ELECTRODE AND ELECTROLYTE DESIGN FOR HIGH-ENERGY-DENSITY LITHIUM BATTERIES

Abstract

The continuous pursuit of higher energy density in lithium batteries has driven intensive research into advanced electrode and electrolyte systems. To satisfy the stringent requirements of electric vehicles and large-scale energy storage, next-generation lithium batteries must simultaneously achieve high capacity, long cycle life, and reliable safety under increasingly harsh operating conditions. Over the past decade, significant progress has been made in the development of high-nickel layered oxides, lithium-rich cathodes, and advanced anode materials such as silicon and lithium metal. However, realizing both high energy density and long-term interfacial stability remains a major challenge, as structural degradation, oxygen evolution, and electrolyte decomposition become more severe at high operating voltages. Electrolyte engineering plays a central role in addressing these interfacial issues. Conventional carbonate-based electrolytes, although widely adopted, often exhibit insufficient stability when paired with high-voltage cathodes or lithium-metal anodes. Consequently, recent efforts have focused on designing novel electrolyte systems with wider electrochemical stability windows and improved interfacial compatibility. Strategies including localized high-concentration electrolytes, fluorinated solvents, functional additives, and solid or gel-state electrolytes have demonstrated considerable potential in stabilizing electrode–electrolyte interfaces and suppressing parasitic side reactions. Looking forward, the development of next-generation high-energy cathode materials—such as cobalt-free and high-voltage layered oxides, lithium-rich disordered rock-salt structures, and high-capacity conversion-type compounds—represents a critical frontier for advancing lithium battery technology. In parallel, rational electrolyte design tailored to regulate interfacial reactions and preserve structural integrity under extreme conditions will be indispensable for fully unlocking the performance of these emerging cathodes. Ultimately, the synergistic optimization of both electrode and electrolyte systems is key to realizing practical, safe, and sustainable high-energy-density lithium batteries. The design and performance of various cathode and electrolyte systems are discussed in detail in Chapters 2 to 4.Chapter 2 investigates the electrochemical performance and stability of a novel electrolyte system based on Lithium cyano(trifluoromethanesulfonyl)imide (LiCTFSI) salt. Commercial LiPF6-based carbonate electrolytes suffer from limited oxidation stability (~4.2 V) and poor moisture tolerance (~10 ppm). Although Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) offers improved resistance to moisture, it causes corrosion of the aluminium current collector at potentials above 3.7 V vs. Li/Li+. To overcome these drawbacks, a non-corrosive and moisture-tolerant LiCTFSI salt was introduced in a 2.0 M LiCTFSI/PC–FEC (7:3 v/v) electrolyte formulation. This electrolyte enables NCM811 cathodes to deliver a high specific capacity of 210 mAh g-1 over a voltage window of 2.8-4.6 V for 500 cycles, demonstrating excellent capacity retention and interfacial stability. When paired with a graphite anode, full cells exhibit a stable cycling performance over 500 cycles with a capacity retention of 77.8% at room temperature. Remarkably, even under harsh conditions with 2000 ppm moisture in the electrolyte, the cells maintain high cycling stability, highlighting the superior water tolerance of LiCTFSI and its potential to reduce the cost associated with dry-room processing. Moreover, the low freezing point of the solvent system and the high thermal stability of LiCTFSI allow NCM811||graphite full cells (2.0 mAh cm-2) to operate effectively over a wide temperature range from -20 °C to 60 °C. The cells deliver 168 mAh g-1 at 0.1 C and retain 92.1% of their capacity after 200 cycles at -20 °C, as well as 94% retention after 100 cycles at 60 °C. In contrast, cells using conventional LiPF6 electrolytes exhibit only 71 mAh g-1 at -20 °C and retain 52.7% of capacity after 100 cycles at 60 °C. These results demonstrate that LiCTFSI represents a cost-effective and robust alternative to conventional lithium salts, offering superior electrochemical performance, enhanced moisture tolerance, and excellent temperature adaptability. This chapter therefore establishes a promising pathway for developing high-performance lithium-ion batteries capable of reliable operation under extreme environmental conditions. Chapter 3 focuses on Li-S batteries, which provide exceptionally high theoretical energy density and economic advantages due to the natural abundance of sulfur. Despite notable progress in enhancing sulfur conductivity and alleviating volume expansion, conventional Li-S systems continue to suffer from intrinsic challenges, including the low redox potential of the S2-/S0 conversion and the severe polysulfide shuttle effect. To address these limitations, a high-valence sulfur chemistry was explored through the construction of an S-Cl redox system. The incorporation of chlorine facilitates the formation of higher-valence sulfur species, thereby increasing the redox potential and alleviating performance constraints imposed by traditional sulfur chemistry. This S-Cl system demonstrates the potential to simultaneously enhance energy density and cycling stability, offering a novel strategy for the practical realization of high-performance Li-S batteries. Chapter 4 extends this investigation from the S-Cl system to fluorine-based electrochemistry, recognizing fluorine as an ideal candidate for next-generation high-energy rechargeable batteries due to its extremely high electronegativity and strong bonding characteristics. Conventional transition-metal fluorides, such as CuF2 and FeF3, have been widely explored as cathode materials because of their high theoretical voltages; however, they suffer from poor reversibility resulting from the irreversible formation of LiF during discharge. To overcome this challenge, the thermodynamic feasibility of mediator-assisted LiF dissociation reactions was systematically analysed by calculating the Gibbs free energy changes associated with fluorides formed by elements across the first to fifth periods of the periodic table. The results reveal a general increase in reaction voltage from left to right across the periodic table, corresponding to a decrease in metallic character and an increase in fluorine affinity. These findings suggest that nonmetal fluorides may possess more favourable thermodynamic and electrochemical properties than conventional transition-metal fluorides. This work highlights the promise of rational fluorine–mediator chemistry as an effective strategy to achieve reversible fluorine redox reactions and enable the development of high-energy-density rechargeable batteries.