High voltage negative active material for a rechargeable lithium battery The invention relates to active material for the negative electrode of secondary rechargeable lithium batteries, wherein the active material is based on doped or undoped carbon-bearing lithium titanium ramsdellite oxide with general formula Li2Ti3O7 or Li2.28Ti3.43O8 Anode materials for rechargeable lithium batteries are generally selected from the carbon group. Carbon materials might have security issues in extreme conditions. First, while charging at very fast rates and/or at low temperature, Li can deposit at the surface of carbon and hence the formation of lithium dendrites can induce soft short. Second, abusive overheating induces the dissolution of the passivation layer made of the reduction products of electrolyte's solvents at the potential of graphite; and the resulting continuous reduction of solvents can be a first step to thermal run away. Numerous efforts have been made to find alternative electrochemical active anode materials to replace graphite. Notably, lithium titanium oxides, such as the spinel phase Li4Ti5O12 as related in Journal of Electrochemical Society 141 (1994) L147, or the ramsdellite phase Li2Ti3O7 as reported in Material Research Bulletin 32 (1997) 993, have been proposed due to several advantages versus carbon: i.e. a higher average voltage around 1.5V vs. Li, improving the security while cycling, a low irreversible loss and a lower polarization. The spinel structure inserts lithium in a two-phase process due to the spinel to rocksalt phase transition, presenting a 1.55V vs. Li plateau, and acquiring a maximum capacity of 175 Ah/kg, whilst the ramsdellite inserts lithium topotactically in a solid solution with a flat S-shape charge-discharge curve corresponding to a one-phase process at a voltage range of 1-2V vs Li. Lithium titanate oxide (Li2Ti3O7) is regarded as promising negative electrode material because of the low cost of production, and the non-toxicity of CONFIRMATION COPY titanium. While the theoretical capacity is 198 Ah/kg, in practice the reversible capacities are between 120 and 130 Ah/kg for low current densities (C/15) and attain only 110 Ah/kg at higher current densities (C). As a consequence the reversible capacity, the polarisation observed upon lithium insertion and the required high temperature for the firing process strongly limit the application field of this compound. A lower synthesis temperature and better cyclability at low current density can be achieved by substitution of a small amount of Ti4+ by Fe3+ in Li2Ti3O7, using a ceramic route. However, the first discharge curve shows a plateau due to the transformation Fe3+/Fe2+, which limits the reversible capacity, and the other performances are not improved, compared with Li2Ti3O7. According to EP1623473 B1, the reversible capacity can be improved to 140 Ah/kg when the lithium titanium oxide having the ramsdellite structure, according to the general formula Li2+vTi3-wFexMyM'zO7. α, is co-substituted with one or two of the following elements: Ti3+, Co2+, Co3+, Ni2+, Ni3+ Cu2+, Mg2+, Al3+, In3+, Sn4+, Sb3+, Sb5+ Substituted materials are obtained at lowered synthesis temperatures, which decreases the production cost. Furthermore, according to PCT/EP2008/009763, when the active material contains a carbon richer phase with general formula Li2+v-4cCcTi3.wFexMyM,zO7-α, and containing two of the following elements: Ti3+, Co2+, Co3+, Ni2+, Ni3+ Cu2+, Mg2+, Al3+, ln3+, Sn4+, Sb3+, Sb5+, the specific reversible capacity is increased to 190 Ah/kg, close to the theoretical value of the ramsdellite lithium titanate . The electrochemical results show an electrochemical curve having a two-step voltage profile, one between 2.2 and 1.6 V and the second under 1.5 V. The material was obtained by grinding and mixing a lithium, titanium and iron compound, a C precursor compound, and a M and M' compound, followed by a sintering process at elevated temperature in a neutral atmosphere. It is an object of the present invention to further improve the performances in terms of high energy and high specific power, whilst respecting the safety of use and the environment, all of this at a reasonable cost. This is obtained by a composite negative active material for a lithium battery, comprising a carbon substituted ramsdellite phase having a general formula Li2-4cCcTi3O7, with 0.1 < c < 0.5, and a spinel phase having a general formula Li1+xTi2-xO4 with 0 0; x+y+z = w and 0 < w < 0.3; 0.1 < c < (2+v)/4, and a is related to the formal oxidation numbers n and n' of M and M' by the relation 2a = -v+4w-3x-ny-n'z, where n and n' are the formal oxidation numbers of M and M' respectively. Preferably M and M' are different metals selected from the list consisting of Ti3+, Co2+ Co3t, Ni2+, Ni3+, Cu2+, Mg2+, Al3+, ln3+, Sn4+, Sb3+, Sb5+; and preferably M= Ni2+ and M' = Al3+ The active material described above preferably has a carbon content of 1.0 to 1.5 wt%. Also preferred is a spinel content between 5 and 16 mole%, and even between 8 and 11 mole%. The invention also covers a secondary rechargeable battery having an anode material described before. The negative active electrode material according to the invention is constituted of a composite material containing principally the undoped or doped C-bearing Li2Ti307 ramsdellite phase and a second phase of spinel type Li1-xTi2-XO4, with 0 0.8 %wt, is compensated by the formation of the lithium rich spinel phase Li4Ti50i2 or LiVxTi2x04 0 0; x+y+z = w and 0 < w < 0.3; 0.1 < c < (2+v)/4; and a is related to the formal oxidation numbers n and n' of M and M' by the relation 2a = -v+4w-3x-ny-n'z, where n and n' are the formal oxidation numbers of M and M' respectively. 3. The active material of claim 2, wherein M and M' are different metals selected from the list consisting of Ti3+ Coz+, Co3+, Ni2+, Ni3+, Cu2+, Mg2+, Al3+ ln3+, Sn4+, Sb3+, Sb5+; and preferably M= Ni2+ and M' = Al3+ 4. The active material of any one of claims 1 to 3, having a carbon content of 1.0 to 1.5 wt%. 5. The active material of any one of claims 1 to 4, having a spinel content between 5 and 16 mole%, and preferably between 8 and 11 mole%. 6. The composite negative active material according to claim 1, comprising at least 99 mole%, and preferably at least 99.9 mole% of both of said carbon substituted ramsdellite phase having a general formula Li2.4cCcTi3O7 and said spinel phase having a general formula Li1+xTi2-xO4 with 0