NEW SILICON BASED ELECTRODE FORMULATIONS FOR LITHIUM-ION BATTERIES AND METHOD FOR OBTAINING IT This invention pertains to new electrode formulations for lithium-ion batteries. Today, lithium-ion batteries are widely used in portable electronic devices. Compared to other rechargeable cells, such as nickel-cadmium and nickel metal hydride, Li-ion cells have higher energy density, higher operating voltages, lower self discharge, and low maintenance requirements. These properties have made Li-ion cells the highest performing available secondary battery. The worldwide energy demand increase has driven the lithium-ion battery community to search for new generation electrode materials with high energy density. One of the approaches is to replace the conventional carbon graphite negative electrode material by metal or metallic alloy based on for example silicon (Si), tin (Sn) and/or aluminum (Al). These materials can provide much higher specific and volumetric capacities than graphite. Electrodes for Li-ion batteries are commonly prepared with standard formulations and routine processing conditions. More and more studies report the impact of formulations, morphology, and processing routines on the electrochemical performance of composite electrode. However, formulations and processing of the composite electrodes are strongly influenced by the experimental and technical parameters such as the physical and chemical properties of materials, mixing sequences of material sources, time, temperature, electrodes thickness,... These parameters should be optimized to enhance the composite electrode stability and its electrochemical performances. Different polymers have been studied as binders' additives for negative/positive composite electrodes and as electrolytes host for lithium-ion batteries. One of the most studied polymers is Poly-(ethylene oxide). However, this polymer has some limitations such as its relatively high operating temperature (- 80°C) and its electrochemical instability above 4 V vs LiVLi. Consequently, polymers with high electrochemical stability such as PTFE, PVdF, and PVdF-HFP copolymer have been widely adopted as a binder for composite electrodes in lithium-ion batteries. Remarkable improvement resulted from the use of the PVdF-HFP copolymer in PLIon™ technology. This copolymer has a good distribution of amorphous and crystalline domains which allows a high uptake of liquid electrolyte and provides good mechanical cohesion. However, due to poor chemical properties (bonding effects), environmental issues, and safety aspects with new negative electrode materials these binders are substituted by new binder types such as silica, gelatin, poly-(acrylonitrile-methyl methacrylate) (PAMMA), poly-(methyl methacrylate) (PMMA), polypyrrole, aromatic polyamides, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR). Since safety, cost and environmental issues arise daily, a switch away from organic process using organic solvents became an obligation. Recently, many attempts have been made to switch from non-aqueous to aqueous processes. Si-based negative electrode materials could significantly enhance the energy density of the commercial lithium ion batteries. Silicon has the largest theoretical gravimetric capacity (3579 mAh/g) corresponding to the following reaction: 15Li + 4Si -, Li15Si4 and a large volumetric capacity (2200 mAh/cm3). Unfortunately, it exhibits poor capacity retention. This poor cycle life is also due to the huge volume expansion (+310%) over cycling, as the particles break up and become non-contacted. Several studies have been made to reduce the capacity fading of Si-based electrodes. Nevertheless, the binder has been found to play a key role in stabilizing capacity retention of Si based electrodes. Chen et al. (in: Z. Chen, L. Christensen, and J.R. Dahn; Journal of Electrochemical Society; (150) 1073, 2003) suggested that cycling stability of Si-based electrodes might benefit from the use of elastic binder materials. An elastomeric binder has generally a higher elasticity modulus than PVdF. This allows the composite electrode to expand and shrink more easily and reduces the forces exerted between particles. Li et al. (in: J. Li, R.B. Lewis, and J.R. Dahn; Electrochemical Solid State Letters; (10); 17 (2007)) showed that Si-based electrodes using Na-CMC as binder experienced good cycling performance although Na-CMC is not elastomeric and has a low elongation to break. In Mazouzi et al. (in Electrochemical and solid-state letters, IEEE Service Center, Piscataway, NJ, US, vol.12, no. 11) a nanosilicon-based composite electrode is described prepared using aqueous processing in an acidic medium. Covalent bonding between Si particles and a CMC binder is promoted. In US2006/237697 an electrode material for a rechargeable lithium battery is disclosed, characterized in that said electrode material comprises a fine powder of a silicon-based material whose principal component is silicon element, said fine powder having an average particle size (R) in a range of 0.1 um 90%). In each suspension nano silicon powder, with a specific surface area of 20m21% and oxygen content of 3wt%, is added. Four different suspensions are prepared with pH equal to 3, 3.5, 4.5 and 5. These suspension are ball milled during 15 minutes using the Fritch Pulverisette. Oxygen levels of the silicon powders varies from 8wt% at pH5 up to 18wt%atpH3. Pastes are prepared at these different pH by adding Na-CMC and acetylene black to the suspension of silicon prepared as described in Example 1. The final paste, having a silicon/CMC/acetylene black ratio of 50/25/25, is ball milled for 30 minutes. Coatings with a thickness between 20 and 30pm are deposited on a copper foil by doctor blade coating. The electrodes are subsequently dried in a vacuum oven at 150° C for three hours. Finally coin cell type batteries are prepared in a glove box using Li-foil as counter electrode. Battery tests are performed under similar conditions as explained in Example 1. Table 1 gives an overview of the resulting capacity at the 5th charge. These values are an average of 3 coin cells. It is shown that high capacity values between 3120 mAh/g and 3550 mAh/g are obtained between pH3 and pH5. Example 6 A water based suspension is prepared by adding acetylene black to an aged water based Na-CMC solution having a pH of 8. This suspension is ball milled during 15 minutes using the Fritch Pulverisette 6. In this way the oxygen level is measured and equals 3wt%. Subsequently, a paste is prepared by adding silicon powder to the acetylene black:Na-CMC suspension. The final paste, having a silican/CMC/acetylene black ratio of 50/25/25, is finally ball milled for 30 minutes. Coatings with a thickness between 20 and 30pm are deposited on a copper foil by doctor blade coating. The electrodes are subsequently dried in a vacuum oven at 150° C for three hours. Finally coin cell type batteries are prepared in a glove box using Li-foil as counter electrode. Battery tests are performed on the electrodes under following conditions: between 0.01 and 1.0V at C/20 in which C is defined as a capacity of 3572mAh/g. This resulted in a capacity at the 5th charge that is clearly higher than the value obtained in Example 1. Counter example 7 A silicon suspension is prepared at pH 2.5, leading to an oxygen level of 23 wt%. A paste and coin cells are prepared and battery tests are performed in a similar way as described in Example 1. The resulting capacity at the 5th charge equals 2600 mAh/g. The capacity level is much lower than for lower oxygen contents and this low capacity is unacceptable. Counter example 8 A water based suspension is prepared by adding silicon powder and acetylene black in one step in a water based Na-CMC solution at pH8. The final paste, having a silicon/CMC/acetylene black ratio of 50/25/25, is finally ball milled for 30 minutes. Coatings with a thickness between 20 and 30pm are deposited on a copper foil by doctor blade coating. The electrodes are subsequently dried in a vacuum oven at 150°C for three hours. Finally coin cell type batteries are prepared in a glove box using Li-foil as counter electrode. Battery tests are performed on the electrodes under following conditions: between 0.01 and 1.0V at C/20 in which C is defined as a capacity of 3572mAh/g. This resulted in a low capacity at the 5th charge that is lower than 85% of the capacity obtained in Example 1. Table 1: Capacity of coin cells vs. oxygen content of the silicon in the electrode composition. The exemplification set out herein illustrates preferred embodiments of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. CLAIMS 1. An electrode assembly for a rechargeable Li-ion battery, comprising a current collector provided with an electrode composition comprising nano silicon powder and carboxymethyl cellulose (CMC) binder material, wherein said nano silicon powder is provided with a SiOx layer, with 0