The finding of nickel extraction material: How nickel ferrocyanide offer excess capacity

In alkali metal ion batteries, insoluble ferrocyanide with few alkali metal ions can provide excess initial charge capacity which is much higher than the contribution from the extraction of alkali metal ions. Why? This commonly encountered fatal problem about Prussian blue analogues is solved in this work. When nickel ferrocyanide cycled in sodium ion half cell, an unexpected side product is formed. On the basis of this fact, we propose nickel ferrocyanide as a new nickel extraction material. According to a systematic spectroscopy investigation, this material has stable structure, small lattice variation and reversible redox couple of C-coordinated Fe³⁺/Fe²⁺, resulting in its excellent electrochemical performance. The electrode thereby provides 83.6% of initial discharge capacity after 300 cycles. The abnormal lattice expansion upon charging is also elaborated. Additionally, we also construct a full nickel-ion cell in configuration of nickel ferrocyanide/NiCl₂ aqueous electrolyte/Ni as a proof of concept. It shows a higher operating voltage than Daniell cell and good cycling performance. This work proposes a new finding for aqueous Ni batteries via uncoupling the excess capacity phenomenon, which will benefit and inspire the development of Prussian blue analogues and aqueous Ni batteries.