Wet-chemical synthesis of thin film materials for 3D all-solid-state Li-ion batteries

Abstract

With this thesis, the use of wet-chemical synthesis routes within the field of 3D thin film all-solid-state batteries was explored. This battery concept combines both high energy density and power density in the form of 3D architectures. Although the concept is simple, deposition of the materials involved is challenging. In this thesis, this topic is addressed with wet-chemical synthesis routes combined with spray-deposition, because of i) low cost, ii) scalabil-ity and iii) ease of multi-metal material deposition. Successful im-plementation of 3D thin film all-solid-state Li-ion battery concept would be very rewarding, since lifetime and safety of batteries would be improved tremendously. Two materials classes were selected to be used as solid electrolyte within this study. First lithium lanthanum titanate (Li0.35La0.55TiO3) was studied. By spin and spray-deposition of a nitrate-citrate pre-cursor, high quality films were obtained with thicknesses between 90 to 120 nm. Although highly conductive tetragonal phase was obtained at higher annealing temperatures (700°C), morphological issues such as pin-holes could not be prevented. While preparing the material in amorphous form at 500°C, a conductivity of 3.8·10-8 S·cm-1 was measured. However, the electrochemical stability was limited due to Ti4+ reduction at 1.5 V (vs. Li/Li+). With spray-coating, two different synthesis routes yielded 3D coatings on vari-ous 3D architectures. With a conformality of over 30%, this proved to be a unique approach to obtain 3D structured thin film solid electrolytes. In an effort to resolve the issue of electrochemical stability of the solid electrolyte, the synthesis of Li-stuffed garnet lithium lanthanum zirconate (Li7La3Zr2O12) was studied. However, due to crystallization issues and demanding thermal budgets, this proved to be a less suitable material. Tungsten oxide (WO3) was prepared as a negative electrode to be combined with the less electrochemically stable lithium lanthanum titanate, in order to study functional half-cells. Two different routes were studied for WO3 thin film synthesis. First, a colloidal suspen-sion was prepared an studied, identifying the reaction mechanism 261 responsible for chloride removal from the tungsten-chloride based precursor. Spray-deposition of the crystalline, rod-shaped WO3 particles yielded smooth WO3 films of 150 nm thickness. Electro-chemical activity of the films was achieved at 400°C and higher, with stable cycling performance shown for 100 cycles on (oxida-tive) TiN current collectors, with capacities over 600 mAh·cm-3. To establish higher capacities, lithium doping of WO3 was studied, yielding the Li2W2O7 phase. Contraction and expansion effects ren-dered this material unsuitable for thin-film Li-ion batteries. For 3D deposition of WO3, a second (citrate) precursor was developed. Within thermal constraints, active WO3 was achieved on 3D archi-tectures. A 2.6 fold 3D capacity enhancement was booked with TiN as a current collector with current densities up to 10C. While using spray-deposited indium tin oxide (ITO) as a current collector, a 4.5 fold capacity increase was achieved at current densities of up to 4C. Finally, the combination of an electrolyte with a 3D structured elec-trode was shown with a lithium lanthanum titanate – WO3 stack. Although rate performance was reduced by the addition of the solid electrolyte, as well as a 30% capacity drop, a unique 3D half-stack was achieved completely by spray-deposition. The research done in this study increases the knowledge on 3D depositions via spray-deposition. Various (gelating) precursors appeared to show similarities in deposition behaviour, whereas colloidal suspensions to be less suitable. In addition, combining materials to form all-solid-state half cells is a unique achievement, especially considering the low-cost and highly scalable methods used. Therefore, this this is considered a major step forward for wet-chemical synthesis of battery materials, as well as realization of a 3D thin-film all-solid-state Li-ion battery.