Lithium insertion into Li₃CrO₄ and related LISICON materials

The electrochemical intercalation of lithium into chromium (V)-containing oxides isostructural with β- and γ-Li₃PO₄ has been studied. β-Li₃CrO₄ can reversibly intercalate around 1 mole of Li ions per formula unit to give Li_{3 + x}CrO₄: 0 < x < 1 with a discharge capacity of 196 mAh g^-1; electrochemical data indicate the possible existence of a two-phase region in the approximate range 0.3 < x < 0.8. Fully intercalated material appears to have a γ-related structure. γ-Li_3.5Cr_0.5Ge_0.5O₄, with a LISICON structure can reversibly intercalate up to 0.5 moles of Li ions per formula unit to give Li_{3.5 + x}Cr_0.5Ge_0.5O₄: 0 < x < 0.5. During intercalation, the γ-phase transforms into a β-phase which is retained on deintercalation. The volume changes associated with (de) intercalation of both β and γ structures are small, ≪ 1%. During the β → γ transition, the orthorhombic c axis, and the unit cell volume, increase by ~ 2%.