To facilitate the characterization of the orange-brown solids, they were prepared on a larger scale, using 500 mL of electrolyte of the same concentration as in the simulated-crossover experiments (Methods S8). The solids were prepared both in the presence and absence of 2,6-DHAQ (Methods S8), and the resulting solids were named FeQ-solids and Fe-solids, respectively (Figure ). The first hypothesis was that these solids contained a mixture of iron (oxy) hydroxides, as this has previously been reported as a ferricyanide degradation product, and some organic impurities. The FeQ-solids and Fe-solids were therefore analyzed by Raman spectroscopy, which can be used to identify different iron minerals such as iron (oxy) hydroxides (Methods S9). The Raman spectra for the FeQ-solids and Fe-solids were very similar (Figure ); both materials had significant peaks at ~232 and ~375 cm -1 , and some weaker, broader peaks ~505 cm -1 and ~683 cm -1 (Figure ). The Raman spectra of these solids did not match well with the spectra of pure iron(III) oxide or iron(III) hydroxide (Figure ). However, comparing the Raman spectra of our FeQ-solids with those of iron oxide/hydroxide minerals on the RRUFF database (Figure ) revealed that they most closely resemble γ-FeOOH (a polymorph of iron oxide hydroxide, also known as Lepidocrocite). The FeQ-solids and Fe-solids were also analyzed using powder X-ray diffraction (PXRD) to characterise any crystalline components of the material (Methods S10). Both solids had PXRD patterns with few, broad peaks, further indicating the materials were amorphous, or that they have a very small particle size (Figure and Figure ). However, some weak PXRD peaks were observed at 2θ of ~26°, ~34°, ~38°, ~47° and ~61° (Figure ). Similar PXRD patterns have previously been assigned to γ-FeOOH particles, further supporting the hypothesis that this is a major component of the FeQ-solids. The FeQ-solid preparation was also repeated using a higher KOH concentration (1.2 M), to replicate possible concentration variations within the battery. The resulting material had strong PXRD peaks at 2θ of 18°, 21° and 26°, which are characteristic of the iron oxide hydroxide polymorph α-FeOOH, or Goethite (Figure ).