Microstructural characterization and multiscale ionic conductivity in lithium and sodium-based solid state electrolytes
Creators: name: Fritsch, Charlotte Alina
Publisher: name: Karlsruher Institut für Technologie (KIT)
Year: 2021
Subjects: solid state electrolyte
garnet electrolyte
hydrogarnet

description: In order to meet the increasing need for energy storage systems in consumer devices, electric vehicles and stationary energy storage devices, the existing battery technologies are constantly being further developed. An important criterion for the performance of the battery is its energy density. Even if the cathode accounts mostly for the weight of a lithium ion battery cell, there is a promising possibility of weight saving on the anode side by replacing graphite with pure lithium metal. To this end, an electrolyte needs to be developed that is stable against the potential of the pure metal. Liquid electrolytes cannot meet this requirement and also involve safety risks as ammability. Solid-state electrolytes are supposed to enable the use of a lithium-metal electrode. Additionally, due to the scarcity of resources for lithium, sodium-based technologies are being developed for use as stationary energy storage devices. Thiophosphates feature the highest ionic conductivities among all solid electrolytes. In many cases, amorphous thiophosphates offer even higher conductivities than their crystalline analogues and their structure can differ, too. A structural investigation of amorphous compounds may not be possible with a standard X-ray analysis. However their local structure has to be enlightened so that a reproducible synthesis can take place. The solid electrolyte Na2P2S6 was synthesized via ball milling in an amorphous state with subsequent crystallization. The structure of the crystalline phase differs markedly compared to the corresponding amorphous phase. A combination of XRD-PDF analysis and 23Na/31P MAS NMR spectroscopy measurements indicate that single PS30-4 tetrahedra and corner-sharing tetrahedra are transformed to edge-sharing-tetrahedra during crystallization of amorphous Na2P2S6 to crystalline Na2P2S6. Impedance spectroscopy shows that amorphous Na2P2S6 has a conductivity of 5.710-8 S cm-1 which is three orders of magnitude higher than crystalline Na2P2S6 (2.6 10-11 S cm-1). The higher conductivity can also be recovered by ball milling crystalline Na2P2S6, inducing a reamorphization. Lithium Lanthanum Zirconium oxide (LLZO) exhibits a high ionic conductivity and stability against lithium metal and is therefore a promising candidate as solid state electrolyte. Yet, specifcations on its conductivity are often not reliable and spread widely. Attempts are made to attribute the differences in reported conductivities to the different substituents, sintering times or surface passivations. A microstructural comparison of four differently substituted samples is performed to elucidate the reasons for the different conductivities. X-ray diffraction revealed that commercial LLZO samples crystallize in the hydrogarnet structure (space group No. 220), which is described for the first time with a substituent on Zr and La sites. Ball milling of Al3+, Nb5+, Ta5+ and W6+ substituted LLZO results in a phase transformation from the garnet structure into the hydrogarnet structure with a lower symmetry. The distribution of lithium ions in the hydrogarnet structure differs from that in the garnet structure which was investigated with 6Li MAS NMR and neutron diffraction. A targeted conversion of the hydrogarnet structure into the garnet structure is proved by calcining the material at 1100 °C for 10 h. With high-temperature X-ray diffraction, an low thermal expansion of the hydrogarnet unit cell is observed in comparison to the greater expansion of the garnet unit cell. The ionic mobility of Li ions in the hydrogarnet structure is examined by means of NMR, in particular line shape analysis, relaxometry and pulsed-field gradient NMR. This combination of techniques shows that the mobility of lithium is significantly reduced on small length scales. In combination with the structural analysis, this can be traced back to the high occupancy of the Li3 position in the hydogarnet structure, blocking long-range lithium diffusion. However, it was not possible to access the long-range mobility of Li in the hydrogarnet structure (at 25 °C). Therefore, the contribution of the ceramic component to the total ionic conductivity of polymer composite electrolytes is evaluated. The question whether the long-range lithium mobility in the hydrogarnet structure is lower compared to the garnet structure is assessed without the necessity for sintering the LLZO to pellets. Impedance spectroscopy shows a conductivity of 1.2 10-6 S cm-1 for a composite electrolyte with a hydrogarnet structure and 3.4 10-6 S cm-1 for a composite electrolyte with a garnet structure. The higher Li mobility of the garnet-based composite electrolyte compared to the hydrogarnet-based electrolyte is verified with PFG-NMR measurements of the diffusion coeffcient: 6.1 10-14 m2 s-1 (garnet), resp. 1.1 10-14 m2 s-1 (hydrogarnet). The measured activation energy of dffusion is also higher in the hydrogarnet composite. The conductivity results measured with impedance spectroscopy are compared with commercial composite electrolytes; a SiO2-ceramic-polymer and a purely polymer-based electrolyte. The next step from optimizing a solid state electrolyte in terms of ionic conductivity is to look at its compatibility with the electrodes, here the cathode. It is tested whether ball milling of LLZO with the established cathode material Lithium Nickel Cobalt Manganese oxide (NCM) results in a good contact of the two materials and consequently a low Li ion diffusion barrier. The interface between LLZO and NCM is investigated by X-ray diffraction, 6Li MAS NMR and transmission electron microscopy. A model system consisting of LLZO and NCM is characterized with impedance spectroscopy for a lithium diffusion barrier sandwiched between an auxiliary electrolyte in order to separate the ionic conductivity from the electrical. An evaluation of only the ionic conductivity apart from the electrical conductivity is not possible due to the high electrical conductivity of the auxiliary electrolyte. The electrolyte Lithium Aluminum Titanium Phosphate (LATP) is examined crystallographically against the background of upscaling of the synthesis. If the process is upscaled, local inhomogeneities of the educts can be expected in a way that varying educt contents have an effect on the product. This applies especially to phosphoric acid, the concentration of which cannot be specified precisely due to its hygroscopy. A Rietveld refinement analysis against X-ray diffraction data of LATP with varying phosphoric acid content during synthesis is performed. An excess of phosphoric acid leads to the formation of AlPO4, which impedes the ionic conductivity. Insufficient phosphoric acid causes the formation LiTiOPO4. TiO2 is formed in this material after a second sintering step. The findings in this work contribute the understanding of structural changes in solid electrolytes during processing and thus contribute to the improvement of future solidstate batteries.