Lithium intercalation of Black Phosphorous: Ex-situ and in-situ Raman Spectroscopy Study*

Black phosphorous is emerging as a promising two-dimensional (2D) material which can be used as a host material for many intercalants. A systematic study on Li intercalation of black phosphorous was done using both in-situ and ex-situ electrochemical cells using black phosphorous as the cathode material. Galvanostatic discharge of dedicated in-situ electrochemical cell for Raman spectroscopy were used to study time evolution of vibrational modes under lithiation. Other than the peak broadening which is a result of structural expansion, it was observed that peaks corresponding to all three atomic vibrational modes A¹_g, B_2g and A²_g were red-shifted as a result of lithiation. Peaks B_2g and A²_g shifted more towards lower wavenumbers whereas peak A¹_g shifted only 50 % with respect to the other two, suggesting that Li ions prefer to be intercalated in-plane, along zig-zag and armchair directions than out-of-plane direction. Additional Transmission Electron Microscopy images were used to confirm the associated structural changes and Density Functional Theory (DFT) calculations were used to build a theoretical model for the phenomena.