Submicron sized silicon powder with low oxygen content TECHNICAL FIELD AND BACKGROUND This invention relates to submicron sized silicon based powders having low oxygen content and the synthesis of this powder using a gas phase technology. Silicon powders are currently developed and used in a wide variety of applications including lithium-ion batteries, printed electronics and solar applications. These applications require ultrafine powders with low oxygen content. Lithium-ion batteries are the most widely used secondary systems for portable electronic devices. Compared to aqueous 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 t o replace the conventional carbon graphite negative electrode material by another better performing active material, being a metal, metalloid or metallic alloy based on silicon (Si), tin (Sn) or aluminum (Al). These materials can provide much higher specific and volumetric capacity compared to graphite. On top of the specific composition of the negative electrode material, the surface properties of the particles are playing a key role in the electrochemical behaviour of the resulting Li-ion battery. Therefore, it is of paramount importance to be able to optimize those parameters in order to enhance the electrochemical performances of the negative electrode. The composite electrode needs to posses mixed conductivity with both ionic lithium and electrons. Such a complex medium is generally obtained by mixing together the active material particles with different additives such as a very fine powder of carbon black and a polymeric binder. The binder additive has a complex role since it not only gives mechanical strength to the composite electrode but also allows for a good adhesion between the electrode layer and the current collector, and it gives the composite electrode a sufficient liquid electrolyte uptake to provide internal ionic percolation. As mentioned 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 Li 15Si4 and a large volumetric capacity (2200 mAh/cm 3). However, the microscopic structure of these materials and their huge volume expansion upon lithium intercalation had never allowed reaching acceptable life characteristics for their use in rechargeable cells. The synthesis of materials at the submicron scale allows to overcome the main drawbacks of these materials and makes them suitable candidates for the replacement of carbon. An interesting method t o prepare submicron powders is plasma technology, as is disclosed in WO 2008/064741 A 1. Unfortunately, these submicron silicon powders rapidly oxidize when exposed to air. This uncontrolled oxidation of submicron sized silicon powder finally results in oxygen contents above 10 wt%. This high oxygen level will have a negative impact on the electrochemical behaviour of these Si based powders in Li-ion batteries, generating high capacity losses during first cycling (the so called irreversible capacity) because of the reduction of this layer. It is an aim of the present invention to improve or even overcome these problems, and t o provide for better negative electrode materials that can be manufactured by a simple and economical process. SUMMARY Viewed from a first aspect, the invention can provide a submicron sized Si based powder having an average primary particle size between 20 nm and 200 nm, wherein the powder has a surface layer comprising SiO , with 0