UC San Diego

Rechargeable lithium-ion batteries (LIBs) have become the frontrunner of energy storage in the 21st century. Transitioning toward more sustainable materials and manufacturing methods will be critical to continue supporting the rapidly expanding market for LIBs. Meanwhile, different energy storage applications are also demanding higher power and energy densities than ever before, with aggressive performance targets like fast charging and greatly extended operating ranges and durations. Due to its high operating voltage and cobalt-free chemistry, the spinel-type LiNi0.5Mn1.5O4 (LNMO) cathode material has attracted great interest as one of the few next-generation candidates capable of addressing this combination of challenges. However, severe capacity degradation and poor interphase stability have thus far impeded the practical application of LNMO. In this thesis study, by combining dry electrode fabrication method and a fluorinated electrolyte formulation, LNMO with stable full cell operation (up to 68% after 1000 cycles under 25 ℃ and up to 70% after 100 cycles under 55 ℃), and ultra-high loading (up to 9.5 mAh/cm2 in half cells) are demonstrated. Firstly, dry-coated LNMO electrode was successfully fabricated using a method without conventional toxic N-Methylpyrrolidone (NMP) solvent. Proven by 90° peel-off test and tensile test, it was found that the dry-LNMO electrodes exhibited more robust mechanical properties compared to conventional slurry-based electrodes. Secondly, plasma focused ion beam scanning electron microscopy (PFIB-SEM) and 2-D modeling on dry electrodes’ cross-sectional surface show that conductive carbon was homogeneously distributed within large electrode volume, hence providing well-established electron transfer pathway. Moreover, to understand dry electrodes’ excellent cell performance more in-depth, interphase property studies are provided. Reduced electrolyte decomposition due to less active sites from the conductive carbon is identified as the key factor to enable the more conformal interphases in both cathode and anode surfaces, which were captured by high resolution transmission electron microscopy (HRTEM). Lastly, fluorinated electrolyte containing fluoroethylene carbonate (FEC) and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) is studied as a novel electrolyte in preventing LNMO’s subsurface phase transformation from hydrofluoric acid (HF) corrosion induced and accelerated by deprotonation at elevated temperature. FEC’s improved oxidation stability such as reduced deprotonation on fully charged LNMO surface was confirmed by first-principles calculations based on density functional theory (DFT). A formation of Mn3O4 phase preventing Li ion transfer from the LNMO’s bulk region was identified by HRTEM. Due to less HF corrosion on both cathode-electrolyte interphase (CEI) and SEI, a reduced amount of transition metal dissolution and redeposition has been proved by EDX and ICP. The prevention of cell crosstalk thereby mitigates the capacity decay in LNMO/graphite full cells under 55 ℃.