The effect of FEC as a co-solvent on the electrochemical performance and surface chemistry of silicon nanowire (SiNW) anodes was thoroughly investigated. Enhanced electrochemical performance was observed for SiNW anodes in alkyl carbonates electrolyte solutions containing fluoroethylene carbonate (FEC). Reduced irreversible capacity losses accompanied by enhanced and stable reversible capacities over prolonged cycling were achieved with FEC-containing electrolyte solutions. TEM studies provided evidence for the complete and incomplete lithiation of SiNW's in FEC-containing and FEC-free electrolyte solutions, respectively. Scanning electron microscopy (SEM) results proved the formation of much thinner and compact surface films on SiNW's in FEC-containing solutions. However, thicker surface films were identified for SiNW electrodes cycled in FEC-free solutions. SiNW electrodes develop lower impedance in electrolyte solutions containing FEC in contrast to standard (FEC-free) solutions. The surface chemistry of SiNW electrodes cycled in FEC-modified and standard electrolytes were investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The impact of FEC as a co-solvent on the electrochemical behavior of SiNW electrodes is discussed herein in light of the spectroscopic and microscopic studies.

The high charge-state dopant Zr4+ improves the structural stability and electrochemical behavior of the lithiated transition metal oxide LiNi0.6Co0.2Mn0.2O2.

LiMnPO 4 nanoparticles synthesized by the polyol method were examined as a cathode material for advanced Li-ion batteries. The structure, surface morphology, and performance were characterized by X-ray diffraction, high resolution scanning electron microscopy, high resolution transmission electron microscopy, Raman, Fourier transform IR, and photoelectron spectroscopies, and standard electrochemical techniques. A stable reversible capacity up to 145 mAh g −1 could be measured at discharge potentials Ͼ4 V vs Li/Li + , with a reasonable capacity retention during prolonged charge/discharge cycling. The rate capability of the LiMnPO 4 electrodes studied herein was higher than that of LiNi 0.5 Mn 0.5 O 2 and LiNi 0.8 Co 0.15 Al 0.05 O 2 ͑NCA͒ in similar experiments and measurements. The active mass studied herein seems to be the least surface reactive in alkyl carbonate/LiPF 6 solutions. We attribute the low surface activity of this material, compared to the lithiated transition-metal oxides that are examined and used as cathode materials for Li-ion batteries, to the relatively low basicity and nucleophilicity of the oxygen atoms in the olivine compounds. The thermal stability of the LiMnPO 4 material in solutions ͑measured by differential scanning calorimetry͒ is much higher compared to that of transition-metal oxide cathodes. This is demonstrated herein by a comparison with NCA electrodes. 2 Among them, LiMnPO 4 is of particular interest as it offers the advantages of a flat discharge voltage profile at 4.1 V vs Li/Li + , the expected safety features, and an abundance of the relevant elements in the earth's crust.1-6 Hence, LiMnPO 4 can be an ideal substitute for the commonly used cathode material, LiCoO 2 , which is expensive, toxic, and demonstrates problematic safety features. The three-dimensional framework of the olivine structure is stabilized by the strong covalent bonds between the oxygen and the P 5+ ions, resulting in PO 4 3− tetrahedral polyanions. As a result, lithium metal phosphate materials do not undergo a structural rearrangement during lithiation and delithiation. This indicates that LiMPO 4 electrodes may demonstrate better stability and capacity retention during prolonged cycling as compared to lithiated transition-metal oxide cathode materials such as LiCoO 2 , LiNiO 2 , LiMnO 2 , and LiMn 2 O 4 . However, LiMPO 4 compounds and LiMnPO 4 , in particular, suffer from poor electronic and ionic ͑Li + ͒ conductivity, which means a limited rate capability ͑especially at low temperatures͒.

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This paper is dedicated to studies of the electrochemical behavior, the structural and thermal features of the Ni-rich LiNi 0.5 Co 0.2 Mn 0.3 O 2 undoped and Al-doped (∼0.01 at.%) materials for positive electrodes of lithium batteries. We have found that structural characteristics of these materials are quite similar from the crystallographic point of view. It was demonstrated that Al substitution in the doped LiNi 0.5 Co 0.2 Mn 0.3 O 2 is preferred at Ni sites over Co sites, and the thermodynamic preference for Al 3+ substitutions follows the order: Ni>Co>Mn. The lower capacity fading of the Al-doped electrodes upon cycling and aging of the cells in a charged state (4.3 V) at 60 • C, as well as more stable mean voltage behavior, are likely due to the chemical and structural modifications of the electrode/solution interface. The Al-doped LiNi 0.5 Co 0.2 Mn 0.3 O 2 electrodes demonstrate also lower resistances of the surface film and charge-transfer as well as lower activation energies for the discharge process. From XPS studies we conclude that the modified stable and less resistive interface on the Al-doped particles comprises the Li + -ion conducting nano-sized centers like LiAlO 2 , AlF 3 , etc., which promote, to some extent, the Li + ionic transport to the bulk. A partial layered-to-spinel transformation was established upon cycling of LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathodes.One of the major challenges in lithium batteries technology is, undoubtedly, the further improvement of battery components -electrodes, solutions, and separators. 1-7 Among several modern strategies to improve electrochemical performance and structural characteristics of materials for positive electrodes, doping has attracted the attention of scientists over the years. This is due to the effectiveness of dopants in stabilizing the structure of materials (even in minute amounts) and thus to increase the electrochemical cycling activity and to diminish the heat evolution of the electrodes in a charged state. A variety of dopant ions, like Co 2+ , Al 3+ , Ti 4+ , Zr 4+ , Zn 2+ , Fe 3+ , Cu 2+ , and Cr 3+ , has been used to improve the stability, morphology and microstructure of cathode materials, to enhance the electrode cycleability and rate capability, and to reduce capacity fading upon cycling. 8-13 For instance, doping of LiNi 0.5 Mn 0.5 O 2 with Co, Al, Ti resulted in decrease of the irreversible capacity loss and in almost no capacity fading of the doped electrodes. 14,15 In a systematic study of the Al-doped Ni-rich electrodes (LiNi 0.8 Co 0.15 Al 0.05 O 2 ), which are promising materials for use in batteries for electromotive applications, the authors have shown high cycling stability of these electrodes upon accelerated testing. 16 Several other doping metals, such as silver, magnesium, cobalt, gallium, lanthanum, bismuth, 17-19 as well as non-metallic ions (boron, fluorine), 20,21 were also explored in an attempt to increase the electrochemical cycling behavior of cathodes (both of layered and spinel structures) and to reduce their in...

In this work, LiClO4 was revisited and explored as a possible electrolyte in Li-ion batteries. LiClO4 and LiPF6 solutions in alkyl carbonate solvent mixtures were compared in several aspects: electrochemical windows with noble metal and aluminum electrodes, anodic stability, surface chemistry developed on negative electrodes (Li, Li–graphite, Li–Si), the electrochemical behavior of graphite anodes and LiMn1/3Ni1/3Co1/3normalO2 cathodes, and thermal behavior (solutions alone and mixtures of solutions and electrode materials). The anodic stability and the aluminum passivation are much better in LiPF6 solutions than in LiClO4 solutions. However, HF contamination in the former solutions worsens the passivation of negative electrodes due to reactions with surface ROCO2Li and ROLi species. Thermal reactions of LiClO4 produce more specific heat than LiPF6 solutions. However, in terms of onset temperatures for thermal runaway, the two electrolytes are equivalent. In conclusion, LiClO4 is still an electrolyte that may be considered for use in lithium-ion batteries.

We investigated the structural characteristics of Li-rich xLi2MnO3·(1-x)Li[MnyNizCow]O2 cathode material (x around 0.5, y:z:w around 2:2:1) and its electrochemical performance in lithium cells at 30 and 60°C. It was established that nanoparticles of the xLi2MnO3·(1-x)Li[MnyNizCow]O2 compound are intergrown on the nano-scale and are built of thin plates of 40–50 Å. We demonstrated that xLi2MnO3·(1-x)Li[MnyNizCow]O2 electrodes exhibited at 60°C high capacities of ∼270 and ∼220 mAh/g at 1C and 2C rates, respectively. They can be cycled effectively at 30 and 60°C providing capacity ∼250 mAh/g in the initial cycles, but it fades upon prolonged cycling due, to some extent, to increasing the electrode impedance (charge-transfer resistance) especially at the elevated temperature. The effective chemical diffusion coefficient of Li+ in these electrodes measured during charge to 4.7 V by potentiostatic intermittent titration technique (PITT) was found to be ∼10−10 cm2/s. From convergent beam electron diffraction and Raman spectroscopy studies we established, for the first time, that partial structural transition from layered-type to spinel-type ordering in xLi2MnO3·(1-x)Li[MnyNizCow]O2 electrodes occurred in the initial charge to 4.7 V and even at the early stages of charging at 4.1 V–4.4 V. The thermal behavior of the xLi2MnO3·(1-x)Li[MnyNizCow]O2 material and electrodes are also discussed.

We report herein on the study of Li and Mn rich Lix[MnNiCo]O2 cathode materials with an emphasis on the effect of AlF3 coating on their electrochemical performance. The initial stoichiometry of these materials was xLi2MnO3.(1-x)LiMnyNizCowO2 where x is in the range 0.4-0.5 and the y:z:w ratio was as we previously reported. Their structure was considered on the basis of two-components model, namely monoclinic Li2MnO3 (C2/m) and rhombohedral LiMO2 (R-3m) (M = Mn, Ni, Co) that are structurally compatible and closely integrated phases. Based on TEM studies we concluded that the coating had a crystalline tetragonal structure t-AlF3 (P4nmm symmetry) and AlF3 nano-crystals were regularly distributed over the particles surface. Amorphous clusters of AlF3 and/or other Al-containing species, like AlFxOy, Al[FOH], etc. may also present, as it follows from solid-state NMR measurements. It was shown that electrodes comprising the AlF3-coated material exhibited higher reversible capacities of ∼250 mAh/g at a C/5 rate, more stable cycling behavior, higher lithium storage capability at 60°C, and lower impedance measured during Li-deinteraclation comparing to electrodes prepared from the uncoated material. An important finding is that Lix[MnNiCo]O2 /AlF3 materials revealed much higher thermal stability both in the pristine (lithiated) and cycled (delithiated) states than their uncoated counterparts.