Influence of the annealing atmosphere on the properties of LiCoPO4–graphitic carbon foams composites

The investigation on the properties of LiCoPO₄–graphitic carbon foams (LCP-GCF) composites is reported in this work. The diffraction analysis (XRD) on powders confirmed the presence of LiCoPO₄ as major crystalline phase and Li₄P₂O₇ and Co₂P as secondary phases. The morphological investigation of the composites shows a layer of crystalline spongy-like material on the surface of the GCF for t?=?0 h and of acicular crystallites with different dimensions (5–50 μm) for t?≥?0.1 h. The voltammetric curves (cyclic voltammogramms) show mean values of reduction potential above 5.0 V independently of the annealing time. The LCP-GCF composites deliver a discharge-specific capacity of 76mAh g^?1 (t?=?0 h) and of 102mAh g^?1 (t?=?0.1 h) at a discharge rate of C/10 and room temperature. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity as a function of the annealing time.     

Investigation of graphitic carbon foams/LiNiPO4 composites

The characterization of composites consisting of graphitic carbon foams coated with a structured lithium nickel phosphate is reported. The LiNiPO₄ as cathode material for lithium-ion batteries is prepared by a Pechini-assisted sol-gel process. The coating is performed by soaking the graphitic carbon foams in aqueous solutions containing lithium, nickel salts, and phosphates at 70?°C for 2–4?h and then by treating in flowing air and nitrogen. The formation of the olivine-like structured LiNiPO₄ is confirmed by X-ray diffraction analysis performed on powders prepared under very similar conditions. However, crystalline reflections attributed to Li₄P₂O₇ and to Ni₃P as secondary phases have been observed. The morphological investigation revealed the presence of a layer on the graphitic foams that consists of interconnected blend of grains with different size. The voltammetric curves show values of the mean peak maxima in the anodic region between 5.1–5.3?V and in the cathodic region at ~4.9?V. The electrochemical measurements deliver a discharge specific capacity of 86?mAhg^?1 (at discharge rate of C/10 and RT). The electrochemical impedance spectroscopy data confirm an increase of the electrical resistance after cycling and the decrease of the ionic contribution which indicate the formation/growth of phases behaving like resistors.     

Investigation on LiCoPO<Subscript>4</Subscript> powders as cathode materials annealed under different atmospheres

An uncomplicated Pechini-assisted sol–gel process in aqueous solutions is used for the synthesis of Li–Co phosphate powders as cathode materials. The powders are annealed under different conditions in flowing nitrogen and in flowing air. The structural, morphological, and electrochemical properties are strongly dependent upon the annealing conditions. After the treatment in air, the X-ray diffraction (XRD) patterns reveal the presence of LiCoPO₄ as a single phase. The morphology of the powders consists of a homogeneous and good interconnected blend of grains with different sizes; the cyclic voltammetry (CV) curves show a very good reversibility with very close values of the mean peak maxima in the cathodic region. The electrochemical measurements deliver a discharge specific capacity of 37 mAhg⁻¹ at a discharge rate of C/25 at room temperature. After annealing in nitrogen, the XRD analysis detects the formation of Li₄P₂O₇ and to Co₂P as secondary phases; the morphological investigation indicated that the LiCoPO₄ particles took shape of prisms with an average size of 2 μm. The CV curves are associated with a large polarization and poor irreversibility. The electrochemical measurements deliver a discharge specific capacity of 42 mAh g⁻¹ at a discharge rate of C/25 at room temperature and lower capacity fade (approx. 35%).     

Investigation of the solid-state electrolyte/cathode LiPON/LiCoO<Subscript>2</Subscript> interface by photoelectron spectroscopy

The electronic structure of a solid electrolyte/solid electrode interface (SESEI) of an all-solid-state thin film battery was investigated. The thin film battery consisted of a LiPON solid electrolyte and a LiCoO₂ cathode. The lithium phosphorus oxynitride (LiPON) electrolyte was RF sputtered in a step-by-step procedure onto the cathode and investigated by photoelectron X-ray-induced spectroscopy after each deposition step. An intermediate layer was found—composed of some new species—that differs in its chemical composition from the cathode as well as the LiPON solid electrolyte material and changes with growing layer thickness. In contrast, the electronic structure of the underlying cathode material remained predominantly unchanged.     

Preparation of LiCoPO<Subscript>4</Subscript> powders and films via sol–gel

Lithium intercalation materials are of special interest as cathodes in rechargeable batteries. An uncomplicated sol–gel process has been used for the synthesis of Li–Co phosphates powders and, for the first time, of LiCoPO₄ films. The powders were prepared from aqueous solutions, containing Li, Co and phosphate precursors to which acid citric and ethylene glycol was added, during the drying process at 75 °C. The X-ray diffraction patterns of the prepared powders confirmed the presence of LiCoPO₄ with an olivine-like structure as main phase. The morphological investigations of the powder showed a platelet-like structure with an average grain size of 0.75 μm. The films of LiCoPO₄ were deposited onto ITO glass substrates with the combination of the dip-coating process under the same conditions. Finally, the films were annealed in inert atmosphere at 300 °C. The morphological investigations reveal a smooth and homogeneous surface of the prepared Li–Co phosphate films. The preliminary electrical investigation on the prepared LiCoPO₄ films showed lithium ions electrochemical activity in the range 3.0–4.5 V.     

The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte-cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation

The stability of the valence state of the 3d transition metal ions and the stoichiometry of LiMO(2) (M = Co, Ni, Mn) layered oxides at the surface-electrolyte interface plays a crucial role in energy storage applications. The surface oxidation/reduction of the cations caused by the contact of the solids to air or to the electrolyte results in the blocking of the Li-transport through the interface that leads to the fast batteries deterioration. The influence of the end-of-charge voltage on the chemical composition and the oxidation state of 3d transition metal ions, as well as the stability of the solid-electrolyte interface formed during the electrochemical Li-deintercalation/intercalation of the LiCoO(2) and Li(Ni,Mn,Co)O(2), have been investigated by X-ray photoelectron spectroscopy. While the chemical composition of the solid-electrolyte interface is similar for both layered oxide surfaces, the electrochemical cycling to some critical voltage values leads to the disappearance of the interface. By the analysis of the shape of the 2p and 3s photoelectron emissions we show that the formation of the solid-electrolyte interface layer correlates with the partial reduction of the trivalent Co ions at the electrolyte-LiCoO(2) interface and the amount of the Co(2+) ions is increased as the solid-electrolyte interface vanishes. In contrast, the Mn(4+), Co(3+) and Ni(2+) ions of the Li(Ni,Mn,Co)O(2) are stable at the interface under the electrochemical cycling to higher end-of-charge voltage. A correlation between deterioration of the LiCoO(2) and Li(Ni,Mn,Co)O(2) batteries and the change of electronic structure at the surface/interface after the electrochemical cycling has been found. The dissolution of the solid-electrolyte interface layer might be the reason for the fast deterioration of the Li-ion batteries.     

Synthesis and characterization of LiMn1-x Fe x PO4/carbon nanotubes composites as cathodes for Li-ion batteries

Carbon nanotubes (CNT) coated with LiMn_1-x Fe _x PO₄ (0.2?≤?x?≤?0.8), as possible cathode materials, was synthesized by using a sol–gel process (Polyol method), after annealing under flowing nitrogen. X-ray diffraction (XRD) patterns of the composites confirmed the formation of the olivine structured LiMn_1-x Fe _x PO₄ phase and no secondary phases were detected. The morphological investigation revealed the formation of agglomerates with particles size ranging between 300 and 700 nm. XRD investigation of composites shows difference of the morphology by doping CNT and carbon black in the composites. Transmission electron microscopy shows the growth of nano-sized particles on CNT (20–70 nm) and the agglomeration of primary particles to form secondary particles. The X-ray photoelectron spectroscopy showed that the Fe and Mn ions are in divalent states in the LiMn_1-x Fe _x PO₄ composites. The cyclic voltamograms showed the oxidation peaks of iron and manganese ions at 3.53–3.63 and 4.05–4.33 V, respectively, while the reduction peaks were found at 3.21–3.42 V (iron reduction) and 3.85–3.93 V (manganese reduction) depending on the iron content in the composition. The LiMn_0.6Fe_0.4PO₄/CNT composite (x?=?0.4) (with 20 %?wt CNT) delivered a specific capacity of 120 mAhg^?1 (at a discharge rate of C/20 and RT).     

Properties of Ca-containing LiCoPO<Subscript>4</Subscript>-graphitic carbon foam composites

LiCo_1???x Ca _x PO₄–graphitic carbon foam composites are prepared using a sol–gel method. The structural analysis reveals LiCoPO₄ as major crystalline phase and Co₂P₂O₇ (for x?=?0.0) and Co₂P, Li₃PO₄, and (Ca,Co)₃(PO₄)₂ (for x?≥?0.05) as secondary phases. The morphology consists of microcrystalline “islands” with acicular crystallites (5–50 μm size). Transmission electron microscopy (TEM) of the powders showed that the Ca is incorporated into the crystal structure evoking exaggerated grain growth. The voltammetric profiles show a decrease of the voltammetric surface between anodic and cathodic sweeps and a shift of the reduction potentials toward higher values (～4.6 V, x?=?0.1). The electrochemical measurements, at a discharge rate of C/10 (room temperature), show an increase of the discharge-specific capacity from 100 mAhg^?1 for x?=?0.0 to 104 mAhg^?1 for x?=?0.1. The ac impedance spectroscopy data revealed an improvement of the Li-ion conductivity at high content of Ca ions (x?=?0.1).     

Synthesis and characterization of LiFePO4/3-dimensional carbon nanostructure composites as possible cathode materials for Li-ion batteries

Composites of three-dimensional (3D) carbon nanostructures coated with olivine-structured lithium iron phosphates (LiFePO₄) as cathode materials for lithium ion batteries have been prepared through a Pechini-assisted reversed polyol process for the first time. The coating has been successfully performed on nonfunctionalized commercially available 3D carbon used as catalysts. Thermal analysis revealed no phase transitions till crystallization occurred at 579 °C. Morphological investigation of the prepared composites showed a very good quality of the coating on the 3D carbon structures. A great enhancement of the crystallinity of the olivine structure and of the composites was revealed by the structural investigation performed on pure LiFePO₄ and composites after annealing at 600 °C for 10 h under nitrogen atmosphere. The cyclic voltammetry curves of the composites show well-defined peaks and smaller value of the polarization overpotential indicating an enhancement of electrode reaction reversibility compared to the LiFePO₄ phase.