Advanced Fluoride-Based Materials for Energy Conversion (eBook)

Henri Groult, Tsuyoshi Nakajima (Herausgeber)

eBook Download: PDF | EPUB

2015 | 1. Auflage
458 Seiten
Elsevier Science (Verlag)
978-0-12-800712-9 (ISBN)

Infos

Advanced Fluoride-Based Materials for Energy Conversion provides thorough and applied information on new fluorinated materials for chemical energy devices, exploring the electrochemical properties and behavior of fluorinated materials in lithium ion and sodium ion batteries, fluoropolymers in fuel cells, and fluorinated carbon in capacitors, while also exploring synthesis applications, and both safety and stability issues. As electronic devices, from cell phones to hybrid and electric vehicles, are increasingly common and prevalent in modern lives and require dependable, stable chemical energy devices with high-level functions are becoming increasingly important. As research and development in this area progresses rapidly, fluorine compounds play a critical role in this rapid progression. Fluorine, with its small size and the highest electronegativity, yields stable compounds under various conditions for utilization as electrodes, electrolytes, and membranes in energy devices. The book is an ideal reference for the chemist, researcher, technician, or academic, presenting valuable, current insights into the synthesis of fluorine compounds and fluorination reactions using fluorinating agents. - Provides thorough and applied information on new fluorinated materials for chemical energy devices - Describes the emerging role of stable energy devices with high-level functions and the research surrounding the technology - Ideal for the chemist, research, technician, or academic seeking current insights into the synthesis of fluorine compounds and fluorination reactions using fluorinating agents

Textauszug

Chapter 2

Electrochemical Behavior of Surface-Fluorinated Cathode Materials for Lithium Ion Battery

Susumu Yonezawa1,2, Jae-Ho Kim1,  and Masayuki Takashima2     1Headquarters for Innovative Society-Academia Cooperation, Fukui University, Fukui, Japan     2Department of Materials Science & Engineering, Faculty of Engineering, University of Fukui, Fukui, Japan

Abstract

Electrochemical properties and thermal stability of carbon-coated LiFePO4 cathode are improved by surface fluorination using NF3 gas at 298 K. The LiFePO4 fluorinated at 6.67 kPa-NF3 showed that the discharge capacity was about 10% higher than that of untreated LiFePO4. Electrochemical impedance spectra indicates the lowest resistance (Rct = 35.55 Ω) and largest exchange current density (i0 = 0.72 mA). However, in the case of LiFePO4 fluorinated at 50.66 kPa-NF3, the electrochemical properties are negatively affected by the formation of resistive fluoride films. Regarding the thermal stability, the decomposition temperature (495 K) of LiFePO4 fluorinated at 6.67 kPa-NF3 was 14 K higher than that of untreated LiFePO4. Surface fluorination using F2 gas improves the electrochemical properties and thermal stability of LiFePO4 as a promising cathode of high power lithium-ion cells for HEVs.

The LiNi0.5Mn1.5O4 is used as a 5 V class cathode for lithium secondary battery. Because of its high redox potential near 5 V versus Li/Li+, it is attracting much attention as high energy density cathode material. The effect of surface fluorination on the electrochemical properties and Mn dissolution from LiNi0.5Mn1.5O4 cathode has been investigated. Fluorinated LiNi0.5Mn1.5O4 particles were prepared at 298 and 373 K by 6.67 kPa-F2 for 1 h. X-ray diffraction and scanning electron microscopy indicated that the surface fluorination with F2 gas did not affect the crystal structure and particle morphology of LiNi0.5Mn1.5O4. However, X-ray photoelectron spectroscopy data proved the existence of fluorinated surface layers at the surface of LiNi0.5Mn1.5O4. The discharge capacity and cycle stability of LiNi0.5Mn1.5O4 fluorinated at 298 K was higher than that of untreated sample. The Mn dissolution of LiNi0.5Mn1.5O4 is also restrained by the surface fluorination. Thus the surface fluorination improves the electrochemical properties of LiFePO4 and LiNi0.5Mn1.5O4 as the promising cathode active materials for lithium-ion cells.

Keywords

Discharge capacity; Fluorination; Impedance; LiFePO4; LiNi0.5Mn1.5O4; Mn dissolution; Thermal stability

Chapter Outline

2.1 Surface Fluorination of LiFePO4 33

2.1.1 Introduction 33

2.1.2 Experimental Details 34

2.1.3 Characterization of Fluorinated LiFePO4 35

2.1.4 Electrochemical Properties of Fluorinated LiFePO4 40

2.2 Surface Fluorination of LiNi0.5Mn1.5O4 43

2.2.1 Introduction 43

2.2.2 Experimental Details 44

2.2.3 Characterization of Surface-Fluorinated LiNi0.5Mn1.5O4 44

2.2.4 Electrochemical Properties of Surface-Fluorinated LiNi0.5Mn1.5O4 46

2.3 Summary 49

References 49

2.1. Surface Fluorination of LiFePO4

2.1.1. Introduction

Lithium secondary battery has been widely studied because of a large terminal voltage and a large energy density. Lithium-containing transition metal oxides, LiCoO2, LiNiO2, LiMn2O4, LiFePO4, and their derivatives have been investigated to obtain the high performance cathode active materials of lithium secondary battery. Among these cathode materials, LiFePO4 has flat voltage profile, good electrochemical and thermal stability, relatively low cost for synthesis, and environmental compatibility with less toxicity than other cathode materials. Due to these advantages, LiFePO4 has been attracting much attention as a promising new cathode electrode material for lithium-ion batteries. However, the low electric conductivity and low diffusion coefficient of Li+ are the main shortcomings that limit its application in industry. Extensive efforts have been performed to improve its electrochemical performance over decades, which can be classified into the following categories: particle size/shape optimizing [1–6], metal doping [7], and mixing with the electronically conductive materials like carbon, metal, and metal oxide [8–10]. Among these methods used for improving electrochemical properties of LiFePO4, carbon coating is one of the most frequently used techniques to improve the specific capacity, rate performance, and cycling life [11,12]. However, it should be further discussed on the optimum conditions with the thickness and formation of carbon layer.

The surface modification of a cathode active material gives strong effects to the battery performance because electrochemical reaction takes place at the interface among active material, carbon as an electro-conductive material and electrolyte. The surface modification of LiMn2O4 with fluorine/carbon nanocomposite was already reported [13,14]. Charge/discharge capacity and cycle ability of LiMn2O4 as a cathode material are enhanced by optimizing the arrangement of nanothickness carbon film and surface fluorination, which is superior to the carbon-coated LiMn2O4. In the present section, the effects of surface fluorination on the electrochemical properties and thermal stability of carbon-coated LiFePO4 are summarized.

2.1.2. Experimental Details

Carbon-coated LiFePO4 particles (SLFP-PD60, anatase; 98% purity) were obtained from Hohsen Corp. Nitrogen trifluoride gas (NF3, 99.5% purity) was supplied by Central glass Co., Ltd. Details of the fluorination apparatus are given in our previous papers [15,16]. Fluorinated LiFePO4 (F-LiFePO4) particles were prepared by direct fluorination using NF3 gas under the following conditions. Reaction temperature, NF3 pressures, and reaction time were set at 298 K, 6.67, and 5066 kPa, and 1 h, respectively.

The structure and chemical bonds of the samples were investigated using powder X-ray diffraction (XRD, XD-6100) and X-ray photoelectron spectroscopy (XPS, XPS-9010). The surface morphology of the samples was observed using a scanning electron microscope (SEM, s-2400; Hitachi Ltd.). Fluorine contents in F-LiFePO4 were determined using an ion chromatography (IC; SD-8022, Tosoh Co.) after dissolving in distilled water.

As shown in Figure 2.1, two-electrode test cell (TOM Cell) was used for the electrochemical measurements. The cathode mixture consists of LiFePO4 sample, acetylene black (AB), and polyvinylidene difluoride (PVDF) in weight ratios of 8:1:1. The mixture was rolled spread to a film with 0.1 mm thickness and the film was cut into a disk with 13 mmϕ. It was then pressed onto a titanium mesh welded on the bottom of SUS304 container (20 mmϕ × 3mmt). The cathode was fully dried under vacuum (∼10−1 Pa) for 12 h at room temperature prior to use. The mixture of propylene carbonate (PC) and dimethoxyethane (DME) (1:1 vol.) containing 1.0 mol dm−3 LiPF6 was used as an electrolyte solution. Li metal foil (0.2 mmt Kyokuto Kinzouku Co. Ltd.) was used as the reference and counter electrodes. After assembling the test cell, AC impedance measurements of test cell containing each sample were carried out using a potentio/galvanostat (1287; Solartron Analytical) and frequency response analyzer (1287; Solartron Analytical). The frequency range was 100 mHz to 10 kHz and the current amplitude was 1 mA cm−2.

Figure 2.1 Schematic illustration of two−electrode test cell.

Charge–discharge test was carried out at the currents of 0.2 °C (discharge rate) and 0.2 °C (charge rate) (Hokuto Denko Co., HJ101SM6). The temperature of cathode was controlled at 25 °C. Cut-off potentials were 3.0 V (discharge) and 4.0 V (charge).

The thermal behavior of the delithiated samples prepared after charging at 4.0 V was investigated using differential scanning calorimetry (DSC, DSC6300; Seiko Instruments Inc.) at a heating rate of 4 K min−1 up to 723 K. The electrode mixture was taken from the cell, washed with tetrahydrofuran, and dried under vacuum, and part of it (approximately 5 mg) was used for the DSC...

Produkt-Details

• Erscheint lt. Verlag: 30.4.2015
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Anorganische Chemie
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-12-800712-5 / 0128007125
• ISBN-13: 978-0-12-800712-9 / 9780128007129