Long Cycle Life, Low Self‐Discharge Sodium–Selenium Batteries with High Selenium Loading and Suppressed Polyselenide Shuttling

Abstract

of sodium. [9,13] In addition, the electrode dynamics and thermodynamic knowledge accrued recently for LIBs could be employed to ensure rapid advances in SIBs. [14][15][16][17][18] Thus, development of viable room-temperature sodium-ion batteries is attracting increasing attention. [19][20][21][22][23][24] Recently, room-temperature (RT) sodium-sulfur (NaS) batteries, based on the conversion reaction chemistry, have triggered extensive research interest due to the high charge-storage capacity and the abundance of both sodium and sulfur. [25][26][27][28][29][30] However, the practical applications of RT NaS batteries are facing two major challenges: (i) sulfur has a low electronic conductivity (5 × 10 −30 S cm −1 at 25 °C), thus leading to sluggish electrochemical reaction processes and low utilization of the active sulfur in the electrode; (ii) severe "polysulfide shuttle effect," i.e., migration of dissolved polysulfide intermediate products through the porous separator between the cathode and the anode, which leads to rapid capacity fade during cycling. [31] Many approaches have been pursued to mitigate these issues, including employing modified separators, [30] porous carbon matrix, [26] etc. However, the intrinsic challenges of RT NaS batteries have been far from completely solved. Selenium is a chemical analogue of S [32,33] and is considered to be an alternative electrode materials for SIBs due to its much higher electronic conductivity (1 × 10 −3 S m −1 ) as indicated in Figure S1 in the Supporting Information, volumetric capacity (3253 A h L −1 ) comparable to that of S (3467 A h L −1 ), and stable function in the relatively low-cost carbonate-based electrolytes. [34][35][36] However, bulk Se particles still suffer from low active material utilization and Coulombic efficiency due to its electronically insulating properties and shuttling of high-order polyselenides. To circumvent these problems, the current efforts are mainly concentrated on improving the electrical conductivity and entrapping the active material within the cathode region, which include microporous/mesoporous carbon-selenium, [37][38][39] slit microporous carbon-selenium, [34] and flexible porous carbon nanofiberselenium. [40,41] With these approaches, the introduction of additional components (such as carbon black and binders) often lowers the energy density of NaSe batteries. Also, fabrication of these electrodes often involves complex multistep processes.In light of these pros and cons, we present herein a facile and scalable strategy to design carbon nanofiber (CNF)/selenium The use of selenium as a cathode in rechargeable sodium-selenium batteries is hampered by low Se loading, inferior electrode kinetics, and polyselenide shuttling between the cathode and anode. Here a high-performance sodiumselenium cell is presented by coupling a binder-free, self-interwoven carbon nanofiber-selenium cathode with a light-weight carbon-coated bifunctional separator. With this strategy, electrodes with a high Se mass loading (4.4 mg cm −2 ) rende...

CNT Interwoven Nitrogen and Oxygen Dual‐Doped Porous Carbon Nanosheets as Free‐Standing Electrodes for High‐Performance Na‐Se and K‐Se Flexible Batteries

“…Many strategies have been attempted to improve the specific capacity and cycling stability of Na‐Se batteries. The most effective approach is hybridization with electrical conductive materials and/or encapsulation Se within a porous matrix . Another strategy is to stabilize Se by chemical bonding and physical encapsulation of Se by carbon .…”

“…To the best of our knowledge, the ultralong cycling life (>700 cycles) and remarkable reversible capacity of Se@NOPC‐CNT electrodes are unprecedented. Table S2 (Supporting Information) compares the electrochemical performance of Se@NOPC‐CNT electrode with other Se‐based electrodes from the literature . The Se@NOPC‐CNT electrode exhibits the best rate capacities and cycling stability for both Na‐Se batteries and K‐Se batteries.…”

Remedies for Polysulfide Dissolution in Room‐Temperature Sodium–Sulfur Batteries

“…The S cathode thus achieved high capacity retention of ≈88.8% with prolonged cycling over 200 cycles and superior rate capability (≈390 and 127 mAh g −1 at 0.1 and 5 A g −1 , respectively) 15b. Nanoengineering hierarchical pores are an effective strategy to optimize carbonaceous S hosts, which can synergistically exploit different types of pores, such as by combining micropores with mesopores,16c,24 and integrating mesopores with macropores 16a,25. For instance, a compact host, multiporous carbon fibers (MPCFs), was synthesized as an S host materials, which took advantages of both mesopores (<10 nm) along the carbon walls and inner micropores (<2 nm).…”

Interface Engineering of MXene Composite Separator for High‐Performance Li–Se and Na–Se Batteries

“…[ 12,13 ] Tremendous efforts have been devoted to the design of advanced cathode host materials to mitigate the shuttle effect. [ 14,15 ] These designed cathode strategies include: employing porous carbon materials as the hosts; [ 7,16–19 ] coating the cathode materials with graphene, metal oxide, or conductive polymers; [ 20–24 ] and heteroatoms doping. [ 25–27 ] However, these cathode engineering would inevitably reduce the mass ratio of selenium by introducing additional components, leading to decreased energy density.…”

“…Recently, functional separators have been prepared to inhibit the shuttle effect in Se‐ or S‐based batteries. [ 17,29–34 ] For example, MXenes (M n +1 X n T x , where M = transition metal, X = carbon/nitrogen, n = 1, 2, and 3, and T x represents surface functional groups) modified separators have been proved to be effective in suppressing the shuttle effect due to their anisotropic shape and large 2D dimensions that increases the diffusion pathway of polysulfide/polyselenide species. [ 35–37 ] In addition, MXene shows strong adsorption to polysulfides due to the Lewis acid–base interaction between polysulfides and Ti sites, [ 38,39 ] which may also work for polyselenides.…”