Patent Application
20240300822
The present invention relates to a rapid, simple and cost-effective process to prepare in-situ carbon coated LiFePO4 (C-LFP) in large scale by adopting solid-state high-energy attrition milling technique. It is an efficient electrode material for various energy storage applications including Li-ion battery application. This is achieved by employing annealing of attritor milled Li(Li2CO3), Fe (FeC2O4), P (NH4H2PO4), and C (C6H8O7) precursors at high temperature (650-700° C.). The powder obtained from attritor milling were pelletized followed by carbonizing at low temperature (350-400° C.) and subsequently heated at high temperature (650-700° C.) to produce highly crystalline carbon coated LFP(C-LFP) with smaller particle size and is a cathode material for Li-ion battery application. The C-LFP so developed shows promising electrochemical properties in terms of high charge discharge capacity, excellent rate capability and long cyclic stability and hence it may be suitable for high energy as well as high power density lithium-ion battery application. The method developed for synthesizing C-LFP in this invention has the advantage of being cost effective, single step, and fast processing due to high kinetic energy system used for milling of powders.
Among the cathode materials reported, LFP which was introduced in 1997 by Goodenough's group (A. K. Padhi et al J. Electrochem. Soc. 144, 1997, 1188; A. K. Padhi et al J. Electrochem. Soc. 144, 1997, 2581) is considered an excellent candidate due to its intriguing features like high theoretical capacity (˜ 170 mA h g−1), single voltage plateau (3.45 V vs. Li+/Li)), non-toxic (cobalt free), safety, structural stability, environmental friendliness economic viability, and long cycle life (Chung et al Nat. Mater. 1, 2002, 123). The high thermal stability of LFP is excellent without exothermic reactions up to 400° C. Despite many advantages, the usage of LFP is hampered by low intrinsic electronic conductivity (10−9 S cm−1) and low ionic conductivity (10−8-10−7 cm2 s−1) (Piana et al Ionics (Kiel). 8, 2002, 17). The limited diffusion pathway restricts lithium-ion diffusion coefficient in the order of 10−14 to 10−16 cm2 s−1. Several efforts including (a) doping/mixing with metal ion (Croce et al Electrochem. Solid State Letters, 5, 2002, A47-A50) (b) reducing the particle size (Yamada et al J. Electrochem. Soc., 148, 2001, A224) and (c) carbon coating (Ravet et al The Electrochemical Society and Electrochemical Society of Japan Meeting Abstract, 99-2, 1999; Huang et al, Electrochem. Solid State Lett., 2001, 4, A170) have been made to alleviate these intrinsic defects. Among these efforts put forward to overcome the intrinsic limitations, carbon coating one of the most effective techniques to improve the specific capacity, rate performance and cycle life of LFP. The main role of carbon coating is to enhance the surface electronic conductivity so that the active materials can be fully utilized at high current rates. Carbon coating not only improves the electronic conductivity, but also prevents metal ion dissolution, avoids direct contact of electrolyte with active material, restricts crystal growth and act as reducing agent to suppress the oxidation of Fe2+ to Fe3+ during sintering. The influence of carbon coating has observed to depend on the structure, uniformity, thickness, loading and precursor of the coating. In general, carbon coating is carried out by either in-situ (Lin et al J. Power Sources 184, 2008, 444) or ex-situ carbon coating process (Wang et al J. Power Sources 184, 2008, 633). In the case of in-situ synthesis, carbon precursors are introduced during the synthesis of LFP whereas in the case of ex-situ process, LFP is post-treated with carbon precursors to form the carbon coating i.e., adding carbon precursors into the synthesized electrode materials followed by heat treatment to obtain carbon coating on electrode materials. Considering the single step and simplicity involved in the process, in-situ carbon coating found to be an ideal process suitable for commercial applications. Hence, in the present invention, in-situ solid state technique was employed to find the appropriate carbon coating process for improving the electrochemical properties of LFP cathode material.
Though many methods have reported to synthesize LFP, the factors such as cost, scalability and simplicity of material technology determine its commercial viability. Considering the above, in the present invention, we have adopted a sustainable and scalable low cost, energy efficient and one step solid state method for the synthesis of LFP that can be suitable for lithium-ion batteries based electric vehicle. Since it is a single step large scale process devoid of solvents and large number of precursors, it may be considered as potential process for commercial application.
Prior-art patents as well as non-patent literature related to the field of present invention available in public domains are discussed below.
CN101607702A invention provides a method for preparing lithium iron phosphate by two step processes using FeC2O4, lithium carbonate, ammonium phosphate, glucose as the source of Fe, Li, P and C respectively. In the first step involved mixing of lithium carbonate, a part of ammonium dihydrogen phosphate in deionized water and subsequent stirring for 2 hr. In the second step, the above mixture was mixed with FeC2O4, with remaining ammonium dihydrogen phosphate, glucose and industrial alcohol with zirconium balls in high-speed ball mill and ran for 6 hrs. Later, the milled raw materials were dried in oven at 60° C. for 8 hrs and subsequent sintering at 750° C. at heating rate of 10° C./min for 18 hrs. The resulting C—LiFePO4 delivers specific capacity of 125 mAh/g.
CN104752717B invention provides a wet chemical method for preparing lithium iron phosphate using ferrous sulfate, phosphoric acid, and a lithium hydroxide as a source for Fe, P and Li respectively. The method involved atomizing and mixing an aqueous solution A containing the divalent iron source and phosphorus source and an aqueous solution B containing the lithium source, and the atomization rate of the aqueous solution A and the aqueous solution B during the atomization process is maintained to control the pH value of the mixed product at 5-7.5. The method for preparing C-LFP further also includes mixing the reaction product with an organic carbon source (citric acid) and spray-drying, and then firing to 180° C. at a heating rate of 1.3° C./min for 5 hrs and subsequent carbonization at high temperature to obtain C-LFP.
CN100567143C invention provides a wet chemical method for synthesizing LiFePO4. In this method, first step involved is the mixing of Fe, P and carbon source, which forms precipitate with inclusion of H2O2. After drying, the solid precipitate was ball milled with Li source for 2 hrs and then sintered at a temperature 100-750° C. for 10-25 hrs under an inert gas and/or a reducing gas at heating rate of 2-4° C./min. The resulting LFP shows discharge capacity of ˜146 mAh/g.
CN102208618A invention provides a wet chemical method for producing a lithium iron phosphate with high conductivity and stable structure. LiFePO4 is prepared in wet chemical process. Initially, FePO4 was prepared by mixing of ammonium dihydrogen phosphate, ferric chloride and organic monomer (aniline) by maintaining pH 4-6. Later, the above prepared FePO4 ball milled with lithium acetate followed by heating at 400-700° C. for 12-30 hrs at a heating at a rate of 1-10° C./min under an inert atmosphere. The highest discharge specific capacity obtained for LFP is 159 mAh/g.
CN102795611A invention provides a method for synthesizing LiFePO4 by a wet chemical process. This method involved, preparation of iron phosphate in an alcohol solution and lithium acetate in an alcohol solution separately and subsequent mixing of the two solutions, followed by reaction in a reducing gas atmosphere or by adding a reducing agent results in LFP after the reaction is completed. Further, the addition of glucose in an alcohol solution with the above mixture and subsequent stirring with a high-speed shear disperser for 10 min., vacuum drying at 100° C., and then sintering under the protection of nitrogen at 700° C. for 8 hours also results LFP powder material. The highest discharge specific capacity obtained for C-LFP is 164 mAh/g.
CN1030224258B invention provides a solid-liquid combined method for the preparation of lithium iron phosphate. The method involved (i) preparation of mixed solution by dissolving lithium hydroxide and cellulose in water, and then add cellulose for reaction to obtain a mixed solution, (ii) addition of FePO4 to the above mixed solution to make a slurry, and then spray-drying at a temperature of 180 ˜220° C. and sintering at 700° C. ±100° C. for 6-8 h to obtain a lithium iron phosphate material. The LFP synthesized in the present invention shows discharge specific capacity of 164 mAh/g with good rate capability.
CN105293458A invention provides a combined wet chemical and spray drying method for the synthesis of C-LFP, which includes (i) the synthesis of ferrous phosphate by spray drying of mixed solution (pH is 9 to 9.5) containing metal iron powder, phosphoric acid solution and lithium hydroxide solution at temperature range of 120 to 240° C., (ii) addition of carbon source with lithium ferrous phosphate, and subsequent sintering at 650 to 750° C. for 4 to 16 hours under oxygen-free conditions. The resulting C-LFP delivers discharge specific capacity of 165 mAh/g.
The various techniques reported in prior art adapts a method of wet chemical/solid-state (wet and dry milling)/spray drying/precipitation or combination of either one or two techniques. In most of the inventions, laboratory ball milling having the overall capacity of few hundred grams of materials was used. Further, the method of synthesis involved mixing of raw materials, use of solvents, filtration & drying of material and subsequent multi-step heating process for long time. Moreover, reactants such as lithium hydroxide, lithium chloride, FePO4, etc., used in this method are expensive and also, most of the methods are based on an inorganic/organic solvent. Accordingly, manufacturing costs are high and thus commercialization of this method has not been realized. On the other hand, the method of the instant invention involves a simple milling of solid raw materials using attrition high energy milling technique, compaction of powder and heating of pellet at high temperature without using any solvent, thus make the process more economic which is attractive for commercial application.
Yan et al [Electrochimica Acta 54 (2009) 5770-5774] discloses a method of production of carbon coated LiFePO4 composite. In this invention, FeC2O4·2H2O and LiH2PO4 used as raw material for Fe and Li & P respectively and citric acid used a s raw material for carbon. The synthesis of C-LFP in this invention involves the wet planetary ball milling of raw materials (Li, Fe, P and C) in alcohol solvent and subsequent drying of milled raw materials at 120° C. Finally, the milled raw materials were calcined at 350° C. and 650° C. under N2/Ar atmosphere to obtain C-LFP composite. Though this invention claims the promising performance of C-LFP composite, the composite like material is nowhere cost competitive compared with solid state synthesized C-LFP composite achieved in the instant invention via simple high energy milling based solid state method without any solvent.
Zhang et al [Electrochimica Acta 55 (2010) 2653] discloses a method of synthesizing C—LiFePO4 by adopting high-energy ball milling of Li2CO3, NH4H2PO4, FeC2O4 raw materials with citric acid as organic carbon source followed by thermal treatment. Further, the effects of ball milling process and annealing time on the phase formation, particulate morphology, carbon content, and consequent electrode performance of LifePO4/C composite were studied. It was concluded that 4 h ball milled C-LFP resulted in the best electrochemical performance due to the proper amount of carbon and proper carbon structure. However, the long milling time, the uses of solvents including acetone, ethanol, toluene, etc. for milling and limited production capability make the process more academic rather than commercial interest.
Mechanical alloying is a basic technique used for milling of homogenous mixture of blended precursors and was used for synthesis of various alloying materials. Particularly, high kinetic horizontal attrition milling unit or vertical attrition milling unit is the one that used for the solid-state milling of composites with realistic of scalability and used for the production of alloys like Fe—Cr, Ni—Cr or milling of high-speed steel for particle size reduction. The high energy milling can be carried out in small shaker mills such as Spex mill for processing a few grams of powder, or in planetary mills for processing 0.1 to 0.5 kg of powder and attritor mills to mill 0.5 kg to about 100 kg of powder. The attritor mills can generate relative velocity of balls similar to planetary mills but with higher throughput. The design and operation of attritor is simple and it is a cost-effective process to carry out high energy ball milling. The main processing principle of milling is to transfer energy from kinetic collision of balls to powder. The continuous collection of milled powders without any wastage and environmentally friendly condition added further advantages to attrition milling (horizontal as well as vertical).
It can be inferred that none of the patents as well as literature matches with the instant invention. Most of the patents were dealt with the synthesis of LFP by wet milling in the presence of a dispersing agent and the synthesis of LFP adopting combined wet-chemical/solid state or wet-chemical/spray drying with long milling and sintering time. Further, reduced atmosphere (H2+Ar) was used to prevent the oxidation of Fe2+ ions and carbon, which expect to increase the production cost of LFP. In contrast, in the present invention, we have adopted attrition milling, in which milling of Li, Fe, P precursors with citric acid, which act as carbon precursor as well complexing agent to prevent the oxidation of Fe2+ to Fe3+ during heating was carried out without reducing atmosphere. Stearic acid used as process control agent (PCA) to prevent the cold-welding during milling process and later it converts into carbon under Ar atmosphere during annealing process.
The objectives of the invention are:
Though large numbers of processes are available for synthesis of carbon coated LiFePO4, they (wet chemical, hydrothermal, solvothermal, precipitation, spray drying, etc.) involve highly complicated synthesis procedures, multi-step processes, use of large amount of solvents & toxic chemicals and expensive techniques. The measures followed to overcome the disadvantages of LifePO4 like poor electronic and ionic conductivity require the addition of one more step, i.e., the requirement of two/multi-step processes making it more complicated and unsuitable for commercial applications. In the instant invention, we developed a simple, economical and energy efficient scalable technique for production of carbon coated LiFePO4 cathode material by a single step solid state-based method of synthesis using Li2CO3, FeC2O4, NH4H2PO4 and Citric acid as precursor for Li, Fe, P and C respectively. Though synthesis of C-LFP from Li2CO3, FeC2O4, NH4H2PO4 and citric acid precursors reported previously by lab scale planetary ball milling, attrition (horizontal as well as vertical) milling process, the most reliable technique for up-scaling without affecting the cost has been adopted by us for the first time to synthesize carbon coated LiFePO4. According to prior patent art and published literatures, many methods of synthesis involve a two-step procedure wherein initially synthesis of LFP was carried out and later ex-situ carbon coating was done to increase the electronic conductivity. Whereas in the instant invention, we have developed in-situ large scale solid state-based process in which, the synthesis of LFP and carbon coating carried out simultaneously. The citric acid used in the present invention plays a dual role, one as carbon precursor and other as complexing agent to prevent the oxidation of Fe2+ to Fe3+ during annealing process which is an added advantage compared to other technique. The method followed here provides the ease of up scaling. This method ensures high-cost effectiveness as the precursors used and the equipment handled is highly cost effective. Accordingly, a method of producing high performance carbon coated LiFePO4 cathode using attrition (horizontal as well as vertical) milling process is disclosed here comprising the following features:
Li2CO3, FeC2O4, NH4H2PO4 and Citric acid precursors are used as lithium, iron, phosphorous and carbon precursors respectively.
Initially Li2CO3, FeC2O4, NH4H2PO4 and Citric acid precursors were blended in pot milling.
Later, milling of blended mixture was carried out in horizontal or vertical attrition milling unit at a speed of 250-550 rpm for 2-12 hrs. to reduce the particle size. Ball to powder ratio was maintained as 1:10 employing 5 mm dia. stainless steel balls as milling medium. A process control agent (PCA) is used to prevent the stacking of powders. Here 2-5 wt. % of stearic acid is used as PCA which also acts as carbon source also.
After milling, the powders are discharged and stored for annealing and characterization.
The powder obtained from attritor milling were pelletized using a 100×100 mm die under 0.5-1 ton pressure using a hydraulic press. When in pellet form, it ensures proper inter-particle contact and more heat transfer making annealing uniform throughout.
The resulting pellet is initially annealed at low temperature (350-400° C.) and subsequently heated at high temperature (650-700° C.) to produce highly crystalline carbon coated LFP with smaller particle size. Annealing was carried out in argon or nitrogen atmosphere in pit furnace. The citric acid used in the instant invention plays a dual role, one as carbon precursor and other as complexing agent to prevent the oxidation of Fe2+ to Fe3+ during annealing process which is an added advantage compared to other technique
Annealed powders were collected and ground to fine powder and stored.
Different citric acid concentrations such as 3%, 5%, 7% and 10% were used to optimize the suitable carbon content required for the synthesis of carbon coated LiFePO4. The resulting carbon coated LiFePO4 materials with 3%, 5%, 7% and 10% carbon content in the precursor are referred as C-LFP-3, C-LFP-5, C-LFP-7 and C-LFP-10 respectively.
On characterization of the resultant powders as cathode material for Li-ion battery application, it was found to give excellent electrochemical performance in terms of high specific capacity, high-rate capability and good cyclic stability.
The method of producing high performance nano sized and carbon coated lithium iron phosphate powders for making the cathode for lithium-ion battery, using horizontal or vertical attrition milling comprising the steps of: a) selecting the Lithium carbonate (Li2CO3), ferrous oxalate (FeC2O4), ammonium dihydrogen orthophosphate (NH4H2PO4) and Citric acid as precursors of Li, Fe, and P respectively as raw material for the process; b) grinding ammonium dihydrogen orthophosphate and citric acid into a fine powder; c) dispersing 0.5-1 wt. % of process control agent, stearic acid into 1.5-2 litres of acetone/isopropanol; d) adding Li2CO3 into the resultant solution and dispersing completely; e) adding ammonium dihydrogen orthophosphate and ferrous oxalate into the above dispersion, in such a way that mole ratio of Li:Fe:P raw materials used for the blending is 1.05:1:1, f) adding citric acid into the above dispersion to obtain the final carbon content of 3-10 wt. %; g) adding 2 to 5% stearic acid as process control agent as well as carbon precursor to the above mixture; h) blending the resultant precursor suspension in a ball mill to get the finely mixed slurry without any lumps; i) drying the blended slurry of raw material glass/stainless steel tray at a temperature of 80° C. for 6-12 h along with the balls followed separating the balls from the powder by sieving; J) milling of the blended mixture in horizontal/vertical attrition milling unit maintained with the ball to powder ratio of 10:1-12:1 at a speed of 250-550 rpm for 2-12 hrs; k) discharging the milled powders from horizontal/vertical attrition milling unit on completion of milling and storing them for annealing in dry form; I) pelletizing the milled powder using a 100×100 mm die at a pressure of 0.5-1 ton using a hydraulic press to ensure proper inter-particle contact, better heat transfer, and thus making the process of annealing uniform throughout; m) annealing of the composite milled and pelletized powder under inert atmosphere of argon/nitrogen in a tubular furnace initially at low temperature (350-400° C.) and subsequently heated at high temperature (650-700° C.) with a heating rate of 2-5° C./min. for a period of 2-10 hrs; and n) grinding the annealed pellets to a fine powder and validate its efficiency as cathode material in half/full cell configuration for lithium ion battery application.
In a preferred embodiment under the invention, C-LFP is produced by Horizontal as well as vertical attrition milling unit using Li2CO3, FeC2O4, NH4H2PO4 and Citric acid are used as lithium, iron, phosphorous and carbon precursors respectively. Citric acid used in the present invention act as carbon precursor as well as complexing agent to prevent the oxidation of Fe2+ to Fe3+ during heat treatment. The usage of citric acid in the prevent invention eliminates the utilization of reducing gases such as H2 for the heat treatment process as the carbon present in citric acid produces a reductive atmosphere in the presence of argon and maintain Fe in +2 oxidation state. Presence of Fe2+ ions and other metal ions (Li+ and P5+) are interpreted from XPS analysis. FE-SEM was done to find out the particle size and was seen that C-LFP with different carbon content produces smaller particles size of around 100-300 nm. Smaller the particle size, smaller is the lithium-ion diffusion path length, therefore smaller size of C-LFP synthesized in the present invention expect to increase the electrochemical performance of LFP. XRD analysis showed the formation of phase pure olivine phase LiFePO4 with orthorhombic structure. Carbon sulfur analysis revealed that the carbon content in LFP materials varied from 2.1% to 5.48% by weight when different concentration (3-10 wt. %) of initial carbon precursors used for the synthesis.
According to an aspect of the invention High-performance nano sized carbon coated lithium iron phosphate powders for making the cathode for lithium-ion battery produced by the method as explained above is having core-shell structured with spherical/distorted spherical crystalline LFP particles as core in the range of 100-300 nm with thin layer of carbon coating. The carbon coated on lithium iron phosphate particles is having more of disordered amorphous (sp3) carbon than ordered carbon (sp2).
According to another aspect of the invention said carbon coated lithium iron phosphate powders the surface areas of the C-LFP having 3, 5, 7 and 10% carbon content in the precursor is ranging from 19-38 m2/g and the ID/IG ratios is ranging from 1.35 to 1.46.
In another aspect of the invention electrochemical efficiency of C-LFP is in the range of 135-146 mA hg−1 at 1 C when electrode is tested in half cell configuration.
According to another aspect of the invention electrochemical cyclic stability of the C-LFP electrode having 10% carbon content in the precursor at 1 C current rate exhibit 97% capacity retention after 1000 cycles. Whereas rate capability of the C-LFP electrode having 10% carbon content in the precursor at 10 C current rate exhibit 97% capacity retention after 1500 cycles.
According to yet another aspect of the invention said C-LFP having 5 wt. % carbon content exhibits specific capacity of 146 mAh/g at 1 C with, rate capability of 132 mAh/g at 5 C and cyclic stability of 90-92% specific capacity retention, after 600 cycles.
According to one more aspect of the invention nano sized carbon coated lithium iron phosphate powders after converting into electrode form, when tested in full cell configuration in combination with graphite as anode delivered a capacity of 1.2 mAh with plateau voltage at 3.2 V whereas in combination with lithium titanate as anode delivered a capacity of 0.3-0.7 mAh with plateau voltage at 1.87 V.
Other features and advantages of the present invention should become apparent from the following description of the preferred process and read in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
In accordance with the invention, high performance in-situ carbon coated lithium iron phosphate (C-LFP) cathode having excellent electrochemical characteristics are developed using Li2CO3, FeC2O4, NH4H2PO4 and Citric acid as are used as precursors of lithium, iron, phosphorous and carbon respectively by adopting a simple, economical and scalable Horizontal or vertical attrition milling technique to achieve highly conducting LFP. Initially Li, Fe, P and C precursors is milled for 2-12 h at 300-550 rpm.
By maintaining ball to powder ratio of 10:1 with stainless steel ball of 5 mm diameter as milling medium. The powder obtained from attritor milling were pelletized using a 100×100 mm die under 0.5-1 ton pressure using a hydraulic press. The resulting pellet is initially carbonized at low temperature (350-400° C.) and subsequently heated at high temperature (650-700° C.) to produce highly crystalline carbon coated LFP with smaller particle size. The method followed here provides the ease of up scaling and ensures high-cost effectiveness as the precursors used and the equipment handled is highly cost effective.
The preferred embodiments of the process under the invention with particular reference to the drawings are as follows.
Lithium iron phosphate (LFP) as an efficient cathode for lithium-ion battery application, according to the invention is synthesized by a simple, facile, fast, and economical and energy efficient Horizontal attrition milling technique. According to the invention, suitable Li, Fe, P and C precursors are used without further purification or treatment. For synthesis of C-LFP, Li2CO3, FeC2O4, NH4H2PO4 in the stoichiometric ratio of 1:1:1 with 5 wt % of extra Lithium carbonate to compensate the lithium loss during heat treatment are transferred to the stainless-steel horizontal vial of attrition milling unit. Further, citric acid with different content was added to the above Li, Fe and P precursors to obtain the carbon content of 3, 5, 7 and 10% in the final LFP materials. Stainless steel ball used as grinding media and with the ball to powder ratio of 10:1-12:1 was used for milling. Milling was carried out for a period of 2-12 hrs with appropriate acceleration and deceleration. Speed of milling was controlled at 300 rpm for vertical attrition milling unit and between 250-550 rpm for horizontal attrition milling unit. The capacity of milling vials is about 1-15 Kg per run, which makes it attractive for large-scale production. Stearic acid was added as the process control agent to avoid stacking and the powders after milling were collected in the form of dry powders and stored properly.
The milled powders were then annealed to produce single-phase Lithium iron phosphate cathode. Prior literatures have reported that temperatures above 600° C. results in the formation of crystalline lithium iron phosphate. In the prior art process mixture of gases, i.e., mixture of argon (90%) and hydrogen (5-10%) was used for annealing of lithium iron phosphate. Mixed gas atmosphere is reported to prevent oxidation of Fe2+ into Fe3+ during annealing of lithium iron phosphate, which is required to synthesize lithium iron phosphate without any impurities. According to the instant invention, C-LFP was synthesized using non-toxic, eco-friendly, cheap Li, Fe, P and C precursors which were available commercially and was used without further purification. Horizontal or vertical attrition milling techniques used in this embodiment of the invention enable proper blending and uniform particle size reduction of Li, Fe, P and C precursors. Due to its high energy produced, it ensures less milling time compared to conventional planetary ball milling technique. The citric acid in the present invention not only acts as carbon source but also helps to prevent the oxidation of Fe2+ into Fe3+ due to its reducing characteristics. The usage of citric acid in the present invention avoids the usage of reducing hydrogen gas, which is expensive and very difficult to handling during annealing of milled LFP powders. This technique thus enables us to produce an efficient cathode material lithium iron phosphate, which can be very much suitable for lithium-ion battery application.
Another embodiment of the invention is the formation of in-situ carbon coating on LFP by adding carbon precursor into Li, Fe, and P precursors followed milling and annealing processes. Milled lithium iron phosphate precursors are pelletized using 100×100 mm die at a pressure of 0.5-1 ton and this ensures proper inter-particle contact. This makes proper crystallization of the bulk powders. The citric acid present along with argon gas creates reducing atmosphere to prevent the oxidation of Fe2+ into Fe3+ during high temperature annealing process.
The above embodiment of the present invention facilitates formation of in-situ carbon coated LFP by a simple and economical Horizontal or vertical attrition milling technique. Lithium iron phosphate synthesized under optimized condition of Li, Fe, P, and C precursors and annealing condition exhibit excellent electrochemical performance in terms of rate capability and cyclic stability showing its better role as a promising material in lithium-ion batteries.
Having described the process of the invention in a general way, now we will further illustrate the mode of execution and demonstrate the characteristics/properties of LFP according to the process under the invention and also its electrochemical properties with the help of the following examples. The present invention is, however, not limited to these examples and various embodiments are possible within the scope thereof.
Lithium carbonate (Li2CO3), ferrous oxalate (FeC2O4), ammonium dihydrogen orthophosphate and citric acid are used as Li, Fe, P and C raw materials respectively for making carbon coated LiFePO4. Raw materials particularly, ammonium dihydrogen orthophosphate and citric acid are grinded into a fine powder. Then certain amount (2-5 wt. %) of process control agent added to 1.5-2 litres of either acetone/isopropanol and dissolve completely. Later Li2CO3 was added into the above solution and completely dispersed. Subsequently other precursors such as ammonium dihydrogen orthophosphate, citric acid and ferrous oxalate were dispersed. The mole ratio of Li Fe:P raw materials used for the blending is 1.05:1:1. Depending on the required carbon content in the final material (C—LiFePO4), the citric acid content will be varied between 3-10 wt. %.
Then the resulting precursor suspension subjected to blending using zirconia balls with sizes of 5-6 mm as milling media. The ball to powder ratio maintained was 1:2-1:4. The blending was carried out with the speed between 100-200 rpm for 5-10 h to get the finely mixed slurry without any lumps.
The wet blended slurry containing raw materials were transferred into glass/stainless steel tray and kept for drying at 80° C. for 6-12 h along with the balls. After drying, the balls were separated by sieving and the resulting powders were used for further processes.
Milling of blended raw materials was carried out in a horizontal attrition milling unit by maintain the ball to powder ratio of 10:1-12:1. The powders used for milling were in the range of 1-2 kg. Stearic acid used as process control agent and 3 wt. % of stearic acid added to the blended raw materials and was subjected to milling process. Stainless steel balls with sizes of 3-6 mm used as milling media. Initially the powders are blended at a speed of 100 rpm for 0.5 h before high energy milling. Then, the powders are milled at a speed of 200-550 rpm. Later the above process of milling repeated to 40 to 48 times in a pattern by increasing (550 rpm) and decreasing (200 rpm) the speed for a period of 0.5 to 2 hrs. Discharging the milled powders subsequently from horizontal attrition milling unit on completion of milling and storing them for annealing in dry form.
Milling of another set of blended raw materials was carried out in a vertical attrition milling unit by maintain the ball to powder ratio of 10:1. The powders used for milling were in the range of 0.5-1 kg. Stearic acid used as process control agent and 3 wt. % of stearic acid added to the blended raw materials and was subjected to milling process. Stainless steel balls with sizes of 5 mm used as milling media. The raw materials along with stearic acid are milled at a speed of 200-300 rpm for 2-12 hrs. Discharging the milled powders subsequently from horizontal attrition milling unit on completion of milling and storing them for annealing in dry form.
10 Kg of blended raw materials were milled in 250 kg capacity vertical attritor milling unit using stainless steel balls with the diameter of 3-6 mm as milling media. The ball to powder ratio maintained was about 10:1.2-3 wt. % stearic acid was used as process control agent and was added to the blended raw materials. The milling was carried out at the speed of 150 rpm for 2-3 h. Discharging the milled powders subsequently from vertical attrition milling unit on completion of milling and storing them for annealing in dry form.
After drying, the milled powders of 250-300 g are kept in square die with the dimensions of 100 mm×100 mmx 80 mm (L×W×H). The punches with dimension of 100 mm×100 mm of are kept in the bottom and top of the milled powders. Later, the die kept in automatic hydraulic machine and was pressed with the applied of 1-2 Tons for 5-10s. Finally, 100 mm×100 mm×40 mm (L×W×H) pellet obtained.
15-20 Nos. of LFP compacts are kept in sample holder which is having three horizontal trays. Then this sample holder is kept inside the retort which was wounded with heating coil in order to get the uniform heated length of about 600 mm. After retort is completely closed with top lid which is having vacuum and gas flow control (inlet and outlet) set up, initially evacuation was done with vacuum pump and subsequently inert gas was introduced into the retort. The above process was repeated at least 10 times to remove the atmospheric gases from the retort. Finally, the positive pressure of 0.1 bar maintained within retort to prevent the entry of atmospheric gases into the chamber.
Furnace was initially heated from room temperature to 350-400° C. with the heating rate of 2-5° C./min. After temperature reaches 350-400° C., it was held for 2 h. During this temperature of 350-400° C., the flow rate of inert gas was maintained between 2-4 L/min. The gases such as CO2, CO, and NH3 which are evolved at this temperature were neutralized by introducing the gas outlet from the furnace into water. Then the temperature increases from 350° C. to 650-700° C. with the heating rate of 3-5° C./min. and held for 3-10 h. The flow rate of inert gas maintained at this temperature was 0.5 L/min. After holding for 10 h, the furnace was cooled to less than 100° C. Then the powder was discharged from the furnace and subsequently weighed to know the material yield after heating process.
After discharging the heated pellets from furnace, it was grinded by commercial grinder. Then the powder was subjected to sieve by keeping the powder into 75 and 45 μm mesh size sieves one after another. The particle free LFP powder was finally obtained with the tap density of 0.5-0.7 g/cc.
The structural, morphological, carbon characteristics and elemental compositions of C—LiFePO4 were measured by XRD, FE-SEM, HR-TEM, BET, C—S, Particle size analyzer and Raman analysis.
In order to validate the material for Li-ion battery application, initially C-LFP containing electrodes was prepared. For making electrode, C-LFP, polyvinylidene fluoride (PVDF) and carbon black (CB) are mixed in different ratios of 80:10:10, 90:6:4, 92:4:4 respectively. Initially PVDF is dispersed in NMP completely and subsequently CB and C-LFP were added into the above dispersion to obtain slurry. This slurry was later coated on current collector, carbon coated Aluminium foil with wet thickness of about 100-120 μm and then dried at 60° C. and 120° C. to get the dry thickness of about 60-80 μm. The electrode with C-LFP mass loadings of 6-10 mg/cm2 was maintained for electrochemical testing. Electrochemical cells were fabricated using 12 mm diameter C-LFP disc as working electrode and 12 mm Li-foil as counter and reference electrode respectively. Electrochemical testing was carried out in half cell configuration by applying different current based on the mass loading of C-LFP. 1M LiPF6 dissolved in 1:1:1, Ethylene Carbonate:Dimethyl carbonate:Ethyl Methyl Carbonate used as electrolyte for electrochemical testing of C-LFP cells. Graphite and lithium titanate (LTO) has been used as anode electrode when C-LFP was tested in full cell configuration. C-LFP to graphite and C-LFP to LTO mass ratio maintained in electrode is about 1:0.55 and 1:1. The same electrolyte has been used for full cell testing as well and testing was carried out by applying different current based on the mass of LFP electrode. Benchmarking studies were carried out in half and full cell configuration under identical experimental conditions using electrode, which were prepared using commercially available LiFePO4 material. The schematic illustration for the synthesis of lithium iron phosphate in the present invention is shown in
Since the in-situ carbon coated LiFePO4 obtained by vertical attritor milling unit shown better electrochemical performance, the structural and morphological characterizations were done only for C-LFP synthesized by vertical attritor milling unit. X-ray diffraction studies were carried out to find the phase formation and crystallinity of the material developed using the invented method and is shown in
Superior electrochemical properties are the final target of the invented technique. The property of C-LFP powder cathode material synthesized by blending, horizontal and vertical attritor milling techniques was tested in half-cell configuration using lithium metal as counter electrode. The crystalline cathode C-LFP material prepared in this invention was used for electrode fabrication to test the efficiency for electrochemical properties.
In order to improve the electrochemical performance of C-LFP, horizontal as well as attritor milling technique was adopted for milling of raw materials in order to bring down the particle size further. The resulting milled LFP precursor materials by vertical (kinetic energy of 300 rpm) as well as horizontal (kinetic energy of 550 rpm) attritor technique was subsequently heated to obtain C-LFP. C-LFP processed by high energy horizontal attritor milling unit were tested for charge-discharge cycles and the results are shown in
Further, C-LFP material processed by vertical attrition milling unit also tested for Li-ion Battery application. Electrochemical results of vertical attritor milled 10% C-LFP shows capacity of 147 mA hg−1, 144 mA hg−1, 138 mA hg−1, 127 mA hg−1, 117 mA hg−1, and 101 mA hg−1 at 0.1 C, 0.2 C, 1 C, 2 C, 5 C and 10 C respectively. In order to analyze the high-power capability of 10% C-LFP, electrochemical cycles were carried out at 10 C for 1500 cycles. 10% C-LFP capacity retention of 87% after 1500 charge-discharge cycles. In order to optimize the carbon content in LFP powder materials, LFP having different carbon content such as 3%, 5% and 7% was prepared and the resulting C-LFP materials were validated for their electrochemical performance in half cell configuration. 3% C-LFP delivered a capacity of 155 mA hg−1, 153 mA hg−1, 142 mA hg−1, 132 mA hg−1, and 115 mA hg−1 at 0.1 C, 0.2 C, 1 C, 2 C, and 5 C respectively whereas 5% C-LFP exhibited capacity of 155 mA hg−1, 153 mA hg−1, 146 mA hg−1, 139 mA hg−1, and 132 mA hg−1 at 0.1 C, 0.2 C, 1 C, 2 C, and 5 C respectively. 7% and 10% C-LFP delivered capacity of 152 mA hg−1, 148 mA hg−1, 142 mA hg−1, 136 mA hg−1, and 121 mA hg−1 at 0.1 C, 0.2 C, 1 C, 2 C, and 5 C respectively. Among 3%, 5%, 7% and 10% C-LFP, 5% C-LFP showing high specific capacity (
The C-LFP material synthesized by solid state milling method as mentioned in Example 1 was tested for electrochemical efficiency in full cell configuration with combination of graphite as well as lithium titanate as an anode. Before full cell testing, the anode materials were tested in half cell configuration in order to balance the capacity of cathode and anode materials in full cell. Based on the capacity of cathode and anode, mass loading ratio is maintained in full cell between cathode and anode. Charge discharge profile and cyclic stability results reveal that commercial grade C-LFP did not show any high capacity at 5 C and failed to produce the voltage profile which shows the potential of indigenous solid-state C-LFP developed in the present invention. Similarly, commercial graphite has tested for half-cell and it shows 15% irreversible capacity loss in the first cycle. Rate capability studies revealed that superior graphite also found to be suitable for high power Li-ion battery application. Further, full cell studies were carried out with different combination of indigenous and commercial electrode materials. Two full-cells were fabricated as follows: Full-cell 1: Commercial LFP//Superior Graphite; Full-cell 2: Solid state LFP//Superior Graphite and the corresponding charge-discharge profiles are shown in
Full-cell studies were also carried out to find the efficiency of the developed cathode for practical applications. Since LFP vs. LTO forms a promising chemistry for EV applications, C-LFP synthesized in the lab scale was used as cathode. A full-cell was also fabricated using commercial lithium titanate as anode for comparison and the weight ratios of anode to cathode were maintained at 1:3 to compensate for 3 lithium ions that can be accommodated by anode for one ion in cathode and to avoid lithium deficiency resulting from the use of lithium for SEI layer formation. Corresponding electrochemical performance results are shown in
Benchmarks Studies of C-LFP with Commercial LFP
As C-LFP developed in the present invention is found to be efficient in terms of electrochemical performance, benchmark studies were carried out to find the performance of the material for practical applications. Commercial lithium iron phosphate powders were fabricated as thin film electrodes on Aluminium foil using same procedure mentioned in example 3. Then it was cut to 12 mm discs to fabricate half-cell for electrochemical studies. Bench mark studies (
We have bought out the novel features of the invention by explaining some of its preferred embodiments thereby enabling any person skilled in the art to understand and visualize our invention. It is also to be understood that the above invention is not limited in its application to the details set forth in the above description or illustrated in the drawings. The phraseology and terminology employed herein are for the purpose of description only and should not be regarded as limiting. Although the invention has been preferred embodiments thereof, variations and modifications can be affected within the scope of the invention as described herein above and as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202011056608 | Dec 2020 | IN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IN2021/051138 | 12/6/2021 | WO |
