Study on electrode materials for lithium-ion batteries by neutron scattering and X-ray diffraction Study on Electrode Materials for Lithium-Ion Batteries by Neutron Scattering and X-ray Diffraction
Study on electrode materials for lithium-ion batteries by neutron scattering and X-ray diffraction
Study on Electrode Materials for Lithium-Ion Batteries by Neutron Scattering and X-ray Diffraction
Mamun S.M.Mostafa Al
マムン エス エム モスタファ アル
Lithium-ion secondary (rechargeable) cell technology is in its infancy. Many improvements including new cathode, anode, and electrolyte materials have been developed since the commercialization of the first Li-ion battery system in 1991 by Sony Corporation. Current commercial lithium-ion cells offer good performance but at a premium price in comparison to conventional rechargeable systems. In order to make Li-ion batteries more competitive, there must be improvements in performance and cost reduction without compromising the safety issues. Accordingly, through innovative cell design and the proper selection of materials, safety and performance can be enhanced and cost lowered in order to broaden its application and acceptance in the commercial market.<br /> Research on the intercalation of carbon materials dates back to the 1950's. Much research has concentrated on the study of graphite intercalation compounds (GIC) for a long time. On the other hand fundamental aspects of electrochemical lithium intercalation into carbon materials other than graphite have been made since the lithium ion battery was first commercialized. People have been facing the difficulties in correlating electrochemical performances and structure because of the many different types of structural parameters that must be characterized in a single material. Therefore, the first task is to understand the structure correctly in order to discuss the intercalation mechanism, then improvement of the carbon anode can be achieved by structural designing.<br /> Three kinds of carbon have been used as anodes for commercial lithium-ion cells: graphite, soft carbon and hard carbon. Graphite is three a three-dimensional ordered crystal that can intercalate up to a maximum of one lithium atom per six carbon atoms (LiC<SUB>6</SUB>) giving a maxumum capacity of 372 mAh/g<SUP>-1</SUP>. Soft carbon and hard carbon constructed with two-dimensional ordered graphene sheets which are randomly stacked have a 'turbostratic' structure. Many soft carbons show a maximum reversible capacity when heat treated around 1200℃ of about 300mAh/g<SUP>-1</SUP>. Lithium can be doped to some hard carbons up to over one lithium atom per six carbon atoms and some of them offer over 500mAh/g<SUP>-1</SUP> of reversible capacity with a small irreversible capacity of about 60m Ah/g<SUP>-1</SUP>.<br /> Hard carbons play a significant role in recent developments of rechargeable Li-ion batteries. In addition to high capacity they also provide a significant performance in the potential range form 0 to 0.1V (vs. Li/Li<SUP>+</SUP>). The capacity below 0.1V looks like a plateau during charge and discharge and is very attractive for anodes of high energy-density batteries. The charge-discharge mechanism studied by <SUP>7</SUP>Li-nuclear magnetic resonance (<SUP>7</SUP>Li-NMR) suggested two types of Li species during lithiation process; one type of Li is the same as those in graphitizable carbons, and the other is quite different. The former is thought to be in the inter-layer space between graphene layers, and the latter is Li clusters with metallic character, which causes a significant capacity below 0.1V. In 1997 Y. Nishi proposed nanopore structure in hard carbons and these pores reversibly intercalate lithium which is the origin of high capacity. To rationalize the high "extra" lithium capacity of hard carbons, still a variety of controversial models and explanations have been suggested in the literature. Yazami et al. (1995) proposed the formation of lithium multilayers on the graphene sheets. Peled et al. (1996) believe that the extra charge is attributed to the accommodation of lithium is "zigzag" and "armchair" faces between two adjacent crystallites and in the vicinity of the defects and impurities. Sato et al. (1995) suggested that lithium occupies nearest neighbour sites in intercalated carbons. <br /> Neutron scattering technique is efficient to understand the structure of nongraphitizable carbons before and after Li intercalation. In the present study, the structures of the hard carbon and lithiated hard carbon were studied by small angle neutron scattering (SANS) to understand the structural changes during the lithiation process. In addition, Swedish natural graphite meso carbon microbeads (MCMB) were also studied as candidates for graphite and soft carbon by SANS and neutron powder diffraction (NPD). In-situ X-ray diffraction (IXRD) experiment was carried on jet-milled swedish graphite to see if there is any influence of jet-milling on staging behaviour. From our study we can conclude the followings: <br /> 1. Small angle neutron scattering (SANS) measurement provided direct observation of nanopores and disordered structure in hard carbon. Lithium can be stored reversibly inside the pores by electrochemical intercalation resulting in higher reversible capacity (〓430 mAh/g) which is substantially greater than the maximum theoretical capacity (372mAh/g) of graphite at LiC<SUB>6</SUB> composition.<br /> 2. During shallow lithiation process lithium first intercalates between the graphene layers of hard carbon which is concluded from the increase in d-spacing between the graphene layes, but no remarkable change in pore size was observed. Upon further deep charging d-spacing dose not increase, but pore size increases indicating lithium intercalation into the pores.<br /> 3. Volume fraction of pores in hard carbon sample was measured 〓16%. We are much confident on this result because SANS technique is capable of probing ever pore (open or closed, small or big) which is impossible by traditional gas adsorption techniques.<br /> 4. At low temperature (96 K) we did not observe any change concerning d-spacing and pore size in fully lithiated hard carbon.<br /> 5. Unlike hard carbon we observed that MCMB and Swedish natural graphite (Woxna Fines) do not contain any pore structure. They offer reversible capacity (305 mAh/g for MCMB and from 340 to 371 mAh/g between crude and jet-milled Swedish natural graphite)compared to hard carbon.<br /> 6. Jet-milling of Woxna Fines did not introduce any pore structure. However, from neutron powder diffraction (NPD) experiment we observed that the rhombohedral (ABC) phase content increased at the expense of hexagonal (AB) phase after jet-milling. This change in phase content increased the reversible capacity from 340 to 371 mAh/g.