Mechanism of abnormal Mg2+ extraction in sodium tetraphenylborate - tributyl phosphate system for Li+/Mg2+ separation

Synergetic extraction system has been extensively applied in Li extraction to alleviate the increasingly severe shortage of lithium production. However, the underlying coordination mechanisms still remain unclear. Herein, a synergistic extraction system of sodium tetraphenylborate (NaBPh 4 ) and tributyl phosphate (TBP) was employed for Li + /Mg 2+ separation. With increasing TBP content in the organic phase, Li + extraction efficiency gradually rose from 88.64% to 92.20%. By contrast, Mg 2+ extraction efficiency presented an abnormal trend, which was an initial decrease followed by a gradual increase. High-resolution electrospray ionization mass spectrometry together with quantum chemical calculations revealed that Mg 2+ mainly formed Mg(BPh 4 ) 2 at the low TBP content whereas the formed complex gradually transitioned to [Mg · 4TBP](BPh 4 ) 2 with increasing TBP content, leading to the initial decline in Mg 2+ extraction efficiency. Our work provides valuable insights into the mechanisms underlying Li + /Mg 2+ separation in the TBP system and offers a promising approach for broader applications.