Patent Application
20100297496
The present invention relates to a process for the preparation of carbon-treated complex oxides having a very low water content and to their use as cathode material.
During the last 20 years, lithium-ion batteries have become the main source of energy for mobile electronic devices by virtue of their high energy density and their high cycle life. However, safety problems related to the use of LiCoO2 for the cathode limit the possibility of developing reliable lithium-ion batteries on a large scale. The replacement of LiCoO2 by a lithium iron phosphate as cathode material has been proposed (cf. U.S. Pat. No. 5,910,382, U.S. Pat. No. 6,391,493 and U.S. Pat. No. 6,514,640). The safety problems are thus solved by virtue of the P—O covalent bond, which stabilizes the completely charged cathode with respect to the release of oxygen. The phosphate LiFePO4, which has an olivine structure, exhibits nonoptimum kinetics induced by the low intrinsic electronic conductivity, which results from the fact that the PO4 polyanions are covalently bonded. However, the use of subnanometric particles (proposed in U.S. Pat. No. 5,910,382), preferably in the form of particles carrying a thin layer of carbon at their surface (as described in U.S. Pat. No. 6,855,273, U.S. Pat. No. 6,962,666, WO02/27823 A1 and WO02/27824 A1), has made it possible to develop and market a phosphate LiFePO4 carrying a deposit of battery-grade carbon which has a high capacity and which can provide a high power. Lithium iron phosphate can in addition be modified by a partial replacement of the Fe cations by isovalent or aliovalent metal cations, such as, for example, Mn, Ni, Co, Mg, Mo, Nb, Ti, Al, Ta, Ge, La, Y, Yb, Sm, Ce, Hf, Cr, Zr, Bi, Zn, Ca and W, or by partial replacement of the PO4 oxyanion by SiO4, SO4 or MoO4 (as described in U.S. Pat. No. 6,514,640).
The lithiated cathode materials prepared in the discharge state are supposed to be stable towards oxygen. Mention may in particular be made of the lithiated oxides of at least one of the following elements: cobalt, nickel or manganese. However, the inventors have noticed that, surprisingly, the quality of LiFePO4 carrying a deposit of carbon, in particular in the form of a powder with a high specific surface or in the form of a coating of a collector in order to form a cathode, can deteriorate during exposure to air or during handling or storage. This results in a detrimental change in the product or in the formation of impurities, which can subsequently exert a harmful effect on the cyclability or the charge potential in the battery comprising the phosphate. The sequence of reactions which brings about the deterioration in the product or the formation of the impurities can be complex and variable, depending on the synthetic route, the methods of deposition of the layer of carbon by pyrolysis, the structure and the form of operation of the battery. The synthesis can be performed, for example, via the hydrothermal route, in the solid state or in the molten state, each route generating specific impurities. A not uncommon deterioration is a conversion of Fe2+ to Fe3+ with formation of related products.
The aim of the present invention is to identify some of the specific impurities formed during the deterioration of the material and to provide a process which makes it possible to avoid the formation of the impurities and the deterioration of the complex oxide.
The results of the research carried out by the inventors has shown, on the one hand, that the deterioration of a complex oxide (in particular LiFePO4) carrying a deposit of carbon is, surprisingly, greater than that of a complex oxide without a deposit of carbon and that it is due essentially to the combination of the presence of oxidizing entities (in particular atmospheric oxygen) and of a relatively high level of humidity in the ambient medium or in the reaction medium. In addition, it has been found that the exposure of the same material to dry air results in no deterioration or in a deterioration which remains at a sufficiently low level not to be harmful.
The inventors have thus developed a process in which the atmosphere around the complex oxide is controlled through the duration of the preparation process, including even a stage of pyrolysis and of formation of conductive carbon but also preferably during storage and use, so as to keep the level of humidity of the carbon-treated complex oxide at a value of less than 1000 ppm throughout all the stages of its preparation, storage and use.
This is why a subject matter of the present invention is a C-AMXO4 material in which the level of humidity is less than 1000 ppm, that is to say a material which is stable over time with regard to oxidation, a process for its preparation and also an electrode which comprises it and the use of this electrode in a lithium battery.
The material which is a subject matter of the present invention, denoted below by “C-AMXO4 material”, is composed of particles of a compound corresponding to the formula AMXO4 which have an olivine structure and which carry, on at least a portion of their surface, a film of carbon deposited by pyrolysis, the formula AMXO4 being such that:
In the material of the invention, the deposit of carbon is a uniform, adherent and nonpowdery deposit. It represents from 0.03 to 15% by weight, preferably from 0.5 to 5% by weight, with respect to the total weight of the material.
The material according to the invention, when used as cathode material, exhibits at least one charge/discharge plateau at approximately 3.4-3.5 V vs Li, characteristic of the Fe2+/Fe3+ couple.
The process according to the present invention consists in preparing the material C-AMXO4 by a process comprising a stage of pyrolysis of a compound which is a source of conductive carbon and it is characterized in that said material C-AMXO4 is placed, immediately after it has been obtained, in a controlled atmosphere and is then kept in said controlled atmosphere, said controlled atmosphere either being an oxidizing atmosphere with a dew point of less than −30° C., preferably of less than −50° C. and more particularly of less than −70° C., or a nonoxidizing atmosphere.
In the continuation of the text, the expression “controlled atmosphere” will denote a nonoxidizing atmosphere or an oxidizing atmosphere with a dew point of less than −30° C.
In a 1st specific embodiment, the material C-AMXO4 is composed of particles of a compound corresponding to the formula AMXO4 which have an olivine structure and which carry, on at least a portion of their surface, a film of carbon deposited by pyrolysis, the formula AMXO4 being such that:
In a 2nd embodiment, the complex oxide AMXO4 comprises less than 1000 ppm of water and less than 1000 ppm of LiOH, of Li3PO4, of Li4P2O7, of lithium polyphosphates, optionally hydrated, or of Li2CO3, preferably less than 500 ppm, and more particularly less than 200 ppm.
In a 3rd embodiment, the complex oxide AMXO4 comprises less than 1000 ppm of water and less than 10 000 ppm of Fe2O3, Li3Fe2(PO4)3, LiFeP2O7, or an Fe3+ compound which can be detected electrochemically, preferably less than 5000 ppm and more particularly less than 2000 ppm.
The level of humidity of the material according to the invention can be measured using devices commonly used in industry. Mention may be made, as example, of Computrac Vapor Pro L, sold by Arizona Instrument LLC (USA), or the moisture measuring devices of Mettler Toledo (USA) or Brinkmann (USA).
The properties of the materials according to the invention can be adapted by appropriately choosing the element or elements partially replacing Fe. For example, in the material in which the complex oxide corresponds to the formula LiFe1-x-yM′xM″yPO4, the choice of M′ from Mn, Ni and Co makes it possible to adjust the average discharge potential of the cathode material. The choice of M″ from Mg, Mo, Nb, Ti, Al, Ca and W makes it possible to adjust the kinetic properties of the cathode material.
Among the above materials, those in which the complex oxide AMXO4 corresponds to the formula LiFe1-xMnxPO4, with 0≦x≦0.5, are particularly preferred.
In a particularly advantageous embodiment, the complex oxide AMXO4 is LiFePO4 and the material comprises less than 1000 ppm of water and less than 1000 ppm of LiOH, of Li3PO4, of Li4P2O7, of lithium polyphosphates, optionally hydrated, or of Li2CO3, preferably less than 500 ppm and more particularly less than 200 ppm. In addition, it is preferable for the material to comprise less than 10 000 ppm of Fe2O3, of Li3Fe2(PO4)3, of LiFePO2O7, or of an Fe3+ compound detectable electrochemically, preferably less than 5000 ppm and more particularly less than 2000 ppm.
In the context of the present invention, the expression “particles” encompasses both individual particles and agglomerates of individual particles. The size of the individual particles is preferably between 10 nm and 3 μm. The size of the agglomerates is preferably between 100 nm and 30 μm. These particle sizes and the presence of the carbon deposit confer, on the material, a high specific surface typically of between 5 and 100 m2/g.
The material C-AMXO4 can be prepared by various processes, before being placed in a controlled atmosphere as defined above. It can be obtained, for example, via a hydrothermal route, via a solid-state thermal route or via a melt route.
In a preferred embodiment, the process of the invention is carried out by reacting, by placing under thermodynamic or kinetic equilibrium, a gas atmosphere with a mixture in the required proportions of the following source compounds a), b), c), d) and e):
In this embodiment, the gas stream and the stream of solid products move countercurrentwise. If the controlled gas atmosphere is dry nitrogen, the material C-AMXO4 recovered at the outlet of the furnace comprises less than 200 ppm of water. This material C-AMXO4 thus obtained is immediately transferred into a controlled atmosphere, such as defined above.
The water content of the final material depends, on the one hand, on the water content of the controlled atmosphere and, on the other hand, on the duration of maintenance in said atmosphere. The water content of the material increases when the duration of maintenance in the controlled atmosphere increases and when the water content of the controlled atmosphere increases.
The process of the invention is of particular use in the preparation of a C—LiFePO4 material comprising less than 1000 ppm of water. For the preparation of this material, the source compound a) is a lithium compound chosen, for example, from the group consisting of lithium oxide, lithium hydroxide, lithium carbonate, the neutral phosphate Li3PO4, the hydrogen phosphate LiH2PO4, lithium ortho-, meta- or polysilicates, lithium sulfate, lithium oxalate, lithium acetate and one of their mixtures. The source compound b) is a compound of iron, for example iron(III) oxide or magnetite, trivalent iron phosphate, lithium iron hydroxyphosphate or trivalent iron nitrate, ferrous phosphate, hydrated or nonhydrated vivianite Fe3(PO4)2, iron acetate (CH3COO)2Fe, iron sulfate (FeSO4), iron oxalate, ammonium iron phosphate (NH4FePO4), or one of their mixtures. The source compound c) is a compound of phosphorus, for example phosphoric acid and its esters, the neutral phosphate Li3PO4, the hydrogen phosphate LiH2PO4, monoammonium or diammonium phosphates, trivalent iron phosphate or manganese ammonium phosphate (NH4MnPO4). All these compounds are additionally a source of oxygen and some of them are sources of at least two elements from Li, Fe and P. The deposition of carbon on the surface of the particles of complex oxide AMXO4 is obtained by pyrolysis of a source compound e).
The pyrolysis of the compound e) can be carried out at the same time as the synthesis reaction between the compounds a) to d) to form the compound AMXO4. It can also be carried out in a stage in succession to the synthesis reaction.
The deposition of the layer of conductive carbon at the surface of the particles of complex oxide AMXO4 can be obtained by thermal decomposition of highly varied source compounds e). An appropriate source compound is a compound which is in the liquid state or in the gas state, a compound which can be used in the form of a solution in liquid solvent, or a compound which changes to the liquid or gas state during its thermal decomposition, so as to more or less completely coat the particles of complex oxide.
The source compound e) can, for example, be chosen from liquid, solid or gaseous hydrocarbons and their derivatives (in particular polycyclic aromatic entities, such as tar or pitch), perylene and its derivatives, polyhydric compounds (for example, sugars and carbohydrates, and their derivatives), polymers, cellulose, starch and their esters and ethers, and their mixtures. Mention may be made, as examples of polymers, of polyolefins, polybutadienes, polyvinyl alcohol, condensation products of phenols (including those obtained from reaction with aldehydes), polymers derived from furfuryl alcohol, from styrene, from divinylbenzene, from naphthalene, from perylene, from acrylonitrile and from vinyl acetate.
When the compound e) is CO or a gaseous hydrocarbon, it is subjected to dismutation, advantageously catalyzed by a transition metal element present in at least one of the precursors a) to c) or by a compound of a transition metal added to the mixture of precursors.
When the source compound e) is a gas or a mixture of gases, such as ethylene, propylene, acetylene, butane, 1,3-butadiene or 1-butene, the thermal decomposition is carried out by cracking in a furnace at a temperature between 100 and 1300° C. and more particularly between 400 and 1200° C., preferably in the presence of an inert carrier gas (cf., for example, US 2002/195591 A1 and US 2004/157126 A1).
The deposition of carbon can, in addition, be carried out by CVD starting from hydrocarbons, as described in JP 2006-302671.
In the various processes for the preparation of the carbon-treated complex oxides C-AMXO4, some stages can bring about sintering or agglomeration of the particles. It is therefore strongly recommended to subject the complex oxide obtained by the synthesis to a milling in order to ensure the homogeneity of the final product and to control the size and optionally the degree of deagglomeration thereof. Jet milling is a convenient means for controlling the sizes of the particles and agglomerates. However, it has been found that, in some cases, it causes irreversible damage to the product. The process of the present invention is thus of particular use when the preparation of the complex oxide intended to form the active material of a cathode has to be subjected to milling in order to have the optimum particle size. The milling is carried out in a controlled atmosphere, such as defined above, and on a material comprising less than 1000 ppm of water, such as obtained at the end of the pyrolysis stage.
In a specific embodiment, the process for the preparation of the material according to the invention comprises a stage of washing the material C-AMXO4 in hot water, for example at a temperature of greater than 60° C. It has been found that the material obtained subsequently, by extraction from the aqueous medium (for example by filtration or by centrifuging) and drying, exhibits high stability when it is used as cathode active material in a lithium ion battery. The specific capacity is equivalent to, indeed even higher than, that which is obtained with an identical material which comprises less than 1000 ppm of water and which has not been subjected to washing.
The irreversible damage to C—LiFePO4 when it is exposed to moist air, that is to say both to oxygen and to water, can be the result of various chemical processes. The following mechanisms are given below but without implied limitation.
LiFePO4+¼O2+½H2O→FePO4+LiOH (LiOH changing to LiCO3)
3LiFePO4+¾O2+½H2O→LiFeP2O7+Fe2O3+LiH2PO4
3LiFePO4+¾O2→Li3Fe2(PO4)3+½Fe2O3
The mechanisms of deterioration might also involve impurities originating from the synthesis of LiFePO4 or impurities produced or modified during the pyrolysis stage carried out in order to deposit the conductive carbon in the core of the particles or at their surface.
The inventors have discovered that the materials C—LiFePO4 carrying a carbon coating are more sensitive to deterioration by moist air than carbon-free materials LiFePO4, in particular when they have a high specific surface. For a material C—LiFePO4, several electrochemical mechanisms of deterioration can be envisaged, because of the threefold contact between the gas phase, the conductive carbon and the compounds which comprise the Fe2+ entity (the complex oxide LiFePO4 and/or various impurities which depend on the route of synthesis of LiFePO4, for example Fe2P or Fe2P2O7). These mechanisms can be represented by
2Fe2+2Fe3++2e−+2Li+
½O2+H2O+2e−+2Li+→2LiOH
The formation of Fe3+ and the formation of LiOH are concomitant. The exact nature of the decomposition products is uncertain, owing to the fact that these products can react with one another or with other elements of an electrochemical cell comprising the material C—LiFePO4. However, their presence and their electrochemical activity are illustrated in the examples described subsequently in the present text.
In other words, when the material C—LiFePO4 is brought into contact with oxygen and water, it can be regarded as a short-circuited carbon-LiFePO4 battery, the water acting as electrolyte, owing to the fact that the surface of the LiFePO4 particles is not completely covered with the carbon, and O2 acting as oxidizing agent. The combination of several electrochemical couples, of a high specific surface and of the activation of the surface by the carbon might explain the specific difficulties encountered during the storage and use of C—LiFePO4 in the presence of moist air.
The possible release of LiOH/Li2CO3 can cause not only an irreversible loss in capacity but also the deterioration of an electrolyte comprising LiPF6 or other elements of a battery.
Even if these specific difficulties exhibit disadvantages, in particular for laboratory experiments, they can be solved on the industrial scale by appropriate technological solutions, in particular by controlling the conditions of exposure of the material to O2 and moisture.
A material C-AMXO4 according to the invention is of particular use as cathode in a lithium battery. The lithium battery can be a solid electrolyte battery in which the electrolyte can be a plasticized or nonplasticized polymer electrolyte, a battery in which a liquid electrolyte is supported by a porous separator or a battery in such the electrolyte is a gel.
The cathode is preferably composed of a composite material applied to a collector, said composite material comprising C-AMXO4, a binder and a material which promotes electronic conduction. The material which promotes electronic conduction is advantageously chosen from carbon black, graphite or carbon fibers (for example in the form of carbon nanotubes or of VGCF (vapor grown carbon fiber) fibers, the growth of which is carried out in the gas phase).
The capacity of the cathode is commonly expressed in mg of electroactive material per cm2 of the surface of the cathode.
When the cathode is intended for a polymer electrolyte battery, the binder is preferably a solvating polymer, preferably the polymer which forms the solvent of the electrolyte.
When the cathode is intended for a liquid electrolyte battery, the binder can be a nonsolvating polymer, for example a PVdF-HFP copolymer or a styrene-butadiene-styrene copolymer.
The cathode is prepared from a material C-AMXO4 having a water content of less than 1000 ppm, used directly after its synthesis or stored in a controlled atmosphere and/or treated in a controlled atmosphere. If it is necessary to mill the particles of C-AMXO4 before incorporating them in the cathode composite material, it is advisable to carry out the milling under a controlled atmosphere. A milling technique which is particularly useful is jet milling.
A material C—LiFePO4 and a material C—LiMPO4 in which M represents Fe partially replaced by Mn or Mg are particularly preferred as cathode active material.
The process according to the invention was employed by way of comparison with the techniques of the prior art in order to demonstrate that restricting the moisture level to very low values has a favorable effect on the performance of the material C-AMXO4 used as cathode material in a lithium battery.
In all the tests, the water contents of the materials were determined using a Computrac Vapor Pro L sold by Arizona Instruments LLC.
The results are reproduced in
A mixture comprising FePO4.(H2O)2 (1 mol, sold by Budenheim, grade E53-81) and Li2CO3 (1 mol, sold by Limtech, level of purity: 99.9%) in stoichiometric amounts and 5% of polyethylene-block-poly(ethylene glycol) comprising 50% of ethylene oxide (sold by Aldrich) was prepared and was introduced into isopropyl alcohol, mixing was carried out for approximately 10 h and then the solvent was removed. In the material thus obtained, the polymer keeps together the particles of phosphate and of carbonate.
The mixture was treated under a stream of nitrogen at 700° C. for 2 hours, in order to obtain a material C—LiFePO4 of battery grade, drying was then carried out under vacuum at 100° C. and the final material was stored in a glovebox under an argon atmosphere at a dew point of −90° C.
The material has a specific surface of 13.6 m2/g and a carbon content of 1.8% by weight.
The material obtained was subjected to jet milling under compressed air for 3 min at a dew point of −70° C. and then the material thus obtained was divided into several fractions.
Each fraction was dried under vacuum at 120° C. for 1 hour (in order to obtain a perfectly dry sample) and then exposed to an atmosphere having a relative water content of 20%. The exposure time is different for each fraction, which made it possible to ascertain the change in the water content as a function of the exposure time. For the measurement, each sample is placed in an airtight septum flask.
It is apparent that, surprisingly, the material C—LiFePO4 adsorbs not insignificant amounts of water, despite the deposition of a surface layer of carbon, which is hydrophobic. The water content is thus approximately 200 ppm after 30 sec, approximately 500 ppm after approximately 10 min and greater than 2000 ppm after 3 hours.
These results show that C—LiFePO4 presents a specific and surprising problem of water reuptake and that the drying and handling conditions have to be under perfect control in order to produce a product of battery quality.
A compound LiFe0.5Mn0.5PO4 was prepared by mixing the precursors LiH2PO4, FeC2O4.2H2O and (CH3COO)2Mn.4H2O in stoichiometric amounts. The mixture was subsequently milled in heptane, then dried and gradually heated up to 40° C. under air in order to decompose the acetate and oxalate groups. This temperature was maintained for 8 hours. During this treatment, the iron(II) is oxidized to iron(III).
The mixture was subsequently remilled in an acetone solution comprising an amount of cellulose acetate (carbon precursor) representing 39.7% by weight of acetyl groups and 5% by weight with respect to the mixture.
After drying, the mixture was heated in a tubular furnace up to 700° C. at the rate of 6° C. per minute. This temperature was maintained for one hour and then the sample was cooled over 40 minutes, i.e. with a cooling rate of approximately 15° C. per minute. The tubular furnace was maintained under flushing with the reducing gas (CO/CO2: 1/1) throughout the duration of the heat treatment (approximately 3 and a half hours).
A material C—LiFe0.5Mn0.5PO4 of battery grade was thus obtained, which was dried under vacuum at 100° C. and then stored in a glovebox under an argon atmosphere at a dew point of −90° C.
This material has a specific surface of 16.2 m2/g and a carbon content of 1.2% by weight.
Under the same conditions as in example 1a, the milled material exhibits a level of moisture of greater than 500 ppm after 15 min of exposure to an atmosphere having a relative water content of 20%.
A compound LiFe0.98Mg0.02PO4 was prepared by a melt process. The compounds Fe2O3, Li2CO3, (NH4)2HPO4 and MgHPO4 were mixed in a molar ratio of 0.49/0.5/0.98/0.02 and then this mixture was brought under argon to 980° C. in a graphite crucible, was maintained at this temperature for 1 hour, in order to melt it, and was then cooled to approximately 50° C. in 3 hours.
The compound LiFe0.98Mg0.02PO4 thus obtained was subsequently milled in 90 cm3 of isopropanol for 10 min with 12 g of zirconia beads having a diameter of 20 mm and then for 90 min with 440 g of zirconia beads having a diameter of 3 mm, in order to obtain a powder with a mean size of 1.12 μm.
The LiFe0.98Mg0.02PO4 powder obtained after milling was mixed with 7% by weight of cellulose acetate and then dried and treated at 700° C. for 1 hour under argon to give a material C—LiFe0.98Mg0.02PO4 comprising a deposit of 1.32% by weight of carbon and exhibiting a specific surface of 19.2 m2/g.
Under the same conditions as in example 1a, the milled material exhibits a level of moisture of greater than 500 ppm after 15 min of exposure to an atmosphere having a relative water content of 20%.
A compound LiFePO4 was prepared by a hydrothermal process, such as described in example 4 of US 2007/054187, from FeSO4, H3PO4 and LiOH as precursors. The LiFePO4 powder thus obtained was mixed with lactose monohydrate, such as described in example 5 of US 2007/054187, and then subjected to a heat treatment in order to pyrolyze the lactose monohydrate, according to the procedure described in example 5 of US 2007/054187. After deagglomeration, a material C—LiFePO4 was thus obtained in the form of particles having a mean size of less than 0.6 μm and having a specific surface of 17.4 m2/g.
Under the same conditions as in example 1a, the milled material exhibits a level of moisture of greater than 800 ppm after 15 min of exposure to an atmosphere having a relative water content of 20%.
Four samples of the material from example 1 were prepared and were treated for 3 hours with different amounts of phenyliodoso diacetate in dry acetonitrile. The relative amounts of phosphate and diacetate were chosen in order to obtain the materials C—Li1-xFePO4 in which x is respectively approximately 0.02, 0.04, 0.06, 0.08 and 0.1. After filtering and washing with dry acetonitrile, the product was dried under vacuum at 80° C. for 3 hours.
Liquid electrolyte batteries were prepared according to the following procedure.
A PVdF-HFP copolymer (supplied by Atochem) and an EBN-1010 graphite powder (supplied by Superior Graphite) were carefully mixed in N-methyl-pyrrolidone for one hour using zirconia beads in a Turbula® mixer in order to obtain a dispersion composed of the PVdF-HFP/graphite/NMP 80/10/10 by weight mixture. The mixture obtained was subsequently deposited, using a Gardner® device, on a sheet of aluminum carrying a carbon-treated coating (supplied by Intellicoat) and the film deposited was dried under vacuum at 80° C. for 24 hours and then stored in a glovebox.
A battery of the “button” type was assembled and sealed in a glovebox, use being made of the carbon-treated sheet of aluminum carrying the coating comprising the material C—LiFePO4, as cathode, a film of lithium, as anode, and a separator having a thickness of 25 μm (supplied by Celgard) impregnated with a 1M solution of LiPF6 in an EC/DEC 3/7 mixture.
In the various batteries assembled according to this procedure, the cathode material comprises either the material C—LiFePO4 directly obtained by the process of example 1 (x=0, fresh), or a material C—LiFePO4 of example 1 after storage, or a material C—Li1-xFePO4 of example 2. The natures of the modified materials are summarized in the following table. The term “fresh” means that the material is used immediately after it has been synthesized according to example 1. The term “new” means that the cathode is mounted in the battery as soon as it is prepared. The capacity C of the cathode of the battery is also shown in the table, said capacity being expressed in mg of electroactive material C—Li1-xFePO4 per cm2 of the surface of the cathode.
Before cycling, the new batteries are subjected to a potential scanning in reduction (20 mV/80 s) from the rest potential up to 2 V. This technique makes it possible to detect the electrochemical activity of Fe(III) impurities present in the starting material C—LiFePO4. This scanning in reduction is followed by a scanning in oxidation up to 3.2 V, which makes it possible to study the reversibility of the couple.
The batteries A2 and B2 were subjected to scanning cyclic voltammetry at ambient temperature with a rate of 20 mV/80 s using a VMP2 multichannel potentiostat (Biologic Science Instruments), first in reduction from the rest potential up to 2 V and then in oxidation between 2 and 3.2 V. The corresponding voltammograms are represented in
The batteries A3 to F3, which have been chemically delithiated, were subjected to cyclic voltammetry under the same conditions as the batteries A2 and B3. The corresponding voltammograms are represented in
In the various batteries assembled in this example, the cathode is prepared with the cathode material comprising C—LiFePO4 directly obtained by the process of example 1. Subsequently, the cathode is mounted directly in the battery after it has been prepared (new) or it is stored under certain conditions before mounting in the battery. In the following table, “Fe(III)” indicates the content of Fe(III) formed with respect to the amount of Fe(II), “ΔFe(III)” indicates the increase in the level of Fe(III) during storage and C denotes the capacity of the cathode of the battery, expressed in mg of electroactive material per cm2 of the surface of the cathode.
The solid electrolyte batteries were prepared according to the following procedure.
2.06 g of C—LiFePO4, 1.654 g of poly(ethylene oxide) having a molecular weight of 400 000 (supplied by Aldrich) and 334 mg of Ketjenblack carbon powder (supplied by Akzo-Nobel) were carefully mixed for 1 hour in acetonitrile using zirconia beads in a Turbula® mixer. The mixture obtained was subsequently deposited, using a Gardner® device, on a sheet of aluminum carrying a carbon-treated coating (supplied by Intellicoat) and the film deposited was dried under vacuum at 80° C. for 12 hours and then stored in a glovebox.
A battery of the “button” typed was assembled and sealed in a glovebox, use being made of the carbon-treated sheet of aluminum carrying the coating comprising the phosphate, as cathode, a film of lithium, as anode, and a film of poly(ethylene oxide) comprising 30% by weight of LiTFSI (supplied by 3M).
The batteries A4 to C4 and A5 to C5, assembled according to this procedure, were subjected to voltammetry as described in example 3. The corresponding voltammograms are represented in
As above, the behavior of the batteries B4 and C4 can be attributed to the production of Fe(III) impurities during the storage in air of the cathodes prepared from “fresh” LiFePO4, said impurities being FePO4 for the battery C4.
As regards the batteries B5 and C5, it is apparent that impurities are not formed during storage under dry argon or under dry air, the appearance of the curve in oxidation not being modified.
The batteries A6, B6 and C6, assembled like the batteries A4 to C4 and A5 to C5, were subjected to C/4 galvanostatic cycling at 60° C. The curves are represented in
The following were prepared according to the process used for the preparation of the batteries A6, B6 and C6 respectively:
The loss in capacity after 100 cycles for each battery is shown in the following table:
The batteries A7 and B7 differ in that the material C—LiFePO4 is used from its preparation in A7 and after milling for 3 min under compressed air with a dew point of −6° C.
The voltammeter curves show the formation of from 1.3 to 2% of Fe(III) phase and the galvanostatic cycling shows a significant increase in the loss in capacity at 60° C. after 100 cycles for the material milled at a dew point of −6° C. (6% instead of 2% for the unmilled material). On the other hand, milling carried out at a dew point of −70° C. does not significantly increase the loss in capacity.
Batteries A7.1 and B7.1 were prepared like A7 and B7 but replacing the material C—LiFePO4 of example 1a with the material C—LiFePO4 of example 1d. The galvanostatic cycling shows a significant increase in the loss in capacity at 60° C. after 100 cycles for the material milled at a dew point of −6° C. (7.8% instead of 1.8% for the unmilled material).
Batteries were assembled according to the procedure of example 3, use being made of compounds C—LiFePO4 which have been subjected to various treatments before the preparation of the cathode. Each treatment was applied to two batteries. The influence of the treatment, with a duration of 10 hours, on the final specific capacity of the corresponding batteries is given in the following table.
The above results clearly show that water (contributed by an oxidizing gas atmosphere) causes deterioration to the material C-AMXO4, which is reflected in general by the formation of Fe3+, in a form which is poorly defined but which is electrochemically active, and by the presence of related products, such as, for example, lithium hydroxide and phosphorus-comprising lithium compounds, which are optionally soluble. The Fe3+ compounds and said lithium compounds are capable of reacting with the components of an electrochemical cell in which the material C-AMXO4 is used as electrode material and of affecting the functioning of is said cell.
On the other hand, it has been found that, surprisingly, washing the material C-AMXO4 with water makes it possible to retain its specific capacity analogous to that of a dry material C-AMXO4, even when the washing is carried out under an oxidizing atmosphere.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2566906 | Oct 2006 | CA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR07/01803 | 10/30/2007 | WO | 00 | 7/8/2009 |
