Monoclinic S8 , an uncommon allotrope of sulfur at room temperature, can be formed when common orthorhombic S8 is heat-treated under enclosed environments in nanometer dimensions. Monoclinic S8 prevents the formation of soluble polysulfides during battery operation, resulting in unprecedented cycling performance over 1000 cycles under the highest sulfur content to date.

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Candidates for high‐energy cathodes in potassium‐ion batteries (KIBs) are selected by fully screening the inorganic compound structure database. The compounds that satisfy the specific conditions for plausible KIB cathodes are further subjected to theoretical and electrochemical verification, and KVP2O7 is finally pinpointed. KVP2O7 can reversibly desert/insert ≈60% of K+ (60 mA h g−1) during either chemical or electrochemical oxidation/reduction. KVP2O7 shows an average discharge potential of ≈4.2 V versus K/K+, which corresponds to an energy density of 253 W h kg−1 at 0.25 C. This high energy density characteristic of KVP2O7 is maintained both during fast charge/discharge (C/D) and prolonged redox cycles. The C/D of KVP2O7 is also accompanied by a phase transition between a monoclinic KVP2O7 (P21/c) and a triclinic K1−xVP2O7(P1true¯). The structure interpretation of a new K1−xVP2O7 phase indicates that K+‐extraction induces a conformational change of two tetrahedral PO4 units in pyrophosphates. The P1true¯ phase of K1−xVP2O7 (x ≈0.6) remains stable during the C/D process, although it returns to the inborn P21/c phase after thermal treatment. It is believed that the data‐mining protocol designed for this study will provide a new strategy for materials discovery and that the pinpointed KVP2O7 can be utilized as a reliable KIB cathode.

3227wileyonlinelibrary.com be adapted almost directly to the sodium system. Sodium equivalents of lithiumcontaining electrode materials, such as oxides, sulfi des, phosphates, pyrophosphates, fl uorophosphates, and alloying metals, have been evaluated as electrode materials for SIBs. [1][2][3][4][5][6] Among the positive electrode materials for SIBs, layered oxides (NaTMO 2 , TM = transition metals) are most attractive due to their large capacity, simple synthesis, and structural stability. [7][8][9][10][11][12][13][14] Various transition metal elements can be substituted into the layered structure, similar to layered lithium compounds, and this will infl uence the structural stability of sodium-ion removal, operating voltage, capacity, and cyclability. In contrast to lithium systems, different stacking structures for Na layered oxides can be examined because of the preferred prismatic coordination of the larger sodium ions. Positive electrode materials with a P2 layered structure by Delmas' notation [ 15 ] have better cyclability and structural stability during electrochemical reactions than those with an O3 layered structure. [ 16 ] The trigonal prismatic site is more favorable for the diffusion of sodium ions, [17][18][19][20] and the diffusion kinetics for Na + can thus be more effi cient in the P2 layered structure compared to those in the O3 layered structure, which is common in lithium intercalation compounds. P2 layered materials with various transition metal compositions for SIBs have been studied, such as Mn-Co, Ni-Mn, and Fe-Mn. [ 9,17,[21][22][23][24][25] Moreover, several approaches for the doping of two-component P2 layered materials with transition metals have also been introduced, [ 8 ] including Co doping on Ni-Mn compounds, [ 26,27 ] Ni doping on Co-Mn systems, [ 28,29 ] and Ni doping on Fe-Mn system. [ 30 ] However, few studies on the Fe-Mn-Co system with P2 stacking have been reported compared to Ni-Fe-Mn or Ni-Co-Mn P2 materials. [ 31,32 ] Co can facilitate the oxidation of Fe atoms, as reported in Li compounds, [ 33 ] and Co can likely stabilize the oxidized state in the layered structure, especially for Fe-containing layered materials. [ 34 ] In addition, Wang et al. reported that Co suppresses the irreversibility of P2-Na 2/3 Mn y Co 1− y O 2 materials. [ 23 ] A similar behavior in the P2-Na-Fe-Co-Mn oxides would be expected.In this paper, P2-Na 0.7 [(Fe 0.5 Mn 0.5 ) 1− x Co x ]O 2 ( x = 0, 0.05, 0.10, and 0.20) was synthesized by a solid-state reaction, and the electrochemical performance of the P2-Fe-Mn-Co system Sodium layered oxides with mixed transition metals have received signifi cant attention as positive electrode candidates for sodium-ion batteries because of their high reversible capacity. The phase transformations of layered compounds during electrochemical reactions are a pivotal feature for understanding the relationship between layered structures and electrochemical properties. A combination of in situ diffraction and ex situ X-ray absorption spectroscopy reveals the phase t...

P′3-type Na0.52CrO2 is proposed as a viable cathode material for potassium-ion batteries (KIBs). The in-situ-generated title compound during the first charge of O3-NaCrO2 in K+-containing electrolytes can reversibly accommodate 0.35 K+-ions with no interference with Na+. In addition to the sequential interlayer slippage that occurs with Na+-insertion, K+-insertion into Na0.52CrO2 induces a sudden phase separation, which ultimately results in a biphasic structure when fully discharged (K+-free O3-NaCrO2 and K+-rich P3-K0.6Na0.17CrO2). A reversible transition between monophasic (Na0.52CrO2) and biphasic states during repeated K+-insertion/deinsertion is also maintained, which contributes to superior electrochemical properties of the title compound when used as a KIB cathode. Na0.52CrO2 delivers a specific capacity of 88 mA h g–1 with an average discharge potential of 2.95 V versus K/K+. This high level of energy density (260 W h kg–1 at 0.05C) is not substantially decreased at fast C-rates (195 W h kg–1 at 5C). When cycled at 2C, the first reversible capacity of 77 mA h g–1 gradually decreases to 52 mA h g–1 during initial 20 cycles, but no further capacity fading is observed for subsequent cycles (51 mA h g–1 after 200 cycles). Density-functional-theory computation reveals that the rearrangement of Na+ is an energetically favored process rather than a homogeneous distribution of Na+/K+.