Chemical reduction method for industrial application of undoped polypyrrole electrodes in lithium-ion batteries

This work describes a new method of chemical reduction of p-doped polypyrrole (Ppy) to make a graphite/Ppy lithium-ion battery. Several reducing agents are analysed (sodium hydrosulfite (Na2S2O4), sodium hydrogensulfite (Na2S2O5), hydroxymethanesulfinic acid monosodium salt (HOCH2SOONa), sodium borohydride (NaBH4), formamidinesulfinic acid (H2NCNHSO2H), stannous chloride (SnCl2) and hydrazine (N2H4)), obtaining the best results when reducing a Ppy electrode with sodium borohydride (NaBH4): at least 85% of the polymer electrode can be reduced without significant degradation. To verify the feasibility of the reduction method two lithium-ion batteries with a Ppy cathode have been assembled using different ways (4.5 cm × 1.9 cm electrodes having similar active electrode material (mg cm−2) values and thicknesses to commercial inorganic oxide electrodes). In one of them the doped Ppy electrode is previously electrochemically reduced, applying consecutive potentials steps before the cell assembling; it would be accurate and easy at laboratory scale, but from an industrial point of view it is complicated and not viable. In the second battery the doped Ppy electrode is chemically reduced resulting in a similar charge–discharge characteristic to the electrochemically reduced one. Therefore, the chemical reduction method can be a step forward for using Ppy as cathode in lithium-ion batteries at industry level.