For over a decade, lithium-ion batteries (LIBs) have been the preferred source of electric power for mobile devices such as digital cameras, smart phones, and laptop computers. This is owing to their high energy density per volume (volumetric) and weight (gravimetric). However, the depletion of petroleum resources coupled with regulations to reduce environmental pollution has created a new demand for large-scale LIBs for electrical vehicles (EVs) and for sustainable energy storage systems (ESSs) to store energy from renewable sources (water, sun, biomass, geothermal, and hydrogen). These large-scale LIBs should ideally exhibit a variety of reversible capacities, cyclabilities, and power capabilities in order to meet this demand.
Graphitic carbon has been widely used as a commercial anode material because of its high coulombic effi-ciency and superior cyclability. However, owing to its relatively low theoretical capacity (372 mAh $g^{-1}$), significant efforts have been dedicated to finding high-capacity anode materials that exhibit high efficiencies and long-term stability. Among the various candidate materials, Si and Li metal have been investigated extensively because of those extremely high theoretical capacities (Si: 4200 mAh $g^{-1}$, Li: 3860 mAh $g^{-1}$).
The practical use of Si-based anodes is hindered by the fact that the Si particles in the anodes undergo pulverization accompanied by large volume changes (up to 300%) during cycling, resulting in electrically isolated and dead Si anodes. This is accompanied by the continuous growth of a solid electrolyte interphase (SEI) layer on the newly exposed surfaces of the Si anodes owing to electrolyte decomposition. Various nanosized or nanostructured Si materials, whose electrochemical performances are significantly better than those of micron-sized Si materials, have been proposed to address this problem. In particular, nanoscaled Si structures such as nanowires, hollow nanoparticles and nanotubes can efficiently accommodate the large strains and stresses that result from the Li-Si alloying reaction. Another alternative material, silicon monoxide (SiO), has emerged because SiO can alleviate the aforementioned fading mechanisms in Si anode operations. In detail, lithium oxide ($Li_{2}O$) is generated during the first lithiation (SiO + 2Li^+ + 2e^- → Li_{2}O + Si) thus creating a microstructure in the active material such that Si nano-domains are embedded in the $Li_{2}O$ matrix. As a result, the $Li_{2}O$ layers can act as buffer zones and thus suppress the side effects originating from the volume changes of Si. Nevertheless, the moderate electrical conductivity of SiO requires certain integration with conductive materials, and carbon-coating has been the simplest approach to address the conductivity issue.
In the case of Li metal anode, it has some problems caused by the dendrites formed on the surface of Li electrode during the deposition of Li ion, which can result in internal short-circuit of a cell. Apart from the safety issue, Li electrode is known to suffer from poor cycling performance caused by the corrosion and passivation. To solve these problems, various approaches have been reported to improve the cycling performance of Li electrode. For instance, the surface of Li electrode was covered by inorganic coating or polymer protective layers to prevent direct contact between Li and liquid electrolyte.
First of all, we investigated Si based anode materials, including nitrogen-doped carbon coated SiO (NC-SiO) and mesoporous Si nanofibers (m-SiNFs). N-doping increases the specific capacity significantly compared to that based on the bare carbon-coated analogue by supporting electronic conductivity and thus enabling a larg-er portion of Si domains to participate in the reactions with Li ions. In the case of the prepared m-SiNFs have a unique structure: the primary Si nanoparticles interconnect and form secondary 3D mesoporous nanofibers. While the nanosized primary particles lead to rapid electronic and ionic diffusion, the structure of the second-ary nanofibers (a few micrometer in length) permits the uniform distribution of nanoparticles, thus allowing for the easy fabrication of electrodes; these issues have been the main barriers to the commercialization of LIBs based on nanostructured Si materials.
In addition, we studied effective fabrication of lithium meal anode coated with inorganic/organic compo-site protective layer (CPL) including inorganic Al2O3 particles and PVdF-HFP as polymeric binder via simple casting method. Lithium-oxygen ($Li-O_2$) batteries are of great interest because of their very high-energy density; however, they present many challenges, one of which is the low cycling stability of Li metal anode. We pro-posed CPL for the Li metal anode that resulted in a dramatic enhancement of the cycling stability of the $Li-O_2$ cells. A cell with the CPL-coated Li metal anode exhibited more than 3 times higher discharge capacity at the 80^{th} cycle compared to a cell without the CPL. The CPL effectively suppressed electrolyte decomposition at the surface of the Li metal anode.
기존 리튬이차전지의 에너지 밀도의 한계성을 극복하기 위해, 고용량의 음극 소재 (실리콘계 음극: 4200 mAh $g^{-1}$, 리튬전극: 3860 mAh $g^{-1}$) 개발의 필요성이 대두되고 있다. 본 연구의 목적은 실리콘 기반의 고용량 음극재의 구조제어 및 표면개질을 통해 리튬이차전지의 성능을 향상시키는 것에 있다. 한편, 기존 리튬이차전지보다 이론적 에너지 밀도가 10배 큰 차세대 전지시스템인 리튬-산소 이차전지의 성능향상에 관한 연구도 진행하였다. 특히, 이 시스템의 음극으로 사용되는 리튬금속의 안정화를 통해 리튬-산소 이차전지의 성능 확보를 위한 연구를 진행하였다.
본 연구에서는 첫째, 실리콘계 음극소재 중 하나인 실리콘 모노옥사이드 입자에 이온성 액체의 코팅 및 탄화를 통해 전기전도성이 우수한 질소도핑된 탄소를 코팅하였다. 이를 통해 얻어진 질소도핑된 탄소 코팅을 포함하는 실리콘계 음극은 우수한 전기화학적 특성 (고출력특성 및 장기사이클 안정화)을 나타내었다. 이러한 질소 도핑된 탄소 코팅법은 전기전도도가 낮은 다른 전극 소재로의 응용가능한 접근법으로 판단된다.
또한, 실리카 나노섬유와 마그네슘을 이용한 환원반응을 통해 다공성 구조의 실리콘 나노섬유를 제조해 이를 고출력, 고성능을 특징으로 하는 리튬이차전지 음극 소재 특성 연구를 진행하였다. 제조된 다공성 실리콘 나노섬유 음극은 수 나노사이즈의 작은 입자가 서로 연결되어 이차구조를 가지고 있으며, 이러한 구조적 장점은 사이클 구동 중에 발생되는 큰 부피변화로 인한 열화 현상을 완화시켜줄 뿐 아니라, 다공성 구조로 인해 리튬이온의 접근성을 용이하게 해줌으로써 음극으로 적용시 고출력 특성 확보를 가능하게 하였다.
마지막으로 차세대 리튬이차전지 시스템(리튬-산소 이차전지)의 핵심 소재인 리튬금속 음극에 유무기 복합보호막을 솔루션 코팅법을 통해 손쉬운 방법으로 도입하였다. 리튬전극의 우수한 사이클 성능을 확보하였다. 유무기 복합보호막이 코팅된 리튬 전극을 음극으로 적용한 리튬 산소 이차전지는 우수한 사이클 특성을 보여주었으며, 표면 분석 및 임피던스 분석을 통해 전해질의 분해를 최소화되는 효과로 인한 것임을 확인하였다. 이러한 리튬 보호막은 리튬전극을 음극으로 사용하는 리튬메탈 이차전지시스템 (리튬-황 전지, 리튬금속 폴리머전지 등)으로의 응용범위를 넓힐 수 있을 것으로 기대된다.
