The improvement of volumetric energy density remains a key area of research to opti-mize Li-ion batteries for applications such as extending the range of electric vehicles. There is still improvement to be made in the energy density in the positive elec-trode materials. The current thesis deals with determining the phase diagrams of the Li-Mn-Ni-O and Li-Co-Mn-O systems in order to better understand the structures and the electrochemistry of these materials. The phase diagrams were made through careful analysis of hundreds of X-ray di raction patterns taken of milligram-scale combinatorial samples. A number of bulk samples were also investigated. The Li-Mn-Ni-O system is of particular interest as avoiding cobalt lowers the cost of the material. However, this system is very complex: there are two large solid-solution regions separated by three two-phase regions as well as two three-phase regions. Comparing quenched and slow cooled samples shows that the system trans-form dramatically when cooled at rates typically used to make commercial materials. The consequences of these results are that much of the system must be avoided in order to guarantee that the materials remain single phase during cooling. This work should therefore impact signi cantly researchers working on composite electrodes. Two new structures were found. The first was Li-Ni-Mn oxide rocksalt structures with vacancies and ordering of manganese which were previously mistakenly identi ed as LixNi2xO2. The other new structure was a layered oxide with metal site vacancies allowing manganese to order on two superlattices. The electrochemistry of both these materials is presented here. Finally, the region where layered-layered composites form during cooling has been determined. These materials were long looked for along the composition line from Li2MnO3 to LiNi0.5Mn0.5O2 and the most significant consequence of the actual locations of the end-members is that one of the structures contains a high concentration of nickel on the lithium layer. Layered-layered nano-composites formed in this system are therefore not ideal positive electrode materials and it will be demonstrated that single-phase layered materials lead to better electrochemistry.

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24925号 / エネ博第467号 / 新制||エネ||87(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 萩原 理加, 教授 佐川 尚, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Ce travail présente les études électrochimiques et structurales menées sur deux systèmes : P2/O3-NaxMn1-yFeyO2 et O’3-NaxNiO2 utilisés en tant que matériaux d’électrode positive pour batteries Na-ion.Concernant le système P2/O3-NaxMn1-yFeyO2, l’étude par diffraction des rayons X menée in situ pendantla charge de batteries a montré de nombreuses transitions structurales. Que leur structure soit de type P2ou O3, les matériaux présentent une phase distordue pour les taux d’intercalation (x) les plus élevés etune phase très peu ordonnée pour les taux d’intercalation les moins élevés. Entre ces deux étatsd’intercalation, les phases de type P2 présentent moins de transitions que les phases de type O3. Celaentraine de meilleures propriétés électrochimiques pour les phases de type P2 (meilleure capacité endécharge, meilleure rétention de capacité…). Les spectroscopies d’absorption des rayons X et Mössbauerdu 57Fe ont montré que les couples redox Mn4+/Mn3+ et Fe4+/Fe3+ sont impliqués lors du cyclage, à bas ethaut potentiel, respectivement.Concernant O’3-NaNiO2, la diffraction des rayons-X menée in situ pendant la charge de batteriesO’3-NaNiO2//Na a montré de nombreuses transitions structurales O’3 ↔ P’3 résultant du glissement desfeuillets MO2. Ces transitions s’accompagnent de mises en ordre Na+ - lacunes dans le matériau. La tailledes grains a montré avoir un intérêt majeur puisqu’elle influe sur le nombre de phases présentessimultanément dans le matériau. Lorsque la batterie est déchargée, la phase limitante Na≈0.8NiO2 estobservée et empêche le retour à O’3-NaNiO2 / This work concerns the electrochemical and structural studies carried out on two systems used aspositive electrode materials for Na-ion batteries: P2/O3-NaxMn1-yFeyO2 and O’3-NaxNiO2. Concerning theP2/O3-NaxMn1-yFeyO2 systems, in situ X-ray diffraction carried out during the charge of the batteriesshowed that both materials undergo several structural transitions. Both the P2 and O3 phases show adistorted phase for the higher intercalation rates (x) and a poorly ordered phase for the lower ones.Between these two states, P2-based materials exhibit less structural transitions than the O3-based ones.This is correlated to the better electrochemical properties the P2-based materials exhibit (better dischargecapacity, better capacity retention…). X-ray absorption and 57Fe Mössbauer spectroscopies showed thatthe Mn4+/Mn3+ and Fe4+/Fe3+ redox couples are active upon cycling, respectively at low and high voltage.Concerning O’3-NaNiO2, in situ X-ray diffraction carried out during the charge of O’3-NaNiO2//Nabatteries showed several structural transition between O’3 and P’3 structures, resulting from slab glidings.These transitions are accompanied by Na+ - vacancies ordering within the “NaO6” slabs. Upon discharge,the material does not come back to its initial state and, instead, the Na≈0.8NiO2 phase represents themaximum intercalated state. The occurrence of this limiting phase prevents O’3-NaNiO2 to be consideredas an interesting material for real Na-ion applications.

Emerging energy storage applications are driving the demand for Li-ion battery positive electrode materials with higher energy densities and lower costs. The recent production of complete pseudo-ternary phase diagrams of the Li-Mn-Ni-O system generated using combinatorial methods has provided a greater understanding of the impact of initial composition, synthesis temperature, and cooling rate on the phases that form in the final materials. This thesis focuses on the synthesis and characterization of gram-scale positive electrode materials in the Li-Mn-Ni-O system. Structural analysis of these samples has resulted in the production of partial pseudo-ternary phase diagrams focusing on the positive electrode materials region of the Li-Mn-Ni-O system at 800°C and 900°C in air for both quenched and slow cooled compositions. These bulk-scale diagrams support the observations of the combinatorial diagrams, and show similar layered and cubic structures contained within several single- and multi-phase regions. The phases that form at each composition are shown to be dependent on both the reaction temperature and cooling rate used during synthesis. The electrochemical characterization of two composition series near Li2MnO3, one quenched and one slow cooled, is presented. The quenched compositions exhibited reversible cycling at 4.4 V, voltage plateaus and small increases in capacity above 4.6 V, and large first cycle irreversible capacity losses at 4.8 V. In the slow cooled series, all but one composition exhibited initial capacities below 100 mAh/g which began to continually increase with cycling, with several compositions exhibiting capacity increases of 300% over 150 cycles at 4.9 V. In both series, analysis of the voltage and differential capacity plots indicated that significant structure rearrangements are taking place in these materials during extended cycling, the possible origins of which are discussed. Finally, high precision coulometry studies of one Li-deficient and two Li-rich single-phase layered compositions are discussed. These materials exhibit minimal oxidation of simple carbonate-based electrolyte when cycled to high potential, with the Li-deficient composition producing less electrolyte oxidation at 4.6 V vs. Li/Li+ than commercial Li[Ni1/3Mn1/3Co1/3]O2 at 4.2 V. The inherent inertness of this composition may make it suitable for use as a thin protective layer in a core-shell particle.