WO2022195602A1

WO2022195602A1 - A process for heteroatom doping in graphene and a heteroatom doped graphene material

Info

Publication number: WO2022195602A1
Application number: PCT/IN2021/050434
Authority: WO
Inventors: Rebti BHUSHAN; Awalendra Kumar Thakur; Prashant Kumar
Original assignee: Indian Institute Of Technology Patna
Priority date: 2021-03-18
Filing date: 2021-05-04
Publication date: 2022-09-22

Abstract

The present invention relates to a process for heteroatom doping in graphene. The process comprises preparing a mixture of graphene and a doping precursor in a weight ratio of 0.5-1: 0.4-1.5 in a solvent and sonicating the mixture to obtain a dispersed solution. The dispersed solution is then subjected to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of heteroatom doped graphene. The invention also relates to a heteroatom doped graphene material comprising elemental nitrogen, elemental boron or a combination of nitrogen-boron resulting in three stable phases of BCN, B5C73N22, B8C76N16, or B10C77N13.

Description

A PROCESS FOR HETERO ATOM DOPING IN GRAPHENE AND A HETEROATOM DOPED GRAPHENE MATERIAL

FIELD OF THE INVENTION

The present invention relates to a process for heteroatom doping in graphene and a heteroatom doped graphene material comprising elemental nitrogen, elemental boron or a combination of nitrogen-boron resulting in three stable phases of BCN, B₅C₇₃N₂₂, B₈C₇₆N₁₆, or B₁₀C₇₇N₁₃.

BACKGROUND OF THE INVENTION

Carbonaceous materials have emerged as a promising material for fabrication of anode for lithium ion battery owing to their superior electronic, thermal, and mechanical properties. Graphene, a sp²-hybridized atomic sheet of carbon possesses high surface area and electronic mobility as well as excellent thermal conductivity. Similar to graphite which is commercially utilized anode material for lithium ion batteries, with emergence of graphene in 2004, utilizing it as an active anode material for lithium ion battery, has become a great dream. However, for anode application, sufficient guest ion site availability and electrochemically active basal planes are prerequisite. An unhindered diffusion of lithium ion through the bulk of anode material is also desirable. Conventionally, graphite is a commercially used anode for lithium ion battery (LIB) which delivers a theoretical capacity of 372 mAhg ¹ for lithium ion intercalation by forming an intercalated graphite i.e. LiC_6.

Li⁺+ 6C +le -^LiC_f, During charging cycle.

LiC₆ - Li⁺ + le + 6C During discharge cycle.

Its capacity is limited and does not satisfy the need of today’s portable consumer electronic devices and electric vehicles. Many carbon-based Vander Waals materials are modified to enhance the gap between the layers which are very weakly bounded in the out-of-plane direction by F(r) = A/r⁶- B/r¹² where r is the distance between layers. Changing the inter-planar distance just by few angstroms can therefore alter interactions among the layers by many orders of magnitude. Heteroatom doping can overcome the limitations of pristine graphene. When doped by pentavalent atoms, it is supposed to have an n-type majority carrier. It has been proved that doping of elements such as nitrogen or boron into carbonaceous materials efficiently facilitates the interaction of ions on the active site and promotes the charge transfer process between electrode and ions, resulting in an improvement in redox action for energy storage. However, the efficiency of charge transfer depends upon the level of doping of carbonaceous material. Tuning the various properties of graphene by introducing an electron- rich atom or electron deficient dopant is known. Most of the reported literature till date on heteroatom doping of graphene has resulted in low elemental doping percentage and poor interlayer exfoliation which degrades the device performance. They also suffer from poor sp² hybridization incorporation of dopant into the lattice of pristine graphene, which is desirable to maintain crystalline behavior and charge mobility of pristine graphene. Moreover, most of these techniques are usually multi-step synthesis, cumbersome, involve uncontrollable reaction and often end up in fragmenting graphene sheets, enhanced oxygen content in the system. Apart from this, these techniques suffer from scalability and tunability issues as well. In brief, precisely tunable ultimate level of doping in graphene with elemental nitrogen, elemental boron in a controlled manner and nitrogen-boron co-doping, has not yet been attained due to several processing related limitations, which remains a technological gap.

CN105060279 relates to a four step technique for graphene doping via grafting method for preparing three-dimensional porous nitrogen-doped graphene where carbon and sulfur bonds of the thioamide compounds are reduced in the presence of Zn-Hg/HCl reducing agent; one reduction product of thioamide compounds has a grafting reaction with the surface of oxidized graphene and is decomposed at a high temperature, and accordingly, nitrogen doping of graphene is obtained. However, the process by-product is highly toxic (H₂S gas), and the synthesis process required an acidic medium. Moreover, a very low level of uncontrollable nitrogen doping (-9.2%) inside graphene was obtained which hampered the mobility of charge carriers. CN102485647 relates to a method for preparing boron doped graphene by utilizing combination of boron element and boron tri bromide as source of boron. A very poor level of boron content (0.1-10% mol%) was doped in graphene.

Also, CN104108712, KR20180099572, CN103072977, R0134113, W02015050353, CN107963627, CN104229781, CN 103058177, CN103833012, CN103840160, and CN109110751 disclose a method of doping graphene with nitrogen or boron. The known methods provide poor level of dopant content. Moreover, the prior arts also suffer from non-sp² doping which disturbs highly crystalline graphene plane and therefore causes charge carrier localization. The prior art methods involve a high manufacturing cost.

SUMMARY OF THE INVENTION

In an aspect, the invention relates to a process for heteroatom doping in graphene. The process comprises preparing a mixture of graphene and a doping precursor in a weight ratio of 0.5-1: 0.4-1.5 in a solvent. Then, the mixture is sonicated to obtain a dispersed solution. The dispersed solution is then subjected to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of heteroatom doped graphene.

In another aspect, the invention relates to a heteroatom doped graphene material comprising elemental nitrogen, elemental boron or a combination of nitrogen- boron resulting in three stable phases of BCN, B5C73N22, B₈C76Ni6, or B10C77N13.

BRIEF DESCRIPTION OF THU DRAWINGS

Figure 1: Flowchart for the process steps followed for heteroatom graphene doping.

Figure 2: (a): Schematic description of microwave reactor graphene doping; Schematic description of graphene doping process for elemental nitrogen (b) or boron doping (c) and co-doping nitrogen-boron (d). Figure 3: Raman spectra, optical image and Raman mapping for; (a) N-doped graphene (Raman Spectra: Sample N2 (Red), Sample N3 (Blue) and Sample N4 (Green); Optical image: Sample N4; Raman mapping: Sample N4Samples N2, N3 and N4), (b) B-doped graphene (Raman Spectra: Sample B1 (Red) and Sample B2 (Green); Optical image: Sample B2; Raman mapping: Sample B2) and (c) NB co doped graphene (Raman Spectra: Sample NB1 (Red), Sample NB2 (Blue) and Sample NB3 (Green); Optical image: Sample NB3; Raman mapping: Sample NB3). Raman mapping of samples w.r.t. G peak of pristine graphene (1580 cm ¹) for an area of 10 pm ^c 10 pm. (Color code: Yellow: 1580cm ¹, Red 1595cm ¹, Violet: 1565cm ¹).

Figure 4: FTIR (Fourier Transform Infra-Red spectroscopy) spectra of (a) N- doped graphene (Sample N1 (Black), Sample N2 (Blue), Sample N3 (Green) and Sample N4 (Red)) and (b) B-doped graphene (Sample B1 (Blue) and Sample B2 (Red)).

Figure 5: (a) XPS (X-ray photoelectron spectroscopy) spectra of N-doped graphene (Samples N2-N4) under microwave irradiation depicting binding energy of nitrogen with graphene lattice, first column Cls spectra and second column Nls fine spectra; (b) XPS spectra of B-doped graphene (Samples B1 and B2) under microwave irradiation depicting binding energy of boron with graphene lattice, first column Cls spectra and second column Bis fine spectra; (c) XPS spectra of NB co-doped graphene (Sample) under microwave irradiation depicting binding of boron and nitrogen with graphene lattice, first column Bis spectra and second column Nls fine spectra.

Figure 6: HRTEM (High-resolution transmission electron microscope) micrographs confirming dopant insertion in graphene lattice, Selected area atomic resolution images and Fast Fourier Transform (FFT) profile of pristine graphene compared with doped graphene at various doping configurations: (a) pristine graphene (b) N-doped graphene (Sample N4) (c) B-doped graphene (Sample B2) and (d) NB co-doped graphene (Sample NB3).

Figure 7: HRTEM image showing elemental mapping for boron-nitrogen co doped graphene (Sample NB3) confirming co-doping in graphene lattice: (a) Atomic profile for all three atoms C, B and N together (Mixed colour), (b) Atomic profile for only carbon (green colour), (c) Atomic profile for only boron (red colour) and (d) Atomic profile for only nitrogen (blue colour).

Figure 8: Cyclic voltammogram of Li || N-doped graphene based 2032-coin cell exhibiting redox action vs. Li/ Li⁺ for; (a) Sample N2, (b) Sample N3 and (c) Sample N4 at a scan rate of lmVs ¹.

Figure 9: Li || N-doped graphene based 2032-coin cell exhibiting anode discharge response with reversibility vs. Li/ Li⁺ for; (a) Sample N2, (b) Sample N3 and (c) Sample N4.

Figure 10: Discharge profile performance of Li || N-doped graphene based 2032- coin cell rate capability performance up to 450 cycles for; (a) Sample N2, (b) Sample N3 and (c) Sample N4.

Figure 11: Electrochemical impedance spectroscopy (EIS) pattern of N-doped graphene cell vs. Li before and after (5 charge discharge cycles) for; (a) Sample N2, (b) Sample N3 and (c) Sample N4. Equivalent electrical circuit (inset).

PET ATT ED DESCRIPTION OF THE INVENTION

In an aspect, the invention relates to a process for heteroatom doping of graphene. The process comprises preparing a mixture of graphene and a doping precursor in a weight ratio of 0.5-1: 0.4-1.5 in a solvent. The mixture is then sonicated to obtain a dispersed solution and the dispersed solution is subjected to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of heteroatom doped graphene. The process of heteroatom doping of graphene has been outlined in Figure 1.

The doping precursor is selected from nitrogen precursor including ammonium hydroxide, polyacronitrile (PAN), ethylene diamine and combination thereof, boron precursor including boric acid, boron trioxide and combination thereof and a combination of nitrogen-boron precursor. The solvent is in a range of 10 ml to 100 ml and is selected from water, ethanol, isopropyl alcohol (IPA), dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof. The solvent preferably is isopropyl alcohol in a range of 30-80 ml.

The mixture of graphene and doping precursor in the solvent is sonicated at a frequency in a range from 20 kHz to 25 kHz for 30 minutes to 2 hours or at a frequency of 20 kHz for one hour. The sonication is carried out in an ultrasonicator at room temperature (RT). The mixture preferably comprises graphene and doping precursor in a weight ratio in a range from 0.5:0.4 to 0.5:1.5.

The dispersed solution is subjected to a pulsed microwave radiation at a power in a range from 400 W to 900 W. The dispersed solution is subjected to a pulsed microwave radiation preferably for 10 seconds to 1 minute for 8-20 times. The microwave reactor for carrying out the process of heteroatom doping of graphene with elemental boron and nitrogen is shown in Figure 2a.

The process also comprises cooling, washing the heteroatom doped graphene with de-ionized water, isopropyl alcohol or ethanol each for 2-5 minutes. The heteroatom doped graphene is then dried at 80°C for around 8 hours.

The process of the invention provides faster, economical and easier doping of graphene with elements such as nitrogen, boron, phosphorous, sulphur, aluminum etc. and simultaneous (co-doping) with an appropriate combination of elements. The pulsed microwave doping process of the invention is pre-dominantly electric field driven unlike conventional doping which is purely a thermally driven diffusion process. Moreover, conventional doping methods suffer from uncontrollable C-dopant (say C-N bond) bond formation mechanism. Graphene being semi-metallic, absorbs microwave and triggers microwave plasma. Hot spots are generated at the graphene-doping precursor interface upon microwave exposure that helps in vacancy formation in graphene. Localized thermal spikes, created during the process at an optimal microwave power, are responsible for breaking of bonds of doping precursor.

The process of invention provides a method of heteroatom doped graphene with yield in the range of 48%-86%. The process provides heteroatom doped graphene with an enhanced interlayer spacing of up to 10 A, preferably up to 9.1 A which is ~3 times larger than commercially used graphite and the elemental nitrogen, elemental boron or a combination of nitrogen-boron is sp² bonded with carbon. The process provides ultrahigh doping of graphene closer to its theoretical doping limit (-37.5%) for elemental nitrogen. The process results in oxygen content to be as low as 4.9% in the heteroatom doped graphene. The process also provides heteroatom doped graphene consisting of electrochemically active sites to support redox reaction. This provides sufficient and reasonably improved host sites favoring electrochemical activity for improved energy storage and high rate capability delivery when incorporated in an anode for a lithium ion battery.

The process does not involve any catalyst and does not have any toxic by products. The process of the invention is scalable and reproducible.

In an embodiment, the process relates to heteroatom doping in graphene with elemental nitrogen. The process comprises preparing a mixture of graphene and a nitrogen doping precursor selected from ammonium hydroxide (NH₄OH), polyacronitrile (PAN), ethylene diamine and combination thereof in 10- 100ml of solvent selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof, wherein the weight ratio of graphene: nitrogen doping precursor is 0.5-1 : 0.4-1.5. The mixture is sonicated at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours to obtain a dispersed solution. Then, the dispersed solution is subjected to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of elemental nitrogen doped graphene, wherein 6.5%-35% of elemental nitrogen is doped into one or more layers of graphene and up to 95% of elemental nitrogen is sp² bonded with carbon, i.e.; graphitic doping. A schematic representation of the process is shown in Figure 2b.

In another embodiment, the process relates to heteroatom doping in graphene with elemental boron. The process comprises preparing a mixture of graphene and a boron doping precursor selected from boric acid, boron trioxide (B₂0₃) and combination thereof in 10- 100ml of solvent selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof, wherein the weight ratio of graphene: boron doping precursor is 0.5-1 : 0.4-1.5. The mixture is sonicated at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours to obtain a dispersed solution. Then, the dispersed solution is subjected to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of elemental boron doped graphene, wherein 4.6%-19.4% of elemental boron is doped into graphene. A schematic representation of the process is shown in Figure 2c.

In another embodiment, the process relates to heteroatom doping in graphene with nitrogen and boron simultaneously. The process comprises preparing a mixture of graphene and a nitrogen-boron doping precursor selected from ammonium hydroxide, polyacronitrile (PAN), ethylene diamine, boric acid, boron trioxide and combination thereof in 10- 100ml of solvent selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof, wherein the weight ratio of graphene: nitrogen-boron doping precursor is 0.5-1: 0.4-1.5. The mixture is sonicated at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours to obtain a dispersed solution. Then, the dispersed solution is subjected to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of nitrogen-boron doped graphene, wherein 23%-27% of nitrogen-boron are doped into graphene resulting in three stable phases of BCN, B.C.JNL· BX.JNL· and B _nC N . A schematic representation of the process is shown in Figure 2d. The BCN is two dimensional (2D). In another aspect, the invention relates to a heteroatom doped graphene material comprising elemental nitrogen, elemental boron or a combination of nitrogen- boron resulting in three stable phases of BCN, B₅C₇₃N₂₂, B₈C₇₆Ni₆, or B₁₀C₇₇N_13. The heteroatom doped graphene material is monolayered or multi-layered.

The heteroatom doped graphene material is included as an anode in lithium ion battery and has a very high reversible capacity against lithium in comparison to commercially available graphite or MCMB (Mesocarbon microbeads) carbon that are used currently as anode in lithium ion batteries.

In an embodiment, the invention relates to a heteroatom doped graphene material comprising 6.5%-35% of nitrogen and up to 95% of elemental nitrogen is sp² bonded with carbon.

In another embodiment, the invention relates to a heteroatom doped graphene material comprising 4.6%-19.4% of boron.

In an embodiment, the invention relates to a heteroatom doped graphene material comprising 23%-27% of nitrogen-boron resulting in three stable phases of BCN B₅C₇₃N₂₂, B₈C₇₆Ni₆, or B₁₀C₇₇N_13. The formula indicates the percentage of each element present in the heteroatom doped graphene material.

The invention also relates to a process for preparation of an anode. The process comprises preparing a slurry of the heteroatom doped graphene material, a binder and a conducting material. The slurry is uniformly coated on a metal foil and then the coated metal foil is rolled and dried.

The binder is a polymeric binder such as poly(vinylidene) fluoride (PVDF). The conducting material is selected from activated carbon, carbon black (available under the tradename Super P) and a combination thereof, preferably the conducting material is carbon black. The metal foil is made of copper and serves as a current collector.

The heteroatom doped graphene material, binder and conducting material are present in the slurry in a weight ratio in a range from 70:10:20 to 95:5:0 or 70:20:10 to 85:10:5. The coated foil is dried at 80°C for around 15 hours.

The invention also relates to an electrochemical cell comprising the anode containing the heteroatom doped graphene material, a cathode, a separator and an electrolyte. The cathode is selected from a group comprising of alkali metal or an alloy thereof. The separator comprises polypropylene carbonate film, glass fiber or combination thereof. The electrolyte comprises lithium hexafluorophosphate (LiPF₆) in ethylene carbonate and diethyl carbonate (EC: DEC) in volume ratio of 1 : 1 and/or similar compositions of organic electrolytes for lithium ion cell.

The alkali metal in the cathode preferably is lithium and the electrochemical cell is a coin cell, preferably CR 2032.

In an embodiment, the electrochemical cell is a 2032-coin cell comprising the anode containing heteroatom doped graphene material comprising elemental nitrogen, lithium cathode, polypropylene separator and lithium hexafluorophosphate in ethylene carbonate and diethyl carbonate in volume ratio of 1:1, as an electrolyte. The electrochemical cell has a diffusion co-efficient of lithium ion (Li ) in range from 1.51x10 cm /s to 3.78x10 cm /s and a charge transfer resistance in range from 46 W to 114 W.

The diffusion co-efficient was higher than other carbonaceous electrodes for lithium ion battery (~1.12xlO ¹⁰ cm²/s for graphite and ~5.24xlO ¹⁰ cm²/s for MCMB). The electrochemical cell delivers a higher capacity than the theoretical capacity of 365-372 mAhg ¹ and much higher than the reversible capacity of 240-270 mAhg ¹ achieved with graphitic or MCMB carbon currently used in commercial LIBs.

EXAMPLES

The process for heteroatom doping in graphene is described in Examples below:

Example 1 (Sample Nl)

A solution of multilayered graphene and ammonium hydroxide was mixed in a ratio of 1:1.5 ratio in 40 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 480 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration the washed solution was dried at 80°C for 8 hours to obtain Sample Nl with yield of synthesis 76.5%. The Sample Nl exhibited 2-4 layers with a nitrogen content of 14.9%, out of which 45% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 5.6 A and bond length between carbon and nitrogen was 1.45 A.

Example 2 (Samples N2)

A solution of multilayered graphene and ammonium hydroxide was mixed in a ratio of 1:1.5 ratio in 40 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 560 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample N2 with yield of synthesis 79%. The Sample N2 exhibited 1-2 layers with a nitrogen content of 22 %, out of which 72% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 8.9 A and bond length between carbon and nitrogen was 1.49 A.

Example 3 (Sample N3)

A solution of multilayered graphene and ammonium hydroxide was mixed in a ratio of 1:1.5 ratio in 40 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 640 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample N3 with yield of synthesis 81%. The Sample N3 exhibited 1-2 layers with a nitrogen content of 28%, out of which 75% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 8.1 A and bond length between carbon and nitrogen was 1.49 A.

Example 4 (Sample N4)

A solution of multilayered graphene and ammonium hydroxide was mixed in a ratio of 1:1.5 ratio in 40 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 720 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample N4 with yield of synthesis 81%. The Sample N4 exhibited mono layer with a nitrogen content of 35%, out of which 95% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 9.1 A and bond length between carbon and nitrogen was 1.46 A.

Example 5 (Sample N5)

A solution of multilayered graphene and ammonium hydroxide was mixed in a ratio of 1:1.5 ratio in 40 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 850 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample N5 with yield of synthesis 81.7%. The Sample N5 exhibited 1-2 layers with a nitrogen content of 22.8%, out of which 68% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 8.9 A and bond length between carbon and nitrogen was 1.48 A. Example 6 (Sample N6)

A solution of multilayered graphene and Polyacronitrile was mixed in a ratio of 0.5:1.5 ratio in 60 ml of toluene solvent. This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hours to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 560 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain N6 with yield 58%. The Sample N6 exhibited 4-6 layers with a nitrogen content of 9% out of which 24% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 3.78 A and bond length between carbon and nitrogen was 1.43 A.

Example 7 (Sample N7)

A solution of multilayered graphene and ethylene diamine was mixed in a ratio of 0.5:1.5 ratio in 60 ml of toluene solvent. This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hours to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 560 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain N7 with yield 51%. The Sample N7 exhibited 4-6 layers with a nitrogen content of 6.5% out of which 35% of nitrogen was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 3.42 A and bond length between carbon and nitrogen was 1.44 A. Example 8 (Sample Bl)

A solution of multilayered graphene and boron trioxide was mixed in a ratio of 1:1 ratio in 50 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 1 -minute shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 480 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample Bl with yield of synthesis 69%. The Sample Bl exhibited 3-5 layers with a boron content of 4.6%, out of which 25% of boron was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 4.6 A and bond length between carbon and boron was 1.44 A.

Example 9 (Sample B2)

A solution of multilayered graphene and boron trioxide was mixed in a ratio of 1:1 ratio in 50 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 30 sec shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 720 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample B2 with yield of synthesis 73%. The Sample B2 exhibited 1 layers with a boron content of 16.3 %, out of which 68% of boron was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 8.8 A and bond length between carbon and boron was 1.42 A. Example 10 (Sample B3)

A solution of multilayered graphene and boron trioxide was mixed in a ratio of 1:1 ratio in 50 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 1 hour to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10 % of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 20 sec shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 850 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample B3 with yield of synthesis 74.2%. The Sample B3 exhibited 1 layer with a boron content of 19.4 %, out of which 82.6% of boron was sp² bonded with carbon and others were sp³ and sp bonded. The interlayer separation was 8.8 A and bond length between carbon and boron was 1.43 A.

Example 11 (Sample NB1)

A solution of multilayered graphene, ammonium hydroxide and boron tri oxide (B₂0₃) was mixed in a ratio of 0.5: 0.43 ratio in 60 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20kHz in an ultrasonicator for 2 hours (120 minutes) to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 30 sec shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 720 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample NB1 with yield of synthesis 48%. The Sample NB1 was monolayered and has the formula B₅C₇₃N_22. The interlayer separation was 7.2 A. The heteroatom doped graphene anode material was monolayered with boron content of 5% and nitrogen content 22%. The combined doping percentage was 27% out of which 74.2% was sp² bonded with carbon and others were sp³ and sp bonded.

Example 12 (Sample NB2)

A solution of multilayered graphene, ammonium hydroxide and boron tri oxide was mixed in a ratio of 0.5: 0.66 ratio in 60 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20kHz in an ultrasonicator for 2 hours to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 30 sec shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 720 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample NB2 with yield of synthesis 86%. The Sample NB2 was monolayered has the formula B₈C76Ni6. The interlayer separation was 6.8 A. The heteroatom doped graphene anode material was monolayered with boron content of 8% and nitrogen content 16%. The combined doping percentage was 24% out of which 70% was sp² bonded with carbon and others were sp³ and sp bonded.

Example 13 (Sample NB3)

A solution of multilayered graphene, ammonium hydroxide and boron tri oxide was mixed in a ratio of 0.5: 1 ratio in 60 ml of isopropyl alcohol (IP A). This mixture was sonicated at 20 kHz in an ultrasonicator for 2 hours to obtain a dispersed solution. The amount of solvent taken was kept in the microwave vessel at 10% of the total volume. The dispersed solution so obtained was exposed to microwave irradiation for 30 sec shot for 10 times. The generated pressure was frequently released up to 2 to 5 minutes. The microwave irradiation was maintained at 720 W. After natural cooling to the room temperature, the microwave exposed dispersed solution was washed with de-ionized water, isopropyl alcohol and ethanol a few times. After washing for the optimized duration, the washed solution was dried at 80°C for 8 hours to obtain Sample NB3 with yield of synthesis 84%. The Sample NB3 was monolayered and has the formula B₁₀C₇₇N_13. The interlayer separation was 6.8 A. The heteroatom doped graphene material was monolayered with boron content of 10% and nitrogen content 13%. The combined doping percentage was 23% out of which 70.2% was sp² bonded with carbon and others were sp³ and sp bonded.

A summary of the process conditions of the examples is shown in Table-1 below:

Table 1: Process conditions for co-doping of pristine graphene with elemental boron and nitrogen

Example 14: Preparation of an anode comprising heteroatom doped graphene anode material of the present invention.

A slurry was prepared using 80% of the heteroatom doped graphene material (active material), 10 % of Polyvinylidene fluoride as a binder and 10% of Super P

(carbon black) as a conducting material. The resulting slurry was uniformly coated onto a copper foil as a current collector and then rolled. The coated copper foil was dried at 80°C for around 15 hours and punched into circle-shaped electrodes.

Example 15: Preparation of an electrochemical cell

A coin cell (CR2032) was fabricated with the anode (working electrode) comprising Samples N2-N5 and lithium as the cathode (counter electrode). LiPF₆ (lithium hexafluorophosphate) in ethylene carbonate (EC), and diethyl carbonate (DEC) 1:1 was used as the electrolyte and Polypropylene was used as the separator. The coin cell was assembled in an air-filled glove box with oxygen wherein moisture level less than lppm. The cell configuration was Li || Polypropylene separator+LiPF₆ (EC:DEC) || nitrogen-doped graphene.

Characterization of the heteroatom doped graphene anode of the present invention

The effect of the process parameters on the graphene structure as a consequence of doping is shown in Table 2 below:

Table 2: Effect of process conditions on co-doping in pristine graphene bulk structure

The hybridization details of the heteroatom doped graphene material as mentioned in the above table show that up to 95% of the dopant element nitrogen was bonded to sp² hybridized carbon. This indicates that heteroatom doping process of the invention did not disturb the crystalline graphene plane thereby preventing charge carrier localization resulting in reduction of resistance to movement of electrons.

Characterization of the heteroatom doped graphene material made by the process of the invention

Structural Characterization

The crystal structure of the heteroatom doped graphene anode materials was studied using Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, optical microscopy and High resolution transmission electron microscopy.

Raman Spectroscopy

Figure 3 shows the Raman spectra, optical image and Raman mapping for heteroatom doped graphene comprising elemental nitrogen (Sample N2-N4) (Figure 3 a), elemental boron (Samples B1 and B2) (Figure 3b) and nitrogen and boron together (Samples NB1-NB3) (Figure 3c), made by the process of the present invention. The Raman spectra confirms the formation of heteroatom doped graphene with elemental nitrogen, elemental boron and nitrogen and boron together. Fourier Transform Infrared (FTIR) Spectroscopy

Figure 4 shows the FTIR spectra for heteroatom doped graphene comprising elemental nitrogen (Samples N1-N4) (Figure 4a) and elemental boron (Samples B1 and B2) (Figure 4b) made by the process of the present invention. The FTIR spectra confirms the formation of heteroatom doped graphene with elemental nitrogen and elemental boron.

X-Ray Photoelectron Spectroscopy (XPS)

Figure 5 shows the XPS spectra for heteroatom doped graphene comprising elemental nitrogen, Samples N2-N4 (Figure 5a), elemental boron, Samples B1 and B2 (Figure 5b) and nitrogen and boron together (Figure 5c), made by the process of the present invention. The figure provides quantitative confirmation of percentage doping and yield of the samples.

High-Resolution Transmission Electron Microscopy (HRTEM)

Figure 6 shows the HRTEM micrographs. Figure 6a shows the micrograph of pristine graphene. While Figure 6b-6d show the micrograph for heteroatom doped graphene comprising elemental nitrogen, elemental boron and nitrogen and boron together, respectively.

The presence of red dots in Figure 6b indicate nitrogen incorporated into the graphene lattice. The presence green dots in Figure 6c indicate boron incorporated into the graphene lattice. While the presence of red and green dots in Figure 6d indicate nitrogen and boron simultaneously incorporated into the graphene lattice.

The co-doping of nitrogen and boron into the graphene lattice was confirmed by HRTEM images showing elemental mapping (Figure 7). Figure 7b-7d shows the atomic profile for individual element-carbon (green colour), boron (red colour) and nitrogen (blue colour), respectively. Figure 7a is the atomic profile for nitrogen-boron co-doped graphene (Sample NB3) made by the process of the present invention: green colour indicates carbon, red colour indicates boron and blue color indicates nitrogen. This confirms the insertion of boron and nitrogen together into the graphene lattice. Electrochemical characterization

The electrochemical performance of the heteroatom doped graphene made by the process of the present invention was investigated in lithium ion electrochemical cell by using cyclic voltammetry, galvanostatic charge-discharge, C-rate performance, cycling studies, and electrochemical impedance analysis. The results for cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy is tabulated in Table 3 below.

Table 3: Summary of lithium ion cell performance

Cyclic Voltammetry

Cyclic Voltammetry (CV) studies were carried out to determine the oxidation- reduction phenomena in the samples via electron transfer among various redox couples. Figure 8 shows the cyclic voltammogram obtained for lithium ion 2032- coin cell having anode comprising heteroatom doped graphene material containing elemental nitrogen (Sample N2, (Figure 8a), Sample N3 (Figure 8b) and Sample N4 (Figure 8c)) at a scan rate of lmVs ¹. From Table 3, it is observed that anode containing Sample N2 provides a diffusion co-efficient for lithium ion of 3.78x 10⁹ cm²/s which is higher in comparison to anodes for lithium ion battery containing graphite (1.12xlO ¹⁰ cm²/s) and MCMB (5.24xlO ¹⁰ cm²/s).

Galvanostatic Charge-Discharge studies and C-rate performance Figure 9 shows the anode discharge response for lithium ion 2032-coin cell having anode comprising heteroatom doped graphene material containing elemental nitrogen (Sample N2, (Figure 9a), Sample N3 (Figure 9b) and Sample N4 (Figure 9c)). Figure 10 shows the discharge profile for lithium ion 2032-coin cell having anode comprising heteroatom doped graphene material containing elemental nitrogen (Sample N2, (Figure 10a), Sample N3 (Figure 10b) and Sample N4 (Figure 10c)). The figure shows rate capability performance up to 450 cycles. The lithium ion coin cell having anode comprising heteroatom doped graphene material Sample N2 at charge-discharge cycles at different load current as high as 1.12 Ag ¹ delivered a very high reversible capacity of 457 mAhg ¹ even during 300^th - 450^th cycle that recovers to 1377 mAhg ¹ when charge-discharge rate is lowered to 73mAg ¹. This capacity delivery was higher than the theoretical capacity (365-372 mAhg ¹) and much higher than the reversible capacity (240-270 mAhg ¹) of graphitic or MCMB carbon currently used in anode of commercial LIBs.

Electrochemical Impedance Analysis Electrochemical impedance spectroscopy (EIS) results depict the mass and charge diffusion kinetics inside the material synthesized by present invention. The Nyquist plot characterizes real and imaginary impedance bestowed by bulk of the material and solid-electrolyte interphase (SEI) for incoming electrolyte ions. It tells the transport mechanism of mass and charge in the electrode material. Figure 11 shows the EIS pattern of a lithium ion electrochemical cell having anode containing heteroatom doped graphene material containing elemental nitrogen (Sample N2, (Figure 11a), Sample N3 (Figure lib) and Sample N4 (Figure 11c) before and after 5 charge discharge cycles.

The above results for the 2032-coin cell having anode comprising heteroatom doped graphene material containing elemental nitrogen in comparison to the commercial anodes for lithium ion battery showed that the mechanism for charge storage in the of heteroatom doped graphene material containing elemental nitrogen was different from the known mechanism described above. The inventors proposed the following mechanism for electrochemical action in the case of heteroatom doped graphene material containing elemental nitrogen.

Li⁺+ 2C +le - L1C2 during charging cycle.

LiC₂ Li⁺+ le + 2C during discharge cycle.

Based on the above electrochemical redox action scheme, the reversible capacity calculation at the C-rate works out to be 1116 mAhg ¹. This is unlike graphitic or MCMB carbon intercalation mechanism where 6 C-atoms per Li are involved in redox activity forming LiC_6.

Among the heteroatom doped graphene material containing elemental nitrogen in the present studies, N2-N4 exhibited lithium (Li⁺) ion diffusion comparable to graphitic or MCMB carbon. Still, their reversible capacity versus Li⁺ ion has been measured to be higher -1377-418 mAhg ¹ with a cyclic stability up to 450 charge discharge cycles. Further, heteroatom doped graphene material containing elemental nitrogen, elemental boron or co-doped with nitrogen and boron were equally good in their electrochemical action.

The result of the tests indicates that by appropriate optimization of key process conditions such as microwave power, exposure duration, and graphene: doping precursor ratio etc., a tailored composition of doping comprising optimal dopant concentration, favorable doping sites and graphene interlayer spacing can deliver an improved lithium intercalation-deintercalation feature forming LiC₂ yielding hitherto unknown reversible capacity in carbonaceous anodes. The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.

Claims

Claims :

1. A process for heteroatom doping in graphene, the process comprising: preparing a mixture of graphene and a doping precursor in a weight ratio of 0.5-1: 0.4-1.5 in a solvent; sonicating the mixture to obtain a dispersed solution; and subjecting the dispersed solution to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of heteroatom doped graphene.

2. The process as claimed in claim 1, wherein the doping precursor is selected from nitrogen precursor including ammonium hydroxide, polyacronitrile (PAN), ethylene diamine and combination thereof, for elemental nitrogen doped graphene, boron precursor including boric acid, boron trioxide and combination thereof, for elemental boron doped graphene, and, a combination of nitrogen-boron precursor, for boron-nitrogen co-doped graphene.

3. The process as claimed in claim 1, wherein the solvent is present in a range of 10ml- 100ml and is selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof.

4. The process as claimed in claim 1, wherein sonication is performed at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours or at 20 kHz for one hour.

5. The process as claimed in claim 1, wherein the dispersed solution is subjected to microwave radiation at a power in range from 400 W to 900 W.

6. The process as claimed in claim 1, wherein the ratio of graphene to doping precursor is in a range from 0.5:0.4 to 0.5:1.5.

7. The process as claimed in claim 1, comprising: washing the heteroatom doped graphene and drying the heteroatom doped graphene at 80°C for 8 hours.

8. The process as claimed in claim 1, wherein the yield of heteroatom doped graphene is 48%-86%.

9. The process as claimed in claim 1, wherein the heteroatom doped graphene has an interlayer spacing of up to 10 A and the elemental nitrogen, elemental boron or combination of nitrogen-boron is sp² bonded with carbon.

10. The process for heteroatom doping in graphene as claimed in claim 1, the process comprising: preparing a mixture of graphene and a nitrogen doping precursor selected from ammonium hydroxide, polyacronitrile (PAN), ethylene diamine and combination thereof in 10- 100ml of solvent selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof, wherein the weight ratio of graphene: nitrogen doping precursor is 0.5-1: 0.4-1.5; sonicating the mixture at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours to obtain a dispersed solution; and subjecting the dispersed solution to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of elemental nitrogen doped graphene, wherein 6.5%-35% of elemental nitrogen is doped into one or more layers of graphene and up to 95% of elemental nitrogen is sp² bonded with carbon.

11. The process for heteroatom doping in graphene as claimed in claim 1, the process comprising: preparing a mixture of graphene and a boron doping precursor selected from boric acid, boron tri oxide and combination thereof in 10- 100ml of a solvent selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof, wherein the weight ratio of graphene: boron doping precursor is 0.5-1: 0.4-1.5; sonicating the mixture at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours to obtain a dispersed solution; and subjecting the dispersed solution to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of elemental boron doped graphene, wherein 4.6%-19.4% of elemental boron is doped into graphene.

12. The process for heteroatom doping in graphene as claimed in claim 1, the process comprising: preparing a mixture of graphene and a nitrogen-boron doping precursor selected from ammonium hydroxide, polyacronitrile (PAN), ethylene diamine, boric acid, boron tri oxide and combination thereof in 10- 100ml of a solvent selected from water, ethanol, isopropyl alcohol, dimethyl sulfoxide (DMSO), benzene, toluene and mixture thereof, wherein the weight ratio of graphene: nitrogen-boron doping precursor is 0.5-1: 0.4-1.5; sonicating the mixture at a frequency in a range from 20 kHz - 25 kHz for 30 minutes to 2 hours to obtain a dispersed solution; and subjecting the dispersed solution to microwave radiation for 10 seconds to 2 minutes for 8 to 20 times to obtain one or more layers of nitrogen-boron doped graphene, wherein 23%-27% of nitrogen-boron are doped into graphene resulting in three stable phases of BCN,

and

^B10^C77^N13 ·

13. A heteroatom doped graphene material comprising elemental nitrogen, elemental boron or a combination of nitrogen-boron resulting in three stable phases of BCN, B₅C₇₃N₂₂, B₈C₇₆N₁₆, or B₁₀C₇₇N₁₃.

14. The heteroatom doped graphene material as claimed in claim 13, wherein the material is monolayered or multi-layered.

15. The heteroatom doped graphene material as claimed in claim 13 comprising 6.5%-35% of nitrogen and up to 95% of elemental nitrogen is sp² bonded with carbon.

16. The heteroatom doped graphene material as claimed in claim 13 comprising 4.6%-19.4% of boron.

17. The heteroatom doped graphene material as claimed in claim 13 comprising 23%-27% of nitrogen-boron resulting in three stable phases of BCN, B₅C₇₃N₂₂, B₈C₇₆N₁₆, or B₁₀C₇₇N₁₃.

18. The heteroatom doped graphene material as claimed in claim 13, wherein the material is included as an anode in lithium ion battery.

19. A process for preparation of an anode comprising: preparing a slurry of heteroatom doped graphene material as claimed in claim 13, a binder and a conducting material; coating the slurry on a metal foil; and drying the coated foil.

20. The process as claimed in claim 19, wherein the binder is a polymeric binder- poly(vinylidene) fluoride (PVDF), the conducting material is selected from activated carbon, carbon black and a combination thereof, and the metal foil is of copper.

21. The process as claimed in claim 19, wherein the heteroatom doped graphene material, binder and conducting material in the slurry is in a weight ratio of 70:10:20 to 95:5:0.

22. The process as claimed in claim 19, wherein the coated foil is dried at 80°C for 15 hours.

23. An electrochemical cell comprising: an anode comprising heteroatom doped graphene material as claimed in claim 13; a cathode selected from a group comprising of alkali metal or an alloy thereof; a separator comprising polypropylene carbonate film, glass fiber or combination thereof; and an electrolyte.

24. The electrochemical cell as claimed in claim 23 wherein the alkali metal is lithium, and the electrolyte comprises lithium hexafluorophosphate (LiPF₆) in ethylene carbonate and diethyl carbonate (EC: DEC) in volume ratio of 1 : 1.

25. The electrochemical cell as claimed in claim 23 wherein the cell is a 2032- coin cell and the anode comprises heteroatom doped graphene material comprising elemental nitrogen as claimed in claim 15 having a diffusion co efficient of lithium ion (Li⁺) in range from 1.51xlO ¹⁰ cm²/s to 3.78 1 O ⁹ cm²/s and a charge transfer resistance in range from 46 W to 114 W.