Grid-scale energy storage systems (ESSs) that can connect to sustainable energy resources have received great attention in an effort to satisfy ever-growing energy demands. Although recent advances in Li-ion battery (LIB) technology have increased the energy density to a level applicable to grid-scale ESSs, the high cost of Li and transition metals have led to a search for lower-cost battery system alternatives. Based on the abundance and accessibility of Na and its similar electrochemistry to the well-established LIB technology, Na-ion batteries (NIBs) have attracted significant attention as an ideal candidate for grid-scale ESSs. Since research on NIB chemistry resurged in 2010, various positive and negative electrode materials have been synthesized and evaluated for NIBs. Nonetheless, studies on NIB chemistry are still in their infancy compared with LIB technology, and further improvements are required in terms of energy, power density, and electrochemical stability for commercialization. Most recent progress on electrode materials for NIBs, including the discovery of new electrode materials and their Na storage mechanisms, is briefly reviewed. In addition, efforts to enhance the electrochemical properties of NIB electrode materials as well as the challenges and perspectives involving these materials are discussed.

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Recent Progress in Electrode Materials for Sodium-Ion Batteries

This comprehensive Review focuses on the key challenges and recent progress regarding sodium-metal anodes employed in sodium-metal batteries (SMBs) The metal anode is the essential component of emerging energy storage systems such as sodium sulfur and sodium selenium, which are discussed as example full-cell applications We begin with a description of the differences in the chemical and physical properties of Na metal versus the oft-studied Li metal, and a corresponding discussion regarding the number of ways in which Na does not follow Li-inherited paradigms in its electrochemical behavior We detail the major challenges for Na-metal systems that at this time limit the feasibility of SMBs The core Na anode problems are the following interrelated degradation mechanisms: An unstable solid electrolyte interphase with most organic electrolytes, "mossy" and "lath-like" metal dendrite growth for liquid systems, poor Coulombic efficiency, and gas evolution Even solid-state Na batteries are not immune, with metal dendrites being reported The solutions may be subdivided into the following interrelated taxonomy: Improved electrolytes and electrolyte additives tailored for Na-metal anodes, interfacial engineering between the metal and the liquid or solid electrolyte, electrode architectures that both reduce the current density during plating-stripping and serve as effective hosts that shield the Na metal from excessive reactions, and alloy design to tune the bulk properties of the metal per se For instance, stable plating-stripping of Na is extremely difficult with conventional carbonate solvents but has been reported with ethers and glymes Solid-state electrolytes (SSEs) such as beta-alumina solid electrolyte (BASE), sodium superionic conductor (NASICON), and sodium thiophosphate (75Na2S·25P2S5) present highly exciting opportunities for SMBs that avoid the dangers of flammable liquids Even SSEs are not immune to dendrites, however, which grow through the defects in the bulk pellet, but may be controlled through interfacial energy modification We conclude with a discussion of the key research areas that we feel are the most fruitful for further pursuit In our opinion, greatly improved understanding and control of the SEI structure is the key to cycling stability A holistic approach involving complementary post-mortem, in situ, and operando analyses to elucidate full battery cell level structure-performance relations is advocated

Sodium Metal Anodes: Emerging Solutions to Dendrite Growth

Rechargeable Na-ion batteries (NIBs) are attractive large-scale energy storage systems compared to Li-ion batteries due to the substantial reserve and low cost of sodium resources. The recent rapid development of NIBs will no doubt accelerate the commercialization process. As one of the indispensable components in current battery systems, organic liquid electrolytes are widely used for their high ionic conductivity and good wettability, but the low thermal stability, especially the easy flammability and leakage make them at risk of safety issues. The booming solid-state batteries with solid-state electrolytes (SSEs) show promise as alternatives to organic liquid systems due to their improved safety and higher energy density. However, several challenges including low ionic conductivity, poor wettability, low stability/incompatibility between electrodes and electrolytes, etc., may degrade performance, hindering the development of practical applications. In this review, an overview of Na-ion SSEs is first outlined according to the classification of solid polymer electrolytes, composite polymer electrolytes, inorganic solid electrolytes, etc. Furthermore, the current challenges and critical perspectives for the potential development of solid-state sodium batteries are discussed in detail.

Solid-State Sodium Batteries

K.H.P. and Q.B. contributed equally to this work. K.H.P., D.H.K., D.Y.O., and Y.S.J. were supported by the Technology Development Program to Solve Climate Changes and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT a Future Planning (Nos. NRF-2017M1A2A2044501 and NRF-2018R1A2B6004996), and by the Materials and Components Technology Development Program of MOTIE/KEIT (10077709). Q.B., Y.Z., and Y.M. were supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, under Award Nos. DE-EE0006860 and DE-EE0007807.

Design Strategies, Practical Considerations, and New Solution Processes of Sulfide Solid Electrolytes for All-Solid-State Batteries

DOI: 10.1002/aenm.201903242 materials in silico,[19–22] high computational costs and poor scaling still limit their effectiveness in exploring unconstrained chemical spaces and/or complex real-world materials. For instance, highthroughput DFT screening works typically limit the search space to hundreds or, at best, thousands of materials, while DFT simulations of materials are mostly limited to typically less than 1000 atoms, i.e., bulk crystals and isolated molecules. ML therefore offers a solution to the materials exploration problem, making predictions of new materials or properties from existing data, which in turn can drive the generation of more data that can be used to further refine the ML models. Here, we will provide an in-depth, critical review of MLguided design and discovery of energy materials, a field where a novel material with superior performance (e.g., higher energy density, higher energy conversion efficiency, etc.) can have a transformative impact on the urgent global problem of climate change. This review is structured along the steps in a typical workflow for materials ML model building, as shown in Figure 1. The next four sections will provide a concise overview of ML concepts designed to give the reader an appreciation of state-of-the-art techniques as well as resources for building ML models for materials. Section 6 reviews the actual application of ML techniques to the discovery and design of various classes of energy materials, from energy storage (e.g., batteries, fuel cells, etc.) to energy conversion (e.g., thermoelectrics, catalysis, etc.). The final section outlines our perspectives on various challenges and opportunities in ML for energy materials design.

A Critical Review of Machine Learning of Energy Materials

The identification of materials for advanced energy-storage systems is still mostly based on experimental trial and error. Increasingly, computational tools are sought to accelerate materials discovery by computational predictions. Here are introduced a set of computationally inexpensive software tools that exploit the bond-valence-based empirical force field previously developed by the authors to enable high-throughput computational screening of experimental or simulated crystal-structure models of battery materials predicting a variety of properties of technological relevance, including a structure plausibility check, surface energies, an inventory of equilibrium and interstitial sites, the topology of ion-migration paths in between those sites, the respective migration barriers and the site-specific attempt frequencies. All of these can be predicted from CIF files of structure models at a minute fraction of the computational cost of density functional theory (DFT) simulations, and with the added advantage that all the relevant pathway segments are analysed instead of arbitrarily predetermined paths. The capabilities and limitations of the approach are evaluated for a wide range of ion-conducting solids. An integrated simple kinetic Monte Carlo simulation provides rough (but less reliable) predictions of the absolute conductivity at a given temperature. The automated adaptation of the force field to the composition and charge distribution in the simulated material allows for a high transferability of the force field within a wide range of Lewis acid–Lewis base-type ionic inorganic compounds as necessary for high-throughput screening. While the transferability and precision will not reach the same levels as in DFT simulations, the fact that the computational cost is several orders of magnitude lower allows the application of the approach not only to pre-screen databases of simple structure prototypes but also to structure models of complex disordered or amorphous phases, and provides a path to expand the analysis to charge transfer across interfaces that would be difficult to cover by ab initio methods.

SoftBV - a software tool for screening the materials genome of inorganic fast ion conductors.

Solid-state fast ionic conductors are of great interest due to their application potential enabling the development of safer high-performance energy and conversion systems ranging from batteries th...

Bond Valence Pathway Analyzer—An Automatic Rapid Screening Tool for Fast Ion Conductors within softBV

Bond-valence site energy modelling, classical molecular dynamics and DFT simulations were employed to clarify Na+ ion migration in monoclinic Na2+δFe2−δ/2(SO4)3, the recently reported first representative of a new promising class of alluaudite-type high voltage cathode materials for sodium-ion batteries. Empirical potential parameters derived from our softBV bond valence parameter set reproduce experimental unit-cell parameters. Migration energy barrier calculations based on both these empirical and on ab initio approaches consistently show a strongly anisotropic and fairly fast Na+ ion mobility along partially occupied Na(3) channels in the c-direction. Nominally fully occupied Na(1) sites are attached to these paths with a moderate activation energy as sources of mobile ions. At elevated temperatures separate parallel Na(2) channels contribute to the ionic conductivity. As such one-dimensional pathways are highly vulnerable to blocking by structural defects, the experimentally observed favourable rate performance can only be understood as a consequence of cross-linking of the channels to a more robust higher-dimensional migration pathway network. Our static and dynamic bond valence pathway models for representative local structure models reveal that this cross-linking is achieved by the iron deficiency of the compound: iron vacancies act as low-lying interstitial sites that can be reached from both types of channels with moderate activation energies. Structural relaxations around the vacancies however reduce the sodium mobility along the channels. An analogous dual effect of blocking migration along the channels and promoting perpendicular migration would result from Na+/Fe2+ antisite defects. Hence, further new alluaudite type transition metal sulphates can only be expected to yield a high rate performance, if their synthesis ensures the presence of a comparable transition metal sub-stoichiometry and/or a suitably tailored concentration of sodium/transition metal antisite defects.

Sodium-ion diffusion mechanisms in the low cost high voltage cathode material Na2+δFe2−δ/2(SO4)3

Electrolytes in current Na-ion batteries are mostly based on the same fundamental chemistry as those in Li-ion batteries – a mixture of flammable liquid cyclic and linear organic carbonates leading to the same safety concerns especially during fast charging. All-solid-state Na-ion rechargeable batteries utilizing non-flammable ceramic Na superionic conductor electrolytes are a promising alternative. Among the known sodium conducting electrolytes the cubic Na3PS4 phase has relatively high sodium ion conductivity exceeding 10−4 S cm−1 at room temperature. Here we systematically study the doping of Na3PS4 with Ge4+, Ti4+, Sn4+ and optimise the processing of these phases. A maximum ionic conductivity of 2.5 × 10−4 S cm−1 is achieved for Na3.1Sn0.1P0.9S4. Utilising this fast Na+ ion conductor, a new class of all-solid-state Na2+2δFe2−δ(SO4)3|Na3+xMxP1−xS4 (M = Ge4+, Ti4+, Sn4+) (x = 0, 0.1)|Na2Ti3O7 sodium-ion secondary batteries is demonstrated that is based on earth-abundant safe materials and features high rate capability even at room temperature. All-solid-state Na2+2δFe2−δ(SO4)3|Na3+xMxP1−xS4|Na2Ti3O7 cells with the newly prepared electrolyte exhibited charge–discharge cycles at room temperature between 1.5 V and 4.0 V. At low rates the initial capacity matches the theoretical capacity of ca. 113 mA h g−1. At 2C rate the first discharge capacity at room temperature is still 83 mA h per gram of Na2+2δFe2−δ(SO4)3 and at 80 °C it rises to 109 mA h per gram with 80% capacity retention over 100 cycles.

Na3+xMxP1−xS4 (M = Ge4+, Ti4+, Sn4+) enables high rate all-solid-state Na-ion batteries Na2+2δFe2−δ(SO4)3|Na3+xMxP1−xS4|Na2Ti3O7

The obvious cost advantage as well as attractive electrochemical properties, including excellent cycling stability and the potential of high rate performance, make sodium-ion batteries prime candidates in the race to technically and commercially enable large-scale electrochemical energy storage. In this work, we apply our bond valence site energy modelling method to further the understanding of rate capabilities of a wide range of potential insertion-type sodium-ion battery cathode materials. We demonstrate how a stretched exponential function permits us to systematically quantify the rate performance, which in turn reveals guidelines for the design of novel sodium-ion battery chemistries suitable for high power, grid-scale applications. Starting from a diffusion relaxation model, we establish a semi-quantitative prediction of the rate-performance of half-cells from the structure of the cathode material that factors in dimensionality of Na+ ion migration pathways, the height of the migration barriers and the crystallite size of the active material. With the help of selected examples, we also illustrate the respective roles of unoccupied low energy sites within the pathway and temperature towards the overall rate capability of insertion-type cathode materials.

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Lee Loong Wong

Papers

SoftBV - a software tool for screening the materials genome of inorganic fast ion conductors.

Bond Valence Pathway Analyzer—An Automatic Rapid Screening Tool for Fast Ion Conductors within softBV

Sodium-ion diffusion mechanisms in the low cost high voltage cathode material Na2+δFe2−δ/2(SO4)3

Na3+xMxP1−xS4 (M = Ge4+, Ti4+, Sn4+) enables high rate all-solid-state Na-ion batteries Na2+2δFe2−δ(SO4)3|Na3+xMxP1−xS4|Na2Ti3O7

Design of fast ion conducting cathode materials for grid-scale sodium-ion batteries.