Development of artificial surface layers for thin film cathode materials
Développement de surface artificielles pour cathode sous forme de couche mince pour accumulateurs Li-ion
Résumé
The present work was based on the investigation of different thin film components of Li ionbatteries. A first part was dedicated to the deposition of cathodes in thin film form of aknown material, LiCoO2, and an alternative one, Li(NiMnCo)O2 employing physical vapordeposition (PVD) and chemical vapor deposition (CVD), respectively. Results on thedeposition of LiCoO2 showed how after cycling there is a reduction of rate capability andincrease in interface resistance. The X-ray diffraction pattern showed the presence of severalorientations related to the known HT phases found in literature for LiCoO2 with lowcrystallinity. On the other hand Li(NixMnyCoz)O2 thin films prepared via aerosol assistedCVD were analyzed with X-ray diffraction and Rietveld refinement using the March-Dollasemodel for the determination of the texturing and microstructural characteristics. Additionallythe morphology of the films was characterized using scanning electron microscopy. Theinvestigation showed that concentration of precursor solution and process pressures have asignificant effect on the film morphology and texturing. A second part was focused on the cathode-electrolyte interface for three case studies: 1) asdeposited LiCoO2 cathode thin film, 2) ZrO2 coated LiCoO2 thin film and 3) LiPON coatedLiCoO2 thin film. The interface cathode-electrolyte of these three cases were studied beforeand after galvanostatic cycling to determine surface layer characteristics and changes arisingon the interface after battery operation. The interface of a bare LiCoO2 layer was furtherstudied after soaking in liquid electrolyte to elucidate the effect of short storage procedures inbatteries.Surface analysis done on LiCoO2 thin films showed changes occurring at the interface layersafter the electrode was in short contact with the electrolyte solution and after galvanostaticcycling. Washing and soaking the electrode material in electrolyte and solvent showed thatsurface reactions start from the first contact. A main component of the electrolyte solution,LiPF6, has critical effect since it can decompose and form HF which reacts with carbonatesand forms LiF on the surface. Given the large amount of LiF, a high reactivity of LiCoO2 withthe decomposed species was observed, as the main components of the film were related to thedecomposed LiPF6 salt.The surface chemistry of the layer formed on LiCoO2 after cycling was mainly based ondecomposed species from the electrolyte salt arising from carbonated and fluorinated species.Artificial surface layers were deposited on LiCoO2 by means of rf sputtering. The thin layersof ZrO2 and LiPON used as coatings had minor effects on the original film morphology andcrystalline structure. An XPS analysis of the interface showed how the nature of each layerafter galvanostatic cycling was different for each case. The resulting artificial surface layerformed from ZrO2 coating showed mainly inorganic species, while the LiPON coatedcathode showed an organic nature. The final surface layers after electrochemical cycling ofthe ZrO2 coated film resembled that of the uncoated LiCoO2. Additionally, LiPON thin films were studied on the basis of structural changes occurringwith nitrogenation and its correlation to a possible mechanism during ion conduction.Composition of phosphate glasses with rf sputtering was proven to be greatly influenced bythe gas ratio employed. The largest variations were observed for lower amounts of N2 in thegas mixture. The IR spectra results showed important differences in the short range order forfilms with a similar amount of lithium. The lithium phosphorus oxynitride films depositedhere presented glassy structures with mainly ortho and pyro-phosphate units with smallamounts of short metaphosphate chains. Nitrogen insertion favors stability of lithium bygiving an environment with lower potential energy, as was evidenced by the far-IR results.
Ce travail porte sur la recherche de différentes compositions de couches minces pouraccumulateurs Li-ion.Une première partie a été dédiée au dépôts de cathode sous forme de couche mince d’unmatériau connu, LiCoO2, et d’un matériau alternatif, Li(NiMnCo)O2 en utilisant le dépôtphysique en phase vapeur (PVD) et le dépôt chimique en phase vapeur (CVD),respectivement. Les résultats (LiCoO2) ont montrés comment, après cyclage, il y a diminutionde la capacité à cycler à régime rapide et augmentation de la résistance à l’interface. Ladiffraction des rayons X a montré la présence de différentes orientations, peu cristallisées,appartenant à la phase LiCoO2 HT selon confirmation par la littérature. Les couches mincesde Li(NixMnyCo1-x-y)O2 ont été préparées par dépôt chimique en phase vapeur assisté paraérosol. La diffraction des rayons X et l’analyse Rietveld utilisant le modèle March-Dollase aété mise en oeuvre pour la détermination de la texture et des caractéristiques microstructurales.La morphologie des films a été caractérisée par microscopie électronique à balayage. L’étudea montré que la concentration de la solution de précurseur et la pression totale ont un effetmajeur sur la morphologie des films et leur texture.Une seconde partie s’est focalisée sur l’interface cathode-électrolyte pour trois cas d’étude : 1)couche mince de matériau de cathode LiCoO2, 2) couche mince de LiCoO2 recouvert de ZrO2et 3) couche mince de LiCoO2 recouvert de LIPON. L’interface cathode-électrolyte de cestrois cas d’étude a été étudiée avant et après cyclage galvanostatique afin de déterminer lescaractéristiques de la couche de surface et les changements provenant à l’interface lors dufonctionnement de l’accumulateur. L’interface des couches minces de LiCoO2 a été étudiéeplus en détail après trempage dans un électrolyte liquide afin de comprendre l’effet desprocédures de stockages courts dans les accumulateurs.De plus, les couches minces de LiPON ont été étudiées sur la base de changementsstructuraux se produisant avec la nitruration et sa corrélation à un possible mécanisme ayantlieu durant la conduction ionique.
Origine : Version validée par le jury (STAR)