe0.25Mr1075PO4). Due to the low amount of the olivine phase (5% or less), the phase quantification is very difficult and cannot be accurate enough. Example 8: Preparation of a Na1Li,FeyMn1_yPO4 (x 0.1, y 0.75, see Table 1) [0075] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. 0.0078 moles of FeSO4.7H20, 0.003 moles of MnSO4= H20, and 0.012 moles of LiOH=H20 were then dissolved in 6.5 mL, 2.5 mL, and 1 mL of deionized water, respectively. First, Mn-solution was added to FeSO4 solution and was mixed. LiOH solution was then added to Fe-Mn solution and mixed under stirring until a homogeneous solution was obtained. After that, 0.012 moles of solution of H3PO4 were added in a graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Fe-Mn-Li solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Fe-Mn-Li-P solution and it was also stirred until getting a homogeneous dense solution. The pH of the final solution (total volume 30 mL) was higher than 10. A Teflon liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. Powders crystallized in the maricite structure were grown in the autoclave under stirring at 200 C for 4 hours. [0076] In this embodiment of the invention, the deionized water was previously bubbled under N2 for 4-5 hours. In addition, the solutions, the mixtures, and the autoclave sealing were carried out in a glove box under N2 atmosphere. After that, the obtained solution was filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. [0077] X-Ray diffraction was used for the structural characterization and phase identification. FIG. 17 corresponds to a XRD LeBail profile matching of the compound obtained after hydrothermal synthesis which reveals that the compound crystallized with the characteristic orthorhombic maricite phase (FIG. 17, vertical blue lines). However, there are some peaks that correspond to olivine phase as impurities (FIG. 17, vertical red lines). The determined unit cell parameters were a = 9.0267(5) A, b = 6.8692(5) A and c = 5.0678(3) A for the maricite phase and a = 10.518(1) A, b = 6.309(1) A and c = 4.9838(6) A for the olivine phase (estimated CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 21 composition NaFe025Mr10.75F04). Due to the low amount of the olivine phase (5% or less), the phase quantification presented some challenges. Example 9 (comparative): Preparation of Nai_xLixFePO4(x 0.1, see Table 1) [0078] Excess NaOH (0.36 moles) was dissolved in 100 mL of deionized water. Then 0.108 moles of FeSO4-7H20 and 0.012 moles of Li01-1-1-120 were dissolved in 90 mL and 10 mL of deionized water, respectively. LiQH solution was added to FeSO4 solution and mixed under stirring until a homogeneous solution was obtained. 0.12 moles of solution of H3PO4 were added in graduated cylinder, which was then filled to 100 mL with deionized water. This last solution was added to the Fe-Li solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Fe-Li-P solution and it was also stirred until a homogeneous dense solution was obtained. The pH of the final solution (total volume 300 mL) was 9-10. A glass liner was filled with the final solution/mixture and the autoclave was sealed under N2 atmosphere. Powders crystallized in the maricite structure were grown in the autoclave under stirring at 200 C for 4 hours (P = 2 MPa). [0079] In this example, the deionized water was previously bubbled under N2 for 4-5 hours. In addition, the solutions, the mixtures, and the autoclave sealing were carried out in a glove box under N2 atmosphere. After that, the obtained solution was filtered and the product was washed with water (3-4 times) and ethanol (1 time). Then, it was dried at 60-75 C overnight. [0080] Due to the high degree of purity of the sample, neutron diffraction was used for the structural characterization. FIG. 18 shows the Rietveld refinement for the obtained compound. In this case the compound crystallizes in the maricite Pnma structure with a = 8.9953(9) A, b = 6.8610(7) A and c = 5.0455(5) A as unit cell parameters, which are the values reported for maricite NaFePO4 [24]. Three different structural models were used in the Rietveld refinement: (1) Li is located in the Na site, (2) Li is located in the Fe site and (3) Li is located in both Na and Fe sites. Due to the low amount of Li in the structure, distinguishing among the structural models presented some challenges. A time of flight ¨ secondary ion mass spectroscopy analysis was carried out at IREQ facilities, which showed that the amount of Li in this structure was 0.2%. Moreover, scanning electron microscopy images showed a high degree of homogeneity, where the particles had a platelet-shape making up the aggregates (FIG. 19). CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 22 [0081] Despite the good Rietveld refinement of the neutron diffraction, it is difficult to determine the Li site due to its lower amount in the structure (assuming that Li ions are located in the Na site as in the previous examples). [0082] In this comparative example, direct synthesis of olivine Na1_xLixFePO4 has not been demonstrated, as maricite Na1_xLi.FePO4 has been obtained instead. Examples 10a and 10b (comparative): Preparation of a NaFeyMn1_yPO4 (y gL-, 0.5 or y 0.75, see Table 1) Example 10a: NaFe0.5Mno 5P01 [0083] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. Then 0.006 moles of FeSO4=7H20, 0.006 moles of MnSO4.H20 were dissolved in 5 mL and 5 mL of deionized water, respectively. Firstly, Mn-solution was added to FeSO4 solution and it was mixed until homogeneous solution was obtained. After that 0.012 moles of solution of H3PO4 were added to a graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Fe-Mn solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Fe-Mn-P solution and it was also stirred until getting a homogeneous dense solution. The pH of the final solution (total volume 30 mL) was higher than 10. A Teflon liner was filled with the final solution/mixture and the autoclave was sealed under 02 atmosphere. Powders crystallized grown in the autoclave under stirring at 200 C for 4 hours. After that a fast cooling (quenching) of the autoclave was carried out. Example'_1(:D: NaFe075Mno25PO4 [0084] Excess NaOH (0.036 moles) was dissolved in 10 mL of deionized water. Then 0.009 moles of FeSO4.7H20 and 0.003 moles of MnSO4.H20 were dissolved in 7.5 mL and 2.5 mL of deionized water, respectively. First, Mn-solution was added to FeSO4 solution and it was mixed until a homogeneous solution was obtained. After that, 0.012 moles of solution of H3PO4 were added to a graduated cylinder, which was then filled to 10 mL with deionized water. This last solution was added to the Fe-Mn solution and was stirred until a homogeneous solution was obtained. Finally, NaOH solution was added to the Fe-Mn-P solution and it was also stirred until CA 03004687 2018-05-08 WO 2017/109707 PCT/IB2016/057840 23 getting a homogeneous dense solution. The pH of the final solution (total volume 30 mL) was higher than 10. A Teflon liner was filled with the final solution/mixture and the autoclave was sealed under 02 atmosphere. Powders crystallized grown in the autoclave under stirring at 200 C for 4 hours. After that a fast cooling (quenching) of the autoclave was carried out. [0085] X-Ray diffraction was used for the structural characterization and phase identification. FIG. 20 shows the X-Ray diffraction patterns for compounds NaFe0 6Mn0 6PO4 (Example 10a, red pattern) and NaFeomMno 26PO4 (Example 10b, green pattern). Also, the patterns have been compared with the characteristic maricite and olivine NaFePO4 database. As mentioned above, this synthesis was used to explore the effect of Li-absence and the 02 atmosphere when the Fe content is higher than 0.25. Both compounds do not crystallize in the characteristic olivine structure. Mainly, NaFe0.76Mr1026PO4 crystallizes in the maricite phase, however there are some peaks attributed to unknown impurities (*). Whereas the NaFe0 6Mn0 26PO4 (Example 10b, green pattern) is more complex, this compound does not crystallize in the maricite phase or the olivine phase. [0086] This comparative example explored the effect of non Li-doping and 02 atmosphere. Direct synthesis of olivine NaFexMn1,PO4 (x 0.5 and x 0.75) was not been achieved. Rather, maricite NaFe076Mn0.6PO4 has been obtained instead, as the higher amount of Fe probably induces the maricite phase. Electrochemical test [0087] First test: preliminary tests were conducted using active material power (Example 1) mixed with Denka carbon with a ratio of 80:20 (active material: carbon) using ball milling. Ball milling was used to decrease the particle size of the prepared material and also to ensure good homogeneity of the mixture. FIG. 21 shows a SEM image after ball milling treatment; a good homogeneity of the sample but also the presence of agglomerates of small particles (mainly, less than 500 nm) can be observed. [0088] Coin cell was assembled using a sodium metal as anode, Watman GF/D borosilicate glass fiber as a separator, and 1M NaPF6 Ethylene carbonate: Dimethyl carbonate (EC:DMC) as an electrolyte, as well as using a MACCOR battery test system. 24 [0089] Na1_xLixMnPO4 (theoretical capacity of 145mA=h/g) was tested versus lithium and sodium metal using galvanostatic charge and discharge curves at C/20 (FIG. 22A and FIG. 22B). While the material is electrochemically active for both ions, both curves exhibit low capacity, far from the theoretical one, as well as a poor cyclability and high polarization. [0090] Second test: a challenge with these new olivine compounds is to find a methodology for the carbon coating process. Temperatures over 400 C are probably to be avoided, since a phase transition from olivine to maricite may occur. A composite may be prepared by mixing Na0.966Lio.034MnPO4 (Example 2) with C65 using a ball milling. Then the mixture may be thermally treated under Argon at 400 C for 3 hours. The compound obtained may be tested versus metallic sodium and compared with the results obtained in the first test. [0091] The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. Date Recue/Date Received 2022-09-20 25 REFERENCES [1] R. A. Huggins. Advanced Batteries: Materials Science Aspects. (Springer, 2009). [2] J. M. Tarascon and M. Armand, Nature 2001, 414, 359. [3] M. Armand and J. M. Tarascon, Nature 2008, 451, 652. [4] M. S. Whittingham, Chem. Rev. 2004, 104,4271. [5] J. B. Goodenough, J. Power Sources 2007, 174, 996. [6] J. W. Fergus, J. Power Sources 2010, 195, 939. [7] A. Padhi, K. S. Nanjuindaswamy and J. B. Goodenough, J. Electrochem. Soc. 1997, 144, 1188. [8] B. L. Ellis, K. T. Lee and L. F. Nazar, Chem. Mater. 2010, 22, 691. [9] H. Hyand, S. C. Yin and L. F. 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Claims (5)

1. 27 CLAIMS: 1. A compound of general formula NahLiiM'JM"kPat, wherein: h=1, 0<i51, 0451, and 05k51, and wherein M' and M" are each independently a metal selected from the group consisting of: Mg, Ca, Mn, Fe, Co, Ni, and Cu.
2. 2. A compound according to claim 1, which is: of general formula NaLi1_xM'xPO4, wherein 0.5x<1; or of general formula NaLi1_xM'xM"yPO4, wherein: 05x<1, 05y<1, and x+y<1.
3. 3. A compound according to claim 1, wherein O<j51 and 0<k51.
4. 4. A compound according to claim 1, which is of general formula NaFe1MnxLiyPO4, wherein: 05x<1, 05y<1, and x+y<1. 5. A compound of general formula Na1_xLixFe1_yMnyPO4; or of general formula Nai_xLixFeyMni_yPO4, wherein: O<x<1 and 0<y<1. 6. A compound according to claim 5, wherein x=0 and 0<y5.0.25. 7. A compound according to claim 5 or 6, which has the formula Nao.9Lio.1Feo.22Mno.79PO4 or Nao.9Lio.1Feo.25Mno.75PO4. 8. A compound of general formula NahLiiM'JM"kPO4, wherein: 04151, 0<i51, 045.1, and 05k5.1, and wherein M' and M" are each independently a metal selected from the group consisting of: Mg, Ca, Mn, Fe, Co, Ni, and Cu, and wherein i is not (1-h). 9. A compound according to any one of claims 1 to 8, which is a sodium phospho- ofivine compound. 10. A compound of general formula NaMn1.xMgxPO4, wherein x=0.1. Date recue/Date received 2023-05-29 28 11. A compound of general formula Na1õMg,MnPO4, wherein 0<x5.1. 12. A method of preparing a compound as defined in any one of claims 5 to 7, the method comprising the steps of: (a) preparing a first aqueous mixture comprising a Li-containing compound, a Fe- containing compound, and a Mn-containing compound; (b) adding a P-containing compound to the first aqueous mixture to obtain a second aqueous mixture; (c) adding a Na-containing compound to the second aqueous mixture to obtain a third aqueous mixture; and (d) introducing the third aqueous mixture into an autoclave to perform crystal growth and obtain the compound. 13. The method according to claim 12, further comprising the steps of: (e) fast-cooling the crystals; and (f) drying the cooled crystals. 14. The method according to claim 13, wherein step (f) is performed at a temperature of 50- 85 C. 15. The method according to claim 13, wherein step (f) is performed at a temperature of 60- 75 C. 16. The method according to any one of claims 13 to 15, wherein step (f) is performed for a period of 6-12 hours. 17. The method according to any one of claims 13 to 15, wherein step (f) is performed for a period of 8-10 hours. 18. The method according to any one of claims 12 to 17, wherein step (a) comprises preparing separate aqueous solutions of the Li-containing compound, the Fe-containing compound and the Mn-containing compound, first mixing two of the aqueous solutions and then adding the third aqueous solution to obtain the first aqueous mixture. Date recue/Date received 2023-05-29 29 19. The method according to any one of claims 12 to 18, wherein each of steps (a)-(c) is performed under stirring. 20. The method according to any one of claims 12 to 19, wherein step (b) comprises preparing an aqueous solution of the Na-containing compound and adding the aqueous solution to the second aqueous mixture. 21. The method according to any one of claims 12 to 20, wherein step (d) is performed under an atmosphere of 02, N2, or a combination thereof. 22. The method according to any one of claims 12 to 21, wherein step (d) is performed at a temperature of 150-250 C. 23. The method according to any one of claims 12 to 21, wherein step (d) is performed at a temperature of about 200 C. 24. The method according to any one of claims 12 to 23, wherein step (d) is performed for a period of 2-6 hours. 25. The method according to any one of claims 12 to 23, wherein step (d) is performed for a period of about 4 hours. 26. The method according to any one of claims 12 to 25, wherein step (d) is performed under a pressure of 1.5-2.
5. 5 MPa. 27. The method according to any one of claims 12 to 25, wherein step (d) is performed under a pressure of about 2 MPa. 28. The method according to any one of claims 12 to 27, wherein water used is deionized water bubbled under N2. 29. The method according to any one of claims 12 to 28, wherein the Li- containing compound is Li0H, the Fe-containing compound is FeSO4, and the Mn-containing compound is FeSO4. Date recue/Date received 2023-05-29 30 30. The method according to any one of claims 12 to 29, wherein the Li- containing compound, the Fe-containing compound, and the Mn-containing compound are each independently a hydrated compound. 31. The method according to any one of claims 12 to 30, wherein the P- containing compound is H3PO4. 32. The method according to any one of claims 12 to 31, wherein the Na- containing compound is NaOH. 33. The method according to any one of claims 12 to 32, wherein the Na- containing compound is used in excess amount. 34. A cathode material for sodium-ion batteries, comprising a compound as defined in any one of claims 1 to 11. 35. A cathode material for sodium-ion batteries, comprising a compound as defined in any one of claims 1 to 11, and a carbon material. 36. A cathode material for sodium-ion batteries, comprising a compound prepared by the method as defined in any one of claims 12 to 33. 37. A cathode material for sodium-ion batteries, comprising a compound prepared by the method as defined in any one of claims 12 to 33, and a carbon material. 38. A cathode for sodium-ion batteries, comprising a cathode material as defined in any one of claims 34 to 37. 39. A sodium-ion battery, comprising a cathode as defined in claim 38. Date recue/Date received 2023-05-29