Understanding the Coupling Mechanism of Intercalation and Conversion Hybrid Storage in Lithium-Graphite Anode.

Anodes with high capacity and long lifespan play an important role in the advanced batteries. However, none of the existing anodes can meet these two requirements simultaneously. Lithium (Li)-graphite composite anode presents great potential in balancing these two requirements. Herein, the working mechanism of Li-graphite composite anode is comprehensively investigated. The capacity decay features of the composite anode are different from those of Li ion intercalation in Li ion batteries and Li metal deposition in Li metal batteries. An intercalation and conversion hybrid storage mechanism are proposed by analyzing the capacity decay ratios in the composite anode with different initial specific capacities. The capacity decay models can be divided into four stages including Capacity Retention Stage, Relatively Independent Operation Stage, Intercalation & Conversion Coupling Stage, Pure Li Intercalation Stage. When the specific capacity is between 340 and 450 mAh g-1, its capacity decay ratio is between that of pure intercalation and conversion model. These results intensify the comprehensive understandings on the working principles in Li-graphite composite anode and present novel insights in the design of high-capacity and long-lifespan anode materials for the next-generation batteries.

Thermally Stable Polymer-Rich Solid Electrolyte Interphase for Safe Lithium Metal Pouch Cells.

Serious safety risks caused by the high reactivity of lithium metal against electrolytes severely hamper the practicability of lithium metal batteries. By introducing unique polymerization site and more fluoride substitution, we built an in situ formed polymer-rich solid electrolyte interphase upon lithium anode to improve battery safety. The fluorine-rich and hydrogen-free polymer exhibits high thermal stability, which effectively reduces the continuous exothermic reaction between electrolyte and anode/cathode. As a result, the critical temperature for thermal safety of 1.0 Ah lithium-LiNi0.5 Co0.2 Mn0.3 O2 pouch cell can be increased from 143.2 °C to 174.2 °C. The more dangerous "ignition" point of lithium metal batteries, the starting temperature of battery thermal runaway, has been dramatically raised from 240.0 °C to 338.0 °C. This work affords novel strategies upon electrolyte design, aiming to pave the way for high-energy-density and thermally safe lithium metal batteries.

An anion-immobilized composite electrolyte for dendrite-free lithium metal anodes.

Lithium metal is strongly regarded as a promising electrode material in next-generation rechargeable batteries due to its extremely high theoretical specific capacity and lowest reduction potential. However, the safety issue and short lifespan induced by uncontrolled dendrite growth have hindered the practical applications of lithium metal anodes. Hence, we propose a flexible anion-immobilized ceramic-polymer composite electrolyte to inhibit lithium dendrites and construct safe batteries. Anions in the composite electrolyte are tethered by a polymer matrix and ceramic fillers, inducing a uniform distribution of space charges and lithium ions that contributes to a dendrite-free lithium deposition. The dissociation of anions and lithium ions also helps to reduce the polymer crystallinity, rendering stable and fast transportation of lithium ions. Ceramic fillers in the electrolyte extend the electrochemically stable window to as wide as 5.5 V and provide a barrier to short circuiting for realizing safe batteries at elevated temperature. The anion-immobilized electrolyte can be applied in all-solid-state batteries and exhibits a small polarization of 15 mV. Cooperated with LiFePO4 and LiNi0.5Co0.2Mn0.3O2 cathodes, the all-solid-state lithium metal batteries render excellent specific capacities of above 150 mAh⋅g-1 and well withstand mechanical bending. These results reveal a promising opportunity for safe and flexible next-generation lithium metal batteries.

A Diffusion--Reaction Competition Mechanism to Tailor Lithium Deposition for Lithium-Metal Batteries.

Lithium metal is recognized as one of the most promising anode materials owing to its ultrahigh theoretical specific capacity and low electrochemical potential. Nonetheless, dendritic Li growth has dramatically hindered the practical applications of Li metal anodes. Realizing spherical Li deposition is an effective approach to avoid Li dendrite growth, but the mechanism of spherical deposition is unknown. Herein, a diffusion-reaction competition mechanism is proposed to reveal the rationale of different Li deposition morphologies. By controlling the rate-determining step (diffusion or reaction) of Li deposition, various Li deposition scenarios are realized, in which the diffusion-controlled process tends to lead to dendritic Li deposition while the reaction-controlled process leads to spherical Li deposition. This study sheds fresh light on the dendrite-free Li metal anode and guides to achieve safe batteries to benefit future wireless and fossil-fuel-free world.

Regulating the Inner Helmholtz Plane for Stable Solid Electrolyte Interphase on Lithium Metal Anodes.

The stability of a battery is strongly dependent on the feature of solid electrolyte interphase (SEI). The electrical double layer forms prior to the formation of SEI at the interface between the Li metal anode and the electrolyte. The fundamental understanding on the regulation of the SEI structure and stability on Li surface through the structure of the electrical double layer is highly necessary for safe batteries. Herein, the interfacial chemistry of the SEI is correlated with the initial Li surface adsorption electrical double layer at the nanoscale through theoretical and experimental analysis. Under the premise of the constant solvation sheath structure of Li+ in bulk electrolyte, a trace amount of lithium nitrate (LiNO3) and copper fluoride (CuF2) were employed in electrolytes to build robust electric double layer structures on a Li metal surface. The distinct results were achieved with the initial competitive adsorption of bis(fluorosulfonyl)imide ion (FSI-), fluoride ion (F-), and nitrate ion (NO3-) in the inner Helmholtz plane. As a result, Cu-NO3- complexes are preferentially adsorbed and reduced to form the SEI. The modified Li metal electrode can achieve an average Coulombic efficiency of 99.5% over 500 cycles, enabling a long lifespan and high capacity retention of practical rechargeable batteries. The as-proposed mechanism bridges the gap between Li+ solvation and the adsorption about the electrode interface formation in a working battery.

Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.

The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applications is included. A general conclusion and a perspective on the current limitations and recommended future research directions of lithium metal batteries are presented. The review concludes with an attempt at summarizing the theoretical and experimental achievements in lithium metal anodes and endeavors to realize the practical applications of lithium metal batteries.

Highly Stable Lithium Metal Batteries Enabled by Regulating the Solvation of Lithium Ions in Nonaqueous Electrolytes.

Safe and rechargeable lithium metal batteries have been difficult to achieve because of the formation of lithium dendrites. Herein an emerging electrolyte based on a simple solvation strategy is proposed for highly stable lithium metal anodes in both coin and pouch cells. Fluoroethylene carbonate (FEC) and lithium nitrate (LiNO3 ) were concurrently introduced into an electrolyte, thus altering the solvation sheath of lithium ions, and forming a uniform solid electrolyte interphase (SEI), with an abundance of LiF and LiNx Oy on a working lithium metal anode with dendrite-free lithium deposition. Ultrahigh Coulombic efficiency (99.96 %) and long lifespans (1000 cycles) were achieved when the FEC/LiNO3 electrolyte was applied in working batteries. The solvation chemistry of electrolyte was further explored by molecular dynamics simulations and first-principles calculations. This work provides insight into understanding the critical role of the solvation of lithium ions in forming the SEI and delivering an effective route to optimize electrolytes for safe lithium metal batteries.

Lithium Nitrate Solvation Chemistry in Carbonate Electrolyte Sustains High-Voltage Lithium Metal Batteries.

The lithium metal anode is regarded as a promising candidate in next-generation energy storage devices. Lithium nitrate (LiNO3 ) is widely applied as an effective additive in ether electrolyte to increase the interfacial stability in batteries containing lithium metal anodes. However, because of its poor solubility LiNO3 is rarely utilized in the high-voltage window provided by carbonate electrolyte. Dissolution of LiNO3 in carbonate electrolyte is realized through an effective solvation regulation strategy. LiNO3 can be directly dissolved in an ethylene carbonate/diethyl carbonate electrolyte mixture by adding trace amounts of copper fluoride as a dissolution promoter. LiNO3 protects the Li metal anode in a working high-voltage Li metal battery. When a LiNi0.80 Co0.15 Al0.05 O2 cathode is paired with a Li metal anode, an extraordinary capacity retention of 53 % is achieved after 300 cycles (13 % after 200 cycles for LiNO3 -free electrolyte) and a very high average Coulombic efficiency above 99.5 % is achieved at 0.5 C. The solvation chemistry of LiNO3 -containing carbonate electrolyte may sustain high-voltage Li metal anodes operating in corrosive carbonate electrolytes.

Ion-Solvent Complexes Promote Gas Evolution from Electrolytes on a Sodium Metal Anode.

Lithium and sodium metal batteries are considered as promising next-generation energy storage devices due to their ultrahigh energy densities. The high reactivity of alkali metal toward organic solvents and salts results in side reactions, which further lead to undesirable electrolyte depletion, cell failure, and evolution of flammable gas. Herein, first-principles calculations and in situ optical microscopy are used to study the mechanism of organic electrolyte decomposition and gas evolution on a sodium metal anode. Once complexed with sodium ions, solvent molecules show a reduced LUMO, which facilitates the electrolyte decomposition and gas evolution. Such a general mechanism is also applicable to lithium and other metal anodes. We uncover the critical role of ion-solvent complexation for the stability of alkali metal anodes, reveal the mechanism of electrolyte gassing, and provide a mechanistic guidance to electrolyte and lithium/sodium anode design for safe rechargeable batteries.

Healing High-Loading Sulfur Electrodes with Unprecedented Long Cycling Life: Spatial Heterogeneity Control.

Self-healing capability helps biological systems to maintain their survivability and extend their lifespan. Similarly, self-healing is also beneficial to next-generation secondary batteries because high-capacity electrode materials, especially the cathodes such as oxygen or sulfur, suffer from shortened cycle lives resulting from irreversible and unstable phase transfer. Herein, by mimicking a biological self-healing process, fibrinolysis, we introduced an extrinsic healing agent, polysulfide, to enable the stable operation of sulfur microparticle (SMiP) cathodes. An optimized capacity (∼3.7 mAh cm-2) with almost no decay after 2000 cycles at a high sulfur loading of 5.6 mg(S) cm-2 was attained. The inert SMiP is activated by the solubilization effect of polysulfides whereas the unstable phase transfer is mediated by mitigated spatial heterogeneity of polysulfides, which induces uniform nucleation and growth of solid compounds. The comprehensive understanding of the healing process, as well as of the spatial heterogeneity, could further guide the design of novel healing agents (e.g., lithium iodine) toward high-performance rechargeable batteries.

Powering Lithium-Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts.

Lithium-sulfur (Li-S) battery system is endowed with tremendous energy density, resulting from the complex sulfur electrochemistry involving multielectron redox reactions and phase transformations. Originated from the slow redox kinetics of polysulfide intermediates, the flood of polysulfides in the batteries during cycling induced low sulfur utilization, severe polarization, low energy efficiency, deteriorated polysulfide shuttle, and short cycling life. Herein, sulfiphilic cobalt disulfide (CoS2) was incorporated into carbon/sulfur cathodes, introducing strong interaction between lithium polysulfides and CoS2 under working conditions. The interfaces between CoS2 and electrolyte served as strong adsorption and activation sites for polar polysulfides and therefore accelerated redox reactions of polysulfides. The high polysulfide reactivity not only guaranteed effective polarization mitigation and promoted energy efficiency by 10% but also promised high discharge capacity and stable cycling performance during 2000 cycles. A slow capacity decay rate of 0.034%/cycle at 2.0 C and a high initial capacity of 1368 mAh g(-1) at 0.5 C were achieved. Since the propelling redox reaction is not limited to Li-S system, we foresee the reported strategy herein can be applied in other high-power devices through the systems with controllable redox reactions.

Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite-Free Lithium Metal Anodes.

Lithium (Li) metal is the most promising electrode for next-generation rechargeable batteries. However, the challenges induced by Li dendrites on a working Li metal anode hinder the practical applications of Li metal batteries. Herein, nitrogen (N) doped graphene was adopted as the Li plating matrix to regulate Li metal nucleation and suppress dendrite growth. The N-containing functional groups, such as pyridinic and pyrrolic nitrogen in the N-doped graphene, are lithiophilic, which guide the metallic Li nucleation causing the metal to distribute uniformly on the anode surface. As a result, the N-doped graphene modified Li metal anode exhibits a dendrite-free morphology during repeated Li plating and demonstrates a high Coulombic efficiency of 98 % for near 200 cycles.

Columnar Lithium Metal Anodes.

The rechargeable lithium metal anode is of utmost importance for high-energy-density batteries. Regulating the deposition/dissolution characteristics of Li metal is critical in both fundamental researches and practical applications. In contrast to gray Li deposits featured with dendritic and mossy morphologies, columnar and uniform Li is herein plated on lithium-fluoride (LiF)-protected copper (Cu) current collectors. The electrochemical properties strongly depended on the microscale morphologies of deposited Li, which were further embodied as macroscale colors. The as-obtained ultrathin and columnar Li anodes contributed to stable cycling in working batteries with a dendrite-free feature. This work deepens the fundamental understanding of the role of LiF in the nucleation/growth of Li and provides emerging approaches to stabilize rechargeable Li metal anodes.

Rational Integration of Polypropylene/Graphene Oxide/Nafion as Ternary-Layered Separator to Retard the Shuttle of Polysulfides for Lithium-Sulfur Batteries.

The reversible electrochemical transformation from lithium (Li) and sulfur (S) into Li2 S through multielectron reactions can be utilized in secondary Li-S batteries with very high energy density. However, both the low Coulombic efficiency and severe capacity degradation limits the full utilization of active sulfur, which hinders the practical applications of Li-S battery system. The present study reports a ternary-layered separator with a macroporous polypropylene (PP) matrix layer, graphene oxide (GO) barrier layer, and Nafion retarding layer as the separator for Li-S batteries with high Coulombic efficiency and superior cyclic stability. In the ternary-layered separator, ultrathin layer of GO (0.0032 mg cm(-2) , estimated to be around 40 layers) blocks the macropores of PP matrix, and a dense ion selective Nafion layer with a very low loading amount of 0.05 mg cm(-2) is attached as a retarding layer to suppress the crossover of sulfur-containing species. The ternary-layered separators are effective in improving the initial capacity and the Coulombic efficiency of Li-S cells from 969 to 1057 mAh g(-1) , and from 80% to over 95% with an LiNO3 -free electrolyte, respectively. The capacity degradation is reduced from 0.34% to 0.18% per cycle within 200 cycles when the PP separator is replaced by the ternary-layered separators. This work provides the rational design strategy for multifunctional separators at cell scale to effective utilizing of active sulfur and retarding of polysulfides, which offers the possibility of high energy density Li-S cells with long cycling life.

3D Mesoporous Graphene: CVD Self-Assembly on Porous Oxide Templates and Applications in High-Stable Li-S Batteries.

A nanostructured carbon with high specific surface area (SSA), tunable pore structure, superior electrical conductivity, mechanically robust framework, and high chemical stability is an important requirement for electrochemical energy storage. Porous graphene fabricated by chemical activation and liquid etching has a high surface area but very limited volume of electrochemically accessible mesopores. Herein, an effective strategy of in situ formation of hierarchically mesoporous oxide templates with small pores induced by Kirkendall diffusion and large pores attributed to evaporation of deliberately introduced volatile metal is proposed for chemical vapor deposition assembly of porous graphene frameworks (PGFs). The PGFs inherit the hierarchical mesoporous structure of the templates. A high SSA of 1448 m(2) g(-1), 91.6% of which is contributed by mesopores, and a mesopore volume of 2.40 cm(3) g(-1) are attained for PGFs serving as reservoirs of ions or active materials in electrochemical energy storage applications. When the PGFs are applied in lithium-sulfur batteries, a very high sulfur utilization of 71% and a very low fading rate of ≈0.04% per cycle after the second cycle are achieved at a current rate of 1.0 C. This work provides a general strategy for the rational construction of mesoporous structures induced by a volatile metal, with a view toward the design of hierarchical nanomaterials for advanced energy storage.

Dendrite-free nanostructured anode: entrapment of lithium in a 3D fibrous matrix for ultra-stable lithium-sulfur batteries.

Entrapment of free lithium in a 3D fibrous Li7 B6 framework allows LiB nanostructured anodes with stable interfaces between the electrolyte and the electrode, which retards the formation of lithium dendrites. A lithium-sulfur cell with a nanostructured anode is created with a high Coulombic efficiency and high capacity retention rate of 36.3% after 2000 cycles.

Electrochemically and Thermally Stable Inorganics-Rich Solid Electrolyte Interphase for Robust Lithium Metal Batteries.

Severe dendrite growth and high-level activity of the lithium metal anode lead to a short life span and poor safety, seriously hindering the practical applications of lithium metal batteries. With a trisalt electrolyte design, an F-/N-containing inorganics-rich solid electrolyte interphase on a lithium anode is constructed, which is electrochemically and thermally stable over long-term cycles and safety abuse conditions. As a result, its Coulombic efficiency can be maintained over 98.98% for 400 cycles. An 85.0% capacity can be retained for coin-type full cells with a 3.14 mAh cm-2 LiNi0.5 Co0.2 Mn0.3 O2 cathode after 200 cycles and 1.0 Ah pouch-type full cells with a 4.0 mAh cm-2 cathode after 72 cycles. During the thermal runaway tests of a cycled 1.0 Ah pouch cell, the onset and triggering temperatures were increased from 70.8 °C and 117.4 °C to 100.6 °C and 153.1 °C, respectively, indicating a greatly enhanced safety performance. This work gives novel insights into electrolyte and interface design, potentially paving the way for high-energy-density, long-life-span, and thermally safe lithium metal batteries.

Heterostructures Regulating Lithium Polysulfides for Advanced Lithium-Sulfur Batteries.

Sluggish reaction kinetics and severe shuttling effect of lithium polysulfides seriously hinder the development of lithium-sulfur batteries. Heterostructures, due to unique properties, have congenital advantages that are difficult to be achieved by single-component materials in regulating lithium polysulfides by efficient catalysis and strong adsorption to solve the problems of poor reaction kinetics and serious shuttling effect of lithium-sulfur batteries. In this review, the principles of heterostructures expediting lithium polysulfides conversion and anchoring lithium polysulfides are detailedly analyzed, and the application of heterostructures as sulfur host, interlayer, and separator modifier to improve the performance of lithium-sulfur batteries is systematically reviewed. Finally, the problems that need to be solved in the future study and application of heterostructures in lithium-sulfur batteries are prospected. This review will provide a valuable reference for the development of heterostructures in advanced lithium-sulfur batteries.

Thermoresponsive Electrolytes for Safe Lithium-Metal Batteries.

Exploring advanced strategies in alleviating the thermal runaway of lithium-metal batteries (LMBs) is critically essential. Herein, a novel electrolyte system with thermoresponsive characteristics is designed to largely enhance the thermal safety of 1.0 Ah LMBs. Specifically, vinyl carbonate (VC) with azodiisobutyronitrile is introduced as a thermoresponsive solvent to boost the thermal stability of both the solid electrolyte interphase (SEI) and electrolyte. First, abundant poly(VC) is formed in SEI with thermoresponsive electrolyte, which is more thermally stable against lithium hexafluorophosphate compared to the inorganic components widely acquired in routine electrolyte. This increases the critical temperature for thermal safety (the beginning temperature of obvious self-heating) from 71.5 to 137.4 °C. The remained VC solvents can be polymerized into poly(VC) as the battery temperature abnormally increases. The poly(VC) can not only afford as a barrier to prevent the direct contact between electrodes, but also immobilize the free liquid solvents, thereby reducing the exothermic reactions between electrodes and electrolytes. Consequently, the internal-short-circuit temperature and "ignition point" temperature (the starting temperature of thermal runaway) of LMBs are largely increased from 126.3 and 100.3 °C to 176.5 and 203.6 °C. This work provides novel insights for pursuing thermally stable LMBs with the addition of various thermoresponsive solvents in commercial electrolytes.

Synchronization of parallel intense electron beam accelerators based on single trigger generator.

In this study, the authors provide results of the precisely synchronized triggering of an intense electron beam accelerator (IEBA). The trigger generator was composed of a fractional-turn ratio saturable-pulse transformer and a compact six-stage Marx generator. The main switch of the IEBA was a corona-stabilized triggered switch (CSTS) based on the stabilized corona mechanism. The output voltage of a single IEBA exceeded 500 kV with less than 4 ns of jitter on a 50 Ω dummy load. We also conducted an experiment on the synchronous triggering of two IEBAs by using two independent trigger generators. The synchronization-related jitter was 6.1 ns, while the average time difference was 1.3 ns. We used this to attempt to trigger two parallel IEBAs by using a single trigger generator. The results showed a reduction in the synchronization-related jitter of 31% to 4.2 ns. The designs of the CSTS and the trigger generator guaranteed a precisely synchronous trigger by using a single trigger generator. Thus, the proposed method appears to be promising for accurately and synchronously triggering multiple IEBAs by using a single trigger generator. This provides an effective method to generate pulses with high power.