Li₁₄Ln₅[Si₁₁N₁₉O₅]O₂F₂ with Ln = Ce, Nd—Representatives of a Family of Potential Lithium Ion Conductors

Abstract

The silicate layers consist of dreier and sechser rings interconnected via common corners, yielding an unprecedented silicate substructure. A topostructural analysis indicates possible 1D ion migration pathways between five crystallographic independent Li positions. The specific Li-ionic conductivity and its temperature dependence were determined by impedance spectroscopy as well as DC polarization/depolarization measurements. The ionic conductivity is on the order of 5 × 10 −5 S/cm at 300°C, while the activation energy is 0.69 eV. Further adjustments of the defect chemistry (e.g., through doping) can make these compounds interesting candidates for novel oxonitridosilicate based ion conductors. ■ INTRODUCTIONAs the need for mobile power supply and efficient energy storage has constantly increased in the last decades, rechargeable batteries have become a key technology in modern society. 1−4 In the effort to meet the demand for effective batteries, lithium based electrochemical devices play a major role; however, most make use of organic liquid electrolytes that require relatively stringent safety precautions and hence are rather inapplicable for a large range of applications (e.g., large-scale systems, medical devices). 1 Nonflammable, nonvolatile solid electrolytes have the potential to overcome these problems, but the assortment of materials that combine all of the basic requirements for all-solid-state batteries, i.e., high ionic conductivity at the operating temperature and a high chemical, electrochemical, and thermal stability, is still restricted. 5,6 Therefore, great effort has been put into the search for new solid electrolytes and a variety of crystalline, glassy, or composite materials have been considered. 1−3,7 Although Li 3 N has one of the highest ionic conductivities at room temperature (6 × 10 −3 S cm −1 ), novel materials are mostly oxides (e.g., Li 2+2x Zn 1-x GeO 4 (−0.36 < x < 0.87) 8 17 reported in the quasi-binary system Li 3 N−Si 3 N 4 , all show lithium ion conduction (for example Li 2 SiN 2 : σ(400 K) = 1 × 10 −3 S cm −1 ; Li 8 SiN 4 : σ(400 K) = 5 × 10 −2 S cm −1 ). 15,20,21 These facts lead us to expect perceptible Li ion conduction also in multinary lithium (oxo)nitridosilicates.In recent years, we have structurally elucidated a number of alkaline earth as well as rare earth (oxo) The complementary electrical conduction measurements prove that these compounds are indeed Li ion conductors with a very large ionic transference number. Yet, further adjustments of the defect chemistry of this material (e.g., through aliovalent cation doping) are required to obtain ionic conductivity values which might be interesting for battery applications.

Structural Relationship between the Mg‐Containing Nitridosilicates Ca2Mg[Li4Si2N6] and Li2Ca2[Mg2Si2N6]

“…As a result, the compound has a very high degree of condensation [i.e., atomic ratio (Li,Si)/N] κ = 1, which is rather uncommon for layered nitridosilicates. Li–N distances vary from 2.12 to 2.20 Å and are in good accordance with those of tetrahedrally coordinated Li and the sum of the ionic radii (2.12 Å) 3,12,13,21. Ca and Mg ions are located between the layers of dreier rings.…”

“… [a] Typical MAPLE values [kJ/mol], for Ca 2+ : 1700–2200; Mg 2+ : 2100–2500; Si 4+ : 9000–10200; N 3– : 4300–6200; Li + : 550–860 1,3,4,10…”

Li18P6N16—A Lithium Nitridophosphate with Unprecedented Tricyclic [P6N16]18− Ions

“…[20] Accordingly, the difference of the conductivity is caused by the differing crystal structure, the coordination of the lithium atoms,a nd the number of chargec arriers available. [20] The conductivity of these lithium nitridophosphates is comparable to other ternary p-block lithium nitride compounds, for example, Li 2 SiN 2 , [52] LiSi 2 N 3 , [52] or Li 14 Ce, Nd), [53] but they are significantly lower than the value found for Li 3 N( s = 8 10 À3 W À1 cm À1 at 400 Ka nd E a = 24.1 kJ mol À1 ). [54,55] It is important to point out that the samples of the lithium nitridophosphatesw ere cold pressed and not sintered.…”

Structural manipulation approaches towards enhanced sodium ionic conductivity in Na-rich antiperovskites

“…All-solid-state batteries employing inorganic solid electrolytes are the best choice for that they will not suffer from leakage or flammability problems of the currently used liquid or polymeric organic electrolytes [5,6]. However, inorganic materials showing superionic conductivities at ambient or moderate temperatures are rare, most of which are still inflammable chalcogenides [7e10] or oxide ceramics requiring high temperature sintering to reduce the grain-boundary resistance [11,12].…”