Atomistic Insight into Ion Transport and Conductivity in Ga/Al-Substituted Li7La3Zr2O12 Solid Electrolytes

How Concerted Are Ionic Hops in Inorganic Solid-State Electrolytes?

Despite being fundamental to the understanding of solid-state electrolytes (SSEs), little is known on the degree of coordination between mobile ions in diffusive events, thus hindering a detailed comprehension and possible rational design of SSEs. Here, we introduce an unsupervised k-means clustering approach that is able to identify ion-hopping events and correlations between many mobile ions and apply it to a comprehensive ab initio MD database comprising several families of inorganic SSEs and millions of ionic configurations. It is found that despite two-body interactions between mobile ions being the largest, higher-order n-ion (2 < n) correlations are most frequent. Specifically, we prove a general exponential decaying law for the probability density function governing the number of concerted mobile ions. For the particular case of Li-based SSEs, it is shown that the average number of correlated mobile ions amounts to 10 ± 5 and that this result is practically independent of the temperature. Interestingly, our data-driven analysis reveals that fast-ionic diffusion strongly and positively correlates with ample hopping lengths and long hopping spans but not with high hopping frequencies and short interstitial residence times. Finally, it is shown that neglection of many-ion correlations generally leads to a modest overestimation of the hopping frequency that roughly is proportional to the average number of correlated mobile ions.

Clarifying the Dopant Local Structure and Effect on Ionic Conductivity in Garnet Solid-State Electrolytes for Lithium-Ion Batteries

The high Li-ion conductivity and wide electrochemical stability of Li-rich garnets (Li7La3Zr2O12) make them one of the leading solid electrolyte candidates for solid-state batteries. Dopants such as Al and Ga are typically used to enable stabilization of the high Li+ ion-conductive cubic phase at room temperature. Although numerous studies exist that have characterized the electrochemical properties, structure, and lithium diffusion in Al- and Ga-LLZO, the local structure and site occupancy of dopants in these compounds are not well understood. Two broad 27Al or 69,71Ga resonances are often observed with chemical shifts consistent with tetrahedrally coordinated Al/Ga in the magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of both Al- and Ga-LLZO, which have been assigned to either Al and/or Ga occupying 24d and 96h/48g sites in the LLZO lattice or the different Al/Ga configurations that arise from different arrangements of Li around these dopants. In this work, we unambiguously show that the side products γ-LiAlO2 and LiGaO2 lead to the high frequency resonances observed by NMR spectroscopy and that both Al and Ga only occupy the 24d site in the LLZO lattice. Furthermore, it was observed that the excess Li often used during synthesis leads to the formation of these side products by consuming the Al/Ga dopants. In addition, the consumption of Al/Ga dopants leads to the tetragonal phase formation commonly observed in the literature, even after careful mixing of precursors. The side-products can exist even after sintering, thereby controlling the Al/Ga content in the LLZO lattice and substantially influencing the lithium-ion conductivity in LLZO, as measured here by electrochemical impedance spectroscopy.

Pathways to Next-Generation Fire-Safe Alkali-Ion Batteries

High energy and power density alkali-ion (i.e., Li+ , Na+ , and K+ ) batteries (AIBs), especially lithium-ion batteries (LIBs), are being ubiquitously used for both large- and small-scale energy storage, and powering electric vehicles and electronics. However, the increasing LIB-triggered fires due to thermal runaways have continued to cause significant injuries and casualties as well as enormous economic losses. For this reason, to date, great efforts have been made to create reliable fire-safe AIBs through advanced materials design, thermal management, and fire safety characterization. In this review, the recent progress is highlighted in the battery design for better thermal stability and electrochemical performance, and state-of-the-art fire safety evaluation methods. The key challenges are also presented associated with the existing materials design, thermal management, and fire safety evaluation of AIBs. Future research opportunities are also proposed for the creation of next-generation fire-safe batteries to ensure their reliability in practical applications.

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.

Unveiling Interfacial Li-Ion Dynamics in Li7La3Zr2O12/PEO(LiTFSI) Composite Polymer-Ceramic Solid Electrolytes for All-Solid-State Lithium Batteries

Unlocking the full potential of solid-state electrolytes (SSEs) is key to enabling safer and more-energy dense technologies than today's Li-ion batteries. In particular, composite materials comprising a conductive, flexible polymer matrix embedding ceramic filler particles are emerging as a good strategy to provide the combination of conductivity and mechanical and chemical stability demanded from SSEs. However, the electrochemical activity of these materials strongly depends on their polymer/ceramic interfacial Li-ion dynamics at the molecular scale, whose fundamental understanding remains elusive. While this interface has been explored for nonconductive ceramic fillers, atomistic modeling of interfaces involving a potentially more promising conductive ceramic filler is still lacking. We address this shortfall by employing molecular dynamics and enhanced Monte Carlo techniques to gain unprecedented insights into the interfacial Li-ion dynamics in a composite polymer-ceramic electrolyte, which integrates polyethylene oxide plus LiN(CF₃SO₂)₂ lithium imide salt (LiTFSI), and Li-ion conductive cubic Li₇La₃Zr₂O₁₂ (LLZO) inclusions. Our simulations automatically produce the interfacial Li-ion distribution assumed in space-charge models and, for the first time, a long-range impact of the garnet surface on the Li-ion diffusivity is unveiled. Based on our calculations and experimental measurements of tensile strength and ionic conductivity, we are able to explain a previously reported drop in conductivity at a critical filler fraction well below the theoretical percolation threshold. Our results pave the way for the computational modeling of other conductive filler/polymer combinations and the rational design of composite SSEs.

Spray Flame Synthesis (SFS) of Lithium Lanthanum Zirconate (LLZO) Solid Electrolyte

A spray-flame reaction step followed by a short 1-h sintering step under O₂ atmosphere was used to synthesize nanocrystalline cubic Al-doped Li₇La₃Zr₂O₁₂ (LLZO). The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline La₂Zr₂O₇ (LZO) pyrochlore phase while Li was present on the nanoparticles’ surface as amorphous carbonate. However, a short annealing step was sufficient to obtain phase pure cubic LLZO. To investigate whether the initial mixing of all cations is mandatory for synthesizing nanoparticulate cubic LLZO, we also synthesized Li free LZO and subsequently added different solid Li precursors before the annealing step. The resulting materials were all tetragonal LLZO (I4₁/acd) instead of the intended cubic phase, suggesting that an intimate intermixing of the Li precursor during the spray-flame synthesis is mandatory to form a nanoscale product. Based on these results, we propose a model to describe the spray-flame based synthesis process, considering the precipitation of LZO and the subsequent condensation of lithium carbonate on the particles’ surface.

Gallium-Doping Effects on Structure, Lithium-Conduction, and Thermochemical Stability of Li7-3x Gax La3 Zr2 O12 Garnet-Type Electrolytes

One of the most promising electrolytes for all-solid-state lithium batteries is Li7 La3 Zr2 O12 . Previously, their thermodynamic stability, Li-ion conductivity, and structural features induced by Ga-doping have not been empirically determined or correlated. Here, their interplay was examined for Li7-3x Gax La3 Zr2 O12 with target xGa=0, 0.25, 0.50, 0.75, and 1.00 atoms per formula unit (apfu). Formation enthalpies, obtained with calorimetry and found to be exothermic at all compositions, linearly decreased in stability with increased xGa. At dilute xGa substitution, the formation enthalpy curve shifted stepwise endothermically, and the conductivity increased to a maximum, coinciding with 0.529 Ga apfu. This correlated with percolation threshold analysis (0.558 Ga apfu). Further substitution (0.787 Ga apfu) produced a large decrease in the stability and conductivity due to a large increase in point defects and blocked Li-migration pathways. At xGa=1.140 apfu, a small exothermic shift was related to defect cluster organization extending the Li hopping distance and decreased Li-ion conductivity.

Al/Ga-Doped Li7La3Zr2O12 Garnets as Li-Ion Solid-State Battery Electrolytes: Atomistic Insights into Local Coordination Environments and Their Influence on 17O, 27Al, and 71Ga NMR Spectra

Li₇La₃Zr₂O₁₂ (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state ¹⁷O, ²⁷Al, and ⁷¹Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both ²⁷Al and ⁷¹Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp ²⁷Al and ⁷¹Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO₄ neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO₄ neighbors. A correlation between the size of the ²⁷Al/⁷¹Ga quadrupolar coupling and the distortion of the doping sites (viz. XO₄/XO₅/XO₆ with X = {Al/Ga}) is established. ¹⁷O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: ¹⁷O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.