A first-principles study of carbon-free two-dimensional cathode materials for Li-air batteries

Abstract

It is widely accepted that global warming is caused by human-induced emission of CO2 and that they must be substantially reduced in order to prevent further climate change. In this regard, it is well-recognized that electrification of transportation would reduce CO2 emissions by minimizing our consumption of fossil fuels. As a result, a transition to road electrification has accelerated. Hybrid electric vehicles are now common, plug-in hybrid electric vehicles are now beginning to sell appreciably, affordable electric vehicles with a limited driving range of nearly 150 kilometers are also selling at a modest pace. While the electric vehicles are commercialized, it is a common sight that there is a lack of marketability of the electric vehicles yet. The major issue confronting the popularization of electric vehicles is simply a battery problem, i.e., developing a safe, long-lived, quick charging and cost-effective battery with sufficient energy density to extend the driving range to cover most daily use. The lithium-oxygen battery is one of the most promising technologies, which would allow electric vehicles to have enough driving range to cover a daily use, due to its extremely high-energy density than conventional lithium-ion battery. However, the battery design still faces many challenges for practical use including the decomposition of cathodes, which are typically composed of carbon-based materials. In this thesis, carbon-free and two-dimensional cathode structures based on group IV elements and metal sulfides are proposed for lithium-oxygen batteries using density functional theory. This thesis explores two strategies to investigate the potential of carbon-free and two-dimensional materials for cathode structure in lithium-oxygen batteries: (i) a theoretical prediction of discharge-charge cycles efficiency, i.e. overpotentials, and (ii) an atomic-level understanding of initial stages of cathode reaction on cathode surface. First, the discharge/charge electrode potentials of monolayer group IV honeycomb structure, i.e. silicene, germanene, and stanene, were calculated along the reaction pathway for cathode in lithium-oxygen battery. In contrast to graphene, oxygen reduction reactions and oxygen evolution reactions can occur on the pristine form of the group IV honeycomb structure without any defect sites. In addition, it was found that reactions on the group IV honeycomb structure strongly correlate with strong adsorptions of the ORR intermediates, which are caused not only by ionic bonding between the oxygen atoms in the ORR intermediates and the group IV honeycomb structure but also by the structural stabilization of the group IV honeycomb structure. Theoretical observations demonstrate the great potential of group IV honeycomb structure, in particular germanene, which has the lowest total overpotentials among the group IV honeycomb structure, as a carbon-free cathode structure for lithium-oxygen batteries and provide further insights for designing a new cathode material architecture based on two-dimensional structured materials. The second part of this thesis investigated the proposed reaction pathway of discharge/charge reactions on two-dimensional metal sulfides for lithium-oxygen battery. Although the dissociative form of LiO2 is more stable than the associative form when the LiO2 adsorbs on SnS2 and MoS2, the discharge reaction proceeds through the associative pathways, resulting in 2e- pathway. This is because the reaction energy barriers for forming the dissociative form of LiO2 are considerably high, and the dissociative LiO2 rarely forms under a conventional battery conditions. It is worth noting that the reaction energy barriers for forming of dissociative LiO2 on cathode surface are very important factor to investigate the reaction pathway. In addition, the calculated charge potential of SnS2 along the 2e- pathway was <4 V, which implies that the SnS2 could also lead to alleviation of the decomposition problem of electrolytes in typical operating conditions of lithium-oxygen battery.|지구 온난화는 인간의 활동이 만들어낸 이산화탄소 배출이 가장 큰 원인이며, 더 이상의 기후 변화를 막기 위해서는 지속적인 배출 감소가 이루어져야 한다고 인식되어 왔다. 이와 관련하여, 교통수단의 ‘전기화(electrification)’를 통해 화석 연료의 소비 감소, 궁극적으로는 이산화탄소 배출을 줄이는 데에 주목하였으며, 이는 결과적으로 교통수단의 전기화를 가속시켰다. 하이브리드 전기차는 이미 주변에서 많이 찾아 볼 수 있으며, 플러그인-하이브리드 전기차 및 순수 전기차도 점차 판매량이 확대되고 있다. 이처럼 많은 노력 끝에 전기차가 상용화에는 성공하였지만, 아직까지는 시장성이 부족하다는 것이 일반적인 시각이다. 이는 전기차의 일상적인 사용에 있어서 필요한 주행거리를 충족시킬 만큼의 충분한 에너지 밀도와 안정성 수명 등을 갖춘 배터리를 합리적인 가격에 공급할 수 없기 때문이다. 리튬-공기 전지는 기존의 리튬-이온 전지에 비해 극히 높은 에너지 밀도를 가지고 있어, 전기차의 일상적인 사용을 커버하는 데 충분한 배터리 기술 중 하나로 주목 받고 있다. 그러나 리튬-공기 전지 또한 실용화 단계에 접어 들지 못하는 여러 문제점들이 남아 있는데, 그 중 하나가 탄소기반으로 되어 있는 양극재의 분해이다. 본 학위 논문에서는, 밀도범함수이론(density functional theory)을 이용하여 주기율표 상 4족에 해당하는 물질 또는 금속황화물을 기반으로 한 탄소를 포함하지 않고, 2차원으로 된 물질의 리튬-공기 전지 양극재로의 가능성에 대한 연구를 하였다. 이러한 가능성에 대한 연구는 다음과 같이 두 개의 핵심 전략을 통해 이루어졌다.

(1) 과전위(overpotential)와 같은 충,방전 효율에 대한 이론적인 예측

(2) 양극 표면에서의 반응 초기단계에 대한 원자단위에서의 이해

먼저, 주기율표 상 4족에 해당하는 물질의 2차원 형태인 실리신(silicene), 게르마닌(germanene), 스태닌(stanene) 단일층 물질에 대해 리튬-공기 전지의 양극 반응에 따라 충,방전 전위를 계산하였다. 이 물질들은 주로 결함 위치를 중심으로 산소환원반응 및 산소발생반응이 발생하는 그래핀(graphene)과는 달리, 결함이 없는 그대로의 형태에서도 위 반응들이 일어난다. 또한 이러한 결과는 반응 중간생성물이 물질 표면에 강하게 흡착되는 것과 관련이 있는데, 이는 단순히 표면과 중간생성물의 산소 원자가 강한 이온 결합을 하고 있을 뿐 아니라, 흡착이 되는 과정에서 물질 구조가 에너지적으로 더 안정한 구조로 변해가는 과정을 거치기 때문이다. 이러한 물질들이 리튬-공기 전지 양극재로서 가능성을 가지는지 이론적인 충,방전 전위 예측을 통해 확인하였으며, 특히 게르마닌의 경우 이들 중 가장 낮은 충,방전 과전위를 보여 효율 면에서 가장 우수한 특성을 보였다. 이 논문의 두 번째 부분에서는 2차원 금속황화물의 리튬-공기 전지 양극 반응에 대한 충,방전 메커니즘에 대해 연구를 하였다. SnS2와 MoS2의 경우, 방전반응의 첫 번째 중간 생성물인 LiO2가 산소 분자가 해리된 형태(dissociative form)로 흡착되는 것이 산소 분자가 결합된 형태(associative form) 보다 에너지적으로 안정함에도 불구하고 전체 반응은 결합된 반응경로(associative pathway)로, 즉 2e- 반응경로로 진행됨이 예측되었다. 이는 기존 연구결과들을 통해 보고된 금속 표면이나 그래핀의 경우와는 상이한 것으로, 반응 초기 단계에서 Li과 O2가 만나서 LiO2가 해리된 형태로 흡착되는데 넘어야 하는 반응에너지 장벽이 일반적인 전지구동 환경에서는 넘기 어려울 만큼 높기 때문이었다. 이는 기존 리튬-공기 양극 반응의 이론적 연구 결과들에서는 중요하게 다루지 않았던 LiO2 형성에 관한 에너지 장벽이 반응경로 예측에 중요한 요소가 된다는 것을 보인 결과이다. 또한 SnS2의 경우 충전 전위가 전해질의 분해가 활발히 일어나기 시작하는 4 V 보다 작은 값을 가질 것으로 예측 되는데, 이는 SnS2가 전해질의 분해 방지 측면에서도 유리할 수 있음을 뜻한다.; It is widely accepted that global warming is caused by human-induced emission of CO2 and that they must be substantially reduced in order to prevent further climate change. In this regard, it is well-recognized that electrification of transportation would reduce CO2 emissions by minimizing our consumption of fossil fuels. As a result, a transition to road electrification has accelerated. Hybrid electric vehicles are now common, plug-in hybrid electric vehicles are now beginning to sell appreciably, affordable electric vehicles with a limited driving range of nearly 150 kilometers are also selling at a modest pace. While the electric vehicles are commercialized, it is a common sight that there is a lack of marketability of the electric vehicles yet. The major issue confronting the popularization of electric vehicles is simply a battery problem, i.e., developing a safe, long-lived, quick charging and cost-effective battery with sufficient energy density to extend the driving range to cover most daily use. The lithium-oxygen battery is one of the most promising technologies, which would allow electric vehicles to have enough driving range to cover a daily use, due to its extremely high-energy density than conventional lithium-ion battery. However, the battery design still faces many challenges for practical use including the decomposition of cathodes, which are typically composed of carbon-based materials. In this thesis, carbon-free and two-dimensional cathode structures based on group IV elements and metal sulfides are proposed for lithium-oxygen batteries using density functional theory. This thesis explores two strategies to investigate the potential of carbon-free and two-dimensional materials for cathode structure in lithium-oxygen batteries: (i) a theoretical prediction of discharge-charge cycles efficiency, i.e. overpotentials, and (ii) an atomic-level understanding of initial stages of cathode reaction on cathode surface. First, the discharge/charge electrode potentials of monolayer group IV honeycomb structure, i.e. silicene, germanene, and stanene, were calculated along the reaction pathway for cathode in lithium-oxygen battery. In contrast to graphene, oxygen reduction reactions and oxygen evolution reactions can occur on the pristine form of the group IV honeycomb structure without any defect sites. In addition, it was found that reactions on the group IV honeycomb structure strongly correlate with strong adsorptions of the ORR intermediates, which are caused not only by ionic bonding between the oxygen atoms in the ORR intermediates and the group IV honeycomb structure but also by the structural stabilization of the group IV honeycomb structure. Theoretical observations demonstrate the great potential of group IV honeycomb structure, in particular germanene, which has the lowest total overpotentials among the group IV honeycomb structure, as a carbon-free cathode structure for lithium-oxygen batteries and provide further insights for designing a new cathode material architecture based on two-dimensional structured materials. The second part of this thesis investigated the proposed reaction pathway of discharge/charge reactions on two-dimensional metal sulfides for lithium-oxygen battery. Although the dissociative form of LiO2 is more stable than the associative form when the LiO2 adsorbs on SnS2 and MoS2, the discharge reaction proceeds through the associative pathways, resulting in 2e- pathway. This is because the reaction energy barriers for forming the dissociative form of LiO2 are considerably high, and the dissociative LiO2 rarely forms under a conventional battery conditions. It is worth noting that the reaction energy barriers for forming of dissociative LiO2 on cathode surface are very important factor to investigate the reaction pathway. In addition, the calculated charge potential of SnS2 along the 2e- pathway was <4 V, which implies that the SnS2 could also lead to alleviation of the decomposition problem of electrolytes in typical operating conditions of lithium-oxygen battery.