Abstract
The present invention discloses a hydrothermal process of preparing lithium iron phosphate (LiFePCO.sub.4) nanoparticles. It further discloses a composite electrode comprising lithium iron phosphate, multiwalled carbon nanotubes (MWCNTs) and polyvinylidene fluoride as well as a method of manufacturing this composite electrode. It also discloses a free-standing composite electrode comprising spinel-Li.sub.4Ti.sub.5O.sub.12, multiwalled carbon nanotubes and carboxymethyl cellulose as well as a method of manufacturing this free-standing composite electrode.
Claims
1. A hydrothermal process of preparing lithium ion phosphate (LiFePO.sub.4) micrometer-scale and nanometer-scale particles, wherein the hydrothermal process comprises the steps of: preparing a precursor solution; mixing de-ionized water with the precursor solution forming a mixture; subjecting the mixture to intensive magnetic stirring; and recovering precipitates of the mixture by a process of centrifugation.
2. The hydrothermal process according to claim 1, wherein the precursor solution comprises: 3 M of LiOOCCH.sub.3; 1 M of FeCl.sub.2; 1 M of L-ascorbic acid; and 1 M of H.sub.3PO.sub.4.
3.-4. (canceled)
5. The hydrothermal process according to claim 2, wherein the L-ascorbic acid acts as a reducing agent to reduce Fe.sup.+ ions to Fe.sup.+ ions, thereby preventing oxidation of Fe.sup.+ ions within the mixture.
6. The hydrothermal process according to claim 1, wherein subjecting the mixture to intensive magnetic stirring comprises stirring the mixture at 800-1000 revolutions per minute for a duration of 1 hour at room temperature.
7.-10. (canceled)
11. The hydrothermal process according to claim 1, wherein recovering the precipitates comprises washing the precipitates three times with deionized water.
12. The hydrothermal process according to claim 1, further comprising drying the recovered precipitates at 80° C. to form LiFePO.sub.4 micrometer-scale and nanometer-scale particles.
13. The hydrothermal process according to claim 1, further comprising confirming a crystal orientation of the recovered precipitate using X-ray diffraction, Raman spectroscopy, or scanning electron microscopy to confirm a crystal orientation of the recovered precipitates.
14. The hydrothermal process according to claim 13, wherein the crystal orientation of the recovered precipitates depends on a duration of the hydrothermal process.
15. (canceled)
16. A composite electrode comprising: lithium ion phosphate (LiFePO.sub.4); multi-walled carbon nanotubes (MWCNT); and polyvinylidene fluoride (PVDF).
17. (canceled)
18. A method of manufacturing the composite electrode according to claim 16, the method comprising the steps of: mixing synthesized LiFePO.sub.4 particles with ethanol to form a mixture; grinding the mixture softly using mortar and pestle forming a slurry; transferring the slurry to a beaker and sonicating the slurry; coating the slurry on copper foil and baking the copper foil in an oven; and detaching the composite electrode from the copper foil.
19. The method of manufacturing the composite electrode according to claim 18, wherein the mixture comprises synthesized lithium ion phosphate (LiFePO.sub.4) particles, multi-walled carbon nanotubes (MWCNT), and polyvinylidene fluoride (PVDF).
20. (canceled)
21. The method of manufacturing the composite electrode according to claim 19, wherein the synthesized lithium ion phosphate (LiFePO.sub.4) particles, the multi-walled carbon nanotubes (MWCNT), and the polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 70:20:10 respectively.
22. The method of manufacturing the composite electrode according to claim 18, wherein the mixture is annealed at 600° C. for 2 hours in an Argon (Ar)-atmosphere.
23. The method of manufacturing the composite electrode according to claim 18, wherein the slurry is sonicated for 10 minutes.
24. The method of manufacturing the composite electrode according to claim 18, wherein the slurry is degasified for 1 minute in a vacuum oven.
25.-29. (canceled)
30. A free-standing composite electrode comprising: spinel-Li.sub.4Ti.sub.5O.sub.12 (LTO); multi-walled carbon nanotubes (MWCNTs); and carboxymethyl cellulose.
31. A method of manufacturing the free-standing composite electrode of claim 30, the method comprising the steps of: mixing Spinel-Li.sub.4Ti.sub.5O.sub.12 (LTO), multi-walled carbon nanotubes (MWCNTs), and carboxymethyl cellulose with water or ethanol to form a slurry; grinding and sonicating the slurry; coating the slurry on copper foil and placing the slurry in an oven at 120° C. forming an electrode; and detaching the electrode from the copper foil to form a free-standing composite electrode.
32. The method of manufacturing according to claim 31, wherein a method for preparing spinel-Li.sub.4Ti.sub.5O.sub.12 (LTO) free-standing composite electrodes comprises: synthesizing a precursor solution through a wet-milling technique resulting in a mixture; drying the mixture in air and calcinating the mixture at 850° C.; and grinding the calcinated mixture.
33. The method of manufacturing according to claim 32, wherein the precursor solution comprises 1.073 g of Li.sub.2CO.sub.3 and 2.897 g of TiO.sub.2.
34. The method of manufacturing according to claim 31, wherein the slurry is ground for 2 minutes and sonicated for 10 minutes.
35.-38. (canceled)
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention, the inventions of which can be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein. The foregoing and other aspects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
[0039] FIG. 1 Schematic diagram illustrating the procedure for fabrication of cathode nanocomposites of LiFePO.sub.4 nano/micro-particles and MWCNTs.
[0040] FIG. 2 illustrates photo of free-standing and flexible cathode nanocomposites of LiFePO.sub.4 particles and MWCNTs.
[0041] FIG. 3 illustrates scanning electron microscope (SEM) image of free-standing and flexible cathode nanocomposites of LiFePO.sub.4 particles and MWCNTs.
[0042] FIG. 4 illustrates photo of free-standing and flexible anode nanocomposites of LTO particles and MWCNTs.
[0043] FIG. 5 illustrates SEM image of free-standing and flexible cathode nanocomposites of LiFePO.sub.4 particles and MWCNTs (unpublished).
[0044] FIGS. 6 A and B illustrate top view (left) and cross-sectional view (right) of PEO/LiFTSI solid electrolyte films.
[0045] FIG. 7 illustrates schematic diagram showing a few steps towards integration of three components to build a full-cell and flexible Li-ion battery.
[0046] FIGS. 8 A and B illustrate SEM images of cross-sectional view of the integrated cathode nanocomposite and PEO-based solid electrolyte films.
[0047] FIG. 9 illustrates schematic diagram showing a few steps designed for ‘Roll-to-Roll’ process towards mass production of flexible Li-ion batteries.
[0048] FIG. 10 illustrates schematic illustration for the preparation of LiFePO.sub.4 particles and LiFePO.sub.4+CNT nanocomposites.
[0049] FIG. 11 illustrates XRD patterns of a) LiFePO.sub.4 micrometer-scale particles synthesized by a hydrothermal process under different reaction hours b) LiFePO.sub.4/MWCNT nanocomposites.
[0050] FIG. 12 illustrates Raman spectra of a) LiFePO.sub.4 micrometer-scale particles synthesized by a hydrothermal process under different reaction hours b) LiFePO.sub.4/MWCNT nanocomposites.
[0051] FIG. 13 illustrates SEM images of LiFePO.sub.4 micrometer-scale particles synthesized by a hydrothermal process under different reaction hours: a) 3 h b) 6 h c) 9 h d) 12 h. SEM images of LiFePO.sub.4/MWCNT nanocomposites for different reaction hours: e) 3 h f) 6 h g) 9 h h) 12 h. All the scale bars are 5 μm.
[0052] FIG. 14 illustrates a) Cyclic voltammograms of LiFePO.sub.4 materials for 3 and 12 h b) Half-cell analysis at different discharge rates for 3 and 12 h.
[0053] FIG. 15 illustrates SEM images of (a) S1; (b) S2; (c) S3; (d) XRD patterns; (e) Raman spectra of composite electrodes.
TABLE-US-00001 Names Composite Electrodes S1 As-prepared LiFePO.sub.4 + CNT + PVDF S2 LiFePO.sub.4 annealed at 600° C. for 2 h under Ar-atmosphere + CNT + PVDF S3 Commercial LiFePO.sub.4 + CNT + PVDF Pristine LiFePO.sub.4 Commercial LiFePO.sub.4 powders
[0054] FIG. 16 illustrates (a) Stress vs strain curves at room temperature; (b) stress vs strain curves at 80° C.; (c) stress; (d) tan δ— tan δ depicts the damping behavior of the composite electrodes. High tan δ value indicated the more energy dissipation potential. Low tan δ value indicated the more load could be stored rather than energy dissipation; (e) modulus behavior with temperature ramping of composite electrodes.
[0055] FIG. 17 illustrates (a) Thermal conductivity and effusivity; (b) Volumetric heat capacity; (c) Electrical conductivity; (d) Cyclic voltammograms of composite electrodes.
[0056] FIG. 18 illustrates (a) XRD patterns and (b) Raman spectra of LTO-COM, LTO-BM, LTO-COM BP and LTO-BM BP.
TABLE-US-00002 Names Compositions LTO-COM Commercial LTO LTO-BM As-prepared LTO LTO-COM BP Commercial LTO composite electrode LTO-BM BP As-prepared LTO composite electrode
[0057] FIG. 19 illustrates SEM images of (a) LTO-COM, (b) LTO-BM, (c) LTO-COM BP and (d) LTO-BM BP.
[0058] FIG. 20 illustrates (a) Cyclic voltammograms at scan rate of 0.1 mVs-1, (b) charge/discharge curves at rate of 0.2 C and inset is showing potential difference between charge and discharge plateaus (c) rate performance at different C-rates and (d) cycling performance at 1C rate for 100 cycles of LTO-COM BP and LTO-BM BP.
[0059] FIG. 21 illustrates the uses of Li-ion batteries, preferably for space applications.
[0060] FIG. 22 illustrates the objectives to develop Li-ion batteries
[0061] FIG. 23 illustrates the methodologies to develop Li-ion batteries.
[0062] FIG. 24 illustrates the development of anode based on CNT composites.
[0063] FIG. 25 illustrates the characterization of anode nanocomposites.
[0064] FIG. 26 illustrates the development of solid electrolyte.
[0065] FIG. 27 illustrates the development of solid electrolyte.
[0066] FIG. 28 illustrates the summary of development of Li-ion batteries.
[0067] FIG. 29 illustrates the on-going development
[0068] FIG. 30 illustrates the future work
DETAILED DESCRIPTION OF THE INVENTION
[0069] The present invention deals with hydrothermal synthesis of LiFePO.sub.4 micro-particles for fabrication of cathode materials based on LiFePO.sub.4/MWCNT nanocomposites for Li-ion batteries.
[0070] Lithium iron phosphate (LiFePO.sub.4) micro-particles (MPs) were synthesized under hydrothermal condition for fabrication of cathode materials based on LiFePO.sub.4 MPs/multi-walled carbon nanotube (MWCNT) nanocomposites. Influence of reaction time for the hydrothermal process on structural, morphological and electrochemical behavior was investigated. Crystal quality was confirmed by X-ray diffraction (XRD) together with Raman analysis. Micrometer scale seeds and capsule-shaped morphology were observed. Such nanocomposite cathodes based on LiFePO.sub.4 MPs/MWCNT were prepared by Surface-engineered Tape Casting technique. The well-crystallized material composed of densely aggregated MPs and interconnected with MWCNTs led to excellent volumetric Li storage properties at a current rate of 0.1 mVs between 2.5 V to 4.3 V. However, the half-cell analysis does not show reasonable capacity values, which may be due to the larger particle size and morphology of synthesized LiFePO.sub.4, resulting in limiting ionic transportation and electronic conduction path.
[0071] Experiment
[0072] All chemicals were analytical grade, and were used as received. The preparation procedures are shown in FIG. 10. LiFePO.sub.4 particles were prepared by a simple hydrothermal process, under which the precursor solution was prepared with the chemicals of 3 M of LiOOCCH.sub.3, 1 M of FeCl.sub.2, 1 M of L-ascorbic acid, 1 M of H.sub.3PO.sub.4 mixed with 5 mL of de-ionized water/ethylene glycol (volumetric ratio 1/1) medium. L-ascorbic acid acted as a reducing agent to reduce the Fe.sup.3+ to Fe.sup.2+ and prevent the oxidation of Fe.sup.2+. Then the mixture subjected to intensive magnetic stirring at 800 RPM for 1 hour at room temperature. The resulting homogeneous mixture was quickly transferred to a 23 mL Teflon lined stainless steel autoclave and placed in a timer controlled oven. The autoclave was heated and maintained at 160° C. for different reaction hours (3, 6, 9 and 12 h) under air atmosphere. Subsequently, the autoclave was cooled down naturally to room temperature. Precipitates were recovered by centrifugation and washed several times with deionized water and dried at 80° C. in an oven under air atmosphere. The resultant precipitates were characterized by X-ray diffraction (XRD, PANalytical X'pert Pro), Raman spectroscopy (Witec Alpha 300RAS) and scanning electron microscopy (SEM, Nova Nanosem, FEI).
[0073] Electrochemical characterization was conducted using 2032 type coin cells. Nanocomposite cathodes based on LiFePO.sub.4 micrometer-scale particles (MPs)/multi-walled carbon nanotubes (MWCNTs) were prepared by tape-casting technique as shown in FIG. 10. Synthesized LiFePO.sub.4 micro-particles and multi-walled carbon nanotubes (MWCNTs) were mixed with the weight ratio of 50:50. Then the mixture was added with Water/Ethanol (volumetric ratio 50/50) solvents and softly grounded for 2 minutes. Further the slurry was transferred to a beaker and sonicated for 10 minutes. While doing sonication, slurry was stirred simultaneously using advanced hot plate stirrers at room temperature for better particles dispersion. LiFePO.sub.4/carbon nanotubes nanocomposites working electrodes has been prepared through surface-engineered tape casting technique with a tape casting blade gap of 3 mm. Before casting, slurry was placed in a vacuum oven for 1 minute for degasification purposes. Further the prepared slurry was coated on copper foil and placed in an oven at 120° C. for 1 h under air atmosphere. After that, the working electrodes has been de-attached from the copper foil and acted as a free-standing working electrode. Cyclic voltammetry was done with multi-channel potentiostat/galvanostat (Princeton Applied Research PMC-1000) without IR compensation. Charge-discharge performances of the prepared coin cells were tested using a battery tester (Maccor Battery Test System Series 4000) inside an environmental chamber (CSZ Model MC-3 Chamber) at a constant temperature of 25° C.
[0074] Results and Discussion
[0075] Structural evolution of the synthesized LiFePO.sub.4 materials and LiFePO.sub.4/MWCNT nanocomposites for different reaction hours (3, 6, 9 and 12 h) was investigated by XRD and shown in FIG. 11 a, b. From the FIG. 11a, main diffraction peaks represented the single phase of orthorhombic olivine structure of pure LiFePO.sub.4 (JCPDS #—811173) with the space group of Pnma. All diffraction peaks look like intense, confirming the high order of crystallinity. Average crystallite size was estimated from XRD data as 33.54, 51.07, 49.98 and 43.23 nm for 3, 6, 9 and 12 hours, respectively. XRD patterns of LiFePO.sub.4/MWCNT nanocomposites are shown in FIG. 11b, which illustrate no change in olivine structure of pure LiFePO.sub.4 after the addition of MWCNTs. However, there is no evidence for the presence of carbon because of its low intensity weak peak. Hence, the presence of carbon in the LiFePO.sub.4/MWCNT nanocomposites was confirmed with Raman spectra shown in FIG. 12. FIG. 12 demonstrates a strong band at 987 cm.sup.−1 due to internal stretching vibrations of the PO4.sup.3− anions of γ-Li.sub.3Fe.sub.2(PO.sub.4).sub.3 and the weak emission band at 1042 cm.sup.−1 because of laser-induced decomposition of olivine LiFePO.sub.4 under air atmosphere. In FIG. 12, strong bands were observed at 1342, 1582 and 2680 cm.sup.−1, which are ascribed to D, G and G′ bands respectively. D-band attributed the defects or disorders in the graphene structure, whereas G-band confirmed the presence of graphite carbon. G′-band depicted the second order two-phonon process. The intensity ratio of D and G band (ID/IG) was used to estimate the degree of disorders in the nanocomposite electrodes. The ratios ID/IG of 3, 6, 9 and 12 h were calculated and found 1, 1, 1.01 and 1 respectively. The higher ID/IG ratios implied more defects of the LiFePO.sub.4/MWCNT nanocomposites.
[0076] SEM micrographs for LiFePO.sub.4 particles (FIG. 13a-d) and LiFePO.sub.4/MWCNT nanocomposites (FIG. 13e-h for different reaction hours (3, 6, 9 and 12 h) are shown in FIG. 13. Micrometer-scale LiFePO.sub.4 MPs with the shape of ‘seed’ and ‘capsule’ were formed from a shorter reaction time (3 h—FIG. 13a) than others. Morphology of the seed-shaped LiFePO.sub.4 MPs turned to micro-capsule morphology (FIG. 13 b, c & d) while increasing the reaction time, that implies the small changes occurred in LiFePO.sub.4 crystal orientation. In addition, the observed high densely distributed MPs with some pores and tightly packed surface morphology was an advantage for efficient charge carrier separation. SEM micrographs of LiFePO.sub.4/MWCNT nanocomposites confirmed that LiFePO.sub.4 particles are perfectly embedded in an extensive network of MWCNTs, facilitating a highly conductive channel for the mobility of electrons.
[0077] Cyclic voltammetry and half cells analysis of LiFePO.sub.4 materials was conducted and the results were shown in FIG. 14. However, the electrochemical performance for all 3, 6, 9 and 12 reaction hours were almost similar. For comparison, we have reported the initial reaction hours (3 h) and final reaction hours (12 h). Synthesized LiFePO.sub.4 materials were tested at a scanning rate of 0.1 mVs between 2.5 V to 4.3 V (versus Li.sup.+/Li) and shown in FIG. 14a. A pair of anodic and cathodic peaks were observed which represented the two phase intercalation and deintercalation of Li.sup.+ ions involving an Fe.sup.2+/Fe.sup.3+ redox couple. Cyclic voltammograms exhibited the corresponding anodic peaks at 3.54 and 3.52 V, cathodic peaks at 3.31 and 3.33 V for 3 and 12 hours respectively. The observed small potential separation of 0.23 V for 3 h and 0.19 V for 12 h, which indicated an excellent reversible electrochemical mechanism and good stability during Li insertion and extraction. Further, according to Randles-Sevcik equation, high current range from the redox reactions indicates high Li-ion diffusion during redox reactions. Here in this case, 12 h nanocomposite showed high current (0.32 mA) range compared to 3 h nanocomposite (0.08 mA) which might be capable for the excellent volumetric Li-ion storage and transportation performances.
[0078] Half-cell analysis for the synthesized LiFePO.sub.4 cathode materials at different discharge rates was shown in FIG. 14b. Well structural, morphological and electrochemical properties was obtained for the synthesized LiFePO.sub.4 cathode materials. Specific capacity values showed a good stable performance as a function of cycle number for each discharge rate. However, low specific capacity values from the half-cell analysis may be due to the larger particle size and morphology, limited ionic transportation and lack of electron conduction path of synthesized LiFePO.sub.4 cathode materials. Because mobility of the Li-ions in LiFePO.sub.4 particles are mainly depending upon the particle size and morphology. Large particle size blocking the lithium diffusion channel by means of defects, impurities and also increased the channel diffusion length. These factors might have restricted the movement of Li-ions inside the LiFePO.sub.4 particles so that Li-ions cannot hop to their neighbor sites, which might have caused the suppression of Li-ion migration and electronic conduction. Such low Li-ion migration and low electronic conduction might have resulted in capacity loss. During charge and discharge, crystallographic changes and phase boundary movement might occur within the particles that also influenced the capacity loss. So specific capacity losses due to channel blocking and high diffusion path could be minimized for the LiFePO.sub.4 cathode materials with smaller particle size and desired morphologies. It was concluded that the specific capacity loss might be attributed to large particle size and different morphologies.
[0079] Conclusion
[0080] In summary, micrometer-scale LiFePO.sub.4 particles as cathode materials for Li-ion batteries were synthesized through a simple, cost-effective hydrothermal process. XRD and Raman studies confirmed the high order crystallinity of LiFePO.sub.4 cathode materials. Morphological changes were investigated with SEM analysis. Cyclic voltammograms indicated an excellent reversible electrochemical mechanism during Li insertion and extraction. Larger particle size and morphology, limited ionic transportation and lack of electron conduction path are the possible reasons for capacity loss.
[0081] Mechanical Thermal and Electrical Properties of LiFePO.sub.4/MWCNTs Composite Electrodes
[0082] Lithium iron phosphate (LiFePO.sub.4)/multi-walled carbon nanotubes (MWCNT) composite electrodes were prepared via a wet-filtration-zipping technique. Mechanical, thermal and electrical properties of the composite electrodes at various temperatures were studied. The composite electrodes exhibited electrical conductivity in the range of 1.1×10.sup.1-3.56×10.sup.1 S/cm. Further, the thermal conductivity, effusivity and volumetric heat capacity were measured. Cyclic voltammograms confirmed good electrochemical performances and high stability during Li.sup.+ ion intercalation/de-intercalation.
[0083] Experiment
[0084] LiFePO.sub.4 particles were synthesized by a simple hydrothermal process. A stoichiometric amount of LiOOCCH.sub.3, FeCl.sub.2, L-ascorbic acid, H.sub.3PO.sub.4 was mixed in a 3:1:1:1 molar ratio and added to 5 mL of de-ionized (DI) water/ethylene glycol (volumetric ratio 1/1) medium. Then the mixture was subjected to intensive magnetic stirring at 800 RPM for 1 h and transferred quickly into a 23 mL Teflon-lined stainless steel autoclave and heated at 160° C. for 12 h. After autoclave was cooled down to room temperature (RT), the precipitates were washed several times with DI water and dried at 80° C. overnight. The samples in two different conditions were prepared as (i) as-prepared (Sample 1-S1) and (ii) annealed at 600° C. for 2 h under Ar-atmosphere (Sample 2—S2).
[0085] Synthesized LiFePO.sub.4, MWCNT and polyvinylidene fluoride (PVDF) were mixed with the weight ratio of 70:20:10. Then the mixture was added to the DI Water/Ethanol (volumetric ratio 50/50) medium and sonicated for 40 minutes. The prepared slurry was placed in a specific filtration mold and the dimensions of the working electrodes could be controlled. Working electrodes prepared with the dimensions of 5×5 cm and were dried in an oven at 90° C. for 24 h. Commercially available LiFePO.sub.4 powders and commercial LiFePO.sub.4-MWCNT composite electrodes were also prepared for comparison studies. A summary of the prepared samples is illustrated in table 1.
TABLE-US-00003 TABLE 1 Names of the composite electrodes Names Composite Electrodes S1 As-prepared LiFePO.sub.4 + CNT + PVDF S7 LiFePO.sub.4 annealed at 600° C. for 2 h under Ar-atmosphere + CNT + PVDF S3 Commercial LiFePO.sub.4 + CNT + PVDF Pristine LiFePO.sub.4 Commercial LiFePO.sub.4 powders
[0086] Composite electrodes were characterized by X-ray diffraction (XRD, PANalytical X'pert Pro), Raman spectroscopy (Witec Alpha 300RAS) and scanning electron microscopy (SEM, Nova Nanosem, FEI). Tension (room temperature and 80° C.) and three-point bending (frequency fixed at 1 Hz) properties were measured with Dynamic Mechanical Analyzer (DMA TA Q800).
[0087] Thermal measurements were carried out using thermal analyzer (TPS 2500S) under RT. Electrical conductivity (RT and 75° C.) was measured using Hall measurement system (Ecopia HMS-5000). Cyclic voltammetry analysis was conducted with multi-channel potentiostat/galvanostat (Princeton Applied Research PMC-1000).
[0088] Results and Discussion
[0089] SEM images shown in FIG. 15 (a-c) confirmed the LiFePO.sub.4 particles were perfectly embedded and incorporated in the cross-linked MWCNT matrix. MWCNT matrix was found to be tightly packed with the LiFePO.sub.4 particles in close proximity, facilitating a highly conductive channel for transport of electrons. This tightly packed surface morphology can be an advantage for efficient charge carrier separation which might lead to enhanced mobility of electrons during Li.sup.+ ions intercalation/de-intercalation. XRD patterns shown in FIG. 15(d) confirmed the presence of LiFePO.sub.4 phase with an ordered olivine structure perfectly matched to the orthorhombic Pnma space group (JCPDS—811173). All diffraction peaks look narrow and sharp, affirmed the high crystalline nature of the composite electrodes. A small peak shifting is observed in LiFePO.sub.4 composite to higher angle due to addition of MWCNT. The presence of carbon in the composite electrodes was evidenced with Raman spectra as shown in FIG. 15(e). Strong bands observed at 1347, 1586 and 2682 cm.sup.−1 contributed to the D, G and G′ bands respectively. D-band attributed to the defects or disorders in the graphene structure, whereas Gband confirmed the presence of graphite carbon. G′-band depicted the second order two-phonon process. The ratios ID/IG of S1, S2, and S3 were calculated as 1.02, 1, and 1 respectively. The higher ID/IG ratios implied more defects of the composite electrodes.
[0090] FIG. 16 (a & b) exhibited the stress-strain curves at RT and 80° C. respectively. Both the plots confirmed S1 is showing high stress and low strain value compared with S2 and S3, which represented the brittle properties of S1. In S2, notable reduction of stress and increase in strain value affirmed the flexibility and ductility properties. Young's modulus was calculated as 182, 61, 30 MPa for 51, S2, S3 respectively at RT and 99, 40, 24 MPa for 51, S2, S3 respectively at 80° C. FIG. 16c showed the gradually decreasing stress with increasing temperature, because vibration of molecules due to their internal energy increased the mean distance between molecules reducing the mechanical stress. FIG. 16d depicted the damping behavior of the composite electrodes. High tan δ value indicated the more energy dissipation potential. Low tan δ value indicated the more load could be stored rather than energy dissipation. High tan δ value from S2 composite electrodes represented that it could be stretched more compared with S1 and S3. Low tan δ value for S1 composite electrode represented it could not be stretched more which resulted in the high stiffness and rigid behavior. From FIG. 16e, storage and loss modulus continuously decreased with increasing temperature. High storage modulus also confirmed the rigid behavior of S1 and low storage modulus supported the ductile behavior of S2 and S3. A similar trend has been observed in loss modulus that confirmed the low loss modulus values of S2 and S3 indicated the ductile behavior. All the mechanical analysis confirmed S2 composite electrodes exhibited good mechanical performances compared with S1.
[0091] Thermal, electrical and electrochemical properties for the composite electrodes are shown in FIG. 17. FIG. 17a indicated the thermal conductivity values in the range from 0.55 to 0.45 W/mK which is higher than that of previous reported value. High thermal effusivity and volumetric specific capacity values confirmed the S2 composite electrode could store a large degree of heat energy without undergoing a phase transition compared with S1. FIG. 17c affirmed the electrical conductivity of the composite electrodes at RT and 75° C. Electrical conductivity has been determined as the range from 1.16 to 2.71×101 S/cm at RT and 1.34 to 3.56×10.sup.1 S/cm at 75° C. It was 2 orders of magnitude higher than those of reported values 2.3×10.sup.−1 S/cm and 1×10.sup.−1 S/cm. Electrical conductivity increased with the increasing of temperature, because electrons oscillated and acquired sufficient energy to move the conduction band from valance band. Thus, the electrons could move freely for conduction and drastically lowered the resistance. FIG. 17d exhibited the cyclic voltammograms of composite electrodes at a scanning rate of 0.1 mV/s between 2.5 V to 4.2 V (versus Li.sup.+/Li). Well-defined anodic and cathodic peaks corresponded to the two phase intercalation/de-intercalation of Li.sup.+ ions involved the Fe.sup.2+/Fe.sup.3+ redox reaction. In addition, small potential peak separation between the anodic and cathodic waves demonstrated the stability of Li.sup.+ ions during intercalation/de-intercalation.
[0092] Conclusions
[0093] LiFePO.sub.4/MWCNT composite electrodes were prepared through a wet-filtration-zipping technique. SEM images confirmed the LiFePO.sub.4 particles were perfectly embedded and incorporated in the cross-linked MWCNT matrix. High damping value and low storage modulus affirmed the high mechanical performance of the composite electrode with annealed LiFePO.sub.4 particles. High thermal, electrical and electrochemical performances promise the S2 composite electrode as an excellent candidate for cathode materials of Li-ion battery.
[0094] Enhanced Electrochemical Performance of MWCNTs Supported Free-Standing LTO Composite Electrode
[0095] Spinel-Li.sub.4Ti.sub.5O.sub.12/multi-walled carbon nanotubes composite electrodes were prepared via novel and cost-effective surface engineered tape casting technique and well compared with commercially available LTO. The structural, morphological and electrochemical properties of LTO and its composite electrodes were studied. The enhanced electrochemical performance of as-prepared LTO is mainly related to the homogeneous distribution of particles and its small size which facilitates large amount of active sites for lithium insertion and also short diffusion paths to operate at high current.
[0096] Experimental
[0097] LTO was synthesized by wet-milling route using 1.073 g of Li.sub.2CO.sub.3 (Sigma-Aldrich) and 2.897 g of TiO.sub.2 (VWR, <500 nm) as lithium and titanium sources, respectively with ethanol (Sigma Aldrich) as media. All the precursors were ball-milled at 400 rpm for 5 h using full directional planetary ball mill (Tencan QXQM-0.4). The resulting mixture was dried in air and later calcined at 850° C. for 26 h in muffle furnace, followed by grinding for 1 h and named as LTO-BM. The commercial LTO (EQ-Lib-LTO-1, MTI Corp, USA) was used for comparison study and named as LTO-COM.
[0098] MWCNTs supported LTO free-standing composite electrodes were prepared by surface engineered tape-casting technique. LTO-BM/LTO-COM, MWCNTs, and carboxymethyl cellulose with the weight ratio of 70:25:5 were mixed with water/ethanol (volumetric ratio 50/50). Later, the slurry was ground for 2 minutes and sonicated for 10 minutes with continuous stirring. The slurry was coated on copper foil and placed in an oven at 120° C. for 1 h. The electrode was de-attached from the copper foil and acted as a free-standing working electrode. The composite electrodes are named as LTO-BM BP and LTO-COM BP for LTO-BM and LTOCOM, respectively and tabulated in Table 1.
TABLE-US-00004 TABLE 1 Sample names and its compositions Names Compositions LTO-COM Commercial LTO LTO-BM As-prepared LTO LTO-COM BP Commercial LTO composite electrode LTO-BM BP As-prepared LTO composite electrode
[0099] LTO and MWCNTs-supported LTO composite electrodes were characterized by powder X-ray diffraction (XRD, PANalytical X'pert Pro), Raman spectroscopy (Witec Alpha 300RAS) and scanning electron microscopy (SEM, Nova Nanosem, FEI). Coin cells of 2032-type were assembled in half-cell (working electrode against lithium) configuration with 1 M LiPF.sub.6 in EC:EMC (1:1 vol %) with 2 wt % FEC electrolyte inside glovebox (MBraun MB-Labstar 1450/780). Cyclic voltammetry (CV) was carried out with multi-channel potentiostat/galvanostat (Princeton Applied Research PMC-1000) at a scanning rate of 0.1 mVs.sup.−1 between 1 V to 2 V (versus Li.sup.+/Li) without IR compensation. Charge-discharge performance was tested using a battery tester (Maccor Battery Test System Series 4000) at room temperature (RT).
[0100] Result and Discussion
[0101] XRD patterns for LTO and MWCNTs-supported LTO composite electrodes were shown in FIG. 18a. All diffraction peaks are sharp confirming the high crystalline and pure phase face-centered cubic spinel LTO with a space group of Fd3m, which is in well-accordance with the JCPDS card no. 00-049-0207. A broad peak around ˜26° was observed for composite electrodes, due to the presence of carbon, and it does not influence the structural changes in LTO. Average crystallite sizes were calculated from Debye-Scherrer equation D=0.9λ/B cos θ and found to be 66 and 71 nm for LTO-COM and LTO-BM, respectively, where ‘D’ is the average crystallite size, ‘λ’ is the wavelength of X-ray, ‘B’ is the full width half maximum value in radian and ‘θ’ is the diffraction angle. Further, affirming the structural information and quality of graphitic carbon, Raman analysis was performed and Raman spectra are shown in FIG. 18b. The characteristic peaks observed for LTO at 676 and 746 cm.sup.−1 were due to the vibrations of Ti—O bonds in TiO.sub.6 octahedral. The peaks at 238, 424 and 350 cm.sup.−1 confirmed the presence of O—Ti—O, Li—O bonds, respectively. Furthermore, strong peaks are observed at 1348, 1583, and 2693 cm−1, corresponding to D, G, and G′ bands respectively, which are the characteristic peaks of carbon materials.
[0102] For the investigation of morphological properties SEM analysis was performed and images are shown in FIG. 19. From FIG. 19 (a, b), it can be clearly seen that LTO-BM has uniform and homogeneous particle size, shape and distribution compared to LTO-COM. The MWCNTs supported LTO composite electrodes are shown in FIG. 19 (c, d) which indicates that LTO particles are perfectly embedded in MWCNTs conductive matrix in a close proximity range, thereby producing a robust inner-connected architecture. In such inner-connected architecture, the MWCNTs act as a fast transmission conductive network to connect the LTO particles, which favors for enhanced electric and ionic transfer during electrochemical reactions. Electrical conductivity was measured using Hall-measurement (Fig. not shown) and found to be 30.5 and 28.3 Scm.sup.−1 for LTO-BM BP and LTO-COM BP respectively, and electrical conductivity of both LTO-BM BP and LTO-COM BP is found to be higher than that of previously reported carbon coated LTO.
[0103] The CV curves of composite electrodes are shown in FIG. 20a which has one pair of reversible redox peaks between 1.0-2.0 V. The LTO-BM BP electrode showed oxidation and reduction peaks at 1.67 and 1.44 V whereas for LTO-COM BP electrode it was observed at 1.69 and 1.38 V. The peak potential difference between oxidation and reduction peaks was 0.23 V for LTO-BM BP and 0.31 V for LTO-COM BP, indicating lesser polarization in LTO-BM BP electrode. Further, higher peak of LTO-BM BP electrode compared with that of LTO-COM BP reflects higher Li ion diffusion and lower internal resistance. The initial voltage profiles at 0.2 C (1C=175 mAhg.sup.−1) are presented in FIG. 20b for both composite electrodes between 1.0 to 3.0 V vs. Li.sup.+/Li. The lengthened voltage plateau for LTO-BM BP electrode is mainly attributed to excellent electrode kinetics and higher electrochemical reactivity of as-prepared LTO exhibiting higher capacity (166 mAhg.sup.−1) than that of LTO-COM BP (137 mAhg.sup.−1). Furthermore, the potential difference (ΔV) in charge/discharge curves for LTO-BM BP electrode is lesser than LTO-COM BP displaying lower polarization potential (shown in inset of FIG. 20b) which is in well accordance with CV analysis in previous section. The rate capability of composite electrodes has been extensively probed from 0.2C to 15C (shown in FIG. 20c). It can be clearly seen that LTO-BM BP electrode shows higher capacity than LTO-COM BP at each Crate. LTO-BM BP electrode exhibited a good rate performance with a capacity of ˜120 mAh g.sup.−1 at a relatively high rate of 5 C, whereas LTO-COM BP electrode showed the same capacity at a very low C rate i.e. 0.5 C. Further, to ensure the capacity retention of both electrodes, cyclic performance have been performed at 1 C rate for 100 cycles and shown in FIG. 20d. The LTOBM BP and LTO-COM BP showed discharge capacity of ˜155 mAhg.sup.−1 and 105 mAhg.sup.−1 at 1C-rate and retains nearly 100% even after 100 C-D cycles. The overall improved performance of LTO-BM compared to LTO-COM, even at high C-D rate is mainly attributed to its uniform, homogeneous and smaller particles, lower polarization resistance, excellent electrode kinetics and electrochemical stability.
[0104] MWCNT-supported LTO free-standing electrodes were prepared through surface-engineered tape casting technique. Structural properties were confirmed with XRD and Raman studies. SEM confirms the formation of uniform and homogeneous of LTO particles and their well crosslinking with MWCNTs matrix. Enhanced performance of as prepared LTO-BM shows its potential application towards commercialization.
