kind of secondary batteries will find promising applications in large-scale energy storage. [3] Owing to the low-cost [4] and low standard potential [5] of potassium (K), KIBs could in principle offer cost-effectiveness and higher energy density, which would lead to a substantial advance in energy storage technology. The fast K + ion diffusion property provides the possibility of saving the charging time of KIBs as the next generation energy storage and transport devices. [6] Then compared with Na-ion battery (NIB), a huge advantage lies in the application of graphite anode for KIB, [7] which is not feasible for Na + insertion into the graphite. [8] And also, the weaker Lewis acidity of K + ions ensures smaller solvated ions, leading to the higher mobility and larger transference number in electrolytes and lower desolvation energy, which offers faster K + ion diffusion kinetics with regard to that of Li + or Na + ions. [4,9] Based on these advantages, many investigations on the available anode materials for KIBs, such as carbonaceous materials, [8a,10] oxides, [11] sulfides, [12] organic materials, [13] alloy, [5] prussian blue analogues, [14] etc., have been reported continuously in the past few decades, [2a,15] however, fewer studies are devoted to the development of cathode. Present cathodes could be summarized as the layered transition metal oxides, [16] polyanions and pyrophosphates, [17] prussian blue analogs, [18] nanostructured iron composite, [19] organic materials, [20] and other potential materials. Up to now, most of the available studies are working on screening for new electrode materials with enhanced electrochemical performance, while the underlying K storage and transport mechanism in the electrode materials is lack of exploration and comprehension. [2a,21] Therefore, studying the electrochemical behavior of potassium ions is very important for building a better KIBs system.In specific, alkali transition metal oxides (AMO 2 : A = Li, Na, K; M = Ni, Co, Mn, Fe, etc.) with layered structure have been widely studied as promising cathodes for rechargeable batteries, [22] because of the topotactic de/intercalation feature. However, the accommodation of the bulky K + ion is less favorable than that of Li + and Na + ion in such compact structures, [2a] which will plausibly hinder the K + ion diffusion kinetics. [23] Previous reports [16e,24] have shown the sluggish diffusion performance of K + ions in KIBs with alkali transition Novel and low-cost rechargeable batteries are of considerable interest for application in large-scale energy storage systems. In this context, K-Birnessite is synthesized using a facile solid-state reaction as a promising cathode for potassium-ion batteries. During synthesis, an ion exchange protocol is applied to increase K content in the K-Birnessite electrode, which results in a reversible capacity as high as 125 mAh g −1 at 0.2 C. Upon K + exchange the reversible phase transitions are verified by in situ X-ray diffraction (XRD) characterization. The underlying mechani...

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The electrochemical insertion of Li into graphite initiates a series of thermodynamic and kinetic processes. An in-depth understanding of this phenomenon will deepen the knowledge of electrode material design and optimize rechargeable Li batteries. In this context, the phase transition from dense stage II (LiC) to stage I (LiC) was comprehensively elucidated in a graphite anode via both experimental characterizations and first-principles calculations. The results indicate that, although the transition from stage II to stage I is thermodynamically allowed, the process is kinetically prohibited because Li ions tend to cluster into stage compounds rather than form a solid solution. Additionally, the phase transitions involve at least three intermediate structures (1T, 2H, and 3R) before reaching the LiC (stage I) phase. These findings provide new insights into the electrochemical behavior of graphite and the electrode process kinetics for rechargeable Li batteries.

Lowland tropical forests with chronic nitrogen (N) deposition and/or abundant N-fixing organisms are commonly rich in N relative to other nutrients. The tropical N richness introduces a paradoxical relationship in which many tropical forests sustain high rates of asymbiotic N fixation despite the soil N richness and the higher energy cost of N fixation than of soil N uptake. However, the mechanism underlying this phenomenon remains unclear. Our study aims to test this phenomenon and examine potential mechanisms of nutrient concentrations vs. substrate stoichiometry in regulating N fixation using multiple linear regression models. We hypothesized that the rates of asymbiotic N fixation would be low in an N-rich forest under N deposition and substrate stoichiometry would explain the variation in N fixation better than nutrient concentrations. We conducted a chronic N-addition experiment in an N-saturated tropical forest in southern China and measured the N fixation rates, carbon (C), N, and phosphorus (P) concentrations, and stoichiometry in different substrates (soil, forest floor, mosses, and canopy leaves). Total N fixation rates were high (10.35-12.43 kg N·ha ·yr ) in this N-saturated forest because of the high substrate C:N and N:P stoichiometry (which explained 13-52% of the variation in N fixation, P < 0.037) rather than substrate nutrient concentrations (P > 0.05). Atmospheric N deposition (34-50 kg N·ha ·yr ) failed to down-regulate asymbiotic N fixation in this forest possibly because the N deposition rate was insufficient to inhibit N fixation or N deposition maintained high N fixation rates by increasing C sequestration in the substrates. Our N-addition experiment showed the insensitivity of N fixation in all the tested substrates to low N addition (50 kg N·ha ·yr ); however, medium and high N addition (100-150 kg N·ha ·yr ) stimulated the moss and foliar N fixation because of the increases in substrate C:N stoichiometry (which explained 30-34% of the variation in N fixation, P < 0.001). Overall, our results emphasize the importance of substrate (particularly mosses and foliage) stoichiometry as a driver of asymbiotic N fixation and sustained N richness in lowland tropical forests.