Hongfa Xiang

Ultrathin Li4Ti5O12 Nanosheets as Anode Materials for Lithium and Sodium Storage

Ultrathin Li4Ti5O12 (LTO) nanosheets with ordered microstructures were prepared via a polyether-assisted hydrothermal process. Pluronic P123, a polyether, can impede the growth of Li2TiO3 in the precursor and also act as a structure-directing agent to facilitate the (Li1.81H0.19)Ti2O5·2H2O precursor to form the LTO nanosheets with the ordered microstructure. Moreover, the addition of P123 can suppress the stacking of LTO nanosheets during calcining of the precursor, and the thickness of the nanosheets can be controlled to be about 4 nm. The microstructure of the as-prepared ultrathin and ordered nanosheets is helpful for Li(+) or Na(+) diffusion and charge transfer through the particles. Therefore, the ultrathin P123-assisted LTO (P-LTO) nanosheets show a rate capability much higher than that of the LTO sample without P123 in a Li battery with over 130 mAh g(-1) of capacity remaining at the 64C rate. For intercalation of larger size Na(+) ions, the P-LTO still exhibits a capacity of 115 mAh g(-1) at a current rate of 10 C and a capacity retention of 96% after 400 cycles.

Chromium-Modified Li4Ti5O12 with a Synergistic Effect of Bulk Doping, Surface Coating, and Size Reducing.

Bulk doping, surface coating, and size reducing are three strategies for improving the electrochemical properties of Li4Ti5O12 (LTO). In this work, chromium (Cr)-modified LTO with a synergistic effect of bulk doping, surface coating, and size reducing is synthesized by a facile sol-gel method. X-ray diffraction (XRD) and Raman analysis prove that Cr dopes into the LTO bulk lattice, which effectively inhibits the generation of TiO2 impurities. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) verifies the surface coating of Li2CrO4 on the LTO surface, which decreases impedance of the LTO electrode. More importantly, the size of LTO particles can be significantly reduced from submicroscale to nanoscale as a result of the protection of the Li2CrO4 surface layer and the suppression from Cr atoms on the long-range order in the LTO lattice. As anode material, Li4-xCr3xTi5-2xO12 (x = 0.1) delivers a reversible capacity of 141 mAh g(-1) at 10 °C, and over 155 mAh g(-1) at 1 °C after 1000 cycles. Therefore, the Cr-modified Li4Ti5O12 prepared via a sol-gel method has potential for applications in high-power, long-life lithium-ion batteries.

Enhanced electrochemical performance and thermal stability of a CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 cathode material for lithium-ion batteries

A layered Li1.2Ni0.13Co0.13Mn0.54O2 cathode is coated with a CePO4 layer via a simple precipitation method. The pristine and CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscope (HR-TEM) and X-ray photoelectron spectroscopy (XPS), and the results indicate that CePO4 has been uniformly coated on the Li1.2Ni0.13Co0.13Mn0.54O2. Charge–discharge tests show that the CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 has an obviously enhanced electrochemical performance compared with the pristine sample: the initial coulombic efficiency from 88.26% to 92.19%, rate capability from 6 to 110 mA h g−1 at 10 C, high-temperature performance from 59.5 to 219.6 mA h g−1 at 55 °C after 20 cycles, and low-temperature performance from 128.3 to 246.7 mA h g−1 at −20 °C. According to the analysis from dc impedance and electrochemical impedance spectra, the improvements on the electrochemical performance are mainly because the coated CePO4 layer can reduce side reactions of Li1.2Ni0.13Co0.13Mn0.54O2 with the electrolyte, and thus form the cathode–electrolyte interface (CEI) layer with enhanced Li+ diffusion. In addition, the CePO4 layer significantly improves the thermal stability of the coexisting systems of the charged cathode with the electrolyte. Therefore, CePO4 coating will be a promising approach to improve the electrochemical performance and thermal stability of Li-rich layered oxide cathode materials.

A trilayer carbon nanotube/Al2O3/polypropylene separator for lithium-sulfur batteries

A trilayer carbon nanotube (CNT)/Al2O3/polypropylene (PP) separator is prepared by means of simple tape casting of Al2O3 and CNT layers onto a commercial PP separator, respectively. The electrochemical performance of the trilayer separator in the lithium-sulfur batteries is compared with those of the PP and the Al2O3/PP separator. The CNT/Al2O3/PP separator exhibits the best electrochemical performance, including the cycling performance and rate capability. The CNT component in the trilayer separator acts as a matrix for capturing the polysulfide species and suppressing the migration of polysulfides to the lithium anode side and also reduces the charge transfer resistance of sulfur cathodes significantly. Meanwhile, the Al2O3 layer in the trilayer separator not only has the advantages on thermal stability and electrolyte intake but also can provide the strong shield for CNT from penetrating the separator.

The Role of Cesium Cation in Controlling Interphasial Chemistry on Graphite Anode in Propylene Carbonate-Rich Electrolytes

Despite the potential advantages it brings, such as wider liquid range and lower cost, propylene carbonate (PC) is seldom used in lithium-ion batteries because of its sustained cointercalation into the graphene structure and the eventual graphite exfoliation. Here, we report that cesium cation (Cs(+)) directs the formation of solid electrolyte interphase on graphite anode in PC-rich electrolytes through its preferential solvation by ethylene carbonate (EC) and the subsequent higher reduction potential of the complex cation. Effective suppression of PC-decomposition and graphite-exfoliation is achieved by adjusting the EC/PC ratio in electrolytes to allow a reductive decomposition of Cs(+)-(EC)m (1 ≤ m ≤ 2) complex preceding that of Li(+)-(PC)n (3 ≤ n ≤ 5). Such Cs(+)-directed interphase is stable, ultrathin, and compact, leading to significant improvement in battery performances. In a broader context, the accurate tailoring of interphasial chemistry by introducing a new solvation center represents a fundamental breakthrough in manipulating interfacial reactions that once were elusive to control.

Effects of propylene carbonate content in CsPF6-containing electrolytes on the enhanced performances of graphite electrode for lithium-ion batteries

The effects of propylene carbonate (PC) content in CsPF6-containing electrolytes on the performances of graphite electrode in lithium half cells and in graphite∥LiNi0.80Co0.15Al0.05O2 (NCA) full cells are investigated. It is found that the performance of graphite electrode is significantly affected by PC content in the CsPF6-containing electrolytes. An optimal PC content of 20% by weight in the solvent mixtures is identified. The enhanced electrochemical performance of graphite electrode can be attributed to the synergistic effects of the PC solvent and the Cs(+) additive. The synergistic effects of Cs(+) additive and appropriate amount of PC enable the formation of a robust, ultrathin, and compact solid electrolyte interphase (SEI) layer on the surface of graphite electrode, which is only permeable for desolvated Li(+) ions and allows fast Li(+) ion transport through it. Therefore, this SEI layer effectively suppresses the PC cointercalation and largely alleviates the Li dendrite formation on graphite electrode during lithiation even at relatively high current densities. The presence of low-melting-point PC solvent improves the sustainable operation of graphite∥NCA full cells under a wide temperature range. The fundamental findings also shed light on the importance of manipulating/maintaining the electrode/electrolyte interphasial stability in various energy-storage devices.

Spinel MgAl2O4 modification on LiCoO2 cathode materials with the combined advantages of MgO and Al2O3 modifications for high-voltage lithium-ion batteries

In order to improve the electrochemical performance of LiCoO2 cathode in a high-voltage range of 3.0–4.5 V, spinel MgAl2O4 has been modified on the surface of LiCoO2 particle by a facile high-temperature solid state reaction. The structure and morphology of the MgAl2O4-modified LiCoO2 are investigated in comparison with the pristine, Al2O3-modified and MgO-modified LiCoO2. The MgAl2O4 modification is highly conformal and uniform just similar as the Al2O3 modification, while the MgO modification is not uniform. In terms of electrochemical performance as a high-voltage cathode material, the MgAl2O4-modified LiCoO2 delivers an initial discharge capacity of 184 mA h g−1 between 3.0 V and 4.5 V at 0.1C (1C-rate = 160 mA g−1) and a capacity retention of 96.8% after 70 cycles at 1C rate. There is a significant improvement on high-voltage cycling stability for the MgAl2O4-modified LiCoO2 since the capacity retention of the pristine LiCoO2 is only 38.7% after 70 cycles. Moreover, the MgAl2O4-modified LiCoO2 exhibits an enhanced rate capability. Compared with the Al2O3 modification and the MgO modification, spinel MgAl2O4 modification has the combined advantages of Al2O3 and MgO modifications on improving the electrochemical performance of the LiCoO2 cathode for high-voltage applications. The modified spinel MgAl2O4 layer can effectively protect the charged Li1−xCoO2 cathode from structural collapse and impede the oxidation decomposition of the electrolyte for the high-voltage application of LiCoO2.

Ethylene ethyl phosphate as a multifunctional electrolyte additive for lithium-ion batteries

The effects of ethylene ethyl phosphate (EEP) as a multifunctional electrolyte additive on safety characteristics and electrochemical performance of lithium-ion batteries are investigated. Based on the flammability test, the self-extinguishing time of the electrolyte with 10% EEP is only less than half of the baseline electrolyte, which indicates that EEP is a highly efficient flame retardant for the electrolyte. During overcharging the LiNi1/3Co1/3Mn1/3O2/Li cells, incorporation of EEP into the electrolyte can postpone the sharp voltage rise. Therefore, EEP is an improver of safety characteristics of lithium-ion batteries, both in terms of flame resistance and overcharge protection. Furthermore, the EEP-containing electrolyte in the half-cells and full-cells both exhibit higher initial coulombic efficiency and cycling stability than the baseline electrolyte. It is concluded that EEP is a good film-formation additive not only for the graphite anode, but also for the LiNi1/3Co1/3Mn1/3O2 cathode. Therefore, EEP is proposed as a promising multifunctional electrolyte additive for lithium-ion batteries.

Effects of porous structure of carbon hosts on preparation and electrochemical performance of sulfur/carbon composites for lithium–sulfur batteries

Three kinds of carbon hosts, Ketjenblack (KB, high surface area and porosity), black pearls 2000 (BP2000, high surface area and moderate porosity), and ordered mesoporous carbon nanospheres (OMCN, low surface area and porosity), have been used as conductive hosts in the sulfur/carbon (S/C) composite cathodes for lithium–sulfur (Li–S) batteries. To correlate the carbon properties (surface area and pore volume), the electrochemical performances of S/C composite cathodes with the same sulfur content (60 wt%) have been investigated in detail. S/KB and S/BP2000 composites with high surface porosity can provide more reactive sites for sulfur, which can result in increasing the utilization rate of sulfur, reducing the polarization, and improving the high-rate capability. Large pore volume can effectively capture the polysulfide species and improve easy passages for ion transport, which can promote long-term cycling stability and reduce the resistance of Li–S batteries.

Lithium Difluorophosphate as a Dendrite-Suppressing Additive for Lithium Metal Batteries

The notorious lithium (Li) dendrites and the low Coulombic efficiency (CE) of Li anode are two major obstacles to the practical utilization of Li metal batteries (LMBs). Introducing a dendrite-suppressing additive into nonaqueous electrolytes is one of the facile and effective solutions to promote the commercialization of LMBs. Herein, Li difluorophosphate (LiPO2F2, LiDFP) is used as an electrolyte additive to inhibit Li dendrite growth by forming a vigorous and stable solid electrolyte interphase film on metallic Li anode. Moreover, the Li CE can be largely improved from 84.6% of the conventional LiPF6-based electrolyte to 95.2% by the addition of an optimal concentration of LiDFP at 0.15 M. The optimal LiDFP-containing electrolyte can allow the Li||Li symmetric cells to cycle stably for more than 500 and 200 h at 0.5 and 1.0 mA cm-2, respectively, much longer than the control electrolyte without LiDFP additive. Meanwhile, this LiDFP-containing electrolyte also plays an important role in enhancing the cycling stability of the Li||LiNi1/3Co1/3Mn1/3O2 cells with a moderately high mass loading of 9.7 mg cm-2. These results demonstrate that LiDFP has extensive application prospects as a dendrite-suppressing additive in advanced LMBs.