Commercially available Li-ion batteries (LIB) with liquid electrolytes are state-of-the-art in many applications due to their high energy density, steadily decreasing cost and long cycle life. However, they still face intrinsic safety issues. Despite continuous progress, improvements are mainly incremental due to materials and interface constraints. Therefore, solid electrolytes are under development in different research labs and companies.
To optimize the interface between lithium and the solid electrolyte Al doped LLZTO (Lithium Lanthanum Tantalum Zirconia Oxide), different kind of interlayer have been introduced. This study reports on FIB/SEM, HRSTEM and EDS analysis of a solid-state electrolyte coated with Sn.
Experimental details
For the evaluation of the morphology and chemical composition of the sample, FIB (focused ion beam) cross sections were prepared in a Zeiss XB550 scanning electron microscope and analysed with an Oxford X-Max 150 EDS (energy dispersive spectroscopy) detector. In a second step, a 4 µm thick lamella was produced by FIB lift out technique and transferred to a copper TEM grid. In order to reduce beam induced damage during EDS, sample oxidation and contamination, the sample was FIB polished under cryogenic conditions (-160°C) with a Quorum PP3010T System. After obtaining a TEM transparent lamella, the sample was warmed up to room temperature and transferred under inert gas conditions (argon) to a TEM vacuum holder. Further investigation of the LLZO sample with nanoscale resolution was performed in a probe corrected Jeol ARM 200F electron microscope equipped with a Jeol Dual EDS system. Since the Li K line is not accessible in conventional EDS, additional EELS (Electron Energy Loss Spectroscopy) data were recorded with a Gatan Quantum ER Spectrometer. Moreover, crystallographic phases were identified by SAED (Selected Area Electron Diffraction).
SEM/EDS analysis
Figure 1 shows a FIB cross section before and after EDS mapping for 60 s with 10 kV EHT (Electron High Tension) at room temperature. After the EDS mapping, several areas with segregation of components are observable. Note that the delamination of the Sn coating in some places was also present before EDS mapping. Electron microscopic investigation of beam sensitive Li containing materials can be challenging. The incident electrons can cause sample degradation by breaking bonds, ionization and create also free radicals and secondary electrons that cause further chemical reactions [1]. An effective method to reduce these damages is to operate under cryogenic conditions. After sample cooling to -160°C, no obvious beam damage is observable during EDS mapping of the lamella (fig. 1(d)). The EDS mapping images in figure 1(e) show the Sn top layer and the distribution of lanthanum, zirconia, oxygen and the dopant elements Ta and Al. In particular, Al seems to be enriched in the precipitates. The big structure in the middle of the lamella represents a cavity in the material. During FIB final milling of the lamella, the cavity was erased and therefore not visible any more in the finished TEM lamella.
TEM observations
The EELS mapping in figure 2(b) reveals Li enrichment in the Sn layer and in the precipitates and Li depletion in the matrix. Two different kind of precipitates were detected, one with Al enrichment and Ta depletion and another one with Zr enrichment and La depletion. Besides elemental information, EELS also provides information on the chemical state of atoms and their interactions with neighbouring atoms, which can be extracted from the near edge fine structure. The STEM image in figure 2(c) demonstrates that the Sn layer is subdivided into two different layers, a dense top layer with bright contrast and a porous bottom layer with mixed dark and bright contrast. Different fine structures were obtained for the different regions by EELS spectra for the Li K and Sn M 4,5 edge as shown in figure 2(d). The comparison of the Sn M 4,5 edge with spectra from literature [2] reveals an partly oxidized Sn bottom layer of SnO2 type and non-oxidized Sn in the top layer. Areas with bright contrast in the Sn-oxide bottom layer correspond to non-oxidized Sn. Based on this information, the bright top layer is expected to be a Li/Sn intermetallic phase [3] and the bottom layer (Li)Sn-oxide.
Poor contact between the solid-state electrolyte and the coating interface is a main issue in the development of interlayer systems for LLZO based battery materials. Sn is known to change its volume during lithiation up to 300% [4]. It is also known, that carbonate rich LLZO surfaces cause contact problems during cycling. The observed extensive delamination of the Sn interface in the SEM-image from figure 1(a) was not found in the thinned TEM sample in figure 2(a). Thereby further investigations of delaminated regions are necessary.
Al doping of LLZO has the purpose to stabilize the high conductive cubic crystallographic phase at lower temperature [5]. Depending on the synthesis conditions, intermixing of the different precursors and Al content, different phases and secondary phases can arise. The Al rich phase was identified as cubic (Li)LaAlO3 by comparison of the d-values obtained from the FFT in figure 3(b) with a reconstructed diffraction pattern (Jems software) from crystallographic data [6]. The Zr-rich second phase needs further investigation because no matching crystallographic data have yet been identified. The matrix phase is assumed to be cubic LLZO from the SAED pattern (fig. 3(c)), compared to a simulated diffraction pattern from crystallographic data [7]. In cubic LLZO reflexions of type h00 are only allowed for h=4*n and the 200 reflexion is only visible in the tetragonal phase (fig. 3(d)). Due to dynamic effects in SAED, comparative measurements with XRD for example should be performed.
Conclusion and outlook
SEM/FIB combined with EDS under cryogenic conditions is a practical way to determine the coating quality and elemental distributions in many beam sensitive materials. For more detailed information on the nanoscale and the distribution of light elements like Li and their chemical state, STEM in combination with EELS is the method of choice (optionally under cryo conditions). Additionally, information on crystallographic phases with high spatial resolution, using TEM/SAD can be a useful tool for optimizing synthesis conditions in material development. Another topic to address in analysing highly reactive materials like battery components is the environmental condition during sample preparation, where we can provide sample handling and transfer to TEM under inert gas conditions with argon.
Contact
Clementine Warres
NMI Natural and Medical Sciences
Institute at the University of Tübingen
Reutlingen, Germany
[email protected]
References
[1] Baker, L. and J. Rubinstein, Radiation Damage in Electron Cryomicroscopy. Methods in enzymology, 2010. 481: p. 371-88.
[2] Lorenz, H., et al., Preparation and structural characterization of SnO2 and GeO2 methanol steam reforming thin film model catalysts by (HR)TEM. Materials Chemistry and Physics, 2010. 122: p. 623-629.
[3] Mayo, M. and A.J. Morris, Structure Prediction of Li–Sn and Li–Sb Intermetallics for Lithium-Ion Batteries Anodes. Chemistry of Materials, 2017. 29(14): p. 5787-5795.
[4] Obrovac, M.N. and V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries. Chemical Reviews, 2014. 114(23): p. 11444-11502.
[5] Buschmann, H., et al., Structure and dynamics of the fast lithium ion conductor “Li7La3Zr2O12”. Physical chemistry chemical physics : PCCP, 2011. 13: p. 19378-92.
[6] Persson, Kristin (2014), Materials Data on LaAlO3 (SG:221) by Materials Project , cif file mp-5304, Available at: https://materialsproject.org/materials/mp-5304/ (Accessed: 03 May 2022)
[7] Persson, Kristin (2016), Materials Data on Li7La3Zr2O12 (SG:142) by Materials Project, cif file mp-942733, Available at: https://materialsproject.org/materials/mp-942733/ (Accessed: 03 May 2022)
