Building a Beyond Concentrated Electrolyte for High‐Voltage Anode‐Free Rechargeable Sodium Batteries

Abstract

Low-cost and scalable sodium ion (Na-ion) batteries serve as an ideal alternative to the current lithium-ion batteries. To compensate for the shortage of energy density, the most accessible solution is developing a high-voltage anode-free configuration comprising a lightweight Al current collector on the anode and a high-voltage sodiumized cathode. However, it imposes stringent Na reversibility and high-voltage stability requirements on the electrolyte. A 3A zeolite molecular sieve film is rationally designed, and a highly aggregated solvation structure is constructed through the size effect. It suppresses the trace but continuous oxidative decomposition and extends the oxidative stability to 4.5 V without sacrificing the Na reversibility of the anode (99.91 %). Thus, we can make anode-free cells with high energy density of 369 and 372 W h kg À 1 for 4.0 and 4.25 V class cells, respectively. Furthermore, this strategy enables a long lifespan (250 cycles) for 4.0 V-class anode-free cells.

Refined Electrolyte and Interfacial Chemistry toward Realization of High-Energy Anode-Free Rechargeable Sodium Batteries

“…To prove this point, ex situ XPS C 1s and F 1s spectra were collected from the surface of the P2-NaNMT cathode after one complete (dis)­charge cycle with either NG2 or NG24 10 . Referring to Figure S18a,b, a C-metal bonding (C-M) signal is observed in the case of NG2 yet absent in the case of NG24 10 . The C-M signal could be attributed to the intimate chemical interaction between CEI organics and transitional metal (TM) cations in the surface lattice of layered oxides.…”

“…9,10 Practically, poor electrolyte compatibility against both electrodes could result in inferior cycling performance and low CE of anode-free RSBs, so the batteries are usually reported with drastic performance decay, energy loss, and failure. 5,11 As discussed above, high (electro)chemical compatibility between the electrolyte and two working electrodes forms the prerequisite for the stable operation of anode-free RSBs. For this sake, reasonable design of electrolyte composition and buildup of a stable electrode−electrolyte interface are equally important.…”

“…G4 with a longer molecule chain was included as the cosolvent, for its improved tolerance against high-voltage layered oxide cathodes (Figure S1, which shows a lower highest occupied molecular orbital (HOMO) energy of G4 than that of G2). 11,13 The cyclic ether molecules, such as 1,3-dioxolane, were not included in the electrolyte, for their tendency to undergo cationic polymerization in the presence of inorganic salt 14 and loss of working-ion conductivity as a result. ).…”

“…Owing to a higher standard electrode potential of the Na + /Na redox pair (−2.71 V vs standard hydrogen electrode, SHE) than that of the Li + /Li pair (−3.04 V vs SHE), building a high-voltage RSB comparable to the Li-ion counterparts would require 0.3 V larger energy separation (or electrochemical ‘window’) of the electrolyte. To maintain a stable cathode–electrolyte interface, a stronger antioxidation property of the electrolyte is required, posing rigorous restrictions on electrolyte selection and formulation. , Practically, poor electrolyte compatibility against both electrodes could result in inferior cycling performance and low CE of anode-free RSBs, so the batteries are usually reported with drastic performance decay, energy loss, and failure. , …”

“…G2 was selected as the main solvent for its ability to facilitate dendrite-free Na plating/stripping with an ultrahigh CE, possibly due to the unique Na + solvation structure, and stronger antireduction ability compared with other ether solvents and anions. G4 with a longer molecule chain was included as the cosolvent, for its improved tolerance against high-voltage layered oxide cathodes (Figure S1, which shows a lower highest occupied molecular orbital (HOMO) energy of G4 than that of G2). , The cyclic ether molecules, such as 1,3-dioxolane, were not included in the electrolyte, for their tendency to undergo cationic polymerization in the presence of inorganic salt and loss of working-ion conductivity as a result.…”