Novel nanomaterials based on electronic and mixed conductive glasses

doi:10.1016/j.ssi.2008.09.021

Solid State Ionics

Volume 180, Issues 6–8, 14 May 2009, Pages 531-536

https://doi.org/10.1016/j.ssi.2008.09.021 Get rights and content

Abstract

Novel nanomaterials based on lithium–vanadate-phosphate (LVP) and lithium–iron-phosphate (LFP) glasses were prepared and their electrical properties were investigated by impedance spectroscopy. It was found out that the electronic conductivity of the original glasses of both systems can be considerably enhanced by appropriate annealing at temperatures close to beginning of crystallization temperatures determined by DSC/DTA methods. Increase in conductivity arises from the modification of the microstructure. It was shown by XRD and SEM studies that by appropriate heat-treatment glasses of both systems can be turned into nanomaterials consisting of crystallites smaller than 100 nm embedded in the glassy matrix. It was postulated that the major role in the conductivity enhancement of these nanomaterials is played by the developed interfacial regions between nanocrystalline and glassy phases, in which the concentration of V⁴⁺–V⁵⁺ or Fe²⁺–Fe³⁺, pairs responsible for electron hopping in those systems, is higher than inside the glassy matrix and the formed nanocrystallites.

Introduction

Lithium–nickel–cobalt oxides, lithium–manganese-oxides, vanadium oxides or phosphates and phospho-olivines belong to the most studied materials for cathodes in Li–ion rechargeable batteries [1], [2], [3], [4]. Majority of these materials are polycrystalline. Substantially less is known about glasses [5] or glassy-crystalline nanomaterials suitable for battery applications. It was shown [6], that glasses of the Li₂O–V₂O₅–P₂O₅ system are mixed electronic–ionic conductors in which the proportions of the electronic-to-ionic components of the total conductivity depend on the ratio between the contents of a glass modifier (Li₂O) and glass formers (V₂O₅ + P₂O₅). Use of these glasses as potential cathode materials for lithium batteries requires that the conduction is mainly electronic. An increase in electronic conductivity of these materials can be achieved after their nanocrystallization, induced by an appropriate annealing process.

The aim of this paper was to demonstrate that electric conductivity of potential glassy cathode materials may be substantially improved by their nanocrystallization. We have studied lithium–vanadate-phosphate (LVP) and lithium–iron-phosphate (LFP) glasses. The latter glasses are amorphous analogs of polycrystalline phospho-olivines intensively studied recently. Unfortunately, besides all their advantages [1], polycrystalline olivines have one serious deficiency—very low electrical conductivity at room temperature (ca. 10^− 10 S cm^− 1). This low conductivity may be considerable increased in nanocrystalline olivines.

In our studies we have used impedance spectroscopy (IS), thermal analyses (DSC/DTA), X-ray diffractometry (XRD) and scanning electron microscopy (SEM).

Section snippets

Experimental

Lithium–vanadate-phosphate (LVP) and lithium–iron-phosphate (LFP) glasses were synthesized by a standard press quenching technique.

Two series of vanadate–phosphate glasses were prepared—one containing lithium and one lithium-free. Their compositions were described by respective formulas: i) xLi₂O·(100 − 2x)V₂O₅·xP₂O₅, for 15 ≤ x ≤ 45, and ii) xV₂O₅·(100 − x)P₂O₅, for 60 ≤ x ≤ 90. Samples were prepared from pre-dried and mixed chemicals: V₂O₅ (ABCR, 99.9%), NH₄H₂PO₄ (POCh, 99.9%) and LiNO₃ (Aldrich,

Results and discussion

Use of mixed conductive LVP or LFP glasses as potential cathode materials for lithium batteries requires that the electric conduction is mainly electronic. This occurs in V₂O₅-rich LVP glasses and all LFP glasses under study.

Conclusions

Electronic and mixed conductive glasses can be promising starting compounds to prepare attractive nanostructured materials. The annealing of LVP glasses leads to their nanocrystallization. The resulting nanomaterials exhibit much higher electronic conductivity (10^− 1 S·cm^− 1 at 300 °C), lower activation energy and better thermal stability than the initial glasses. It was found that thermal nanocrystallization of LFP glasses also leads to the electronic conductivity enhancement and therefore it

Acknowledgements

The authors thank Dr. I.Gorzkowska (Department of Chemistry, Warsaw University of Technology) for her assistance in DTA measurements and Dr. S.Gierlotka (Institute of High Pressure Physics PAS) for his help in SEM studies.

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Novel nanomaterials based on electronic and mixed conductive glasses

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Solid State Ionics

Solid State Ionics

Solid State Ionics

Solid State Ionics

Chem.Rev.

J. Electrochem. Soc.

Electrochem. Solid-State Lett.