UC Berkeley

Lithium-ion batteries (LIBs) have become integral to modern life, powering a wide range of applications, from consumer electronics to electric vehicles. As their usage expands, so does the demand for reliable performance across diverse operating conditions, including low temperatures. However, LIBs exhibit significant performance degradation at sub-zero temperatures, posing a barrier to their widespread adoption in colder climates. Addressing this challenge, the U.S. Department of Energy (DoE) has set a target of retaining 70% of room temperature capacity during a three-hour discharge under low-temperature conditions.This thesis investigates the factors limiting low-temperature performance in LIBs and leverages operando electrochemical impedance spectroscopy (EIS) to quantify their contributions. It highlights the utility of EIS in diagnosing various performance bottlenecks, while addressing the limitations of traditional equivalent circuit modeling in capturing the complexities of low-temperature behavior. Instead, distribution of relaxation times (DRT) analysis is employed to accurately deconvolute constituent resistances, providing deeper insights into interfacial and transport phenomena. A novel methodology is introduced to deconvolute charge transfer resistance on both graphite and NMC sides from standard two-electrode full coin cells, significantly reducing experimental time. Through a novel combination of EIS and overpotential analysis, this work demonstrates that charge transfer resistance, traditionally attributed to desolvation energy, is also significantly influenced by lithium diffusion within NMC cathodes. High transport overpotentials at the end of discharge were identified as the primary cause of poor low-temperature performance, stemming from diffusion limitations in NMC particles. These findings establish a critical link between kinetic and transport limitations, providing new insights into the interplay between charge transfer resistance and electrode diffusion. Additionally, this work explores strategies to mitigate charge transfer resistance and enhance low-temperature performance through electrolyte design. Using a baseline electrolyte of 1.0M LiPF6 in a 3:7 EC:EMC mixture, I systematically varied salt concentration, solvent composition, and incorporated additives to optimize performance. Salt concentration studies revealed the interplay between ionic conductivity, concentration gradients, and solvation/desolvation energies in determining low-temperature behavior. Moreover, through DEMS analysis of SEI composition, a clear relationship between SEI structure and charge transfer resistance emerged, highlighting its critical role in low-temperature performance. This finding motivated the exploration of additives, including a range of FEC concentrations (1-10%) in the baseline electrolyte, with 4% FEC yielding the best performance. The underlying reasons were examined through dQdV analysis and SEI characterization. Similarly, the inclusion of 0.75 wt% fumed silica nanoparticles (SiNPs) demonstrated remarkable improvements, achieving 82% capacity retention at low temperatures and significantly reducing charge transfer resistance. These findings further reinforce the critical role of SEI composition in influencing charge transfer resistance, underscoring its impact on low-temperature performance. The SEI composition analysis, conducted through salt concentration and additive studies, provides valuable insights that can guide the design of tailored interfaces for various applications. Finally, I explored the use of alternative solvents to develop EC-lean electrolytes. We focused on solvents with low melting points yet high viscosity and dielectric constants to reduce desolvation energy on the NMC side. Solvents such as γ-butyrolactone and ethyl acetate demonstrated potential in lowering desolvation energy; however, their incorporation into the SEI, as confirmed by dQdV analysis, introduced significant challenges. In contrast, acetonitrile (ACN) did not participate in SEI formation, allowing for the design of an SEI optimized specifically for low-temperature performance. These efforts culminated in the development of an electrolyte capable of delivering exceptional capacity retention under sub-zero conditions, marking a significant advancement in the application of LIBs in cold climates. Finally, I discuss how these two key aspects—overpotential analysis and electrolyte design—contribute to the current body of literature and their potential applications in the industry. Together, these methodologies not only advance our fundamental understanding of lithium-ion battery performance but also present practical solutions for industrial implementation.