Mg-Ion Battery Electrode: An Organic Solid's Herringbone Structure Squeezed upon Mg-Ion Insertion.

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg2+ and Ca2+, in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalent nature of Mg2+ causes unique squeezing deformation of the electrode structure, where it contracts and expands in different crystallographic directions when hosting the inserted Mg-ions. This phenomenon is revealed experimentally by ex situ X-ray diffraction and transmission electron microscopy, and is investigated theoretically by first-principles calculations. Interestingly, hosting one Mg2+ ion requires the coordination from three PTCDA molecules in adjacent columns of stacked molecules, which rotates the columns, thus reducing the (011) spacing but increasing the (021) spacing. We demonstrate that a PTCDA Mg-ion electrode delivers a reversible capacity of 125 mA h g-1, which may include a minor contribution of hydronium storage, a good rate capability by retaining 75 mA h g-1 at 500 mA g-1 (or 3.7 C), and a stable cycle life. We also report Ca2+ storage in PTCDA, where a reversible capacity of over 80 mA h g-1 is delivered.

Hydronium-Ion Batteries with Perylenetetracarboxylic Dianhydride Crystals as an Electrode.

We demonstrate for the first time that hydronium ions can be reversibly stored in an electrode of crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). PTCDA exhibits a capacity of 85 mAh g-1 at 1 A g-1 after an initial conditioning process. Ex situ X-ray diffraction revealed reversible and significant structure dilation upon reduction of PTCDA in an acidic electrolyte, which can only be ascribed to hydronium-ion intercalation. The lattice expansion upon hydronium storage was theoretically explored by first-principles density functional theory (DFT) calculations, which confirmed the hydronium storage in PTCDA.

Lithium Dendrite Suppression with a Silica Nanoparticle-Dispersed Colloidal Electrolyte.

Developing a safe and long-lasting lithium (Li) metal battery is crucial for high-energy applications. However, its poor cycling stability due to Li dendrite formation and excessive Li pulverization is the major hurdle for its practical applications. Here, we present a silica (SiO2) nanoparticle-dispersed colloidal electrolyte (NDCE) and its design principle for suppressing Li dendrite formation. SiO2 nanoclusters in the NDCE play roles in enhancing the Li+ transference number and increasing the Li+ diffusivity in the vicinity of the Li-plating substrate. The NDCE enables less-dendritic Li plating by manipulating the nucleation-growth mode and extending Sand's time. Moreover, SiO2 can interplay with the electrolyte components at the Li-metal surface, enriching fluorinated compounds in the solid electrolyte interface layer. The initial control of the Li plating morphology and SEI structure by the NDCE leads to a more uniform and denser Li deposition upon subsequent cycling, resulting in threefold enhancement of the cycle life. The efficacy of the NDCEs has been further demonstrated by the practical battery design, featuring a commercial-level cathode and thin Li-metal (40 µm) anode.

Toward Higher Capacities of Hydrocarbon Cathodes in Dual-Ion Batteries.

We report the electrochemical anion storage properties of a group of molecular solids of polycyclic aromatic hydrocarbons (PAHs): coronene, perylene, and triphenylene. We discover an interesting trend of progressively lower potentials for these three molecular solids. Our DFT calculations reveal that the inserted PF6- anions preferably bind with the edge sites of the coronene molecules as opposed to being sandwiched between two coronene molecular planes. For smaller PAHs, the more edge sites in the solids may facilitate higher capacity values. However, small PAHs do face a greater challenge of dissolution in the nonaqueous electrolyte, which affects the cycling stability.

A ZnCl2 water-in-salt electrolyte for a reversible Zn metal anode.

We report a low-cost water-in-salt electrolyte, of 30 m ZnCl2, which enables a dendrite-free Zn metal anode to possess a high coulombic efficiency (CE). In asymmetric Zn‖Zn cells with a limited mass of plated Zn as the working electrode, the ZnCl2 WiSE improves the average CE of the Zn anode to 95.4% from 73.2% in 5 m ZnCl2.

Hard carbon anodes of sodium-ion batteries: undervalued rate capability.

The rate capability of hard carbon has long been underestimated in prior studies that used carbon/Na two-electrode half-cells. Through a three-electrode cell setup, we discover that it is the overpotential of the sodium counter electrode that drives the half-cells to the lower cutoff potential prematurely during hard carbon sodiation, particularly at high current rates, which prevents the hard carbon anode from being fully sodiated.