Crescita ALD di materiali catodici in film sottili per batterie al litio: caratterizzazione strutturale ed elettroinica di un ossido di maganese e litio (LixMn2O4) con struttura spinello tramite spettroscopia di assorbimento raggi X.

Owing to their high energy density, lithium-ion batteries (LIBs) are currently the method of choice for energy storage for numerous applications and are dominating the rechargeable batteries market. However, it is well accepted that the interfaces of the LIBs electrode materials are the source of problems in terms of capacity fading and safety. Atomic layer deposition (ALD) is considered as a great technique to address these shortcomings, allowing the production of highly uniform and conformal films with an accurate thickness and growth control at the sub-Å level, making it an enabling technology for 3D all-solid state thin-film LIBs. Among the available cathode materials for these batteries, lithium manganese oxide spinel is a promising candidate due to its low cost, low toxicity, high voltage, high specific capacity, and minimal structural distortion during chargedischarge cycling. In the present study, various thin-films samples of Li x Mn 2 O 4 spinel were synthesized by means of ALD. The synthesis process consisted of the deposition of a 100-nm parent MnO 2 thin-film on a Si (100) wafer that was subsequently lithiated using a precursor of lithium tert-butoxide and water (LiO t Bu + H 2 O), enabling the conversion of MnO 2 into Li x Mn 2 O 4 spinel. Samples of varying lithium content were produced by using different lithiation cycles, i.e., 10, 50, 100, 200 and 300 cycles. To gain a fundamental understanding of this processing technique, the local atomic and local electronic structure of the samples were investigated by means of X-ray absorption spectroscopy (XAS). Experimental measurements were done at the CLÆSS beamline from ALBA synchrotron radiation facility in Barcelona, Spain. The spectra were collected at the Mn K-edge in fluorescence and total electron yield mode to account for any difference between the bulk and surface of the lithiated samples. A XANES (X-ray absorption near edge structure) analysis was performed to qualitatively determine the changes in the Mn average oxidation state (AOS) upon increasing lithiation cycles. Likewise, an EXAFS (extended X-ray absorption fine structure) analysis was done to obtain quantitative information of the geometrical changes near the Mn atom by fitting the EXAFS region of the spectra with theoretical models. From the XANES analysis, it was demonstrated that the average oxidation state of Mn decreases as a function of lithiation cycles, with no large differences between the bulk and surface of the samples. From the EXAFS analysis, it is noticeable the presence of three distinct peaks in the R-space plot, corresponding to the Mn-O interaction for the first coordination shell, and a main Mn-Mn interaction for the second and third coordination shell. The position of the first peak is related to the average Mn-O bond length within the primary [MnO 6 ] octahedron. This distance appears to be slightly longer and increasing for more lithiated samples due to the Mn reduction. The position of the second and third peaks correlates mainly to the average Mn-Mn distance between edge-sharing [MnO6] octahedra and between corner-sharing [MnO6] octahedra respectively. The third peak is found to be less visible for more lithiated samples, indicating the reduction of corner-sharing octahedra upon lithiation.