Patent Application
20090130560
The present invention relates to the positive lithium insertion electrode for lithium ion battery and in particular, to the different steps required to make lithium manganese phosphate electrochemically active.
Insertion compounds are those that act as a solid host for the reversible insertion of guest atoms. Cathode materials that will reversibly intercalate lithium have been studied extensively in recent years for use as electrode materials in advanced high energy density batteries and they form the cornerstone of the emerging lithium-ion battery industry. Lithium-ion batteries have the greatest gravimetric (Wh/kg) and volumetric (Wh/L) energy densities of currently available conventional rechargeable systems (i.e., NiCd, NiMH, or lead acid batteries) and represent a preferred rechargeable power source for many consumer electronics applications. Additionally, lithium ion batteries operate around 3.6 volts enabling a single cell to operate in the correct voltage window for many consumer electronic applications.
Lithium ion batteries use two different insertion compounds: for the active cathode and for the anode materials. In a lithium-ion battery, lithium is extracted from the cathode material while lithium is concurrently inserted into the anode on charge of the battery. Lithium atoms travel, or “rock”, from one electrode to the other in the form of ions dissolved in a non-aqueous electrolyte. The associated electrons travel in the circuit external to the battery. Layered rock-salt compounds such as LiCoO.sub.2 and LiNiO.sub.2 (1) are proven cathode materials. Nonetheless, Co and Ni compounds have economic and environmental problems that leave the door open for alternative materials.
LiMn.sub.2O.sub.4 is a particularly attractive cathode material candidate because manganese is environmentally benign and significantly cheaper than cobalt and/or nickel. LiMn.sub.2O.sub.4 refers to a stoichiometric lithium manganese oxide with a spinel crystal structure. A spinel LiMn.sub.2O.sub.4 intercalation cathode is the subject of intense development work (2), although it is not without faults. The specific capacity obtained (120 mAh/g) is 15-30% lower than Li(Co,Ni)O.sub.2 cathodes, and unmodified LiMn.sub.2O.sub.4 exhibits an unacceptably high capacity fade. Several researchers have stabilized this spinel by doping with metal or alkali cations (3,4). While the dopants successfully ameliorated the capacity decline, the initial reversible capacity is no better than 115 mAh/g, and the running voltage of the cell is no better than the usual 3.5 V.
Recently, olivine-structured LiMPO.sub.4 where M=Fe, Mn, Co, Cu, V have been gaining interest as candidate materials for rechargeable lithium batteries (5,6 & Goodenough patent). They have a theoretical capacity of up to 170 mAh/g, which would increase the energy density compared to LiCoO.sub.2 or LiMn.sub.2O.sub.4.
In particular Lithium iron phosphate (LiFePO.sub.4) has established its position as a potential next generation cathode material. LiFePO.sub.4 has advantages in terms of material cost, chemical stability and safety. However, the Fe.sup.3+/Fe.sup.2+ couple in LiFePO.sub.4 has a significantly lower voltage (3.45V versus Li/Li.sup.+) when compared to the (4.05 V versus Li/Li.sup.+) in the standard LiCoO.sub.2 based lithium ion batteries and this lowers the energy available for the LiFePO.sub.4 system. In addition LiFePO.sub.4 has low electronic conductivity which leads to initial capacity loss and poor rate capability associated with diffusion-controlled kinetics of the electrochemical process. Morphological modification at the nano-scale level appears to be the best tool to control these undesired phenomena. The use of olivine type LiMnPO.sub.4 would also be of interest because of the position of the Mn.sup.3+/Mn.sup.2+ couple which creates a potential of 4.05V versus Li/Li.sup.+ which is compatible with the present LiCoO.sub.2 based lithium ion batteries. However, LiMnPO.sub.4 is an insulator with ca. 2 eV spin exchange band gap and this significantly lowers the electrochemical activity compared to LiFePO.sub.4 which is a semiconductor with ca. 0.3 eV crystal field band gap. Furthermore the two-phase Mn.sup.3+/Mn.sup.2+ redox character also prohibits the introduction of mobile electrons or holes into the band.
Sol-gel processing can control the structure of the material on a nanometre scale from the earliest stages of processing. This technique of material synthesis is based on some organometallic precursors, and the gels may form by network growth from an array of discrete particles or by formation of an interconnected 3-D network by the simultaneous hydrolysis and polycondensation of organometallic precursors.
Based on thermodynamics and kinetics that govern the precipitation of pure phosphate phases, Delacourt et al. (6, 8) described a low-temperature preparation of optimised phosphates.
Dominko et al. synthesized micro-sized porous LiMnPO.sub.4/C composite (where M stands for Fe and/or Mn) using a sol-gel technique (9). However, the materials obtained via these “chimie douce” methods, gave disappointing electrochemical performances ˜70 mAh/g at C/20 are the maximum obtained.
The origin of this poor performance is ascribed to both slow Li diffusion in the solid phase and a poor electronic and/or ionic conductivity of the material. (Delacourt, C; Laffont, L; Bouchet, R; Wurm, C; Leriche, J B; Morcrette, M; Tarascon, J M; Masquelier, C. Journal of the electrochemical society (2005), 152 (5): A913-A921)
A novel approach is required that addresses these issues concurrently, if higher performances are to be achieved.
The primary object of the invention is to obtain LiMnPO.sub.4 of an excellent crystallinity and a high purity via a “chimie douce” reaction and low sintering temperatures. In order to achieve the above object, the invention is a method for manufacturing lithium manganese phosphate (LiMnPO.sub.4). As such the primary object of the invention is to describe a synthetic preparation method. More particularly, the primary object of the present invention is to provide a sol-gel synthesis route resulting in a pure well-crystallised phase of LiMnPO.sub.4.
According to an embodiment of the present invention, by covering surfaces of the particles of lithium manganese phosphate (LiMnPO.sub.4) with acetylene black by high-energy milling, the electrochemical properties of the material as a positive electrode in Lithium ion battery are improved.
The third object of the invention is to describe an electrode preparation of the lithium manganese phosphate/carbon composite. This process is very important to reach the correct electrochemical performances.
The present invention will be described in detail with reference to the drawing showing the preferred embodiment of the present invention, wherein:
Hereinafter, a method for manufacturing lithium manganese phosphate (LiMnPO.sub.4) according to the invention and a method for manufacturing a positive electrode active material will be detailed.
A. Method for Manufacturing LiMnPO.sub.4
Firstly, a method for manufacturing LiMnPO.sub.4 according to the invention will be described. The present invention discloses sol gel methods to prepare lithium manganese phosphate (LiMnPO.sub.4). LiMnPO.sub.4. The success of the alkoxides as precursors of the sol-gel process is their facility to undergo hydrolysis because the hydrolysis is the main step in the transformation of alkoxides to oxides. All metals are capable of forming an alkoxide in which an alkyl group is bonded to the metal by means of oxygen atom.
The choice of the alkyl group may be made according to availability, but one must be careful because the reaction rate varies throughout the process depending on the alkyl group used.
The method for manufacturing LiMnPO.sub.4 according to the invention is a method of obtaining LiMnPO.sub.4 by carrying out the steps of mixing and gel formation, drying and calcination.
Hereinafter, the respective steps in the invention will be detailed.
1. Mixing Formation
The present invention discloses sol-gel methods to prepare lithium manganese phosphate (LiMnPO.sub.4). Most sol-gel processes are preferably accomplished in a common solvent. Water is the solvent chosen in this system. A mixing of different optimised precursors together is done in the solvent. The final solution should be stirred long enough to ensure homogeneity.
Hydrolysis and polycondensation reactions of metal alkoxides lead to the formation of metal oxides. The fundamental chemical process involved in this processing is influenced by several parameters, which allow the control of homogeneity and the nanostructure of the derived materials.
A hydrolysis—condensation reaction must take place: precursors of metal alkoxides will be hydrolysed in presence of water. Lithium acetate dihydrate (C.sub.2H.sub.3O.sub.2 Li, (H.sub.2O).sub.2), manganese (II) acetate tetrahydrate (C.sub.4H.sub.6O.sub.4Mn, (H.sub.2O).sub.4) and ammonium dihydrogen phosphate (H.sub.6NO.sub.4P) are used as precursors. The starting materials were dissolved in distilled water at room temperature.
Hydrolysis rates of highly reactive alkoxides can be control by using chelating organic ligands such as organic acids. The formation of the metal complex with a multidentated ligand will decrease the hydrolysis rate. As chelating agents glycolic acid (C.sub.2H.sub.4O.sub.3), oxalic acid (C.sub2H.sub.2O.sub.4), citric acid (C.sub6H.sub.8O.sub.7), etc were used.
In addition to water, an acid can also be used to hydrolyse the solution. HNOsub.3 is used to acidify the solution from pH 4.5 to 1.5. This solution was further heated to 60-90° C. to form the gel.
2. Drying Process
Once the solution has been condensed into a gel, solvent removal must be carried out. Drying is the term used for the removal of solvent. After drying, a porous and homogeneous aerogel is obtained. Once the gel has been dried, a sintering step is needed to collapse the pore structure and solidify the gel. This gel was dried 14 hours at 80° C. and 10 hours at 120° C. under air.
3. Thermal Decomposition
In this step, the complex decompositions of organic precursor take place, and the organic substances added for the preparation of the gel are almost completely removed, leading to amorphous powders. DTA-TGA experiments are performed to study the decompositions of organic precursors, from which one can determine a minimal calcination temperature. In the case of LiMnPO.sub.4 sol-gel synthesis the temperature is fixed at 350° C.
4. Sintering
During calcination, pore-formation occurs via a process of particle bonding by thermal energy. The driving force behind sintering is a reduction in the surface area. The calcination step is required to obtain the desired crystallinity. The powder was heated at different temperatures (400-900° C.) from 1 to 5 hours in air. The resulting powder was ground in a mortar and characterised by X-ray diffraction study. Measuring the cell parameters of the orthorhombic structure indicates measurements as follow: a=10.4539(6)Å, b=6.1026(4)Å, c=4.7469(3)Å. The specific surface area is about 7 to 20 m.sub.2/g (particle size is about 260 to 90 nm).
B. Method for Manufacturing a Positive Electrode Active Material
Next, a method for manufacturing a positive electrode active material according to the invention will be described. The method for manufacturing the positive electrode active material according to the invention is characterized by blending a conductive agent with the LiMnPO.sub.4 obtained according to the above method for manufacturing the LiMnPO.sub.4 and.
LiMnPO.sub.4 used in the invention, being obtained according to a manufacturing method described in the “A. Method for manufacturing LiMnPO.sub.4”, is omitted from describing here.
Furthermore, the conductive agent used in the invention, as far as it can improve the electrical conductivity, is not particularly restricted. For instance, graphite or carbon black such as acetylene black can be cited.
The conductive agent is added in the range of 5 to 25 parts by weight, preferably in the range of 10 to 20 parts by weight to 100 parts by weight of LiMnPO.sub.4. When an amount of the conductive agent is less than necessary, the electrical conductivity may not be sufficiently improved, and, when it is more than necessary, since an amount of LiMnPO.sub.4 becomes relatively less, the performances as the positive electrode active material may be deteriorated. In the invention, a method of blending the LiMnPO.sub.4 and the conductive agent is not particularly restricted. However, for instance, the physical blending is preferable and the mechanical blending is particularly preferable. Specifically, a ball mill pulverizing method or the like can be cited. Furthermore, applications of the positive electrode active material obtained according to the invention are not particularly restricted. However, it can be used in, for instance, lithium secondary batteries.
The present invention discloses improved electrochemical performances of LiMnPO.sub.4/carbon composite. This composite was obtained by high energy milling of LiMnPO.sub.4 with acetylene black in a stainless steel container using a planetary ball mill for several hours.
The present invention also discloses electrode preparation of LiMnPO.sub.4/C composite to improve electrochemical performances. Electrode of LiMnPO.sub4/C active material was prepared by mixing of the active material (composite) with a carbon black and a binder in N-methyl-2-pyrrolidinon. The slurry was then coated on an aluminium foil, serving as the current collector. The N-methyl-2-pyrrolidinon was subsequently evaporated in air on titanium hot plate.
Hereinafter, the invention will be more specifically described with reference to examples.
In 300 mL of distilled water,
X-ray spectrum of this material indicates a pure phase of lithium manganese phosphate (LiMnPO.sub.4).
The powder of LiMnPO.sub.4 was placed in a 250 mL stainless steel container and ball milled with a planetary ball mill using 9 stainless steel balls of 20 mm diameter for one hour. In addition, 20% in weight of acetylene black was added to the milled LiMnPO.sub.4 and ball milled again for 3 hours. A composite of LiMnPO.sub.4/C was then obtained.
A positive electrode composition of LiMnPO.sub4/C active material was prepared by mixing of the active material (composite) with a carbon black (C55 from Shawinigan) and a binder (polyvinylidene difluoride —PVDF) with the mass ratio (90:5:5), in N-methyl-2-pyrrolidinon. The slurry was then coated on an aluminium foil, serving as the current collector. The N-methyl-2-pyrrolidinon was subsequently evaporated in air at 100° C. for 1 hour and 120° C. for 30 minutes on titanium hot plate. The electrode was then dry at 160° C. overnight under vacuum.
The positive electrode of example 3 was tested in standard laboratory Swagelok test cells versus Li metal. Microporous Celgard membrane served as separator. The electrolyte was made of 1M of LiPF.sub.6 dissolved in a 1:1:3 by volume mixture of dried and purified propylene carbonate (PC), ethylene carbonate (EC) and dimethyl carbonate (DMC).
The electrochemical properties of LiMnPO.sub4/C electrodes were measured using an Arbin BT 2000 electrochemical measurement system by galvanostatic charge/discharge and cyclic voltammetry.
The battery prepared above was charged under a current density of 0.03 mA/cm.sup.2 until a termination voltage of 4.7 volt was reached. Then the charged battery was discharged at a current density of 0.03 mA/cm.sup.2 until a termination voltage of 2.3 volt was reached.
In 300 mL of distilled water,
X-ray spectrum of this material indicates a pure phase of lithium manganese phosphate (LiMnPO.sub.4).
The composite LiMnPO.sub.4/C, cell preparation and test conditions were performed according Example 2, 3 and 4.
In 300 mL of distilled water,
X-ray spectrum of this material indicates a pure phase of lithium manganese phosphate (LiMnPO.sub.4).
The composite LiMnPO.sub.4/C, cell preparation and test conditions were performed according to those in Example 2, 3 and 4.
In 300 mL of distilled water,
X-ray spectrum of this material indicates a pure phase of lithium manganese phosphate (LiMnPO.sub.4).
The composite LiMnPO.sub.4/C, cell preparation and test conditions were performed according to those in Example 2, 3 and 4.
In 300 mL of distilled water,
X-ray spectrum of this material indicates a pure phase of lithium manganese phosphate (LiMnPO.sub.4).
The composite LiMnPO.sub.4/C, cell preparation and test conditions were performed according to those in Example 2, 3 and 4.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/050483 | 2/13/2006 | WO | 00 | 10/23/2008 |
