Nanotechnology Prospect for Rechargeable Li-ion Batteries

Transcription

1 Nanotechnology Prospect for Rechargeable Li-ion Batteries Chien-Min Wang, Jin-Ming Chen Materials Research Laboratories, Industrial Technology Research Institute Hsinchu 310, Taiwan, R.O.C. ABSTRACT Some interesting results from the research areas of applying nanotechnology for rechargeable Li-ion batteries at Materials Research Laboratories, Industrial Technology Research Institute are summarized in this paper. After discussing current materials and product issues, we will provide examples of novel nanostructured anode and cathode materials, especially on fabrication of lithium ion cells with nano-composite anode covering from discrete component level to prototype cell with a specific energy density of 180 Wh/kg. It is shown that although the nanomaterials technology is still in its infancy, nanotechnology may offer tremendous prospect for high energy storage devices. However, commercialization of this nanostructured Li-ion battery needs further integration of interdisciplinary developments involving mass-producible synthesis and processing techniques, fine instrument as well as precision machine technologies. Keywords: nanotechnology, Li-ion batteries, anode, cathode, capacity 1. Introduction During the last two decades, rechargeable lithium metal (anode)/lithium intercalation compounds (cathode) batteries using nonaqueous electrolytes have been intensively studied [1,2,3] and entered to market [4] in very short time because of their excellent energy density. However, the formation of dendrites on the surface and the changes in morphology of lithium anode, can lead to serious safety problems. [5] In order to eliminate these problems, a novel lithium ion battery has been put forward [6,7] and successfully commercialized by Sony Energytec in The principle of Li-ion battery (LIB) is based on the rocking-chair concept, in which a low potential Li insertion anode as carbonaceous material is matched with a high potential Li insertion cathode. Various materials have been developed for high performance, more safety, low cost and availability considerations. So far, the best choice for electrode reactions for commercial Li-ion batteries is Li intercalation compounds such as LiCoO 2 for cathode material and graphite for anode material. The discharge process is the lithium ion moves out of the intercalated carbon and into another lithium intercalation compound. The overall electrochemical reaction follows the equation Li x C 6 + Li (1-x) MO 2 C 6 + LiMO 2 In order to satisfy the human needs of both environmental protection and miniaturization of consumer products, Li-ion batteries have rapidly become the rechargeable battery of choice for many applications [8] and are available in a variety of configuration, size, and

2 capacity. Generally, there are three types of Li-ion products developed in the past 10 years from cylindrical and prismatic Li-ion batteries to Li-ion polymer batteries (LIPBs). In the remarkable diversification and spread of portable appliances, the cellular phone and notebook computer are two big promising markets for LIB applications and occupy over 85% of market shares. Other battery demands for camcorders, digital cameras, PDA, Bluetooth wireless headset..are still small and for HEV, UPS and load leveling not ready. The capacity requirement of cylindrical Li-ion style batteries for next generation notebook computers is above 2300 mah as shown in Fig. 1. Volumetric energy density demands for 3G requirement of cellular phones are above 450 Wh/l as shown in Fig. 2. The technology trends of rechargeable batteries for portable device utilization are higher density, more safety, long cycle and lighter. Therefore, searching for better performance and novel materials of LIBs becomes urgently important. One of intensive researches is to apply nanotechnology for lithium ion batteries. 2. Materials System and Nanotechnology 2.1 Cathode Materials Requirement The typical cell structure of LIBs is C-Cu foil / electrolyte / LiMO 2 -Al foil. Among the many choices for cathode of lithium ion cells, LiCoO 2, LiNiCoO 2, and LiMn 2 O 4 are the three most popular materials. [6,7,9,10,11,12,13] LiCoO 2 is the most successfully commercial material and has trigonal layer structure with one layer Li and one layer O. The working voltage and specific capacity of these materials are as follows: LiCoO 2 3.7Vx145mAh/g, LiMn 2 O 4 3.8Vx120 mah/g, and LiNi 0.85 Co 0.15 O 2 3.6Vx180mAh/g. The general requirements of good LiCoO 2 cathode are (1) high mobility of Li + : high crystallinity and small particle size of LiCoO 2, little impurity (ex. Li 2 CO 3 ), high mobility in binder & electrolyte, and thin electrode; (2) high reversible capacity: high crystallinity of LiCoO 2, little fine particle, little impurity (ex. Li 2 CO 3, LiOH), high packing density of LiCoO 2, high degree of dispersion & mixing of components (LiCoO 2, carbon ); and (3) low impedance: little insulation content (ex.li 2 CO 3 ), high degree of dispersion & mixing of components, high adhesion of electrode & collector ( Al foil ), and high packing density. 2.2 Anode Materials Requirement Among the alternatives for the anode material in the past decade, the choice is almost exclusively limited to carbons [14]. Perhaps the two major reasons for the widespread use of carbon in many electrochemical technologies are its (a) reasonable high electrical conductivity and (b) good corrosion resistance in many electrolytes. Other important factors are its low cost and availability in different physical structures, which are easily fabricated into electrode. Carbons can be roughly classified into three of common forms: diamond, the graphitic (soft) and disordered /non-graphitic (hard) carbons. Graphite and disordered carbons possess the unique combination of chemical, electrical, mechanical, and thermal properties that are attractive in electrochemical technologies. The formation of passive film at the interface between the carbon anode and the organic electrolyte during the initial period of intercalation is noted as the

3 solid electrolyte interphase (SEI). [15,16] There has been a gradual progressing of the carbon material used from coke to graphite in order to dramatically decrease the irreversible capacity and enhance cycle stability of the electrode. Two major problems were then identified regarding the application of graphite intercalation compounds in LIBs: the solvent co-intercalation causing graphite exfoliation and the solubility in the electrolyte, especially in PC containing system. [16,17] For finding new substitutes for metallic Li or carbon materials as anode materials, various oxide ternary (R-V-O) [18], amorphous tin composite oxide (ATCO) [19,20], or Sn/Si/Al-based alloys [21] that can display reversible capacities far greater than those obtained for the lithiated compound LiC 6 (372 mah/g) are arousing interest world-wide in recent years (see Fig. 3). However, they all have substantial expansion coefficients upon the uptake of lithium, exhibited substantial first cycle irreversibility as well as then are limited their use in practical cells. The cycling stability can be improved by diluting the reacting component in a more or less inactive matrix, by a proper design of morphology (small particle size, porosity, even down to the nano-scale), by use of multi-phase instead of single phase, or by the use of inter-metallic compounds. 2.3 Nanotechnology Consideration Nanotechnology is working at atomic, molecular and supramolecular levels, in the length scale of approximately 1~100 nm range, in order to create materials, devices and systems with fundamentally new properties and functions because of their small structure. [22] The microstructure of nano-crystalline materials depends on the methods of preparation. Basically there are two kinds of preparation methods: top-down and bottom-up. The forward one includes: high energy mechanical milling/alloy and nano-lithography ; the latter one includes: gas phase condensation, liquid phase chemical precipitation, sol-gel, hydrothermal, vapor phase chemical reaction, spray conversion process, transformation assisted consolidation, self-assembly, and supercritical fluid. In this paper conventional solid state reaction, chemical precipitation, sol-gel coating, and spray forming are applied. Synthesis issues means to achieve monodispersity: The stability of the collected nanoparticle powders against agglomeration, sintering, and compositional changes can be ensured by collecting the nanoparticle in liquid suspension. In general, the research and development progress in LIBs between current technology and nanotechnology summarized as follows. [23,24,25] Current Technology Nanotechnology (1) Anode Materials MCMB B-MCF Li (?) (1) Anode Materials Nano-SnO 2, Sn, SnSb, Si Nano-particle doped carbon fiber, CNT transition metal Nano-Li 3-x Co x N, CoO, oxides, or others (2) Cathode Materials LiCoO 2 + Super P LiMn 2 O 4 + Super P LiNi x Co 1-x O 2 (3) Electrolytes LiPF 6 /EC+DEC+DMC PVDF/HFP-based (4) Slim Batteries Li-polymer battery Cu 6 Sn 5 (2) Cathode Materials Nano-V 2 O 5 Nano-LiNiCoO 2 Nano-LiM x Mn 2-x O 4 (3) Electrolytes [26] Nano-TiO 2,SiO 2, powders in PEO-LiX electrolyte (4) Slim Batteries Nanostructure thin film is

4 Thin-film battery (?) Solid state battery Some nanotechnologies have already been applied in the existing LIB industry. For examples, in order to increase electron transfer rate and the lubricating effect, cathode materials must be mixed with a conductive agent such as super-fine/nano-scale carbon powders or acetylene black. Others, PVDF/HFP-based mesoporous electrolytes for the Bellcore s LIPB process can be made by both the plasticization/extraction and controlled evaporation [27] methods. The cylindrical mesoporous structure of the membranes is very uniform and has the pore diameter of ~20 nm and the length of no larger than 40 nm. Latter on, we will focus on three areas: A) high-capacity and low-cost anode materials such as nanocomposite graphite anode; B) high-performance and safer cathode materials such as fine LiCoO 2 cathode with high rate capability and nano-coating LiNiCoO 2 cathode with high capacity; and C) newly nano-fiber materials. 3. Nanostructured Composite Anodes 3.1 Nanostructured SnO 2 /C Composites In order to prevent graphite exfoliation, there are three approaches: (1) for active material particles: coating with a thin nanostructure layer on the graphite surface; (2) for organic electrolyte: selecting an appropriate solvent compatible with PC; (3) for enhancing electronic conductivity: keeping high and homogeneous over the whole electrode area. Nanostructured SnO 2 /C composites used as anode materials were prepared by sol-gel synthesis and mesophase carbon microbead (MCMB, average size of µm) powders, were employed as base materials. Surface characteristics of the SnO 2 /C nanocomposite were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). From Fig. 4, it was found that nanocrystalline SnO 2 with a grain size of nm was uniformly dispersed on the carbon surface. Because of nanocrytalline SnO 2 coated onto carbon, the discharge capacity of Li-ion cells showed an increase up to 23 %, i.e., from 300 to 370 mah/g at a current density of 0.6 ma/cm 2. As shown in Fig. 5 and Fig. 6, the nanocomposite anode can achieve a fairly stable discharge capacity even preventing the graphite sheet exfoliation for the PC-based electrolyte systems, and excellent Coulombic efficiency (> 99.5 %) over 50 cycles. Cyclic voltammograms in Fig. 6 indicated that the improvements on capacity and cycleability were due to reversible alloying of nanosized Sn and Li on carbon surface. Impedance spectroscopy is one of the most promising tools for the modeling and diagnosis of interfacial reactions between the anode and electrolytes. From the impedance test results of nano-composite anode for LIBs, as we can see from Fig. 7, the film resistances of nano-sn/c and nano-sno 2 composites are similarly low around 10~12 Ω cm 2 while that of conventional graphite is three times about 30.5 Ω cm 2 after 10 cycle test. This reason is due to ultra-thin Li 2 O film (~ m) formation which is stable/resistive (with PC) and has good ionic conductivity. 3.2 Nanocomposite CoO/C Systems Novel transition-metal oxides used as anodes for LIBs were initiated by Tarascon s group. [28,29] Thus, nano-composite anode of MCMB-CoO ideal [30] was proposed. Fig. 8

5 shows the acidified MCMB is coated by PEG polymer and nano-coo particle. High specific capacity of MCMB-CoO composite with equal amounts is ~590 mah/g while that of MCMB-PEG-CoO composite is ~ 380 mah/g. 3.3 Nanofiber System Epoxy-based carbon nanofibers were prepared from template synthesis method (TSM). The anodic aluminum oxide (AAO) membrane employed as template, had a nominal pore size of about 200 nm and thickness of about 50 µm. Experimental results showed that the nanofibers had amorphous and well-defined 1-D structures. It shows that as the concentration reduced, the thickness of pore wall of the nanofibers became thinner at the same immersed time. Compared with other methods, the advantages of this method are convenient, material general, efficiency, and low cost. The carbon nanofibers can be extensively applied in areas such as electron emitters, battery research, biochemistry, hydrogen storage media, and chemical sensor. Nano-SnO 2 fibers and nano-composite SnO 2 /C fibers for anode materials in LIBs were synthesized by similar template means. [25] From Fig. 9, it appears that precise control of nano-fiber diameter (50 nm) and low cost for template removal (thermal decomposition substitute oxygen plasma) was demonstrated (vs. Martin group). The electrochemical results of applying nano-sno 2 fiber as anode can achieve high reversible capacity (>740 mah/g), lower irreversible capacity (from 328 mah/g to 131 mah/g), and high charge & discharge rate (>10 C rate, commercial carbon C&D rate: 2C). 4. Nano-LiNi 1-x-y Co x O 2 Cathodes The commercial grade of LiCoO 2 cathode material for Li-ion cells has the capacity limitation up to 145mAh/g. Various novel cathode materials were developed for enhancing high density. The solid state reaction method has been used to prepare the LiNi 1-x-y Co x (M y )O 2 cathode material. The high-capacity and safe LiNi 1-x-y Co x (M y )O 2 cathode material is made by the surface modification of nano-coated oxide on the surface of grains. As shown in Fig. 10, the micrograph contains the nano-coated oxide layer with thickness of 10~15 nm and sub-particle of ~300 nm. This structure decreases the exothermic heat of the LiNi 1-x-y Co x (M y )O 2 material (DSC material exothermic heat <4.5 W/g, the same as LiCoO 2 exothermic heat) and enhances the safety of Ni-based cathode materials. From the electrochemical results of Fig. 11 and Fig. 12, high cathode specific capacity ( >180 mah/g, compared with LiCoO 2 specific capacity: 140 mah/g) can be achieved. Although nanotechnology can enhance the performance of Li-ion cells significantly from preliminary R&D results, a lot of future challenge ahead for commercialization [31] might be highlighted as follows. (1) Large scale synthesis and processing (e.g., uniform dispersion, mixing, coating) (2) Control nanostructure size distribution and composition assembly (3) Whether natural stability is sufficient or additionally stabilizes against changes. (4) Control of the interfaces and distribution of nano-components (5) Analytical or characterization instrumenttation needed

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