Intercalation compounds form the basis of essentially all lithium rechargeable batteries. They exhibit a wide range of electronic and crystallographic structures. The former varies from metallic conductors to excellent insulators. The latter often have layer structures or have open tunnel structures that can act as the hosts for the intercalation of a wide range of metal cation and other guest species. They are fascinating materials with almost infinitely variable properties, with the crystal structure controlling the identity and the amount of the guest species that may be intercalated and subsequently removed. The electronic structure controls not only the degree of electron transfer to the host, but also defines the degree of the electrostatic interactions a mobile ion experiences; thus, a metallic host will provide a minimizing of those interactions, whereas in an ionic lattice the interactions will be much greater and the mobile ion will experience a much higher activation energy for motion. This becomes more important for multivalent cations such as Mg. Today's lithium batteries are limited in capacity, because less than one lithium ion is reversibly intercalated per transition metal redox center. There may be an opportunity to increase the storage capacity by utilizing redox centers that can undergo multielectron reactions. This might be accomplished by intercalating multiple monovalent cations or one multivalent cation. In this Account, we review the key theoretical and experimental results on lithium and magnesium reversible intercalation into two prototypical materials: titanium disulfide, TiS, and vanadyl phosphate, VOPO. Both of these materials exist in two or more phases, which have different molar volumes and/or dimensionalities and thus are expected to show a range of diffusion opportunities for battery active guest ions such as lithium, sodium, and magnesium. One major conclusion of this Account is that reversibly intercalating two lithium ions into a host lattice while maintaining its crystal structure is possible. A second major conclusion is that theoretical studies are now sufficiently mature that they can be relied upon to predict the key free energy values of simple intercalation reactions, i.e., the energy that might be stored. This could help to focus future choices of battery couples. In hindsight, theory would have predicted that magnesium-based intercalation cells are not a viable electrochemical option, relative to lithium cells, from either power or energy density considerations. However, the fundamental study of such reactions will lead to a better understanding of intercalation reactions in general, and of the critical importance of crystal structure in controlling the rates and degree of chemical reactions.
By controlling the morphology and particle size of the epsilon polymorph of vanadyl phosphate, ε-VOPO4, it can fully reversibly intercalate two Li-ions and reach the theoretical capacity of 305 mA h g-1 over two voltage plateaus at about 4.0 and 2.5 V.
A flexible solid-state supercapacitor based on vapor phase polymerized (VPP) PEDOT into cellulose paper matrix (PEDOT/CP) was successfully fabricated. The PEDOT/CP composite material worked as both current collector and electrode in constructed test cells. It had a low sheet resistance of 14 Ω/square and survived the Scotch tape test for adhesion. It also showed excellent stability with no significant conductivity drop after 1000 cycles of bending. The PEDOT from electrode obtained the mass specific capacitance of 179 F/g at scan rate of 10 mV/s, which was among the highest specific capacitances ever reported. This high capacitance was attributed to the combination of the VPP technique and the porous fibrous structure of the cellulose matrix. The EDOT vapor penetrated and polymerized through the CP matrix made of nanometer to micrometer level CP fibers. The highest electrode volumetric capacitance achieved was 13.7 F/cm3. The whole device achieved an energy density of 0.76 mWh/cm3 and a power density of 0.01 W/cm3. Bending the supercapacitor to 90° or rotating to 45° caused no major change in capacitance. Owing to the all nonmetallic materials used to construct the supercapacitor, it can be easily disposed. The incineration of the supercapacitor does not release significant hazardous exhaust.
The pressing demand in electrical vehicle (EV) markets for high-energy-density lithium-ion batteries (LIBs) requires further increasing the Ni content in high-Ni and low-Co cathodes. However, the commercialization of high-Ni cathodes is hindered by their intrinsic chemomechanical instabilities and fast capacity fade. The emerging single-crystalline strategy offers a promising solution, yet the operation and degradation mechanism of single-crystalline cathodes remain elusive, especially in the extremely challenging ultrahigh-Ni (Ni > 90%) regime whereby the phase transformation, oxygen loss, and mechanical instability are exacerbated with increased Ni content. Herein, we decipher the atomic-scale stabilization mechanism controlling the enhanced cycling performance of an ultrahigh-Ni single-crystalline cathode. We find that the charge/discharge inhomogeneity, the intergranular cracking, and oxygen-loss-related phase degradations that are prominent in ultrahigh-Ni polycrystalline cathodes are considerably suppressed in their single-crystalline counterparts, leading to improved chemomechanical and cycling stabilities of the single-crystalline cathodes. Our work offers important guidance for designing next-generation single-crystalline cathodes for high-capacity, long-life LIBs.
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