Yulin Ma

Nanosized core/shell silicon@carbon anode material for lithium ion batteries with polyvinylidene fluoride as carbon source

A nanosized anode material for lithium ion batteries with silicon as core and amorphous carbon as shell was synthesized by dispersing nanosized silicon in polyvinylidene fluoride solution and a subsequent pyrolysis process. The amorphous nature of the carbon in the composite was detected by X-ray diffraction and Raman spectroscopy. The core/shell structure was further identified by transmission electron microscopy. High reversible capacity and acceptable rate capability were exhibited compared with pristine silicon. The reversible capacity of the silicon@carbon nanocomposite at 50 mA g−1 after 30 cycles is 1290 mAh g−1 with a capacity retention of 97%. A stable reversible capacity of 450 mAh g−1 was delivered even at 1000 mA g−1. These improvements are attributed to the amorphous carbon shell, which suppresses the agglomeration of nanosized silicon, reduces the cell impedance, buffers the volume changes and stabilizes the electrode structure during charge/discharge cycles.

An Li-rich oxide cathode material with mosaic spinel grain and a surface coating for high performance Li-ion batteries

Lithium-rich layered oxides are considered to be one of the most promising cathode materials for lithium ion batteries due to their extremely high reversible capacity. Here, we report the design of a novel heterostructured Li1.2Ni0.13Co0.13Mn0.54O2 material with mosaic spinel nanograins and a surface coating, which is synthesized by a facile and green one-step solid-phase surface-modification process. We propose that the chemical Li leaching from Li2MnO3 simultaneously induces the formation of a fluorite coating and the layer-to-spinel phase transformation at high temperatures. The fluorite coating protects the lithium-rich oxides from direct exposure to the highly active electrolyte. The spinel phase provides an efficient path for Li+ mobility and also facilitates the suppression of the initial irreversible capacity loss. This unique heterostructured Li1.2Ni0.13Co0.13Mn0.54O2 material thus exhibits an outstanding initial Coulombic efficiency, superior rate capability and excellent cyclability. The design concept and facile synthetic strategy can be applied to both advanced lithium ion batteries and other high-performance energy storage devices.

Lithium-rich Li1.2Ni0.13Co0.13Mn0.54O2 oxide coated by Li3PO4 and carbon nanocomposite layers as high performance cathode materials for lithium ion batteries

Lithium-rich layered oxide Li1.2Ni0.13Co0.13Mn0.54O2 (LNCMO) coated with a nanocomposite layer of Li3PO4 and carbon (LNCMO@Li3PO4/C) is designed and facilely prepared as the cathode material for rechargeable lithium ion batteries. The structure and morphology of the LNCMO@Li3PO4/C material are characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, and its electrochemical performance is measured by the constant current charge and discharge, electrochemical impedance spectroscopy and cyclic voltammetry. It is clearly revealed that the LNCMO surface is uniformly coated by the Li3PO4/C nanocomposite layer. Moreover, the coating process induces the layer-to-spinel phase transformation, leading to the formation of a spinel nanophase in the LNCMO@Li3PO4/C material. The presence of Li3PO4/C composite coating with high ionic and electronic conductivity and the spinel nanophase synergistically contribute to the electrochemical properties. Therefore, the LNCMO@Li3PO4/C material shows a high discharge capacity of 124.4 mA h g−1 even at a current density of 1000 mA g−1, a remarkable capacity retention of 87.3% after 200 cycles, and a desirable initial coulombic efficiency of 87.0%. The LNCMO@Li3PO4/C material represents an attractive alternative to high-rate and long-life electrode materials for lithium-ion batteries.

Understanding undesirable anode lithium plating issues in lithium-ion batteries

Lithium-ion batteries (LIBs) are attractive candidates as power sources for various applications, such as electric vehicles and large-scale energy storage devices. However, safety and life issues are still great challenges for the practical applications of LIBs. Metallic lithium plating on the negative electrode under critical charging conditions accelerates performance degradation and poses safety hazards for LIBs. Therefore, anode lithium plating in LIBs has recently drawn increased attention. This article reviews the recent research and progress regarding anode lithium plating of LIBs. Firstly, the adverse effects of anode lithium plating on the electrochemical performance of LIBs are presented. Various in situ and ex situ techniques for characterizing and detecting anode lithium plating are then summarized. Also, this review discusses the influencing factors that induce anode lithium plating and approaches to mitigating or preventing anode lithium plating. Finally, remaining challenges and future developments related to anode lithium plating are proposed in the conclusion.

Improved electrochemical performance and capacity fading mechanism of nano-sized LiMn0.9Fe0.1PO4 cathode modified by polyacene coating

Nano-sized LiMn1−xFexPO4 (x = 0 and 0.1) was prepared by a solvothermal method in a mixed solvent of water and ethanol. LiMn0.9Fe0.1PO4–polyacene (PAS) composite exhibits a high conductivity (0.15 S cm−1), resulting in an excellent rate performance and good cycle life. The LiMn0.9Fe0.1PO4–PAS composite delivers a discharge capacity of 161, 141, and 107 mA h g−1 at 0.1 C, 1 C and 10 C, respectively. The well-distributed conductive polyacene surrounding the LiMn0.9Fe0.1PO4 nanoplates enhances the electronic contact of the nanosized crystalline particles and suppresses the manganese dissolution related to the structure evolution during cycling. Specifically, the manganese dissolution, electrolyte decomposition and the antisite defects are the most significant factors that impact the capacity degradation of olivine iron-doped lithium manganese phosphate cathode materials.

Electrochemical performance degeneration mechanism of LiCoO2 with high state of charge during long-term charge/discharge cycling

Electrochemical performance degeneration of LiCoO2 electrodes under high state of charge (SOC) during long-term cycling was studied using LiCoO2/MCMB batteries. The batteries were charged/discharged at 0.6C with 30% depth of discharge (DOD) for 100, 400, 800, 1600, 2000 and 2400 cycles, respectively, and then disassembled to analyze the evolution of morphology, element content, microstructure and electrochemical performance. Through energy dispersive spectrometer (EDS), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) characterization, it was confirmed that the formation of discontinuous solid electrolyte interface (SEI) layer consisting of Li2CO3, RCOOLi and LiF led to the increase of electrochemical charge transfer resistance (Rct). Although the X-ray diffraction (XRD) refined results showed that there was no new phases were formed during the long-term cycling, the actually increased Li/Co exchange ratio of LiCoO2 from 1.6% at 800th to 2.1% at 2400th resulted in the decrease of lithium ion diffusion coefficient and deterioration of the rate performance.

Lithium deposition on graphite anode during long-term cycles and the effect on capacity loss

Lithium deposition on the surface of a graphite anode during long-term cycles was evaluated using a LiCoO2/graphite battery. The batteries were charged/discharged at 1 C and 25 °C within the voltage range of 2.75–4.2 V for 600, 700, 800, 900 and 1000 cycles. Scanning electron microscopy (SEM) results indicated that both solid electrolyte interphase (SEI) film and lithium deposition appeared on the surface of the cycled graphite anode. Dendritic and granular lithium deposits grew on the anode non-uniformly. Metallic lithium existed in the deposition according to differential scanning calorimetry (DSC) results. Capacity declined distinctly from the 800th cycle, corresponding with the growth of lithium deposits. An SEI film was formed on the surface of the lithium deposits. Results of X-ray photoelectron spectroscopy (XPS) test indicated that the composition of SEI film on the surface of the lithium deposits was the same as that of the SEI film on the surface of cycled graphite. Capacity loss from the electrolyte consumed by the formation of the SEI film was 23.61%, while the loss from other battery components was 76.39%. Formation of lithium deposits consumed active lithium in the battery and led to capacity loss. According to test results of the three-electrode cell, the average anode potential at the end of constant-current charging for full battery became more negative with the cycling, and this phenomenon was related to the generation of lithium deposits.

Facilitating the redox reaction of polysulfides by an electrocatalytic layer-modified separator for lithium–sulfur batteries

Although physical confinement and chemical adsorption have been adopted for trapping sulfur species within cathodes, there still exist some drawbacks, including low charge/discharge coulombic efficiency and unsatisfied cycleability in terms of the slow kinetic process of polysulfide conversion. Herein, we propose a KB@Ir-modified separator with a catalytic layer to facilitate the redox reaction of polysulfide intermediates and to achieve improved electrochemical performance in lithium sulfur batteries. The iridium nanoparticles not only exhibit strong chemical interaction with the polysulfides, but also efficiently accelerate the kinetic process for polysulfide conversion, especially for the reduction of soluble polysulfides towards insoluble Li2S2/Li2S. A high initial capacity of 1508 mA h g−1 along with 90.0% utilization of sulfur could be achieved under a charge/discharge rate of 0.2C. Also the cell showed a low capacity decay rate of 0.105% per cycle over 500 cycles at 1.0C. This strategy from the point of view of electrocatalysis is expected to be effective for achieving high-energy lithium–sulfur batteries with excellent electrochemical performance.

Lithium compound deposition on mesocarbon microbead anode of lithium ion batteries after long-term cycling.

Lithium compound deposition on mesocarbon microbead (MCMB) anode after long-term cycling was studied in LiCoO2/MCMB battery. Lithium compound deposition did not generate on the activated MCMB anode, but it generated unevenly on the long-term cycled anode. Gray deposition composed of dendrites and particles was formed on the lower surface of the MCMB layer first, then on the upper surface. The deposition and MCMB layer peeled off from the current collector, and a bump was formed in the cycled anode. The exfoliation and thick deposition increased the ohmic resistance, film resistance, and charge transfer resistance of the cell and decreased the capacity significantly. Metallic lithium did not exist in either the upper or the lower deposition layer according to the results of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), the discharge curve, and anode potential. The outer region of both the lower and the upper deposition layers consisted of Li2CO3, LiOH, ROCO2Li, and ROLi. The inner region of the etched lower deposition layer mainly consisted of Li2O, LiF, and Li2CO3, and that of the etched upper deposition layer mainly consisted of Li2CO3, ROCO2Li, ROLi, and LiF. Solid electrolyte interphase (SEI) film hindering the intercalation of lithium ions into carbon layers and LiCoO2 cathode providing lithium source for the deposition were the two reasons leading to the formation of lithium compound deposition during long-term cycles. Because SEI film on the lower surface of MCMB layer was thicker than that on the upper surface, lithium compound deposition generated on the lower surface first.

Effects of VC-LiBOB binary additives on SEI formation in ionic liquid–organic composite electrolyte

A safe electrolyte based on an ionic liquid–organic composite with binary additives was prepared. The stable solid electrolyte interphase (SEI) forms on the surface of a carbon anode by addition of vinylene carbonate (VC) and lithium bis(oxalato) borate (LiBOB) binary additives in ionic liquid–organic electrolyte mixture. The stable SEI effectively prevents the co-intercalation of PP13+ cations, thus leading to an obvious improvement in the performance of the cell.