Halide perovskites have gained a lot of attention in energy storage applications due to their high electrical and ionic conductivity, large diffusion coefficients and ease of structural dimensions. However, the use of toxic solvents and lead in the synthesis processes has limited their potential for commercial development [1]. To overcome these challenges, we developed the mechanosynthesis of lead-free 2D and 3D perovskite powders for supercapacitor applications. Using this scalable and solvent-free ball-milling technique, we have synthesized methylammonium copper bromide (MA₂CuBr₄), cesium bismuth chloride (Cs₂Bi₃Cl₉) perovskites, and MA₂CuBr₄-graphite composite powders which were composed of aggregates of nanograins. After mechanosynthesis, MA₂CuBr₄ powders were washed and centrifuged in methyl acetate several times to obtain the orthorhombic crystal structure (with Pbca space group) and confirmed by the x-ray diffraction analysis [2]. MA₂CuBr₄ was first studied as an electrode material for supercapacitor and we have tested two electrolytes/working electrode systems. In the optimized LiPF₆ electrolyte/Ti electrode system, MA₂CuBr₄ showed stable and high average specific capacitance of 205 Fards/gram (Fg^-1) for 20 cycles as a working electrode thanks to the 2D layered morphology suitable for lithium intercalation. To the best of our knowledge, the performance of MA₂CuBr₄ perovskite was not studied for supercapacitor electrode application but is comparable to the recently reported lead-free bismuth-based perovskite (Cs₃Bi₂I₉) supercapacitor with a specific capacitance of ~242 Fg^-1 [3]. A maximum current of up to 20 miliamperes (mA) was observed at voltage of 1.72 V during the large operating voltage range of 0.3 V to 3.2 V. Witnessing the high performance of MA₂CuBr₄ as a supercapacitor, perovskite-graphite composites were synthesized using one-pot and two-pot schemes. In graphite-based composite powders, the graphite few layers were observed to distribute homogeneously within powders, indicating strong graphite-perovskite interaction. The effect of graphite addition was also studied for improving the electrochemical performance. These results demonstrated at first the great potential of mechanosynthesis, which is a green and easy-scalable powder synthesis technique to develop electrode materials for supercapacitors. This was confirmed by further experiments with another mechanosynthesized perovskite, Cs₂Bi₃Cl₉. The cyclovoltametric and charge/discharge experiments with Cs₂Bi₃Cl_9, MA₂CuBr_4, and MA₂CuBr₄-graphite composite have allowed for better evidencing the interest of such layered materials. Thus, mechanosynthesis shows a promise for creating eco-friendly, high-efficiency lead-free hybrid perovskite devices for sustainable energy storage applications.

Keywords : lead-free perovskites, green chemistry, solvent-free mechanosynthesis, electrode materials, supercapacitors, energy storage