Ni based cathode material for rechargeable lithium-ion batteries TECHNICAL FIELD AND BACKGROUND This invention relates to a high Ni-excess "NMC" cathode material having a particular composition. By "NMC" we refer to lithium nickel manganese cobalt oxide. The high Ni-excess NMC powder can be preferably used as a cathode active material in rechargeable lithium-ion batteries. Batteries containing the cathode material of the invention show excellent performance, such as high reversible capacity, improved thermal stability during high temperature storage, and good long-term cycle stability when cycled at a high charge voltage. Lithium-ion battery technology is currently the most promising energy storage means for both electro-mobility and stationary power stations. LiCo02 (doped or not - hereafter referred to as "LCO"), which previously was the most commonly used as a cathode material, has a good performance but is expensive. In addition, since cobalt resources are gradually depleted, lithium nickel cobalt aluminum oxide or lithium nickel manganese cobalt oxide (hereafter referred to as "NCA" and "NMC" respectively - note that both can be doped) have become prospective candidates of replacing LCO. These materials have a high reversible capacity, a relatively high volumetric energy density, good rate capability, long-term cycle stability, and low cost. NMC cathode materials can (approximatively) be understood as a solid state solution of LiCo02, LiNio.5Mno.5O2 and LiNi02, corresponding to the general formula Lii+a[Niz(Ni0.5 Mn0.5)yCox]i-aO2, where "z" stands for the so-called Ni-excess, as is defined below, as Ni is 100% divalent (Ni2+) in LiNi0.5Mno.502 and Ni is 100% trivalent (Ni3+) in LiNi02. At 4.3 V the nominal capacity of LiCo02 and LiNi0.5Mn0.5O2 is about 160mAh/g, against 220mAh/g for that of LiNi02. Typical NMC based materials are expressed as LiM'02, where M'=NiX'MnyCozand can be referred to as "111" material with M'=N'\1/3Mn1/3Co1/3, "442" with M'=Ni0.4Mno.4COo.2, "532" with M'=Ni0.5Mno.3COo.2, or "622" with M'=Nio.6Mn0.2COo.2. M' can be doped with dopants "A" such as Al, Ca, Ti, Mg, W, Zr, B, and Si, resulting in the formula Li1_a((Niz(Ni0.5 Mn0.5)yCox)1-k Ak)1+a02. The reversible capacity of (undoped) NMC cathode materials can be roughly estimated from these capacities. For example, NMC 622 is comprehended as 0.2 LiCo02 + 0.4 LiNio.5Mn0.502 + 0.4 LiNi02. The expected capacity equals 0.2 x 160 + 0.4 x 160 + 0.4 x 220=184mAh/g. The capacity increases with "Ni-excess". For example, the Ni-excess is 0.4 in NMC 622. If we assume lithium stoichiometry with Li/(Ni+Mn+Co) = 1.0, then "Ni-excess" is the fraction of 3-valent Ni. Figure 1 shows the expected capacities as a function of Ni-excess. Here, the x-axis is the Ni-excess ("z") and the y-axis is the calculated reversible capacity. Additionally, the price of Ni and Mn is much lower than that of Co. Therefore, the cost of the cathode per unit of delivered energy is greatly reduced by using Ni and Mn instead of Co. According to '2020 cathode materials cost competition for large scale applications and promising LFP best-in-class performer in term of price per kWh' announced at the OREBA 1.0 conference on May 27, 2014, the metal price per cathode capacity of LCO is 35 $/kWh, while for NMC 111 it is 22 $/kWh. As the Ni content of NMC increases, the metal price per cathode capacity also increases because the Ni price is higher than the Mn price, but it does not reach the cost of LCO. Therefore, Ni-excess NMC with higher energy density and lower process cost -by contrast to LCO- is more preferred in today's battery market. Large-scale manufacturing of NMC demands that it is easy to prepare and produce high-quality cathode materials. As the Ni-excess in the cathode materials is increased - which is desired from a capacity point of view - the production becomes more difficult. As an example - very high Ni-excess cathode materials like NCA - LiNio.sCoo.15Alo.05O2 cannot be prepared in air or using Li2C03 as a lithium source. If Li2C03 is used as a lithium precursor, the carbonate needs to decompose and C02 is released into the gas phase. However, the C02 equilibrium partial pressures of very high Ni-excess cathode materials are very small. Thus, the gas phase transport of C02 limits the reaction kinetics and the C03 decomposition occurs very slowly - even in pure oxygen. Furthermore, very high Ni-excess cathodes have low thermodynamic stability. A fully reacted and fully lithiated very high Ni-excess cathode will even decompose when heated in normal air. The C02 partial pressure of air is high enough so that the C02 extracts lithium from the crystal structure and forms Li2C03. Therefore C02 free gas, typically oxygen, is required during the production of very high Ni-excess cathodes. This causes higher production cost. Additionally, as the use of Li2C03 is not possible as the lithium source, lithium precursors like Li20, LiOH-H20 or LiOH need to be applied instead of the cheaper Li2C03, which increases production cost further. In addition, the transition metal precursors - for example mixed transition metal hydroxide - need to be free of carbonate. Finally, when using lithium hydroxide (LiOH-H20 or LiOH), the low melting point of lithium hydroxide is a point of concern. Whereas Li2C03 tends to react before melting, lithium hydroxide tends to melt before reacting. This causes many unwanted effects during a mass production process, like inhomogeneity of products, impregnation of the ceramic saggers with molten LiOH, and etc. In addition, during the manufacturing of high Ni-excess NMC, Ni ions tend to migrate into the Li site which severely limits the actual capacity, so it is difficult to have an appropriate stoichiometry. This problem also affects the reversibility of the intercalation mechanism, leading to capacity fading. It can be summarized that the increased capacity of the very high Ni-excess cathode materials like NCA comes at a significant production cost. Another issue of very high Ni-excess cathodes is the content of soluble base. The concept of "soluble base" is explicitly discussed in e.g. WO2012-107313: the soluble base refers to surface impurities like Li2C03 and LiOH. Because of the low thermodynamic stability of Li in Ni-excess cathode materials, remaining carbonate decomposes very slowly or C02 being present in the air is easily adsorbed and forms Li2C03 on the surface of cathodes. Additionally, in the presence of water or moisture, Li is easily extracted from the bulk, resulting in formation of LiOH. Thus, undesired "soluble bases" occur easily on the surface of very high Ni-excess cathodes like NCA. In the case of very high Ni-excess, there are many possible sources of carbonate impurity. Specifically, the soluble bases can originate from the mixed transition metal hydroxides that are used as the transition metal source in the production. The mixed transition metal hydroxide is usually obtained by co-precipitation of transition metal sulfates and an industrial grade base such as sodium hydroxide (NaOH). Thus, the hydroxide can contain a C032" impurity. During sintering with the lithium source, the residual CO32" reacts with lithium and creates Li2C03. As LiM'02 crystallites grow during sintering, the Li2C03 base will be accumulated on the surface of these crystallites. Thus, after sintering at high temperature in a high Ni-excess NMC, like NMC 622, carbonate compounds remain on the surface of the final product. This base can dissolve in water, and therefore the soluble base content can be measured by a technique called pH titration, as discussed in US7,648,693. Soluble bases, in particular residual Li2C03, are a major concern since they are the cause of poor cycle stability in lithium ion batteries. Also, it is not clear if very high Ni-excess is sustainable during large-scale preparation, because materials used as precursors are air sensitive. Therefore, the preparation of very high Ni-excess cathode materials is performed in C02 free oxidizing gas (typically 02) to reduce the soluble base content at increasing temperature. LiOH-H20 is also used as the lithium source instead of Li2C03 to reduce the soluble base content. A typical process to prepare high Ni-excess NMC using LiOH-H20 is for example applied in US2015/0010824. LiOHH20 with a low Li2C03 impurity as the lithium source is blended with the mixed transition metal hydroxide at the target composition, and sintered at high temperature under an air atmosphere. In this process, the base content of high Ni-excess NMC final product (like NMC 622) is much reduced. There are two major trends to achieve a high energy density with Ni-excess in NMC. One trend is to increase the Ni-excess up to very high values in order to achieve high capacities at normal change voltage. The second trend is to increase the charge voltage in order to achieve high capacities with less Ni-excess. NCA, for example, has a very high Ni-excess of around 0.8 as all Ni is 3-valent. In NC91 (LiNio.9Coo.1O2), the Ni-excess is even 0.9. These cathode materials have very high capacities even at relatively low charge voltage. As an example - NC91 has a capacity as high as 220mAh/g at 4.3V in a coin cell testing with lithium as a counter electrode. As discussed before, it is difficult to produce such cathode materials in a mass production process at reasonable cost. Additionally, we observe the issue of poor safety. The safety issue of charged batteries is a general concern. The safety is related to a process called thermal runaway. Due to exothermic reaction, the battery heats up and the reaction rate inside the battery increases, causing the battery to explode by thermal runaway. The thermal runaway is mostly caused by electrolyte combustion. If the battery is fully charged and the cathodes are in the delithiated state, the values of "x" in the resulting Lii-xM'02 are high. These highly delithiated cathodes are very unsafe when in contact with electrolyte. The delithiated cathode is an oxidizer and can react with the electrolyte which acts as the reducing agent. This reaction is very exothermic and causes thermal runaway. In the ultimate case, the battery will explode. In a simple way, it can be explained that the electrolyte is combusted using the oxygen which is available from the delithiated cathode. Once a certain temperature in the battery has been reached the cathodes decompose and deliver oxygen which combusts the electrolyte. After the reaction - as Ni is stable in the divalent state and there is large Ni-excess - most of the transition metal is 2 valent. Schematically - each mol of cathode can deliver one mol oxygen to combust the electrolyte: Ni02 + electrolyte -> NiO + combustion products (H20, C02). The other trend to achieve a high energy density is to set the Ni-excess at more intermediate values but to apply a high charge voltage. Typical values for the Ni-excess range from about 0.4 to about 0.6. This region we will be referred as "high Ni-excess". The reversible capacity at 4.2 or 4.3V of high Ni-excess NMC is less than that of "very high" Ni-excess compound (with Ni-excess >0.6). To achieve the same state of charge (i.e. remaining Li in the delithiated cathode) like very high Ni-excess cathode (fx. NCA), a battery with high Ni-excess cathode (fx. NMC622) needs to be charged to a higher voltage. A similar state of charge could, for example, be obtained at 4.2V for NCA and 4.35V using NMC622. Thus, to improve the capacity of "high Ni-excess" NMC, higher charge voltages are applied. Even at the high charge voltage, the resulting delithiated high Ni-excess cathodes are safer than the delithiated very high Ni-excess cathodes mentioned above at lower voltage. Whereas Ni based cathodes tend to form NiO during the oxygen combustion reaction, Ni-M' tends to form more stable M'304 compounds during the delithiation process. These compounds have a higher final oxygen stoichiometry thus less oxygen is available to combust the electrolyte. A schematic example for a cathode without Ni-excess is LiMno.5Nio.5O2 -> Mno.5Nio.5O2 + electrolyte -> 0.5 NiMn03 + combustion products (H20, C02). In this case, 0.5 oxygen is available to combust the electrolyte as only 50% of the transition metal is divalent after the combustion reaction. This is different from the case of very high Ni-excess cathodes discussed above, where almost 1 mol is available. In principle, the 2nd trend could be extended towards still less Ni-excess cathodes. Cathode materials with only a small Ni-excess could be charged to still higher voltages. As an example, NMC532 could be charged to about 4.45V or NMC442 to about 4.55V to achieve a similar capacity. In this case - due to the lower content of Ni the safety of the delithiated cathodes is expected to improve further and also the production process is simplified. However, this approach is not feasible as current electrolytes are not working well at these very high charge voltages, and thus, a poor cycle stability is observed. The current invention refers to the 2nd trend, applying higher charge voltages to cathode materials not having very high (> 0.6) but only high Ni-excess (0.4-0.6). As both the Ni content and the charge voltage increase, it is difficult to obtain good safety and a cheap preparation process. From the prior art it is thus known that high Ni excess materials have many issues for a successful preparation and application in Li ion batteries. Therefore, to make high Ni excess materials acceptable, it is necessary to provide such cathode materials having optimized NMC compositions and enhanced battery performances, where a high reversible capacity is achieved together with good cycle stability and safety. SUMMARY Viewed from a first aspect, the invention can provide a positive electrode material for lithium ion batteries, comprising a lithium transition metal-based oxide powder having a general formula Li1+a ((Niz (Ni0.5Mn0.5)yCOx)i-kAk)1-a 02, wherein A is a dopant, with -0.025 M'(OH)2 + Na2S04" takes place. Many precipitation device designs are possible. A continuously stirred tank reactor (CSTR) process provides a continuous process which both supplies the feed solution and collects the overflow continuously. Alternatively, the design can be a batch-process where the precipitation is stopped after the reactor is filled. It can also be the combination of batch and thickening processes where more precipitate accumulates in the reactor, because liquid (after sedimentation or filtering) is removed, but the majority of solid remains in the reactor during the process. In this way, the feed of M'S04 and NaOH into the reactor can continue for a longer time. During the precipitation reaction conditions like RPM of the stirrer, pH of the tank, flow rates and flow rate ratios, residence time and temperature etc. are kept well controlled to obtain a high quality mixed transition metal hydroxide product. After precipitation the obtained mixed transition metal hydroxide is filtered, washed and dried. Thus, the mixed transition metal hydroxide is achieved. The mixed transition precursor will be the precursor for the sintering process that follows. As the mixed transition metal precursors may be prepared by a precipitation method, the target transition metal composition M' in the precipitated M'-hydroxide has a Co content of 0.18 to 0.22 mol and it contains a Ni-excess (= Ni - Mn) of 0.42 to 0.52. Furthermore the Ni to Mn ratio is between 3.15 to 4.25. The transition metal composition can thus be written as Niz(Nio.5Mno.5)yCOx where 0.422 (Li/M'=1.01) is obtained according to the same method as in EX1.1, except that M' in M'(OH)2 is Nio.65Mn0.ioCo0.25 (Ni-excess=0.55) and the 2nd sintering temperature is 800°C. Comparative Example 3 CEX3 with a composition Lii. 005M0.995O2 (Li/M'—1.01) is prepared according to the same method as in EX1.1, except that M' in M'(OH)2 is Nio.e5Mno.175Coo.175 (Ni-excess=0.48) and the 2nd sintering temperature is 825°C. Comparative Example 4 CEX4 with a composition Li l.oosM'o.ggsC^ (Li/M'—1.01) is obtained according to the same method as in EX1.1, except that M' in M'(OH)2 is Nio.6Mn0.2Coo.2 (Ni-excess=0.4) and the 2nd sintering temperature is 860°C. Comparative Example 5 CEX5 with a composition LiM'02 (Li/M'=1.00) is obtained according to the same method as in EX1.1, except that M' in M'(OH)2 used as precursor is Nio.esMno.^Coo^ (Ni-excess=0.56) and the 2nd sintering temperature is 820°C. Comparative Example 6 CEX6 with formula Lio.ggsM'i.oosC^ (Li/M'=0.99) is obtained according to the same method as in EX1.1, except that M' in M'(OH)2 is Nio.7Mno.15Coo.15 (Ni-excess=0.55) and the 2nd sintering temperature is 830°C. The initial discharge capacities and capacity fading of comparative examples CEX1 to 6 are measured according to the same method as in EX1. So too are the slope of the example, which means the cycle stability, the storage property at 80°C for 2 weeks, and the carbon content. The initial discharge capacity, capacity fading, slope, recovered capacity, and carbon content are shown in Table 5. Example 2 EX2.1, which is an industrial scale product, is prepared according to the above-mentioned "Manufacturing Example". A mixed nickel-manganese-cobalt hydroxide (M'(OH)2) is used as a precursor, where M'(OH)2 is prepared by a co-precipitation process in a large-scale continuous stirred tank reactor (CSTR) with mixed nickel-manganese-cobalt sulfates, sodium hydroxide and ammonia. In the 1st blending step, 5.5kg of the mixture of M'(OH)2, wherein M'=Ni0.625Mn0.175Coo.2o (Ni-excess=0.45), and Li2C03 with Li/M' ratio of 0.8 is prepared. The 1st blend is sintered at 885°C for 10 hours under the dry air atmosphere in a chamber furnace. The resultant lithium deficient sintered precursor is blended with LiOH-H20 in order to prepare 4.5 kg of the 2nd blend of which Li/M' is 1.045. The 2nd blend is sintered at 840°C for 10 hours in a dry air atmosphere in a chamber furnace. The above prepared EX2.1 has the formula Lii.022M'0.978O2 (Li/M'=1.045). EX2.2, which is an aluminum coated lithium transition metal oxide, is prepared by the following procedure. 1.3kg of EX2.1 is blended with 0.26g of aluminum oxide. The blend is heated at 750°C for 7 hours in a chamber furnace. The heated aluminum coated lithium transition metal oxide is sieved with a 270 mesh (ASTM) sieve. EX2.3, which is an aluminum coated lithium transition metal oxide containing LiNaS04 as a secondary phase, is prepared by the following procedure. 4.0kg of EX2.1 is blended with 8.0g of aluminum oxide to prepare the 1st blend. The 1st blend is blended with a Na2S208 solution (48g Na2S208 powder in 140ml water) by a high RPM blender to prepare the 2nd blend. The 2nd blend is heated at 375°C for 6 hours. The heated aluminum coated lithium transition metal oxide containing LiNaS04 as a secondary phase is sieved using a 270 mesh (ASTM) sieve. The initial capacities and capacity fading of EX2.1, EX2.2 and EX2.3 are measured according to the same method as in EX1 and are shown in Table 5. Full cell testing of EX2.1, EX2.2 and EX2.3 are performed following the above mentioned full cell testing method, yielding a number of cycles at 80% of recovered capacity that is given in Table 5. Comparative Example 7 CEX7.1, which is an industrial scale product, is prepared according to the above-mentioned "Manufacturing Example". A mixed nickel-manganese-cobalt hydroxide (M'(OH)2) is used as a precursor, where M'(OH)2 is prepared by a co-precipitation process in a large-scale continuous stirred tank reactor (CSTR) with mixed nickel-manganese-cobalt sulfates, sodium hydroxide and ammonia. In the 1st blending step, 5.5kg of the mixture of M'(OH)2, wherein M'=Nio.6Mn0.2Co0.2 (Ni-excess=0.40), and Li2C03 with a Li/M' ratio of 0.85 is prepared. The 1st blend is sintered at 900°C for 10 hours under a dry air atmosphere in a chamber furnace. The resultant lithium deficient sintered precursor is blended with LiOH-H20 in order to prepare 3.0kg of the 2nd blend with a Li/M' ratio of 1.055. The 2nd blend is sintered at 855°C for 10 hours under a dry air atmosphere in a chamber furnace. The above prepared CEX7.1 has the formula Li1027 M'0.973O2(Li/M'=1.055). CEX7.2, which is an aluminum coated lithium transition metal oxide, is prepared by the following procedure. 1.3kg of EX7.1 is blended with 0.26g of aluminum oxide. The blend is heated at 750°C for 5 hours in a chamber furnace. The heated aluminum coated lithium transition metal oxide is sieved with a 270 mesh (ASTM) sieve. Comparative Example 8 CEX8, which is an industrial scale product, is prepared according to the above-mentioned "Manufacturing Example". A mixed nickel-manganese-cobalt hydroxide (M'(OH)2) is used as a precursor, where M'(OH)2 is prepared by a co-precipitation process in a large-scale continuous stirred tank reactor (CSTR) with mixed nickel-manganese-cobalt sulfates, sodium hydroxide and ammonia. In the 1st blending step, 5.5kg of the mixture of M'(OH)2, wherein M'=Nio.7oMn0.i5Co0.i5 (Ni-excess=0.55), and LiOH-H20 with Li/M' ratio of 0.85 is prepared. The 1st blend is sintered at 800°C for 10 hours under an oxygen atmosphere in a RHK (roller hearth kiln). The resulting lithium deficient sintered precursor is blended with LiOH-H20 in order to prepare 3.0kg of a 2nd blend with a Li/M' ratio of 0.99. The 2nd blend is sintered at 830°C for 10 hours under an oxygen atmosphere in a chamber furnace. The above prepared CEX8 has the formula Lio.995 M'i.oo502 (Li/M'=0.99). Initial capacities and capacity fading of CEX7.1, CEX7.2 and CEX8 are measured according to the same method as in EX1 and are shown in Table 5. Full cell testing of CEX7.1, CEX7.2 and CEX8 are performed following the abovementioned full cell testing method, yielding a number of cycles at 80% of recovered capacity that is given in Table 5 and in Figure 9. As shown in Table 5, EX1.1 is compared with examples with higher and lower Co content. First, if the Co content is higher, such as for CEX2, the cycle stability decreases due to its lower Mn content. Conversely, if the Co content is lower, such as for CEX3, structural stability during cycling is deteriorated. Even though CEX3 has high Ni-excess of 0.48, it has a lower discharge capacity and worse cycle stability to keep the fixed charge capacity. Next, EX1.1 is compared to examples with low and high Ni-excess. If the Ni-excess is lower, such as CEX4, the capacity at a fixed voltage is lower. Additionally, to achieve the high charge capacity (200mAh/g), higher charge voltage is applied, resulting in poor cycle stability. Conversely, if the Ni-excess is higher, such as CEX5 and CEX6, they have a higher discharge capacity. Accordingly, to obtain the high charge capacity, a lower charge voltage is applied. However, the safety still deteriorates and the cycle stability is lower compared to the EX1.1. In addition, a higher Ni-excess NMC compound (CEX5) exhibits poor thermal stability. Furthermore, EX1.1 is compared with examples with higher and lower molar ratio of Ni/Mn. As shown in Table 5, if the ratio of Ni/Mn is too high, such as for CEX2, the discharge capacity is high but the cycle stability deteriorates. Conversely, if the ratio of Ni/Mn is too low, such as CEX4, the discharge capacity is low even at high voltage. Accordingly, NMC compounds, such as EX1.1, with Ni/Mn of 3.15-4.25, show higher capacity and better cycle stability. Figure 3 shows the discharge capacities of the examples measured by "Testing Method 1". The values of DQ1 are indicated by the shading in the different regions using commercial software Origin 9.1 - contour plot. In this figure, the x-axis is for the Ni-excess (z) and the y-axis is for the Co/M' (mol/mol%) in the NMC compounds. As the Ni-excess increases, the capacity also increases. The NMC compounds that have a discharge capacity above about 180mAh/g correspond to compositions with high capacity. We observe an optimum of capacity at Co/M' = 20 mol/mol%, higher capacities are achieved with less Ni-excess. Next, Figure 4 shows the capacity fade rate of the examples measured by "Testing Method 1". The values of 1C/1C QFad. in % per 100 cycles are indicated by the shading in the different regions using commercial software Origin 9.1 - contour plot. In this figure, the x-axis is for Ni-excess (z) and the y-axis is for the Co/M' content (mol/mol%) in the sample. The samples that have a capacity fading below about 20 mol/mol% have a composition with improved cycle life. We observe a certain optimum of Co composition. With increasing Ni-excess, better cycle stabilities are observed at about 20 mol/mol% Co/M'. Moreover, Figures 5a, 5b (exploded view of upper left corner of Fig. 5a) & 6 show the slope of the examples measured by "Testing Method 2". In Figure 5a & 5b, the x-axis gives the cycle number and the left and right y-axis are for discharge capacity and real cut-off charge voltage, respectively. In these figures, the values of slope (mV/cycle) are calculated according to the equation in "Testing Method 2". For example, EX1.1 has 4.6317V at cycle 14 and its number of cycles (N) is 23 until reaching 4.7V. The cycle stability of EX1.1 is measured by a slope (S) calculated as follows: (4.7000 V - 4.63177 at 14 cycles) 1000 (mV) S = ^—TT^ r^ x —7-777;— = 7.6 mV/cycle 23-14 (cycle) 1 (V) ' y Furthermore, CEX3 has 4.6045V at cycle 14 and its number of cycles is 19 until reaching 4.7V. The slope of CEX3 is calculated as follows: (4.7000 V - 4.6045 at 14 cycles) 1000 (mV) s = ~ 77<—77T7 rv—" -x—, ,„s = 19.1 mV/cycle 19 - 14 (cycle) 1 (V) ' y In Figure 6, the values of slope (mV/cycle) are indicated by the shading in the different regions using commercial software Origin 9.1 - contour plot. In this figure, the x-axis is for Ni-excess (z) and the y-axis is for the Co/M' content (mol/mol%) in the sample. As shown the figures, the samples that have a slope below about 16 mV have a composition with enhanced cycle stability. We observe that the slope gets worse as Ni-excess decreases, if the Ni-excess is below 0.42 and Co is below 0.18 or above 0.22 the slope is too large. Additionally, Figure 7 shows the recovered capacity of the examples measured by "Testing Method 3". The values of R.Q. in % are indicated by the shading in the different regions using commercial Software Origin 9.1 - contour plot. In this figure, the x-axis is for Ni-excess (z) and the y-axis is for the Co/M' content (mol/mol%). The samples that have a recovered capacity above about 70% have a composition having a good storage property at high temperature. It can be concluded from Figures 3 to 7 that the best one of the optimized compositions is that of samples having a Co/M' content of 20 mol/mol% and z=0.45, as all the criteria described above are met by this composition. Figure 8 shows the DSC spectra of EX1.1, CEX4 and CEX5. In this figure, the x-axis is for temperature (°C) and the y-axis is for heat flow (W/g). The main exothermic peak, starting at about 180°C and reaching a maximum at about 250°C to 264°C, results from structural changes of the delithiated cathode, accompanied by oxygen release and subsequent combustion of the electrolyte by oxygen. Especially, as the Ni content in NMC increases, the temperature of the main peak continuously decreases and the evolved exothermic heat continuously increases, which indicates a worse safety. CEX5 with high Ni-excess (0.56) has a lower exothermic peak temperature and higher exothermic reaction enthalpy than the other examples. These examples show that as the Ni-excess increases the thermal stability of the charged cathode materials significantly deteriorates. Therefore, an increased capacity not only reduces the cycle stability but also reduces the safety. Accordingly, from these examples EX1.1 has an optimized composition with enhanced cell performances and high thermal stability. To further identify the electrochemical properties of the samples of Example 1, NMC samples having various Li/M' ratio are investigated by "Testing method 1" and "Carbon Analysis". As described in Table 5, if the ratio of Li/M' is too high, such as CEX1, the reaction between the mixed transition metal source and the lithium source doesn't finish and results in unreacted and molten lithium sources. Therefore, the remaining lithium cause a large amount of carbon to exist in the final NMC product, and a low discharge capacity results. On the other hand, if the ratio of Li/M' is too low, i.e. below 0.95, the lithium stoichiometry within the crystal structure is less than desired. XRD diffraction data (not shown here) allow to conclude that as a result of the low Li/M' more transition metals are located on lithium sites thus blocking the Li diffusion pathways. This causes a lower reversible capacity as well as poor cycle life. Therefore, the samples in EX1 with Li/M' of 0.95-1.05 have a specific composition with enhanced electrochemical performance, such as high capacity, good cycle stability and high thermal stability. EX2.1, CEX7.1, 7.2 and CEX8 were prepared at a scale using processes which are compatible with industrial production. The results of coin cell tests by the test method 1 and full cell tests (see Fig. 9) indicate that the above conclusion about the Ni-excess of around 0.45 being the best amongst the optimized NMC compositions is still valid in the industrial products. Figure 9 and Table 5 further show that EX2.2 and EX2.3 have superior electrochemical properties, which indicates that the electrochemical performance can be further improved by surface modification technologies such as an aluminum coating. Figure 10 shows the correlation between capacity fading (1C/1C QFad.) from coin cell test method 1 and full cell cycle life. The x-axis is the capacity fading (1C/1C QFad.) in %/100 cycles from coin cell test method 1 and the y-axis is the number of cycles at 80% of the initial full-cell discharge capacity. It indicates that the results from coin cell test method 1 can represent the electrochemical properties of real batteries. We Claim: 1. A positive electrode material for lithium ion batteries, comprising a lithium transition metal-based oxide powder having a general formula Li1+a ((Niz(Ni0.5Mn0.5)yCOx)i-kAk)1-a 02, wherein A is a dopant, -0.025