A robust, highly reversible, mixed conducting sodium metal anode

Multivariate Distribution Structured Anisotropic Inorganic Polymer Composite Electrolyte for Long-Cycle and High-Energy All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes are promising candidates for solid-state Li metal batteries owing to their favorable rheological properties and interfacial compatibility with cathodes and Li anodes. However, their limited ionic conductivity and low modulus lead to inferior electrochemical performance and dendrite growth. Herein, we developed a composite solid-state electrolyte comprising vermiculite sheets and a poly(vinylidene fluoride) (PVDF) matrix with multivariate distribution and an anisotropic structure. Within this assembly, some vermiculite sheets were suspended in the PVDF matrix to facilitate Li salt dissociation and Li+ transport, while others were tiled on the electrolyte surface, generating a dense, high-modulus Li2SiO3-rich solid electrolyte interphase via in situ electrochemical reduction, which further improved interfacial kinetics and suppressed dendrite growth. As a result, a high conductivity of 1.38 mS cm-1 was achieved at room temperature, and the Li||Li cells displayed robust stability over 3000 h. The LiNi0.6Co0.2Mn0.2O2||Li full cells delivered a specific capacity of 172 mAh g-1 at 0.2 C and 86% capacity retention after 500 cycles at 0.5 C. Additionally, practical cycle performance at a high loading (4.4 mAh cm-2) was achieved in pouch cells. Overall, multivariate distribution and anisotropic structuring offers a novel perspective for the preparation of high-performance solid-state electrolytes.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

Oxygen-regulated spontaneous solid electrolyte interphase enabling ultra-stable solid-state Na metal batteries

Solid-state sodium metal batteries utilizing inorganic solid electrolytes (SEs) hold immense potentials such as intrinsical safety, high energy density, and environmental sustainability. However, the interfacial inhomogeneity/instability at the anode-SE interface usually triggers the penetration of sodium dendrites into the electrolyte, leading to short circuit and battery failure. Herein, confronting with the original nonuniform and high-resistance solid electrolyte interphase (SEI) at the Na-Na3Zr2Si2PO12 interface, an oxygen-regulated SEI innovative approach is proposed to enhance the cycling stability of anode-SEs interface, through a spontaneous reaction between the metallic sodium (containing trace amounts of oxygen) and the Na3Zr2Si2PO12 SE. The oxygen-regulated spontaneous SEI is thin, uniform, and kinetically stable to facilitate homogenous interfacial Na+ transportation. Benefitting from the optimized SEI, the assembled symmetric cell exhibits an ultra-stable sodium plating/stripping cycle for over 6600 h under a practical capacity of 3 mAh cm-2. Quasi-solid-state batteries with Na3V2(PO4)3 cathode deliver excellent cyclability over 500 cycles at a rate of 0.5 C (1 C = 117 mA cm-2) with a high capacity retention of 95.4%. This oxygen-regulated SEI strategy may offer a potential avenue for the future development of high-energy-density solid-state metal batteries.

Enhancing Interfacial Strength and Wettability for Wide-Temperature Sodium Metal Batteries

Development of high-performance sodium metal batteries (SMBs) with a wide operating temperature range (from -40 to 55 °C) is highly challenging. Herein, an artificial hybrid interlayer composed of sodium phosphide (Na3 P) and metal vanadium (V) is constructed for wide-temperature-range SMBs via vanadium phosphide pretreatment. As evidenced by simulation, the VP-Na interlayer can regulate redistribution of Na+ flux, which is beneficial for homogeneous Na deposition. Moreover, the experimental results confirm that the artificial hybrid interlayer possesses a high Young's modulus and a compact structure, which can effectively suppress Na dendrite growth and alleviate the parasitic reaction even at 55 °C. In addition, the VP-Na interlayer exhibits the capability to knock down the kinetic barriers for fast Na+ transportation, realizing a 30-fold decrease in impedance at -40 °C. Symmetrical VP-Na cells present a prolonged lifespan reaching 1200, 500, and 500 h at room temperature, 55 °C and -40 °C, respectively. In Na3 V2 (PO4 )3 ||VP-Na full cells, a high reversible capacity of 88, 89.8, and 50.3 mAh g-1 can be sustained after 1600, 1000, and 600 cycles at room temperature, 55 °C and -40 °C, respectively. The pretreatment formed artificial hybrid interlayer proves to be an effective strategy to achieve wide-temperature-range SMBs.

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

The sodium (Na)-metal anode with high theoretical capacity and low cost is promising for construction of high-energy-density metal batteries. However, the unsatisfactory interface between Na and the liquid electrolyte induces tardy ion transfer kinetics and dendritic Na growth, especially at ultralow temperature (-40 °C). Herein, an artificial heterogeneous interphase consisting of disodium selenide (Na₂ Se) and metal vanadium (V) is produced on the surface of Na (Na@Na₂ Se/V) via an in situ spontaneous chemical reaction. Such interphase layer possesses high sodiophilicity, excellent ionic conductivity, and high Young's modulus, which can promote Na-ion adsorption and transport, realizing homogenous Na deposition without dendrites. The symmetric Na@Na₂ Se/V cell exhibits outstanding cycling life span of over 1790 h (0.5 mA cm^-2 /1 mAh cm^-2 ) in carbonate-based electrolyte. More remarkably, ab initio molecular dynamics simulations reveal that the artificial Na₂ Se/V hybrid interphase can accelerate the desolvation of solvated Na⁺ at -40 °C. The Na@Na₂ Se/V electrode thus exhibits exceptional electrochemical performance in symmetric cell (over 1500 h at 0.5 mA cm^-2 /0.5 mAh cm^-2 ) and full cell (over 700 cycles at 0.5 C) at -40 °C. This work provides an avenue to design artificial heterogeneous interphase layers for superior high-energy-density metal batteries at ambient and ultralow temperatures.

Ion Hopping: Design Principles for Strategies to Improve Ionic Conductivity for Inorganic Solid Electrolytes

Solid electrolytes are considered as an ideal substitution of liquid electrolytes, avoiding the potential hazards of volatilization, flammability, and explosion for liquid electrolyte-based rechargeable batteries. However, there are significant performance gaps to be bridged between solid electrolytes and liquid electrolytes; one with a particular importance is the ionic conductivity which is highly dependent on the material types and structures. In this review, the general physical image of ion hopping in the crystalline structure is revisited, by highlighting two main kernels that impact ion migration: ion hopping pathways and skeletons interaction. The universal strategies to effectively improve ionic conductivity of inorganic solid electrolytes are then systematically summarized: constructing rapid diffusion pathways for mobile ions; and reducing resistance of the surrounding potential field. The scoped strategies offer an exclusive view on the working principle of ion movement regardless of the ion species, thus providing a comprehensive guidance for the future exploitation of solid electrolytes.

Room-Temperature Sodium-Sulfur Batteries: Rules for Catalyst Selection and Electrode Design

Seeking an optimal catalyst to accelerate conversion reaction kinetics of room-temperature sodium-sulfur (RT Na-S) batteries is crucial for improving their electrochemical performance and promoting the practical applications. Herein, theoretical calculations of interfacial interactions of catalysts and polysulfides in terms of the surface adsorption state, interfacial ions migration, and electronic concentration around the Fermi level are systematically proposed as guiding principles of catalyst selection for RT Na-S batteries. As a case, MoN catalyst is accurately selected from transition metal nitrides with different d orbital electrons, and for experiment, it is introduced into the carbon nanofibers as a dual-functioning host (MoN@CNFs). The MoN@CNFs can effectively anchor polysulfides and accelerate their conversion reaction. In addition, for the sodium anode, the MoN@CNFs can also induce uniform deposition of Na and inhibit dendrite growth, which are supported by in situ characterizations and finite element simulation technique. As a result, the as-prepared RT Na-S battery displays high reversible capacity of 990 mAh g^-1 at 0.2 A g^-1 after 100 cycles and long lifespan over 1500 cycles at 2 A g^-1 . Even with high S loading of 5 mg cm^-2 , the RT Na-S battery still exhibits a high areal capacity of 2.5 mAh cm^-2 .

Stabilizing Metallic Na Anodes via Sodiophilicity Regulation: A Review

This review focuses on the Na wetting challenges and relevant strategies regarding stabilizing sodium-metal anodes in sodium-metal batteries (SMBs). The Na anode is the essential component of three key energy storage systems, including molten SMBs (i.e., intermediate-temperature Na-S and ZEBRA batteries), all-solid-state SMBs, and conventional SMBs using liquid electrolytes. We begin with a general description of issues encountered by different SMB systems and point out the common challenge in Na wetting. We detail the emerging strategies of improving Na wettability and stabilizing Na metal anodes for the three types of batteries, with the emphasis on discussing various types of tactics developed for SMBs using liquid electrolytes. We conclude with a discussion of the overlooked yet critical aspects (Na metal utilization, N/P ratio, critical current density, etc.) in the existing strategies for an individual battery system and propose promising areas (anolyte incorporation and catholyte modifications for lower-temperature molten SMBs, cell evaluation under practically relevant current density and areal capacity, etc.) that we believe to be the most urgent for further pursuit. Comprehensive investigations combining complementary post-mortem, in situ, and operando analyses to elucidate cell-level structure-performance relations are advocated.

Stabilizing the cycling stability of rechargeable lithium metal batteries with tris(hexafluoroisopropyl)phosphate additive

The application of rechargeable lithium metal batteries (LMBs) has been hindered by the fast growth of lithium dendrites during charge and the limited cycling life because of the decomposition of the electrolyte at the interface. Here, we have developed a non-flammable triethyl phosphate (TEP)-based electrolyte with tris(hexafluoroisopropyl)phosphate (THFP) as an additive. The polar nature of the C-F bonding and the rich CF₃ groups in THFP lowers its LUMO energy and HOMO energy to help form a stable, LiF-rich solid electrolyte interphase (SEI) layer through the reduction of THFP and increases the binding ability of the PF₆^- anions, which significantly suppresses lithium dendrite growth and reduces the electrolyte decomposition. Moreover, THFP participates in the formation of a thin, C-F rich electrolyte interphase (CEI) layer to provide the stable cycling of the cathode at a high voltage. The symmetric Li||Li and full Li/NCM622 cells with THFP additive have small polarization and long cycling life, which demonstrates the importance of the additive to the application of the LMBs.

Artificial Heterogeneous Interphase Layer with Boosted Ion Affinity and Diffusion for Na/K-Metal Batteries

Metallic Na (K) are considered a promising anode materials for Na-metal and K-metal batteries because of their high theoretical capacity, low electrode potential, and abundant resources. However, the uncontrolled growth of Na (K) dendrites severely damages the stability of the electrode/electrolyte interface, resulting in battery failure. Herein, a heterogeneous interface layer consisting of metal vanadium nanoparticles and sodium sulfide (potassium sulfide) is introduced on the surface of a Na (K) foil (i.e., Na₂ S/V/Na or K₂ S/V/K). Experimental studies and theoretical calculations indicate that a heterogeneous Na₂ S/V (K₂ S/V) protective layer can effectively improve Na (K)-ion adsorption and diffusion kinetics, inhibiting the growth of Na (K) dendrites during Na (K) plating/stripping. Based on the novel design of the heterogeneous layer, the symmetric Na₂ S/V/Na cell displays a long lifespan of over 1000 h in a carbonate-based electrolyte, and the K₂ S/V/K electrode can operate for over 1300 h at 0.5 mA cm^-2 with a capacity of 0.5 mAh cm^-2 . Moreover, the Na full cell (Na₃ V₂ (PO₄ )₃ ||Na₂ S/V/Na) exhibits a high energy density of 375 Wh kg^-1 and a high power density of 23.5 kW kg^-1 . The achievements support the development of heterogeneous protective layers for other high-energy-density metal batteries.