Impact of Morphology of Conductive Agent and Anode Material on Lithium Storage Properties
In this study, the impact of morphology of conductive agent and anode material (Fe3O4) on lithium storage properties was throughly investigated. Granular and belt-like Fe3O4 active materials were separately blended with two kinds of conductive agents (i.e., granular acetylene black and multi-walled carbon nanotube) as anodes in lithium-ion batteries (LIBs), respectively. It was found that the morphology of conductive agent is of utmost importance in determining LIBs storage properties. In contrast, not as the way we anticipated, the morphology of anode material merely plays a subordinate role in their electrochemical performances. Further, the morphology-matching principle of electrode materials was discussed so as to render their utilization more rational and effective in LIBs.
KeywordsLithium-ion batteries Morphology Conductive agent Anode material
Long-lasting and green rechargeable energies are in high demand for solving the dilemma of global environmental pollution and energy shortage [1, 2, 3]. Among various renewable batteries, lithium-ion batteries (LIBs) have attracted increasing attention for their applications in portable electronics and electric vehicles in recent years [4, 5, 6, 7, 8]. For the requirement of high-performance LIBs, novel anode materials have become one of the research hotspots nowadays. Magnetite (Fe3O4) is such a potential candidate because of its attractive theoretical capacity of 924 mAh g−1, nontoxicity, natural abundance, low cost, and high electronic conductivity (2 × 104 S m−1) [9, 10]. Along these lines, many efforts have been devoted to obtaining high-performance Fe3O4 anode material with various morphologies, including hollow spheres [11, 12], arrays , belts , rods , fibers , etc. Although controlling the morphology of anode material may initially seem like a scientific curiosity, its impact goes far beyond esthetic appeal. For example, the porous morphology with higher surface area is much needed because of the intercalation capacities and affinities for lithium ions (Li+) to the more exposed holes in the surface, which could then shorten the diffusion length of Li+ . Besides, the inner pores also allow the material to effectively buffer the stress induced during the charge–discharge process . Other desirable morphologies involve one-dimensional nanostructures, like nanorods and nanowires, which can offer a small diameter to enhance lithium diffusion and yet still provide a limited surface area to prevent excessive side reactions . All of these morphological features of anode materials play a significant role in determining the discharge characteristics , and thus they are essential to apply in high-performance LIBs.
Apart from anode materials, the types and morphologies of conductive agents are other determinants to LIBs storage performances. Generally, granular carbon black [acetylene black (AB)] is seen as a universal conductive agent with high conductivity. In addition, other types and morphologies of carbon species may be more desirable, and several research efforts have been conducted in this regard. Wang et al.  enabled Ni(OH)2 nanocrystals to grow on graphene sheets for potential energy storage applications. They found that graphene sheets with low oxidation are qualified to impart excellent electrical conductivity to the macroscopic ensemble of the composite materials (without the need of carbon black additives). The highly conducting graphene network allows rapid and effective charge transport between the Ni(OH)2 nanoplates in the macroscopic ensemble and the current collector, allowing for fast energy storage and release. Likewise, Liu et al.  utilized carbon-coated ZnO nanorod arrays as anode material without the extra addition of conductive agents. They found the coated carbon arrays not only ensure good electrical contact of ZnO with the current collector and enhance the charge transfer/Li+ transport, but also effectively alleviate the strains caused by the volume variation of ZnO nanorod cores and prevent the disintegration. Despite the progress achieved to date, the optimization of morphology of conductive agents with anode materials is not sufficiently discussed. And what is more, very little is known about the morphology-matching principle between the used conductive agents and anode materials.
In this study, we utilize Fe3O4 anode material (with granular and belt-like morphologies) and two kinds of conductive agent [i.e., granular AB and multi-walled carbon nanotube (MWNT)] as prototype systems to investigate their morphology impact on lithium storage performances. The selection criteria of multi-morphological conductive agents and anode materials are also proposed according to our electrochemical measurements and analyses.
2.1 Preparation of Fe3O4 Anode Material
Analytical ferrous sulfate (FeSO4) and sodium carbonate (Na2CO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and used as received without further purification. In a typical synthesis, 2 mmol of FeSO4 was dissolved into 50 mL of deionized water at room temperature until a homogeneous solution was formed. After that, 1 g of Na2CO3 powder was added to the solution with continuous magnetic stirring. Then the mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 100 mL for hydrothermal treatment at 160 °C for 20 h (sample 1, marked as S160) and 200 °C for 8 h (sample 2, marked as S200) separately. The as-obtained precipitates were repeatedly washed with deionized water and ethanol, and finally dried at 60 °C for 6 h.
X-ray diffraction (XRD) patterns were recorded on a powder X-ray diffractometer (Rigaku D/max-rA) equipped with a rotating anode and a Cu Ka1 radiation source (λ = 1.5406Å) at a step width of 0.02°. Field emission scanning electron microscope (FE-SEM) images were collected on a field emission scanning electron microscope (JEOL JSM-6700F). Transmission electron microscopy (TEM) images were performed on a JEM-2100 TEM with operating voltage at 200 kV.
2.3 Electrochemical Measurements
The electrochemical measurements were carried out at 25 °C using 2032 coin-type cells with pure lithium metal as the counter and reference electrodes. The working electrode consists of active material (as-synthesized Fe3O4 products), conductive agents (AB or MWNT), and sodium carboxymethyl cellulose binder (CMC, 800–1200 mPa s, DS 0.7) in a weight ratio of 60: 20: 20, using deionized water as the dispersion medium. The mixture was spread on a Cu foil and dried under vacuum at 120 °C for 8 h. The electrolyte used was 1.0 mol L−1 LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (1: 1 by volume). Cell assembly was carried out in an Ar-filled glove box with the concentrations of moisture and oxygen below 1 ppm. The cells were cycled at different current rates of 0.2, 0.5, 1, 2, and 5 C (1 C = 924 mA g−1) between 0.01 and 3 V using a LAND battery tester. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a CHI660D electrochemical workstation by applying a sine wave with an amplitude of 10.0 mV over a frequency range of 100 kHz to 10 mHz. The voltages mentioned in this study were referred to a Li/Li+ redox couple.
3 Results and Discussion
The formation of Li2O and Fe in the forward reaction is thermodynamically favorable during the discharge process. However, the extraction of Li+ ion from Li2O in the reverse process is more difficult, which suggests that a certain extent of irreversibility is inevitable. This conversion reaction provides the dominant contribution to the lithium storage capacity of Fe3O4 material and gives rise to a high initial discharge capacity of 1417, 1163, 1097, and 970 mAh g−1 for S160@MWNT/Li, S160@AB/Li, S200@MWNT/Li, and S200@AB/Li cells, respectively, corresponding to 12.3, 10.1, 9.5, and 8.4 mol consumption of Li per mole of Fe3O4 anode material. A reversible charge capacity of 859, 224, 528, and 305 mAh g−1 can be delivered after the 5th cycle, which leads to irreversible capacity loss of 39.4, 80.7, 51.9, and 68.6 % for S160@MWNT/Li, S160@AB/Li, S200@MWNT/Li, and S200@AB/Li cells, respectively. Such initial irreversible capacity loss is commonly ascribed to the formation of solid-electrolyte interface layer and some other side reactions .
To further validate the above statements, rate capabilities of S160@MWNT (1.86 mg)/Li, S160@AB (2.04 mg)/Li, S200@MWNT (2.34 mg)/Li, and S200@AB (1.92 mg)/Li cells were conducted in the voltage range of 0.01 and 3 V at different cycling rates (with the same rates for both charge and discharge). Figure 4b shows the S160@MWNT/Li cell exhibits the most superior rate capability, the next is S200@MWNT/Li cell, followed by S200@AB/Li and S160@AB/Li cells, which is in the same priority order as their cycling performances. Notably, the cyclability and rate capability of MWNT-involved cells (i.e., S160@MWNT/Li and S200@MWNT/Li cells) greatly outperform those of AB-involved cells (i.e., S160@AB/Li and S200@AB/Li cells). This is because micron-sized soft MWNT not only facilitates the rapid and effective electron transport between electrode materials and the current collector, but also tightly twists and traps the active material, so that the strain caused by the volume variation of Fe3O4 during charge–discharge cycles can be effectively alleviated. This observation strongly evidences that the morphology of conductive agent, rather than that of anode material, is of utmost importance in determining LIBs storage performances. More specifically, if the topology of electronic pathways is such that all the particles of conductive agent are effectively wired, much faster charging and discharging rates are achieved, and thus leading to superior cyclability and rate capability. Otherwise, even though the morphology of Fe3O4 anode material is conducive for Li+ ions to travel (e.g., S160@AB/Li cell), the electrochemical properties of such cells are inferior even at low current densities (Fig. 4).
Based on the above discussion, if the morphology of conductive agent of each cell is identical, the electrochemical performances of these cells are mainly determined by the morphology of anode material. However, the morphology of anode material plays a subordinate role on the LIBs storage performance. For instance, by using the same type of MWNT conductive agent, the cyclability and rate capability of S160@MWNT/Li cell is much superior to those of the S200@MWNT/Li cell. This is because the two-dimensional Fe3O4 belt presents a finite lateral size and enhanced open-edges, which facilitate lithium-ion and electron diffusion through active materials and better withstand the large volume change during the charge/discharge process [32, 33]. However, the electrochemical performances of S160@AB/Li and S200@AB/Li cells seem to run counter to the above trend, hinting at the presence of morphology-matching principle for the electrode materials. Under the condition of any given type of conductive agent, the contact degree of the conductive agent and anode material actually determines the LIBs electrochemical performances. In comparison to polyhedral Fe3O4 particles, the rigid Fe3O4 belts cannot integrate AB particulates well (Fig. 1g) and hence resulting in the worst cyclability and rate capability (Fig. 4).
Electrochemical impedance spectroscopy fitting results for cells after 50th-cycled capacity retention and 60th-cycled rate capability test
Capacity retention test
After 60th-cycled rate
In summary, we utilize granular and belt-like Fe3O4 anode materials and two types of conductive agents (including AB and MWNT) as prototype systems to investigate their morphology impact on lithium storage performances. We find that the morphology of conductive agent plays a decisive role on electrochemical performances. After 50th cycle, the capacity of MWNT-involved cells (i.e., 970 mAh g−1 for S160@MWNT/Li cell and 380 mAh g−1 for S200@MWNT/Li cell) is much higher than that of the AB-involved cells (i.e., 182 and 96 mAh g−1 for S200@AB/Li and S160@AB/Li cells, respectively). Provided that the morphology of conductive agent of each cell is identical, the electrochemical performances of these cells are mainly determined by the morphology of anode material as well as the contact degree of these electrode materials.
The authors are grateful to the financial aid from the National Natural Science Foundation of China (NSFC No. 51472133).
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