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Numerous preparation techniques for perovskite-based MIEC materials have been developed over the last years. Apart from the conventional methods of cathode preparation, including high temperature calcination, mechanical grinding of powders, screen printing and sintering of thick fi lm layers, alternative synthesis and deposition routes are available. Nanocrystalline LSC or La 1−x Sr x Co 1−y Fe y O 3− δ thin fi lms on electrolyte substrates were already fabricated by pulsed laser deposition, [6][7][8][9] DC, and RF sputtering, [ 10,11 ] spray pyrolysis, [ 12 ] and sol-gel deposition. [13][14][15][16][17] In this work the chemical and structural properties and the stability of nanoscaled La 0.6 Sr 0.4 CoO 3− δ LSC thin fi lm cathodes on polycrystalline Gd 0.1 Ce 0.9 O 1.95 (CGO) substrates were investigated thoroughly. The fabrication by a low-temperature sol-gel technique is favorable for tailoring the grain size, porosity and cathode-fi lm thickness. [ 18 ] The electrochemical properties of the nanoscaled LSC thin fi lm cathodes were already reported by Hayd et al. who presented an outstanding electrochemical performance and extraordinary low area specifi c resistances as low as ASR chem = 0.023 Ω cm 2 at an operating temperature of 600 ° C. [ 19,20 ] Interestingly enough, theoretical models predicted ASR chem values up to one order of magnitude larger than the experimental data. For obvious reasons, the excellent performance of our nanoscaled cathodes does not solely depend on microstructure (porosity, increased inner surface area) as reported in previous work, [ 21 ] but presumably on the enhanced catalytic properties of the La 0.6 Sr 0.4 CoO 3− δ . However, the outstanding electrochemical performance motivates the analysis of the microstructure and phase composition of the LSC cathodes presented in this work.The nanoscaled LSC thin-fi lm cathodes were studied by transmission and scanning transmission electron microscopy (TEM/STEM) combined with energy-dispersive X-ray spectroscopy (EDXS). Moreover, high-angle annular dark-fi eld (HAADF) STEM tomography was used to study the distribution of the pores and quantify the porosity, which was not reported for nanoscaled (La,Sr)CoO 3 -based materials in literature before. Although several techniques like μ -tomography, [ 22 ] FIB-tomography, [ 23,24 ] transmission X-ray microscope based X-ray tomography, [ 12 , 25 ] mercury intrusion, [ 22 , 26 ] nitrogen adsorption, [ 27 ] or the Archimedes method [ 22 , 28 ]

Combining the use of nickel-rich layered oxide cathode materials with the implementation of aqueous electrode processing can pave the way to cost-reduced and environmentally friendly electrodes and simultaneously increase the energy density of cells. Herein, LiNi0.33Co0.33Mn0.33O2 (NCM111), LiNi0.6Co0.2Mn0.2O2 (NCM622), LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.8Co0.15Al0.05O2 (NCA) were evaluated in terms of their response to aqueous processing under the same conditions to facilitate a direct comparison. The results illustrate that mainly nickel driven processes lead to lithium leaching which is combined with the increase of the pH value in the alkaline region. For NCA an additional aluminum-involving lithium leaching mechanism is assumed, which could explain the highest amount of leached lithium and the additional detection of aluminum. Electrochemical tests show a reduced capacity for cells containing water-based electrodes compared to reference cells for the NCM-type materials which increases during the first cycles indicating a reversible Li+/H+-exchange mechanism. In contrast, the NCA cells were completely electrochemically inactive making NCA the most water sensitive material tested in this report. By comparing the cycling performance of cells containing aqueous processed electrodes, a more pronounced capacity fade for nickel-rich cathode materials as compared to their reference cells can be observed.

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The successful implementation of an aqueous-based electrode manufacturing process for nickel-rich cathode active materials is challenging due to their high water sensitivity. In this work, the surface of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) was modified with a lithium phosphate coating to investigate its ability to protect the active material during electrode production. The results illustrate that the coating amount is crucial and a compromise has to be made between protection during electrode processing and sufficient electronic conductivity through the particle surface. Cells with water-based electrodes containing NCA with an optimized amount of lithium phosphate had a slightly lower specific discharge capacity than cells with conventional Nmethyl-2-pyrrolidone-based electrodes. Nonetheless, the cells with optimized water-based electrodes could compete in terms of cycle life. the active material by applying surface coatings seems to be very promising. [6-11,16] LiNiCoAlO 2 (NCA) has attracted significant attention as a cathode active material because of its high energy density. [17] However, NCA is known to be extremely sensitive to moisture, [15,18-20] making it a difficult candidate for the aqueous electrode processing. According to the results in the literature, it is assumed that aqueous processing of NCA will not be successful without additional surface modifications, prior to electrode fabrication [10,11] or in situ surface modification during processing [8,9] or in a combination of both. [21] The strong PÀ O-bonding energy in the PO 4 3À ion gives metal phosphates high structural stability against chemical attack. [22-24] Various phosphate coatings such as Ni 3 (PO 4) 2 , [25] FePO 4 , [26] LiMnPO 4 , [27] MgHPO 4 , [28] BiPO 4 , [29] Li 1.3 Al 0.3 Ti 1.7 (PO 4) 3 , [30] Li 3 PO 4 , [23,31] Co 3 (PO 4) 2 , [32,33] LiFePO 4 , [34] and AlPO 4 [31,33,35] have been studied on NCA and resulted in improved electrochemical performance. However, to the best of the authors' knowledge, a phosphatecoated NCA has never been used in a combination with a waterbased electrode manufacturing process. Amongst the phosphate coatings mentioned above, Li 3 PO 4 is relatively easy to synthesize and, in contrast to other metal phosphates such as Ni 3 (PO 4) 2 , Co 3 (PO 4) 2 , BiPO 4 , FePO 4 , MgHPO 4 , and AlPO 4, a lithium-ion conductor. [36,37] The latter aspect might comparatively facilitate the migration of lithium ions through the particles surface. Therefore, in this study, the surface of LiNi 0.8 Co 0.15 Al 0.05 O 2 particles was modified by applying Li 3 PO 4 coatings via a simple precipitation reaction. The modified particles are compared with pristine NCA in terms of their processability in water and their electrochemical performance in cells. Finally, the cycle stability of cells with electrodes prepared via an aqueous and the conventional NMP route as reference is investigated.