Xiaoqi Sun

A high capacity thiospinel cathode for Mg batteries

Magnesium batteries are energy storage systems that potentially offer high energy density owing to their ability to employ magnesium metal as a negative electrode. Their development, however, has been thwarted by a paucity of functional positive electrode materials after the seminal discovery of the Mo6S8 Chevrel phase over 15 years ago. Herein, we report the second such material – a thiospinel – and demonstrate fully reversible Mg2+ electrochemical cycling vs. a Mg anode, which is complemented by diffraction and first principles calculations. The capacity approaches 80% of the theoretical value at a practical rate (C/5) at 60 °C, and yields a specific energy of 230 Wh kg−1, twice that of the Chevrel benchmark. Our results emphasize the advantage in employing “soft” anions to achieve practical divalent cation mobility.

Prussian Blue MgLi Hybrid Batteries

The major advantage of Mg batteries relies on their promise of employing an Mg metal negative electrode, which offers much higher energy density compared to graphitic carbon. However, the strong coulombic interaction of Mg2+ ions with anions leads to their sluggish diffusion in the solid state, which along with a high desolvation energy, hinders the development of positive electrode materials. To circumvent this limitation, Mg metal negative electrodes can be used in hybrid systems by coupling an Li+ insertion cathode through a dual salt electrolyte. Two “high voltage” Prussian blue analogues (average 2.3 V vs Mg/Mg2+; 3.0 V vs Li/Li+) are investigated as cathode materials and the influence of structural water is shown. Their electrochemical profiles, presenting two voltage plateaus, are explained based on the two unique Fe bonding environments. Structural water has a beneficial impact on the cell voltage. Capacities of 125 mAh g−1 are obtained at a current density of 10 mA g−1 (≈C/10), while stable performance up to 300 cycles is demonstrated at 200 mA g−1 (≈2C). The hybrid cell design is a step toward building a safe and high density energy storage system.

Ultra-rapid microwave synthesis of triplite LiFeSO4F

Quick, effective synthesis of the 4 V Li-ion battery cathode material, triplite LiFeSO4F, takes place via facile conversion of the defect-peppered nanocrystalline tavorite precursor that forms on ultra-rapid microwave heating (10 min) of FeSO4·H2O/LiF. We propose a mechanism for its unique phase transformation to the triplite that occurs as a consequence of the disorder and hydroxyl defects induced by the fast nucleation. The electrochemical properties of the resultant triplite exhibits a doubling of its practical gravimetric capacity compared to the material prepared by conventional methods.

Screening for positive electrodes for magnesium batteries: a protocol for studies at elevated temperatures

The well-known all phenyl complex (APC) electrolyte for magnesium batteries is studied for the first time at high temperature using tetraglyme as a solvent. Combined with a molybdenum current collector, this enables the examination of positive electrode materials for Mg batteries at temperatures as high as 180 °C and up to 2 V vs. Mg, allowing discovery of the auspicious properties of CuS as a conversion cathode.

High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors

Metal oxides have attracted renewed interest as promising electrode materials for high energy density supercapacitors. However, the electrochemical performance of metal oxide materials deteriorates significantly with the increase of mass loading due to their moderate electronic and ionic conductivities. This limits their practical energy. Herein, we perform a morphology and phase-controlled electrodeposition of MnO2 with ultrahigh mass loading of 10 mg cm-2 on a carbon cloth substrate to achieve high overall capacitance without sacrificing the electrochemical performance. Under optimum conditions, a hierarchical nanostructured architecture was constructed by interconnection of primary two-dimensional ε-MnO2 nanosheets and secondary one-dimensional α-MnO2 nanorod arrays. The specific hetero-nanostructures ensure facile ionic and electric transport in the entire electrode and maintain the structure stability during cycling. The hierarchically structured MnO2 electrode with high mass loading yields an outstanding areal capacitance of 3.04 F cm-2 (or a specific capacitance of 304 F g-1) at 3 mA cm-2 and an excellent rate capability comparable to those of low mass loading MnO2 electrodes. Finally, the aqueous and all-solid asymmetric supercapacitors (ASCs) assembled with our MnO2 cathode exhibit extremely high volumetric energy densities (8.3 mWh cm-3 at the power density of 0.28 W cm-3 for aqueous ASC and 8.0 mWh cm-3 at 0.65 W cm-3 for all-solid ASC), superior to most state-of-the-art supercapacitors.

Monovalent versus Divalent Cation Diffusion in Thiospinel Ti2S4

Diffusion coefficients (D) for both Li+ and Mg2+ in Ti2S4 were measured using the galvanic intermittent titration technique (GITT) as a function of both ion concentration (x) and temperature. During discharge at 60 °C, DLi descends gradually from 2 × 10-8 cm2/s at xLi ≈ 0 to 2 × 10-9 cm2/s at xLi ≈ 1.9. In contrast, DMg decreases sharply from 2 × 10-8 to 1 × 10-12 cm2/s by xMg ≈ 0.8. This kinetic factor limits the maximum practical discharge capacity of MgxTi2S4. The difference in behavior vis a vis Li+ implies that either increasing Mg2+ occupation of the tetrahedral site at xMg > 0.6 and/or interactions between diffusing cations play a larger role in mediating the diffusion of divalent compared to monovalent cations. Diffusion activation energies (Ea) extracted from the temperature-dependent data revealed that Ea,Mg (540 ± 80 meV) is about twice that of Ea,Li (260 ± 50 meV), explaining the poorer electrochemical performance of MgxTi2S4 at room temperature.

Stabilization of Lithium Transition Metal Silicates in the Olivine Structure

While olivine LiFePO4 shows amongst the best electrochemical properties of Li-ion positive electrodes with respect to rate behavior owing to facile Li+ migration pathways in the framework, replacing the [PO4]3- polyanion with a silicate [SiO4]4- moiety in olivine is desirable. This could allow additional alkali content and hence electron transfer, and increase the capacity. Herein we explore the possibility of a strategy toward new cathode materials and demonstrate the first stabilization of a lithium transition metal silicate (as a pure silicate) in the olivine structure type. Using LiInSiO4 and LiScSiO4 as the parent materials, transition metal (Mn, Fe, Co) substitutions on the In/Sc site were investigated by computational modeling via atomic scale simulation. Transition metal substitution was found to be only favorable for Co, a finding confirmed by the successful solid state synthesis of olivine LixInyCo2-x-ySiO4. Stabilization of the structure was achieved by entropy provided by cation disorder.