Structural limitations for optimizing garnet-type solid electrolytes: a perspective

Tuning Ion Mobility in Lithium Argyrodite Solid Electrolytes via Entropy Engineering

The development of improved solid electrolytes (SEs) plays a crucial role in the advancement of bulk-type solid-state battery (SSB) technologies. In recent years, multicomponent or high-entropy SEs are gaining increased attention for their advantageous charge-transport and (electro)chemical properties. However, a comprehensive understanding of how configurational entropy affects ionic conductivity is largely lacking. Herein we investigate a series of multication-substituted lithium argyrodites with the general formula Li6+x[M1aM2bM3cM4d]S5I, with M being P, Si, Ge, and Sb. Structure-property relationships related to ion mobility are probed using a combination of diffraction techniques, solid-state nuclear magnetic resonance spectroscopy, and charge-transport measurements. We present, to the best of our knowledge, the first experimental evidence of a direct correlation between occupational disorder in the cationic host lattice and lithium transport. By controlling the configurational entropy through compositional design, high bulk ionic conductivities up to 18 mS cm-1 at room temperature are achieved for optimized lithium argyrodites. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors via entropy engineering, overcoming compositional limitations for the design of advanced electrolytes and opening up new avenues in the field.

Dopant-induced modulation of lithium-ion conductivity in cubic garnet solid electrolytes: a first-principles study

Cubic garnet Li₇La₃Zr₂O₁₂ (c-LLZO) is a promising solid electrolyte for all-solid-state batteries, often doped with Ga, Al, and Fe to stabilize the structure and enhance Li-ion conductivity. Despite introducing the same amount of Li vacancies, these dopants with +3 classical charge yield different Li-ion conductivities by around an order of magnitude. In this study, we used density functional theory (DFT) calculations to investigate the impact of Ga, Fe, and Al dopants on Li chemical potential and Li-ion conductivity variations. We identified the energetically favorable dopant location in c-LLZO and determined the optimal U value of 7.5 eV for DFT+U calculations for dopant Fe in c-LLZO. Our calculations showed that Ga or Fe doping enhances the Li chemical potential by 0.05-0.08 eV, reducing Li-ion transfer barriers and increasing Li-ion conductivity, while Al doping lowers the Li chemical potential by 0.08 eV, reducing Li-ion conductivity. To determine the cause of Li chemical potential variations, we performed a combined analysis of the projected density of states, charge density, and Bader charge. The distinct charge distribution from dopant atoms to neighboring O atoms is critical for determining the Li-ion chemical potential. Ga and Fe dopants retain more electrons, which consequently makes the adjacent O atoms acquire a more positive charge that destabilizes Li ions by reducing the restraining force acting on them, thereby enhancing Li-ion conductivity. In contrast, Al doping transfers more electrons to neighboring O atoms, resulting in greater attraction forces to Li ions and reducing Li-ion conductivity. Additionally, Fe-doped LLZO exhibits extra states in the bandgap, potentially causing Fe reduction, as observed in experiments. Our findings provide valuable insights into the design of solid electrolytes and highlight the importance of the local charge distribution around the dopant and Li atoms in determining Li-ion conductivity. This insight could serve as a guiding principle for future materials design and optimization in solid-state electrolyte systems.

Study of Codoping Effects of Ta5+ and Ga3+ on Garnet Li7La3Zr2O12

Garnet Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for all-solid-state Li-ion batteries because of its outstanding performance. However, LLZO exists in two polymorphic phases, tetragonal (∼10^-3 mS cm^-1) and cubic (1-10^-1 mS cm^-1), where the cubic phase exhibits higher Li-ion conductivity but is thermodynamically unstable at ambient room temperature. To stabilize the cubic phase with high ionic conductivity, we fabricated mono- and codoped garnet with Ta⁵⁺ and Ga³⁺ (Li_7-3x-z=6.4Ga _x La₃Zr_2-z Ta _z O₁₂) and investigated the doping effects. The doping effects on the crystal structure and ionic conductivity were systematically investigated using X-ray diffraction, Raman scattering, scanning electron microscopy, alternative current (AC) impedance, and direct current (DC) polarization methods. The characterization results revealed that Ta-doping favors higher occupation of Li-ions on the mobile octahedral (LiO₆) site and improves ionic conductivity of the grain boundary. However, it showed poor total ionic conductivity (2.044 × 10^-4 S cm^-1 at 1100 °C for 12 h) due to the low sinterability [relative density (RD): ∼80.3%]. On the other hand, Ga-doping provides better sinterability (RD: ∼93.1%) and bulk conductivity. Considering the beneficial effects of Ga- and Ta-doping, codoped Li_6.4Ga_0.133La₃Zr_1.8Ta_0.2O₁₂ garnet with enhanced ionic conductivity (6.141 × 10^-4 S cm^-1) and improved high-density microstructure (RD: ∼95.7%) was obtained.

Strategies for Enhancing Lithium-Ion Conductivity of Triple-Layered Ruddlesden-Popper Oxides: Case Study of Li2-xLa2-yTi3-zNbzO10

We report strategies of enhancing the ionic conductivity of triple-layered Ruddlesden-Popper oxides through design and synthesis of seven compounds belonging to the series A₂A'₂B₃O₁₀ (A = Li, A' = La, B = Ti/Nb), investigated by neutron diffraction, impedance spectroscopy, and dielectric analyses. We demonstrate, for the first time, that lithium diffusion in triple-layered Ruddlesden-Popper oxides is a result of cooperative effect of both inter- and intrastack sites, i.e., A and A'. As shown by neutron diffraction, the structure of these materials comprises triple-layered stacks of octahedra (BO₆), separated by A-site cations, while A' ions reside in intrastack spaces. We first synthesized Li₂La₂Ti₃O₁₀ and showed that its lithium-ion conductivity can be systematically enhanced by incorporation of cation deficiency in interstack sites through synthesis of Li_1.9La₂Ti_2.9Nb_0.1O₁₀, Li_1.8La₂Ti_2.8Nb_0.2O₁₀, and Li_1.75La₂Ti_2.75Nb_0.25O₁₀. The latter represents the limit of cation deficiency on the A-site and has the highest conductivity among the A-site-deficient materials. We then investigated the enhancement of lithium-ion conductivity by incorporation of cation defects in intrastack A'-sites through synthesis of Li₂La_1.9Ti_2.7Nb_0.3O₁₀ and Li₂La_1.8Ti_2.4Nb_0.6O₁₀, where the latter represents the limit of cation deficiency on the A'-site and has the best conductivity among the A'-deficient materials. Finally, we hypothesized that cooperative effect of defects in both inter- and intrastack sites should have an even higher impact on ionic conductivity. This hypothesis was confirmed by synthesis of Li_1.9La_1.9Ti_2.6Nb_0.4O₁₀, which showed the highest conductivity among all materials synthesized in this work. Detailed analysis of real and imaginary components of impedance spectroscopy, as well as dielectric and loss tangent, have been conducted. This systematic study is aimed at answering a fundamental question related to materials chemistry of Ruddlesden-Popper oxides, namely, determination of the sites that contribute to ionic conductivity.

Space-Charge Effects at the Li7La3Zr2O12/Poly(ethylene oxide) Interface

Composites consisting of garnet-type Li₇La₃Zr₂O₁₂ (LLZO) ceramic particles dispersed in a solid polymer electrolyte based on poly(ethylene oxide) (PEO) have recently been investigated as a possible electrolyte material in all solid state Li ion batteries. The interface between the two materials, that is, LLZO/PEO, is of special interest for the transport of lithium ions in the composite. For obtaining the desired high ionic conductivity, Li⁺ ions have to pass easily across this interface. However, previous research found that the interface is highly resistive. Here, we further investigate the interface between Al-substituted LLZO and PEO-LiClO₄ electrolytes in the frame of a theoretical description, which is based on space-charge layers. By theoretical calculations supported by experiments, we find that the interface is highly resistive. From the results, we have deduced that the highest contribution to this resistance comes from a high activation energy and not from electrostatic repulsion of lithium.

Tailored high cycling performance in a solid polymer electrolyte with perovskite-type Li0.33La0.557TiO3 nanofibers for all-solid-state lithium ion batteries

Solid polymer electrolytes (SPEs) have drawn considerable attention owing to their reliable safety performance, electrochemical stability and exceptional flexibility, which make them superior to conventional liquid electrolytes. Here, we report a novel composite electrolyte which is composed of homogeneously dispersed Li ion-conducting Li_0.33La_0.557TiO₃ (LLTO) nanowires in a poly(ethylene oxide) (PEO)/LiClO₄ matrix. It is demonstrated that only 3 wt% LLTO nanofibers are needed for the optimal performance of SPEs. The PEO-based composite electrolyte shows an excellent Li ion conductivity of 4.01 × 10^-4 S cm^-1 at 60 °C. In addition, it is worth mentioning that the all-solid-state lithium battery based on this composite electrolyte exhibits a specific capacity of 140 mA h g^-1 and an excellent capacity retention of 92.4% after running 100 cycles at a rate of 1C and 60 °C. The study offers a superior alternative for the design of PEO-based solid composite electrolytes.

An ion redistributor for dendrite-free lithium metal anodes

Lithium (Li) metal anodes have attracted considerable interest due to their ultrahigh theoretical gravimetric capacity and very low redox potential. However, the issues of nonuniform lithium deposits (dendritic Li) during cycling are hindering the practical applications of Li metal batteries. Herein, we propose a concept of ion redistributors to eliminate dendrites by redistributing Li ions with Al-doped Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) coated polypropylene (PP) separators. The LLZTO with three-dimensional ion channels can act as a redistributor to regulate the movement of Li ions, delivering a uniform Li ion distribution for dendrite-free Li deposition. The standard deviation of ion concentration beneath the LLZTO composite separator is 13 times less than that beneath the routine PP separator. A Coulombic efficiency larger than 98% over 450 cycles is achieved in a Li | Cu cell with the LLZTO-coated separator. This approach enables a high specific capacity of 140 mAh g^-1 for LiFePO₄ | Li pouch cells and prolonged cycle life span of 800 hours for Li | Li pouch cells, respectively. This strategy is facile and efficient in regulating Li-ion deposition by separator modifications and is a universal method to protect alkali metal anodes in rechargeable batteries.

Competing Structural Influences in the Li Superionic Conducting Argyrodites Li6PS5- xSe xBr (0 ≤ x ≤ 1) upon Se Substitution

Lithium-ion conducting argyrodites have recently attracted significant interest as solid electrolytes for solid-state battery applications. In order to enhance the utility of materials in this class, a deeper understanding of the fundamental structure-property relationships is still required. Using Rietveld refinements of X-ray diffraction data and pair distribution function analysis of neutron diffraction data, coupled with electrochemical impedance spectroscopy and speed of sound measurements, the structure and transport properties within Li₆PS_{5- x}Se _xBr (0 ≤ x ≤ 1) have been monitored with increasing Se content. While it has been previously suggested that the incorporation of larger, more polarizable anions within the argyrodite lattice should lead to enhancements in the ionic conductivity, the Li₆PS_{5- x}Se _xBr transport behavior was found to be largely unaffected by the incorporation of Se^2- due to significant structural modifications to the anion sublattice. This work affirms the notion that, when optimizing the ionic conductivity of solid ion conductors, local structural influences cannot be ignored and the idea of "the softer the lattice, the better" does not always hold true.

Gallium-Doped Li7La3Zr2O12 Garnet-Type Electrolytes with High Lithium-Ion Conductivity

Owing to their high conductivity, crystalline Li_7-3xGa_xLa₃Zr₂O₁₂ garnets are promising electrolytes for all-solid-state lithium-ion batteries. Herein, the influence of Ga doping on the phase, lithium-ion distribution, and conductivity of Li_7-3xGa_xLa₃Zr₂O₁₂ garnets is investigated, with the determined concentration and mobility of lithium ions shedding light on the origin of the high conductivity of Li_7-3xGa_xLa₃Zr₂O₁₂. When the Ga concentration exceeds 0.20 Ga per formula unit, the garnet-type material is found to assume a cubic structure, but lower Ga concentrations result in the coexistence of cubic and tetragonal phases. Most lithium within Li_7-3xGa_xLa₃Zr₂O₁₂ is found to reside at the octahedral 96h site, away from the central octahedral 48g site, while the remaining lithium resides at the tetrahedral 24d site. Such kind of lithium distribution leads to high lithium-ion mobility, which is the origin of the high conductivity; the highest lithium-ion conductivity of 1.46 mS/cm at 25 °C is found to be achieved for Li_7-3xGa_xLa₃Zr₂O₁₂ at x = 0.25. Additionally, there are two lithium-ion migration pathways in the Li_7-3xGa_xLa₃Zr₂O₁₂ garnets: 96h-96h and 24d-96h-24d, but the lithium ions transporting through the 96h-96h pathway determine the overall conductivity.

Synthesis, Crystal Structure, and Stability of Cubic Li7-xLa3Zr2-xBixO12

Li oxide garnets are among the most promising candidates for solid-state electrolytes in novel Li ion and Li metal based battery concepts. Cubic Li₇La₃Zr₂O₁₂ stabilized by a partial substitution of Zr⁴⁺ by Bi⁵⁺ has not been the focus of research yet, despite the fact that Bi⁵⁺ would be a cost-effective alternative to other stabilizing cations such as Nb⁵⁺ and Ta⁵⁺. In this study, Li_7–xLa₃Zr_2–xBi_xO₁₂ (x = 0.10, 0.20, ..., 1.00) was prepared by a low-temperature solid-state synthesis route. The samples have been characterized by a rich portfolio of techniques, including scanning electron microscopy, X-ray powder diffraction, neutron powder diffraction, Raman spectroscopy, and ⁷Li NMR spectroscopy. Pure-phase cubic garnet samples were obtained for x ≥ 0.20. The introduction of Bi⁵⁺ leads to an increase in the unit-cell parameters. Samples are sensitive to air, which causes the formation of LiOH and Li₂CO₃ and the protonation of the garnet phase, leading to a further increase in the unit-cell parameters. The incorporation of Bi⁵⁺ on the octahedral 16a site was confirmed by Raman spectroscopy. ⁷Li NMR spectroscopy shows that fast Li ion dynamics are only observed for samples with high Bi⁵⁺ contents.

The cubic modification of Li₇La₃Zr₂O₁₂ can be stabilized by a by a partial substitution of Zr⁴⁺ by Bi⁵⁺. The incorporation of Bi⁵⁺ leads to an increase in the unit-cell parameters. Samples prepared by a low-temperature preparation route are sensitive to CO₂ and H₂O from air, causing a protonation of the garnet phase. ⁷Li NMR spectroscopy shows that fast translational Li ion dynamics are only observed for samples with high Bi⁵⁺ contents.

Fast Li-Ion-Conducting Garnet-Related Li7-3x Fe x La3Zr2O12 with Uncommon I4̅3d Structure

Fast Li-ion-conducting Li oxide garnets receive a great deal of attention as they are suitable candidates for solid-state Li electrolytes. It was recently shown that Ga-stabilized Li₇La₃Zr₂O₁₂ crystallizes in the acentric cubic space group I4̅3d. This structure can be derived by a symmetry reduction of the garnet-type Ia3̅d structure, which is the most commonly found space group of Li oxide garnets and garnets in general. In this study, single-crystal X-ray diffraction confirms the presence of space group I4̅3d also for Li_7-3x Fe _x La₃Zr₂O₁₂. The crystal structure was characterized by X-ray powder diffraction, single-crystal X-ray diffraction, neutron powder diffraction, and Mößbauer spectroscopy. The crystal-chemical behavior of Fe³⁺ in Li₇La₃Zr₂O₁₂ is very similar to that of Ga³⁺. The symmetry reduction seems to be initiated by the ordering of Fe³⁺ onto the tetrahedral Li1 (12a) site of space group I4̅3d. Electrochemical impedance spectroscopy measurements showed a Li-ion bulk conductivity of up to 1.38 × 10^-3 S cm^-1 at room temperature, which is among the highest values reported for this group of materials.