Solid-state batteries are on the roadmap for commercialization as the next generation of batteries because of their potential for improved safety, power density, and energy density compared with conventional Li-ion batteries. However, the interfacial reactivity and resulting resistance between the cathode and solid-state electrolyte (SSE) lead to deterioration of cell performance. Although reduction of the cathode/SSE interfacial impedance can be achieved using cathode coatings, optimizing their compositions remains a challenge. In this work, we employ a computational framework to evaluate and screen Li-containing materials as cathode coatings, focusing on their phase stability, electrochemical and chemical stability, and ionic conductivity. From this tiered screening, polyanionic oxide coatings were identified as exhibiting optimal properties, with LiH2PO4, LiTi2(PO4)3, and LiPO3 being particularly appealing candidates. Some lithium borates exhibiting excellent (electro)chemical stability at various interfaces are also highlighted. These results highlight the promise of using optimized polyanionic materials as cathode coatings for solid-state batteries. The flammability of organic liquid electrolytes in Li-ion batteries is a serious safety risk. Solid-state batteries (SSBs) replace the liquid with an inorganic solid, dramatically improving the safety. Unfortunately, solid-solid contacts at the cathode/electrolyte interface are often unstable, leading to high interfacial impedance that grows during operation. Buffering this interface with another layer effectively improves stability; however, optimized buffer materials have not been found by previous research. This work develops and uses a computational framework to screen a wide range of chemistries for use as buffer layers between oxide cathodes and sulfide solid electrolytes and identifies the key factors such as Li content and oxygen bonding covalency that affect the stability of these materials. Many of these materials are polyanionic oxides that substantially outperform conventional oxide buffers. Our work provides guidance for material selection for the next-generation SSBs. Solid-state batteries are considered the next generation of batteries, but they still suffer from high interfacial resistance. One strategy to mitigate the problem is to use coating. We performed a computational screening to find promising cathode coating compositions. Polyanionic oxides are highlighted for good overall characteristics, and LiH2PO4, LiTi2(PO4)3, and LiPO3 are particularly appealing candidates. Furthermore, factors including oxygen bond covalency and Li content are identified to affect oxidation stability of polyanionic oxide coatings.

The key component in lithium solid-state batteries (SSBs) is the solid electrolyte composed of lithium superionic conductors (SICs). Lithium oxide SICs offer improved electrochemical and chemical stability compared with sulfides, and their recent advancements have largely been achieved using materials in the garnet- and NASICON (sodium superionic conductor)- structured families. In this work, using the ion-conduction mechanisms in garnet and NASICON as inspiration, a common pattern of an “activated diffusion network” and three structural features that are beneficial for superionic conduction: a 3D percolation Li diffusion network, short distances between occupied Li sites, and the “homogeneity” of the transport path are identified. A high-throughput computational screening is performed to search for new lithium oxide SICs that share these features. From this search, seven candidates are proposed exhibiting high room-temperature ionic conductivity evaluated using ab initio molecular dynamics simulations. Their structural frameworks including spinel, oxy-argyrodite, sodalite, and LiM(SeO3)2 present new opportunities for enriching the structural families of lithium oxide SICs.

Low active material loading in the composite electrode of all-solid-state batteries (SSBs) is one of the main reasons for the low energy density in current SSBs. In this work, it is demonstrated with both modeling and experiments that in the regime of high cathode loading, the utilization of cathode material in the solid-state composite is highly dependent on the particle size ratio of the cathode to the solid-state conductor. The modeling, confirmed by experimental data, shows that higher cathode loading and therefore an increased energy density can be achieved by increasing the ratio of the cathode to conductor particle size. These results are consistent with ionic percolation being the limiting factor in cold-pressed solid-state cathode materials and provide specific guidelines on how to improve the energy density of composite cathodes for solid-state batteries. By reducing solid electrolyte particle size and increasing the cathode active material particle size, over 50 vol% cathode active material loading with high cathode utilization is able to be experimentally achieved, demonstrating that a commercially-relevant, energy-dense cathode composite is achievable through simple mixing and pressing method.

Recent theoretical work has uncovered that a body-centered-cubic (bcc) anion arrangement leads to high ionic conductivity in a number of fast lithium-ion conducting materials. Using this structural feature as a screening criterion, we find that the I4 material LiZnPS4 contains such a framework and has the potential for very high ionic conductivity. In this work, we apply ab initio computational techniques to investigate in detail the ionic conductivity and defect properties of this material. We find that while the stoichiometric structure has poor ionic conductivity, engineering of its composition to introduce interstitial lithium defects is able to exploit the low migration barrier of the bcc anion framework. Our calculations predict a solid-solution regime extending to x = 0.5 in Li1+2xZn1-xPS4, and yield a new ionic conductor with exceptionally high lithium-ion conductivity, potentially exceeding 50 mS cm-1 at room temperature.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Inorganic solid-state ionic conductors with high ionic conductivity are of great interest for their application in safe and high-energy-density solid-state batteries. Our previous study reveals that the crystal structure of the ionic conductor Li7P3S11 contains a body-centered-cubic (bcc) arrangement of sulfur anions and that such a bcc anion framework facilitates high ionic conductivity. Here, we apply a set of first-principles calculations techniques to investigate A7P3X11-type (A = Li, Na; X = O, S, Se) lithium and sodium superionic conductors derived from Li7P3S11, focusing on their structural, dynamic and thermodynamic properties. We find that the ionic conductivity of Na7P3S11 and Na7P3Se11 is over 10 mS cm-1 at room temperature, significantly higher than that of any known solid Na-ion sulfide or selenide conductor. However, thermodynamic calculations suggest that the isostructural sodium compounds may not be trivial to synthesize, which clarifies the puzzle concerning the experimental problems in trying to synthesize these compounds.

Solid-state batteries (SSBs) using a solid electrolyte show potential for providing improved safety as well as higher energy and power density compared with conventional Li-ion batteries. However, two critical bottlenecks remain: the development of solid electrolytes with ionic conductivities comparable to or higher than those of conventional liquid electrolytes and the creation of stable interfaces between SSB components, including the active material, solid electrolyte and conductive additives. Although the first goal has been achieved in several solid ionic conductors, the high impedance at various solid/solid interfaces remains a challenge. Recently, computational models based on ab initio calculations have successfully predicted the stability of solid electrolytes in various systems. In addition, a large amount of experimental data has been accumulated for different interfaces in SSBs. In this Review, we summarize the experimental findings for various classes of solid electrolytes and relate them to computational predictions, with the aim of providing a deeper understanding of the interfacial reactions and insight for the future design and engineering of interfaces in SSBs. We find that, in general, the electrochemical stability and interfacial reaction products can be captured with a small set of chemical and physical principles.

Li9S3N (LSN) is investigated as a new lithium ion conductor and barrier coating between an electrolyte and Li metal anode in all solid state lithium ion batteries. LSN is an intriguing material since it has a 3-dimensional conduction channel, high lithium content, and is expected to be stable against lithium metal. The conductivity of LSN is measured with impedance spectroscopy as 8.3 × 10-7 S cm-1 at room temperature with an activation energy of 0.52 eV. Cyclic voltammetry (CV) scans showed reversible Li plating and striping. First principles calculations of stability, nudged elastic band (NEB) calculations, and ab initio molecular dynamics (AIMD) simulations support these experimental results. Substitution as a means to enhance conductivity is also investigated. First-principles calculations predict that divalent cation substituents displace a lithium from a tetrahedral site along the migration pathway, and reduce the migration energy for the lithium ions in the vicinity of the substituent. A percolating path with low migration energies (∼0.3 eV) can be formed throughout the crystal structure at a concentration of Li8.5M0.25S3N (M = Ca2+, Zn2+, or Mg2+), resulting in predicted conductivities as high as σ300 K = 2.3 mS cm-1 at this concentration. However, the enhanced conductivity comes at the expense of relatively large substitution energy. Halide substitution, such as Cl on a S site (in Kröger-Vink notation), has a relatively low energy cost, but only provides a modest improvement in conductivity.

The interfacial instability between a thiophosphate solid electrolyte and oxide cathodes results in rapid capacity fade and has driven the need for cathode coatings. In this work, the stability, evolution, and performance of uncoated, Li2ZrO3-coated, and Li3B11O18-coated LiNi0.5Co0.2Mn0.3O2 cathodes are compared using first-principles computations and electron microscopy characterization. Li3B11O18 is identified as a superior coating that exhibits excellent oxidation/chemical stability, leading to substantially improved performance over cells with Li2ZrO3-coated or uncoated cathodes. The chemical and structural origin of the different performance is interpreted using different microscopy techniques which enable the direct observation of the phase decomposition of the Li2ZrO3 coating. It is observed that Li is already extracted from the Li2ZrO3 in the first charge, leading to the formation of ZrO2 nanocrystallites with loss of protection of the cathode. After 50 cycles separated (Co, Ni)-sulfides and Mn-sulfides can be observed within the Li2ZrO3-coated material. This work illustrates the severity of the interfacial reactions between a thiophosphate electrolyte and oxide cathode and shows the importance of using coating materials that are absolutely stable at high voltage.

The reactivity of mixtures of high voltage spinel cathode materials Li₂NiMn₃O₈, Li₂FeMn₃O₈, and LiCoMnO₄ cosintered with Li_1.5Al_0.5Ti_1.5(PO₄)₃ and Li_6.6La₃Zr_1.6Ta_0.4O₁₂ electrolytes is studied by thermal analysis using X-ray-diffraction and differential thermoanalysis and thermogravimetry coupled with mass spectrometry. The results are compared with predicted decomposition reactions from first-principles calculations. Decomposition of the mixtures begins at 600 °C, significantly lower than the decomposition temperature of any component, especially the electrolytes. For the cathode + Li_6.6La₃Zr_1.6Ta_0.4O₁₂ mixtures, lithium and oxygen from the electrolyte react with the cathodes to form highly stable Li₂MnO₃ and then decompose to form stable and often insulating phases such as La₂Zr₂O₇, La₂O₃, La₃TaO₇, TiO₂, and LaMnO₃ which are likely to increase the interfacial impedance of a cathode composite. The decomposition reactions are identified with high fidelity by first-principles calculations. For the cathode + Li_1.5Al_0.5Ti_1.5(PO₄)₃ mixtures, the Mn tends to oxidize to MnO₂ or Mn₂O₃, supplying lithium to the electrolyte for the formation of Li₃PO₄ and metal phosphates such as AlPO₄ and LiMPO₄ (M = Mn, Ni). The results indicate that high temperature cosintering to form dense cathode composites between spinel cathodes and oxide electrolytes will produce high impedance interfacial products, complicating solid state battery manufacturing.