Room-Temperature All-Liquid-Metal Batteries Based on Fusible Alloys with Regulated Interfacial Chemistry and Wetting.

Liquid metal batteries are regarded as potential electrochemical systems for stationary energy storage. Currently, all reported liquid metal batteries need to be operated at temperatures above 240 °C to maintain the metallic electrodes in a molten state. Here, an unprecedented room-temperature liquid metal battery employing a sodium-potassium (Na-K) alloy anode and gallium (Ga)-based alloy cathodes is demonstrated. Compared with lead (Pb)- and mercury (Hg)-based liquid metal electrodes, the nontoxic Ga alloys maintain high environmental benignity. On the basis of improved wetting and stabilized interfacial chemistry, such liquid metal batteries deliver stable cycling performance and negligible self-discharge. Different from the conventional interphase between a typical solid electrode and a liquid electrolyte, the interphase between a liquid metal and a liquid electrolyte is directly visualized via advanced 3D chemical analysis. Insights into this new type of liquid electrode/electrolyte interphase reveal its important role in regulating charge carriers and stabilizing the redox chemistry. With facile cell fabrication, simplified battery structures, high safety, and low maintenance costs, room-temperature liquid metal batteries not only show great prospects for widespread applications, but also offer a pathway toward developing innovative energy-storage devices beyond conventional solid-state batteries or high-temperature batteries.

Size-, Water-, and Defect-Regulated Potassium Manganese Hexacyanoferrate with Superior Cycling Stability and Rate Capability for Low-Cost Sodium-Ion Batteries.

Potassium manganese hexacyanoferrate (KMHCF) is a low-cost Prussian blue analogue (PBA) having a rigid and open framework that can accommodate large alkali ions. Herein, the synthesis of KMHCF and its application as a high-performance cathode in sodium-ion batteries (NIBs) is reported. High-quality KMHCF with low amounts of crystal water and defects and with homogeneous microstructure is obtained by controlling the nucleation and grain growth by using a high-concentration citrate solution as a precipitation medium. The obtained KMHCF exhibits superior cycling and rate performance as a NIB cathode, showing 80% capacity retention after 1000 cycles at 1 C and a high capacity of 95 mA h g-1 at 20 C. Unlike conventional single-cation batteries, the hybrid NIB with KMHCF as cathode and Na as anode in Na-ion electrolyte displays three reversible plateaus that involve stepwise insertion/extraction of both K+ and Na+ in the PBA framework. In later cycling, the K+ -Na+ cointercalated phase is partially converted into a cubic sodium manganese hexacyanoferrate (NaMHCF) phase due to the increasing replacement of Na+ for K+ .

A Liquid-Metal-Enabled Versatile Organic Alkali-Ion Battery.

Despite the high specific capacity and low redox potential of alkali metals, their practical application as anodes is still limited by the inherent dendrite-growth problem. The fusible sodium-potassium (Na-K) liquid metal alloy is an alternative that detours this drawback, but the fundamental understanding of charge transport in this binary electroactive alloy anode remains elusive. Here, comprehensive characterization, accompanied with density function theory (DFT) calculations, jointly expound the Na-K anode-based battery working mechanism. With the organic cathode sodium rhodizonate dibasic (SR) that has negligible selectivity toward cations, the charge carrier is screened by electrolytes due to the selective ionic pathways in the solid electrolyte interphase (SEI). Stable cycling for this Na-K/SR battery is achieved with capacity retention per cycle to be 99.88% as a sodium-ion battery (SIB) and 99.70% as a potassium-ion battery (PIB) for over 100 cycles. Benefitting from the flexibility of the liquid metal and the specially designed carbon nanofiber (CNF)/SR layer-by-layer cathode, a flexible dendrite-free alkali-ion battery is achieved with an ultrahigh areal capacity of 2.1 mAh cm-2 . Computation-guided materials selection, characterization-supported mechanistic understanding, and self-validating battery performance collectively promise the prospect of a high-performance, dendrite-free, and versatile organic-based liquid metal battery.

Na3MnZr(PO4)3: A High-Voltage Cathode for Sodium Batteries.

Sodium batteries have been regarded as promising candidates for large-scale energy storage application, provided cathode hosts with high energy density and long cycle life can be found. Herein, we report NASICON-structured Na3MnZr(PO4)3 as a cathode for sodium batteries that exhibits an electrochemical performance superior to those of other manganese phosphate cathodes reported in the literature. Both the Mn4+/Mn3+ and Mn3+/Mn2+ redox couples are reversibly accessed in Na3MnZr(PO4)3, providing high discharge voltage plateaus at 4.0 and 3.5 V, respectively. A high discharge capacity of 105 mAh g-1 was obtained from Na3MnZr(PO4)3 with a small variation of lattice parameters and a small volume change on extraction of two Na+ ions per formula unit. Moreover, Na3MnZr(PO4)3 exhibits an excellent cycling stability, retaining 91% of the initial capacity after 500 charge/discharge cycles at 0.5 C rate. On the basis of structural analysis and density functional theory calculations, we have proposed a detailed desodiation pathway from Na3MnZr(PO4)3 where Mn and Zr are disordered within the structure. We further show that the cooperative Jahn-Teller distortion of Mn3+ is suppressed in the cathode and that Na2MnZr(PO4)3 is a stable phase.

Room-Temperature Liquid Na-K Anode Membranes.

The Na-K alloy is a liquid at 25 °C over a large compositional range. The liquid alloy is also immiscible in the organic-liquid electrolytes of an alkali-ion rechargeable battery, providing dendrite-free liquid alkali-metal batteries with a liquid-liquid anode-electrolyte interface at room temperature. The two liquids are each immobilized in a porous matrix. In previous work, the porous matrix used to immobilize the alloy was a carbon paper that is wet by the alloy at 420 °C; the alloy remains in the paper at room temperature. Here we report a room-temperature vacuum infiltration of the alloy into a porous Cu or Al membrane and a reversible stripping/plating of the liquid alloy with the immobilized organic-liquid electrolyte; no self-diacharge is observed since the liquid Na-K does not dissolve into the liquid carbonate electrolytes. The preparation and stripping/plating of the liquid alkali-metal anode can both now be done safely at room temperature.

A Perovskite Electrolyte That Is Stable in Moist Air for Lithium-Ion Batteries.

Solid-oxide Li+ electrolytes of a rechargeable cell are generally sensitive to moisture in the air as H+ exchanges for the mobile Li+ of the electrolyte and forms insulating surface phases at the electrolyte interfaces and in the grain boundaries of a polycrystalline membrane. These surface phases dominate the total interfacial resistance of a conventional rechargeable cell with a solid-electrolyte separator. We report a new perovskite Li+ solid electrolyte, Li0.38 Sr0.44 Ta0.7 Hf0.3 O2.95 F0.05 , with a lithium-ion conductivity of σLi =4.8×10-4  S cm-1 at 25 °C that does not react with water having 3≤pH≤14. The solid electrolyte with a thin Li+ -conducting polymer on its surface to prevent reduction of Ta5+ is wet by metallic lithium and provides low-impedance dendrite-free plating/stripping of a lithium anode. It is also stable upon contact with a composite polymer cathode. With this solid electrolyte, we demonstrate excellent cycling performance of an all-solid-state Li/LiFePO4 cell, a Li-S cell with a polymer-gel cathode, and a supercapacitor.

Garnet Electrolyte with an Ultralow Interfacial Resistance for Li-Metal Batteries.

Garnet-structured Li7La3Zr2O12 is a promising solid Li-ion electrolyte for all-solid-state Li-metal batteries and Li-redox-flow batteries owing to its high Li-ion conductivity at room temperature and good electrochemical stability with Li metal. However, there are still three major challenges unsolved: (1) the controversial electrochemical window of garnet, (2) the impractically large resistance at a garnet/electrode interface and the fast lithium-dendrite growth along the grain boundaries of the garnet pellet, and (3) the fast degradation during storage. We have found that these challenges are closely related to a thick Li2CO3 layer and the Li-Al-O glass phase on the surface of garnet materials. Here we introduce a simple method to remove Li2CO3 and the protons in the garnet framework by reacting garnet with carbon at 700 °C; moreover, the amount of the Li-Al-O glass phase with a low Li-ion conductivity in the grain boundary on the garnet surface was also reduced. The surface of the carbon-treated garnet pellets is free of Li2CO3 and is wet by a metallic lithium anode, an organic electrolyte, and a solid composite cathode. The carbon post-treatment has reduced significantly the interfacial resistances to 28, 92 (at 65 °C), and 45 Ω cm2 at Li/garnet, garnet/LiFePO4, and garnet/organic-liquid interfaces, respectively. A symmetric Li/garnet/Li, an all-solid-state Li/garnet/LiFePO4, and a hybrid Li-S cell show small overpotentials, high Coulombic efficiencies, and stable cycling performance.

A High-Energy-Density Potassium Battery with a Polymer-Gel Electrolyte and a Polyaniline Cathode.

A safe, rechargeable potassium battery of high energy density and excellent cycling stability has been developed. The anion component of the electrolyte salt is inserted into a polyaniline cathode upon charging and extracted from it during discharging while the K+ ion of the KPF6 salt is plated/stripped on the potassium-metal anode. The use of a p-type polymer cathode increases the cell voltage. By replacing the organic-liquid electrolyte in a glass-fiber separator with a polymer-gel electrolyte of cross-linked poly(methyl methacrylate), a dendrite-free potassium anode can be plated/stripped, and the electrode/electrolyte interface is stabilized. The potassium anode wets the polymer, and the cross-linked architecture provides small pores of adjustable sizes to stabilize a solid-electrolyte interphase formed at the anode/electrolyte interface. This alternative electrolyte/cathode strategy offers a promising new approach to low-cost potassium batteries for the stationary storage of electric power.

Cathode Dependence of Liquid-Alloy Na-K Anodes.

Alkali ions can be plated dendrite-free into a liquid alkali-metal anode. Commercialized Na-S battery technology operates above 300 °C. A low-cost Na-K alloy is liquid at 25 °C from 9.2 to 58.2 wt% of sodium; sodium and/or potassium can be plated dendrite-free in the liquid range at room temperature. The co-existence of two alkali metals in an anode raises a question: whether the liquid Na-K alloy acts as a Na or a K anode. Here we show the alkali-metal that is stripped from the liquid Na-K anode is dependent on the preference of the cathode host. It acts as the anode of a sodium rechargeable cell if the cathode host structure selectively accepts only Na+ ions; as the anode of a potassium rechargeable cell if the cathode accepts K+ ions in preference to Na+ ions. This dual-anode behavior means the liquid Na-K alkali-alloy can be applied as a dendrite-free anode in Na-metal batteries as well as K-metal batteries.

A Plastic-Crystal Electrolyte Interphase for All-Solid-State Sodium Batteries.

The development of all-solid-state rechargeable batteries is plagued by a large interfacial resistance between a solid cathode and a solid electrolyte that increases with each charge-discharge cycle. The introduction of a plastic-crystal electrolyte interphase between a solid electrolyte and solid cathode particles reduces the interfacial resistance, increases the cycle life, and allows a high rate performance. Comparison of solid-state sodium cells with 1) solid electrolyte Na3 Zr2 (Si2 PO4 ) particles versus 2) plastic-crystal electrolyte in the cathode composites shows that the former suffers from a huge irreversible capacity loss on cycling whereas the latter exhibits a dramatically improved electrochemical performance with retention of capacity for over 100 cycles and cycling at 5 C rate. The application of a plastic-crystal electrolyte interphase between a solid electrolyte and a solid cathode may be extended to other all-solid-state battery cells.

Low-Cost High-Energy Potassium Cathode.

Potassium has as rich an abundance as sodium in the earth, but the development of a K-ion battery is lagging behind because of the higher mass and larger ionic size of K+ than that of Li+ and Na+, which makes it difficult to identify a high-voltage and high-capacity intercalation cathode host. Here we propose a cyanoperovskite KxMnFe(CN)6 (0 ≤ x ≤ 2) as a potassium cathode: high-spin MnIII/MnII and low-spin FeIII/FeII couples have similar energies and exhibit two close plateaus centered at 3.6 V; two active K+ per formula unit enable a theoretical specific capacity of 156 mAh g-1; Mn and Fe are the two most-desired transition metals for electrodes because they are cheap and environmental friendly. As a powder prepared by an inexpensive precipitation method, the cathode delivers a specific capacity of 142 mAh g-1. The observed voltage, capacity, and its low cost make it competitive in large-scale electricity storage applications.

Hybrid Polymer/Garnet Electrolyte with a Small Interfacial Resistance for Lithium-Ion Batteries.

Li7 La3 Zr2 O12 -based Li-rich garnets react with water and carbon dioxide in air to form a Li-ion insulating Li2 CO3 layer on the surface of the garnet particles, which results in a large interfacial resistance for Li-ion transfer. Here, we introduce LiF to garnet Li6.5 La3 Zr1.5 Ta0.5 O12 (LLZT) to increase the stability of the garnet electrolyte against moist air; the garnet LLZT-2 wt % LiF (LLZT-2LiF) has less Li2 CO3 on the surface and shows a small interfacial resistance with Li metal, a solid polymer electrolyte, and organic-liquid electrolytes. An all-solid-state Li/polymer/LLZT-2LiF/LiFePO4 battery has a high Coulombic efficiency and long cycle life; a Li-S cell with the LLZT-2LiF electrolyte as a separator, which blocks the polysulfide transport towards the Li-metal, also has high Coulombic efficiency and kept 93 % of its capacity after 100 cycles.

Graphene Sandwiched by Sulfur-Confined Mesoporous Carbon Nanosheets: A Kinetically Stable Cathode for Li-S Batteries.

The practical use of lithium-sulfur batteries for the next-generation energy storage, especially the automobiles, was hindered by low electronic conductivity of sulfur and the resulting poor rate capabilities. Here, we report a sulfur-carbon composite by confining S into a graphene sandwiched in mesoporous carbon nanosheets with a two-dimensional ultrathin morphology, suitable mesopore size and large pore volume, and excellent electronic conductivity. Serving as cathode material for a Li-S battery, the elaborately designed S/C composite leads to "kinetically stable" transmissions of Li ions and electrons, triggering a stable electrochemistry and a record-breaking rate performance. In this way, the S/C composite has been proved a promising cathode material for high-rate Li-S batteries targeted at automobile storage.

NaxMV(PO4)3 (M = Mn, Fe, Ni) Structure and Properties for Sodium Extraction.

NASICON (Na+ super ionic conductor) structures of NaxMV(PO4)3 (M = Mn, Fe, Ni) were prepared, characterized by aberration-corrected STEM and synchrotron radiation, and demonstrated to be durable cathode materials for rechargeable sodium-ion batteries. In Na4MnV(PO4)3, two redox couples of Mn3+/Mn2+ and V4+/V3+ are accessed with two voltage plateaus located at 3.6 and 3.3 V and a capacity of 101 mAh g-1 at 1 C. Furthermore, the Na4MnV(PO4)3 cathode delivers a high initial efficiency of 97%, long durability over 1000 cycles, and good rate performance to 10 C. The robust framework structure and stable electrochemical performance makes it a reliable cathode materials for sodium-ion batteries.

Mastering the interface for advanced all-solid-state lithium rechargeable batteries.

A solid electrolyte with a high Li-ion conductivity and a small interfacial resistance against a Li metal anode is a key component in all-solid-state Li metal batteries, but there is no ceramic oxide electrolyte available for this application except the thin-film Li-P oxynitride electrolyte; ceramic electrolytes are either easily reduced by Li metal or penetrated by Li dendrites in a short time. Here, we introduce a solid electrolyte LiZr2(PO4)3 with rhombohedral structure at room temperature that has a bulk Li-ion conductivity σLi = 2 × 10-4 S⋅cm-1 at 25 °C, a high electrochemical stability up to 5.5 V versus Li+/Li, and a small interfacial resistance for Li+ transfer. It reacts with a metallic lithium anode to form a Li+-conducting passivation layer (solid-electrolyte interphase) containing Li3P and Li8ZrO6 that is wet by the lithium anode and also wets the LiZr2(PO4)3 electrolyte. An all-solid-state Li/LiFePO4 cell with a polymer catholyte shows good cyclability and a long cycle life.

Liquid K-Na Alloy Anode Enables Dendrite-Free Potassium Batteries.

A K-Na liquid alloy allows a dendrite-free high-capacity anode; its immiscibility with an organic liquid electrolyte offers a liquid-liquid anode-electrolyte interface. Working with a sodiated Na2 MnFe(CN)6 cathode, the working cation becomes K+ to give a potassium battery of long cycle life with an acceptable capacity at high charge/discharge rates.

Aligned carbon nanotube-silicon sheets: a novel nano-architecture for flexible lithium ion battery electrodes.

Aligned carbon nanotube sheets provide an engineered scaffold for the deposition of a silicon active material for lithium ion battery anodes. The sheets are low-density, allowing uniform deposition of silicon thin films while the alignment allows unconstrained volumetric expansion of the silicon, facilitating stable cycling performance. The flat sheet morphology is desirable for battery construction.

Carbon-coated Si nanoparticles dispersed in carbon nanotube networks as anode material for lithium-ion batteries.

Si has the highest theoretical capacity among all known anode materials, but it suffers from the dramatic volume change upon repeated lithiation and delithiation processes. To overcome the severe volume changes, Si nanoparticles were first coated with a polymer-driven carbon layer, and then dispersed in a CNT network. In this unique structure, the carbon layer can improve electric conductivity and buffer the severe volume change, whereas the tangled CNT network is expected to provide additional mechanical strength to maintain the integrity of electrodes, stabilize the electric conductive network for active Si, and eventually lead to better cycling performance. Electrochemical test result indicates the carbon-coated Si nanoparticles dispersed in CNT networks show capacity retention of 70% after 40 cycles, which is much better than the carbon-coated Si nanoparticles without CNTs.