LITHIUM SILICATE CATHODES FOR LITHIUM-ION BATTERIES

Abstract

An improved nanocomposite cathode material for lithium-ion batteries and method of making the same. The nanocomposite cathode material includes lithium iron silicate based nanoparticles with a conductive matrix of graphene sheets. The nanoparticles may be doped with at least one anion or cation.

Claims

1. An electrode material comprising: a conductive matrix comprising a plurality of graphene sheets; a plurality of lithium iron silicate based nanoparticles coupled to the conductive matrix.

2. The material of claim 1, wherein the nanoparticles include at least one dopant, the nanoparticles having the formula:
Li.sub.2Fe.sub.1−dY.sub.cSiO.sub.4−bX.sub.a wherein: X=an anion dopant; Y=a cation dopant; a≥0; b=f(a) c≥0; and d=f(c).

3. The material of claim 2, wherein X is selected from the group consisting of: fluorine, chlorine, and bromine.

4. The material of claim 2, wherein Y is selected from the group consisting of: titanium, manganese, copper, terbium, niobium, and molybdenum.

5. The material of claim 2, wherein a>0 and c>0.

6. The material of claim 2, wherein a>c.

7. The material of claim 2, wherein: an anion doping ratio of X to oxygen is about 0% to about 10% by weight; and a cation doping ratio of Y to iron is about 0% to about 10% by weight.

8. The material of claim 2, wherein: X is fluorine; and b=a/2.

9. The material of claim 2, wherein: Y is manganese or copper; and d=c.

10. The material of claim 2, wherein: Y is niobium; and d=2.5c.

11. The material of claim 1, wherein the material comprises about 1 wt. % to about 10 wt. % of the graphene sheets.

12. The material of claim 11, wherein the material comprises about 2 wt. % of the graphene sheets.

13. A battery comprising an electrode with the material of claim 1.

14. The battery of claim 13, wherein the battery is configured for use in a portable electronic device, an electric vehicle, or an energy storage device.

15. The battery of claim 13, wherein the electrode lacks cobalt, nickel, and manganese.

16. A method of making an electrode material comprising: preparing a solution containing a lithium oxide, a silicon oxide, and iron; adding graphene oxide to the solution; heating the solution to produce a plurality of lithium iron silicate based nanoparticles over the graphene oxide; and sintering the nanoparticles to reduce the graphene oxide to graphene and produce a nanocomposite material.

17. The method of claim 16, wherein the lithium oxide of the preparing step is lithium hydroxide, the silicon oxide is silica, and the iron compound is iron dichloride.

18. The method of claim 16, further comprising adding at least one dopant to the solution before the heating step.

19. The method of claim 16, wherein the temperature of the heating step is about 160° C. or more.

20. The method of claim 16, further comprising constructing a cathode with the nanocomposite material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above-mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

[0014] FIG. 1 is a schematic view of a Li-ion battery of the present disclosure;

[0015] FIG. 2 is a schematic view of an exemplary Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite material of the present disclosure;

[0016] FIG. 3 is a flow chart illustrating an exemplary method for preparing the Li.sub.2FeSiO.sub.4-based/Graphene material;

[0017] FIGS. 4A-4C are transmission electron microscopy images of Li.sub.2FeSiO.sub.4-based particles alone, with graphene, and with graphene and fluorine dopants, respectively, in accordance with the examples;

[0018] FIGS. 5A-5D graphically illustrate electrochemical test results for electrodes made from the Li.sub.2FeSiO.sub.4-based particles of FIGS. 4A-4C, in accordance with the examples;

[0019] FIG. 6 graphically illustrates capacity data at different fluorine doping ratios in accordance with the examples;

[0020] FIG. 7 graphically illustrates cycle life data at different fluorine doping ratios in accordance with the examples;

[0021] FIGS. 8A-8C graphically illustrate electrochemical test results for electrodes made with graphene and fluorine dopants in accordance with the examples;

[0022] FIGS. 9A-9C graphically illustrate electrochemical test results for electrodes made with graphene and manganese dopants in accordance with the examples;

[0023] FIG. 10 graphically illustrates capacity data at different manganese doping ratios in accordance with the examples;

[0024] FIG. 11 graphically illustrates cycle life data at different niobium doping ratios in accordance with the examples; and

[0025] FIG. 12 graphically illustrates cycle life data at different titanium doping ratios in accordance with the examples.

[0026] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION

[0027] FIG. 1 provides a Li-ion battery 100 including an anode 102 coupled to an anode current collector 103, a cathode 104 coupled to a cathode current collector 105, and an electrolyte-filled separator 106. In one particular example, the anode 102 comprises Li metal or graphite and the electrolyte comprises a solution of LiPF.sub.6 in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), but sulfone-based, ionic liquid-based, nitrile-based, or other electrolytes can also be used. The illustrative battery 100 is a pouch cell, but the battery 100 may also be a cylindrical cell, a coin cell, or a prismatic cell, for example. The battery 100 may be configured for use in a portable electronic device, an electric vehicle, an energy storage device, or other electronic devices.

[0028] FIG. 2 provides an improved Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 for cathode 104 of battery 100 (FIG. 1). Advantageously, the Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 may have an initial specific energy of 600 Wh/kg, may have a cycle life of at least 1,000 cycles, and may lack cobalt, nickel, and manganese.

[0029] As shown in FIG. 2, the Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 includes Li.sub.2FeSiO.sub.4-based nanoparticles 112 (e.g., nanorods) formed upon a conductive matrix of graphene sheets 114. The graphene sheets 114 may improve the electrical conductivity of the nanocomposite cathode material 110 compared to the Li.sub.2FeSiO.sub.4-based nanoparticles 112 alone. Additionally, the graphene sheets 114 may provide a structural matrix to anchor and stabilize the Li.sub.2FeSiO.sub.4-based nanoparticles 112 and reduce grain stress during charge/discharge cycling, leading to better cycle life. As shown in FIG. 2, each graphene sheet 114 includes a single-layer of graphene with sp.sup.2-bonded carbon atoms arranged in a honeycomb crystal structure and can be viewed as an individual atomic plane of a graphite structure. Each carbon atom in the graphene uses 3 of 4 valence band (2s, 2p) electrons (which occupy the sp.sup.2 orbits) to form 3 covalent bonds with the neighboring carbon atoms in the same plane. Each carbon atom in the graphene contributes its fourth lone electron (occupying the p.sub.z orbit) to form a delocalized electron system, a long-range π-conjugation system shared by all carbon atoms in the graphene plane. Such a long-range π-conjugation in graphene yields extraordinary electrical, mechanical, and thermal properties in the nanocomposite cathode material 110. The nanocomposite cathode material 110 exhibits improved intraparticle electronic conduction because of good electrical conductivity of graphene, and Li.sup.+ ion diffusion is improved because diffusion length is shortened. Furthermore, the small grain size of the Li.sub.2FeSiO.sub.4-based nanoparticles 112 in the nanocomposite cathode material 110 reduces internal stress, leading to better structure stability and cycle life. According to an exemplary embodiment of the present disclosure, the nanocomposite cathode material 110 may comprise about 1 wt. % to about 10 wt. % graphene, more specifically about 1 wt. % to about 5 wt. % graphene, more specifically about 2 wt. % graphene. The graphene content should be sufficiently low to maintain the graphene as single sheets and avoid re-stacking. Additional information regarding the incorporation of graphene sheets 114 is disclosed in U.S. Publication No. 2015/0380732, the disclosure of which is expressly incorporated herein by reference in its entirety.

[0030] In certain embodiments, the graphene sheets 114 may be modified with one or more functional groups (e.g., —OH, —COOH, —NH). For example, the functional groups may be covalently grafted onto the surface of the graphene sheets 114 through diazonium salt via a diazonium reaction. The diazonium reaction-based functionalization may provide a simple and cost-effective way to transform the pure graphene sheets 114 into hierarchical and functional materials that can provide the desired properties (i.e. hydrophobicity, Li.sup.+/e.sup.− conductivity, Li.sup.+ diffusivity, nanoparticle dispersion, local electric field, etc.) to enhance binding with the adjacent Li.sub.2FeSiO.sub.4-based nanoparticles 112.

[0031] The Li.sub.2FeSiO.sub.4-based nanoparticles 112 may also include one or more optional dopants, including anion dopants (X) and/or cation dopants (Y). The dopants X, Y, may alter the crystalline structure and, consequently, the electrochemical performance of the Li.sub.2FeSiO.sub.4-based nanoparticles 112. The types of dopants X, Y, the doping order, and the amount of dopants X, Y may be varied to achieve a desired specific capacity/energy and cycle life. According to an exemplary embodiment of the present disclosure, the Li.sub.2FeSiO.sub.4-based nanoparticles 112 may have an anion doping ratio of anion dopants X to oxygen (O) and/or a cation doping ratio of cation dopants Y to iron (Fe). The doping ratios may be about 30% or less by weight, more specifically about 2% by weight to about 20% by weight, and more specifically about 2% by weight to about 10% by weight.

[0032] Examples of suitable anion dopants X include halogen ions, such as fluorine ions (F.sup.−), chlorine ions (Cl.sup.−), and bromine ions (Br.sup.−). Such anion dopants X may have a larger electronegativity than oxygen (O) and may reduce the formation of ligand holes during delithiation, which may stabilize the crystalline structure by reducing the extent of an O.sup.2−.fwdarw.O.sub.2.sup.2− reaction during delithiation. Also, such anion dopants X may also have a high redox potential (e.g., 2.87 V for F), which may help to increase the discharge potential of the nanocomposite cathode material 110 in the Li-ion battery 100 (FIG. 1).

[0033] Examples of suitable cation dopants Y include titanium ions (Ti.sup.+4), manganese ions (Mn.sup.+2), copper ions (Cu.sup.+2), terbium ions (Tb.sup.+3), niobium ions (Nb.sup.+5), and molybdenum ions (Mo.sup.+4). Such cation dopants Y may improve the structure stability of the Li.sub.2FeSiO.sub.4-based nanoparticles 112 by enhancing the coupling effect among the Li.sub.2FeSiO.sub.4-based tetrahedra via strong d-orbital hybridization. In this way, the dopants Y may function like springs to contain the tetrahedra and prohibit structural fracture. Consequently, the dopants Y may achieve much improved cycle life.

[0034] The Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 may have the following Formula (F-I):

Li.sub.2Fe.sub.1−dY.sub.cSiO.sub.4−bX.sub.a/Graphene  (F-I)

[0035] wherein: [0036] X=anion dopant; [0037] Y=cation dopant; [0038] a≥0; [0039] b=f(a) [0040] c≥0; and [0041] d=f(c).

[0042] Examples of suitable Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode materials 110 are listed in Table 2 below.

TABLE-US-00002 TABLE 2 Material X b Y d Li.sub.2FeSiO.sub.4/Graphene — 0 — 0 Li.sub.2FeSiO.sub.4−a/2Fa/Graphene F a/2 — 0 Li.sub.2Fe.sub.1−cMn.sub.cSiO.sub.4/Graphene — 0 Mn c Li.sub.2Fe.sub.1−cCu.sub.cSiO.sub.4/Graphene — 0 Cu c Li.sub.2Fe.sub.1−2.5cNb.sub.cSiO.sub.4/Graphene — 0 Nb 2.5c Li.sub.2Fe.sub.1−2.5cNb.sub.cSiO.sub.4−a/2F.sub.a/Graphene F a/2 Nb 2.5c

[0043] FIG. 3 provides an exemplary method 200 for synthesizing the Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 (FIG. 2) and constructing the cathode 104 (FIG. 1). The method 200 may involve a hydrothermal synthesis process.

[0044] In step 202, a first solution is prepared including a lithium oxide, specifically lithium hydroxide (LiOH), and a silicon oxide, specifically silica (SiO.sub.2), in a suitable solvent such as distilled water. The SiO.sub.2 may be provided in the form of nanoparticles, also referred to herein as nano-SiO.sub.2.

[0045] In step 204, a second solution is prepared including an iron compound, specifically iron dichloride (FeCl.sub.2), in a suitable solvent such as distilled water.

[0046] In step 206, the first and second solutions are combined. In certain embodiments, the second solution is added dropwise to the first solution. The combined solutions may be stirred together for about 30 minutes, about 1 hour, or longer before proceeding to the next step.

[0047] In step 208, a graphene oxide (GO) solution is added to the combined solution from step 206. The GO solution may be prepared using a modified Hummer's method, as disclosed in Example 1 below, for example. In certain embodiments, the GO solution is added dropwise to the combined solution from step 206. The resulting solution may be stirred together for about 30 minutes, about 1 hour, or longer before proceeding to the next step.

[0048] In step 210, a salt of any desired dopant X, Y is added to the resulting solution from step 208. Examples of suitable salts include ammonium fluoride (NH.sub.4F) for the F-dopant, cupric chloride (CuCl.sub.2) for the Cu-dopant, and niobium hydroxide (Nb(OH).sub.5) for the Nb-dopant. The dopant X, Y may be present in a desired concentration relative to the Li.sub.2FeSiO.sub.4.

[0049] In step 212, the solution is reacted to produce optionally doped, Li.sub.2FeSiO.sub.4-based nanoparticles 112 (FIG. 2) over the GO. This reacting step 210 may be a hydrothermal step that is performed in an autoclave or another suitable heated environment. The temperature of the reacting step 210 may be about 160° C., about 180° C., about 200° C., or more, but this temperature may vary. The duration of the reacting step 210 may be about 5 hours, about 10 hours, about 15 hours, or more, but this duration may vary. After the reacting step 210 is completed, the autoclave may be allowed to return to room temperature before removing the products.

[0050] The reacting step 212 may produce Li.sub.2FeSiO.sub.4-based nanoparticles 112 upon the GO according to Reactions (R-I) thru (R-III), represented overall by Reaction (R). The Reaction (R) may be modified as appropriate to incorporate any desired dopants.

2LiOH+SiO.sub.2.fwdarw.Li.sub.2SiO.sub.3.H.sub.2O  (R-I)

FeCl.sub.2+2LiOH.fwdarw.Fe(OH).sub.2+2LiCl  (R-II)

Fe(OH).sub.2+Li.sub.2SiO.sub.3.H.sub.2O.fwdarw.Li.sub.2FeSiO.sub.4+2H.sub.2O  (R-III)

4LiOH+SiO.sub.2+FeCl.sub.2.fwdarw.Li.sub.2FeSiO.sub.4+2H.sub.2O+2LiCl  (R)

[0051] In step 214, the Li.sub.2FeSiO.sub.4-based nanoparticles 112 produced during the reacting step 212 are separated and cleaned. This step 214 may involve: removing the precipitated nanoparticles 112 from the aqueous solution, such as by filtering or drying; rinsing the nanoparticles 112 with deionized water or another suitable rinsing agent; and drying the nanoparticles 112, such as by subjecting the nanoparticles 112 to an elevated temperature and/or a vacuum environment for several hours or more.

[0052] In step 216, the Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 is formed by sintering the Li.sub.2FeSiO.sub.4-based nanoparticles 112 upon the GO, which reduces the GO to graphene 114 (FIG. 2). The sintering step 112 may be performed in an inert atmosphere, such as argon (Ar). The temperature of the sintering step 216 may be about 500° C., about 600° C., about 700° C., or more, but this temperature may vary. The duration of the sintering step 216 may be about 5 hours, about 10 hours, about 15 hours, or more, but this duration may vary.

[0053] In step 218, the cathode 104 (FIG. 1) is constructed using the Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110 from the sintering step 216. The constructing step 218 may involve preparing a slurry. In one example, the slurry contains about 80 wt. % Li.sub.2FeSiO.sub.4-based/Graphene nanocomposite cathode material 110, about 10 wt. % polyvinylidence difluoride (PVDF), and about 10 wt. % carbon black. Next, the slurry may be sprayed onto or otherwise applied to the cathode current collector 105, such as a 10 μm thick aluminum (Al) foil. Then, the cathode 104 may be dried in a vacuum oven, such as at a temperature of about 90° C. and a duration of about 24 hours.

[0054] While this invention has been described as having exemplary designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

Examples

1. Preparation of GO Solution

[0055] A GO solution was prepared using a modified Hummer's method. 2 grams of graphite flakes were mixed with 10 mL of concentrated H.sub.2SO.sub.4, 2 grams of (NH.sub.4).sub.2S.sub.2O.sub.8, and 2 grams of P.sub.2O.sub.5. The obtained mixture was heated at 80° C. for 4 hours under constant stirring. Then the mixture was filtered and washed thoroughly with DI water. After drying in an oven at 80° C. overnight, this pre-oxidized graphite was then subjected to oxidation using the Hummer's method. 2 grams of pre-oxidized graphite, 1 gram of sodium nitrate and 46 mL of sulfuric acid were mixed and stirred for 15 minutes in an iced bath. Then, 6 grams of potassium permanganate was slowly added to the obtained suspension solution for another 15 minutes. After that, 92 mL DI water was slowly added to the suspension, while the temperature was kept constant at about 98° C. for 15 minutes. After the suspension has been diluted by 280 mL DI water, 10 mL of 30% H.sub.2O.sub.2 was added to reduce the unreacted permanganate. Finally, the resulted suspension was centrifuged several times to remove the unreacted acids and salts. The purified GO were dispersed in DI water to form a 0.2 mg/mL solution by sonication for 1 hour. Then the GO dispersion was subjected to another centrifugation in order remove the un-exfoliated GO. The resulted GO dilute solution could remain in a very stable suspension without any precipitation for a few months.

2. Synthesis of Li.sub.2FeSiO.sub.4

[0056] First, 16 mmol LiOH and 4 mmol nano-SiO.sub.2 were dissolved in 30 mL distilled water to produce Solution 2-A, and 4 mmol FeCl.sub.2.4H.sub.2O was dissolved in another 20 mL distilled water to produce Solution 2-B. After 1 h stirring, the aqueous Solution 2-B was added dropwise in Solution 2-A with continued stirring for 4 h to produce Solution 2-C.

[0057] Then, Solution 2-C was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li.sub.2FeSiO.sub.4.

[0058] Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The pure Li.sub.2FeSiO.sub.4 was sintered at 600° C. for 10 h in argon (Ar) atmosphere.

[0059] Using transmission electron microscopy (TEM), the Li.sub.2FeSiO.sub.4 particles measured about 15 nm in diameter (FIG. 4A).

3. Synthesis of Undoped Li.sub.2FeSiO.sub.4/Graphene

[0060] First, 16 mmol LiOH and 4 mmol nano-SiO.sub.2 were dissolved in 30 mL distilled water to produce Solution 3-A, and 4 mmol FeCl.sub.2.4H.sub.2O was dissolved in another 20 mL distilled water to produce Solution 3-B. After 1 h stirring, the aqueous Solution 3-B was added dropwise in Solution 3-A with continued stirring for 4 h to produce Solution 3-C.

[0061] Next, a 5 mg/mL GO gel was prepared according to Example 1. 20 mL of the GO gel was dropped in Solution 3-C with continued stirring for 1 h.

[0062] Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li.sub.2FeSiO.sub.4 upon the GO.

[0063] Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The Li.sub.2FeSiO.sub.4/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene.

[0064] Using TEM, the undoped Li.sub.2FeSiO.sub.4/Graphene particles appeared as nanorods and measured about 5 nm in diameter and about 10-30 nm in length (FIG. 4B).

4. Synthesis of 2% Mn-Doped Li.sub.2Fe.sub.0.98Mn.sub.0.02SiO.sub.4/Graphene

[0065] First, 16 mmol LiOH and 4 mmol nano-SiO.sub.2 were dissolved in 30 mL distilled water to produce Solution 4-A, and 3.92 mmol FeCl.sub.2.4H.sub.2O and 0.08 mmol MnCl.sub.2.4H.sub.2O as the Mn-dopant were dissolved in another 20 mL distilled water to product Solution 4-B. After 1 h stirring, the aqueous Solution 4-B was added dropwise in Solution 4-A with continued stirring for 4 h to produce Solution 4-C.

[0066] Next, a 5 mg/mL GO gel was prepared according to Example 1. 20 mL of the GO gel was dropped in Solution 4-C with continued stirring for 1 h.

[0067] Then, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li.sub.2Fe.sub.0.98Mn.sub.0.02SiO.sub.4 upon the GO according to the following Mn-doped variation of Reaction (R):

4LiOH+SiO.sub.2+(1−d)FeCl.sub.2+cMnCl.sub.2.4H.sub.2O.fwdarw.Li.sub.2Fe.sub.1−dMn.sub.cSiO.sub.4+(4c+2)H.sub.2O+2(1−d+c)LiCl

[0068] wherein: [0069] c=0.02; and [0070] d=c=0.02.

[0071] Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The Li.sub.2Fe.sub.0.98Mn.sub.0.02SiO.sub.4/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene. The doping ratio of Mn/Fe was 2% by weight, calculated as (0.02*54.938)/(0.98*55.845).

5. Synthesis of 15% F-Doped Li.sub.2FeSiO.sub.3.76F.sub.0.48/Graphene and 6% F-Doped Li.sub.2FeSiO.sub.3.9F.sub.0.2/Graphene

[0072] First, 16 mmol LiOH and 3.76 mmol nano-SiO.sub.2 were dissolved in 30 mL distilled water to produce Solution 5-A, and 4 mmol FeCl.sub.2.4H.sub.2O was dissolved in another 20 mL distilled water to produce Solution 5-B. After 1 h stirring, the aqueous Solution 5-B was added dropwise in Solution 5-A with continued stirring for 4 h to produce Solution 5-C.

[0073] Next, a 5 mg/mL GO gel was prepared according to Example 1. 20 mL of the GO gel was dropped in Solution 5-C with continued stirring for 1 h. In addition to the GO, 0.48 mmol NH.sub.4F as the F-dopant was added to Solution 5-C and stirred for 5 mins.

[0074] Then, the mixture was quickly transferred to a 100 mL Teflon-lined stainless-steel autoclave. After sealing, the autoclave was maintained at 180° C. for 12 h to produce Li.sub.2FeSiO.sub.3.76F.sub.0.48 upon the GO according to the following F-doped variation of Reaction (R):

4LiOH+SiO.sub.2+FeCl.sub.2+aNH.sub.4F.fwdarw.Li.sub.2FeSiO.sub.4−bF.sub.a+aNH.sub.3↑+(2+b)H.sub.2O+2LiCl

[0075] wherein: [0076] a=0.48; and [0077] b=a/2=0.24.

[0078] Finally, when the reaction was completed, the autoclave was cooled to room temperature naturally. The precipitates were filled and washed with DI water several times and finally dried at 60° C. for 12 h in vacuum. The Li.sub.2FeSiO.sub.3.76F.sub.0.48/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene. The doping ratio of F/O was 15% by weight, calculated as (0.48*18.998)/(3.76*15.999).

[0079] Using TEM, the F-doped Li.sub.2FeSiO.sub.3.76F.sub.0.48/Graphene particles appeared as nanorods (FIG. 4C) and measured even smaller in particle size than the undoped Li.sub.2FeSiO.sub.4/Graphene particles (FIG. 4B).

[0080] A similar process was performed to produce F-doped particles with other doping ratios from 2% by weight to 10% by weight. For example, F-doped Li.sub.2FeSiO.sub.3.9F.sub.0.2/Graphene particles were produced having a doping ratio of F/O of 6% by weight, calculated as (0.2*18.998)/(3.9*15.999).

6. Preparation of Li.sub.2FeSiO.sub.4—Based Electrodes

[0081] Cathodes were prepared using the various Li.sub.2FeSiO.sub.4-based nanocomposite cathode materials from Examples 2-5. Each material was slurried with 10% carbon black (Super P™ Conductive Carbon Black, TIMCAL) and 10% PVDF, sprayed onto a 10 μm thick Al foil, placed in a vacuum oven, and allowed to dry at 90° C. for 24 hours. The resulting cathodes were assembled into R2016 coin cells using Li metal anodes and dielectric separators with electrolytes including 1.0 M LiPF.sub.6 in a 3:7 by weight solvent mixture of EC and EMC.

7. Electrochemical Performance of F-Doped Electrodes

[0082] The F-doped Li.sub.2FeSiO.sub.4-based electrodes were subjected to electrochemical testing. The 6% F-doped Li.sub.2FeSiO.sub.3.9F.sub.0.2/Graphene electrodes from Example 5 (labeled F-LFSO-G) exhibited better overall performance than the undoped Li.sub.2FeSiO.sub.4/Graphene electrodes from Example 3 (labeled LFSO-G), which exhibited better overall performance than the pure Li.sub.2FeSiO.sub.4 electrodes from Example 2 (labeled LFSO-blank). The electrochemical test results are presented in FIGS. 5A-5D and are summarized in Table 3 below.

TABLE-US-00003 TABLE 3 Doping Discharge Ratio Capacity Cycle Life Specific Energy Diffusion Electrode Material (to O) (mAh/g) (at 600 Wh/kg) (Wh/kg; at ⅓ C) (cm.sup.2/s) Li.sub.2FeSiO.sub.4 0% 188 <10 <250 Li.sub.2FeSiO.sub.4/Graphene 0% 247 42 <550 2.90 × 10.sup.−16 Li.sub.2FeSiO.sub.3.9F.sub.0.2/Graphene 6% 305 63 720 2.21 × 10.sup.−12

[0083] A surprising phenomenon was seen in the specific capacity/energy of the Li.sub.2FeSiO.sub.3.9F.sub.0.2/Graphene electrodes, where the specific energy increased during the first 25 cycles from 830 Wh/kg to 1020 Wh/kg before decreasing (FIG. 5B).

[0084] The capacity results for the other F-doped Li.sub.2FeSiO.sub.4-based electrodes referenced in Example 5 are presented in FIG. 6. The 6% F-doped electrode showed the highest capacity. However, the additional F-dopant had a negative impact on cycle life, as shown by comparing the 4% F-doped results to the 6% F-doped results in FIG. 7.

[0085] The 6% F-doped electrode was subjected to further electrochemical testing at 0.1C, and the results are presented in FIGS. 8A-8C.

8. Electrochemical Performance of Mn-Doped Electrodes

[0086] The 2% Mn-doped Li.sub.2FeSiO.sub.4-based electrodes from Example 4 were subjected to electrochemical testing at 0.1C, and the results are presented in FIGS. 9A-9C.

[0087] Similar Mn-doped Li.sub.2FeSiO.sub.4-based electrodes were prepared with different Mn-doping ratios from 0.5% by weight to 30% by weight, and the comparative capacity results are presented in FIG. 10. The 2% Mn-doped electrode showed the highest capacity.

9. Electrochemical Performance of Other Cation-Doped Electrodes

[0088] Other Li.sub.2FeSiO.sub.4-based electrodes were prepared with cations other than Mn at different doping ratios. The comparative capacity results are presented in Table 4.

TABLE-US-00004 TABLE 4 Doping Ratio Specific Capacity (mAh/g) (to Fe) Ti Cu Nb Mo 2% 259 258 271 169 5% 142 269 282 151 8% 248 286 307 236

10. Electrochemical Performance of Hybrid-Doped Electrodes

[0089] Other Li.sub.2FeSiO.sub.4-based electrodes were prepared with both anion and cation dopants, specifically 6% F-dopant and from 2% to 10% cation dopant. The initial cycle life data for 6% F-2% Ti hybrids and 6% F-2% Cu hybrids is presented in Table 5. The cycle life data for 6% F-2% Nb, 6% F-8% Nb, and 6% F-10% Nb hybrids is presented in FIG. 11. The cycle life data for 6% F-8% Ti hybrids is presented in FIG. 12. In general, the cation dopants may improve cycle life compared to F-dopants alone.

TABLE-US-00005 TABLE 5 Specific Capacity (mAh/g) Dopants 1 2 3 6%F—2%Ti 259 240 246 6%F—2%Cu 258 235 243