High Energy Materials for Li-Ion Batteries and Beyond

Recently, renewable energy (solar, wind, etc) and electric vehicles (EV) are becoming more competitive with conventional energy technologies and vehicles that rely on fossil fuels (oil, gas, and coal). However, to enable these clean energy technologies, there is an urgent need to develop new generation energy storage systems. Among all currently available energy storage technologies, lithium-ion batteries (LIB) possess the highest gravimetric and volumetric energy density. Hence, lithium-ion batteries have become the dominant energy storage technology especially for powering portable electronic devices. LIBs have also been considered as energy storage source for emerging applications in the transportation systems, including electrified automobiles and smart grids. Unlike mobile applications, the emerging application for automobiles and grids required substantially higher requirements on various aspects of lithium-ion batteries; these requirements include significantly higher energy density, longer cycle/calendar life, better safety characteristics, and lower cost. Major R&D activities are undergoing to tackle the above technological barriers for state-of-the-art lithium-ion technology to meet the demanding requirements for transportation and grid applications. State-of-the-art lithium-ion batteries cannot satisfy the increasing energy demand worldwide because of the low specific capacity of the graphite anode. Silicon and phosphorus both show much higher specific capacity; however, their practical use is significantly hindered by their large volume changes during charge/discharge. Although significant efforts have been made to improve their cycle life, the initial coulombic efficiencies of the reported Si-based and P-based anodes are still unsatisfactory (<90%). In chapter 2, by using a scalable high-energy ball milling approach, we report a practical hierarchical micro/nanostructured P-based anode material for high-energy lithium-ion batteries, which possesses a high initial coulombic efficiency of 91% and high specific capacity of ~ 2500 mAh g-1 together with long cycle life and fast charging capability. In situ high-energy X-ray diffraction and in situ single-particle charging/discharging were used to understand its superior lithium storage performance. Moreover, proof-of-concept full-cell lithium-ion batteries using such an anode and a LiNi0.6Co0.2Mn0.2O2 cathode were assembled to show their practical use. The findings presented in this chapter can serve as a good guideline for the future design of high-performance anode materials for lithium-ion batteries. Sustaining a sound structure in Si-based anodes is extremely challenging because of the high volumetric expansion that occurs upon cycling. To maintain high capacities over many cycles, new designs rely on engineering-specific hierarchical geometries and/or optimized composite compositions such that at least one of the multiple elements serves as a buffer and/or electron conductive pathway in the electrodes. In chapter 3, we report an innovative design in which alternate layers of atomic structures involving multiple elements form a new anode material for lithium-ion batteries. A superlattice-structured film containing Si, Mo, and Cu is fabricated by a simple and scalable magnetron sputtering process for the first time. With the help of the formation of a continuous and repetitive superlattice along the film thickness, a homogeneous stress-strain distribution is attained. In the superlattice thin film, the Si atoms are distributed along the film thickness within the alternate Mo-Cu layers, which act as inactive-conductive layers and as a backbone web to handle the volume expansion of active Si while restricting electrochemical agglomeration. This nano-functional superlattice approach enables harnessing the high energy density of Si while maintaining its structural stability. As a result, the electrode exhibits high energy density and capacity retention even at high cycling rates. The possible use of the film in a full cell is also evaluated using LiMn1.5Ni0.5O4 cathodes. The full cell maintained a stable capacity of about 900 mAh ganode-1 (~93 mA cathode-1) over 150 cycles at the ~600 mA g-1 rate. The remarkable performance of this nanostructured, multifunctional superlattice film in anodes is found to be promising for applications that require high energy, long calendar life, and excellent abuse tolerance, such as electric vehicle batteries. Lithium-sulfur batteries are also attractive for automobile and grid applications due to their high theoretical energy density and the abundance of sulfur. Despite the significant progress in cathode development, lithium metal degradation and polysulfide shuttle remain two critical challenges in the practical application of Li-S batteries. The development of advanced electrolytes has become a promising strategy to simultaneously suppress lithium dendrite formation and prevent polysulfide dissolution. In chapter 4, we report on a new class of concentrated siloxane-based electrolytes, demonstrating significantly improved performance over the widely investigated ether-based electrolytes in terms of stabilizing the sulfur cathode and Li metal anode as well as minimizing flammability. Through a combination of experimental and computational investigation, we found that siloxane solvents can effectively regulate a hidden solvation-ion-exchange process in the concentrated electrolytes that results from the interactions between cations/anions (e.g., Li+, TFSI-, and S2-) and solvents. As a result, it could invoke a quasi-solid-solid lithiation and enable reversible Li plating/stripping and robust solid-electrolyte interphase chemistries. The solvation-ion-exchange process in the concentrated electrolytes is a key factor in understanding and designing electrolytes for other high-energy lithium metal batteries. Despite the tremendous research and development in the field of Li ion battery to increase the energy density. It is still not sufficient to meet the demand of new markets such as pure electric vehicles (EV). Therefore, there is a need to pursue new electrochemical systems with higher energy densities; Li‐air (Li-O2) batteries are considered one of the promising candidates. However, the practical application for such batteries is still challenging. The discharge and charge mechanisms of the rechargeable Li-O2 batteries have been recently under extensive investigation. However, they are not yet fully understood. In chapter 5, a systematic study of the morphology transition of the Li2O2 from a single crystalline to a toroid like particle during the discharge-charge cycle is reported in addition to theoretical modeling to explain the evolution of the Li2O2 at different stages of this process. The model suggests that the transition starts in the first monolayer of Li2O2, which followed by a transformation of particle growth to film growth if the applied current exceeds the exchange current for the oxygen reduction reaction in a Li-O2 cell. Furthermore, a sustainable mass transport of diffusive active species (e.g. O2 and Li+) and the underlying evolving interfaces is critical to dictate a desirable oxygen reduction (discharge) and evolution (charge) reactions in a porous carbon electrode of Li-O2 cell. Other issues related to nonaqueous lithium-oxygen batteries such as the function of the electrolytes and the oxidation of the lithium oxides during the charging progress are still not understood. In chapter 6, we propose a detailed study to investigate the charge mechanism of oxide materials in different electrolyte systems. Commercially available lithium peroxide and lithium oxide have been employed as cathodes to determine how the lithium oxides (both lithium oxide and lithium peroxide) and electrolyte change during the charge process. The result showed that the Li2O2 decomposed to lithium and oxygen and the electrolyte has a significant influence during the charge process. Furthermore, while most of the Li2O is found to participate in the side reactions with the electrolyte, some of it is found to delithiate and crumble in structure.