Patent Application
20170170479
The invention relates to cathode materials containing olivine structured nanocomposites for lithium batteries. In particular, the olivine structured nanocomposites include a mixture of lithium metal phosphates.
Lithium iron phosphate (LiFePO4 or LFP, for short) that belongs to the olivine group has recently emerged as a critical cathode material for a new generation of rechargeable batteries for use in computers, power tools, mobility products, consumer electronics, cellphones, large-scale power storage applications, and hybrid electric vehicles. Due to the stable and safe olivine structure of LFP materials afford, an increasing attention has been paid to lithium rechargeable batteries due to the continuous growing needs on energy conversion and storage for portable electronic devices, electric vehicles, hybrid electric vehicles, etc. This material has been known for its low cost, non-toxicity, and remarkable thermal stability for some time.
However, the olivine LFP shows some intrinsically disadvantages as a cathode material. Low electronic conductivity and slow lithium ion diffusion coefficient due to its 1D channel for Li+ insertion and extraction result in a poor rate capability. Many efforts have been made to overcome the above shortcomings and to improve the electrochemical performance. For example, carbon coating of the particles to overcome their low intrinsic electronic conductivity, reduction of the size of the particles, and the recent progress to free the material from impurities are few initiatives to improve the performance of LFP as cathode materials.
Whichever synthesis method is employed, the final product should fulfill the following three fundamental requirements in order to achieve an excellent electrochemical performance: (1) Li channels that are not blocked; (2) particles small enough to provide a high surface area and short diffusion paths for ionic transport and electron tunneling; (3) a complete, but thin coating with a conductive phase to ensure that the LiFePO4 particles get electrons from all directions and that ions can penetrate through the coat without appreciable polarization.
Present invention is based on the inventors' surprising finding that a combination of two olivine structured cathode materials, each of them having a general formula LiMPO4 where M is Fe, Mn, of identical or different compositions (Fe and Mn contents), prepared under different conditions and having different characteristics/morphology, shows improved energy storage performances as compared to the respective individual material. Here, the first material A may be a commercially available material and the second material B may be produced in-house using specific techniques. The mixture of A and B performs better than A and B used separately, which generates a synergetic effect between them. The weight fraction of B in the A and B mixture can range between 5% and 95%.
Thus, in accordance with a first aspect of the invention, there is provided a cathode material, comprising:
a first olivine structured nanocomposite having a formula of LiFePO4 or LiFeyMn1-yPO4, wherein 0.2≦y≦0.4; and
a second olivine structured nanocomposite having a formula of LiFexMn1-xPO4, wherein 0.2≦x≦0.4.
In a second aspect of the invention, a lithium rechargeable battery comprising a cathode material of the first aspect is disclosed.
According to a third aspect of the invention, there is provided a method for forming a cathode, comprising:
grinding to powder form a first olivine structured nanocomposite having a formula of LiFePO4 or LiFeyMn1-yPO4, wherein 0.2≦y≦0.4;
grinding to powder form a second olivine structured nanocomposite having a formula of LiFexMn1-xPO4, wherein 0.2≦x≦0.4;
dispersing the first olivine structured nanocomposite powder and the second olivine structured nanocomposite powder in N-methyl-2-pyrrolidone (NMP);
stirring the dispersion to form a slurry;
coating the slurry on a conductive foil; and
drying the coating to form the cathode.
A method for preparing an olivine structured nanocomposite having a formula of LiFexMn1-xPO4, wherein 0.2≦x≦0.4, the method comprising:
providing in solid-state a mixture comprising a manganese precursor, an iron precursor, a lithium and phosphate precursor, and a carbon source;
mechanically working the mixture;
pelletizing the resultant mixture to form pellets; and
sintering the pellets in an inert gas environment to obtain the olivine structured nanocomposite.
In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.
The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
According to a first aspect of the invention, a cathode material is herein disclosed. In present context, the cathode is a positive electrode for use in a lithium-ion secondary or rechargeable battery.
The cathode material includes a first olivine structured nanocomposite having a formula of LiFePO4 or LiFeyMn1-yPO4, wherein 0.2≦y≦0.4.
In present context, an olivine structured nanocomposite refers to a nanocomposite that has an olivine crystal structure.
In present context, LiFePO4 refers to lithium iron phosphate and may be abbreviated by LFP.
In present context, LiFeyMn1-yPO4 refers to lithium iron manganese phosphate and may be abbreviated by LFMP.
In various embodiments, the first olivine structured nanocomposite may consist of only LiFePO4.
In alternative embodiments, the first olivine structured nanocomposite may consist of only LiFeyMn1-yPO4, wherein 0.2≦y≦0.4. For example, y may be 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, or 0.4.
In one embodiment, the first olivine structured nanocomposite may consist of LiFe0.33Mn0.67PO4. In another embodiment, the first olivine structured nanocomposite may consist of LiFe0.2Mn0.8PO4.
In yet further embodiments, the first olivine structured nanocomposite may include both LiFePO4 and LiFeyMn1-yPO4.
The cathode material further includes a second olivine structured nanocomposite having a formula of LiFexMn1-xPO4, wherein 0.2≦x≦0.4.
In present context, similar to the definition of LiFeyMn1-yPO4, LiFexMn1-xPO4 refers to lithium iron manganese phosphate and may be abbreviated by LFMP.
In various embodiments, the second olivine structured nanocomposite may include LiFexMn1-xPO4, wherein x is 0.2, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, or 0.4.
In one embodiment, the second olivine structured nanocomposite may include LiFe0.3Mn0.7PO4.
LiFePO4 may be synthesized by any known technique. For example, existing LiFePO4 synthesis technique includes solid-state method, hydrothermal, sol-gel, and co-precipitation, just to name a few.
In general, in the solid-state method for synthesizing LiFePO4, LiF, Li2CO3, LiOH.2H2O and CH3COOLi are used as the lithium source, FeC2O4.2H2O, Fe(CH3COO2)2 and FePO4(H2O)2 are used as the iron source, NH4H2PO4 and (NH4)2HPO4 are used as the phosphorus source and details of the synthesis may be found in Y. Zhang, Q.-y. Huo, P.-p. Du, L.-z. Wang, A.-q. Zhang, Y.-h. Song, Y. Lv and G.-y. Li, Synthetic Metals, 2012, 162, 1315-1326; C. Lai, Q. Xu, H. Ge, G. Zhou and J. Xie, Solid State Ionics, 2008, 179, 1736-1739; Y. Z. Dong, Y. M. Zhao, Y. H. Chen, Z. F. He and Q. Kuang, Materials Chemistry and Physics, 2009, 115, 245-250; S. Luo, Z. Tang, J. Lu and Z. Zhang, Ceramics International, 2008, 34, 1349-1351, the contents of which are herein incorporated in their entirety by reference.
In general, in the hydrothermal method for synthesizing LiFePO4, LiOH.H2O, FeSO4.7H2O and H3PO4 (85 wt. % solution) may be used as the starting material, and the optimized molar ratio of Li:Fe:P in the starting material may be 3:1:1. Polyethylene glycol (PEG) is added during the hydrothermal reaction (S. Tajimi, Y. Ikeda, K. Uematsu, K. Toda and M. Sato, Solid State Ionics, 2004, 175, 287-290). Alternatively, LiFePO4 may be prepared by rheological phase reaction using PEG as carbon source, and the starting materials may be Li2CO3, FeC2O4.2H2O, NH4H2PO4 and PEG. The precursor was heated at 500° C. for 12 h in Ar atmosphere to get the LiFePO4/C powders (L. N. Wang, X. C. Zhan, Z. G. Zhang and K. L. Zhang, Journal of Alloys and Compounds, 2008, 456, 461-465). In yet another example, FeSO4.7H2O, (NH4)2HPO4, LiC6H5O7.4H2O and phenanthroline are used as the starting materials at 300° C. by hydrothermal route (Z. Wang, S. Su, C. Yu, Y. Chen and D. Xia, Journal of Power Sources, 2008, 184, 633-636). The contents of references cited above are herein incorporated in their entirety by reference.
In general, in the co-precipitation method for synthesizing LiFePO4, lithium and phosphate compounds in mixed precursor solutions are co-precipitated by controlling the pH values. The co-precipitated slurries are then filtered, washed, and dried under N2 atmosphere. During that process, dried precursors may form amorphous LiFePO4. Crystalline LiFePO4 powders are obtained by carrying out the calcinations at 500 to 800° C. for 12 h under N2 or argon flow. Depending on the precursors and other processing conditions, the particle sizes of the synthesized LiFePO4 powders can range from 100 nm to several microns (O. Toprakci, H. A. K. Toprakci, L. Ji and X. Zhang, KONA Powder and Particle Journal, 2010, 28, 50-73; G. Arnold, J. Garche, R. Hemmer, S. Ströbele, C. Vogler and M. Wohlfahrt-Mehrens, Journal of Power Sources, 2003, 119-121, 247-251; J. C. Zheng, X. H. Li, Z. X. Wang, H. J. Guo and S. Y. Zhou, Journal of Power Sources, 2008, 184, 574-577). The contents of references cited above are herein incorporated in their entirety by reference.
The second olivine structured nanocomposite LiFexMn1-xPO4 may be synthesized by any known technique for synthesizing LiFePO4 as described above. For example, such synthesis technique includes the solid-state method, hydrothermal, sol-gel, and co-precipitation, just to name a few, but with the addition of a manganese precursor in the starting material. Details of the various synthesis techniques for LiFexMn1-xPO4 will be described in the example section below.
According to another aspect of present disclosure, a method for preparing an olivine structured nanocomposite having a formula of LiFexMn1-xPO4, wherein 0.2≦x≦0.4, is provided (i.e. the second olivine structured nanocomposite). The method includes providing in solid-state a mixture comprising a manganese precursor, an iron precursor, a lithium and phosphate precursor, and a carbon source. In other words, the manganese precursor, the iron precursor, and the lithium and phosphate precursor are reacted via a solid-state reaction. Preferably, the manganese precursor, the iron precursor, the lithium and phosphate precursor are mixed in stoichiometric ratio with the carbon source, although not necessarily so.
The manganese precursor may, for example, be MnCO3 or MnC2O4.2H2O.
The iron precursor may, for example, be Fe(C2O4)2.2H2O.
The lithium and phosphate precursor may, for example, be LiH2PO4, or Li2CO3 and NH4H2PO4.
The carbon source may be selected from the group consisting of carbon black (acetylene black), sucrose, citric acid, and malconic acid.
The method further includes mechanically working the mixture. In various embodiments, mechanically working the mixture may include ball milling the mixture, such as for a period of 7 hours at 300 rpm.
The method further includes pelletizing the resultant mixture to form pellets and sintering the pellets in an inert gas environment to obtain the olivine structured nanocomposite. The inert gas environment may include argon and additionally hydrogen.
By forming the second olivine structured nanocomposite via the solid-state method described above, the nanocomposite particles obtained thereof can be in nanometre in size. This is shown in
In various embodiments, the second olivine structured nanocomposite may be present in 5% to 95% based on the total weight of the first olivine structured nanocomposite and the second olivine nanocomposite. For example, the second olivine structured nanocomposite may be present in 40% based on the total weight of the first olivine structured nanocomposite and the second olivine nanocomposite, although other weightage is also possible.
A challenge to the use of LFP and LFMP in batteries is the insulating behavior of the phosphate. This can be overcome to a certain extent by coating the particles with a conducting layer of carbon, for example. Thus, in preferred embodiments, at least one of the first olivine structured nanocomposite and the second olivine structured nanocomposite is coated with carbon, more preferably both the first and second olivine structured nanocomposites are coated with carbon.
It is known that LFP and LFMP nanocomposites exhibit different morphology and therefore different properties, behaviours and characteristics when prepared under different synthesis conditions and different synthesis routes.
However, present invention is based on the inventors' surprising finding that a combination of two olivine structured cathode materials, each of them having a general formula LiMPO4 where M is Fe, Mn, of identical or different compositions (Fe and Mn contents), prepared under different conditions and having different characteristics/morphology, shows improved energy storage performances as compared to the respective individual material. In other words, instead of solely using one type of olivine structured cathode material that shows superior performance on its own, by combining it with another type of olivine structured cathode material whose performance may not be as good, the combination results in a better performance than each of the olivine structured cathode material itself.
To demonstrate this synergistic effect, a method for forming a cathode is herein disclosed.
The method includes grinding to powder form a first olivine structured nanocomposite having a formula of LiFePO4 or LiFeyMn1-yPO4, wherein 0.2≦y≦0.4. The method further includes grinding to powder form a second olivine structured nanocomposite having a formula of LiFexMn1-xPO4, wherein 0.2≦x≦0.4.
For convenience, the first olivine structured nanocomposite may be obtained from a commercial source while the second olivine structured nanocomposite may be prepared by any one of the known synthesis methods described herein.
After obtaining the powder form of the first and second olivine structured nanocomposites, the first olivine structured nanocomposite powder and the second olivine structured nanocomposite powder are dispersed in N-methyl-2-pyrrolidone (NMP) and stirred to form a slurry.
Next, the slurry is coated on a conductive foil such as an aluminium foil. For example, the slurry may be coated on an aluminium foil using doctor blade equipment.
After coating the slurry on the conductive foil, the coating is dried to form the cathode.
In various embodiments, in the dispersing step a carbon source such as, but is not limited to, carbon black, acetylene black, or Super-P®, may be added to the mixture of the first olivine structured nanocomposite powder and the second olivine structured nanocomposite powder.
In preferred embodiments, in the dispersing step polyvinylidene fluoride is also added to the mixture of the first olivine structured nanocomposite powder, the second olivine structured nanocomposite powder, and the carbon source.
The cathode disclosed herein or formed by the method disclosed herein is suitable for use in a lithium rechargeable battery due to the following advantages:
In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.
Experimental Information for Cathode Fabrications
Materials
Hydrothermal Method
FeSO4.7H2O (Aldrich, >99%), MnSO4.H2O (Aldrich, >99%), Li2SO4.H2O (Aldrich, 99.9%), LiOH (Aldrich, >98%), H3PO4 (Aldrich, purity >85%), Ascorbic acid (Aldrich, >99%), Sucrose (Aldrich, >99%).
Solid State Method
MnCO3 or MnC2O4.2H2O (Aldrich, >99%), Fe(C2O4).2H2O (Aldrich, 99.9%), LiH2PO4 or Li2CO3 and NH4H2PO4 (Aldrich, >99%) were used as olivine precursors. Carbon black (acetylene black), Sucrose (Aldrich, 99%), citric acid (Aldrich, 99%) and malconic acid (Aldrich, 99%) were used as carbon sources. Also, commercially available carbon coated LiFePO4 and LiMn0.67Fe0.33PO4 powders; products of Clariant were used as received.
Co-Precipitation Method
(NH4)H2PO4 (Aldrich, >98%), CH3COOLi.2H2O (Aldrich, >99%), (CH3COO)2Mn.4H2O (Aldrich, 98%), Fe(CH3COO)2 (Aldrich, 95%).
Synthesis of Lithium Iron Manganese Phosphate (LFMP)
Hydrothermal Synthesis
The LiFePO4 was prepared by hydrothermal reaction in 50 ml containers. Specifically the starting materials were FeSO4.7H2O (98% Aldrich), MnSO4.H2O (98% Aldrich), H3PO4 (85 wt. % solution Aldrich), LiOH (98% Aldrich). The molar ratio of the Li:M(Fe, Mn):P was 3:1:1, and a typical concentration of FeSO4 was 22 g·l−1 of water. Sugar and/or I-ascorbic acid (99% Aldrich) were added as an in situ reducing agent to minimize the oxidation of ferrous to ferric, 1.3 g·l−1 was used. The mixture was vigorously stirred for 1 min and transferred in a Teflon-lined stainless steel autoclave and heated at 190° C. for 7 h. The autoclave was then cooled to room temperature and the precipitated products were filtrated and finally dried at 100° C. for 10 h. The heat treatment was carried out in Ar—H2 atmosphere at 700° C. (5° C./min) for 10 h to obtain the crystalline phase and to carbonize the reducing agent, thereby obtaining a carbon film that homogeneously covers the grains.
One-Step Solid State Synthesis and Carbon Coating
The C—LiMn0.7Fe0.3PO4 and C—LiMn0.8Fe0.2PO4 compound with an olivine structure were synthesized by a solid-state reaction between MnCO3, Fe(C2O4).2H2O, and LiH2PO4, which were mixed thoroughly in stoichiometric ratio with carbon source. Three types of carbon source were used; carbon black (acetylene black), sucrose and citric acid. The mixture was reground by high energy ball milling with 300 rpm speed for 7 hours. The ball to powder ratio was kept constant at 30 and a combination of large and small balls were used. The sintering was performed under Ar and Ar—H2 atmosphere. The samples were pressed into pellets and sintered at 500 to 700° C. for 10 h.
Physical Mixing of Commercial C-LFP and C-LFMP and In-House C-LFMP Fabricated Using Solid State Method
After sintering of in-house LFMP products, the powders were grinded manually using mortar and pestle and mixed with commercial C-LFP and C-LFMP. The weight fraction of in-house/commercial nanocomposites in the mixture can range between 5% and 95%. In this example, results of in-house/commercial nanocomposite ratio of 40/60 is demonstrated. For optimum mixing, the powders were dispersed in ethanol or N-methyl-2-pyrrolidone (NMP) and stirred overnight. When ethanol was used, the powders were dried at 80° C. under vacuum condition. The mixed powders were grinded before slurry preparation.
Co-Precipitation Method
To fabricate LFMP powders using co-precipitation method, stoichiometric amounts of CH3COOLi.2H2O, (CH3COO)2Mn.4H2O and Fe(CH3COO)2 (Aldrich, 95%) were added to 100 cc of absolute ethanol while stirring. The ratio of Fe:Mn precursors was maintained as 30:70. (NH4)H2PO4 was added to 3-4 cc of water and dissolved using ultrasonication. The water solution was added to ethanol solution dropwise under stirring to start precipitation. The solution was stirred overnight. To apply carbon coating, 10 wt. % of sucrose was added to ethanol solution. Using a rotary evaporator the ethanol was evaporated and precipitates were dried in vacuum oven at 100° C. overnight. To crystallize the products, the fabricated powder was sintered under Ar—H2 atmosphere at 700° C. for 10 h.
Sample Characterization
The sample morphology was examined using a field-emission scanning electron microscopy (FESEM; JEOL, JSM-7600F). The elemental compositions of the samples were characterized with energy-dispersive X-ray spectroscopy (EDX) which is attached to the SEM instrument. Crystallographic data of the specimen was collected using powder X-ray diffractometer (Bruker, Cu KR radiation with λ=1.5406 Å). The determination of the phase was done using the Match software. For TEM characterization, the samples were dispersed in ethanol. After ultrasonication for 2-10 mins, the solution was drop cast onto carbon coated 200 mesh Cu grids. TEM/HRTEM was obtained by using a JEOL 2010 system operating at 200 kV.
Cathode Preparation
80 wt % of active material prepared by different methods, 10 wt % carbon black (acetylene black) and 10 wt % polyvinylidene fluoride (PVDF) were mixed in a mortar. Then N-methyl-2-pyrrolidone (NMP) was added to prepare slurry, which was coated on a piece of Al foil using doctor blade equipment. The thickness of coated thin films was controlled at 50 μm. The coated foils were pressed using roll press and punched to 1.4 cm circles. After drying at 110° C. for 6 hours, the prepared cathode was pressed again using the roll press and the mass of the active material was accurately measured.
Property Measurement of Lithium Ion Battery
The coin cells were assembled inside an Ar-filled glove box with oxygen and moisture content less than 1.00 ppm. The prepared electrodes were used as the working electrode. The lithium foils were used as counter/reference electrodes and the electrolyte was a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1, w/w). For the electrochemical measurement coin battery cells were installed and galvanostically tested using a NEWARE battery tester between 2.7 and 4.4 V (vs. Li/Li+).
Result and Discussion of Cathode
Sample Characterization
Characterization of LFP and LFMP Obtained by Hydrothermal Synthesis
Solution methods for synthesizing LiFePO4 and LiMnPO4 provide intimate mixing of the starting ingredients at the atomic level, thus allowing finer particles of high purity to be produced by rapid homogeneous nucleation. Such methods are also faster and more economical than solid-state approaches. Therefore, many solution methods including co-precipitation, sol-gel processing, and hydrothermal synthesis, have been utilized to prepare olivines. Of these, hydrothermal synthesis offers the promise of simplicity, and scalability.
A key challenge in using aqueous solutions is to prevent the oxidation of ferrous to ferric. A reducing agent, such as hydrazine, has been used historically with mixed success in the formation of ferrous phosphates. Here, it is explored a few reducing agents such as ascorbic acid (vitamin C) and sugar for the hydrothermal formation of LiFePO4 and LiFexMn1-xPO4.
In the initial hydrothermal attempt, only iron precursor was used. This experiment was performed for comparison purposes and also for the simplification of the fabrication process.
Preliminary results using ascorbic acid (vitamin C) showed the criticality of synthesis temperature and the presence of a reducing agent such as ascorbic acid. According to SEM observation shown in
A challenge to the use of LiFePO4 in batteries is the insulating behavior of the phosphate. This can be overcome by coating the particles with a conducting layer of carbon, for example. An attempt was therefore made to generate such a coating during the hydrothermal process. Ascorbic acid and sugar were added to the hydrothermal reactor, and a black product was formed. According to EDS, the sample has approximately 10 wt. % carbon.
As all the samples prepared in the presence of the surfactant were sintered at 700° C. (5.0° C. min−1) in Ar—H2, the presence of a carbonaceous phase, due to the decomposition of the ascorbic acid and sucrose after washing and filtering, has to be expected. Nevertheless, there is no evidence of such phase in the diffraction patterns (see
In present disclosure, a mixed lithium metal phosphate is fabricated. Specifically, both manganese and iron precursors were used to fabricate olivine products.
The SEM microphotographs, reported in
Characterization of Commercial Products
XRD examination of commercial powders (obtained from Clariant) is shown in
The morphology of commercial C-LFP particles is shown in
The microstructure of commercial C-LFMP powders is shown in
Characterization of Samples Obtained by One-Step Ball Milling
It is widely known and accepted that carbon coating is critical in olivine cathodes and a thin, uniform carbon coating with good contact with active materials is essential for achieving good electrochemical properties. In order to achieve superior battery performance three types of carbon coating were tested; carbon black (acetylene black), sucrose, combination of sucrose and citric acid. C-LFMP using 10 wt % sucrose was fabricated. The XRD analysis exposes formation of olivine compounds with an orthorhombic structure (space group Pnma) (see
The morphology of C-LFMP particles is shown in
The amount of carbon source used is another important factor. It is possible that by increasing the carbon contents of the electrode a more uniform and complete coating is achieved and the conductivity of the cathode is improved. To evaluate this parameter another set of powders was fabricated using the same fabrication conditions and adding 20 wt % sucrose to the precursor chemicals.
The microstructure of the fabricated powders is shown in
In the next attempt, the amount of carbon source was decreased to 10 wt % and a combination of sucrose (6 wt %) and citric acid (4 wt %) was applied. The purpose of using citric acid is to increase the surface area of the active material by forming mesopores. The formation of these mesopores is due to the decomposition of citric acid during the sintering process. The formation of mesoporous agglomerates was observed in the SEM examination shown in
To study the influence of addition of carbon black as carbon source during ball milling, the powders were fabricated using the same precursor and ball milling conditions. However, in this set of experiment the precursors were mixed in a respective ratio to fabricate C—LiFe0.3Mn0.7PO4. The samples were fabricated in the presence of 10 wt. % carbon black.
XRD examination shown in
The nanosized particles reduce the solid-state diffusion path, thus expediting the lithium-ion transport. However, to achieve high specific capacity, especially at high current density high porosity to enable penetration of electrolyte into the structure and reduction in the diffusion distance are required, Additionally, a uniform carbon coating is required on the particle surface to enhance electronic conductivity.
Characterization of Mixed Commercial and In-House Fabricated Olivine
C—LiFe0.3Mn0.7PO4 powders fabricated using solid state process was discussed in the previous section. It is herein evaluated whether a mixture of A and B performs better than A and B used separately. It is confirmed that mixing of A and B in present case generates a synergetic effect between the components. The procedure is to mix C—LiFe0.3Mn0.7PO4 powder fabricated in laboratory using solid state with industrial grade C—LiFePO4/C—LiFe0.33Mn0.67PO4 powder. This was performed with the aim to elevate the electrochemical performance.
Two sets of samples were prepared. The first set was fabricated by mixing in-house fabricated C—LiFe0.3Mn0.7PO4 with commercial C—LiFePO4 and the second set by mixing it with commercial C—LiFe0.33Mn0.67PO4. As can be seen in SEM images shown in
Characterization of LFMP Obtained by Co-Precipitation
The XRD pattern of LFMP powders fabricated by simple co-precipitation process is shown in
The simplicity of the co-precipitation process, the purity of the products and the nanometer particle size makes this process an attractive method of fabrication. Additionally, the possibility of its application in large quantity and its use in industrial application should not be ignored.
The Electrochemical Properties of Olivine Electrodes
Electrochemical Performance of Samples Obtained by Hydrothermal Method
As explained in the characterization section, carbon coated LFMP was fabricated by hydrothermal method.
Electrochemical Performance of Samples Obtained by One-Step Ball Milling
As explained in the previous section, another set of samples was fabricated using a combination of sucrose and citric acid as the carbon source.
The rate capability of the C-LFMP electrode coated with sucrose-citric acid was also examined. Similar to other two cathodes, the electrode was examined at current densities from 17 mA/g (0.1 C) to 85.5 mA/g (5 C) (see
The rate capability of the C-LFMP electrode was further examined at current densities from 17 mA/g (0.1 C) to 85.5 mA/g (5 C) (see
Electrochemical Performance of Commercial Olivine Powders
The rate capability of pure commercial C—LiFePO4 and C—LiFe0.33Mn0.67PO4 electrode was also examined at current densities from 0.1 to 0.5 C (see
In case of commercial C-LFMP an initial charge capacity of 121 mAh/g with 90% Coulombic efficiency was achieved which improved to 135 mAh/g in the second charge cycle. Thereafter, a stable capacity of 130 mAh/g, 107 mAh/g, and 40 mAh/g was delivered at 0.2, 0.5 and 1 C, respectively. However, at 2 C and 5 C the delivered capacity is very low. However, after applying such high rates to the electrode, when the rate was again decreased to 0.1 C the C-LFMP cathode recovers and delivers a high charge capacity of 114 mAh/g and then 140 mAh/g and stays almost unchanged thereafter. This observation demonstrates that although the C-LFMP does not function well at high rates, the structure is highly stable and it can recover well after application of high current densities.
Electrochemical Performance of Physically Mixed Commercial C-LFP and C-LFMP and in House C-LFMP Fabricated Using Solid State Method
The rate capability of the mixed electrode is also promising (see
The second electrode was prepared by mixing commercial C—LiFe0.33Mn0.67PO4 and in-house fabricated C—LiFe0.3Mn0.7PO4 produced by ball milling process. The first cycle gives a charge capacity of 184 mAh/g and a subsequent discharge capacity of 107 mAh/g, this results gives an initial Coulombic efficiency of 58% (see
Fabrication of lithium iron manganese phosphate was performed using different fabrication processes such as hydrothermal, solid-state and co-precipitation. The electrochemical investigation of each fabricated product was performed. It was observed that solid state delivers the best electrochemical performance. In an attempt to elevate electrochemical performance of olivine cathodes, C—LiFe0.3Mn0.7PO4 powder fabricated by solid state method were physically mixed with industrial grade C—LiFePO4 and C—LiFe0.33Mn0.67PO4. Battery performance of both set of mixed cathodes was studied in details and based on cycling performance and rate capability of two mixed electrodes, it can be concluded that mixed industrial grade olivine and in-house C-LFMP deliver superior performance compared to their individual constituents.
By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.
By “consisting of” is meant including, and limited to, whatever follows the phrase “consisting of”. Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory, and that no other elements may be present.
The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including”, “containing”, etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.
By “about” in relation to a given numerical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.
The invention has been described broadly and generically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.
Other embodiments are within the following claims and non-limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.
This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/944,709, filed Feb. 26, 2014, the contents of which being hereby incorporated by reference in its entirety for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2015/000057 | 2/26/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61944709 | Feb 2014 | US |
