ARTICLE pubs.acs.org/JPCC
Surface Behaviors of Conductive Acetylene Black for Lithium-Ion Batteries at Extreme Working Temperatures Jie Shu,* Miao Shui, Fengtao Huang, Dan Xu, Yuanlong Ren, Lu Hou, Jia Cui, and Jinjin Xu Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, People’s Republic of China ABSTRACT: The surface physical and chemical behaviors of conductive additive acetylene black (AB) are studied after the samples are cycled at different working temperatures (20, 20, and 60 °C). Working at low temperature (20 °C), AB shows high polarization and poor electrochemical property. In addition, plenty of inorganic compounds are formed on the surface of AB. For comparison, the surface film on AB after being worked at 60 °C is comprised of an outer organic layer and an inner inorganic layer. Moreover, it consumes large electrical energy and results in irreversible trapped lithium for surface film formation. Furthermore, much more organic components can induce lower thermal stability of conductive additive. During repeated electrochemical cycles, the surface structure of AB experiences a series of changes of surface electrolyte decomposition products. It is found that the organic polymer in outer layer will decompose into other stable species, and partial surface fluorides in the inner layer will transform into other fluorides. The working temperature has a great effect on the final transformation products.
1. INTRODUCTION In lithium-ion batteries, many anode or cathode materials, such as Co3O4, SnO2, LiFePO4, and Li2MnSiO4, show low electronic conductivity and poor high-power ability. Therefore, conductive additive becomes an important component in the lithium-ion batteries, especially for high-power use. In previous studies, many different kinds of conductive additives, such as acetylene black, ketjen black, carbon black, carbon filaments, carbon fiber, carbon nanotube, graphite, and metal powder, etc.,16 are presented as the bridges between particles and/or the current collector for electron transportation. It is obvious that different conductive additives show different structural and physical characteristics. As a result, they have different effects on the improvement of electrochemical property of active material. Therefore, it is essential to study the physical and chemical behaviors of conductive additive in lithium-ion batteries. As well-known, carbon black is an useful conductive additive to improve the rate performance of anode or cathode materials.4,6,7 Dominko et al. reported the influence of carbon black content and carbon black distribution on the performance of oxide-based cathodes.4 It is clear that high content and uniform distribution of carbon black can bring excellent electrochemical properties to oxide materials. Fransson et al. reported the electrochemical property and thermal stability of carbon black at room temperature.8 It is found that carbon black shows an irreversible capacity of 70% in the voltage range 0.011.5 V and poor thermal stability in the temperature range 30300 °C. Although the electrochemical performance and conductive behavior of carbon black were thoroughly studied at room temperature, the physical and chemical properties of carbon black are rarely investigated under extreme working conditions. Moreover, r 2011 American Chemical Society
lithium-ion batteries will be working at high temperature or low temperature in certain regions, such as summer in Kuwait or winter in Russia. Therefore, it is necessary to study the physical and chemical properties of carbon black under extreme working conditions. Acetylene black (AB) is a special type of conductive carbon black formed by thermal decomposition of acetylene. In this Article, the surface characteristics, electrochemical behavior, and thermal stability of AB are reported under extreme working conditions (60 and 20 °C).
2. EXPERIMENTAL SECTION 2.1. Reagents and Electrode Preparation. AB was provided by Ningbo Shanshan Co., Ltd., China. The preparation of electrode for repeated cycling and physical characterization can be found elsewhere.9 The active electrode is comprised of AB (90 wt %) and polyvinylidene fluoride (PVDF, 10 wt %). For repeated cycling and physical characterization, the Swagelock battery is assembled by using the AB film as working electrode, Whatman GF/D glass fiber filter as separator, lithium metal disk as counter electrode, and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v) as electrolyte in Ar-filled glovebox (M. Braun, Germany). 2.2. Physical and Chemical Characterization. Galvanostatic chargedischarge cycling is tested on a multichannel Land Battery Test System (Wuhan Jinnuo Co., Ltd., China). All Received: January 6, 2011 Revised: March 5, 2011 Published: March 17, 2011 6954
dx.doi.org/10.1021/jp200167x | J. Phys. Chem. C 2011, 115, 6954–6960
The Journal of Physical Chemistry C
ARTICLE
Figure 1. XRD (a), SEM (a), and TEM (b) of AB.
Swagelock batteries are charged and discharged at a constant current of 25 mAg1. Furthermore, the batteries are tested at the working temperatures of 60, 20, and 20 °C, respectively. The X-ray diffraction (XRD) pattern for the sample is carried out with Bruker AXS D8 FOCUS X-ray diffractometer using nickel-filtered Cu KR radiation (λ = 1.5408 Å). The surface morphology is characterized by means of scanning electron microscopy (SEM, Hitachi S-3400, Japan). The grain morphology is observed with a JEM-2010 high resolution transmission electron microscopy (HRTEM), performing at 200 kV (JEOL, Japan). Raman behaviors are taken on a SPEX-1403 Raman spectrophotometer (SPEX, USA) from 1000 to 2000 cm1. Fourier transform infrared (FTIR) spectra are collected on a Shimadzu FTIR-8900 infrared spectrophotometer from 400 to 2000 cm1. X-ray photoelectron spectroscopy (XPS) collected on sample is carried out by using an ESCALAB MKII photoelectron spectrometer (VG, Great Britain). Thermal reactivity tests are performed by a thermo-gravimetric differential scanning calorimetry (TG-DSC) apparatus STA-449F3 (Netzsch, Germany). All of the samples are prepared in the Ar-filled glovebox.
3. RESULTS AND DISCUSSION 3.1. Morphology, Structure, and Electrical Property. As is well-known, AB is obtained by an exothermic decomposition of acetylene at moderate or high temperature. It is a special type of conductive carbon black. In Figure 1a, AB shows the varying particle size between 20 and 50 nm. Furthermore, it also exhibits spherical morphology and well-dispersed particles. Uniform distribution of superfine AB is beneficial to achieve good electronic conductivity between active materials and/or current collector in lithium-ion batteries. The XRD pattern (Figure 1a) of AB merely shows two broad (002) and (100) peaks at 25.2° and 44.5° in the 2θ range from 5° to 80°, which indicates disordered-carbon structure of AB similar to that of pyrolyzed hard carbon.10 As shown in Figure 1b, the HRTEM image reveals many parallel small-sized graphene sheets and disordered zones in the structure of AB, which is different from the structure of asreported hard carbon nanospherules.11 Usually, amorphous carbon-coated active particles have excellent electronic conductivity for high-performance lithium-ion batteries. Here, AB shows surface physical and chemical characteristics similar to those of amorphous carbon. Therefore, it will be a kind of good conductive additive as described by other researchers.1214
Figure 2. Charge/discharge curves of AB cycled at 60, 20, and 20 °C.
Usually, the ionic conductivity of organic electrolyte depends on the change of working temperature.15 It indicates that the inner ohm resistance of the battery will increase with the decrease of storage temperature. In the experiment, the open circuit voltages of fresh Swagelock batteries after storage at 60, 20, and 20 °C for 10 h are 3.28, 3.23, and 2.51 V, respectively. This information implies different electrochemical characteristics and surface behaviors of AB can be achieved under different working conditions. Figure 2 shows the charge and discharge curves of Swagelock batteries cycled at different working temperatures. The initial discharge capacities for AB cycled at 60, 20, and 20 °C are 964, 563, and 108.8 mAh g1, respectively. The corresponding charge capacities for samples cycled at 60, 20, and 20 °C are 260.6, 231, and 23.2 mAh g1, respectively. Working at 60 °C, AB shows the smallest polarization and the largest Li-storage capacity among the three samples. Besides, it exhibits three slopes at 1.88, 0.84, and 0.11 V, which is different from the behavior of graphite (
