F. Croce

Nanocomposite polymer electrolytes for lithium batteries

Ionically conducting polymer membranes (polymer electrolytes) might enhance lithium-battery technology by replacing the liquid electrolyte currently in use and thereby enabling the fabrication of flexible, compact, laminated solid-state structures free from leaks and available in varied geometries. Polymer electrolytes explored for these purposes are commonly complexes of a lithium salt (LiX) with a high-molecular-weight polymer such as polyethylene oxide (PEO). But PEO tends to crystallize below 60 °C, whereas fast ion transport is a characteristic of the amorphous phase. So the conductivity of PEO–LiX electrolytes reaches practically useful values (of about 10−4 S cm−1) only at temperatures of 60–80 °C. The most common approach for lowering the operational temperature has been to add liquid plasticizers, but this promotes deterioration of the electrolytes mechanical properties and increases its reactivity towards the lithium metal anode. Here we show that nanometre-sized ceramic powders can perform as solid plasticizers for PEO, kinetically inhibiting crystallization on annealing from the amorphous state above 60 °C. We demonstrate conductivities of around 10−4 S cm−1 at 50 °C and 10−5 S cm−1 at 30 °C in a PEO–LiClO4 mixture containing powders of TiO2 and Al2O3 with particle sizes of 5.8–13 nm. Further optimization might lead to practical solid-state polymer electrolytes for lithium batteries.

A Novel Concept for the Synthesis of an Improved LiFePO4 Lithium Battery Cathode

This paper describes the synthesis and the properties of a kinetically improved LiFePO 4 cathode material. The novel aspect of the synthesis is based on a critical step involving the dispersion of metal (e.g., copper or silver) at a very low concentration (1 wt %). This metal addition does not affect the structure of the cathode but considerably improves its kinetics in terms of capacity delivery and cycle life. Such an enhancement of the electrochemical properties has been ascribed to a reduction of the particle size and to an increase of the bulk intra- and interparticle electronic conductivity of LiFePO 4 , both effects being promoted by the finely dispersed metal powders. This improved conductivity favors the response of LiFePO 4 , thus substantiating its interest as new cathode for advanced lithium ion batteries.

Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes

A model to account for the role of the ceramic fillers in enhancing the transport properties of PEO-based composite polymer electrolytes is here proposed. The model is supported by a series of specifically addressed electrochemical tests which included the determination of the conductivity and of the lithium transference number of various composite electrolyte samples differing from the type of the surface states of the ceramic filler.

Kinetics and stability of the lithium electrode in poly(methylmethacrylate)-based gel electrolytes

Abstract The transport and electrochemical properties of gel-type ionic conducting membranes formed by immobilizing liquid solutions of lithium salts in a poly(methylmethacrylate) matrix have been determined. In particular, the conductivity, the lithium ion transference number and the electrochemical stability window are evaluated and discussed. Finally, particular attention is devoted to the phenomena occuring at the interface between these ionic membranes and the lithium metal electrode.

Impedance Spectroscopy Study of PEO‐Based Nanocomposite Polymer Electrolytes

The addition of nanometric fillers (e.g., , ) to polymer electrolytes induces consistent improvement in the transport properties. The increase in conductivity and in the cation transference number is attributed to the enhancement of the degree of the amorphous phase in the polymer matrix, as well as to some acid‐base Lewis type, ceramic‐electrolyte interactions. This model is confirmed by results obtained from a detailed impedance spectroscopy study carried out on poly(ethylene oxide) [P(EO)]‐based polymer electrolyte samples with and without ceramic fillers.

Transport and interfacial properties of composite polymer electrolytes

Lithium polymer electrolytes formed by dissolving a lithium salt LiX in poly(ethylene oxide) PEO, may find useful application as separators in lithium rechargeable polymer batteries. The main problems, which are still to be solved for a complete successful operation of these materials, are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70°C. In this paper we demonstrate that a successful approach for overcoming these problems, is the dispersion of selected ceramic powders in the polymer mass, with the aim of developing new types of composite PEO–LiX polymer electrolytes characterized by enhanced interfacial stability, as well as by improved ambient temperature transport properties.

A High-Rate, Nanocomposite LiFePO4 ∕ Carbon Cathode

We describe here a new type of template-prepared nanostructured LiFePO4 electrode, a nanocomposite consisting of monodispersed nanofibers of the LiFePO4 electrode material mixed with an electronically conductive carbon matrix. This unique nanocomposite morphology allows these electrodes to deliver high capacity, even when discharged at the extreme rates necessary for many pulse-power applications. For example, this nanocomposite electrode delivers almost 100% of its theoretical discharge capacity at the high discharge rate of 3 C, and 36% of its theoretical capacity at the enormous discharge rate of 65 C. This new nanocomposite electrode shows such excellent rate capabilities because the nanofiber morphology mitigates the problem of slow Li+-transport in the solid state, and the conductive carbon matrix overcomes the inherently poor electronic conductivity of LiFePO4.

Enhancement of ion transport in polymer electrolytes by addition of nanoscale inorganic oxides

Abstract The effect of addition of nanoparticle inorganic oxides to poly(ethylene oxide) (PEO) complexed with LiClO 4 on cation transport properties has been explored by electrochemical and 7 Li nuclear magnetic resonance (NMR) methods. The presence of the nanoparticles generally increases the ionic conductivity and the cation transference number, the effect being greatest for TiO 2 . The enhancement in cation transference number is directly correlated with increased Li diffusivity measured by NMR. The NMR results also demonstrate that the increased ionic conductivity is not attributable to a corresponding increase in polymer segmental motion, but more likely a weakening of the polyether-cation association induced by the nanoparticles.

Progress in lithium polymer battery R&D

In this paper the characteristics and performance of composite polymer electrolytes formed by dispersing selected ceramic (e.g. γ-LiAlO2, Al2O3, SiO2) powders in poly(ethylene oxide)–lithium salt (e.g. PEO–LiCF3SO3) matrices, are reported and discussed. Particular emphasis is devoted to the role of these composite electrolytes in providing the conditions for stabilizing the interface with the lithium metal electrode, as well as for enhancing the electrolyte’s overall transport properties. Finally, results based on tests of practical prototypes demonstrate that these unique properties allow the development of new types of high performance, rechargeable lithium polymer batteries.

Investigation of the O2 Electrochemistry in a Polymer Electrolyte Solid‐State Cell

The oxygen electrode has been the subject of intense activity for more than 150 years, since the report of the first fuel cell in 1842 by Grove. Recently, there had been a revived interest in the utilization of air as oxidizer in the positive electrode of air secondary batteries, for example, lithium–air batteries. Very high specific energies can be obtained from these systems (at 2 kWh kg 1 of reactant for lithium), and these high numbers justify the present excitement for the lithium–air battery. While systems in which lithium is totally protected from the outside environment by an impervious vitroceramic electrolyte are akin to fuel-cell electrodes as the medium is aqueous, a good fraction of relevant research is geared towards the implementation of a reversible O2 electrode in organic aprotic solvents that are somehow compatible with lithium. In contrast with aqueous systems, in which the ultimate reduction product requires four electrons, to H2O or OH , the most often observed product in dismantled electrodes or in situ observations of Li/O2 organic electrolyte batteries is Li2O2 (Eo = 2.96 V), that is, there is no cleavage of the O O bond. It is well known that in aprotic systems the oxygen molecule is easily, and in some solvents reversibly, reduced to the radical anion O2 , provided that the counterion is a large cation of “onium” type (e.g. tetrabutylammonium) or even heavier alkali ions. 12] In the presence of a more polarizing countercation, O2 · rapidly disproportionates or accepts another electron. Accordingly, the reduction of molecular oxygen in aprotic solvents may be assumed to proceed as in Scheme 1. Most research on the Li–air system has focused on the capacity, expressed per gram of carbon used as collector in the positive electrode. This approach has several drawbacks, as the electrically insulating products Li2O2 and Li2O, besides mechanically clogging the electrode owing to insolubility, impair the diffusion of oxygen and of electrolyte, and the polarizations observed are difficult to translate into a particular electrode process. Moreover, the choice of electrolytes is often questionable. O2 , O2 2 , and O are very strong bases, with pKA values close to 30 and 40 on the Bordwell DMSO acidity scale that has to be used in aprotic media, and all three are very strong nucleophiles. As now demonstrated by various authors, the “unsinkable” carbonate esters should be avoided totally, 14, 15] and even the almost universal solvent DMSO would be deprotonated by “naked” O2 2 and O if the very small solubility product of the Li salt did not prevent such reaction. The solute anion PF6 is also prone to substitution, given the polarity and lability of the P F linkage. These issues hamper the study of the electrochemical processes of the oxygen electrode in cells using conventional carbonate ester electrolytes. Herein we depart totally from this conventional approach by moving to a solid-state cell using a poly(ethylene oxide)–lithium triflate (PEOLiCF3SO3) solvent-free polymer electrolyte. The main purpose was to assure a cell structure in which reactions of the products of the electrochemical process (and, particularly, of the singlet oxygen) with electrolyte could be totally avoided. Therefore, PEO-based electrolytes were chosen, as the resistance of the PEO ether linkage is well-documented (e.g. Grignard reagents) and LiCF3SO3 is stable towards nucleophiles, especially under the low current densities used in this study. PEO, owing to the tendency of the polymer to arrange into trans-gauche-trans helices to accommodate ions, is far more solvating in the solid state than even its shorter liquid homologues, such as the recently used tetraglyme. Another advantage of the solid state is the stability of the triple contact between the carbon, the electrolyte, and the surrounding O2, especially under low charge passage. The main goal of this work is then to use a PEO-based Li–O2 solidstate cell to investigate the electrochemical processes occurring at the oxygen electrode. It is expected that, owing to the favorable properties of the solid electrolyte, the cell is able to provide reliable results. The PEO-based polymer electrolyte, hereafter referred to as PCE (polymer composite electrolyte), the cell configuration, and the experimental setup are described in detail in the Experimental Section. Our study was performed using electrochemical techniques, in particular potentiodynamic cycling with galvanostatic acceleration (PCGA) and cyclic Scheme 1. Oxygen-molecule reduction in aprotic systems.