Influence of interstitial Li on the electronic properties of Li$_{x}$CsPbI$_{3}$ for photovoltaic and battery applications

TL;DR

This work investigates how interstitial Li affects the stability and electronic structure of CsPbI3 for potential photo-battery applications. Using density functional theory with the SCAN+rVV10 functional and spin-orbit coupling, it analyzes two limiting CsPbI3 structures, the high-symmetry alpha phase and a distorted gamma-prime phase, to map Li uptake limits and band-gap changes. A key finding is that the alpha structure can accommodate only a small Li content, whereas the gamma-prime structure supports Li up to unity per formula unit with favorable formation energies; the band gap increases with Li largely due to structural distortions in the Pb-I-Pb framework, with a modest contribution from extra electrons. The results highlight structural distortions as the dominant mechanism for band-gap tuning in Li-doped CsPbI3, which has implications for solar-cell performance and motivates exploring two-dimensional perovskites as robust photo-battery materials.

Abstract

The integrated device of a perovskite solar cell with a Li-ion battery is an innovative solution for decentralized energy storage in smart electronic devices. In this study, we examine the stability of Li ions intercalated in a CsPbI$_3$ perovskite and their effect on the electronic structure of Li$_x$CsPbI$_3$ compounds using first-principles density functional theory. Our simulations demonstrate that the insertion of Li at concentrations up to $x$ = 1 into CsPbI$_3$ is energetically possible. Moreover, we identify that the distortion of the Pb-I octahedra has the strongest impact on the change in the electronic band gap. Specifically, an increase in the amount of intercalated Li causes larger structural distortions, which in turn lead to an increasing band gap as function of the Li content.

Figures (9)

• Figure 1: The two limiting models of the dynamic crystal structure of CsPbI$_3$, illustrating the degrees of structural distortion. The B-site atoms are located on the lattice points of the cubic (pseudo-cubic) structure. The off-center displacement of the A-site atoms ($\Delta {\rm r_{Cs}}$) and the tilts of B-X octahedra described by the averaged Pb-I-Pb bond angle difference ($\Delta_{\text{Pb}\hbox{-}\text{I}\hbox{-}\text{Pb}}$) quantify the structural distortion.
• Figure 2: a) Li interstitial sites in the $\alpha$ structure, located in the center of an octahedron (O) or a tetrahedron (T) formed by host lattice atoms. b) Formation energies of interstitial Li as function of the concentration $x$. The horizontal dotted line labels 0 eV. Square symbols indicate results obtained for a fixed simulation cell ($V_\text{fixed}$), circle symbols indicate that the volume of the cell was optimized, but the rectangular shape of the cell was maintained during optimization (V$_\text{opt}$), and the triangle symbols indicate structural relaxation in all degrees of freedom (E$_\text{opt}$). The point symmetry of the interstitial Li at the O site ($D_{4h}$) or T site ($C_{3v}$) is maintained for all Li concentrations.
• Figure 3: The relationship between the formation energy of interstitial Li and the structural distortion $\Delta_{\text{Pb}\hbox{-}\text{I}\hbox{-}\text{Pb}}$ with varying Li concentration $x$ in the $\alpha$ structure at fixed volume and cell shape. Blue and green symbols indicate interstitial Li on O and T sites, respectively. The different Li concentrations are indicated by these symbols: "+" for x=1/8, $\triangle$ for x=1/4, and $\hexago$ for x=1. Filled and open symbols represent high-symmetry and low-symmetry structures of the Li$_x$CsPbI$_3$ compounds, respectively.
• Figure 4: The relationship between the formation energy of interstitial Li and the structural distortion $\Delta_{\text{Pb}\hbox{-}\text{I}\hbox{-}\text{Pb}}$ for the most stable O and T sites with varying Li concentration $x$ in the $\gamma$' structure. The different Li concentrations are indicated by these symbols: "+" for x=1/8, $\triangle$ for x=1/4, $\square$ for x=1/2, $\largestar$ for x=3/4, and $\hexago$ for x=1.
• Figure 5: The relationship between the formation energy of Li ions at T sites of the $\gamma$’ structure and the concomitant distortion of the pseudo-cubic perovskite structure, for different Li concentrations up to $x$=2. The distortion is quantified by the average displacement of Pb atoms at B sites, and the Li concentration is illustrated by the color code. Formation energies for Li ions at O sites are not included in this figure because their values are considerably higher. The symbols denote consecutive ($\square$), pairwise ($\bigcirc$) and all in one unit ($\Delta$) distributions of Li atoms, respectively. The vertical dashed line at d=0.6 Å$$ distinguishes perovskite-type crystal structures for d$<$0.6 Å, which are weakly distorted by the inserted Li, from strongly distorted, no more perovskite-type crystal structures for d$>$0.6 Å.