Sodium Incorporation in CsPbBr$_{3-x}$I$_x$ Nanocrystal Electrodes: Lattice Contraction and the Suppression of Field-Driven Iodine Expulsion

Abstract

Mixed-halide perovskite nanocrystal electrodes fail in supercapacitors through field-driven halide segregation. In undoped $\mathrm{CsPbBr_2I}$, this appears as a capacitance that climbs to 218% of its first-cycle value by cycle 1378 and then collapses to 39% by cycle 2500, with complete loss of the iodine signal from the cycled electrode. This work tests whether sodium incorporation suppresses that failure mode. Na-doped $\mathrm{CsPbBr_{3-x}I_x}$ ($x = 0, 1, 2$) nanocrystals were prepared by ligand-assisted reprecipitation at a Na/Pb precursor ratio of 1.25:1.00 and compared with undoped analogues in 0.1 M tetrabutylammonium tetrafluoroborate in anhydrous dichloromethane. Sodium contracts the pseudocubic lattice parameter of $\mathrm{CsPbBr_3}$ from $5.908 \pm 0.029$ to $5.851 \pm 0.019$ $\mathring{\mathrm{A}}$ after correction for specimen displacement. Specific capacitance at 0.3 A g$^{-1}$ rises for every composition, from 42 to 75, from 63 to 96, and from 56 to 84.5 F g$^{-1}$. Na-$\mathrm{CsPbBr_2I}$ gives the lowest charge-transfer resistance at 175 $Ω$ and the highest ion diffusion coefficient at $1.6 \times 10^{-16}$ m$^2$ s$^{-1}$, and power-law exponents between 0.33 and 0.45 at all potentials examined show that charge storage is limited by ion transport through the pore network rather than by the interfacial process. In the sodium-containing (Na-$\mathrm{CsPbBr_2I}$) electrode, the capacitance rise reaches only 115% at cycle 600, no collapse follows, and 97% is retained at 2500 cycles, the excess above the first-cycle value being reduced by a factor of 7.9. Iodine is retained at unchanged binding energy, and the Br:I ratio measured by elemental mapping is 2.30 after cycling against 2.25 before.