Due to its outstanding safety and high energy density, all‐solid‐state lithium‐sulfur batteries (ASLSBs) are considered as a potential future energy storage technology. The electrochemical reaction ...pathway in ASLSBs with inorganic solid‐state electrolytes is different from Li‐S batteries with liquid electrolytes, but the mechanism remains unclear. By combining operando Raman spectroscopy and ex situ X‐ray absorption spectroscopy, we investigated the reaction mechanism of sulfur (S8) in ASLSBs. Our results revealed that no Li2S8, Li2S6, and Li2S4 were formed, yet Li2S2 was detected. Furthermore, first‐principles structural calculations were employed to disclose the formation energy of solid state Li2Sn (1≤n≤8), in which Li2S2 was a metastable phase, consistent with experimental observations. Meanwhile, partial S8 and Li2S2 remained at the full lithiation stage, suggesting incomplete reaction due to sluggish reaction kinetics in ASLSBs.
Utilizing operando Raman spectroscopy and ex situ synchrotron X‐ray absorption spectroscopy, the electrochemical reaction mechanisms of the sulfur cathode in all‐solid‐state batteries are revealed. The results demonstrated that no Li2S6 and Li2S4 is formed but Li2S2 is observed. Furthermore, first‐principles calculations disclose the formation energy of solid state Li2Sn (1≤n≤8), in which Li2S2 is a metastable phase.
Electrochemical in situ X‐ray absorption spectroscopy (XAS) of 2D titanium carbide MXene is used to probe the mechanism of high capacitance of Ti3C2Tx MXene. Changes in the Ti oxidation state are ...detected during cycling, confirming that most of the capacitance of Ti3C2Tx is due to changes in the Ti oxidation state, i.e., pseudocapacitance.
2D vanadium carbide MXene containing surface functional groups (denoted as V2CTx, where Tx are surface functional groups) is synthesized and studied as anode material for Na‐ion batteries. V2CTx ...anode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V2CTx material during Na+ intercalation/deintercalation and the redox reaction of vanadium are studied using a combination of synchrotron based X‐ray diffraction, hard X‐ray absorption near edge spectroscopy (XANES), and soft X‐ray absorption spectroscopy (sXAS). Experimental evidence of a major contribution of redox reaction of vanadium to the charge storage and the reversible capacity of V2CTx during sodiation/desodiation process are provided through V K‐edge XANES and V L2,3‐edge sXAS results. A correlation between the CO32− content and the Na+ intercalation/deintercalation states in the V2CTx electrode observed from C and O K‐edge in sXAS results implies that some additional charge storage reactions may take place between the Na+‐intercalated V2CTx and the carbonate‐based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V2CTx as anode material for Na‐ion batteries and capacitors.
Na‐ion intercalation and charge storage mechanism of 2D vanadium carbide MXene are investigated by using a combination of synchrotron‐based X‐ray techniques. It is demonstrated, for the first time, that the redox reaction at the transition metal site in MXene is responsible for the reversible charge storage. The reversible formation/decomposition of carbonate species at the surface upon sodiation/desodiation is also discussed in detail.
Exploring materials with regulated local structures and understanding how the atomic motifs govern the reactivity and durability of catalysts are a critical challenge for designing advanced ...catalysts. Herein we report the tuning of the local atomic structure of nickel–iron layered double hydroxides (NiFe‐LDHs) by partially substituting Ni2+ with Fe2+ to introduce Fe‐O‐Fe moieties. These Fe2+‐containing NiFe‐LDHs exhibit enhanced oxygen evolution reaction (OER) activity with an ultralow overpotential of 195 mV at the current density of 10 mA cm−2, which is among the best OER catalytic performance to date. In‐situ X‐ray absorption, Raman, and electrochemical analysis jointly reveal that the Fe‐O‐Fe motifs could stabilize high‐valent metal sites at low overpotentials, thereby enhancing the OER activity. These results reveal the importance of tuning the local atomic structure for designing high efficiency electrocatalysts.
Iron OER: Fe2+ sites are introduced to the NiFe layered double hydroxide (LDH) structure to construct Fe‐O‐Fe couples. These couples stabilize high‐valent metal species at low overpotentials, thereby enhancing the oxygen evolution reaction activity.
A comprehensive understanding of the solid-electrolyte interphase (SEI) composition is crucial to developing high-energy batteries based on lithium metal anodes. A particularly contentious issue ...concerns the presence of LiH in the SEI. Here we report on the use of synchrotron-based X-ray diffraction and pair distribution function analysis to identify and differentiate two elusive components, LiH and LiF, in the SEI of lithium metal anodes. LiH is identified as a component of the SEI in high abundance, and the possibility of its misidentification as LiF in the literature is discussed. LiF in the SEI is found to have different structural features from LiF in the bulk phase, including a larger lattice parameter and a smaller grain size (<3 nm). These characteristics favour Li
transport and explain why an ionic insulator, like LiF, has been found to be a favoured component for the SEI. Finally, pair distribution function analysis reveals key amorphous components in the SEI.
Thermal stability of charged LiNixMnyCozO2 (NMC, with x + y + z = 1, x:y:z = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using ...combined in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS) techniques upon heating up to 600 °C. The TR-XRD/MS results indicate that the content of Ni, Co, and Mn significantly affects both the structural changes and the oxygen release features during heating: the more Ni and less Co and Mn, the lower the onset temperature of the phase transition (i.e., thermal decomposition) and the larger amount of oxygen release. Interestingly, the NMC532 seems to be the optimized composition to maintain a reasonably good thermal stability, comparable to the low-nickel-content materials (e.g., NMC333 and NMC433), while having a high capacity close to the high-nickel-content materials (e.g., NMC811 and NMC622). The origin of the thermal decomposition of NMC cathode materials was elucidated by the changes in the oxidation states of each transition metal (TM) cations (i.e., Ni, Co, and Mn) and their site preferences during thermal decomposition. It is revealed that Mn ions mainly occupy the 3a octahedral sites of a layered structure (R3̅m) but Co ions prefer to migrate to the 8a tetrahedral sites of a spinel structure (Fd3̅m) during the thermal decomposition. Such element-dependent cation migration plays a very important role in the thermal stability of NMC cathode materials. The reasonably good thermal stability and high capacity characteristics of the NMC532 composition is originated from the well-balanced ratio of nickel content to manganese and cobalt contents. This systematic study provides insight into the rational design of NMC-based cathode materials with a desired balance between thermal stability and high energy density.
The high‐energy‐density, Li‐rich layered materials, i.e., xLiMO2(1‐x)Li2MnO3, are promising candidate cathode materials for electric energy storage in plug‐in hybrid electric vehicles (PHEVs) and ...electric vehicles (EVs). The relatively low rate capability is one of the major problems that need to be resolved for these materials. To gain insight into the key factors that limit the rate capability, in situ X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) studies of the cathode material, Li1.2Ni0.15Co0.1Mn0.55O2 0.5Li(Ni0.375Co0.25 Mn0.375)O2·0.5Li2MnO3, are carried out. The partial capacity contributed by different structural components and transition metal elements is elucidated and correlated with local structure changes. The characteristic reaction kinetics for each element are identified using a novel time‐resolved XAS technique. Direct experimental evidence is obtained showing that Mn sites have much poorer reaction kinetics both before and after the initial activation of Li2MnO3, compared to Ni and Co. These results indicate that Li2MnO3 may be the key component that limits the rate capability of Li‐rich layered materials and provide guidance for designing Li‐rich layered materials with the desired balance of energy density and rate capability for different applications.
In the cathode material Li1.2Ni0.15Co0.1Mn0.55O2 0.5Li(Ni0.375Co0.25Mn0.375)O2·0.5Li2MnO3 the capacity contributed from different components and elements is elucidated and correlated with the local structure changes. The reaction kinetic characteristics for each element are been identified and differentiated. It is observed that Li2MnO3 may be the key component determining the rate capability of the Li‐rich layered materials.
Lithium-ion battery (LIB) technology is the most attractive technology for energy storage systems in today’s market. However, further improvements and optimizations are still required to solve ...challenges such as energy density, cycle life, and safety. Addressing these challenges in LIBs requires a fundamental understanding of the reaction mechanisms in various physical/chemical processes during LIB operation. Advanced in situ/operando synchrotron-based X-ray characterization techniques are powerful tools for providing valuable information about the complicated reaction mechanisms in LIBs. In this review, several state-of-the-art in situ/operando synchrotron-based X-ray techniques and their combination with other characterization tools for battery research are introduced. Various in situ cell configurations and practical operating tips for cell design and experimental set-ups are also discussed.
Cobalt‐ and nickel‐free cathode materials are desirable for developing low‐cost sodium‐ion batteries (SIBs). Compared to the single P‐type and O‐type structures, biphasic P/O structures become a ...topic of interest thanks to improved performance. However, the added complexity complicates the understanding of the storage mechanism and the phase behavior is still unclear, especially over consecutive cycling. Here, the properties of biphasic P2(34%)/O3(60%) Na0.8Li0.2Fe0.2Mn0.6O2 and its behavior at different states of charge/discharge are reported on. The material is composed of single phase O3 and P2/O3 biphasic particles. Sodium occupies the alkali layers, whereas lithium predominantly (95%) is located in the transition metal layer. An initial reversible capacity of 174 mAh g‐1 is delivered with a retention of 82% dominated by Fe3+/Fe4+ along with contributions from oxygen and partial Mn3+/4+ redox. Cycling leads to complex phase transitions and ion migration. The biphasic nature is nevertheless preserved, with lithium acting as the structure stabilizer.
A Co‐ and Ni‐free layered oxide with a P2/O3 biphasic structure is prepared by using a sol‐gel method and used as the cathode for sodium ion batteries. Its electrochemical and structural properties, ion storage mechanism, phase transition, and ion migration are investigated using a set of techniques involving XRD, ssNMR, XANES, EXAFS before and after cycling.
The lithium-sulfur (Li-S) battery is a promising next-generation energy storage technology because of its high theoretical energy and low cost. Extensive research efforts have been made on new ...materials and advanced characterization techniques for mechanistic studies. However, it is uncertain how discoveries made on the material level apply to realistic batteries due to limited analysis and characterization of real high-energy cells, such as pouch cells. Evaluation of pouch cells (>1 A h) (instead of coin cells) that are scalable to practical cells provides a critical understanding of current limitations which enables the proposal of strategies and solutions for further performance improvement. Herein, we design and fabricate pouch cells over 300 W h kg
−1
, compare the cell parameters required for high-energy pouch cells, and investigate the reaction processes and their correlation to cell cycling behavior and failure mechanisms. Spatially resolved characterization techniques and fluid-flow simulation reveal the impacts of the liquid electrolyte diffusion within the pouch cells. We found that catastrophic failure of high-energy Li-S pouch cells results from uneven sulfur/polysulfide reactions and electrolyte depletion for the first tens of cycles, rather than sulfur dissolution as commonly reported in the literature. The uneven reaction stems from limited electrolyte diffusion through the porous channels into the central part of thick cathodes during cycling, which is amplified both across the sulfur electrodes and within the same electrode plane. A combination of strategies is suggested to increase sulfur utilization, improve nanoarchitectures for electrolyte diffusion and reduce consumption of the electrolytes and additives.
Reaction heterogeneity was discovered as a main reason for Li-S pouch cell degradation at practical high energy.
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