There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In the critical area of sustainable energy storage, solid-state batteries have attracted considerable attention due to their potential safety, energy-density and cycle-life benefits. This Review describes recent progress in the fundamental understanding of inorganic solid electrolytes, which lie at the heart of the solid-state battery concept, by addressing key issues in the areas of multiscale ion transport, electrochemical and mechanical properties, and current processing routes. The main electrolyte-related challenges for practical solid-state devices include utilization of metal anodes, stabilization of interfaces and the maintenance of physical contact, the solutions to which hinge on gaining greater knowledge of the underlying properties of solid electrolyte materials. Solid-state batteries are attractive due to their potential safety, energy-density and cycle-life benefits. Recent progress in understanding inorganic solid electrolytes considering multiscale ion transport, electrochemical and mechanical properties, and processing are discussed.

Fundamentals of inorganic solid-state electrolytes for batteries

Commercial lithium-ion (Li-ion) batteries suffer from low energy density and do not meet the growing demands of the energy storage market. Therefore, building next-generation rechargeable Li and Li-ion batteries with higher energy densities, better safety characteristics, lower cost and longer cycle life is of outmost importance. To achieve smaller and lighter next-generation rechargeable Li and Li-ion batteries that can outperform commercial Li-ion batteries, several new energy storage chemistries are being extensively studied. In this review, we summarize the current trends and provide guidelines towards achieving this goal, by addressing batteries using high-voltage cathodes, metal fluoride electrodes, chalcogen electrodes, Li metal anodes, high-capacity anodes as well as useful electrolyte solutions. We discuss the choice of active materials, practically achievable energy densities and challenges faced by the respective battery systems. Furthermore, strategies to overcome remaining challenges for achieving energy characteristics are addressed in the hope of providing a useful and balanced assessment of current status and perspectives of rechargeable Li and Li-ion batteries.

Guidelines and trends for next-generation rechargeable lithium and lithium-ion batteries.

Over the past several years, important strides have been made in demonstrating protonic ceramic fuel cells (PCFCs). Such fuel cells offer the potential of environmentally sustainable and cost-effective electric power generation. However, their power outputs have lagged behind predictions based on their high electrolyte conductivities. Here we overcome PCFC performance and stability challenges by employing a high-activity cathode, PrBa_(0.5)Sr_(0.5)Co_(1.5)Fe_(0.5)O_(5+δ) (PBSCF), in combination with a chemically stable electrolyte, BaZr_(0.4)Ce_(0.4)Y_(0.1)Yb_(0.1)O_3 (BZCYYb4411). We deposit a thin dense interlayer film of the cathode material onto the electrolyte surface to mitigate contact resistance, an approach which is made possible by the proton permeability of PBSCF. The peak power densities of the resulting fuel cells exceed 500 mW cm^(−2) at 500 °C, while also offering exceptional, long-term stability under CO_2.

Exceptional power density and stability at intermediate temperatures in protonic ceramic fuel cells

Carbonate electrolytes are commonly used in commercial non-aqueous Li-ion batteries. However, the high affinity between the solvents and the ions and high flammability of the carbonate electrolytes limits the battery operation temperature window to −20 to + 50 °C and the voltage window to 0.0 to 4.3 V. Here, we tame the affinity between solvents and Li ions by dissolving fluorinated electrolytes into highly fluorinated non-polar solvents. In addition to their non-flammable characteristic, our electrolytes enable high electrochemical stability in a wide voltage window of 0.0 to 5.6 V, and high ionic conductivities in a wide temperature range from −125 to + 70 °C. We show that between −95 and + 70 °C, the electrolytes enable LiNi0.8Co0.15Al0.05O2 cathodes to achieve high Coulombic efficiencies of >99.9%, and the aggressive Li anodes and the high-voltage (5.4 V) LiCoMnO4 to achieve Coulombic efficiencies of >99.4% and 99%, respectively. Even at −85 °C, the LiNi0.8Co0.15Al0.05O2 || Li battery can still deliver ~50% of its room-temperature capacity. Batteries generally do not perform well at extreme temperatures, and electrolytes are mainly to blame. Here, the authors dissolve fluorinated electrolytes in highly fluorinated non-polar solvents, enabling batteries that can operate at a wide temperature range (−125 to +70 °C).

All-temperature batteries enabled by fluorinated electrolytes with non-polar solvents

Na-based batteries have shown substantial progress in recent years and are promising candidates for mitigating the supply risks associated with Li-based batteries. In this Review, Na and Li batteries are compared in terms of fundamental principles and specific materials. Principles for the rational design of a Na battery architecture are discussed. Recent prototypes are surveyed to demonstrate that Na cells offer realistic alternatives that are competitive with some Li cells in terms of performance. Sodium batteries are promising candidates for mitigating the supply risks associated with lithium batteries. This Review compares the two technologies in terms of fundamental principles and specific materials, and assesses the performance of commercial prototype sodium cells.

Fundamentals, status and promise of sodium-based batteries

BACKGROUND Developing high-performance, affordable, and durable batteries is one of the decisive technological tasks of our generation. Designing such batteries requires more than just the identification of electroactive storage materials with desirable properties such as high voltage, high capacity, and sufficient stability. These materials must also be assembled with ion- and electron-conducting phases into a composite electrode architecture, and this step is of equal significance; the size, shape, and spatial distribution of the various phases have a decisive influence on the charging and discharging rate capability of the electrode. Because the combined motion of ions and electrons within the solid is notoriously sluggish at room temperature, reduction of the transport length by downsizing the storage particles is indispensable. However, this necessity shifts the transport problems to the electrode’s internal circuitry. That is, every electroactive particle must be part of a network that rapidly provides both ions from an electrolyte and electrons from an electronic current collector. If nanosized particles are used, then a myriad of particles and connections are required, resulting in a network with a degree of nanoscale intricacy comparable to that of electronic circuits in information technology or bioelectrochemical networks in living systems (see the figure). While electronic circuits are based on electron transport and bioelectrochemistry relies on ion motion, battery electrodes require a combination of electron-conducting, ion-conducting, and mixed-conducting phases. ADVANCES Criteria for rational electrode design are based on transport and dimensional parameters. The optimal size of the storage particles can be estimated from the chemical diffusivity and the (dis)charging rate needed in the application. An important strategy is to implement two different length scales over which ions and electrons must diffuse within the storage phase to reach the ionic (electrolyte) and electronic current collector phases. The optimal values of these two “wiring lengths” depend on the ionic and electronic conductivities of the storage material. Nanoscale structures with different dimensionalities such as dots, fibers, and sheets are often useful for various reasons. If the current-collecting phases exhibit appreciable resistance due to tiny feature sizes, a hierarchical structure is helpful. Such considerations help to optimize the morphology and simplify the circuitry. They also provide a conceptual framework for organizing the vast literature on nanostructured battery materials and for reviewing characteristic topologies in a systematic way. To realize the targeted electrode architectures, substantial progress has been achieved by adapting various chemical and physical preparation methods. The figure shows lithium-storing tin particles embedded in a 2-μm-diameter carbon fiber prepared by electrospinning. Often a synthesized structure must deviate from the general design criteria to address material-specific stability issues such as interfacial reactivity, crack formation, agglomeration, and dendrite growth. Structural evolution during battery cycling is being elucidated by new methods based on x-ray absorption, electron microscopy, and other nondestructive probes that can provide in operando analysis at a variety of length scales. OUTLOOK More powerful multiscale computational approaches are needed to adequately model the mixed-conducting electrode networks. Such numerical treatments will be helpful in optimizing electrode structures beyond the semiquantitative design rules reviewed here. The further development of synthetic methods is also vital, so that the desired complex and hierarchical architectures can be systematically reduced into practice. Whether or not even self-organized networks may be realized—as is the case in bioelectrochemical systems—remains to be seen. Finally, continued development of in operando characterization methods will surely boost our still-limited understanding of combined ion and electron transport in the presence of complicating factors such as mechanical strain, slow interfacial reactions, crystallographic anisotropy, phase transformations, and morphological instability.

The nanoscale circuitry of battery electrodes

https://iopscience.iop.org/article/10.1149/2.1121910jes/pdf

Transport and Charge Carrier Chemistry in Lithium Oxide

Transport and Charge Carrier Chemistry in Lithium Sulfide

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aenm.202001455

Robert Usiskin

Papers

Fundamentals, status and promise of sodium-based batteries

The nanoscale circuitry of battery electrodes

Transport and Charge Carrier Chemistry in Lithium Oxide

Transport and Charge Carrier Chemistry in Lithium Sulfide

Interfacial Effects in Lithium and Sodium Batteries