A Versatile Strategy for Manufacturing Separator-Free Li-Ion Battery Cells: Combination of Dry Process for Electrodes and Solvent-Based Impregnation of Polymer Electrolytes
Authors: Brändel Timo , Hsieh Yi-Chen , Klein Sven 
Published: February 2024

A Separator for conventional Lithium-Ion Batteries (LiBs) typically consists of functionalized Polypropylene or Polyethylene to avoid short circuiting and host the liquid electrolyte[1]. The successful removal of the separator would not only facilitate the cell assembly process, but would also reduce the process cost related to z-stacking machines, which corresponds to around 12 % of total process cost[2]. Therefore, a different strategy needs to be applied in this case to prevent short circuits while maintaining sufficient ionic conduction.
An often-mentioned strategy to achieve the separator-free architecture is to impregnate and overcoat the electrodes with a polymer electrolyte dissolved in a solvent. However, a meticulous investigation of a compatible combination of binder-polymer electrolyte, conductive salt and solvent is usually necessary to avoid negative impacts of this strategy, with the delamination of the electrodes, the insufficient impregnation of the electrodes or the emergence of high interfacial resistances being only some examples of potential problems which can occur during this process.
Here, we present a novel approach to overcome the above-mentioned barriers, by combining different manufacturing techniques. Firstly, in order to avoid the delamination of the electrodes, electrodes with adjustable porosity are prepared by a fully dry process using fibrillated polytetrafluorethylene (PTFE) as a binder. Due to the non-solubility of PTFE in a plethora of different solvents, the choice of solvent is much less limited and a wider range of solvents can be used to tune the viscosity during the impregnation process. Into the porous electrodes, a , dissolved in a solvent, is impregnated via a standard doctor blade-coating method. Furthermore, using dry process electrodes mitigates the impact of inhomogeneous vertical distribution, avoiding self-aggregation in electrodes and forming non-uniform pores[3]. The mentioned flexibility in the choice of solvent type and amount ensures a complete impregnation into the pores of the electrode for different mass loadings and various types of active material. A sufficient thickness of the polymer electrolyte layer during the coating step leads to an overcoating of both electrodes of around 8-10 µm, which is close to the range of commercial separators, enabling practically required high energy densities for battery applications.
In our proof-of-concept study, we fabricated and tested LiB cells composed of polymer-impregnated LFP and Graphite electrodes without any separators based on the above mentioned concept. The resulting cells showed promising cyclability and high coulombic efficiencies for more than 100 cycles at intermediate C-rates. The flexibility of the concept is presented by varying the solvent, used during the impregnation step and tailoring the properties of the specialized fluoropolymer electrolyte via addition of ceramic fillers. In addition to the electrochemical performance, an efficient impregnation process of the polymer electrolyte into the porous cathodes, which can be confirmed by SEM, was successfully implemented.
In summary, this study provides not only a strategy for the rational design of separator-free architecture, but also creates a novel possibility for solid-state battery applications.
[1] Huang, Xiaosong “Separator technologies for lithium-ion batteries”, J Solid State Electrochem
15, 649–662 (2011).
[2] Orangi, Stina, et al. “A Techno-Economic Model for Benchmarking the Production Cost of Lithium-Ion Battery Cells”, Batteries, 8, 83, (2022).
[3] Kumberg, Jana, et al. "Drying of lithium‐ion battery anodes for use in high‐energy cells: influence of electrode thickness on drying time, adhesion, and crack formation." Energy Technology 7.11 (2019).

Figure 1