This review summarizes the recent progress and remaining challenges of polyanion-type cathodes, providing guidelines towards high-performance cathodes for sodium ion batteries.

has been made to explore suitable cathode materials, which are expected to have high capacity, high potential, stable lattice framework, and fast sodium-ion diffusivity. By far, various cathode materials including layered transition-metal oxides, Prussian blue analogues (PBAs), and polyanion-type compounds have been proposed. Although the layered transition-metal oxide materials normally exhibit high theoretical capacity, they suffer from structural instability and unsatisfactory cycle-life. [5][6][7] On the other hand, the applications of PBAs are limited by their low volume density, inferior thermal stability, and toxicity of cyanide. [8][9][10] In this context, great attention has been paid on the polyanion-type materials due to their robust crystal framework with high-level thermal stability, and the moderate capacity with tunable high redox potential, which can achieve high energy density. [11][12][13] Recently, the Na superionic conductor (NASICON)-structured polyanion-type materials are of significant interest because of their unique crystal framework that is constructed by units of MO 6 octahedrons (M represents transition metals) and XO 4 tetrahedrons (X = P, S, Si, As), building 3D ion channels for fast Na + migration. [14][15][16][17][18] Among diverse NASICON-structured compounds, Na 3 V 2 (PO 4 ) 3 is a hotspot, which can deliver a highly reversible capacity over 110 mAh g −1 , and an energy density of over 370 Wh kg −1 as a result of a flat voltage plateau located at 3.3-3.4 V. [19] Although massive work has been reported to promote the development of Na 3 V 2 (PO 4 ) 3 by nanosizing and/or optimizing its poor electronic conductivity, [20][21][22][23] in order to meet the demand of practical applications of Na 3 V 2 (PO 4 ) 3 , improving its operating voltage to reach a higher energy density is urgent. In this regard, researchers have focused on ion-doping strategy to partially substitute V with other elements. Lavela and co-workers used Cr to replace 10% of V and the as-synthesized Na 3 V 1.8 Cr 0.2 (PO 4 ) 3 showed a short plateau above 3.8 V. [24] This high-potential plateau can be prolonged by increasing the amount of Cr to 50%, forming a new material Na 3 VCr(PO 4 ) 3 , which has been reported by Yang and co-workers. [25] In addition, introducing F into Na 3 V 2 (PO 4 ) 3 has been proven effective and the resulting Na 3 V 2 (PO 4 ) 2 F 3 could exhibit an average potential at 3.7-3.8 V, [26][27][28][29] apparently surpassing Na 3 V 2 (PO 4 ) 3 . However, both Cr and F are too expensive and environmentally hazardous to scale-up Na 3 V 2 (PO 4 ) 3 has attracted great attention due to its high energy density and stable structure. However, in order to boost its application, the discharge potential of 3.3-3.4 V (vs Na + /Na) still needs to be improved and substitution of vanadium with other lower cost and earth-abundant active redox elements is imperative. Therefore, the Na superionic conductor (NASICON)structured Na 4 MnV(PO 4 ) 3 seems to be more attractive due to its lower toxicity and higher volt...

Aqueous zinc-ion batteries have drawn increasing attention due to the intrinsic safety, costeffectiveness and high energy density. However, parasitic reactions and non-uniform dendrite growth on the Zn anode side impede their application. Herein, a multifunctional additive, ammonium dihydrogen phosphate (NHP), is introduced to regulate uniform zinc deposition and to suppress side reactions. The results show that the NH 4 + tends to be preferably absorbed on the Zn surface to form a "shielding effect" and blocks the direct contact of water with Zn. Moreover, NH 4 + and (H 2 PO 4 ) À jointly maintain pH values of the electrode-electrolyte interface. Consequently, the NHP additive enables highly reversible Zn plating/stripping behaviors in Zn//Zn and Zn//Cu cells. Furthermore, the electrochemical performances of Zn//MnO 2 full cells and Zn//active carbon (AC) capacitors are improved. This work provides an efficient and general strategy for modifying Zn plating/stripping behaviors and suppressing side reactions in mild aqueous electrolyte.

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The development of high energy/power density sodium-ion batteries (SIBs) is still challenged by the high redox potential of Na/Na + and large radius of Na + ions, thus requiring extensive further improvement to, in particular, enhance the capacity and voltage of cathode materials. Among the various types of cathodes, the polyanion cathodes have emerged as the most pragmatic option due to their outstanding thermostability, unique inductive effect, and flexible structures. In this Review, a critical overview of the design principles and engineering strategies of polyanion cathodes that could have a pivotal role in developing high energy/power density SIBs are presented. Specifically, the engineering of polyanion cathode materials for higher voltage and specific capacity to increase energy density is discussed. The way in which morphology control, architectural design, and electrode processing have been developed to increase power density for SIBs is also analyzed. Finally, the remaining challenges and the future research direction of this field are presented.

Phase engineering of nanomaterials (PEN) is critically important for the preparation of nanomaterials with new phases, which are key for the investigation of phase‐dependent physicochemical properties and applications. This essay presents the state‐of‐the‐art development of PEN for the unique properties of nanomaterials with unconventional phases and their applications in energy storage and conversion and catalytic reactions. Finally, personal perspectives on the challenges and future opportunities of PEN in various applications are also provided.

etc., is essentially crucial to help combat these hazards and build a sustainable society. Among them, rechargeable battery has been regarded as a key technology. In the past decade, we have witnessed that the prevailing lithium-ion batteries (LIBs) made our society more portable, intelligent, and cleaner. [17][18][19] Nevertheless, the limited lithium resources and rising cost hinder their applications in the long run, especially in the field of large-scale stationary energy storage for renewable energy resources (e.g., solar, tide, and wind power). Thus, it is a huge stimulus for researchers to explore more sustainable rechargeable battery systems, which are expected to involve abundant and nontoxic metals to reduce the cost and impacts on environment.Diversified rechargeable batteries such as, the monovalent sodium-ion batteries (SIBs), [20][21][22][23][24] potassium-ion batteries (PIBs), [25][26][27] bivalent zinc-ion batteries (ZIBs), [28][29][30][31] magnesium-ion batteries (MIBs), [32][33][34][35] calcium-ion batteries (CIBs), [36][37][38][39] and trivalent aluminum-ion batteries (AIBs), [40][41][42] have emerged and shown great energy storage promise. As depicted in Figure 1a, those nonlithium metals are much more abundant than Li, especially Al, Ca, Na, K, and Mg, all of which rank the top-8 abundant elements in earth crust. For SIBs and PIBs, since Al would not form alloys with Na and K, Al foil can be used as anode collector, which further lowers the prices of SIBs and PIBs. On the other hand, the higher standard potential of Na/Na + (−2.71 V vs standard hydrogen electrode, SHE) and K/K + (−2.93 V) and their heavier atomic weights make energy densities of SIBs and PIBs intrinsically lower than that of LIBs. For multivalent-ion batteries, the multielectron transfer enables their volumetric capacities (e.g., 5857 and 8056 mA h cm −3 for Zn and Al, respectively) higher than that of Li (2042 mA h cm −3 ). [43,44] Additionally, the small cation radius of Zn 2+ (0.74 Å), Mg 2+ (0.72 Å), and Al 3+ (0.54 Å) indicate that many intercalation electrode materials typical in LIBs may be also potential hosts for reversible intercalation of these multivalent ions. Combining all the above merits, one can anticipate that these emerging rechargeable batteries would be considered as promising alternatives to LIBs.In quest of safe, cost-effective, and high-performance rechargeable batteries, two technical routes have been

Metrics & MoreArticle Recommendations CONSPECTUS: Gold (Au), a transition metal with an atomic number of 79 in the periodic table of elements, was discovered in approximately 3000 B.C. Due to the ultrahigh chemical stability and brilliant golden color, Au had long been thought to be a most inert material and was widely utilized in art, jewelry, and finance. However, it has been found that Au becomes exceptionally active as a catalyst when its size shrinks to the nanometer scale.With continuous efforts toward the exploration of catalytic applications over the past decades, Au nanomaterials show critical importance in many catalytic processes. Besides catalysis, Au nanomaterials also possess other promising applications in plasmonics, sensing, biology and medicine, due to their unique localized surface plasmon resonance, intriguing biocompatibility, and superior stability. Unfortunately, the practical applications of Au nanomaterials could be limited because of the scarce reserves and high price of Au. Therefore, it is quite essential to further explore novel physicochemical properties and functions of Au nanomaterials so as to enhance their performance in different types of applications.Recently, phase engineering of nanomaterials (PEN), which involves the rearrangement of atoms in the unit cell, has emerged as a fantastic and effective strategy to adjust the intrinsic physicochemical properties of nanomaterials. In this Account, we give an overview of the recent progress on crystal phase control of Au nanomaterials using wet-chemical synthesis. Starting from a brief introduction of the research background, we first describe the development history of wet-chemical synthesis of Au nanomaterials and especially emphasize the key research findings. Subsequently, we introduce the typical Au nanomaterials with untraditional crystal phases and heterophases that have been observed, such as 2H, 4H, body-centered phases, and crystal-phase heterostructures. Importantly, crystal phase control of Au nanomaterials by wet-chemical synthesis is systematically described. After that, we highlight the importance of crystal phase control in Au nanomaterials by demonstrating the remarkable effect of crystal phases on their physicochemical properties (e.g., electronic and optical properties) and potential applications (e.g., catalysis). Finally, after a concise summary of recent advances in this emerging research field, some personal perspectives are provided on the challenges, opportunities, and research directions in the future.