Thursday, April 16, 2015

US app '713 on lithium ion batteries

From the specification:

[0002] The ideal battery for supplying power to standalone electrical devices (such as telephones and laptop computers, portable tools, standalone sensors) or for traction of electrical vehicles would have a long life, would be capable of storing large quantities of energy and power, and would not exhibit any risk of overheating or explosion.

[0003] At the present time, these electrical devices are powered essentially by lithium ion batteries (herein called "Li-ion" batteries) that have the best energy density among the various disclosed storage technologies. However, Li-ion batteries can be made using different architectures and with different chemical compositions of electrodes.

[0004] Processes for making Li-ion batteries are presented in many articles and patents and the "Advances in Lithium-Ion Batteries" book (published by W. van Schalkwijk and B. Scrosati) in 2002 (Kluever Academic/Plenum Publishers) gives a good inventory of these processes.

[0005] Li-ion battery electrodes can be made using coating techniques (particularly roll coating, doctor blade, tape casting). With these processes, active materials used to make electrodes are in the form of powders with an average size of between 5 and 15 .mu.m diameter. These particles are integrated into an ink that is composed of these particles and deposited on the surface of a substrate.

[0006] These techniques can be used to make deposits between 50 and 400 .mu.m thick. The power and energy of the battery can be modulated, by varying the thickness of the deposits and their porosity and the size of active particles.

[0007] Inks (or pastes) deposited to form electrodes contain particles of active materials and also binders (organic), carbon powder to make the electrical contact between particles, and solvents that are evaporated during the electrode drying step. A calendering step is performed on the electrodes to improve the quality of electrical contacts between particles and to compact the deposits. After this compression step, active particles of the electrodes occupy about 60% of the volume of the deposit, which means that there is usually 40% porosity between particles.

[0008] The contact between each particle is essentially a point contact and the electrode structure is porous. Pores are filled by an electrolyte that may be liquid (aprotic solvent in which a lithium salt is dissolved) or in the form of a more or less polymerized gel impregnated with a lithium salt. Li-ion battery electrodes are usually between 50 and 400 .mu.m thick, and lithium ions are transported through the thickness of the electrode through pores filled with electrolyte (containing lithium salts). The lithium diffusion rate in the thickness of the electrode varies depending on the quantity and size of the pores.

[0009] Lithium ions must diffuse both in the thickness of the particle and in the thickness of the electrode (i.e. the coating), to ensure smooth functioning of the battery. Diffusion of active material in the particle is slower than in the electrolyte. Thus, the particle size must be reduced to guarantee good power performances of batteries, and it is between 5 and 15 .mu.m in standard li-ion batteries.

[0010] The power and energy of a battery may be varied by varying the thickness of the deposits, and the size and density of active particles contained in the ink. An increase in the energy density is necessarily at the detriment of the power density. High power battery cells necessitate the use of thin very porous electrodes and separators, while on the contrary an increase in the energy density is achieved by increasing these thicknesses and reducing the porosity. The article "Optimization of Porosity and Thickness of a Battery Electrode by Means of a Reaction-Zone Model" by John Newman, published in J. Electrochem. Soc., Vol. 142, No. 1 in January 1995, shows the corresponding effects of electrode thicknesses and their porosity on their discharge rate (power) and energy density.

[0011] However, increasing the porosity in electrodes tends to reduce the energy density of the battery. If the energy density of the electrodes is to be increased, the porosity has to be reduced. However, in existing Li-ion batteries, the main means by which lithium ions diffuse in the electrode is through pores filled with electrolyte and located between the active particles. If there are no pores filled with electrolyte, lithium ions are transported from one particle to the next only through contact between particles, and this is essentially a point contact. The resistance to transport of lithium ions is such that the battery cannot function.

[0012] Furthermore, in order to function satisfactorily, all the electrode pores must be filled with electrolyte. This is only possible if the pores are open. Furthermore, impregnation of the electrode with electrolyte may become very difficult or even impossible depending on the size of the pores and their tortuosity. When the porosity impregnated with electrolyte reduces, the electrical resistance of the deposit reduces and its ion resistance increases. When the porosity drops below 30% or even 20%, the ion resistance increases strongly because some pores are then likely to close which prevents wetting of the electrode by the electrolyte.

[0013] Consequently, when it is required to make electrode films with no porosity to increase the energy density, the thickness of these films should be limited to less than 20 .mu.m, and preferably less than 10 .mu.m to enable fast diffusion of the lithium ions in the solid without any power loss.

[0014] However, the existing deposition techniques described above are incapable of precisely controlling the thickness of the deposit. Furthermore, the dry extracts used and the associated ink viscosities make it impossible to reduce the thickness below 20 .mu.m.

[0015] Another embodiment of electrode films was disclosed. The objective was to use a vacuum technique to deposit a thin film of electrode materials with lithium insertion. This technique can give dense films with no porosity, and consequently with excellent energy densities and temperature resistance. 


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