Unveiling the key factor for enhancing capacity in iron-borate glass anodes for Li-ion batteries

We studied the electrochemical properties of the 40Fe₂O₃–60B₂O₃ (FeB) (mol%) glasses as anode materials in lithium-ion batteries (LIBs) by varying their redox state of iron. We found that the precipitation of Fe₂O₃ crystals in the FeB glass during melt-quenching was inhibited by carbon. The introduced carbon reduced all ferric to ferrous ions in FeB melt. The FeB glass-ceramic as anode material exhibited a lithium-ion storage capacity of 290 mA h g ^{− 1} at a current density of 1A h g ^{− 1} even after 600 cycles. However, the capacity decreased by almost half when 2 wt% carbon was added, ascribed to both the absence of Fe₂O₃ crystals and the deficiency of ferric ions. To identify the key factor for determining the capacity, we prepared a crystal-free FeB glass via rapid cooling. This glass exhibited a notably lower capacity than FeB glass-ceramic, suggesting that Fe₂O₃ crystals are the key factor for enhancing capacity.