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| Home > Publications database > Optimization of NaSICON-type lithium- ion conductors for solid-state batteries |
| Book/Dissertation / PhD Thesis | FZJ-2025-02774 |
2025
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-824-7
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Please use a persistent id in citations: urn:nbn:de:0001-2506230903296.203072864795 doi:10.34734/FZJ-2025-02774
Abstract: The continuing depletion of fossil fuels, rising oil prices and the need to reduce CO2 emissions have stimulated intensive research into alternative energy technologies based on renewable and clean sources. Among the various technologies, electrochemical energy storage in rechargeable lithium-ion batteries (LIBs) plays an important role, both for powering a wide range of electronic devices and electric vehicles, and for storing electricity generated from alternative energy sources such as solar and wind. Commercial LIBs using organic liquid electrolytes dominate the market. However, the reactivity of liquid electrolytes at very positive or negative potentials limits the choice of suitable electrode materials and thus the available energy density. In addition, the toxicity and flammability of organic electrolytes raise serious safety concerns. As an alternative to conventional liquid electrolyte LIBs, solid-state battery (SSB) concepts using solid electrolytes (SEs) are currently under intense investigation. Among numerous classes of materials, ceramic SEs are particularly attractive due to their non-flammability, relatively high ionic conductivity at room temperature (RT) and high chemical stability in air, resulting in potentially very high intrinsic safety of the batteries. In particular, Li-containing NaSICONs (Na Super Ionic CONductors) are currently receiving a great deal of attention due to the large structural variability and high ionic conductivity that can be achieved by substituting the lattice framework with various elements. Within this class of materials, lithium aluminum germanium phosphate Li1+xAlxGe2-x(PO4)3 (LAGP) and lithium aluminum titanium phosphate Li1+xAlxTi2-x(PO4)3 (LATP) are solid-state Li-ion conductors with the highest ionic conductivity at RT. However, the main drawback of germanium and titanium-containing materials is the low electrochemical stability at negative potentials caused by the reduction of Ge4+ to Ge2+ and Ti4+ to Ti3+, which prevents the use of lithium metal anodes and reduces the energy density of the battery. To overcome this problem, the aim of this work was to increase the reduction stability of NaSICON compounds by replacing Ge and Ti with more reduction stable ions. To this end, various compositions of the type Li1+xM3+xZr2-x(PO4)3, where M3+ = Al3+, Sc3+, Y3+, were synthesized by solution-assisted solid-state reaction. The effect of Substitution on crystallographic parameters, relative density, sintering temperature, ionic conductivity, and electrochemical stability was systematically investigated. The cationic substitution of M3+ (M = Al, Sc, Y) for Zr4+ in LiZr2(PO4)3 (LZP) stabilizes the rhombohedral NaSICON structure (space group 𝑅3̅ c) at RT and increases the ionic conductivity significantly. Here, at 25 °C and with a comparable relative density of 94-96 %, an ionic conductivity of 2.7 × 10-5 S cm-1, 6.7 × 10-5 S cm-1 and 3.6 × 10-6 S cm-1 was achieved with the compositions Li1.2Sc0.2Zr1.8(PO4)3, Li1.2Y0.2Zr1.8(PO4)3 and Li1.2Al0.2Zr1.8(PO4)3, respectively. Compared to Li1+xScxZr2-x(PO4)3, the Y3+ substitution in LZP slightly enhanced the ionic conductivity and marked the maximum Li+-ion conductivity at RT with composition x = 0.2 in the whole system Li1+xM3+xZr2-x(PO4)3. However, the Al3+-substitution decreased the ionic conductivity at RT. In addition to cationic substitution, the effect of polyanionic substitution on ionic conductivity was investigated in the two materials series Li3+xSc2SixP3-xO12 (0 ≤ x ≤ 0.6) and Li1.2+xSc0.2Zr1.8SixP3-xO12 (0.3 ≤ x ≤ 2.8). The substitution of (PO4)3- with (SiO4)4- in the system Li2O-Sc2O3-ZrO2-SiO2-P2O5 effectively lowers the sintering temperature. Moreover, the introduction of zirconium can limit the evaporation of lithium species even at high sintering temperature. Silicate substitution in Li3+xSc2SixP3-xO12 (0 ≤ x ≤ 0.6) has been shown to stabilize the monoclinic Symmetry (space group P21/n) resulting in the increase of total ionic conductivity. The ionic conductivity at 25 °C increased from 2 × 10-6 S cm-1 for x = 0 to 1.2 × 10-5 S cm-1 for x = 0.15, which is the highest ionic Conductivity of the investigated compositions in the Li2O-Sc2O3-ZrO2-SiO2-P2O5 system. The purity of the NaSICON materials has a strong influence on the grain boundary resistance and thus on the ionic conductivity. Selected ceramic NaSICON electrolytes such as LZP , Li1.2Y0.2Zr1.8(PO4)3, and Li3Sc2(PO4)3 (LSP) were found to be more stable with respect to lithium than LATP and LAGP. LSP proved to be chemically and electrochemically very stable and might act as an anode protection material in combination with lithium metal.
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