With extinguishing petrochemical natural resources and the environmental pollution ascribed to their use; it has become inevitable that the automobile industry should try to shift the paradigm of fueling the automobiles through rechargeable battery systems. Among several rechargeable battery systems, the burden of powering portable devices as well as automobiles is being bared mainly by the Li-ion batteries from the last few decades. However the energy density of Li-ion batteries have reached its saturation point as a result of significant efforts done by the research fraternity. The typical energy density of Li-ion battery (387 Wh/kg) is quite low for the automobiles to travel a long distance. Moreover, the high cost of cathodes for Li-ion batteries made this battery even more expensive to use. Thus once again research on finding next-generation high energy density Li batteries was embarked.
Among several candidates, Lithium-Sulfur (Li-S) batteries due to its high theoretical capacity (2500 Wh/g), low cost and low toxicity of earth abundant sulfur has gained much attention since the last few decades. However, the associated problems with Li-S battery such as low electrical conductivity of sulfur, dissolution of Li polysulfides in the electrolyte and volume expansion upon cycling makes this battery system unable to deliver high discharge capacity. The polysulfide dissolution is one of the main problem in which the active material loss and polysulfide shuttle phenomenon causes severe capacity fading. To overcome polysulfide dissolution, several physical and chemical entrapment strategies has been employed. The physical entrapment couldn’t hold the polysulfides for a long time since movement of polysulfides is driven by the electric field as well as due to poor non-polar ? polar interaction between carbon hosts and lithium polysulfides. However, the chemical adsorption of polysulfides by polar oxides, functional polymer coatings and functionalized/doped carbon frameworks had worked better for maintaining longer and stable cycling in Li-S battery. But due to the insulating nature of oxides and polymers, the overall electrical conductivity of the cathode decreases. However, 0D, 1D and 2D carbon host such as carbon black, carbon nanotubes and graphene structure can provide good electrical conductivity but non-polar carbon interaction can lose the polar polysulfides during the course of cycling. Thus the best way is to use functionalized carbon framework to simultaneously obtain higher electronic conductivity as well as polysulfide entrapment through functional groups attached to the carbon host.
Moreover, among carbon hosts graphene oxide has exquisite benefits of high mechanical stability as well as superior electrochemical stability thus improving the ionic and electronic transportation with decreased diffusion pathways. Thus in this study, GO was taken as an initial carbon host for sulfur infiltration to make cathode material for Li-S battery. However GO has poor electronic conductivity because of the presence of excessive oxide functional groups on its surface thus GO was chemically reduced by a non-toxic reducing agent, dopamine. Dopamine additionally added amino and hydroxyl functional groups (known to entrap polysulfides) on the rGO surface to chemically bind the polysulfides. Moreover, the dopamine reduced rGO was compared with GO (containing hydroxyl and epoxy functional groups but low electronic conductivity) and thermally reduced rGO (high electronic conductivity but no significant functional groups) based sulfur composites to emphasize that for stable performance both high electronic conductivity and polysulfide binding functional groups are required. The material characterization was done using SEM, TEM, XRD, TGA and Raman Spectroscopy. Moreover the electrochemical characterization was performed by galvanostatic charge/discharge, cyclic voltammetry and electrochemical impedance spectroscopy. Since Li polysulfides exist in different colors in the electrolyte; in-situ UV-Vis spectroscopy was performed on GO-sulfur (GOS) composite and dopamine reduced GO-sulfur composite (c-rGOS) to seek evidence of polysulfide binding with polydopamine. Previously no such reports are present on in-situ characterization of polydopamine binding effect with polysulfides. Two different in-situ UV-Vis cell assemblies were fabricated to show the change in the electrolyte as well as the separator when the cell is discharge till 1.0V. It was clearly shown that no significant polysulfide dissolution took place in the polydopamine modified c-rGOS composite whereas severe polysulfide dissolution was observed in GOS composite both in the electrolyte and on the separator. Due to the binding role of polydopamine, very stable discharge capacity of 600 mAh/g at 0.5C after 300 cycles was obtained in c-rGOS composite however for GOS, it’s dropped to 277 mAh/g after just 100 cycles. This strategy to make c-rGOs composite is also helpful in making large quantity of sulfur composite since the c-rGO synthesis involves only low temperature stirring of GO with dopamine for 24h followed by simple grinding with sulfur before heat treatment at $155 ^\circ C$.
본 연구에서는 리튬-황 전지의 양극 물질로 쓰인 화학적으로 환원된 그래핀 옥사이드(c-rGO)가 전지의 특성에 미치는 효과를 화학적으로 처리 되지 않은 그래핀 옥사이드(GO)와의 비교를 통해 입증하고자 하였다. 폴리도파민을 환원과 기능화의 작용을 모두 하는 물질로 사용하여, 환원과 동시에 기능화된 그래핀 옥사이드를 저온에서 합성하였다. 또한, 양극 물질의 합성 시간과 비용을 감소시키기 위해 용융 확산 방식을 통해 복합체에 황을 침투시켰다. 폴리도파민을 통해 기능화된 환원 그래핀 옥사이드-황 복합체(c-rGOS)의 효과를 확인하기 위해, 열을 통해 환원된 그래핀 옥사이드(t-rGO)와 황의 복합체(t-rGOS) 또한 합성하여 비교하였다. 양극 물질의 전체적인 전도도는 c-rGOS복합체가 GOS복합체보다 화학적 환원을 통해 더 증가했음을 확인하였으며, t-rGOS복합체의 경우 c-rGOS와 GOS보다 저항이 낮음을 발견하였다. 이는 t-rGO가 표면에 어떠한 작용기도 갖고 있지 않기 때문에 그래핀 층을 통한 전자의 전도가 용이한 것이 원인이다. 기대한 바와 같이, 폴리도파민의 catecholamine 작용기가 그래핀-황 복합체에 아미노 작용기와 하이드록실 작용기를 형성하게 하여 높고 안정된 방전 용량을 달성할 수 있었다. 방전 용량은 0.5 C rate에서 100번째와 300번째 사이클일 때 각각 648 mAh/g와 600 mAh/g를 기록했다. 이와 대조적으로, GOS와 t-rGOS의 경우 0.5 C rate에서 100번째 사이클 일 때 각각 277 mAh/g와 345 mAh/g를 넘지 못했다. 따라서, 복합체에 쓰이는 탄소 재료는 우수한 전자 전도성을 띠고 효과적인 화학적 작용기를 갖는 것이 전지의 성능을 높이는 데 필수적이라고 할 수 있다.
폴리도파민의 폴리설파이드 흡착 효과를 강조하기 위해, GOS복합체와 c-rGOS 복합체에 대하여 in-situ UV-Vis 분광 분석을 진행하였으며, c-rGOS가 GOS에 비해 폴리설파이드 용출이 적음을 확인하였다. GOS 복합체의 경우에는 폴리설파이드의 용출량이 현저히 많아 shuttle effect를 유발하였으며 이는 용량의 감소를 야기했다. 그러나 c-rGOS는 폴리도파민의 catecholamine 작용기와 폴리설파이드가 화학적으로 결합하여 폴리설파이드의 용출을 억제했고 이는 우수하고 안정적인 리튬-황 전지의 수명 특성을 통해 나타났다. 폴리도파민과 폴리설파이드의 상호작용에 대한 근본적인 메커니즘은 분광 분석을 통해 추후에 더 심도 있게 분석할 것이다. 나아가, 폴리도파민과 같이 폴리설파이드의 용출을 제어할 수 있는 비슷한 환원제를 찾는 것이 리튬-황 전지의 고성능화를 위한 중요한 열쇠가 될 것이다.