• Korean Chemical Engineering Research
• /
• 제55권4호
• /
• pp.483-489
• /
• 2017

The loss of the sulfur cathode material through dissolution of the polysulfide into electrolyte causes a significant capacity reduction of the lithium-sulfur cell during the charge-discharge reaction, thereby debilitating the electrochemical performance of the cell. We addressed this problem by using a chemical and physical approach called reduction of polysulfide dissolution through direct coating functional inorganic (graphene oxide) or organic layer (polyethylene oxide) on electrode, since the deposition of external functional layer can chemically interact with polysulfide and physically prevent the leakage of lithium polysulfide out of the electrode. Through this approach, we obtained a composite electrode for a lithium-sulfur battery (sulfur: 60%) coated with uniform and thin external functional layers where the thin external layer was coated on the electrode by solution coating and drying by a subsequent heat treatment at low temperature (${\sim}80^{\circ}C$). The external functional layer, such as inorganic or organic layer, not only alleviates the dissolution of the polysulfide electrolyte during the charging/discharging through physical layer formation, but also makes a chemical interaction between the polysulfide and the functional layer. As-formed lithium-sulfur battery exhibits stable cycling electrochemical performance during charging and discharging at a reversible capacity of 700~1187 mAh/g at 0.1 C (1 C = 1675 mA/g) for 30 cycles or more.

• 한국수소및신에너지학회논문집
• /
• 제13권4호
• /
• pp.259-269
• /
• 2002

A battery based on the lithium/elemental sulfur redox couple has the advantage of high theoretical specific capacity of 1,675 mAh/g-sulfur. However, Li/S battery has bad cyclic durability at room temperature due to sulfur active material loss resulting from lithium polysulfide dissolution. To improve the cycle life of Li/S battery, PEGDME (Poly(ethylene glycol) dimethyl ether) 500 containing 1M LiTFSI salt which has high viscosity was used as electrolyte to retard the polysulfide dissolution and nano-sized $Mg_{0.6}Ni_{0.4}O$ was added to sulfur cathode as additive to adsorb soluble polysulfide within sulfur cathode. From experimental results, the improvement of the capacity and cycle life of Li/S battery was observed( maximum discharge capacity : 1,185 mAh/g-sulfur, C50/C1 = 85 % ). Through the charge-discharge test, we knew that PEGDME 500 played a role of preventing incomplete charge-discharge $behavior^{1,2)$. And then, in sulfur dissolution analysis and rate capability test, we first confirmed that nano-sized $Mg_{0.6}Ni_{0.4}O$ had polysulfide adsorbing effect and catalytic effect of promoting the Li/S redox reaction. In addition, from BET surface area analysis, we also verified that it played the part of increasing the porosity of sulfur cathode.

• Korean Chemical Engineering Research
• /
• 제60권2호
• /
• pp.184-192
• /
• 2022

수열합성을 통해 합성한 다공성 NiO는 리튬 폴리설파이드의 용출을 억제하기 위하여 리튬-황 전지의 전극에 사용되었다. 리튬-황 전지의 전극은 경제적이고 간단한 진공 여과 방법을 이용하여 집전체와 바인더가 없는 프리스탠딩 플렉서블 전극으로 제작되었다. 다공성 NiO를 첨가한 S/CNT/NiO 전극은 순수 S/CNT 전극에 비해 125 mA h g^-1 증가한 877 mA h g^-1 (0.2 C)의 초기 방전용량과 200 사이클 후 84% (S/CNT: 66%)의 우수한 용량 유지율을 나타내었다. 이는 방전 과정 중에서 NiO와 리튬 폴리설파이드의 강한 화학적 결합에 의하여 리튬 폴리설파이드의 전해질로 용출되는 것을 억제하여 나타난 결과이다. 또한 S/CNT/NiO 전극의 유연성 테스트를 위해 1.6 × 4 cm²의 파우치셀로 제작하여 폴딩한 상태와 하지 않은 상태에서 모두 620 mA h g^-1의 안정적인 사이클 특성을 나타내었다.

• 청정기술
• /
• 제28권2호
• /
• pp.97-102
• /
• 2022

Lithium sulfur (Li-S) batteries have attracted considerable attention as a promising candidate for next-generation power sources due to their high theoretical energy density, low cost, and eco-friendliness. However, the poor electrical conductivity of sulfur and its insoluble discharging products (Li₂S₂/Li₂S), large volume changes, severe self-discharge, and dissolution of lithium polysulfide intermediates result in rapid capacity fading, low Coulombic efficiency, and safety risks, hindering Li-S battery commercial development. In this study, a three-dimensionally (3D) connected hierarchical porous carbon framework (HPCF) derived from waste sunflower seed shells was synthesized as a sulfur host for Li-S batteries via a chemical activation method. The natural 3D connected structure of the HPCF, originating from the raw material, can effectively enhance the conductivity and accessibility of the electrolyte, accelerating the Li⁺/electron transfer. Additionally, the generated micropores of the HPCF, originated from the chemical activation process, can prevent polysulfide dissolution due to the limited space, thereby improving the electrochemical performance and cycling stability. The HPCF/S cell shows a superior capacity retention of 540 mA h g^-1 after 70 cycles at 0.1 C, and an excellent cycling stability at 2 C for 700 cycles. This study provides a potential biomass-derived material for low-cost long-life Li-S batteries.

• Carbon letters
• /
• 제19권
• /
• pp.40-46
• /
• 2016

Nitrogen-atom doped graphene oxide was considered to prevent the dissolution of polysulfide and to guarantee the enhanced redox reaction of sulfur for good cycle performance of lithium sulfur cells. In this study, we used urea as a nitrogen source due to its low cost and easy preparation. To find the optimum urea content, we tested three different ratios of urea to graphene oxide. The morphology of the composites was examined by field emission scanning electron microscope. Functional groups and bonding characterization were measured by X-ray photoelectron spectroscopy. Electrochemical properties were characterized by cyclic voltammetry in an organic electrolyte solution. Compared with thermally reduced graphene/sulfur (S) composite, nitrogen-doped graphene/S composites showed higher electroactivity and more stable capacity retention.

• Korean Chemical Engineering Research
• /
• 제61권3호
• /
• pp.426-438
• /
• 2023

차세대 전지로 주목받는 리튬-황 전지는 높은 에너지 밀도를 갖는 반면, 황의 절연 특성, 셔틀 현상 그리고 부피팽창으로 인하여 상용화에 어려움이 있다. 본 연구에서는 경제적이고 간단한 진공여과 방법으로 바인더와 집전체가 없는 프리스탠딩 전극을 제조하였고 탄소나노튜브(CNT)를 황의 전기전도도 향상을 위하여 사용하였다. 여기서 CNT는 집전체와 도전재 역할을 동시에 수행하였다. 추가로 리튬폴리설파이드의 흡착에 용이한 금속산화물(MO_x, M=Ni, Mg)을 CNT/S 전극에 첨가함으로써 리튬-황 전지의 셔틀반응을 억제하였다. MO_x@CNT/S 전극은 금속산화물을 도입하지 않은 CNT/S 전극에 비해 높은 용량 특성과 사이클 안정성을 나타내었으며, 이는 금속산화물의 우수한 리튬폴리설파이드 흡착 특성으로 인하여 황 활물질의 손실을 억제한 결과이다. MO_x@CNT/S 전극 중에서 NiO를 도입한 NiO@CNT/S 전극은 1 C에서 780 mAh g^-1의 높은 방전용량을 나타내었고 200 사이클 후 134 mAh g^-1으로 극심한 용량 감소가 나타났다. MgO@CNT/S 전극은 비록 초기 사이클에 544 mAh g^-1의 낮은 방전용량을 나타냈지만, 200 사이클까지 용량을 90% 유지하는 우수한 사이클 안정성을 나타내었다. 고용량과 사이클 안정성 확보를 위하여 Ni:Mg를 0.7:0.3의 비율로 혼합한 Ni_0.7Mg_0.3O@CNT/S 전극은 755 mAh g^-1 (1 C)의 초기 방전용량과 200 사이클 후에도 90% 이상의 용량 유지율을 나타내었다. 따라서 이원 금속산화물의 CNT/S 프리스탠딩으로의 적용은 고용량 특성뿐만 아니라 가장 큰 문제인 리튬폴리설파이드의 용출을 효과적으로 개선하여 경제적이고 고성능 리튬-황 전지의 개발이 가능함을 시사한다.

• Journal of Electrochemical Science and Technology
• /
• 제14권4호
• /
• pp.361-368
• /
• 2023

In lithium-sulfur (Li-S) batteries, encapsulation of sulfur in activated carbon (AC) materials is a promising strategy for preventing the dissolution of lithium polysulfide into electrolytes and enhancing cycle life, because instead of solid-liquid-solid reactions, quasi-solid-state (QSS) reactions occur in the AC micropores. While a high weight fraction of sulfur in S/AC composites is essential for achieving a high energy density of Li-S cells, the deterioration mechanisms under such conditions are still unclear. In this study, we report the deterioration mechanisms during charge-discharge cycling when the discharge products overflow from the AC. Analysis using scanning electron microscopy and energy-dispersive X-ray spectrometry confirms that the sulfur in the S/AC composites migrates outside the AC as cycling progresses, and it is barely present in the AC after 20 cycles, which corresponds to the capacity decay of the cell. Impedance analysis clearly shows that the electrical resistance of the S/AC composite and the charge-transfer resistance of QSS reactions significantly increase as a result of sulfur migration. On the other hand, the charge-discharge cycling performance under limited-capacity conditions, where the discharge products are encapsulated inside the AC, is extremely stable. These results reveal the degradation mechanism of a Li-S cell with micro-porous carbon and provide crucial insights into the design of a S/AC composite cathode and its operating conditions needed to achieve stable cycling performance.