Synthesis, Structure and Electrochemistry of positive Insertion materials for rechargeable Lithium Batteries

Raekelboom, Emmanuelle Angeline (2002) Synthesis, Structure and Electrochemistry of positive Insertion materials for rechargeable Lithium Batteries. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Lithium copper oxides Li_xCu₂O₄ (x = 2,3,4) have been synthesised for lithium battery application using solid state and solution reactions under various conditions. Li₂CuO₂(Immm) has been prepared in air at 800^oC for 10-15 h from a stoichiometric mixture of copper oxide and lithium hydroxide. Synthesis using high pressure oxygen (250 bar, 4 h, 700^oC) and hydrothermal (1.5 kbar, 10 h, 600^oC) was used for the formation of mixed-valence cuprate Li₃Cu₂O₄ (C2/m) and the isostructural Li₂NaCu₂O₄ (250 bar in O₂, 4 h, 700^oC) from simple oxides. The latter has been characterised using powder neutron diffraction and crystallises in the space group C2/m (a = 10.2733(2), b = 2.80324 (3), c = 7.58532(9) A and β = 119.6903 (8)^o). The lithium ions occupy the tetrahedral positions whereas the sodium ions are found to be exclusively in octahedral environment. LiCuO₂ can not be obtained directly; it was synthesised by chemical lithium extraction of the lithium rich oxide, Li₃Cu₃O₄ using Br₂ in CH₃CN. The latter, was found to crystallise in the space group C2/m. The structures of all the lithium copper oxides prepared are composed of one dimensional infinite chains of edge-sharing CuO₄ square planes coordinated to lithium in tetrahedral (Li₂CuO₂, Li₃Cu₂O₄) and octahedral (LiCuO₂, Li₃Cu₂O₄) positions.

Electrochemical testing was carried out in a two-electrode cell using composite electrodes containing the oxide materials, carbon black to enhance the electronic conductivity and polytetrafluoroethylene as Teflon^® binder. Lithium foil has been used as reference and counter electrode and 1 M LiPF6 in EC-DMC as the electrolyte. The slow galvanostatic charging of Li₂CuO₂ until 4.5 V yields charge specific capacity of 320 mAhg^-1 (theoretical capacity of 490 mAhg^-1) at a cycling rate of 11 mAg^-1 which is followed by a large specific discharge capacity of 250 mAhg^-1 (0.8 Li⁺) distributed mostly between 2 and 3 V. Application as additive material with a positive electrode LiMn₂O₄ was found to be successful as it compensates the initial loss of specific charge capacity due to the formation of the passivation on the negative electrode. The slow galvanostatic cycling of Li₃Cu₂O₄ (theoretical capacity of 380 mAhg^-1) between 1.6 and 4.5 V yields a charge density of 160 mAhg^-1 (0.6 Li⁺) at a cycling rate of 10 mAhg^-1. The results for Li₂NaCu₂O₄ indicates the presence of sodium that may disrupt the lithium ion pathway. LiCuO₂ (theoretical capacity of 263 mAhg^-1) provides between 1 and 4.3 V a specific capacity of 500 mAhg^-1 with an average voltage of 2.5 V. This discharge is thought to involve the formation of Cu¹ or Cu^II in this material.