Layer spacing gradient (NaLi)1−xCoO2 for electrochemical Li extraction

Summary Designing materials with high Li to Na selectivity is the key for direct extraction of Li from unconventional sources, such as seawater, without complex separations. However, material discovery is hindered by an insufficient understanding on how a material's composition and structure relate to Li selectivity during electrochemical co-intercalation. Here, we synthesized a core-shell structured (NaLi)1−xCoO2 with a core Li phase (Li0.94CoO2), a shell Na phase (Na0.51CoO2), and a transition intermediate phase via Na ion-exchange of parent Li1−xCoO2. We reveal that the chemical compositions and spatial relation of these phases play critical roles in governing high Li to Na selectivity. The core Li phase restricts large layer spacing expansion and inhibits Na intercalation. The shell Na phase serves to maintain the electrode stability against further Na ion exchange. With the layer spacing gradient design, we achieved a high Li selectivity of 1.5 × 105 with a recovery of ∼7.6:1 Li to Na from 1:20,000 initial ratio.