Core-shell lithium transition metal oxides. TECHNICAL FIELD AND BACKGROUND The invention relates to cathode material for rechargeable lithium batteries, particularly lithium nickel manganese cobalt oxides being ceated with a fluorine containing polymer and heat treated afterwards. Previously LiCoO2 was the most used cathode material for rechargeable lithium batteries. However, recently a substitution of LiCoO2 by lithium nickel oxide based cathodes and by lithium nickel manganese cobalt oxides is in full progress. In these substitute materials, depending on the choice of metal composition, different limitations occur or challenges need to be solved. For simplicity reasons, the term "Lithium nickel oxide based cathodes" will be further referred to as TNO", and "lithium nickel manganese cobalt oxides" will be further referred to as "LMNCO". One example of an LNO material is LiNio.soCoo.15Alo.05O2 - It has a high capacity, however it is difficult to prepare, since typically a carbon dioxide free atmosphere (oxygen) is needed and special carbonate free precursors like lithium hydroxide are used instead of lithium carbonate. Hence such manufacturing restraints tend to increase the cost of this material considerably. LNO is a very sensitive cathode material. It is not fully stable in air, which makes large scale battery production more difficult, and - caused by its lower thermodynamic stability - in real batteries it is responsible for a poor safety record. Finally, it is very difficult to produce lithium nickel oxide with a low content of soluble base. By "soluble base" is meant lithium located near to the surface that is less stable thermodynamically and goes into solution, whilst lithium in the bulk is thermodynamically stable and cannot be dissolved. Thus a gradient of Li stability exists, between lower stability at the surface and higher stability in the bulk. The presence of "soluble base" is a disadvantage because a high base content is often connected with problems during battery manufacturing: during slurry making and coating high base causes a degradation of the slurry {slurry instability, gelation) and high base is also a responsible for poor high temperature properties, like excessive gas generation (swelling of the batteries) during high temperature exposure. By determining the "soluble base" content by pH titration, based on the ion exchange reaction (LiMO2 + dH+ ? Li1-dHdMO2 + d Li+), the Li gradient can be established. The extent of this reaction is a surface property. In US2009/0226810A1 the problem of soluble base is further discussed: LiMO2 cathode material is prepared using mixed transition metal hydroxides as precursors. These are obtained by co- precipitating transition metal sulphates and technical grade bases like NaOH, which is the cheapest industrial route for LiM02 precursor preparation. This base contains CO32" anion in the form of Na2CO3, which is trapped in the mixed hydroxide - the mixed hydroxide typically containing between 0.1 and 1 wt% of CO32-. Besides the transition metal precursor, the lithium precursor Li2CO3, or a technical grade LiOH*H2O, containing at least 1wt% of Li2CO3 is used. In the case of high nickel cathodes LNO, when the lithium and transition metal precursors are reacted at high temperature, typically above 700°C, the Li2CO3 impurity remains in the resulting lithium transition metal oxide powder, especially on its surface. When higher purity materials are used, less Li2CO3 impurity is found, but there is always some LiOH impurity that reacts with CO2 in the air to form Li2CO3. Such a solution is proposed in JP2003-142093, however the use of expensive precursors of very high purity is not preferred. An example of LMNCO is the well known Li1+xM1-xO2 with M=Mn1/3Ni1/3Co1/3O2 , where the manganese and nickel content is about the same. "LMNCO" cathodes are very robust, easy to prepare, have a relatively low content of cobalt and thus generally tend to cost less. Their main drawback is a relatively low reversible capacity. Typically, between 4.3 and 3.0V the capacity is less than or about 160 mAh/g, compared with 185-195 mAh/g for LNO cathodes. A further drawback of LMNCO compared with LNO is the relatively low crystallographic density, so the volumetric capacity is also less; and a relatively low electronic conductivity. In between LNO and LMNCO type materials we can situate "Nickel rich lithium nickel manganese cobalt oxides" Lii+xMi-xO2 where M=Ni1-x.yMnxCoy or M=Ni1-x.y-2MnxCoyAlz , with Ni: Mn larger than 1, having typically values for Ni:Mn of 1.5 to 3, and a Co content "y" typically between 0.1 and 0.35. For simplicity we refer to this class of materials as "LNMO". Examples are M=Ni0.5Mn0.3Co0.2, M=Ni0.67Mn0.22Co0.11, and M=Ni0.6Mn0.2Co0.2. Compared with LNO, LNMO can be prepared by standard processes (using a Li2CO3 precursor) and no special gas (such as oxygen as mentioned above) is needed. Compared to LMNCO, LNMO has a much higher intrinsic capacity and possibly a lower tendency to react with electrolyte (which is normally characterized by dissolution of Mn) at elevated temperature. Thus it becomes apparent that LNMO will possibly play a major role in the substitution of LiCoO2. Generally, the base content increases, and the safety performance tends to deteriorate with increasing Ni:Mn ratio. On the other hand it is widely accepted that high Mn content helps to improve safety. A high base content is related to moisture sensitivity. In this regard LNMO is less moisture sensitive than LNO but more sensitive than LMNCO. Directly after preparation, a well prepared LNMO sample has a relatively low content of surface base, and if it is well prepared most of the surface base is not L2CO3 type base. However, in the presence of moisture, airborn CO2 or organic radicals reacts with LiOH type base to form Li2CO3 type base. Similar, the consumed LiOH is slowly re-created by Li from the bulk, thus increasing the total base (total base= mol of Li2CO3 + LiOH type base). At the same time, the moisture (ppm H2O) increases. These processes are very bad for battery making. Li2CO3 and moisture are known to cause severe swelling, and to deteriorate the slurry stability. Hence it is desired to decrease the moisture sensitivity of LNMO and LNO materials. In US2009/0194747A1 a method to improve the environmental stability of LNO cathode materials is described. The patent discloses a polymer coating of nickel based cathode materials, in the form of a single layer of non-decomposed polymer. The polymers (e.g. PVDF) are chosen from binders typically used in the manufacturing (slurry making for electrode coating) of lithium ion batteries. Thermal stability (safety) is related to interfacial stability between electrolyte and cathode material. A typical approach to improve the surface stability is by coating. Many different examples of coatings are available in literature and especially in patent literature. There are different ways to categorize coatings. For example, we can distinguish between ex-situ and in- situ coating, in ex-situ coating a layer is coated onto the particles. The coating can be obtained by dry or wet coating. Generally the coating is applied in a separate process involving at least the coating step and generally an additional heating step. Thus the total cost of the process is high. Alternatively, in some cases an in-situ coating - or self organized coating - is possible. In this case the coating material is added to the blend before cooking and during cooking separate phases form, preferable the coating phase becomes liquid, and if the wetting between LiMO2 and the coating phase is strong then a thin and dense coating phase ultimately covers the electrochemical active LiMO2 phase. Evidently, in-situ coating is only efficient if the coating phase wets the core. One can also distinguish between cationic and anionic coating. An examples for cationic coating is Al2O3 coating. Examples for anionic coating are fluoride, phosphate, silicate coating and the like. Fluoride coating is especially preferred because a protecting film of LiF is formed. Thermodynamically LiF is very stable, and does not react with electrolyte, thus LiF coating is very promising to achieve a good stability at high temperature and voltage. A typical method, such as used by Croguennec et al. in Journal of The Electrochemical Society, 156 (5) A349-A355 (2009), is the addition of LiF to the lithium transition metal oxide to achieve the protecting LiF film. However, due to the high melting point of LiF and also due to poor wetting properties,, it is not possible to obtain a thin and dense UF film. Croguennec reports that, instead of a coating, small particles or 'sheets' can be found in the grain boundaries of the LiMO2 particles. Further possible methods are the use of MgF2, AIF3 or lithium cryolite. One can further distinguish between inorganic and organic coating. An example of organic coating is a polymer coating. One advantage of polymer coating is the possibility of obtaining an elastic coating. On the other hand, problems arise from poor electronic conductivity, and sometimes the poor transport of lithium across the polymer. Generally, polymer coating more or less adheres to the surface, but it does not chemically change the surface. There cannot be found any experimental data in the prior art that would show that the above described approaches are effective to improve the cited problems of LNO and LNMO materials. To summarize: 1) LMNCO is a robust material but has severe capacity limitations. 2) It is desired to increase the thermal stability and to reduce the base content of LNO. 3) It is desired to increase the thermal stability and reduce the base content of LNMO. It is an aim of the present invention to improve or even overcome the problems cited before, and to provide for high capacity alternatives for LMNCO materials. SUMMARY Viewed from a first aspect, the invention can provide a lithium transition metal (M) oxide powder for use in a rechargeable battery composed of primary particles having a surface coated with LiF, characterized in that the soluble base content of said lithium transition metal oxide powder, determined by pH titration, is less than 60%, and preferably less than 50%"of the soluble base content of said lithium transition metal oxide powder having uncoated primary particles. In one embodiment, the surface of said lithium transition metal oxide powder is substantially free of lithium hydroxide and lithium carbonate. The LiF layer consists of a reaction product of a fluorine-containing polymer and the primary particle surface, where the lithium of the LiF originates from the primary particles surface. The fluorine in the reaction product LiF originates from fully decomposed fluorine-containing polymer. In one embodiment the LiF film has a thickness of at least 0.5 nm, or at least 0.8 nm, and even at least 1 nm. An example of the lithium transition metal oxide can be either one of: - LiCodMeO2, wherein M is either one or both of Mg and Ti, with e<0.02 and d+e=1; - Li1+aM1-aO2±b M1kSm with -0.030, b">0, c">0 and a"+b"+c"=1; and a"/b">l. In another embodiment 0.51 are particularly suitable for use in lithium-ion prismatic or polymer batteries. In another example of cathode material, the fluorine-containing polymer contains at least 50% by weight of fluorine. A typical example for such a polymer is a PVDF homopolymer or PVDF copolymer (such as HYLAR ® or SOLEF® PVDF, both from Solvay SA, Belgium). Another known PVDF based copolymer is for example a PVDF-HFP (hexa-fluoro propylene). Such polymers are often known under the name "Kynar®". Teflon - or PTFE - could also be used as polymer. Viewed from a second aspect, the invention can provide a process for covering a lithium transition metal (M) oxide powder with a LiF coating, comprising the steps of: - providing a transition metal precursor, such as MOOH, prepared from the co-precipitation of transition metal sulphates with a base, such as NaOH, - providing a lithium precursor being either one of LiOH*H2O or LiNO3, both containing a carbonate impurity, or said lithium precursor being Li2CO3, - reacting said transition metal precursor and said lithium precursor at a temperature above 600°C, thereby obtaining said lithium transition metal (M) oxide powder carrying a Li2CO3 impurity, - mixing said powder with a fluorine-containing polymer, and - heating said powder-polymer mixture at a temperature of at least 140eC, and at most 300 or even at most 210°C, above the melting temperature of said fluorine-containing polymer. The NaOH used generally also contains a carbonate ion impurity, which is especially relevant for LNO materials for reasons described above, and which presence is also neutralized by the process provided by the invention. In this example process a Li2CO3 impurity reacts with said polymer, said polymer is decomposed, and a UF coating is formed on the surface of said transition metal (M) oxide powder. In particular, the Li2CO3on or near to the surface of the transition metal oxide powder reacts with the polymer, providing lithium for the LiF layer and CO2 gas. In an example process embodiment, the amount of fluorine-containing polymer in the powder- polymer mixture is between 0.1 and 2 wt%. In another embodiment it is between 0.2 and 0.5 wt%. In another example embodiment the LiF film has a thickness of at least 0.5 nm, or at least 0,8 nm, and evert at least 1 nm. One example process uses a fluorine-containing polymer such a PVDF, and the powder-polymer mixture is heated at a temperature higher than 325°C and less than 380°C for at least one hour, and, in a particular embodiment, between 340 and 360T for at least one hour. An example of the lithium transition metal oxide used in the process is either one of: - LiCodMeO2, wherein M is either one or both of Mg and Ti, with e<0.02 and d+e=1; - Li1+aM'1-aO2±b M1kSm with -0.030, b">0, c">0 and a"+b"+c"=1; and a"/b">1. In another embodiment 0.5sa"<0.7, 0.10, b">0, c">0 and a"+b"+c"=1; and a'Vb" > 1. 6. The lithium transition metal oxide powder of clause 5, wherein 0.50, b">0, c">0 and a"+b"+c"=1; and a'Vb" > 1. 18. The process according to clause 16, wherein 0.5sa"s0.7, 0.1 Mn, Co, Mg and Ti; M1 comprises one or more elements of Ca, Sr, Y, La, Ce and Zr, with 00, b">0, c">0 and a"+b"+c"=1; and a"/b" > 1. 5. A lithium transition metal oxide powder according to claim 4, characterized in that 0.5=a"=0.7, 0.10, b">0, and a"+b"+c"=1; and a"/b" > 1. 12. A process according to claim 10, characterized in that 0.5=a"=0.7, 0.1