Mankind has started taking the global warming problem into consideration, and has been doing so for decades. The relevance of the problem is slowly reaching everyone’s mind, which has resulted in fast growth in important related research topics, among which energy materials. One of the most important challenges has to do both with transportation and the production of polluting gases. In the last 3 decades, lithium ion batteries have grown to be a key technology, and are without doubt one of the most promising energy carriers for transportation and grid energy storage. Despite the technology being well established, the performance of lithium ion batteries still could (and should) be improved on a number of points, such as energy density, lifespan, efficiency and safety. These are mainly materials challenges. This thesis looks at the positive electrode material as a way of obtaining a higher energy density in lithium ion batteries. This thesis was written within the EuroLiion project, which has the goal to develop a new Li- ion cell for traction purposes, such as a car pulling up, with the following characteristics: High energy density of at least 200 Wh/kg Low cost, i.e. a maximum of 150 Euro/kWh. Chapter 1 provides a history of batteries, introduces some generalities and basic knowledge about batteries. In the end, it presents an overview of the EuroLiion project. Chapter 2 gives an overview of the known technologies in terms of lithium nickel manganese oxide (LNMO), as well as an extensive list of the coatings and dopings that can be applied to this material in order to improve it. From this research and its results, in terms of particle size and its effect on performance, the effects of the different coatings, etc..., the positive electrode material for the whole project is chosen. Chapter 3 describes the production of different variations of the material, and an evaluation of their behaviour. The results of an in-situ X-Ray absorption Spectroscopy study are presented. The effect of the oxidation state of the chromium doping on the LNMO, while the battery is cycled, is studied. SEM and EDX pictures of the material and its doping are also shown. In chapter 4, in order to measure the enhancement due to the chromium doping, while having a topographic picture of the interface between the positive electrode and the electrolyte, an AFM analysis is presented. A new way is introduced to monitor ‘in vivo’ the ion and electronic transport between the positive electrode and the electrolyte. Chapter 5 is a direct continuation of chapter 4. Indeed, due to lack of results to explain the enhancement of the material by the chromium doping, more structural measurements and surface characterizations, such as SEM, EDX, XAS, and XPS, are presented and explained. The correlation between the lithium content and the nickel content at different states of charge is also explained. Finally, at this stage of the project, the material has been improved and the reasons and scientific explanations of why this material is better are known. Development is now finalized and the material is in production by a Dutch company and sold to the contributors of the EuroLiion project for testing in prototypes. Chapter 6 explains the whereabouts of the whole project, including external work on the other parts of the project, such as electrolyte, anode, packaging and economic details. In conclusion, during these 4 years of research, I synthetized, developed and tested a material that gathers all the desired performances. When my material has been chosen to be implemented and used by the EuroLiion partners, I also had a technical and scientific support for the industrial scale production of the material, which was well very rewarding and allowed me to gain some knowledge and be in close cooperation with the industry. To create a more powerful and efficient battery, the other elements need to be improved, in order for all parts of the cell to work in harmony and deliver the best performances possible.