Polymorphism, which describes the occurrence of different lattice structures in a crystalline material, is a critical phenomenon in materials science and condensed matter physics. Recently, configuration disorder was compositionally engineered into single lattices, leading to the discovery of high-entropy alloys and high-entropy oxides. For these novel entropy-stabilized forms of crystalline matter with extremely high structural stability, is polymorphism still possible? Here by employing in situ high-pressure synchrotron radiation X-ray diffraction, we reveal a polymorphic transition from face-centred-cubic (fcc) structure to hexagonal-close-packing (hcp) structure in the prototype CoCrFeMnNi high-entropy alloy. The transition is irreversible, and our in situ high-temperature synchrotron radiation X-ray diffraction experiments at different pressures of the retained hcp high-entropy alloy reveal that the fcc phase is a stable polymorph at high temperatures, while the hcp structure is more thermodynamically favourable at lower temperatures. As pressure is increased, the critical temperature for the hcp-to-fcc transformation also rises.

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To meet the demanding requirements in plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs), higher energy density materials, such as the Li-rich, layered manganese-based oxides (LLOs) with the general formula xLi 2 MnO 3 ·(1-x)LiTMO 2 (TM = Mn, Ni, Co, etc.), are promising candidates as they possess higher reversible capacity (>250 mAh g −1 ), improved safety and much reduced cost. [4][5][6][7][8][9] Recent microscopic evidence reveals the intergrowth of rhombohedral LiTMO 2 (R-3m) and the monoclinic Li 2 MnO 3 -like layered structure (C/2m) at the atomic scale in the oxide grains. [10] The Li 2 MnO 3 component serves as an electrochemically active phase for Li storage when cycled above 4.5 V versus Li/Li + . [8,[11][12][13][14] Nevertheless, these LLO materials undergo steady voltage/capacity decay when cycled above 4.5 V, resulting in a substantial decrease of the cathode energy density. [15][16][17][18] The origin of voltage/capacity decay upon cycling stems from cation migration between TM layers and Li layers and subsequent phase transformation. [19,20] The cationic doping with other metallic cations (such as Mg, [21] Al, [22] Ti, [23] Sn, [24] Ru, [25] Y, [26] Zn, [27] etc.) and polyanion doping based on nonmetal elements, such as BO 4 5− , [28] SiO 4 4− , [29] PO 4 3-, [30] etc., have been employed to improve the cyclic durability by weakening the TM-O covalency in the oxygen closepacked structure. In addition, surface coatings using metal oxides, [31][32][33][34] fluorides and phosphates, [35][36][37] LiNiPO 4 and Li 3 VO 4 , [38][39][40] have been applied to protect the surface structure from side reactions with the electrolyte under high voltage and to restrain the layered-to-spinel transformation which occurs preferentially on the crystal surface and leads to capacity fading of LLO materials. However, the ionic dopants and coating materials are mostly electrochemically inactive, so the improved cycling stability is achieved at the expense of reduced specific capacity/energy density of the cathode. Moreover, a conformal and continuous coating on the surface of oxide particles is rather difficult to obtain practically. Hence, advancing the structural and cycling stability in both the bulk material and the surface structure through a simple way is highly desired for potential applications of LLO materials.Herein, we develop a novel LLO material with a nanoscaled spinel-like surface layer through gradient doping of polyanions Surface Structural Transition Induced by Gradient Polyanion-Doping in Li-Rich Layered Oxides: Implications for Enhanced Electrochemical PerformanceYing Zhao, Jiatu Liu, Shuangbao Wang, Ran Ji, Qingbing Xia, Zhengping Ding, Weifeng Wei,* Yong Liu, Peng Wang,* and Douglas G. Ivey Lithium-rich layered oxides (LLOs) exhibit great potential as high-capacity cathode materials for lithium-ion batteries, but usually suffer from capacity/ voltage fade during electrochemical cycling. Herein, a gradient polyaniondoping strategy is developed to initiate surface structural trans...

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Lithium-ion capacitors have attracted tremendous attention among various electrochemical energy storage systems, benefitting from the merits of high energy density, high power output, long cycle life and favorable chemical stability.

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HIGHLIGHTS • The scalable graphitic carbon quantum dots (CQDs) are obtained with a high yield of more than 50%. • The CQDs are utilized to synthesize novel NS-CQDs/NiCo 2 S 4 composite which shows enhanced electrochemical properties. • The assembled novel alkaline aqueous battery delivers superior electrochemical performances. ABSTRACT Carbon quantum dots (CQDs) as a new class of emerging materials have gradually drawn researchers' concern in recent years. In this work, the graphitic CQDs are prepared through a scalable approach, achieving a high yield with more than 50%. The obtained CQDs are further used as structure-directing and conductive agents to synthesize novel N,S-CQDs/ NiCo 2 S 4 composite cathode materials, manifesting the enhanced electrochemical properties resulted from the synergistic effect of highly conductive N,S-codoped CQDs offering fast electronic transport and unique micro-/nanostructured NiCo 2 S 4 microspheres with Faradaic redox characteristic contributing large capacity. Moreover, the nitrogen-doped reduced graphene oxide (N-rGO)/Fe 2 O 3 composite anode materials exhibit ultrahigh specific capacity as well as significantly improved rate property and cycle performance originating from the high-capacity prism-like Fe 2 O 3 hexahedrons tightly wrapped by highly conductive N-rGO. A novel alkaline aqueous battery assembled by these materials displays a specific energy (50.2 Wh kg −1), ultrahigh specific power (9.7 kW kg −1) and excellent cycling performance with 91.5% of capacity retention at 3 A g −1 for 5000 cycles. The present research offers a valuable guidance for the exploitation of advanced energy storage devices by the rational design and selection of battery/capacitive composite materials.