Mechanistic Understanding and Electrode Designs for Improved Reversibility: Case Study for Lithium Titanate Electrodes and Lithium-Oxygen Batteries

Abstract

With the increasing demand for green energy, environmental-friendly power sources, like geothermal, wind and solar start to replace the fossil fuel. Electric vehicle (EV) is a representative example of this trend. The proportion of the EVs is small at this moment, however, the growth rate of the number of the EVs is very rapid. In addition, because the green energy sources suffer from a large supply fluctuation, energy storage system (ESS) is required to buffer the energy production. Enormous potential market of the EVs and the ESSs will lead to the increased consumption of the medium- and large-scale batteries. Lithium-ion batteries (LIBs) is the best system to store the energy with a large capacity due to its high energy capacity and power density. However, two problematic issues still remain to be solved for the LIBs. First, accidental explosions and fires of the EVs and ESSs are appeared occasionally. Second, despite of the high performance of LIBs, the market demands batteries with the higher energy density. To fulfill the requirement for the consumers, new type of electrodes should be developed. Here, lithium titanate (Li4Ti5O12, LTO) anode material with a high performance is proposed for the safe and high-power LIBs and lithium-oxygen battery (LOB) is studied to develop the next-generation battery system with the high energy density. LTO anode is a replacement for the graphite anode of the LIBs. Spinel-framework structure of LTO induces a zero-strain lithium insertion/deinsertion and fast diffusion kinetics. Therefore, LTO exhibits a long lifespan and high power density. In addition, a high operating potential (1.55 V vs Li) and lacks of carbon material make the LTO battery safe. However, a low conductivity of LTO inhibits the operations at a high rate. Nanographene-surounded LTO hybridization structure is reported here for the high rate of lithium storage. The hybridization structure was arranged by the interfacial interactions from amphipathic solvent. The amphipathic solvent acted as a bridge between the hydrophilic LTO and the hydrophobic NG. The LTO-NG hybridization was synthesized via a redox coupling between adsorbed LTO and NG. The large contact area between the LTO core and the NG sheet resulted in a high electron-conducting path. It allowed the rapid kinetics for the lithium storage and also resulted in a cyclic performance stability. LOB is expected to overcome the energy density issue of the large-scale batteries for EVs and ESSs due to its potential high energy density. However, an irreversibility of charge/discharge reactions induces a low coulombic efficiency and lifespan. To improve the reversibility of the reactions, two type of methods were studied to control the parasitic reaction: thermodynamic approach and kinetic approach. For the thermodynamic approach, a platinum catalyst supported on zirconia is proposed as a cathode in LOBs. Superoxide and peroxide materials occurred during battery operation, decompose the electrolyte and the carbon electrode, resulting the low reversibility of LOBs. Experimental and theoretical studies show that zirconia suppresses the reactivity of these superoxide and peroxide materials. Therefore, it is able to enhance the reversibility and lifespan in LOBs. For the kinetic approach, the composition of the discharge product is controlled by the blockage of the reaction pathway. Stacking process of the discharge products on the electrode was investigated. And it is revealed that morphological structure of the cathode catalyst influences the composition of the discharge products. During discharge, LiO2-like species with a low overpotential is converted to Li2O2 with a high overpotential. The catalyst with large pore (> 100 nm) can inhibit the development of Li2O2, because the conversion reaction is suppressed with the large pore. Therefore, catalyst with large pore results in the low overpotential and reversible reaction. This observation provides a clue to understand the behavior and kinetics of the discharge product for LOBs.

녹색 에너지에 대한 수요와 관심의 증가에 따라 전지자동차와 에너지 그리드용 에너지 저장장치의 시장이 매우 커질 것으로 예상되며, 이는 곧 중·대형 배터리 시장의 확대로 이어질 것이다. 리튬이온배터리는 에너지와 출력 밀도가 높아 대용량 에너지를 저장하는 데에 가장 적합한 시스템이다. 그러나 중·대형 배터리로 사용되기 위해서는 안전 문제를 해결할 필요가 있으며, 더 높은 에너지 밀도를 확보하기 위한 방안이 필요하다. 본 논문에서는 안전한 리튬이온배터리를 위해 고성능의 리튬 티타네이트 (LTO) 양극재를 제안하고, 고에너지 밀도의 차세대 배터리 시스템으로 리튬산소전지를 연구하였다. 높은 안전성을 보이는 LTO의 단점인 낮은 전도성을 향상시키기 위하여 LTO-나노그래핀 혼성화 구조를 제안하였다. 양친매성 용매를 사용하여 친수성 LTO와 소수성 나노그래핀 사이에 가교를 이어줄 수 있었고, 산화환원쌍을 통하여 그래핀으로 뒤덮인 LTO를 합성하였다. 그 결과 만들어진 물질은 그래핀의 전도성으로 인하여 높은 출력 특성을 보였다. 리튬산소전지는 에너지 밀도가 높아, 한계에 다다른 리튬이온전지의 성능을 뛰어넘을 수 있을 것으로 예상된다. 그러나 충방전 반응의 가역성이 낮아 수명이 짧다는 문제점이 존재한다. 따라서 반응의 가역성을 개선하고 부반응을 억제하기 위하여 리튬산소전지의 반응기작에 대한 열역학적 접근법과 속도론적 접근법을 연구하였다. 열역학적 접근법에서는 백금을 지르코니아에 담지한 촉매를 사용하였다. 리튬산소전지의 충방전 과정 중, 친핵성이 높아 부반응을 일으키는 초과산화물과 과산화물이 생성되는데, 지르코니아 담지체가 두 물질과 결합하여 안정화시키고, 부반응을 억제시킨다는 것을 확인하였다. 운동학적 접근법에서는 방전 생성물의 조성을 변화시키는 방법을 연구하였다. 촉매의 구조적인 제어를 통하여 전해질의 확산을 억제하고 반응물 간의 접촉을 막아, 높은 과전압을 보이는 방전 생성물이 형성되는 속도를 감소시켰다. 그 결과 낮은 전압에서 충전이 진행되어 리튬산소전지의 수명이 증가하였다.