2 S-Max® patents, page-27

@loisl1978 "4N" is four 9s (ie 99.99), and a term commonly used in the chemical/mining industry. 4N (and above) purities are preferred for xEV applications due to reduced dendrite growth and greater energy densities.

Producing high-purity lithium metal at the level suitable for energy storage technology (>99.99%) is challenging and expensive. If LPD can reliably produce 9N Li, then it will have a competitive advantage as a lost cost supplier. There are a number of projects that have be shuttered due to falling LCE prices. This is one of the reasons why the current patent program (and the rights to the patents) is important.

Lithium metal with purities in the range of 99.5−99.99% can be manufactured (mostly from brine sources) using electrowinning from molten eutectic LiCl−KCl salt electrolysis at around 500 °C. The energy requirements (35−40 kWh kg-Li), combined with the production of anhydrous lithium chloride feedstock, make for an energy-intensive process.

For development into form factors suitable as electrodes, the lithium is extruded from an ingot into a foil typically limited to thicknesses above 100 μm and often prepared as a lithium−aluminum alloy foil, to maintain mechanical strength.

There are inherent difficulties associated with preparing extruded thin lithium metal films <100 μm, with little possibility for control over microstructure. An alternative, that of a pure thin film-on-substrate, would be highly desirable for next-generation battery researchers and
lithium metal battery manufacturers alike.

Here is a paper discussing the matter.
https://pubs.acs.org/doi/pdf/10.1021/acsomega.7b01501