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As one of the most outstanding high-efficiency and environmentally friendly energy storage devices, the supercapacitor has received extensive attention across the world. As a member of transition metal oxides widely used in electrode materials, manganese dioxide (MnO₂) has a huge development potential due to its excellent theoretical capacitance value and large electrochemical window. In this paper, MnO₂ was prepared at different temperatures by a liquid phase precipitation method, and polyaniline/manganese dioxide (PANI/MnO₂) composite materials were further prepared in a MnO₂ suspension. MnO₂ and PANI/MnO₂ synthesized at a temperature of 40 °C exhibit the best electrochemical performance. The specific capacitance of the sample MnO₂-40 is 254.9 F/g at a scanning speed of 5 mV/s and the specific capacitance is 241.6 F/g at a current density of 1 A/g. The specific capacitance value of the sample PANI/MnO₂-40 is 323.7 F/g at a scanning speed of 5 mV/s, and the specific capacitance is 291.7 F/g at a current density of 1 A/g, and both of them are higher than the specific capacitance value of MnO₂. This is because the δ-MnO₂ synthesized at 40 °C has a layered structure, which has a large specific surface area and can accommodate enough electrolyte ions to participate the electrochemical reaction, thus providing sufficient specific capacitance.