Meinan He

POLYANTHRAQUINONE-BASED ORGANIC CATHODE FOR HIGH-PERFORMANCE RECHARGEABLE MAGNESIUM-ION BATTERIES

A rechargeable magnesium ion electrochemical cell comprising an anode, a cathode, and a non-aqueous magnesium electrolyte disposed between the anode and the cathode is described herein. The cathode comprises a redox-active anthraquinone-based polymer comprising one or more of 1,4-polyanthraquinone or 2,6-polyanthraquinone. Both 2,6-polyanthraquinone and 1,4-polyanthraquinone can operate with 1.5-2.0 V with above 100 mAh/g capacities at a reasonable rate, higher than the state-of-the-art Mg—Mg6S8 battery. More than 1000 cycles with very small capacity loss can be realized.

Pure Inorganic Separator for Lithium Ion Batteries

Battery safety is critical for many applications including portable electronics, hybrid and electric vehicles, and grid storage. For lithium ion batteries, the conventional polymer based separator is unstable at 120 °C and above. In this research, we have developed a pure aluminum oxide nanowire based separator; this separator does not contain any polymer additives or binders; additionally, it is a bendable ceramic. The physical and electrochemical properties of the separator are investigated. The separator has a pore size of about 100 nm, and it shows excellent electrochemical properties under both room and high temperatures. At room temperature, the ceramic separator shows a higher rate capability compared to the conventional Celgard 2500 separator and life cycle performance does not show any degradation. At 120 °C, the cell with the ceramic separator showed a much better cycle performance than the conventional Celgard 2500 separator. Therefore, we believe that this research is really an exciting scientific breakthrough for ceramic separators and lithium ion batteries and could be potentially used in the next generation lithium ion batteries requiring high safety and reliability.

Mechanistic Insight in the Function of Phosphite Additives for Protection of LiNi0.5Co0.2Mn0.3O2 Cathode in High Voltage Li-Ion Cells.

Triethlylphosphite (TEP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) have been evaluated as electrolyte additives for high-voltage Li-ion battery cells using a Ni-rich layered cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) and the conventional carbonate electrolyte. The repeated charge/discharge cycling for cells containing 1 wt % of these additives was performed using an NCM523/graphite full cell operated at the voltage window from 3.0-4.6 V. During the initial charge process, these additives decompose on the cathode surface at a lower oxidation potential than the baseline electrolyte. Impedance spectroscopy and post-test analyses indicate the formation of protective coatings by both additives on the cathode surface that prevent oxidative breakdown of the electrolyte. However, only TTFP containing cells demonstrate the improved capacity retention and Coulombic efficiency. For TEP, the protective coating is also formed, but low Li(+) ion mobility through the interphase layer results in inferior performance. These observations are rationalized through the inhibition of electrocatalytic centers present on the cathode surface and the formation of organophosphate deposits isolating the cathode surface from the electrolyte. The difference between the two phosphites clearly originates in the different properties of the resulting phosphate coatings, which may be in Li(+) ion conductivity through such materials.

Caramel Popcorn Shaped Silicon Particle with Carbon Coating as a High Performance Anode Material for Li-Ion Batteries

Silicon is a very promising anode material for lithium ion batteries. It has a 4200 mAh/g theoretical capacity, which is ten times higher than that of commercial graphite anodes. However, when lithium ions diffuse to Si anodes, the volume of Si will expand to almost 400% of its initial size and lead to the crack of Si. Such a huge volume change and crack cause significant capacity loss. Meanwhile, with the crack of Si particles, the conductivity between the electrode and the current collector drops. Moreover, the solid electrolyte interphase (SEI), which is generated during the cycling, reduces the discharge capacity. These issues must be addressed for widespread application of this material. In this work, caramel popcorn shaped porous silicon particles with carbon coating are fabricated by a set of simple chemical methods as active anode material. Si particles are etched to form a porous structure. The pores in Si provide space for the volume expansion and liquid electrolyte diffusion. A layer of amorphous carbon is formed inside the pores, which gives an excellent isolation between the Si particle and electrolyte, so that the formation of the SEI layer is stabilized. Meanwhile, this novel structure enhances the mechanical properties of the Si particles, and the crack phenomenon caused by the volume change is significantly restrained. Therefore, an excellent cycle life under a high rate for the novel Si electrode is achieved.

1,4-Bis(trimethylsilyl)-2,5-dimethoxybenzene: a novel redox shuttle additive for overcharge protection in lithium-ion batteries that doubles as a mechanistic chemical probe

A novel redox shuttle additive, 1,4-bis(trimethylsilyl)-2,5-dimethoxybenzene (BTMSDB), is shown to deliver superb overcharge protection of LiFePO4 electrode in Li-ion batteries. Using this molecule as a chemical probe, we trace the cause of the eventual failure of this additive to the gradual loss of steric protection in the corresponding radical cation, providing the much needed mechanistic insight in the factors controlling the long-term efficiency of overcharge protection.

Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes

Through coupled experimental analysis and computational techniques, we uncover the origin of anodic stability for a range of nonaqueous zinc electrolytes. By examination of electrochemical, structural, and transport properties of nonaqueous zinc electrolytes with varying concentrations, it is demonstrated that the acetonitrile-Zn(TFSI)2, acetonitrile-Zn(CF3SO3)2, and propylene carbonate-Zn(TFSI)2 electrolytes can not only support highly reversible Zn deposition behavior on a Zn metal anode (≥99% of Coulombic efficiency) but also provide high anodic stability (up to ∼3.8 V vs Zn/Zn(2+)). The predicted anodic stability from DFT calculations is well in accordance with experimental results, and elucidates that the solvents play an important role in anodic stability of most electrolytes. Molecular dynamics (MD) simulations were used to understand the solvation structure (e.g., ion solvation and ionic association) and its effect on dynamics and transport properties (e.g., diffusion coefficient and ionic conductivity) of the electrolytes. The combination of these techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties in nonaqueous zinc electrolytes.

Oxidatively stable fluorinated sulfone electrolytes for high voltage high energy lithium-ion batteries

New fluorinated sulfones were synthesized and evaluated in high voltage lithium-ion batteries using a LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode. Fluorinated sulfones with an α-trifluoromethyl group exhibit enhanced oxidation stability, reduced viscosity and superior separator wettability as compared to their non-fluorinated counterparts. Their improved performance in high voltage NMC532/graphite cells makes them promising high voltage electrolytes for next generation high voltage high energy lithium-ion batteries.

The Role of MgCl2 as a Lewis Base in ROMgCl–MgCl2 Electrolytes for Magnesium‐Ion Batteries

A series of strong Lewis acid-free alkoxide/siloxide-based Mg electrolytes were deliberately developed with remarkable oxidative stability up to 3.5 V (vs. Mg/Mg(2+)). Despite the perception of ROMgCl (R=alkyl, silyl) as a strong base, ROMgCl acts like Lewis acid, whereas the role of MgCl2 in was unambiguously demonstrated as a Lewis base through the identification of the key intermediate using single crystal X-ray crystallography. This Lewis-acid-free strategy should provide a prototype system for further investigation of Mg-ion batteries.

A high performance lithium–sulfur battery enabled by a fish-scale porous carbon/sulfur composite and symmetric fluorinated diethoxyethane electrolyte

A high performance lithium–sulfur (Li–S) battery comprising a symmetric fluorinated diethoxyethane electrolyte coupled with a fish-scale porous carbon/S composite electrode was demonstrated. 1,2-Bis(1,1,2,2-tetrafluoroethoxy)ethane (TFEE) was first studied as a new electrolyte solvent for Li–S chemistry. When co-mixed with 1,3-dioxolane (DOL), the DOL/TFEE electrolyte suppressed the polysulfide dissolution and shuttling reaction. When coupled with a fish-scale porous carbon/S composite electrode, the Li–S cell exhibited a significantly high capacity retention of 99.5% per cycle for 100 cycles, which is far superior to the reported numerous systems.

Functionality Selection Principle for High Voltage Lithium-ion Battery Electrolyte Additives

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li+/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives.