High pressure–high temperature (HP/HT) methods are utilized to introduce structural modifications in the layered lithium transition metal oxides LiCoO2 and Li[NixLi1/3−2x/3Mn2/3−x/3]O2 where x = 0.25 and 0.5. The electrochemical property to structure relationship is investigated combining computational and experimental methods. Both methods agree that the substitution of transition metal ions with Li ions in the layered structure affects the compressibility of the materials. We have identified that following high pressure and high temperature treatment up to 8.0 GPa, LiCoO2 did not show drastic structural changes, and accordingly the electrochemical properties of the high pressure treated LiCoO2 remain almost identical to the pristine sample. The high pressure treatment of LiNi0.5Mn0.5O2 (x = 0.5) caused structural modifications that decreased the layered characteristics of the material inhibiting its electrochemical lithium intercalation. For Li[Li1/6Ni1/4Mn7/12]O2 more drastic structural modifications are observed following high pressure treatment, including the formation of a second layered phase with increased Li/Ni mixing and a contracted c/a lattice parameter ratio. The post-treated Li[Li1/6Ni1/4Mn7/12]O2 samples display a good electrochemical response, with clear differences compared to the pristine material in the 4.5 voltage region. Pristine and post-treated Li[Li1/6Ni1/4Mn7/12]O2 deliver capacities upon cycling near 200 mA h g−1, even though additional structural modifications are observed in the post-treated material following electrochemical cycling. The results presented underline the flexibility of the structure of Li[Li1/6Ni1/4Mn7/12]O2; a material able to undergo large structural variations without significant negative impacts on the electrochemical performance as seen in LiNi0.5Mn0.5O2. In that sense, the Li excess materials are superior to LiNi0.5Mn0.5O2, whose electrochemical characteristics are very sensitive to structural modifications.