A rechargeable calcium-oxygen battery that operates at room temperature.

Calcium-oxygen (Ca-O2) batteries can theoretically afford high capacity by the reduction of O2 to calcium oxide compounds (CaOx) at low cost1-5. Yet, a rechargeable Ca-O2 battery that operates at room temperature has not been achieved because the CaOx/O2 chemistry typically involves inert discharge products and few electrolytes can accommodate both a highly reductive Ca metal anode and O2. Here we report a Ca-O2 battery that is rechargeable for 700 cycles at room temperature. Our battery relies on a highly reversible two-electron redox to form chemically reactive calcium peroxide (CaO2) as the discharge product. Using a durable ionic liquid-based electrolyte, this two-electron reaction is enabled by the facilitated Ca plating-stripping in the Ca metal anode at room temperature and improved CaO2/O2 redox in the air cathode. We show the proposed Ca-O2 battery is stable in air and can be made into flexible fibres that are weaved into textile batteries for next-generation wearable systems.

Scalable production of high-performing woven lithium-ion fibre batteries.

Fibre lithium-ion batteries are attractive as flexible power solutions because they can be woven into textiles, offering a convenient way to power future wearable electronics1-4. However, they are difficult to produce in lengths of more than a few centimetres, and longer fibres were thought to have higher internal resistances3,5 that compromised electrochemical performance6,7. Here we show that the internal resistance of such fibres has a hyperbolic cotangent function relationship with fibre length, where it first decreases before levelling off as length increases. Systematic studies confirm that this unexpected result is true for different fibre batteries. We are able to produce metres of high-performing fibre lithium-ion batteries through an optimized scalable industrial process. Our mass-produced fibre batteries have an energy density of 85.69 watt hour per kilogram (typical values8 are less than 1 watt hour per kilogram), based on the total weight of a lithium cobalt oxide/graphite full battery, including packaging. Its capacity retention reaches 90.5% after 500 charge-discharge cycles and 93% at 1C rate (compared with 0.1C rate capacity), which is comparable to commercial batteries such as pouch cells. Over 80 per cent capacity can be maintained after bending the fibre for 100,000 cycles. We show that fibre lithium-ion batteries woven into safe and washable textiles by industrial rapier loom can wirelessly charge a cell phone or power a health management jacket integrated with fibre sensors and a textile display.

Suppressing metal corrosion through identification of optimal crystallographic plane for Zn batteries.

Direct use of metals as battery anodes could significantly boost the energy density, but suffers from limited cycling. To make the batteries more sustainable, one strategy is mitigating the propensity for metals to form random morphology during plating through orientation regulation, e.g., hexagonal Zn platelets locked horizontally by epitaxial electrodeposition or vertically aligned through Zn/electrolyte interface modulation. Current strategies center around obtaining (002) faceted deposition due to its minimum surface energy. Here, benefiting from the capability of preparing a library of faceted monocrystalline Zn anodes and controlling the orientation of Zn platelet deposits, we challenge this conventional belief. We show that while monocrystalline (002) faceted Zn electrode with horizontal epitaxy indeed promises the highest critical current density, the (100) faceted electrode with vertically aligned deposits is the most important one in suppressing Zn metal corrosion and promising the best reversibility. Such uniqueness results from the lowest electrochemical surface area of (100) faceted electrode, which intrinsically builds upon the surface atom diffusion barrier and the orientation of the pallets. These new findings based on monocrystalline anodes advance the fundamental understanding of electrodeposition process for sustainable metal batteries and provide a paradigm to explore the processing-structure-property relationships of metal electrodes.

A Gradient Composite Structure Enables a Stable Microsized Silicon Suboxide-Based Anode for a High-Performance Lithium-Ion Battery.

The practical application of microsized anodes is hindered by severe volume changes and fast capacity fading. Herein, we propose a gradient composite strategy and fabricate a silicon suboxide-based composite anode (d-SiO@SiOx/C@C) consisting of a disproportionated microsized SiO inner core, a homogeneous composite SiOx/C interlayer (x ≈ 1.5), and a highly graphitized carbon outer layer. The robust SiOx/C interlayer can realize a gradient abatement of stress and simultaneously connect the inner SiO core and carbon outer layer through covalent bonds. As a result, d-SiO@SiOx/C@C delivers a specific capacity of 1023 mAh/g after 300 cycles at 1 A/g with a retention of >90% and an average Coulombic efficiency of >99.7%. A full cell assembled with a LiNi0.8Co0.15Al0.05O2 cathode displays a remarkable specific energy density of 569 Wh/kg based on total active materials as well as excellent cycling stability. Our strategy provides a promising alternative for designing structurally and electrochemically stable microsized anodes with high capacity.

Tunneling Interpenetrative Lithium Ion Conduction Channels in Polymer-in-Ceramic Composite Solid Electrolytes.

Polymer-in-ceramic composite solid electrolytes (PIC-CSEs) provide important advantages over individual organic or inorganic solid electrolytes. In conventional PIC-CSEs, the ion conduction pathway is primarily confined to the ceramics, while the faster routes associated with the ceramic-polymer interface remain blocked. This challenge is associated with two key factors: (i) the difficulty in establishing extensive and uninterrupted ceramic-polymer interfaces due to ceramic aggregation; (ii) the ceramic-polymer interfaces are unresponsive to conducting ions because of their inherent incompatibility. Here, we propose a strategy by introducing polymer-compatible ionic liquids (PCILs) to mediate between ceramics and the polymer matrix. This mediation involves the polar groups of PCILs interacting with Li+ ions on the ceramic surfaces as well as the interactions between the polar components of PCILs and the polymer chains. This strategy addresses the ceramic aggregation issue, resulting in uniform PIC-CSEs. Simultaneously, it activates the ceramic-polymer interfaces by establishing interpenetrating channels that promote the efficient transport of Li+ ions across the ceramic phase, the ceramic-polymer interfaces, and the intervening pathways. Consequently, the obtained PIC-CSEs exhibit high ionic conductivity, exceptional flexibility, and robust mechanical strength. A PIC-CSE comprising poly(vinylidene fluoride) (PVDF) and 60 wt % PCIL-coated Li3Zr2Si2PO12 (LZSP) fillers showcasing an ionic conductivity of 0.83 mS cm-1, a superior Li+ ion transference number of 0.81, and an elongation of ∼300% at 25 °C could be produced on meter-scale. Its lithium metal pouch cells show high energy densities of 424.9 Wh kg-1 (excluding packing films) and puncture safety. This work paves the way for designing PIC-CSEs with commercial viability.

Exploring Carbonization Temperature to Create Closed Pores for Hard Carbon as High-Performance Sodium-Ion Battery Anodes.

Biomass-derived porous carbon materials are meaningful to employ as a hard carbon precursor for anode materials of sodium-ion batteries (SIBs) from a sustainability perspective. Here, a straightforward approach is proposed to develop rich closed pores in pinenut-derived carbon, with the aim of improving Na+ plateau storage by adjusting the pyrolysis temperature. The optimized sample, namely the pinenut-derived carbon at 1300 °C, demonstrates remarkable reversible specific capacity of 278 mAh g-1, along with a high initial Coulomb efficiency of 85% and robust cycling stability (with a capacity retention of 89% after 800 cycles at 0.2 A g-1). In situ and ex situ analyses unveil that the developed closed pores play a significant role in enhancing the plateau capacity, providing compelling evidence for the "adsorption-filling" mechanism. Moreover, the corresponding full-cell achieves a high energy density of 245.7 Wh kg-1 (based on the total weight of both electrode active materials) and exhibits outstanding rate capability (191.4 mAh g-1 at 3 A g-1).

Perspective on Iron-Based Phosphate Cathode for Commercial Sodium-Ion Cells.

Sodium (Na)-ion batteries (SIBs) have been considered as a potential device for large-scale energy storage. To date, some start-up companies have released their first-generation SIBs cathode materials. Among them, phosphate compounds, particularly iron (Fe)-based mixed phosphate compounds, present great potential for commercial SIBs owing to its low cost, environment friendly. In this perspective, a brief historical retrospect is first introduce to the development of Fe-based mixed phosphate cathodes in SIBs. Then, the recent development about this kind of cathode has been summarized. One of the iron-based phosphate materials, Na3 Fe2 (PO4 )P2 O7 , is used as an example to roughly calculate the energy density and estimate the cost at the cell level to highlight their advantages. Finally, some strategies are put up to further increase the energy density of SIBs. This timely perspective aims to educate the community on the critical benefits of the Fe-based mixed phosphate cathode and provide an up-to-date overview of this emerging field.

Stacking Order Induced Anion Redox Regulation for Layer-Structured Na0.75 Li0.2 Mn0.7 Cu0.1 O2 Cathode Materials.

Stacking order plays a key role in defining the electrochemical behavior and structural stability of layer-structured cathode materials. However, the detailed effects of stacking order on anionic redox in layer-structured cathode materials have not been investigated specifically and are still unrevealed. Herein, two layered cathodes with the same chemical formula but different stacking orders: P2-Na0.75 Li0.2 Mn0.7 Cu0.1 O2 (P2-LMC) and P3-Na0.75 Li0.2 Mn0.7 Cu0.1 O2 (P3-LMC) are compared. It is found that P3 stacking order is beneficial to improve the oxygen redox reversibility compared with P2 stacking order. By using synchrotron hard and soft X-ray absorption spectroscopies, three redox couples of Cu2+ /Cu3+ , Mn3.5+ /Mn4+ , and O2- /O- are revealed to contribute charge compensation in P3 structure simultaneously, and two redox couples of Cu2+ /Cu3+ and O2- /O- are more reversible than those in P2-LMC due to the higher electronic densities in Cu 3d and O 2p orbitals in P3-LMC. In situ X-ray diffraction reveals that P3-LMC exhibits higher structural reversibility during charge and discharge than P2-LMC, even at 5C rate. As a result, P3-LMC delivers a high reversible capacity of 190.3 mAh g-1 and capacity retention of 125.7 mAh g-1 over 100 cycles. These findings provide new insight into oxygen-redox-involved layered cathode materials for SIBs.

Chemical physics of electrochemical energy materials.

Probing the chemistry and materials science of electrochemical energy materials is a central topic in both chemical physics and energy chemistry due to the increasingly important role of energy devices in the current and future energy system and industry. Especially, understanding the chemical physics of electrochemical energy materials is the key to enhance the performance of energy storage and conversion devices such as batteries, fuel cells, electrolyzers, and supercapacitors. This special topic focuses on the fundamental understanding of electrochemical energy applications, including electrochemistry fundamentals, structural dynamics and degradation mechanism of materials, optimization strategies for improving electrochemical performance of energy devices, and emerging simulation and characterization methods applied to advanced energy materials.

Constructing Solid Electrolyte Interphase for Aqueous Zinc Batteries.

Problems of zinc anode including dendrite and hydrogen evolution seriously degrade the performance of zinc batteries. Solid electrolyte interphase (SEI), which plays a key role in achieving high reversibility of lithium anode in aprotic organic solvent, is also beneficial to performance improvement of zinc anode in aqueous electrolyte. However, various studies about interphase for zinc electrode is quite fragmented, and lack of deep understanding on root causes or general design rules for SEI construction. And water molecules with high reactivity brings serious challenge to the effective SEI construction. Here, we reviewed the brief development history of zinc batteries firstly, then summarized the approaches to construct SEI in aqueous electrolyte. Furthermore, the formation mechanisms behind approaches are systematically analyzed, together with discussion on the SEI components and evaluation on electrochemical performance of zinc anode with various types of SEI. Meanwhile, the challenge between lab and industrialization are also discussed.

Tuning Mass Transport in Electrocatalysis Down to Sub-5†nm through Nanoscale Grade Separation.

Nano and single-atom catalysis open new possibilities of producing green hydrogen (H2 ) by water electrolysis. However, for the hydrogen evolution reaction (HER) which occurs at a characteristic reaction rate proportional to the potential, the fast generation of H2 nanobubbles at atomic-scale interfaces often leads to the blockage of active sites. Herein, a nanoscale grade-separation strategy is proposed to tackle mass-transport problem by utilizing ordered three-dimensional (3d) interconnected sub-5 nm pores. The results reveal that 3d criss-crossing mesopores with grade separation allow efficient diffusion of H2 bubbles along the interconnected channels. After the support of ultrafine ruthenium (Ru), the 3d mesopores are on a superior level to two-dimensional system at maximizing the catalyst performance and the obtained Ru catalyst outperforms most of the other HER catalysts. This work provides a potential route to fine-tuning few-nanometer mass transport during water electrolysis.

Pilot-Scale Synthesis Sodium Iron Fluorophosphate Cathode with High Tap Density for a Sodium Pouch Cell.

Sodium-ion batteries (SIBs) have attracted wide interest for energy storage because of the sufficient sodium element reserve on the earth; however, the electrochemical performance of SIBs cannot achieve the requirements so far, especially, the limitation of cathode materials. Here, a kilogram-scale route to synthesize Na2 FePO4 F/carbon/multi-walled carbon nanotubes microspheres (NFPF@C@MCNTs) composite with a high tap density of 1.2 g cm-3 is reported. The NFPF@C@MCNTs cathode exhibits a reversible specific capacity of 118.4 mAh g-1 at 0.1 C. Even under 5 C with high mass loading (10 mg cm-2 ), the specific capacity still maintains at 56.4 mAh g-1 with a capacity retention rate of 97% after 700 cycles. In addition, a hard carbon||NFPF@C@MCNTs pouch cell is assembled and tested, which exhibits a volumetric energy density of 325 Wh L-1 and gravimetrical energy density of 210 Wh kg-1 (base on electrode massing), and it provides more than 200 cycles with a capacity retention rate of 92%. Furthermore, the pouch cell can operate in an all-climate environment ranging from -40 to 80 °C. These results demonstrate that the NFPF@C@MCNTs microspheres are a promising candidate cathode for SIBs and facilitate its practical application in sodium cells.

Promoting Rechargeable Batteries Operated at Low Temperature.

ConspectusBuilding rechargeable batteries for subzero temperature application is highly demanding for various specific applications including electric vehicles, grid energy storage, defense/space/subsea explorations, and so forth. Commercialized nonaqueous lithium ion batteries generally adapt to a temperature above -20 °C, which cannot well meet the requirements under colder conditions. Certain improvements have been achieved with nascent materials and electrolyte systems but have mainly been restrained to discharge and within a small rate at temperatures above -40 °C. Moreover, the recharging process of batteries based on the graphite anode still faces huge challenges from the simultaneous Li+ intercalation and potential Li stripping at subzero temperatures. Revealing the temperature-dependent evolution of physicochemical and electrochemical properties will greatly benefit our understanding of the limiting factors at low temperature, which is of significant importance.Herein, we dissect the ion movements in the liquid electrolyte and solid electrode as well as their interphase to analyze the temperature effect on Li+-diffusion behavior during charging/discharging processes. An electrolyte is the vital factor, and its ionic conductivity guarantees the smooth operation of the battery. However, it is the sluggish diffusion in the solid, especially the charge transfer at the solid electrolyte/electrode interfaces (SEI), that greatly limits the kinetics at low temperature. Many strategies have been put forward to tame electrolytes for low-temperature application. From a macroscopic point of view, multiple solvents are mixed to adjust the liquid temperature range and viscosity. With respect to the microscopic nature, research is focusing on the solvation structure by formulating the ratio of Li+ ions to solvent molecules. The binding energy of the Li+-solvent complex is crucial for the desolvation process at low temperature, which is manipulated with fluorinated solvents or other weakly solvating electrolytes. On the basis of an optimized electrolyte, electrodes and their reaction mechanism need to be coupled carefully because different materials show totally different responses to temperature change. To avoid the sluggish desolvation process or slow diffusion in the bulk intercalation compounds, several kinds of materials are summarized for low temperature use. The intercalation pseudocapacitive behavior can compensate for the kinetics to some extent, and a metal anode is a good candidate for replacing a graphite anode to build high-energy-density batteries at subzero temperature. It is also a wise choice to develop nascent battery chemistry based on the co-intercalation of solvent molecules into electrodes. Furthermore, the interfacial resistance contributes a lot at low temperature, which need be modified to accelerate the Li+ diffusion across the film. This will be linked to the electrolyte, exactly speaking, the solvation structure, to regulate the organic and inorganic components as well as the structure. Although it is difficult to investigate SEI on a graphite anode owing to its poor performance at low temperature, great efforts on Li metal anodes have offered some valuable information as reference. It is worth mentioning that the improvement in low-temperature performance calls for not only a change in the single composition but also the synergetic effect of each part in the whole battery. The elementary studies covered in this account could be taken as insight into some key strategies that help advance the low-temperature battery chemistry.

Synergy of Weakly-Solvated Electrolyte and Optimized Interphase Enables Graphite Anode Charge at Low Temperature.

Graphite anode suffers from great capacity loss and even fails to charge (i.e. Li+ -intercalation) under low temperature, mainly arising from the large overpotential including sluggish de-solvation process and insufficient ions movement in the solid electrolyte interphase (SEI). Herein, an electrolyte is developed by utilizing weakly solvated molecule ethyl trifluoroacetate and film-forming fluoroethylene carbonate to achieve smooth de-solvation and high ionic conductivity at low temperature. Evolution of SEI formed at different temperatures is further investigated to propose an effective room-temperature SEI formation strategy for low-temperature operations. The synergetic effect of tamed electrolyte and optimized SEI enables graphite with a reversible charge/discharge capacity of 183 mAh g-1 at -30 °C and fast-charging up to 6C-rate at room temperature. Moreover, graphite||LiFePO4 full cell maintains a capacity retention of 78 % at -30 °C, and 37 % even at a super-low temperature of -60 °C. This work offers a progressive insight towards fast-charging and low-temperature batteries.

VPO4 F Fluorophosphates Polyanion Cathodes for High-Voltage Proton Storage.

Proton batteries are emerging in electrochemical energy storage because of the associated fast kinetics, low cost and high safety. However, their development is hindered by the relatively low energy density due to the limited choice of cathode materials. Herein, metal phosphate polyanion cathodes are proposed as the proton cathode for the first time. Combining experimental results and theoretical simulations, a universal criterion for the proton cathode was put forward. Vanadium fluorophosphate (VPO4 F) was demonstrated as a promising high-voltage proton cathode material with a specific capacity of 116 mAh g-1 at a high potential of 1.0 V (vs. SHE). The proton insertion/extraction mechanism in the VPO4 F electrode was also verified through X-ray diffraction (XRD) and photoelectron spectroscopy (XPS). Furthermore, the stability of VPO4 F was investigated in various electrolytes and the optimized electrolyte enabled the stable operation of VPO4 F for 300 cycles. This work provides new inspiration in the exploitation of new electrode materials for electrochemical proton storage devices.

Redox mediators as charge agents for changing electrochemical reactions.

Redox mediators (RMs) play pivotal roles in enhancing the performance of electrochemical energy storage and conversion systems. Unlike the widely explored areas of electrode materials, electrolytes, separators, and electrolyte additives, RMs have received little attention. This review provides a comprehensive discussion toward understanding the effects of RMs on electrochemical systems, underlying redox mechanisms, and reaction kinetics both experimentally and theoretically. Our discussion focuses on the roles of RMs in various electrochemical systems such as lithium-ion batteries, Li-O2 batteries, Li-S batteries, decoupling electrolysis, supercapacitors, and microbial fuel cells. Depending on the reaction regions where the RMs become active, we can classify them into bulk, solid-solid interfacial, solid-liquid interfacial, and cell-unit RMs. The prospect of developing RMs with effective charge transfer properties along with minimal side-effects is an exciting research direction. Moreover, the introduction of an efficient RM into an electrochemical system can fundamentally change its chemistry; in particular, the electrode reaction polarization can be considerably decreased. In this context, we discuss the key properties of RMs applied for various purposes, and the main issues are addressed.

A Desolvation-Free Sodium Dual-Ion Chemistry for High Power Density and Extremely Low Temperature.

The development of conventional rechargeable batteries based on intercalation chemistry in the fields of fast charge and low temperature is generally hindered by the sluggish cation-desolvation process at the electrolyte/electrode interphase. To address this issue, a novel desolvation-free sodium dual-ion battery (SDIB) has been proposed by using artificial graphite (AG) as anode and polytriphenylamine (PTPAn) as cathode. Combining the cation solvent co-intercalation and anion storage chemistry, such a SDIB operated with ether-based electrolyte can intrinsically eliminate the sluggish desolvation process. Hence, it can exhibit an extremely fast kinetics of 10 Ag-1 (corresponding to 100C-rate) with a high capacity retention of 45 %. Moreover, the desolvation-free mechanism endows the battery with 61 % of its room-temperature capacity at an ultra-low temperature of -70 °C. This advanced battery system will open a door for designing energy storage devices that require high power density and a wide operational temperature range.

Advanced Electrolyte Design for High-Energy-Density Li-Metal Batteries under Practical Conditions.

Given the limitations inherent in current intercalation-based Li-ion batteries, much research attention has focused on potential successors to Li-ion batteries such as lithium-sulfur (Li-S) batteries and lithium-oxygen (Li-O2 ) batteries. In order to realize the potential of these batteries, the use of metallic lithium as the anode is essential. However, there are severe safety hazards associated with the growth of Li dendrites, and the formation of "dead Li" during cycles leads to the inevitable loss of active Li, which in the end is undoubtedly detrimental to the actual energy density of Li-metal batteries. For Li-metal batteries under practical conditions, a low negative/positive ratio (N/P ratio), a electrolyte/cathode ratio (E/C ratio) along with a high-voltage cathode is prerequisite. In this Review, we summarize the development of new electrolyte systems for Li-metal batteries under practical conditions, revisit the design criteria of advanced electrolytes for practical Li-metal batteries and provide perspectives on future development of electrolytes for practical Li-metal batteries.

Li/Garnet Interface Stabilization by Thermal-Decomposition Vapor Deposition of an Amorphous Carbon Layer.

Applying interlayers is the main strategy to address the large area specific resistance (ASR) of Li/garnet interface. However, studies on eliminating the Li2 CO3 and LiOH interfacial lithiophobic contaminants are still insufficient. Here, thermal-decomposition vapor deposition (TVD) of a carbon modification layer on Li6.75 La3 Zr1.75 Ta0.25 O12 (LLZTO) provides a contaminant-free surface. Owing to the protection of the carbon layer, the air stability of LLZTO is also improved. Moreover, owing to the amorphous structure of the low graphitized carbon (LGC), instant lithiation is achieved, and the ASR of the Li/LLZTO interface is reduced to 9â€…Ω cm2 . Lithium volatilization and Zr4+ reduction are also controllable during TVD. Compared with its high graphitized carbon counterpart (HGC), the LGC-modified Li/LLZTO interface displays a higher critical current density of 1.2 mA cm-2 , as well as moderate Li plating and stripping, which provides enhanced polarization voltage stability.

Tuning P2-Structured Cathode Material by Na-Site Mg Substitution for Na-Ion Batteries.

Most P2-type layered oxides suffer from multiple voltage plateaus, due to Na+/vacancy-order superstructures caused by strong interplay between Na-Na electrostatic interactions and charge ordering in the transition metal layers. Here, Mg ions are successfully introduced into Na sites in addition to the conventional transition metal sites in P2-type Na0.7[Mn0.6Ni0.4]O2 as new cathode materials for sodium-ion batteries. Mg ions in the Na layer serve as "pillars" to stabilize the layered structure, especially for high-voltage charging, meanwhile Mg ions in the transition metal layer can destroy charge ordering. More importantly, Mg ion occupation in both sodium and transition metal layers will be able to create "Na-O-Mg" and "Mg-O-Mg" configurations in layered structures, resulting in ionic O 2p character, which allocates these O 2p states on top of those interacting with transition metals in the O-valence band, thus promoting reversible oxygen redox. This innovative design contributes smooth voltage profiles and high structural stability. Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]O2 exhibits superior electrochemical performance, especially good capacity retention at high current rate under a high cutoff voltage (4.2 V). A new P2 phase is formed after charge, rather than an O2 phase for the unsubstituted material. Besides, multiple intermediate phases are observed during high-rate charging. Na-ion transport kinetics are mainly affected by elemental-related redox couples and structural reorganization. These findings will open new opportunities for designing and optimizing layer-structured cathodes for sodium-ion batteries.