Patent ID: 12206108

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

FIG.1provides a Li-ion battery100including an anode102coupled to an anode current collector103, a cathode104coupled to a cathode current collector105, and an electrolyte-filled separator106. In one particular example, the anode102comprises Li metal or graphite and the electrolyte comprises a solution of LiPF6in 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 battery100is a pouch cell, but the battery100may also be a cylindrical cell, a coin cell, or a prismatic cell, for example. The battery100may be configured for use in a portable electronic device, an electric vehicle, an energy storage device, or other electronic devices.

FIG.2provides an improved Li2FeSiO4-based/Graphene nanocomposite cathode material110for cathode104of battery100(FIG.1). Advantageously, the Li2FeSiO4-based/Graphene nanocomposite cathode material110may 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.

As shown inFIG.2, the Li2FeSiO4-based/Graphene nanocomposite cathode material110includes Li2FeSiO4-based nanoparticles112(e.g., nanorods) formed upon a conductive matrix of graphene sheets114. The graphene sheets114may improve the electrical conductivity of the nanocomposite cathode material110compared to the Li2FeSiO4-based nanoparticles112alone. Additionally, the graphene sheets114may provide a structural matrix to anchor and stabilize the Li2FeSiO4-based nanoparticles112and reduce grain stress during charge/discharge cycling, leading to better cycle life. As shown inFIG.2, each graphene sheet114includes a single-layer of graphene with sp2-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 sp2orbits) 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 pzorbit) 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 material110. The nanocomposite cathode material110exhibits improved intraparticle electronic conduction because of good electrical conductivity of graphene, and Li+ion diffusion is improved because diffusion length is shortened. Furthermore, the small grain size of the Li2FeSiO4-based nanoparticles112in the nanocomposite cathode material110reduces internal stress, leading to better structure stability and cycle life. According to an exemplary embodiment of the present disclosure, the nanocomposite cathode material110may 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 sheets114is disclosed in U.S. Publication No. 2015/0380732, the disclosure of which is expressly incorporated herein by reference in its entirety.

In certain embodiments, the graphene sheets114may 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 sheets114through diazonium salt via a diazonium reaction. The diazonium reaction-based functionalization may provide a simple and cost-effective way to transform the pure graphene sheets114into hierarchical and functional materials that can provide the desired properties (i.e. hydrophobicity, Li+/e−conductivity, Li+diffusivity, nanoparticle dispersion, local electric field, etc.) to enhance binding with the adjacent Li2FeSiO4-based nanoparticles112.

The Li2FeSiO4-based nanoparticles112may 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 Li2FeSiO4-based nanoparticles112. 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 Li2FeSiO4-based nanoparticles112may 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.

Examples of suitable anion dopants X include halogen ions, such as fluorine ions (F−), chlorine ions (Cl−), and bromine ions (Br−). 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 O2−→O22−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 material110in the Li-ion battery100(FIG.1).

Examples of suitable cation dopants Y include titanium ions (Ti+4), manganese ions (Mn+2), copper ions (Cu+2), terbium ions (Tb+3), niobium ions (Nb+5), and molybdenum ions (Mo+4). Such cation dopants Y may improve the structure stability of the Li2FeSiO4-based nanoparticles112by enhancing the coupling effect among the Li2FeSiO4-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.

The Li2FeSiO4-based/Graphene nanocomposite cathode material110may have the following Formula (F-I):
Li2Fe1−dYcSiO4−bXa/Graphene  (F-I)

wherein:X=anion dopant;Y=cation dopant;a≥0;b=f(a)c≥0; andd=f(c).

Examples of suitable Li2FeSiO4-based/Graphene nanocomposite cathode materials110are listed in Table 2 below.

TABLE 2MaterialXbYdLi2FeSiO4/Graphene—0—0Li2FeSiO4−a/2Fa/GrapheneFa/2—0Li2Fe1−cMncSiO4/Graphene—0MncLi2Fe1−cCucSiO4/Graphene—0CucLi2Fe1−2.5cNbcSiO4/Graphene—0Nb2.5cLi2Fe1−2.5cNbcSiO4−a/2Fa/GrapheneFa/2Nb2.5c

FIG.3provides an exemplary method200for synthesizing the Li2FeSiO4-based/Graphene nanocomposite cathode material110(FIG.2) and constructing the cathode104(FIG.1). The method200may involve a hydrothermal synthesis process.

In step202, a first solution is prepared including a lithium oxide, specifically lithium hydroxide (LiOH), and a silicon oxide, specifically silica (SiO2), in a suitable solvent such as distilled water. The SiO2may be provided in the form of nanoparticles, also referred to herein as nano-SiO2.

In step204, a second solution is prepared including an iron compound, specifically iron dichloride (FeCl2), in a suitable solvent such as distilled water.

In step206, 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.

In step208, a graphene oxide (GO) solution is added to the combined solution from step206. 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 step206. The resulting solution may be stirred together for about 30 minutes, about 1 hour, or longer before proceeding to the next step.

In step210, a salt of any desired dopant X, Y is added to the resulting solution from step208. Examples of suitable salts include ammonium fluoride (NH4F) for the F-dopant, cupric chloride (CuCl2) for the Cu-dopant, and niobium hydroxide (Nb(OH)5) for the Nb-dopant. The dopant X, Y may be present in a desired concentration relative to the Li2FeSiO4.

In step212, the solution is reacted to produce optionally doped, Li2FeSiO4-based nanoparticles112(FIG.2) over the GO. This reacting step210may be a hydrothermal step that is performed in an autoclave or another suitable heated environment. The temperature of the reacting step210may be about 160° C., about 180° C., about 200° C., or more, but this temperature may vary. The duration of the reacting step210may be about 5 hours, about 10 hours, about 15 hours, or more, but this duration may vary. After the reacting step210is completed, the autoclave may be allowed to return to room temperature before removing the products.

The reacting step212may produce Li2FeSiO4-based nanoparticles112upon 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+SiO2→Li2SiO3·H2O  (R-I)
FeCl2+2LiOH→Fe(OH)2+2LiCl  (R-II)
Fe(OH)2+Li2SiO3·H2O→Li2FeSiO4+2H2O  (R-III)
4LiOH+SiO2+FeCl2→Li2FeSiO4+2H2O+2LiCl  (R)

In step214, the Li2FeSiO4-based nanoparticles112produced during the reacting step212are separated and cleaned. This step214may involve: removing the precipitated nanoparticles112from the aqueous solution, such as by filtering or drying; rinsing the nanoparticles112with deionized water or another suitable rinsing agent; and drying the nanoparticles112, such as by subjecting the nanoparticles112to an elevated temperature and/or a vacuum environment for several hours or more.

In step216, the Li2FeSiO4-based/Graphene nanocomposite cathode material110is formed by sintering the Li2FeSiO4-based nanoparticles112upon the GO, which reduces the GO to graphene114(FIG.2). The sintering step112may be performed in an inert atmosphere, such as argon (Ar). The temperature of the sintering step216may be about 500° C., about 600° C., about 700° C., or more, but this temperature may vary. The duration of the sintering step216may be about 5 hours, about 10 hours, about 15 hours, or more, but this duration may vary.

In step218, the cathode104(FIG.1) is constructed using the Li2FeSiO4-based/Graphene nanocomposite cathode material110from the sintering step216. The constructing step218may involve preparing a slurry. In one example, the slurry contains about 80 wt. % Li2FeSiO4-based/Graphene nanocomposite cathode material110, 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 collector105, such as a 10 μm thick aluminum (Al) foil. Then, the cathode104may be dried in a vacuum oven, such as at a temperature of about 90° C. and a duration of about 24 hours.

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

A GO solution was prepared using a modified Hummer's method. 2 grams of graphite flakes were mixed with 10 mL of concentrated H2SO4, 2 grams of (NH4)2S2O8, and 2 grams of P2O5. 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% H2O2was 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 Li2FeSiO4

First, 16 mmol LiOH and 4 mmol nano-SiO2were dissolved in 30 mL distilled water to produce Solution 2-A, and 4 mmol FeCl2.4H2O 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.

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 Li2FeSiO4.

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 Li2FeSiO4was sintered at 600° C. for 10 h in argon (Ar) atmosphere.

Using transmission electron microscopy (TEM), the Li2FeSiO4particles measured about 15 nm in diameter (FIG.4A).

3. Synthesis of Undoped Li2FeSiO4/Graphene

First, 16 mmol LiOH and 4 mmol nano-SiO2were dissolved in 30 mL distilled water to produce Solution 3-A, and 4 mmol FeCl2.4H2O 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.

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.

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 Li2FeSiO4upon the GO.

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 Li2FeSiO4/GO was sintered at 600° C. for 10 h in Ar atmosphere to reduce the GO to graphene.

Using TEM, the undoped Li2FeSiO4/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 Li2Fe0.98Mn0.02SiO4/Graphene

First, 16 mmol LiOH and 4 mmol nano-SiO2were dissolved in 30 mL distilled water to produce Solution 4-A, and 3.92 mmol FeCl2.4H2O and 0.08 mmol MnCl2.4H2O 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.

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.

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 Li2Fe0.98Mn0.02SiO4upon the GO according to the following Mn-doped variation of Reaction (R):
4LiOH+SiO2+(1−d)FeCl2+cMnCl2·4H2O→Li2Fe1−dMncSiO4+(4c+2)H2O+2(1−d+c)LiCl

wherein:c=0.02; andd=c=0.02.

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 Li2Fe0.98Mn0.02SiO4/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 Li2FeSiO3.76F0.48/Graphene and 6% F-Doped Li2FeSiO3.9F0.2/Graphene

First, 16 mmol LiOH and 3.76 mmol nano-SiO2were dissolved in 30 mL distilled water to produce Solution 5-A, and 4 mmol FeCl2.4H2O 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.

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 NH4F as the F-dopant was added to Solution 5-C and stirred for 5 mins.

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 Li2FeSiO3.76F0.48upon the GO according to the following F-doped variation of Reaction (R):
4LiOH+SiO2+FeCl2+aNH4F→Li2FeSiO4−bFa+aNH3↑+(2+b)H2O+2LiCl

wherein:a=0.48; andb=a/2=0.24.

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 Li2FeSiO3.76F0.48/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).

Using TEM, the F-doped Li2FeSiO3.76F0.48/Graphene particles appeared as nanorods (FIG.4C) and measured even smaller in particle size than the undoped Li2FeSiO4/Graphene particles (FIG.4B).

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 Li2FeSiO3.9F0.2/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 Li2FeSiO4—Based Electrodes

Cathodes were prepared using the various Li2FeSiO4-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 LiPF6in a 3:7 by weight solvent mixture of EC and EMC.

7. Electrochemical Performance of F-Doped Electrodes

The F-doped Li2FeSiO4-based electrodes were subjected to electrochemical testing. The 6% F-doped Li2FeSiO3.9F0.2/Graphene electrodes from Example 5 (labeled F-LFSO-G) exhibited better overall performance than the undoped Li2FeSiO4/Graphene electrodes from Example 3 (labeled LFSO-G), which exhibited better overall performance than the pure Li2FeSiO4electrodes from Example 2 (labeled LFSO-blank). The electrochemical test results are presented inFIGS.5A-5Dand are summarized in Table 3 below.

TABLE 3DopingDischargeRatioCapacityCycle LifeSpecific EnergyDiffusionElectrode Material(to O)(mAh/g)(at 600 Wh/kg)(Wh/kg; at ⅓ C)(cm2/s)Li2FeSiO40%188<10<250Li2FeSiO4/Graphene0%24742<5502.90 × 10−16Li2FeSiO3.9F0.2/Graphene6%305637202.21 × 10−12

A surprising phenomenon was seen in the specific capacity/energy of the Li2FeSiO3.9F0.2/Graphene electrodes, where the specific energy increased during the first 25 cycles from 830 Wh/kg to 1020 Wh/kg before decreasing (FIG.5B).

The capacity results for the other F-doped Li2FeSiO4-based electrodes referenced in Example 5 are presented inFIG.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 inFIG.7.

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

8. Electrochemical Performance of Mn-Doped Electrodes

The 2% Mn-doped Li2FeSiO4-based electrodes from Example 4 were subjected to electrochemical testing at 0.1C, and the results are presented inFIGS.9A-9C.

Similar Mn-doped Li2FeSiO4-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 inFIG.10. The 2% Mn-doped electrode showed the highest capacity.

9. Electrochemical Performance of Other Cation-Doped Electrodes

Other Li2FeSiO4-based electrodes were prepared with cations other than Mn at different doping ratios. The comparative capacity results are presented in Table 4.

TABLE 4Doping RatioSpecific Capacity (mAh/g)(to Fe)TiCuNbMo2%2592582711695%1422692821518%248286307236
10. Electrochemical Performance of Hybrid-Doped Electrodes

Other Li2FeSiO4-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 inFIG.11. The cycle life data for 6% F-8% Ti hybrids is presented inFIG.12. In general, the cation dopants may improve cycle life compared to F-dopants alone.

TABLE 5Specific Capacity (mAh/g)Dopants1236%F—2%Ti2592402466%F—2%Cu258235243