Functional materials to enable durable and high loading lithium-sulfur batteries

Abstract

Lithium-sulfur batteries (LSBs) are considered a promising alternative energy storage device to commercial lithium-ion batteries (LIBs). LSB research has received considerable attention due to high theoretical gravimetric energy density, the natural abundance of sulfur and the low cost. These promising features make LSB a suitable candidate for next-generation rechargeable battery technology. However, the shuttle effect (diffusion of polysulfides (PS) between anode and cathode) causes large volume expansion, enhances resistivity and hinders Li + diffusion during cycling of LSBs. This limits the chance of LSB technology to be commercially viable. There are different ways to overcome the above technical challenges of LSB including through modification of the cathode, anode, electrolyte, and separator. This doctoral research aims to explore the modifications of separator and electrode using different heteroatoms and functional polar group associated materials to mitigate the associated LSBs technical limitations at high sulfur loading.Recently, most of the researches focus on the progress of LSBs using non-polar conductive carbon based materials with high surface area to physically confine the PS in inhibiting the shuttle at low sulfur loading. The physical confinement of PS in the high surface area of non-polar carbon seems not to be effective due to weak intermolecular interactions between the PS (0.1–0.7 eV) and non-polar hosts. As a result, the physically encapsulated PS eventually diffuses out from the porous carbon structure during cycling and shuttles between the anode and cathode again. On the other hand, the pore structure of the porous PS host materials (mesopores (2–50 nm) and macropores ≥50 nm) larger than the PS size of ~ 2 nm is another challenge to effectively control the shuttle effect. It necessitates the proper design of host materials to physically block the PS. Different functional heteroatoms or functional polar groups associated with PS hosts also could be a promising solution to chemically attract the PS to limit the shuttle effect and improve the electrochemical performance at high sulfur loading. In this thesis, the impact of different functional polar groups associated with chemically interactive materials with PS and micropore nature significantly contributed to control the shuttle effect at high sulfur loading. The contributions of this thesis are summarized as follows:1.    In the first experiemental chapter, to improve the performance of LSBs, the ketjen black (KB) and Nafion composite was used to coat the Celgard PP separator. The motive of this research was to achieve the low degradation and high areal capacity (mAh cm-2) at high sulfur loading by the conjugal effect of the high surface area of KB to physically confine the PS and -SO3- to chemically interact the PS through polar-polar interactions. Different electrochemical characterizations were carried out to reveal the mechanistic understanding of the coating composites to interact with the PS. In addition, the density functional theory calculation was carried out to understand the adsorption energy of -SO3- group with the PS. From this research, the maximum areal capacity of 6.70 mAh cm-2 and low degradation of 6% was achieved at 4.81 mg cm-2 sulfur loading. 2.    In the second experimental chapter, to further improve the areal capacity of LSBs at high sulfur loading, heteroatom (Nitrogen) doped multilayer graphene and Nafion composite was considered to coat the Celgard PP separator. The layer by layer distances of graphene less than the PS dimension was considered to physically inhibit the PS. Further, the high electronegativity of N and -SO3- group significantly contributed to chemically interact with the PS. The adsorption energy of N and -SO3- contained graphene with the PS species were confirmed using density functional theory calculation. By the multifunctional effect of electronegative N and polar -SO3-, the maximum areal capacities of 12.12 mAh cm-2 and 10.97 mAh cm-2, respectively, were achieved at 12 mg cm-2 and 15 mg cm-2 sulfur loading. This areal capacity limit is almost two times higher than the anticipated areal capacity of LSBs to compete with the state of art LIBs.3.    In the third experimental chapter, the dopants (metal Zn and N) and micropores (<2 nm) possessed PS hosts are considered significant to inhibit the PS diffusions through chemical interactions and physical block. On the other hand, the PS host with the ultra-high surface area also has benefits to physically confine the PS. Considering these parameters, two different materials such as Zinc and N contained carbonized zeoliticzeolitic imidazolate frameworks-8 (ZnN-cZIF-8) and the ultra-high surface area contained carbonized zeolitic imidazolate frameworks-8 (UHS-cZIF-8) are designed carefully to produce an architecture with a micro-porosity (<2 nm) with chemical binding sites (N and Zn) and ultra-high high surface area with no chemically interactive sites, respectively. The main motivation of this research is to realize their relative impact of micropore and dopants over the ultra-high surface area to inhibit the shuttle of LSB. The ZnN-cZIF-8 has an initial specific capacity (Cs) of 929 mAh g-1 and maintains a stable cycling behavior with the degradation of 0.13 % per cycle after 200 cycles with a 5.74 mg cm-2 sulfur loading. In comparison, the UHS-cZIF-8 shows a relatively low specific capacity (841 mAh g-1) with a higher degradation of 0.21 % per cycle with a 5.34 mg cm-2 sulfur loading.  The better cycling performance of the ZnN-cZIF-8 LSB is attributed to the dual role of the micropores in addition to the two chemisorption sites which effectively enhance the charge storage performance in the LSB over UHS-cZIF-8.4.    In the fourth experimental chapter, realizing the impact of chemically interactive sites and micropore (<2 nm), an oriented Lewis acidic Zn containing antiferroelectric perovskite dimethylammonium zinc format (DMAZF) MOF [(CH3)2 NH2] Zn(HCO2)3 is designed as an effective molecular sieve to block polysulfide (PS) diffusion in lithium-sulfur batteries (LSBs) with a tiny window of 6 A. The purpose of this research was to effectively control the PS to achieve high areal capacity at high sulfur loading. DMAZF/CNTs/sulfur electrode with 5 mg cm-2 sulfur loading delivers high areal capacity of 6.3 mAh cm-2 at 0.05 C and 5.03 mAh cm-2 at 0.1 C with a fading of 0.07 % after 120 cycles. Even at 7 mg cm-2 sulfur loading, the electrode exhibits a fading of 0.12 % per cycle even after 500 cycles at 0.5 C.In summary, this thesis successfully demonstrates the effectiveness of the chemically interactive functional materials to achieve high-performance LSB at high sulfur loading. The achieved areal limit is significantly higher than the areal capacity of commercial LIBs (4 mAh cm-2), which will direct the new pathway for electric vehicles (EVs) application. ​​​​​​​