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The extraction and insertion of lithium in LiFePO4 has been investigated in practical Li-ion intercalation electrodes for Li-ion batteries using Fe K-edge X-ray absorption spectroscopy (XAS). A versatile electrochemical in situ reaction cell was utilized, specifically designed for long-term X-ray experiments on battery electrodes during the lithium-extraction/insertion process in electrode materials for Li-ion batteries. The electrode contained about 7.7 mg of LiFePO4 on a 20 microm-thick Al foil. In order to determine the charge compensation mechanism and structural perturbations occurring in the system during cycling, in situ X-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted on the cell at a moderate rate using typical Li-ion battery operating voltages (3.0-4.1 V versus Li/Li+). XAS studies of the LiFePO4 electrode measured at the initial state (LiFePO4) showed iron to be in the Fe(II) state corresponding to the initial state (0.0 mAh) of the battery, whereas in the delithiated state (FePO4) iron was found to be in the Fe(III) state corresponding to the final charged state (3 mAh) of the battery. The X-ray absorption near-edge structure (XANES) region of the XAS spectra revealed a high-spin configuration for the two states [Fe(II), d6 and Fe(III), d5]. The XAFS data analysis confirmed that the olivine structure of the LiFePO4 and FePO4 is retained by the electrodes, which is in agreement with the X-ray diffraction observations on these compounds. The XAFS data that were collected continuously during cycling revealed details about the response of the cathode to Li insertion and extraction. These measurements on the LiFePO4 cathode show that the material retains good structural short-range order leading to superior cycling.

In recent years, lithium/sulfur (Li/S) cells have attracted great attention as a candidate for the next generation of rechargeable batteries due to their high theoretical specific energy of 2600 W·h kg(-1), which is much higher than that of Li ion cells (400-600 W·h kg(-1)). However, problems of the S cathode such as highly soluble intermediate species (polysulfides Li2Sn, n = 4-8) and the insulating nature of S cause poor cycle life and low utilization of S, which prevents the practical use of Li/S cells. Here, a high-rate and long-life Li/S cell is proposed, which has a cathode material with a core-shell nanostructure comprising Li2S nanospheres with an embedded graphene oxide (GO) sheet as a core material and a conformal carbon layer as a shell. The conformal carbon coating is easily obtained by a unique CVD coating process using a lab-designed rotating furnace without any repetitive steps. The Li2S/GO@C cathode exhibits a high initial discharge capacity of 650 mA·h g(-1) of Li2S (corresponding to the 942 mA·h g(-1) of S) and very low capacity decay rate of only 0.046% per cycle with a high Coulombic efficiency of up to 99.7% for 1500 cycles when cycled at the 2 C discharge rate.

The electrolyte, a key component for the successful operation of energy materials, is greatly affected by its solvents. The influence of solvents on the electrochemical performance of a LiFePO4/C composite cathode was investigated at various operating temperatures. The reaction kinetics of the LiFePO4/C composite electrode, including changes of rate capability, redox potential, polarization degree, electrode reaction process, exchange current densities, and activation energies, were evaluated using various techniques. The composition and volume ratio of solvents greatly affect the electrode kinetics. In the mixed solvents of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), EMC is beneficial for the room temperature performance, while the substitution of 20 vol % of EMC by ethyl acetate (EA) is good for the low temperature performance. When 30 vol % of DMC is substituted by 10 vol % of EMC and 20 vol % of EA, the exchange current density increases from 0.022 to 0.038 mA cm−2 at −20 °C, while the activation energy of the charge-transfer process decreases from 48.36 to 33.01 kJ mol−1. Possible mechanisms for improving the electrochemical performance using different solvents have been analyzed. These results are significant for the exploration of appropriate electrolytes for the extensive applications of LiFePO4/C composite electrodes. [ABSTRACT FROM AUTHOR]

The X-ray absorption spectroscopy technique has been applied to study different stages of the lithium/sulfur (Li/S) cell life cycle. We have investigated how speciation of S in Li/S cathodes changes upon the introduction of CTAB (cetyltrimethylammonium bromide, CH3(CH2)15N+(CH3)3Br-) and with charge/discharge cycling. The introduction of CTAB changes the synthesis reaction pathway dramatically due to the interaction of CTAB with the terminal S atoms of the polysulfide ions in the Na2Sx solution. For the cycled Li/S cell, the loss of electrochemically active sulfur and the accumulation of a compact blocking insulating layer of unexpected sulfur reaction products on the cathode surface during the charge/discharge processes make the capacity decay. A modified coin cell and a vacuum-compatible three-electrode electro-chemical cell have been introduced for further in-situ/in-operando studies. [ABSTRACT FROM AUTHOR]

Current lithium-ion batteries are predicted to be unable to provide the specific energy required to meet the ever-increasing demands of rapidly emerging technologies. Due to a high theoretical specific capacity of 1675 mAh/g, sulfur has gained much attention as a promising positive electrode material for high specific energy rechargeable batteries. Although the lithium/sulfur cell has been studied for many years and continues to receive much attention today as an alternative power source for zero-emission vehicles and advanced electronic devices, the realization of this novel cell's promise as a commercial product has yet to be successful. The major problems with sulfur electrodes involve: (1) the dissolution of sulfur (as polysulfides) and the resulting diffusion of dissolved polysulfides and (2) the deposition of insulating products (including Li2S) on both the negative and the positive electrodes. These solid deposits can physically block the electrode reaction sites, thus passivating the electrode surfaces. Another important problem is the large volume change that occurs with the conversion of S to Li2S. It is important to understand that the performance of Li/S cells is hampered by linked chemical and mechanical degradations and both degradation mechanisms must be correctly alleviated in order to markedly improve current-technology Li/S cells. In this study, improved cycling performance via the reactive functional groups on graphene oxide to successfully immobilize sulfur and lithium polysulfides during operation has been demonstrated. The use of a new electrolyte and binder leads to improved cell performance in terms of high-rate capability (up to at least 2 C) and good reversibility (S ↔ Li2S), yielding at least 800 cycles have also been demonstrated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2749-2756, 2015 [ABSTRACT FROM AUTHOR]

After the recent upsurge of interest in the layered LiNi1/3Co1/3Mn1/3O2 system for use as a cathode material for rechargeable lithium batteries, we have studied the charge compensation mechanism and structural perturbations occurring during cycling of the novel layered system of Li1.05Ni0.35Co0.25Mn0.4O2 coated with nano-crystallized LiCoO2, which effectively improved the rate capability. In addition, the LiCoO2 evidently suppressed any structural change of the Li1.05Ni0.35Co0.25Mn0.4O2. In situ X-ray absorption spectroscopy (XAS) measurements were performed utilizing a novel in situ electrochemical cell, specifically designed for long-term X-ray experiments. The cell was cycled at a moderate rate through a typical Li-ion battery operating voltage range (2.8-4.8 V). The electrode contained 1.9 mg of Li1.05Ni0.35Co0.25Mn0.4O2 on a 25 μm Al foil, and had an area of 0.79 cm2. XAS measurements were performed at different states of charge (SOC) during cycling, at the Ni, Co, and the Mn edges, revealing details about the response of the cathode to Li insertion and extraction processes. Extended X-ray absorption fine structure region of the spectra revealed the changes of bond distance and coordination number of Ni, Co, and Mn absorbers as a function of the SOC of the material. We found that the oxidation states of the transition metals in the system are Ni2+, Co3+, and Mn4+ in the fully discharged condition. During charging the Ni2+ is oxidized to Ni4+ through an intermediate stage of Ni3+, Co3+ is oxidized almost to Co4+ Utilizing a combination of Faraday's calculation and XAS results, the Co was found to be at Co3.89+ at the end of the charge, whereas Mn was found to be electrochemically inactive and remains as Mn4+ These measurements on this cathode material confirmed that the material retains its symmetry and good structural short-range order leading to superior cycling. [ABSTRACT FROM AUTHOR]

Abstract: A carbon particle supported sulfur (C–S) composite cathode material was prepared by a chemical deposition method in an aqueous solution. The performance of the C–S cathode material with carbon particles (CP) and carbon nanofibers (CNFs) as electrical conductors and CMC+SBR as binder was evaluated in Li/S cells using cyclic voltammetry, constant current cycling, and electrochemical impedance spectroscopy. The C–S material and the electrodes were examined by scanning electron microscopy. It is found that the C–S cathode with CNFs shows improvement of not only discharge capacity but also cycling durability. It exhibits an initial discharge capacity of 1200mAhg−1, or 72% of theoretical, and retains 668mAhg−1, or 40% of the sulfur theoretical capacity after 50 cycles. The C–S electrode with CNFs has a three-dimensional network structure with regular pores, which can suppress the agglomeration of sulfur or lithium sulfide. Thus, the cell performance is improved. [Copyright &y& Elsevier]

Abstract: 7Li MAS NMR spectroscopy was used to study the failure mechanisms of LiNi0.8Co0.15Al0.05O2 electrodes in Li-ion cells. Three sets of electrodes with different degrees of power fade (0%, 9% and 23%) were studied. The three electrodes were charged to various states of charge (0%, 40%, 60%, 80% and 100%) in pouch cells which were subsequently disassembled for NMR analysis. The lithium NMR shifts of the positive electrodes in the different states of charge were investigated. The results indicate that NMR spectroscopy can be used to probe particle isolation in these electrodes. Particle isolation is responsible for the capacity and power fades since some of the active material particles are disconnected from the matrix. This study also clearly showed the loss of electrochemically active lithium as the power fade increased. [Copyright &y& Elsevier]

As a typical transition metal dichalcogenide, MoS2 offers numerous advantages for nanoelectronics and electrochemical energy storage due to its unique layered structure and tunable electronic properties. When used as the anode in lithium-ion cells, MoS2 undergoes intercalation and conversion reactions in sequence upon lithiation, and the reversibility of the conversion reaction is an important but still controversial topic. Here, we clarify unambiguously that the conversion reaction of MoS2 is not reversible, and the formed Li2S is converted to sulfur in the first charge process. Li2S/sulfur becomes the main redox couple in the subsequent cycles and the main contributor to the reversible capacity. In addition, due to the insulating nature of both Li2S and sulfur, a strong relaxation effect is observed during the cycling process. This study clearly reveals the electrochemical lithiation-delithiation mechanism of MoS2, which can facilitate further developments of high-performance MoS2-based electrodes. [ABSTRACT FROM AUTHOR]

Abstract: A sulfur-coated carbon nanofiber (CNF–S) composite cathode material was prepared by a chemical deposition method in an aqueous solution. This CNF–S material was evaluated as the cathode material in lithium/sulfur cells with three different binders. The results of the SEM and TGA measurements reveal that CNF–S has a typical core–shell structure, containing 75.7 w/o sulfur coated uniformly on the surface of the CNFs. The effects of different binders on the potential profiles, electrode capacity and capacity retention with cycling were investigated. The electrode prepared with CMC+SBR binder has the best performance compared with PVdF and PEO binders, exhibiting a specific capacity of up to 1313mAhg−1 S at the initial discharge and a specific capacity of 586mAhg−1 S after 60 cycles. [Copyright &y& Elsevier]

Abstract: The lithium(1s) K-edge X-ray absorption spectra of lithium-ion battery relevant materials (Li metal, Li3N, LiPF6, LiC6, and LiMn1.90Ni0.10O4) are presented. The Li and LiC6 spectra are discussed and compared with literature data. The Li in lithium-intercalated carbon LiC6, typically used as anode battery electrode material, could be clearly identified in the spectrum, and a presumed purely metallic character of the Li can be ruled out based on the chemical shift observed. The Li in corresponding cathode electrode materials, LiMn1.90Ni0.10O4, could be detected with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, but the strong (self-) absorption of the spinel lattice provides an obstacle for quantitative analysis. Owing to its ionic bonding, the spectrum of the electrolyte salt LiPF6 contains a sharp π-resonance at 61.8eV, suggesting a distinct charge transfer between Li and the hexafluorophosphate anion. In addition, LiPF6 resembles many spectral features of LiF, making it difficult to discriminate both from each other. Residual electrolyte on anodes or cathodes poses a problem for the spectroscopic analysis of the electrodes, because its Li spectrum overshadows the spectral features of the Li in the anode or cathode. The electrolyte must be removed from electrodes prior to spectroscopic analysis. [Copyright &y& Elsevier]

Abstract: Poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) is the most common active material with a stable radical molecular structure used for lithium rechargeable cells. Because it is an organic active material, PTMA can be dissolved easily in various solvents. Using a solution of active material in the electrode making process, we formed an extremely large surface area between the active material and the conducting additive. The electrode has excellent power capability and can be charged and discharged in 1min. We believe that this cell can be used like an electric double layer capacitor. We also believe that the cell, whose cycle life is good, retaining 89% of original capacity after 1000 cycles, can be put into practical use. [Copyright &y& Elsevier]

Abstract: Stable nitroxyl radical polymers, such as poly (2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA), are known to be effective as cathode active materials for lithium rechargeable batteries. A water-based slurry was used to prepare PTMA composite electrodes, enabling successful production of homogeneous electrodes. This improved utilization of active material and enabled us to obtain specific capacities almost equal to the theoretical value (111mAhg−1). Lithium half cells were fabricated using these electrodes and using graphite as the anode active material, and their cycling stability, temperature dependence, rate capability and self-discharge rate were measured in detail. Although they showed 24% irreversible capacity on the first cycle, the capacity stabilized after the second cycle. [Copyright &y& Elsevier]

Abstract: Fe K-edge X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) have been performed on electrodes containing LiFePO4 to determine the local atomic and electronic structure and their stability with electrochemical cycling. A versatile electrochemical in situ cell has been constructed for long-term soft and hard X-ray experiments for the structural investigation on battery electrodes during the lithium-insertion/extraction processes. The device is used here for an X-ray absorption spectroscopic study of lithium insertion/extraction in a LiFePO4 electrode, where the electrode contained about 7.7mg of LiFePO4 on a 20μm thick Al-foil. Fe K-edge X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) have been performed on this electrode to determine the local atomic and electronic structure and their stability with electrochemical cycling. The initial state (LiFePO4) showed iron to be in the Fe2+ state corresponding to the initial state (0.0mAh) of the cell, whereas in the delithiated state (FePO4) iron was found to be in the Fe3+ state corresponding to the final charged state (3mAh). XANES region of the XAS spectra revealed a high spin configuration for the two states (Fe (II), d6 and Fe (III), d5). The results confirm that the olivine structure of the LiFePO4 and FePO4 is retained by the electrodes in agreement with the XRD observations reported previously. These results confirm that LiFePO4 cathode material retains good structural short-range order leading to superior cycling capability. [Copyright &y& Elsevier]