Oxidant-dependent foreign-cation incorporation in lithium-proton exchange recycling of NCA and its impact on resynthesis.

Mild aqueous Li+-H+ exchange is a promising route for recycling layered positive electrode materials because it selectively removes Li while suppressing transition-metal dissolution. However, foreign alkali-metal cations originating from oxidants can be incorporated into the exchanged MOOH phase and deteriorate subsequent resynthesis. Here, we investigate the effect of oxidant species on Li+-H+ exchange recycling of Ni-rich LiNi0.88Co0.09Al0.03O2 (NCA). Replacing K2S2O8 with alkali-metal-free (NH4)2S2O8 suppressed detectable K incorporation and produced a MOOH intermediate with reduced alkali-metal contamination. Control experiments using alkali-metal salts showed that the structural evolution of MOOH depends strongly on the foreign cation species, with K+ promoting formation of an alkali-containing phase. After relithiation with Li2CO3, the (NH4)2S2O8-treated sample recovered the layered NCA structure and delivered a higher initial reversible capacity than the K2S2O8-treated sample. These results demonstrate that oxidant-counter-cation selection is critical for avoiding foreign-cation entrapment and improving initial capacity recovery in topochemical Li+-H+ exchange recycling.