The propensity of metals to form irregular and nonplanar electrodeposits at liquid-solid interfaces has emerged as a fundamental barrier to high-energy, rechargeable batteries that use metal anodes. ...We report an epitaxial mechanism to regulate nucleation, growth, and reversibility of metal anodes. The crystallographic, surface texturing, and electrochemical criteria for reversible epitaxial electrodeposition of metals are defined and their effectiveness demonstrated by using zinc (Zn), a safe, low-cost, and energy-dense battery anode material. Graphene, with a low lattice mismatch for Zn, is shown to be effective in driving deposition of Zn with a locked crystallographic orientation relation. The resultant epitaxial Zn anodes achieve exceptional reversibility over thousands of cycles at moderate and high rates. Reversible electrochemical epitaxy of metals provides a general pathway toward energy-dense batteries with high reversibility.
LiNi0.6Mn0.2Co0.2O2 (NMC622) is one of the most promising Li-ion battery cathodes as it delivers high capacity at high potentials. However, high potentials also lead to decreases in capacity ...retention where the disintegration of the secondary particles has been implicated as a major driving force of this capacity fade. This has been attributed to anisotropic lattice changes and increased microstrain during cycling. To probe how these factors affect capacity fade, Li/NMC622 batteries were cycled from 3 to 4.3 or 4.7 V and probed with operando X-ray diffraction (XRD) over the 1st, 2nd, and 101st cycles. Further characterization with scanning electron microscopy and inductively coupled plasma-optical emission spectroscopy was also performed. The use of operando XRD over many cycles allowed for the collection of detailed structural information in real time over a time frame in which fading can be observed. During the first two cycles, the cells charged to 4.7 V exhibit increased anisotropic lattice changes as compared to the cells charged to 4.3 V. Upon the 101st cycle, when significant fade has been observed, the cells charged to 4.3 and 4.7 V show identical lattice changes to one another, while the 4.7 V charge limit induces more microstrain. This shows that elevated microstrain at high charge limits is a major driver for particle disintegration in NMC622 cathodes. This study provides important insights into the mechanisms of particle disintegration and capacity fade in NMC/Li-ion batteries, which will enable the design of NMC electrodes that deliver both higher capacities and exhibit better capacity retention.
The activator Bi3+ has been successfully incorporated into the anti-perovskite oxy-fluoride host lattice Sr3MO4F (M = Al, Ga) to form rare earth-free phosphors of the composition Sr3-xBi2x/3AlO4F, 0 ...≤ x ≤ 0.1, and Sr3-xBi2x/3GaO4F, 0 ≤ x ≤ 0.048. These phases absorb in the UV region (λex = 240-326 nm) and exhibit broad emission in the blue region of the visible spectrum (λem = 446.5-455 nm). The optimum compositions for maximum photoluminescent intensity were determined to be Sr2.976Bi0.016AlO4F and Sr2.976Bi0.016GaO4F before concentration quenching occurs. Full structural characterization based upon PXRD and NPD data were performed with DFT calculations suggesting that Bi3+ ions are preferentially incorporated on the ten coordinate Sr(1) site.
Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging ...storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.
Aqueous Zn/α‐MnO2 batteries have attracted immense interest owing to their high energy density, low cost, and safety, making them desirable for future large‐scale energy application. Despite these ...merits, the comprehensive understanding of their reaction mechanism has been elusive due to the limitations of standard bulk characterization. Here, via transmission electron microscopy, the dissolution‐mediated reaction mechanism of a Zn/α‐MnO2 system is discovered and explored in full scope to involve reversible formation of Zn4SO4(OH)6·xH2O and “birnessite‐like” Zn‐MnOx phase upon cycling. Overall, α‐MnO2 acts primarily as a source for cell activation through dissolution and thus is not directly involved in the Zn redox chemistry. This microscopic study offers a unique knowledge on the unconventional reaction chemistry of Zn/α‐MnO2 batteries.
Via transmission electron microscopy, the dissolution‐mediated reaction mechanism of a Zn/α‐MnO2 system for aqueous zinc‐ion batteries is discovered to form Zn4SO4(OH)6·xH2O that undergoes reversible formation with “birnessite‐like” Zn‐MnOx upon electrochemical cycling. α‐MnO2 acts primarily as a source for cell activation through dissolution that happens from the surface likely promoted by guest ion insertion at the surface.
Rechargeable aqueous Zn/α-MnO2 batteries are a possible alternative to lithium ion batteries for scalable stationary energy storage applications due to their low cost, safety and environmentally ...benign components. A critical need for advancement of this battery system is a full understanding of the electrochemical reaction mechanisms, which remain unclear. In this report, operando, spatiotemporal resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO2 cell provided direct evidence of a Mn dissolution-deposition faradaic mechanism that governs the electrochemistry. Simultaneous visualization and quantification of the Mn distribution in the electrolyte revealed the formation of aqueous Mn species during discharge and depletion on charge. The findings are supported by ex situ transmission electron microscopy (TEM), X-ray diffraction, Mn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The elucidated mechanism is fundamentally different from the previously proposed Zn2+ insertion or conversion reactions. These findings provide a foundation for developing dissolution- deposition chemistries suitable for scalable stationary energy storage with aqueous electrolyte.
Conspectus Future advances in energy storage systems rely on identification of appropriate target materials and deliberate synthesis of the target materials with control of their physiochemical ...properties in order to disentangle the contributions of distinct properties to the functional electrochemistry. This goal demands systematic inquiry using model materials that provide the opportunity for significant synthetic versatility and control. Ideally, a material family that enables direct manipulation of characteristics including composition, defects, and crystallite size while remaining within the defined structural framework would be necessary. Accomplishing this through direct synthetic methods is desirable to minimize the complicating effects of secondary processing. The structural motif most frequently used for insertion type electrodes is based on layered type structures where ion diffusion in two dimensions can be envisioned. However, lattice expansion and contraction associated with the ion movement and electron transfer as a result of repeated charge and discharge cycling can result in structural degradation and amorphization with accompanying loss of capacity. In contrast, tunnel type structures embody a more rigid framework where the inherent structural design can accommodate the presence of cations and often multiple cations. Of specific interest are manganese oxides as they can exhibit a tunneled structure, termed α-MnO2, and are an important class of nanomaterial in the fields of catalysis, adsorption–separation, and ion-exchange. The α-MnO2 structure has one-dimensional 2 × 2 tunnels formed by corner and edge sharing manganese octahedral MnO6 units and can be readily substituted in the central tunnel by a variety of cations of varying size. Importantly, α-MnO2 materials possess a rich chemistry with significant synthetic versatility allowing deliberate synthetic control of structure, composition, crystallite size, and defect content. This Account considers the investigation of α-MnO2 tunnel type structures and their electrochemistry. Examination of the reported findings on this material family demonstrates that multiple physiochemical properties influence the electrochemistry. The retention of the parent structure during charge and discharge cycling, the material composition including the identity and content of the central cation, the surface condition including oxygen vacancies, and crystallite size have all been demonstrated to impact electrochemical function. The selection of the α-MnO2 family of materials as a model system and the ability to control the variables associated with the structural family affirm that full investigation of the mechanisms related to active materials in an electrochemical system demands concerted efforts in synthetic material property control and multimodal characterization, combined with theory and modeling. This then enables more complete understanding of the factors that must be controlled to achieve consistent and desirable outcomes.
Samples of Mg-todorokite with mixed nanosheet & nanowire (referred to as nanosheets for the sake of simplicity) and pure nanowire morphology were prepared to probe the morphological impact on battery ...relevant electrochemistry. The samples had similar physicochemical properties where the Mg-todorokite nanosheets had a composition of Mg0.20MnO2 with a crystallite size of 9 nm and water content of 0.29 moles of H2O per formula unit, while the nanowires had a composition of Mg0.19MnO2 with a crystallite size of 14 nm and water content of 0.28. The electrochemistry of the Mg-todorokite materials was evaluated using cyclic voltammetry, galvanostatic cycling, and rate capability testing. The Mg-todorokite nanosheets showed higher discharge capacity than the nanowires in lithium, sodium and magnesium-based electrolytes. Notably, the nanowire materials exhibited excellent cycling stability compared to the nanosheets in lithium and sodium-based batteries. X-ray absorption spectroscopy (XAS) of lithiated samples suggests that Mn in the nanosheet containing material was more highly reduced to Mn3+ after the 1st full discharge explaining the higher initial capacities observed. However, a MnO-like structural distortion was observed after extended cycling in the nanosheet material consistent with its poorer capacity retention.
Lithium nickel manganese cobalt oxide (NMC) is a commercially successful Li-ion battery cathode due to its high energy density; however, its delivered capacity must be intentionally limited to ...achieve capacity retention over extended cycling. To design next-generation NMC batteries with longer life and higher capacity the origins of high potential capacity fade must be understood.
Operando
hard X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment of NMC as the material (de)lithiates in a functional battery. This perspective outlines recent developments in the elucidation of capacity fade mechanisms in NMC through hard X-ray probes, surface sensitive soft X-ray characterization, and isothermal microcalorimetry. A case study on the effect of charging potential on NMC811 over extended cycling is presented to illustrate the benefits of these approaches. The results showed that charging to 4.7 V leads to higher delivered capacity, but much greater fade as compared to charging to 4.3 V.
Operando
XRD and SEM results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade.
Operando
hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni
3+
); however, minimal differences were observed between the cells charged to 4.3 and 4.7 V. Additional XAS analyses using soft X-rays revealed significant surface reconstruction after cycling to 4.7 V, revealing another contribution to fade.
Operando
isothermal microcalorimetry (IMC) indicated that the high voltage charge to 4.7 V resulted in a doubling of the heat dissipation when compared to charging to 4.3 V. A lowered chemical-to-electrical energy conversion efficiency due to thermal energy waste was observed, providing a complementary characterization of electrochemical degradation. The work demonstrates the utility of multi-modal X-ray and microcalorimetric approaches to understand the causes of capacity fade in lithium-ion batteries with Ni-rich NMC.
Combining calorimetry with hard and soft X-ray characterization elucidates bulk and surface phenomena responsible for capacity fade in LiNi
0.8
Mn
0.1
Co
0.1
O
2
cathodes.
Localized high-concentration electrolytes (LHCEs) combine a diluent with a high-concentration electrolyte, offering promising properties. The ions, solvent, and diluent interact to form complex ...heterogeneous liquid structures, where high salt concentration clusters are embedded in the diluent. Optimizing LHCEs for desired electrolyte properties like high ionic conductivity, low viscosity, and effective solid electrolyte interphase (SEI) formation ability within the vast chemical and compositional design space requires deeper understanding and theoretical guidance. We investigated the structures and conductivities of LHCEs based on a fluorinated solvent with two different diluents at varying concentrations. 2,2,3,3-Tetrafluoropropyl trifluoroacetate (TFPTFA) enters the solvation cluster due to its stronger Li-ion interactions, whereas 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFETFE) enters only at extremely high diluent concentrations. The ionic conductivity increases with decreasing diluent concentrations, with a slope change during cluster percolation. Overall, TFETFE demonstrates higher effectiveness than TFPTFA, forming higher local salt concentration clusters and resulting in higher ionic conductivity.
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