Improving the Energy Density of Lithium-Ion Batteries with a Dry-Processed Anode via the Synergistic Effects of the FEC-Derived SEI
Authors: Lee Juhyun , Han Seungmin , Lee Seungho , Yang Subi , Jeong Woojin , Roh Kwang Chul , Choi Junghyun 
Published: December 2024

Sustainable energy has been focused on by the whole world for the earth-friendly development toward the goal of net-zero. Vehicle electrification is one of the most efficient strategies. However, lithium-ion batteries (LiBs) which are the primary power source used in electric vehicles need to overcome the obstacles of current LiBs, especially their high cost and low energy density. Therefore, it is crucial to improve the energy of LIBs to make them a practical replacement for internal combustion engines. Commercial electrodes are typically fabricated via slurry casting process. This involves mixing the active material, conductive additives, and polymer binder in a solvent: water for the anode and N-methyl-2-pyrrolidone (NMP) for the cathode, to form a slurry. NMP as a solvent, which incurs carbon dioxide (CO2) emissions and leads to increased manufacturing costs due to high-temperature drying and solvent recovery processes. Recently, dry-processed electrodes have attracted attention as next generation method due to its cost reduction, eco-friendliness, and ability to realize extremely high mass loading electrode. Thick electrode design approaches can substantially increase the active material loading by decreasing the inactive component ratio at the cell level, thereby leading to increased battery energy density as well as decreased cost. In the fabrication of thick electrode through wet process, solvent evaporation can cause binder migration due to unhomogeneous distribution of binder during the drying of high mass loading slurry. Additionally, Li-ion diffusivity and electrolyte permeability are compromised due to the clogging of the electrode surface caused by migrated binder. On the other hand, dry process does not need a solvent for manafaturing electorde, removes the drying process required for solvent evaporation, thus reducing carbon emissions and processing costs. Especially, the fabrication of thick electrodes via dry process ensures uniform dispersion of active materials. A simple change in the electrode manufacturing process effectively overcome the aforementioned challenges in the slurry process and could provide numerous advantages to the LIBs system. The typically employed binder in dry-process polytetrafluoroethylene (PTFE) which has sticky properties under high stress to form PTFE fibrils. The obtained fibrils can act as a net to support active materials and conductive additives. The dry process innovates the routes of electrode fabrication form the traditional "powder-slurry-film" route to the simple "powder-film" route. However, the lowest unoccupied molecular orbital (LUMO) of PTFE has a high affinity for electrons , electrochemically unstable when used as binder for anodes. During the initial lithiation process, PTFE undergoes defluorination, resulting in the formation of lithium fluoride and amorphous carbon. This irreversible decomposition causes lower the initial coulombic efficiency (ICE) and diminishes the binding properties of PTFE, which significantly deteriorates long-term cycling performances. In this work, we examined fluoroethylene carbonate (FEC) as an electrolyte additive to explore how an FEC-derived solid electrolyte interphase (SEI) influences the irreversibility of PTFE and the electrochemical stability of a dry-processed graphite. The FEC-derived SEI reduced the irreversible side reactions of the PTFE and improved the ICE. The SEI also preserved the structural robustness of the PTFE. The alleviated PTFE deterioration not only achieved high cycle stability and high-rate performance but also suppressed graphite volume changes during cycling. The FEC additive improved the ICE of full cells from 72.5% (without FEC) to 83.8% (with FEC) and improved capacity retention from 53.7% (without FEC) to 85.7% (with FEC) in the full cells after 100 cycles. Furthermore, the dry-processed anode under ultrahigh areal capacity (7.5 mAh cm−2) exhibited excellent cycling stability for up to 200 cycles in a full cell with LFP. This study presents a promising strategy to alleviate PTFE degradation, enabling the widespread application of PTFE binders for cost-effective and eco-friendly dry battery manufacturing.