The demand for lithium batteries with energy densities beyond those of lithium-ion has driven the recent studies on lithium metal anode. High-efficiency electrochemical cycling of lithium requires ...improved lithium deposition morphology and reduced parasitic reactions between lithium and the liquid electrolyte. A protective layer on lithium metal is expected to reduce contact between lithium metal and the organic solvent, exert compressive mechanical force on the anode, and improve the selectivity and uniformity of lithium ion transport at the electrode surface. This review covers recent advancements in this topic. We first establish the design criteria for an effective coating followed by a brief description of the methods for depositing the layer, characterizing its structure and morphology, and evaluating its electrochemical performance. Our discussion of the literature is organized on resultant layer composition and corresponding ion conduction mechanisms. In the case of polymeric materials, the polarity difference between the polymer and electrolyte solvents determines the degree of swelling and selectivity of lithium ion transport. We conclude by advocating for the need of increased mechanistic study for the functioning mechanism, improved understanding of layer degradation, and demonstration of the protective function in realistic cell environment, namely lean electrolytes and coupled with appropriate cathodes.
•This review covers recent advancements in protective coatings for Li metal anode.•The design criteria for an effective coating are proposed.•Ion conduction mechanisms of inorganic and polymeric coatings are discussed.•Demonstration of the protective function in realistic cell condition is advocated.
Abstract
Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable ...batteries. The promising P2-layered sodium transition metal oxides (P2-Na
x
TmO
2
) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-Na
x
TmO
2
have not yet been clearly elucidated. Herein, taking P2-Na
0.67
MnO
2
and P2-Na
0.67
Ni
0.33
Mn
0.67
O
2
as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-Na
x
TmO
2
in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-Na
x
TmO
2
is determined by the amount of extracted Na
+
, which is mainly compensated by Na
+
/H
+
exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-Na
x
TmO
2
is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable Na
x
TmO
2
cathodes.
Due to their high specific capacities beyond 250 mA h g
−1
, lithium-rich oxides have been considered as promising cathodes for the next generation power batteries, bridging the capacity gap between ...traditional layered-oxide based lithium-ion batteries and future lithium metal batteries such as lithium sulfur and lithium air batteries. However, the practical application of Li-rich oxides has been hindered by formidable challenges. To address these challenges, the understanding of their electrochemical behaviors becomes critical and is expected to offer effective guidance for both materials and cell development. This review aims to provide fundamental insights into the reaction mechanisms, electrochemical challenges and modification strategies of lithium-rich oxides. We first summarize the research history, the pristine structures, and the classification of lithium-rich oxides. Then we review the critical reaction mechanisms that are closely related to their electrochemical features and performances, such as lattice oxygen oxidation, oxygen vacancy formation, transition-metal migration, layered to spinel transitions, 'two-phase mechanism', and lattice evolution. These discussions are coupled with state-of-the-art characterization techniques. As a comparison, the anionic redox reactions of layered sodium transition metal oxides are also discussed. Finally, after a brief overview of the correlation among the aforementioned mechanisms, we provide perspectives on the rational design of lithium-rich oxides with high energy densities and long-term cycling stability.
This review summarizes the history and critical working mechanisms of Li-rich oxides with a special focus on anionic redox reactions.
Lattice oxygen can play an intriguing role in electrochemical processes, not only maintaining structural stability, but also influencing electron and ion transport properties in high-capacity oxide ...cathode materials for Li-ion batteries. Here, we report the design of a gas-solid interface reaction to achieve delicate control of oxygen activity through uniformly creating oxygen vacancies without affecting structural integrity of Li-rich layered oxides. Theoretical calculations and experimental characterizations demonstrate that oxygen vacancies provide a favourable ionic diffusion environment in the bulk and significantly suppress gas release from the surface. The target material is achievable in delivering a discharge capacity as high as 301 mAh g(-1) with initial Coulombic efficiency of 93.2%. After 100 cycles, a reversible capacity of 300 mAh g(-1) still remains without any obvious decay in voltage. This study sheds light on the comprehensive design and control of oxygen activity in transition-metal-oxide systems for next-generation Li-ion batteries.
Potassium‐ion batteries (PIBs) are promising alternatives to lithium‐ion batteries because of the advantage of abundant, low‐cost potassium resources. However, PIBs are facing a pivotal challenge to ...develop suitable electrode materials for efficient insertion/extraction of large‐radius potassium ions (K+). Here, a viable anode material composed of uniform, hollow porous bowl‐like hard carbon dual doped with nitrogen (N) and phosphorus (P) (denoted as N/P‐HPCB) is developed for high‐performance PIBs. With prominent merits in structure, the as‐fabricated N/P‐HPCB electrode manifests extraordinary potassium storage performance in terms of high reversible capacity (458.3 mAh g−1 after 100 cycles at 0.1 A g−1), superior rate performance (213.6 mAh g−1 at 4 A g−1), and long‐term cyclability (205.2 mAh g−1 after 1000 cycles at 2 A g−1). Density‐functional theory calculations reveal the merits of N/P dual doping in favor of facilitating the adsorption/diffusion of K+ and enhancing the electronic conductivity, guaranteeing improved capacity, and rate capability. Moreover, in situ transmission electron microscopy in conjunction with ex situ microscopy and Raman spectroscopy confirms the exceptional cycling stability originating from the excellent phase reversibility and robust structure integrity of N/P‐HPCB electrode during cycling. Overall, the findings shed light on the development of high‐performance, durable carbon anodes for advanced PIBs.
A viable anode material composed of nitrogen/phosphorus co‐doped hollow porous bowl‐like hard carbon is developed for potassium ion batteries. The resulting anode manifests prominent merits in structure, endowing it with extraordinary K+ storage capability. The K+ storage mechanisms are revealed through in‐depth studies by combining in situ TEM studies, ex situ microscopic, and Raman spectroscopy in conjunction with DFT calculations.
The Li-excess oxide compound is one of the most promising positive electrode materials for next generation batteries exhibiting high capacities of >300 mA h g
−1
due to the unconventional ...participation of the oxygen anion redox in the charge compensation mechanism. However, its synthesis has been proven to be highly sensitive to varying conditions and parameters where nanoscale phase separation may occur that affects the overall battery performance and life. In addition, several thermodynamic and kinetic drawbacks including large first cycle irreversible capacity, poor rate capability, voltage fading, and surface structural transformation need to be addressed in order to reach commercialization. This review will focus on the recent progress and performance trends over the years and provide several guidelines and design considerations based on the library of work done on this particular class of materials.
The Li-excess layered oxide compound is one of the most promising positive electrode materials for next generation batteries exhibiting high capacities of >300 mA h g
−1
due to the unconventional participation of the oxygen anion redox in the charge compensation mechanism.
Hydrogels are investigated broadly in flexible sensors which have been applied into wearable electronics. However, further application of hydrogels is restricted by the ambiguity of the sensing ...mechanisms, and the multi-functionalization of flexible sensing systems based on hydrogels in terms of cost, difficulty in integration, and device fabrication remains a challenge, obstructing the specific application scenarios. Herein, cost-effective, structure-specialized and scenario-applicable 3D printing of direct ink writing (DIW) technology fabricated two-dimensional (2D) transition metal carbides (MXenes) bonded hydrogel sensor with excellent strain and temperature sensing performance is developed. Gauge factor (GF) of 5.7 (0 - 191% strain) and high temperature sensitivity (-5.27% °C
) within wide working range (0 - 80 °C) can be achieved. In particular, the corresponding mechanisms are clarified based on finite element analysis and the first use of in situ temperature-dependent Raman technology for hydrogels, and the printed sensor can realize precise temperature indication of shape memory solar array hinge.
A variety of nanoporous transition metals, Fe, Co, Au, Cu, and others, have been readily formed by a scalable, room-temperature synthesis process. Metal halide compounds are reacted with ...organolithium reductants in a nonpolar solvent to form metal/lithium halide nanocomposites. The lithium halide is then dissolved out of the nanocomposite with a common organic solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a nanoporous structure. This approach is applicable to both noble metals (Cu, Au, Ag) and less-noble transition metals (Co, Fe, Ni). The microstructures of these nanoporous transition metals are tunable, as controlling the formation of the metal structure in the nanocomposite dictates the final metal structure. Microscopy studies and nitrogen adsorption analysis show these materials form pores ranging from 2 to 50 nm with specific surface areas from 1.0 m2/g to 160 m2/g. Our analysis also shows that pore size, pore volume, and filament size of the nanoporous metal networks depend on the mobility of target metal and the amount of lithium halide produced by the conversion reaction. Further, it has been demonstrated that hybrid nanoporous structures of two or more metals could be synthesized by performing the same process on mixtures of precursor compounds. Metals (e.g., Co and Cu) have been found to stabilize each other in nanoporous forms, resulting in smaller pore sizes and higher surface areas than each element in their pure forms. This scalable and versatile synthesis pathway greatly expands our access to additional compositions and microstructures of nanoporous metals.
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