Metal halide perovskites have gained significant interest for use in solar cells and light-emitting devices. Recently, this material has also gained significant interest for its potential in energy storage devices, particularly lithium-ion batteries and photo-batteries, due to their long charge carrier diffusion length, high charge mobility, high light absorption capacity, non-rigid structure, and variable bandgap. This perspective highlights key properties of metal halide perovskites used as electrodes in lithium-ion batteries. The primary discussion is divided into four sections: an explanation of the structure and properties of metal halide perovskites, a very brief description of the operation of a conventional lithium-ion battery, lithium-ion interaction with metal perovskite halides, and the evolution and progress of perovskite halides as electrodes and photo-electrodes. The purpose of this perspective is to build awareness of recent advancements and provide an outlook on this relatively new subfield in order to motivate continued research and development of batteries and photo-batteries containing metal halide perovskites.

These devices use perovskites with an ABX 3 structure as the active emitting layer, where A is Cs, MA or FA, B is Pb or Sn, and X is the halides. In recent years, the EQE for this device type has surged from less than 1% to over 20% for green, red and infra-red peLEDs. [7][8][9][10] Blue LEDs are also needed for displays and lighting applications, but thus far have lagged in EQE, color stability and lifetime, despite recent advances. [1] Blue (460 -480 nm) or sky-blue (480 -500 nm) peLEDs can be achieved by two possible methods: halide tuning, by adding chloride into a bromide-based perovskite (e.g., CsPbBr 3 ), [11,12] and more recently, through the quantum confinement effect, using 0D nanocrystals (NCs) [13][14][15][16][17][18][19] or quasi-2D layered perovskites. For example, You et al. reported a blue peLED incorporating EA into the Cs + site in PEA 2 (CsPbBr 3 ) 2 PbBr 4 to form passivated 2D/3D perovskite devices showing a record high EQE of 12.1% at 488 nm. [20] A key challenge for blue peLEDs is overcoming the issue of spectral (color) instability, especially when a mixed halide strategy is used to achieve blue emitting films. These perovskites, whether in the form of bulk, [21] quasi-2D, [22] or 0D nanocrystals, [23,24] often suffer from unwanted phasesegregation under photo-or electrical excitation, [25][26][27] in which the emission peak can shift or multiple peaks are observed due to ion migration. [28] Efforts to achieve spectral stability in blue peLEDs include A-site cation alloying, [29] tailoring bulky cations, [30,31] passivation, [32] and interface engineering. [33] Additionally, metal doping has long been investigated as an effective approach for enhancing perovskite optoelectronic performance, and this work has extended to peLEDs as well. [32,[34][35][36] Hou et al. reported a nanocrystal peLED doped with Mn that emitted at 466 nm with an EQE of 2.12%. [13] Most recently, Wang et al. reported yttrium (III) chloride doped quaisi-2D CsPbBr 3 :PEACl showing EQE of 11.0% and a maximum brightness of 9040 cd m -2 in the sky-blue region (roughly 485 -500 nm). [37] In this work, we investigate the activity of divalent metal ions as passivation materials in PEA-based 2D/3D CsPbX 3 perovskites, where X is mixed Br and Cl. The above Perovskite light-emitting devices (peLEDs) have recently gained widespread attention as candidates for next-generation display technologies.Red and green peLEDs have achieved over 20% external quantum efficiency (EQE), however blue peLEDs still lag behind. One proposed solution for improving the performance of blue peLEDs has been doping the perovskite films with metal ions, ostensibly to passivate surface trap sites and improve the EQE. Despite a few recent reports, however, the many roles of the metal ion dopants have not yet been fully explored. Here improvements in the EQE and spectral stability of quasi-2D perovskites, often called 2D/3D or layered perovskites are reported. CsPbBr 3−x Cl x skyblue emitting peLEDs, with a maximum EQE of 7.5%, and color stable spectra at ...

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