Binder chemistry sets the interfacial balance constant in CsPbBr$_3$ nanocrystal supercapacitor electrodes

Abstract

Previous work on lead-free tin halide perovskites showed that the binder instead sets the electrolyte concentration at which capacitance is maximised, following the relationship $100 \times [\mathrm{Li}^+]_{\mathrm{opt}} + \mathrm{PVDF}_{\mathrm{wt}\%} = ξ_{\mathrm{Int}}$ with $ξ_{\mathrm{Int}} = 25 \pm 2.5$, established by varying the loading of a single polymer. Whether $ξ_{\mathrm{Int}}$ is universal or specific to that polymer has not been tested. Here, four binders spanning fluorinated (PVDF), carboxylic (PAA), cellulosic (CMC) and sulfonic (PEDOT:PSS) chemistry are compared on $\mathrm{CsPbBr_3}$ nanocrystal electrodes at four LiTFSI concentrations in acetonitrile, with binder loading fixed at 15 wt% and all other formulation variables held constant. The relationship applies to $\mathrm{CsPbBr_3}$: PVDF at 15 wt% gives an optimum at 0.10--0.15 M and 112 F g$^{-1}$, against 126 F g$^{-1}$ reported for $\mathrm{CsSnCl_3}$ under the same conditions, extending the result to a different B-site cation, halide and crystal system. PVDF and CMC optimise at 0.10 M, giving $ξ_{\mathrm{Int}} = 25$, while PAA and PEDOT:PSS optimise at 0.15 M, giving $ξ_{\mathrm{Int}} = 30$, with maximum values of 188 F g$^{-1}$ for PAA and 146 mF cm$^{-2}$ for PEDOT:PSS. The two binders showing the shift carry ionisable acid groups at high density, indicating that $ξ_{\mathrm{Int}}$ expressed in weight percent requires a binder-specific value. In all four electrodes, $\mathrm{CsPbBr_3}$ converts to $\mathrm{PbBr_2}$ and CsBr during electrochemical characterisation, and the surface lead content of the PEDOT:PSS electrode decreases.