Chemistry of Defects in Crystalline Na₂Se: Implications for the Na–Se Battery

Abstract

Selenium (Se) is a promising cathode material for next-generation alkali metal−ion batteries due to its high volumetric capacity and good conductivity. Using a first-principles approach, the present study systematically studies the electronic structure and the chemistry of defects of the final discharge product Na 2 Se for Na−Se batteries. It is found that Na 2 Se is insulating for free electron transfer. The charged native defects can act as carriers to transfer charges through crystalline Na 2 Se. The hole polaron p + has a very low diffusion barrier of 76 meV. However, p + makes a minor contribution to the conductivity of Na 2 Se due to its high formation energy of 1.92 eV. In the Na 2 Se crystal, the negatively charged Na vacancy (V Na − ) is the dominant defect with a formation energy of 0.71 eV, and its diffusion barrier is 258 meV which is much lower than that for other vacancies and interstitial defects. According to the present theoretical study, V Na − is the main charge carrier, and the corresponding ionic conductivity is 10 −13 S cm −1 at the room temperature. Such a low conductivity can limit the utilization of the active material and reduce the discharge voltage of Na−Se batteries, especially for Na−Se batteries based on the solid−solid reaction mechanism.

Amorphous Se Restrained by Biomass-Derived Defective Carbon for Stable Na–Se Batteries

“…After full discharging, the binding energy of Se 3d shifts to lower values of 53.6 and 54.5 eV, which can be attributed to the reduction of Se 0 to Se 2− for forming the Na 2 Se species. 19,41 In addition, two newly generated peaks around 1073.3 and 1073.2 eV appear in the Na 1s spectra after discharge, suggesting the formation of the Na−Se bond and solid-state electrolyte interphase (SEI) layer. Se 3d and Na 1s have completely returned to the initial state after being charged to 2.8 V, which advantageously illustrates a highly reversible conversion reaction between Se 0 and Se 2− during chargingdischarging.…”

Theoretical calculation guided materials design and capture mechanism for Zn–Se batteries via heteroatom‐doped carbon

“…This phenomenon is commonly observed in Li/Na‐Se batteries and can sharply reduce the capacity of Zn–Se batteries. [ 16,22–24 ] Furthermore, the weak ion and electron transport properties of Se cause the inferior electrochemical performance of Zn–Se batteries. [ 25,26 ]…”

“…This phenomenon is commonly observed in Li/Na-Se batteries and can sharply reduce the capacity of Zn-Se batteries. [16,[22][23][24] Furthermore, the weak ion and electron transport properties of Se cause the inferior electrochemical performance of Zn-Se batteries. [25,26] Confining Se to various carbon materials with high electronic conductivity has been made to solve the low conductivity of Se; for instance, hierarchically porous carbon, carbon nanofibers, graphene, and multiwalled carbon nanotubes.…”

A Multifunctional Interphase Layer Enabling Superior Sodium‐Metal Batteries under Ambient Temperature and −40 °C

“…On the other hand, owing to the high electrical insulation of Na 2 Se, [52,53] the artificial hybrid Na 2 Se/V interphase presents extremely low electrical conductivity (σ) of ≈90 S cm −1 on the basis of the four-point probe method (σ (V) ≈ 3.0 × 10 4 S cm −1 and σ (Na 2 Se) ≈ 20 S cm −1 ), therefore the tunneling of electrons through the artificial hybrid Na 2 Se/V interlayer is blocked, and the Na + deposition only occurs at the interface of artificial hybrid layer/Na. The kinetics behavior of Na + diffusion through SEI is critical to the success of the sodium-metal battery.…”