The present invention relates to a method for producing a lithium insertion material for a cathode in a Li-ion battery, the material comprising iron, lithium, silicon, and carbon.
Lithium iron orthosilicate, Li2FeSiO4, has increasingly become a material of interest for cathodes in lithium ion batteries due to its promising electrochemical properties demonstrated the first time by Nyten et al. and also due to the low synthesis cost compared to that of the cobalt based cathodes (Nyten 2005, 2006). A limiting factor with polyanion materials is their poor conductivity. Synthesis conditions have a large influence on the electrochemical performances of Li2FeSiO4, and many studies have been done to find out the key factor for optimized electrochemical performances. Recent trends on Li2FeSiO4 are focused on development of active materials with nano-sized particles to improve the electrochemical performance by different synthesis techniques such as solid-state, sol-gel, hydrothermal, and hydrothermal assisted sol-gel.
The present invention relates to a new, cheaper and rapid combustion method, based on using various carbon sources as fuel for the combustion, adequate for preparing homogeneous nano-sized materials. The method is based on an oxidation-reduction reaction between soluble precursor salts (oxidizers) and soluble, sacrificial, carbonaceous compounds (fuels). In general terms, a combustion reaction may be controlled by several basic parameters: type of fuel and oxidizer, fuel-to-oxidizer ratio value, temperature of initiation of combustion, and relative volume of the evolved gaseous products. The method relies on using metal nitrates as oxidizers and soluble carbonaceous compounds as fuels, to synthesize an inexpensive, nano-sized, silicate cathode material.
According to the invention, nano-sized Li2FeSiO4/C powders have been synthesized by a novel combustion process in which a very low-cost carbonaceous material, such as lactose, maltose, maltodextrine, sucrose, or citric acid, is used as fuel. As the amount of carbonaceous compound increases from half-stoichiometric to triple stoichiometric, the purity and morphology of the products is affected. XRD analysis shows that the amount of Li2SiO3 and Fe1-xO impurities increase with increasing the fuel (e.g. sucrose) amount. Combined SEM and TEM micrographs and BET analysis show that the addition of sucrose is favourable for increasing the surface area while the particle size decreases. The best electrochemical performance is reached for a sample with 50% excess of sucrose as compared to the stoichiometric amount, which delivered an attractive capacity of 130 mAh/g at C/20 rate with stable cycling performance even at 2C, owing to stable crystallinity and phase purity.
According to the invention, a simple combustion method based on carbonaceous fuels is employed to synthesize pure Li2FeSiO4/C. This leads to an improved cycling performance of this silicate cathode material. The process offers good sample homogeneity and allows synthesizing samples with small particle size.
In one aspect, the invention relates to a process for preparing a Li—Fe—Si—O—C material. Said material is a lithium insertion material, useful in cathodes for Li-ion batteries.
The process leading to the Li—Fe—Si—O—C material can be described as follows: LiNO3 and Fe (NO3)3.9H2O are used as the oxidant precursors, a carbonaceous compound, such as saccharides, polysachharides, dextrines, or organic acids, used as fuel and mixed with SiO2 nanoparticles (fumed silica, Sigma-Aldrich, or silica sols, such as Bindzil®820DI, EKA Chem. AB), and the chemical reaction can be described as follows:
48LiNO3+24Fe(NO3)3+24SiO2+13C12H22O11→24Li2FeSiO4+60N2+156CO2+144H2O
Saccharides may be e.g. lactose, sucrose, maltose. Polysaccharides could be the hydrolyzed derivates of starch or celloluse, such as e.g. dextrines or maltodextrines. Organic acids may be e.g. ascorbic acid, malic acid, adipic acid or citric acid. The silica sol is preferably of low pH, and has a low concentration of alkaline ions. With said precursors, the reaction yields nano-particles (i.e. having a size of about 25-100 nm) of Li2FeSiO4/C. This means that the material has the stoichiometric amounts of Li, Fe, Si and O, further containing carbon, e.g. in an amount of about 12% by weight of carbon.
The precursors (e.g. Li, Fe, and Si sources such as LiNO3, Fe (NO3)3, and SiO2 are dissolved and/or dispersed in distilled water, and the fuel (e.g. sucrose) is added to the solution. The mixture is then kept at elevated temperature to evaporate excess water. The water may be removed by other means, e.g. spray drying, pyrolysis, or vacuum drying. During the process of continued heating, the mixture forms a syrup, accompanied by a colour change from red to green, whereby the syrup forms into a brown foam. During further heating, the foam starts to burn or decompose spontaneously and transforms into a light, downy, brown-black powder. This powder may optionally be ground and further heat treated, optionally under a gas mixture of a composition chosen to maintain the iron in a +II state.
Consequently, in one aspect, the invention provides a method for producing a lithium insertion material for a cathode in a Li-ion battery, the method comprising the following steps:
It should be noted that SiO2 is not soluble in water, so the term “dissolve” or “dissolving” is not literally applicable to SiO2. SiO2 nanoparticles are instead suspended or dispersed in solution, but the term “dissolve” or “dissolving”, as used herein, may also include the meanings “suspend”, “suspending”, “disperse” or “dispersing”. SiO2 nano-sized particles such as fumed silica or solvent-stabilized silica sols may be used as the silicon source.
In one embodiment, the molar ratio of LiNO3 and Fe(NO3)3 can be in the interval from 2:1 to 2.2:1.
In a further embodiment, the value for the (F:O)-ratio of the oxidizing and reducing valence of the metal nitrates (O) and the fuel (F), or carbonaceous compound e.g. sucrose, is between 1.2 and 1.7, or preferably between 1.4 and 1.6, or more preferably 1.5.
In one embodiment, the heat treatment is conducted at a temperature above 600° C. for at least 5 hours under a mixture of CO gas and CO2 gas. The temperature may be as high as 800° C., and the heat treatment may last as long as 10 hours or more.
In another aspect, the invention provides a product obtained by the method according to the invention, comprising Li2FeSiO4, and carbon in an amount of 6-14 by weight, optionally having an average particle size of 70-100 nm, and optionally a surface area (SBET) of 55-65 m2/g, and/or the resulting particles having a crystallite size of 25-40 nm.
In a preferred embodiment the product obtained by the method has a carbon content of 6-14%, or preferably 12%.
All starting materials used were of 99.99% purity. Li2FeSiO4/C samples were prepared by the so-called combustion method using LiNO3 (Sigma-Aldrich) and Fe(NO3)3.9H2O (Sigma-Aldrich) as the oxidant precursors, sucrose (Sigma-Aldrich) as fuel and fumed SiO2 nanoparticles (Sigma-Aldrich). Typically, the reaction can be described as follows:
48LiNO3+24Fe (NO3)3+24SiO2+13C12H22O11→24Li2FeSiO4+60N2+156CO2+144H2O
Briefly, the preset stoichiometric amounts of reagent grade Li, Fe and Si sources were dissolved (or, in the case of fumed SiO2, suspended and/or dispersed) in at least the minimum amount of distilled water; the dissolved fuel (sucrose) was then added to the solution. The beaker containing the reaction mixture was placed on an electric heater and kept at 120° C. for 2 hours to evaporate the excess water. The liquid adopted a syrup consistency and the colour changed from red to green while the syrup swelled up and transformed into brown foam. On continuing heating, this foamy mass started to burn spontaneously without flame and transformed finally to light and downy brownish-black powder. The as-formed powder was collected, ground in an agate mortar, and further heat-treated at 800° C. for 10 h under a flowing gas mixture (CO/CO2: 50/50).
The mixture of the oxidants and sucrose was calculated on the basis of the total oxidizing and reducing valence of the metal nitrates (O) and the sucrose (F) according a F/O=1. Experiments with varying sucrose amounts were also carried out to investigate the influence of different F/O ratios (F/O=0.5; 1.5; 2; 2.5 and 3) The samples thus prepared are referred to hereafter as 0.5Sc, 1Sc, 1.5Sc, 2Sc, 2.5Sc and 3Sc, respectively, where the coefficient denotes the F/O ratio and Sc stands for sucrose.
All samples were characterized by X-ray diffraction (XRD) using a Siemens D5000 diffractometer with Cu Kα radiation. The diffraction patterns were recorded in [10-120]° (2θ) angular range, using a 0.02° (2θ) step and a constant counting time of 10s. The lattice parameters and cation distributions were refined by the Rietveld method using the Fullprof program (J. Rodriguez-Carvajal, Fullprof, Program for Rietveld Refinement, version 3.7, LLB JRC (1997)). X-ray diffraction patterns of Li2FeSiO4/C composites (0.5Sc, 1Sc, 1.5Sc, 2Sc, 2.5Sc and 3Sc) are compared in
In order to accurately determine the structure of these materials, refinements by Rietveld method of X-ray data was performed using the Fullprof program. The Rietveld refinement was used to determine the lattice and structural parameters as well as the cationic distribution between the lithium and iron sites.
Firstly, a full pattern matching refinement allowed determining the lattice parameters and the profile parameters of the Pseudo-Voigt function used to describe the shape of the diffraction lines. Then, the structural refinement was carried out by considering the structure of Li2FeSiO4/C composites can be indexed in a monoclinic cell (P21/n space group). As described in the P21/n space group, the Li, Fe, Si, and O occupy 4e sites. All the crystallographic sites were constrained to be fully occupied. The isotropic atomic displacement parameters (Biso (Å2)) were refined. Cell parameters obtained for the six-Li2FeSiO4/C composites are compared in Table 1. The P21/n space group was used. Detailed results of the X-ray diffraction pattern refinement by the Rietveld method are given as example in Table 2 for 0.5Sc, whereas
The possible antisite defect between lithium and iron ions in the 4e sites was also checked. The refinement of all our XRD patterns assuming this structural hypothesis, i.e. P21/n space group, shows that no exchange occurs between the Li and Fe ions.
High Resolution Scanning Electron Microscopy was used to check trends in powder grain size and morphology with increasing sucrose amount. A high resolution scanning electron microscope (HRSEM LEO 1550) was used.
In order to study the effect of sucrose content on the carbon coating status, Transmission electron microscopy (TEM) studies were performed on JEOL JEM-2100F microscope, equipped with field emission gun operated at 200 kV. The results are presented in
It is clear that different amounts of sucrose results in different carbon distributions on Li2FeSiO4 particle surface, as well as different particle size of Li2FeSiO4. TEM images of samples 0.5Sc and 1Sc, containing only 0.76% and 5.44% carbon, respectively, do not provide information about carbon distribution, see
Electrochemical measurements were performed in aluminium pouch cells. Positive electrodes were prepared by spreading a mixture of 75% active material, 15% carbon black, and 10% of PVDF [poly(vinylidene fluoride)] dissolved in NMP (1-methyl-2-pyrrolidone) onto an aluminium foil. Circular electrodes (area: 3.14 cm2) were dried under vacuum at 120 C in an argon-filled glove box (<3 ppm H2O and O2) before cell assembly. Batteries comprising the dried positive electrode, a glass fibre separator soaked in electrolyte, and a lithium metal counter electrode (0.38 mm thick) were assembled and packed in the polymer-coated aluminium pouch in a so called “Coffee-bag” configuration (T. Gustafsson, J. O. Thomas, R. Koksbang and G. C. Farrington, Electrochim. Acta 37 (1992) 1639). The electrolyte was 1 M LiPF6 (Tomyama, dried over night at 80° C. in a vacuum furnace in the glove box) in an EC/DEC (Merck, battery grade and used as received) 2:1 by volume mixture. Charge—discharge tests were performed using a Digatron BTS600 battery testing system with different rates at 60° C.
To clarify the effect of the amount of sucrose on the electrochemical performance of the Li2FeSiO4/C, some electrochemical tests were carried out.
In order to determine the capacity and cyclability of 1.5Sc electrode, charge/discharge cycling on Li2FeSiO4/C electrodes at different rates was performed.
Number | Date | Country | Kind |
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11169811 | Jun 2011 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2012/060809 | 6/7/2012 | WO | 00 | 12/13/2013 |
Publishing Document | Publishing Date | Country | Kind |
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WO2012/171847 | 12/20/2012 | WO | A |
Number | Date | Country |
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101540394 | Sep 2009 | CN |
101582495 | Nov 2009 | CN |
Entry |
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20140145121 A1 | May 2014 | US |
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