| Summary: | Lithium-ion batteries are leading the path for the power sources for various portable applications, such as laptops and cellular phones, which is due to their relatively high energy density, stable and long cycle life. However, the cost, safety and toxicity issues are restricting the wider application of early generations of lithium-ion batteries. Recently, cheaper and less toxic cathode materials, such as LiFePO₄ and a wide range of derivatives of LiMn₂O₄, have been successfully developed and commercialized. Furthermore, cathode material candidates, such as LiCoPO₄, which present a high redox potential at approximately 4.8 V versus Li⁺/Li, have received attention and are being investigated. However, the theoretical capacity of all of these materials is below 170 mAh g⁻¹, which cannot fully satisfy the requirements of large scale applications, such as hybrid electric vehicles and electric vehicles. Therefore, alternative high energy density and inexpensive cathode materials are needed to make lithium batteries more practical and economically feasible.
Elemental sulfur has a theoretical specific capacity of 1672 mAh g⁻¹, which is higher than that of any other known cathode materials for lithium batteries. Sulfur is of abundance in nature (e.g., sulfur is produced as a by-product of oil extraction, and hundreds of millions of tons have been accumulated at the oil extraction sites) and low cost, and this makes it very promising for the next generation of cathode materials for rechargeable batteries. Despite the mentioned advantages, there are several challenges to make the sulfur cathode suitable for battery use, and the following are the main: (i) sulfur has low conductivity, which leads to low sulfur utilization and poor rate capability in the cathode; (ii) multistep electrochemical reduction processes generate various forms of soluble intermediate lithium polysulfides, which dissolve in the electrolyte, induce the so-called shuttle effect, and cause irreversible loss of sulfur active material over repeat cycles; (iii) volume change of sulfur upon cycling leads to its mechanical rupture and, consequently, rapid degradation of the electrochemical performance.
A variety of strategies have been developed to improve the discharge capacity, cyclability, and Coulombic efficiency of the sulfur cathode in Li/S batteries. Among those techniques, preparation of sulfur/carbon and sulfur/conductive polymer composites has received considerable attention. Conductive carbon and polymer additives enhance the electrochemical connectivity between active material particles, thereby enhancing the utilization of sulfur and the reversibility of the system, i.e., improving the cell capacity and cyclability. Incorporation of conductive polymers into the sulfur composites provides a barrier to the diffusion of polysulfides, thus providing noticeable improvement in cyclability and hence electrochemical performance.
Among the possible conductive polymers, polypyrrole (PPy) is one of the most promising candidates to prepare electrochemically active sulfur composites because PPy has a high electrical conductivity and a wide electrochemical stability window (0-5 V vs Li/Li⁺). In the first part of this thesis, the preparation of a novel nanostructured S/PPy based composites and investigation of their physical and electrochemical properties as a cathode for lithium secondary batteries are reported. An S/PPy composite with highly developed branched structure was obtained by a one-step ball-milling process without heat-treatment. The material exhibited a high initial discharge capacity of 1320 mAh g⁻¹ at a current density of 100 mA g⁻¹ (0.06 C) and retained about 500 mAh g⁻¹ after 40 cycles. Alternatively, in situ polymerization of the pyrrole monomer on the surface of nano-sulfur particles afforded a core-shell structure composite in which sulfur is a core and PPy is a shell. The composite showed an initial discharge capacity of 1199 mAh g⁻¹ at 0.2 C with capacity retention of 913 mAh g⁻¹ after 50 cycles, and of 437 mAh g⁻¹ at 2.5 C. Further improvement of the electrochemical performance was achieved by introducing multi-walled carbon nanotubes (MWNT), which provide a much more effective path for the electron transport, into the S/PPy composite. A novel S/PPy/MWNT ternary composite with a core-shell nano-tubular structure was developed using a two-step preparation method (in situ polymerization of pyrrole on the MWNT surface followed by mixing of the binary composite with nano-sulfur particles). This ternary composite cathode sustained 961 mAh g⁻¹ of reversible specific discharge capacity after 40 cycles at 0.1 C, and 523 mAh g⁻¹ after 40 cycles at 0.5 C. Yet another structure was prepared exploring the large surface area, superior electronic conductivity, and high mechanical flexibility graphene nanosheet (GNS). By taking advantage of both capillary force driven self-assembly of polypyrrole on graphene nanosheets and adhesion ability of polypyrrole to sulfur, an S/PPy/GNS composite with a dual-layered structure was developed. A very high initial discharge capacity of 1416 mAh g⁻¹ and retained a 642 mAh g⁻¹ reversible capacity after 40 cycles at 0.1 C rate. The electrochemical properties of the graphene loaded composite cathode represent a significant improvement in comparison to that exhibited by both the binary S/PPy and the MWCNT containing ternary composites.
In the second part of this thesis, polyacrylonitrile (PAN) was investigated as a candidate to composite with sulfur to prepare high performance cathodes for Li/S battery. Unlike polypyrrole, which, in addition of being a conductive matrix, works as physical barrier for blocking polysulfides, PAN could react with sulfur to form inter- and/or intra-chain disulfide bonds, chemically confining sulfur and polysulfides. In the preliminary tests, PAN was ballmilled with an excess of elemental sulfur and the resulting mixture was heated at temperatures varying from 300°C to 350°C. During this step some H₂S gas was released as a result of the formation of rings with a conjugated π-system between sulfur and polymer backbone. These cyclic structures could ‘trap’ some of the soluble reaction products, improving the utilization of sulfur, as it was observed experimentally: the resulting S/PAN composite demonstrated a high sulfur utilization, large initial capacity, and high Coulombic efficiency. However, the poor electronic conductivity of the S/PAN binary composite compromises the rate capability and sulfur utilization at high C-rates. These issues were addressed by doping the composite with small amounts of components that positively affected the conductivity and reactivity of the cathode. Mg₀.₆Ni₀.₄O prepared by self-propagating high temperature synthesis was used as an additive in the S/PAN composite cathode and considerably improved its morphology stability, chemical uniformity, and electrochemical performance. The nanostructured composite containing Mg₀.₆Ni₀.₄O exhibited less sulfur agglomeration upon cycling, enhanced cathode utilization, improved rate capability, and superior reversibility, with a second cycle discharge capacity of over 1200 mAh g⁻¹, which was retained for over 100 cycles. Alternatively, graphene was used as conductive additive to form an S/PAN/Graphene composite with a well-connected conductive network structure. This ternary composite was prepared by ballmilling followed by low temperature heat treatment. The resulting material exhibited significantly improved rate capability and cycling performance delivering a discharge capacity of 1293 mAh g⁻¹ in the second cycle at 0.1 C. Even at up to 4 C, the cell still achieved a high discharge capacity of 762 mAh g⁻¹.
Different approaches for the optimization of sulfur-based composite cathodes are described in this thesis. Experimental results indicate that the proposed methods constitute an important contribution in the development of the high capacity cathode for rechargeable Li/S battery technology. Furthermore, the innovative concept of sulfur/conductive polymer/conductive carbon ternary composites developed in this work could be used to prepare many other analogous composites, such as sulfur/polyaniline/carbon nanotube or sulfur/polythiophene/graphene, which could also lead to the development of new sulfur-based composites for high energy density applications. In particular, exploration of alternative polymeric matrices with high sulfur absorption ability is of importance for the attainment of composites that possess higher loading of sulfur, to increase the specific energy density of the cathode. Note that the material preparation techniques described here have the advantage of being reproducible, simple and inexpensive, compared with most procedures reported in the literature.
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