Suppressing Formation of Zn─Mn─O Phases by In Situ Ti Decoration of MnO2 for Long Lifespan MnO2-Zn Battery.

Mildly-acidic MnO2-Zn batteries are considered as a promising alternative for large-scale energy storage systems for their low toxicity, high safety, and low cost. Though, the degradation of MnO2 with cycling still hinders the further development of the batteries. In this study, it is observed that the decrease in available capacity of MnO2 with charge and discharge is accompanied by a structural transformation with the emergence of Zn─Mn─O phases. An electrodeposition test indicates that the Zn─Mn─O phase is formed from a co-precipitation of Zn and Mn during the charge process. Further, the structural change of MnO2 is suppressed and its cycle stability is improved with the addition of TiOSO4 as a facile electrolyte additive. As a result, under a current of 1200 mA g-1, the MnO2 electrode still gives a capacity of 230 mAh g-1 for over 1500 cycles. Capacity retention is 75% after 10 000 cycles under a current rate of 4800 mA g-1. These findings provide fundamental insights on the degradation mechanism of MnO2 and a new strategy to improve the electrochemical performance of aqueous MnO2-Zn batteries.

Dual-Atom Co/Ni Electrocatalyst Anchored at the Surface-Modified Ti3C2Tx MXene Enables Efficient Hydrogen and Oxygen Evolution Reactions.

Dual-atom catalytic sites on conductive substrates offer a promising opportunity for accelerating the kinetics of multistep hydrogen and oxygen evolution reactions (HER and OER, respectively). Using MXenes as substrates is a promising strategy for depositing those dual-atom electrocatalysts, if the efficient surface anchoring strategy ensuring metal-substrate interactions and sufficient mass loading is established. We introduce a surface-modification strategy of MXene substrates by preadsorbing L-tryptophan molecules, which enabled attachment of dual-atom Co/Ni electrocatalyst at the surface of Ti3C2Tx by forming N-Co/Ni-O bonds, with mass loading reaching as high as 5.6 wt %. The electron delocalization resulting from terminated O atoms on MXene substrates, N atoms in L-tryptophan anchoring moieties, and catalytic metal atoms Co and Ni provides an optimal adsorption strength of intermediates and boosts the HER and OER kinetics, thereby notably promoting the intrinsic activity of the electrocatalyst. CoNi-Ti3C2Tx electrocatalyst displayed HER and OER overpotentials of 31 and 241 mV at 10 mA cm-2, respectively. Importantly, the CoNi-Ti3C2Tx electrocatalyst also exhibited high operational stability for both OER and HER over 100 h at an industrially relevant current density of 500 mA cm-2. Our study provided guidance for constructing dual-atom active metal sites on MXene substrates to synergistically enhance the electrochemical efficiency and stability of the energy conversion and storage systems.

High-Performance Layered Ni-Rich Cathode Materials Enabled by Stress-Resistant Nanosheets.

Layered O3-type transition metal oxides are promising cathode candidates for high-energy-density Li-ion batteries. However, the structural instability at the highly delithiated state and low kinetics at the fully lithiated state are arduous challenges to overcome. Here, a facile approach is developed to make secondary particles of Ni-rich materials with nanosheet primary grains. Because the alignment of the primary grains reduces internal stress buildup within the particle during charge-discharge and provides straightforward paths for Li transport, the as-synthesized Ni-rich materials do not undergo cracking upon cycling with higher overall Li+ ion diffusion rates. Specifically, a LiNi0.75Co0.14Mn0.11O2 cathode with nanosheet grains delivers a high reversible capacity of 206 mAh g-1 and shows ultrahigh cycling stability, e.g., 98% capacity retention over 500 cycles in a full cell with a graphite anode.

A Shuttle-Free Solid-State Cu-Li Battery Based on a Sandwich-Structured Electrolyte.

Cu-Li batteries leveraging the two-electron redox property of Cu can offer high energy density and low cost. However, Cu-Li batteries are plagued by limited solubility and a shuttle effect of Cu ions in traditional electrolytes, which leads to low energy density and poor cycling stability. In this work, we rationally design a solid-state sandwich electrolyte for solid-state Cu-Li batteries, in which a deep-eutectic-solvent gel with high Cu-ion solubility is devised as a Cu-ion reservoir while a ceramic Li1.4 Al0.4 Ti1.6 (PO4 )3 interlayer is used to block Cu-ion crossover. Because of the high ionic conductivity (0.55 mS cm-1 at 25 °C), wide electrochemical window (>4.5 V vs. Li+ /Li), and high Cu ion solubility of solid-state sandwich electrolyte, a solid-state Cu-Li battery demonstrates a high energy density of 1 485 Wh kgCu -1 and long-term cyclability with 97 % capacity retention over 120 cycles. The present study lays the groundwork for future research into low-cost solid-state Cu-Li batteries.

Improvement of stability and capacity of Co-free, Li-rich layered oxide Li1.2Ni0.2Mn0.6O2 cathode material through defect control.

Layered oxides based on manganese (Mn), rich in lithium (Li), and free of cobalt (Co) are the most promising cathode candidates used for lithium-ion batteries due to their high capacity, high voltage and low cost. These types of material can be written as xLi2MnO3·(1 - x) LiTMO2 (TM = Ni,Mn,etc.). Though, Li2MnO3 is known to have poor cycling stability and low capacity, which hinder its industrial application commercially. In this work, Li1.2Ni0.2Mn0.6O2 materials with different amounts of structural defects was successfully synthesized using powder metallurgy followed by different cooling processes in order to improve its electrochemical properties. Microstructural analyses and electrochemical measurements were carried out on the study samples synthesized by a combination of X-ray diffraction, transmission electron microscopy, and cyclic voltammetry. It is found that the disorder of the transition metal layer in Li2MnO3 promotes its electrochemical activity, whereas the Li/Ni antisites of the Li layer maintain the stability of its local structure. The material with optimal amount of structural defects had an initial capacity of 188.2 mAh g-1, while maintaining an excellent specific capacity of 144.2 mAh g-1 after 500 cycles at 1C. In comparison, Li1.2Ni0.2Mn0.6O2 without structural defect only gives a capacity of 40.8 mAh g-1 after cycling. This microstructural control strategy provides a simple and effective route to develop high-performance Co-free, Li-rich Mn-based cathode materials and scale-up manufacturing.

Thermal Stability of Graphite Electrode as Cathode for Dual-Ion Batteries.

The increasing number of accidents relating to battery fire and explosion is raising people's attention towards safety of batteries. Abnormal battery operation can generate much heat and cause thermal runaway due to the exothermic reactions of the electrodes and electrolyte. Recently, dual-ion battery (DIB) has gained many interests because of its low cost and high working voltage compared with traditional lithium-ion battery (LIB). However, investigation on thermal stability of DIB is rare. In this paper, differential scanning calorimetry (DSC) was used to study the thermal stability of DIB using graphite as cathode with different states of charge (SOC) and with different amount of fluoroethylene carbonate as co-solvent in the electrolyte. Then, the thermal stability of graphite cathode for DIB was compared with those of LiCoO2 and LiNi0.5 Mn1.5 O4 at their fully charged states. Specifically, charged DIB using graphite as cathode in 3 m LiPF6 EMC showed superior thermal stability with no exothermic peaks in DSC tests, in contrast to traditional lithium-containing cathodes for LIB, which gave out significant amount of heat at evaluated temperature. In addition, the thermal stability of graphite depended on the type of intercalation species in it. While PF6 - intercalated graphite [Cn (PF6 )] showed an endothermic peak at about 320 °C, Li+ intercalated graphite [Cn (Li)] showed an exothermic peak at about 300 °C. Moreover, the type of electrolyte also affected heat generation from the charged electrodes and should be properly designed to improve thermal stability in the future.

Chelating Polymer-Coated Separators with a BaTiO3 Filler To Improve Reversibility and Round-Trip Efficiency of a 3.3 V Copper-Lithium Battery.

A novel 3.3 V copper-lithium battery using a copper foil as the cathode is a potential candidate for next-generation energy storage system due to its simple manufacturing process. However, the cross-over of copper ions from the cathode to the anode limits the reversibility of the battery. Herein, we suppress self-discharge and migration of copper ions in the cell using a commercial polypropylene separator with a coating of polyacrylic acid (PAA), a chelating polymer. Fourier transform infrared spectroscopy confirms that the PAA layer traps the copper ions and prevents them from passing through. The addition of barium titanate nanoparticles into the PAA layer further enhances ionic transfer through the separator and reduces polarization of the cell at high current rates during charge and discharge. The use of a chelating agent with an inorganic filler as a coating layer on the separator is a cost-effective way to improve reversibility and round-trip efficiency of copper-lithium batteries.

Defect-Rich Amorphous Iron-Based Oxide/Graphene Hybrid-Modified Separator toward the Efficient Capture and Catalysis of Polysulfides.

The sluggish sulfur reduction reaction, severe shuttle effect, and poor conductivity of sulfur species are three main problems in lithium-sulfur (Li-S) batteries. Functional materials with a strong affinity and catalytic effect toward polysulfides play a key role in addressing these issues. Herein, we report a defect-rich amorphous a-Fe3O4-x/GO material with a nanocube-interlocked structure as an adsorber as well as an electrocatalyst for the Li-S battery. The composition and defect structure of the material are determined by X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy measurements. The distinctive open framework architecture of the as-engineered composite inherited from the metal-organic framework precursor ensures the stability and activity of the catalyst during extended cycles. The oxygen defects in the amorphous structure are capable of absorbing polysulfides and similarly work as catalytic centers to boost polysulfide conversion. Taking advantage of a-Fe3O4-x/GO on the separator surface, the Li-S battery shows a capacity over 610 mA h g-1 at 1 C and a low decay rate of 0.12% per cycle over 500 cycles and superior rate capability. The functional material made via the low-cost synthesis process provides a potential solution for advanced Li-S batteries.

Dilute Aqueous-Aprotic Hybrid Electrolyte Enabling a Wide Electrochemical Window through Solvation Structure Engineering.

The application of superconcentrated aqueous electrolytes has shown great potential in developing high-voltage electrochemical double-layer capacitors (EDLCs). However, the broadening of the electrochemical window of such superconcentrated electrolytes is at the expense of their high cost, low ionic conductivity, high density, and narrow operating temperature range. Herein, the electrochemical window of water (>3 V) at low salt concentration (3 m) is expanded by using an aprotic solvent, i.e., trimethyl phosphate (TMP), to regulate the solvation structure of the electrolyte. Benefiting from the low salt concentration, such electrolyte is simultaneously featured with high ionic conductivity, low density, and wide temperature compatibility. Based on the dilute hybrid electrolyte, EDLCs constructed by using porous graphene electrodes are able to operate within an enlarged voltage range of 0-2.4 V at a wide range of temperatures from -20 to 60 °C. They also present excellent rate capability and cycle stability, i.e., 83% capacitance retention after 100 000 cycles. Density functional theory calculations verify that TMP induces a significant electronic modulation for the bonding environment of the electrolyte. This enables the stronger binding of Na+ -H2 O with freely migrating TMP to expand the voltage window to exceed the potential limitation of aqueous electrolytes.

MOF-Derived CoS2/N-Doped Carbon Composite to Induce Short-Chain Sulfur Molecule Generation for Enhanced Sodium-Sulfur Battery Performance.

Dissolution of intermediate sodium polysulfides (Na2Sx; 4≤x≤8) is a crucial obstacle for the development of room-temperature sodium-sulfur (Na-S) batteries. One promising strategy to avoid this issue is to load short-chain sulfur (S2-4), which could prohibit the generation of soluble polysulfides during the sodiation process. Herein, unlike in the previous reported cases where short-chain sulfur was stored by confinement within a small-pore-size (≤0.5 nm) carbon host, we report a new strategy to generate short-chain sulfur in larger pores (>0.5 nm) by the synergistic catalytic effect of CoS2 and appropriate pore size. Based on density functional theory calculations, we predict that CoS2 can serve as a catalyst to weaken the S-S bond in the S8 ring structure, facilitating the formation of short-chain sulfur molecules. By experimentally tuning the pore size of the CoS2-based hosts and comparing their performances as cathodes in Na-S and Li-S batteries, we conclude that such a catalytic effect depends on the proximity of sulfur to CoS2. This avoids the generation of soluble polysulfides and results in superior electrochemical properties of the composite materials introduced here for Na-S batteries. As a result, the optimized CoS2/N-doped carbon/S electrode showed excellent electrochemical performance with high reversible specific capacities of 488 mA h g-1 (962 mA h g(s)-1) after 100 cycles (0.1 A g-1) and 403 mA h g-1 after 1000 cycles (1 A g-1) with a superior rate performance (262 mA h g-1 at 5.0 A g-1).

Hierarchical CoS2/N-Doped Carbon@MoS2 Nanosheets with Enhanced Sodium Storage Performance.

We introduce a hierarchical nanostructure of CoS2/N-doped carbon@MoS2 comprising two transition-metal sulfides CoS2 and MoS2, with enhanced sodium storage performance in sodium-ion batteries. A micron-sized Co metal-organic framework (MOF) is transformed into a CoS2/N-doped carbon composite, followed by a solvothermal growth of MoS2 nanosheets on the surface. The resulting composite material offers several specific advantages for sodium storage: (i) accelerated sodium-ion diffusion kinetics due to its heterogeneous interface; (ii) shortened ion diffusion path and exposed active sites for sodium storage due to its hierarchical nanosheet architecture; and (iii) homogeneous nitrogen doping of the MOF-derived carbon, which is beneficial for electronic conductivity. Due to these merits, this composite exhibits excellent electrochemical performance with a specific capacity of 596 mAh g-1 after 100 cycles at 0.1 A g-1 and 395 mAh g-1 at 5.0 A g-1.

Engineering Solvation Complex-Membrane Interaction to Suppress Cation Crossover in 3 V Cu-Al Battery.

Metal-metal batteries such as the 3 V Cu-Al system are highly desirable for large-scale energy storage owing to their low cost and excellent scalability of Cu and Al foils. However, the dissolved Cu cations will crossover from the cathode to the anode leading to poor electrochemical performance. In this work, it is demonstrated that the reversibility of the Cu-Al battery depends strongly on the interaction of the Cu ions with the electrolyte solvent and subsequently the affinity of the solvated Cu ion with the membrane separator. Specifically, a series of common carbonate-based electrolyte solvents are investigated via molecular dynamics and contact angle measurements to understand the interaction between the solvents and a polypropylene (PP) membrane, as well as that between cations and solvent. Among different solvents, fluoroethylene carbonate (FEC) is shown to drastically enhance the coulombic efficiency to 97%, compared to that of 27% with dimethyl carbonate. Remarkable cyclability of a 3 V Cu-Al battery with 3 m LiTFSI FEC and PP membrane up to 1000 cycles is further demonstrated. This finding opens new opportunities for the development of low-cost, high performance Cu-Al systems for stationary applications.

Novel structurally-stable Na-rich Na4V2O7 cathode material with high reversible capacity by utilization of anion redox activity.

A sodium-rich vanadium compound, Na4V2O7, is investigated as a cathode material for sodium-ion batteries, which delivers a high reversible capacity of 194 mA h g-1 after activating to 4.7 V. By limiting the cut-off voltage to 4.4 V, a good capacity retention of 93% after 50 cycles is achieved. The material exhibits a negligible volume variation of 1.04% during Na+ (de-)intercalation, demonstrating that Na4V2O7 is structurally-stable.

Metal-Organic Framework Derived CoS2 Wrapped with Nitrogen-Doped Carbon for Enhanced Lithium/Sodium Storage Performance.

Transition metal sulfides are attractive electrode materials for both lithium-ion (LIBs) and sodium-ion batteries (SIBs). Starting from micron-sized Co(IPC)·H2O (IPC: 4-(imidazole-1-yl) phthalic acid) and polydopamine as the metal-organic framework (MOF) precursor and carbon source, respectively, we produced a CoS2/C/C composite constituting CoS2 nanoparticles decorated with N-doped carbon layers and subjected to sulfurization. N-doped carbon layers provided a robust network for the CoS2 nanoparticles, enhancing the structural integrity and electronic conductivity of the resulting CoS2/C/C composite, which exhibited electrochemical performance superior to most existing CoS2 composites, and was one of the best among all MOF derived CoS2 anodes for LIBs and SIBs. The use of nanosized CoS2 particles reduced the diffusion length for the transfer of Li+/Na+ ions, resulting in a high specific capacity at a higher current rate. N-doped carbon layers derived from the MOF precursor and polydopamine provided an electrically conductive network between the CoS2 nanoparticles, thus preventing their aggregation and inhibiting adverse side reactions between the electrolyte and the surface of the electrode, and the high pseudocapacitive contribution resulted in the enhanced rate performance of the CoS2/C/C electrodes.

Leveraging Titanium to Enable Silicon Anodes in Lithium-Ion Batteries.

Silicon is a promising anode material for lithium-ion batteries because of its high gravimetric/volumetric capacities and low lithiation/delithiation voltages. However, it suffers from poor cycling stability due to drastic volume expansion (>300%) when it alloys with lithium, leading to structural disintegration upon lithium removal. Here, it is demonstrated that titanium atoms inside the silicon matrix can act as an atomic binding agent to hold the silicon atoms together during lithiation and mend the structure after delithiation. Direct evidence from in situ dilatometry of cosputtered silicon-titanium thin films reveals significantly smaller electrode thickness change during lithiation, compared to a pure silicon thin film. In addition, the thickness change is fully reversible with lithium extraction, and ex situ post-mortem microscopy shows that film cracking is suppressed. Furthermore, Raman spectroscopy measurements indicate that the Si-Ti interaction remains intact after cycling. Optimized Si-Ti thin films can deliver a stable capacity of 1000 mAh g-1 at a current of 2000 mA g-1 for more than 300 cycles, demonstrating the effectiveness of titanium in stabilizing the material structure. A full cell with a Si-Ti anode and LiFePO4 cathode is demonstrated, which further validates the readiness of the technology.

Robust Micron-Sized Silicon Secondary Particles Anchored by Polyimide as High-Capacity, High-Stability Li-Ion Battery Anode.

Silicon is an attractive high-capacity anode material for lithium-ion battery. With the help of nanostructures, cycling performance of silicon anode has improved significantly in the past couple of years. However, three major shortcomings associated with nanostructures still need to be addressed, namely, their high surface area, low tap density, and poor scalability. Herein, we present a facile and practical method to produce micron-sized Si secondary particle cluster (SiSPC) with a high tap density and a low surface area from bulk Si by high-energy ball-milling. By coupling SiSPC with a mechanically robust polyimide binder, more than 95% of the initial capacity is retained after 500 cycles at 3500 mA g-1 (1C rate). Reversibility of electrode thickness change is confirmed by in situ dilatometry. In addition, the polyimide binder suppresses the surface reaction of the particles with electrolyte, resulting in a high Coulombic efficiency of 99.7%. Excellent cycling performance is obtained even for thick electrodes with an areal capacity of 3.57 mAh cm-2, similar to those in commercial lithium-ion batteries. The presented Si electrode system has a high volumetric capacity of 598 mAh cm-3, which is higher than that of the commercial graphite anode materials.

Vapor-Infiltration Approach toward Selenium/Reduced Graphene Oxide Composites Enabling Stable and High-Capacity Sodium Storage.

Emerging sodium-selenium batteries suffer from volume expansion of the selenium cathode and shuttling effects of soluble intermediates. Confining selenium within the carbon matrix is the most adopted strategy to address these two issues, which is generally realized via a melt-infusion method. Herein, we developed a vapor-infiltration method to fabricate selenium/carbon composites that are advantageous over the melt-infusion route in terms of several aspects: it relieves the requirement of intensive mechanical mixing and simplifies the ratio optimization between selenium and carbon; it avoids selenium aggregation and makes it possible to utilize all of the surface and pores of the carbon host. Utilizing this method, we fabricated a selenium/graphene composite from thermally reduced graphene oxide with a selenium loading equal to 71 wt %, thus approaching the record value. The obtained composite achieved the highest reported to date initial Coulombic efficiency of 88% among various selenium cathodes, with superior rate and cycle performance (410 and 367 mA h g-1 at 0.1 and 1 A g-1; capacity decay <10% after 800 cycles at 2 A g-1) enabled by the supporting graphene framework and the use of the ether electrolyte. In view of the distinct advantages of the vapor-infiltration method and the significant influence of the ether electrolyte on both initial Coulombic efficiency and cyclability of the batteries, we believe the introduced approach will be frequently adopted to incorporate selenium into various host materials, and the ether electrolyte will be widely considered for selenium-based electrodes.

Carbon-Supported Nickel Selenide Hollow Nanowires as Advanced Anode Materials for Sodium-Ion Batteries.

Carbon-supported nickel selenide (Ni0.85 Se/C) hollow nanowires are prepared from carbon-coated selenium nanowires via a self-templating hydrothermal method, by first dissolving selenium in the Se/C nanowires in hydrazine, allowing it to diffuse out of the carbon layer, and then reacting with nickel ions into Ni0.85 Se nanoplates on the outer surface of the carbon. Ni0.85 Se/C hollow nanowires are employed as anode materials for sodium-ion batteries, and their electrochemical performance is evaluated via the cyclic voltammetry and electrochemical impedance spectroscopy combined with ex situ X-ray photoelectron spectroscopy and X-ray diffraction measurements. It is found that Ni0.85 Se/C hollow nanowires exhibit greatly enhanced cycle stability and rate capability as compared to Ni0.85 Se nanoparticles, with a reversible capacity around 390 mA h g-1 (the theoretical capacity is 416 mA h g-1 ) at the rate of 0.2 C and 97% capacity retention after 100 cycles. When the current rate is raised to 5 C, they still deliver capacity of 219 mA h g-1 . The synthetic methodology introduced here is general and can easily be applied to building similar structures for other metal selenides in the future.

GeO2 Thin Film Deposition on Graphene Oxide by the Hydrogen Peroxide Route: Evaluation for Lithium-Ion Battery Anode.

A peroxogermanate thin film was deposited in high yield at room temperature on graphene oxide (GO) from peroxogermanate sols. The deposition of the peroxo-precursor onto GO and the transformations to amorphous GeO2, crystalline tetragonal GeO2, and then to cubic elemental germanium were followed by electron microscopy, XRD, and XPS. All of these transformations are influenced by the GO support. The initial deposition is explained in view of the sol composition and the presence of GO, and the different thermal transformations are explained by reactions with the graphene support acting as a reducing agent. As a test case, the evaluation of the different materials as lithium ion battery anodes was carried out revealing that the best performance is obtained by amorphous germanium oxide@GO with >1000 mAh g-1 at 250 mA g-1 (between 0 and 2.5 V vs Li/Li+ cathode), despite the fact that the material contained only 51 wt % germanium. This is the first demonstration of the peroxide route to produce peroxogermanate thin films and thereby supported germanium and germanium oxide coatings. The advantages of the process over alternative methodologies are discussed.