Lidan Xing

Theoretical Investigations on Oxidative Stability of Solvents and Oxidative Decomposition Mechanism of Ethylene Carbonate for Lithium Ion Battery Use

The electrochemical oxidative stability of solvent molecules used for lithium ion battery, ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in the forms of simple molecule and coordination with anion PF(6)(-), is compared by using density functional theory at the level of B3LYP/6-311++G (d, p) in gas phase. EC is found to be the most stable against oxidation in its simple molecule. However, due to its highest dielectric constant among all the solvent molecules, EC coordinates with PF(6)(-) most strongly and reaches cathode most easily, resulting in its preferential oxidation on cathode. Detailed oxidative decomposition mechanism of EC is investigated using the same level. Radical cation EC(*+) is generated after one electron oxidation reaction of EC and there are five possible pathways for the decomposition of EC(*+) forming CO(2), CO, and various radical cations. The formation of CO is more difficult than CO(2) during the initial decomposition of EC(*+) due to the high activation energy. The radical cations are reduced and terminated by gaining one electron from anode or solvent molecules, forming aldehyde and oligomers of alkyl carbonates including 2-methyl-1,3-dioxolane, 1,3,6-trioxocan-2-one, 1,4,6,9-tetraoxaspiro[4.4]nonane, and 1,4,6,8,11-pentaoxaspiro[4.6]undecan-7-one. The calculation in this paper gives a detailed explanation on the experimental findings that have been reported in literatures and clarifies the mechanism on the oxidative decomposition of EC.

Density functional theory study of the role of anions on the oxidative decomposition reaction of propylene carbonate.

The oxidative decomposition mechanism of the lithium battery electrolyte solvent propylene carbonate (PC) with and without PF(6)(-) and ClO(4)(-) anions has been investigated using the density functional theory at the B3LYP/6-311++G(d) level. Calculations were performed in the gas phase (dielectric constant ε = 1) and employing the polarized continuum model with a dielectric constant ε = 20.5 to implicitly account for solvent effects. It has been found that the presence of PF(6)(-) and ClO(4)(-) anions significantly reduces PC oxidation stability, stabilizes the PC-anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. The primary oxidative decomposition products of PC-PF(6)(-) and PC-ClO(4)(-) were CO(2) and acetone radical. Formation of HF and PF(5) was observed upon the initial step of PC-PF(6)(-) oxidation while HClO(4) formed during initial oxidation of PC-ClO(4)(-). The products from the less likely reaction paths included propanal, a polymer with fluorine and fluoro-alkanols for PC-PF(6)(-) decomposition, while acetic acid, carboxylic acid anhydrides, and Cl(-) were found among the decomposition products of PC-ClO(4)(-). The decomposition pathways with the lowest barrier for the oxidized PC-PF(6)(-) and PC-ClO(4)(-) complexes did not result in the incorporation of the fluorine from PF(6)(-) or ClO(4)(-) into the most probable reaction products despite anions and HF being involved in the decomposition mechanism; however, the pathway with the second lowest barrier for the PC-PF(6)(-) oxidative ring-opening resulted in a formation of fluoro-organic compounds, suggesting that these toxic compounds could form at elevated temperatures under oxidizing conditions.

Theoretical insight into oxidative decomposition of propylene carbonate in the lithium ion battery.

The detailed oxidative decomposition mechanism of propylene carbonate (PC) in the lithium ion battery is investigated using density functional theory (DFT) at the level of B3LYP/6-311++G(d), both in the gas phase and in solvent. The calculated results indicate that PC is initially oxidized on the cathode to a radical cation intermediate, PC(*+), and then decomposes through three pathways, generating carbon dioxide CO(2) and radical cations. These radical cations prefer to be reduced on the anode or by gaining one electron from PC, forming propanal, acetone, or relevant radicals. The radicals terminate by forming final products, including trans-2-ethyl-4-methyl-1,3-dioxolane, cis-2-ethyl-4-methyl-1,3-dioxolane, and 2,5-dimethyl-1,4-dioxane. Among all the products, acetone is most easily formed. The calculations in this paper give detailed explanations of the experimental findings that have been reported in the literature and clarify the role of intermediate propylene oxide in PC decomposition. Propylene oxide is one of the important intermediates. As propylene oxide is formed, it isomerizes forming a more stabile product, acetone.

Triple-shelled Mn2O3 hollow nanocubes: force-induced synthesis and excellent performance as the anode in lithium-ion batteries

In this paper, we report a novel structure of Mn2O3, the triple-shelled Mn2O3 hollow nanocube, as the anode material for high-energy lithium-ion batteries, synthesized through a programmed annealing treatment with cubic MnCO3 as precursor. This hierarchical structure is developed through the interaction between the contraction force from the decomposition of MnCO3 and the adhesion force from the formation of Mn2O3. The structure has been confirmed by characterization with XRD, FESEM, TEM, and HRTEM. The charge–discharge tests demonstrate that the resulting Mn2O3 exhibits excellent cycling stability and rate capability when evaluated as an anode material for lithium-ion batteries. It delivers a reversible capacity of 606 mA h g−1 at a current rate of 500 mA g−1 with a capacity retention of 88% and a remaining capacity of 350 mA h g−1 at 2000 mA g−1.

On the Atomistic Nature of Capacitance Enhancement Generated by Ionic Liquid Electrolyte Confined in Subnanometer Pores.

The capacitance enhancement experimentally observed in electrodes with complex morphology of random subnanometer wide pores is an intriguing phenomena, yet the mechanisms for such enhancement are not completely understood. Our atomistic molecular dynamics simulations demonstrate that in subnanometer slit-geometry nanopores, a factor of 2 capacitance enhancement (compared to a flat electrode) is possible for the 1-ethyl-3-methylimidazolium (EMIM)-bis(trifluoro-methylsulfonyl)imide (TFSI) ionic liquid electrolyte. This capacitance enhancement is a result of a fast charge separation inside the nanopore due to abrupt expulsion of co-ions from the pore while maintaining an elevated counterion density due to strong screening of electrostatic repulsive interactions by the conductive pore. Importantly, we find that the capacitance enhancement can be very asymmetric. For the negatively charged 7.5 Å wide pore, the integral capacitance is 100% larger than on a flat surface; however, on the positive electrode, almost no enhancement is observed. Detailed analysis of structure and composition of electrolyte inside nanopores shows that the capacitance enhancement and the shape of differential capacitance strongly depend on the details of the ion chemical structure and a delicate balance of ion-surface and ion-ion interactions.

A novel electrolyte with the ability to form a solid electrolyte interface on the anode and cathode of a LiMn2O4/graphite battery

To improve the cyclability of a LiMn2O4/graphite lithium ion battery at elevated temperature, a carbonate-based electrolyte using prop-1-ene-1,3-sultone (PES) as additive was developed. The cycling performance of the LiMn2O4/graphite cell, based on the developed electrolyte at 60 °C, was evaluated by a constant current charge/discharge test, with comparison of the electrolyte using vinylene carbonate (VC) as additive. It was found that the cell based on the developed electrolyte exhibits better cyclability and exhibits better dimensional stability at elevated temperatures. The capacity retention is 91% and the swell value in thickness is 3.4% for the cell with PES after 150 cycles at 60 °C, while the respective values were 68% and 36.4% for the cell without additive, and 82% and 9.1% for the cell with VC. The results obtained from scanning electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, thermal gravimetric analysis, and molecular energy level calculations show that PES favors the formation of a stable solid electrolyte interphase, not only on the anode but also on the cathode of the LiMn2O4/graphite battery, effectively preventing electrolyte decomposition.

Crystallographic facet- and size-controllable synthesis of spinel LiNi0.5Mn1.5O4 with excellent cyclic stability as cathode of high voltage lithium ion battery

We report a novel synthesis of spinel LiNi0.5Mn1.5O4, in which cubic and porous Mn2O3 nanoparticles, obtained from cubic MnCO3, are used as templates to induce the formation of crystallographic facet- and size-defined spinel. This is done to accomplish excellent cyclic stability of the spinel as a cathode of a high voltage lithium ion battery. The uniformly dispersed pores in the template, whose size can be controlled by limiting the annealing time of MnCO3, facilitate the incorporation of lithium and nickel ions and ensure the formation of spinel with a predominant (111) facet, while the spinel inherits the particle size of the template under controlled temperatures. The characterizations from SEM, TEM and XRD confirm the structure and morphology of the precursors and the resulting product. The charge–discharge test demonstrates the excellent cyclic stability of the resulting products, especially at elevated temperatures: capacity retention of 78.1% after 3000 cycles with 10 C rate at room temperature and that of 83.2% after 500 cycles with 5 C rate at 55 °C.

Porous LiMn2O4 cubes architectured with single-crystalline nanoparticles and exhibiting excellent cyclic stability and rate capability as the cathode of a lithium ion battery

Porous LiMn2O4 was fabricated with cubic MnCO3 as precursor and characterized in terms of structure and performance as the cathode of a lithium ion battery. The characterizations from SEM, TEM and XRD demonstrate that the fabricated product has a cubic morphology with an average edge of 250 nm, which it inherits from the precursor, and a porous structure architectured with single-crystalline spinel nanoparticles of 50 nm, which imitates the Mn2O3 that results from the thermal decomposition of the precursor. The charge–discharge tests show that the synthesized product exhibits excellent rate capability and cyclic stability: delivering a reversible discharge capacity of 108 mA h g−1 at a 30 C rate and yielding a capacity retention of over 81% at a rate of 10 C after 4000 cycles. The superior performance of the synthesized product is attributed to its special structure: porous secondary cube particles consisting of primary single-crystalline nanoparticles. The nanoparticle reduces the path of Li ion diffusion and increases the reaction sites for lithium insertion/extraction, the pores provide room to buffer the volume changes during charge–discharge and the single crystalline nanoparticle endows the spinel with the best stability.

Influence of temperature on the capacitance of ionic liquid electrolytes on charged surfaces

In this work using molecular dynamics simulations we examine the temperature dependence of the differential capacitance of room temperature ionic liquid electrolytes near electrified surfaces. For electrodes with atomically flat surfaces our simulations show very weak temperature dependence of the differential capacitance (DC) with a slight decrease of DC with increasing temperature. For atomically corrugated surfaces where the ion dimensions are comparable to the size of the surface corrugation patterns, the influence of temperature on DC is much more pronounced. At low temperatures the DC dependence on electrode potential shows large variations with well-defined maxima and minima. However, with increasing temperature these features are significantly flattened. Also for these corrugated surfaces an abnormal positive slope of DC vs. temperature is observed in the narrow range of relatively low voltages. Analysis of changes in the electric double layer structure as a function of temperature allowed us to propose a new mechanism explaining the observed trends in capacitance as a function of temperature and surface topography. The obtained simulation results are discussed in light of available experimental data and help to discriminate between contradictory experimentally observed trends in DC temperature dependence reported for ionic liquid based electrolytes in the literature.

Sulfur loaded in curved graphene and coated with conductive polyaniline: preparation and performance as a cathode for lithium–sulfur batteries

We report a composite (CG-S@PANI), sulfur (S) loaded in curved graphene (CG) and coated with conductive polyaniline (PANI), as a cathode for lithium–sulfur batteries. CG is prepared by splitting multi-wall carbon nanotubes and loaded with S via chemical deposition and then coated with polyaniline via in situ polymerization under the control of ascorbic acid. The physical and electrochemical performances of the resulting CG-S@PANI are investigated by nitrogen adsorption–desorption isotherms, X-ray powder diffraction, thermogravimetric analysis, transmission electron microscopy, electrochemical impedance spectroscopy, charge–discharge tests, and electronic conductivity measurements. CG-S@PANI as a cathode for lithium–sulfur batteries delivers an initial discharge capacity of 851 mA h g−1 (616 mA h g−1 on the basis of the cathode mass) at 0.2 C with a capacity retention of over 90% after 100 cycles. This nature is attributed to the co-contribution of CG and conductive PANI to the concurrent improvement in electronic conductivity and chemical stability of the sulfur cathode.