(6R,7S)-7-(2-((difluoromethyl)thio)acetamido)-3-(((1-(2-hydroxyethyl)-1H-tetrazol-5-yl)thio)methyl)-7-methoxy-8-oxo-5-oxa-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, sodium salt