Early-stage latent thermal failure of single-crystal Ni-rich layered cathode

High nickel content worsens the thermal stability of layered cathodes for lithium-ion batteries,raising safety concerns for their applications.Thoroughly understanding the thermal failure process can offer valuable guidance for material optimization on thermal stability and new opportunities in monitoring battery thermal runaway(TR).Herein,this work comprehensively investigates the thermal failure process of a single-crystal nickel-rich layered cathode and finds that the latent thermal failure starts at∼120℃far below the TR temperature(225℃).During this stage of heat accumulation,sequential structure transition is revealed by atomic resolution electron microscopy,which follows the layered→cation mixing layered→LiMn_(2)O_(4)-type spinel→disordered spinel→rock salt.This progression occurs as a result of the continuous migration and densification of transition metal cations.Phase transition generates gaseous oxygen,initially confined within the isolated closed pores,thereby not showing any thermal failure phenomena at the macro-level.Increasing temperature leads to pore growth and coalescence,and eventually to the formation of open pores,causing oxygen gas release and weight loss,which are the typical TR features.We highlight that latent thermal instability occurs before the macro-level TR,suggesting that suppressing phase transitions caused by early thermal instability is a crucial direction for material optimization.Our findings can also be used for early warning of battery thermal runaway.

Deciphering the critical effect of cathode-electrolyte interphase by revealing its dynamic evolution

Cathode electrolyte interphase(CEI)layer plays a crucial role in determining the electrochemical performance of lithium-ion batteries.Limited by the sensitive nature of CEI and the lack of characterization techniques,its dynamic evolution during cycling,its formation mechanism,and its specific impact on battery performance are not yet fully understood.Herein,we systematically investigate the dynamic evolution of CEI layer and its critical effect on the cycling performance of LiCoO_(2)cathode by diverse characterization techniques.We find that cycling voltage plays a key role in affecting CEI formation and evolution,and a critical potential(4.05 V vs.Li)is identified,which acts as the switching potential between CEI deposition and decomposition.We show that CEI starts deposition in the discharge process when the potential is below 4.05 V,and CEI decomposition occurs when the potential is higher than 4.05 V.When the battery is cycled below such a critical potential,a stable CEI layer is developed,which leads to superior cycling stability.When the battery is cycled above such a critical potential,a CEI-free cathode interface is observed,which also demonstrates good cycle stability.However,when the critical potential falls in the cycling voltage range,CEI deposition and decomposition are repeatedly switched on during cycling,leading to the dynamically unstable CEI layer.The unstable CEI layer causes continuous interfacial reaction and degradation,resulting in battery performance decay.Our work deepens the understanding of the CEI formation and evolution mechanisms,and clarifies the critical effect of CEI layer on cycling performance,which provides new insights into stabilizing the electrode-electrolyte interface for high-performance rechargeable batteries.

Regulating local coordination environment of Mg-Co single atom catalyst for improved direct methanol fuel cell cathode

Fuel cells operated with a reformate fuel such as methanol are promising power systems for portable electronic devices due to their high safety,high energy density and low pollutant emissions.However,several critical issues including methanol crossover effect,CO-tolerance electrode and efficient oxygen reduction electrocatalyst with low or non-platinum usage have to be addressed before the direct methanol fuel cells(DMFCs)become commercially available for industrial application.Here,we report a highly active and selective Mg-Co dualsite oxygen reduction reaction(ORR)single atom catalyst(SAC)with porous N-doped carbon as the substrate.The catalyst exhibits a commercial Pt/C-comparable half-wave potential of 0.806 V(versus the reversible hydrogen electrode)in acid media with good stability.Furthermore,practical DMFCs test achieves a peak power density of over 200 m W cm^(-2)that far exceeds that of commercial Pt/C counterpart(82 m W cm^(-2)).Particularly,the Mg-Co DMFC system runs over 10 h with negligible current loss under 10 M concentration methanol work condition.Experimental results and theoretical calculations reveal that the N atom coordinated by Mg and Co atom exhibits an unconventional d-band-ditto localized p-band and can promote the dissociation of the key intermediate*OOH into*O and*OH,which accounts for the near unity selective 4e-ORR reaction pathway and enhanced ORR activity.In contrast,the N atom in SAC–Co remains inert in the absorption and desorption of*OOH and*OH.This local coordination environment regulation strategy around active sites may promote rational design of high-performance and durable fuel cell cathode electrocatalysts.

Origin of Bismuth-Rich Strategy in Bismuth Oxyhalide Photocatalysts

Recently,the bismuth-rich strategy via increasing the bismuth content has been becoming one of the most appealing approaches to improve the photocatalytic performance of bismuth oxyhalides.However,insights into the mechanism behind the encouraging experiments are missing.Herein,we report the results of the theory-led comprehensive picture of bismuth-rich strategy in bismuth oxyhalide photocatalysts,selecting Bi_(5)O_(7)X(X=F,Cl,Br,I)as a prototype.First-principle calculations revealed that the strategy enables good n-type conductivity,large intrinsic internal electric field,high photoreduction ability and outstanding harvest of visible light,and particularly ranked the intrinsic activity of this family:Bi_(5)O_(7)F>Bi_(5)O_(7)I>Bi_(5)O_(7)Br>Bi_(5)O_(7)Cl.Designed experiments confirmed the theoretical predictions,and together,these results are expected to aid future development of advanced photocatalysts.

Selective core-shell doping enabling high performance 4.6 V-LiCoO_(2)

Constructing robust surface and bulk structure is the prerequisite for realizing high performance high voltage LiCoO_(2)(LCO).Herein,we manage to synthesize a surface Mg-doping and bulk Al-doping coreshell structured LCO,which demonstrates excellent cycling performance.Half-cell shows 94.2%capacity retention after 100 cycles at 3.0-4.6 V(vs.Li/Li^(+))cycling,and no capacity decay after 300 cycles for fullcell test(3.0-4.55 V).Based on comprehensive microanalysis and theoretical calculations,the degradation mechanisms and doping effects are systematically revealed.For the undoped LCO,high voltage cycling induces severe interfacial and bulk degradations,where cracks,stripe defects,fatigue H2 phase,and spinel phase are identified in grain bulk.For the doped LCO,Mg-doped surface shell can suppress the interfacial degradations,which not only stabilizes the surface structure by forming a thin rock-salt layer but also significantly improves the electronic conductivity,thus enabling superior rate performance.Bulk Al-doping can suppress the lattice"breathing"effect and the detrimental H3 to H1-3 phase transition,which minimizes the internal strain and defects growth,maintaining the layered structure after prolonged cycling.Combining theoretical calculations,this work deepens our understanding of the doping effects of Mg and Al,which is valuable in guiding the future material design of high voltage LCO.

LiCoO_(2) Epitaxial Film Enabling Precise Analysis of Interfacial Degradations

Interfacial structure evolution and degradation are critical to the electrochemical performance of LiCoO_(2)(LCO),the most widely studied and used cathode material in lithium ion batteries.To understand such processes requires precise and quantitative measurements.Herein,we use well-defined epitaxial LCO thin films to reveal the interfacial degradation mechanisms.Through our systematical investigations,we find that surface corrosion is significant after forming the surface phase transition layer,and the cathode electrolyte interphase(CEI)has a double layer structure,an inorganic inner layer containing CoO,LiF,LiOH/Li_(2)O and Li_(x)PF_(y)O_(2),and an outmost layer containing Li2CO_(3) and organic carbonaceous components.Furthermore,surface cracks are found to be pronounced due to mechanical failures and chemical etching.This work demonstrates a model material to realize the precise measurements of LCO interfacial degradations,which deepens our understanding on the interfacial degradation mechanisms.

The interphasial degradation of 4.2 V-class poly(ethylene oxide)-based solid batteries beyond electrochemical voltage limit

Solid-state polymer electrolytes(SPEs)have attracted increasing attention due to good interfacial contact,light weight,and easy manufacturing.However,the practical application of SPEs such as the most widely studied poly(ethylene oxide)(PEO)in high-energy solid polymer batteries is still challenging,and the reasons are yet elusive.Here,it is found that the mismatch between PEO and 4.2 V-class cathodes is beyond the limited electrochemical window of PEO in the solid Li Ni_(1/3)Mn_(1/3)Co_(1/3)O_(2)(NMC)-PEO batteries.The initial oxidation of PEO initiates remarkable surface reconstruction of NMC grains in solid batteries that are different from the situation in liquid electrolytes.Well-aligned nanovoids are observed in NMC grains during the diffusion of surface reconstruction layers towards the bulk in solid batteries.The substantial interphasial degradation,therefore,blocks smooth Li+transport across the NMC-PEO interface and causes performance degradation.A thin yet effective Li F-containing protection layer on NMC can effectively stabilize the NMC-PEO interface with a greatly improved lifespan of NMC|PEO|Li batteries.This work deepens the understanding of degradations in high-voltage solid-state polymer batteries.

Improvement of valley splitting and valley injection efficiency for graphene/ferromagnet heterostructure

The valley splitting has been realized in the graphene/Ni heterostructure with the splitting value of 14 meV,and the obtained valley injecting efficiency from the heterostructure into graphene was 6.18%[Phys.Rev.B 92115404(2015)].In this paper,we report a way to improve the valley splitting and the valley injecting efficiency of the graphene/Ni heterostructure.By intercalating an Au monolayer between the graphene and the Ni,the split can be increased up to 50 meV.However,the valley injecting efficiency is not improved because the splitted valley area of graphene moves away from the Fermi level.Then,we mend the deviation by covering a monolayer of Cu on the graphene.As a result,the valley injecting efficiency of the Cu/graphene/Au/Ni heterostructure reaches 10%,which is more than 60%improvement compared to the simple graphene/Ni heterostructure.Then we theoretically design a valley-injection device based on the Cu/graphene/Au/Ni heterostructure and demonstrate that the valley injection can be easily switched solely by changing the magnetization direction of Ni,which can be used to generate and control the valley-polarized current.

Interface catalytic reduction of alumina by nickle for the aluminum nanowire growth: Dynamics observed by in situ TEM

Metal nanowires show promise in a broad range of applications and can be fabricated via a number of methods,such as vapor–liquid–solid process and template-based electrodeposition.However,the synthesis of Al nanowires(NWs)is still challenging from the stable alumina substrate.In this work,the Ni-catalyzed fabrication of Al NWs has been realized using various Al_(2)O_(3) substrates.The growth dynamics of Al NWs on Ni/Al_(2)O_(3) was studied using in situ transmission electron microscopy(TEM).The effect of alumina structures,compositions,and growth temperature were investigated.The growth of Al NWs correlates with the Na addition to the alumina support.Since no eutectic mixture of nickel aluminide was formed,a mechanism of Ni-catalyzed reduction of Al_(2)O_(3) for Al NWs growth has been proposed instead of the vapor–liquid–solid mechanism.The key insights reported here are not restricted to Ni-catalyzed Al NWs growth but can be extended to understanding the dynamic change and catalytic performance of Ni/Al_(2)O_(3) under working conditions.

Superb creep lives of Ni-based single crystal superalloy through size effects and strengthening heterostructure γ/γ' interfaces

This study presents a design strategy to enhance the high-temperature creep resistance of Ni-based superalloys.This strategy focuses on two principles:(1)minimizing the dimensions ofγ/γ′interfaces andγchannels by reducing the size of theγ′phase;(2)key alloy composition control to strengthen the heterostructureγ/γ′interfaces.This strategy proved very effective by the designed three superalloys'prolonged creep lives.An alloy exhibits ultra-long creep life by 388 h at 1100°C/137 MPa,which runs at the highest level among those alloys without Ru addition.With Ru addition,an alloy that lasted for 748 h with a creep strain of~6%at 1110°C/137 MPa is developed.This study provides a new route of high-temperature creep lives through heterostructure interfacial design with size effects and key alloying elements.

Isotype Heterojunction‑Boosted CO_(2) Photoreduction to CO

Photocatalytic conversion of CO_(2) to high-value products plays a crucial role in the global pursuit of carbon–neutral economy.Junction photocatalysts,such as the isotype heterojunctions,offer an ideal paradigm to navigate the photocatalytic CO_(2) reduction reaction(CRR).Herein,we elucidate the behaviors of isotype heterojunctions toward photocatalytic CRR over a representative photocatalyst,g-C_(3)N_(4).Impressively,the isotype heterojunctions possess a significantly higher efficiency for the spatial separation and transfer of photogenerated carriers than the single components.Along with the intrinsically outstanding stability,the isotype heterojunctions exhibit an exceptional and stable activity toward the CO_(2) photoreduction to CO.More importantly,by combining quantitative in situ technique with the first-principles modeling,we elucidate that the enhanced photoinduced charge dynamics promotes the production of key intermediates and thus the whole reaction kinetics.

Conductive metal-organic frameworks promoting polysulfides transformation in lithium-sulfur batteries

Metal organic frameworks(MOFs) have been extensively investigated in Li-S batteries owing to high surface area, adjustable structures and abundant catalytic sites. Nevertheless, the insulating nature of traditional MOFs render retarded kinetics of polysulfides conversion, leading to insufficient utilization of sulfur. In comparison, conductive MOFs(c-MOFs) show great potential for promoting polysulfides transformation due to superb electronic conductivity. In this work, a nickel-catecholates based c-MOF, NiHHTP(HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), is designed to regulate surface chemistry of self-supported carbon paper for advanced Li-S batteries. Taking advantage of the porous structure and high conductivity, the as-prepared Ni-HHTP is conducive to synergising strengthening the chemisorption of polysulfides and accelerating the reaction kinetics in Li-S batteries, significantly mitigating the polysulfides diffusion from the non-encapsulated sulfur cathode, therefore promoting polysulfides transformation in Li-S batteries. This work points out a promising modification strategy for developing advanced sulfur cathode in Li-S batteries.

A new insight into the promoting effects of transition metal phosphides in methanol electrooxidation

The construction of highly active catalysts for methanol oxidation reaction(MOR)is central to direct methanol fuel cells.Tremendous progress has been made in transition metal phosphides(TMPs)based catalysts.However,TMPs would be partially damaged and transformed into new substances(e.g.,Pt-M-P composite,where M represents a second transition metal)during Pt deposition process.This would pose a large obstacle to the cognition of the real promoting effects of TMPs in MOR.Herein,Co_(2)P co-catalysts(Pt-P/Co_(2)P@NPC,where NPC stands for N and P co-doped carbon)and Pt-Co-P composite catalysts(Pt-CoP/NPC)were controllably synthesized.Electrocatalysis tests show that the Pt-Co-P/NPC exhibits superior MOR activity as high as 1016 m A/mg_(Pt),significantly exceeding that of Pt-P/Co_(2)P@NPC(345 m A/mg_(Pt)).This result indicates that the promoting effect is ascribed primarily to the resultant Pt-Co-P composite,in sharply contrast to previous viewpoint that Co_(2)P itself improves the activity.Further mechanistic studies reveal that Pt-Co-P/NPC exhibits much stronger electron interaction and thus manifesting a remarkably weaker CO absorption than Pt-P/Co_(2)P@NPC and Pt/C.Moreover,Pt-Co-P is also more capable of producing oxygen-containing adsorbate and thus accelerating the removal of surface-bonded CO^(*),ultimately boosting the MOR performance.

Stable cycling of practical high-voltage LiCoO_(2)pouch cell via electrolyte modification

Nitriles as efficient electrolyte additives are widely used in high-voltage lithium-ion batteries.However,their working mechanisms are still mysterious,especially in practical high-voltage LiCoO_(2)pouch lithium-ion batteries.Herein,we adopt a tridentate ligandcontaining 1,3,6-hexanetricarbonitrile(HTCN)as an effective electrolyte additive to shed light on the mechanism of stabilizing high-voltage LiCoO_(2)cathode(4.5 V)through nitriles.The LiCoO_(2)/graphite pouch cells with the HTCN additive electrolyte possess superior cycling performance,90%retention of the initial capacity after 800 cycles at 25℃,and 72%retention after 500 cycles at 45℃,which is feasible for practical application.Such an excellent cycling performance can be attributed to the stable interface:The HTCN molecules with strong electron-donating ability participate in the construction of cathode-electrolyte interphase(CEI)through coordinating with Co ions,which suppresses the decomposition of electrolyte and improves the structural stability of LiCoO_(2)during cycling.In summary,the work recognizes a coordinating-based interphase-forming mechanism as an effective strategy to optimize the performance of high voltage LiCoO_(2)cathode with appropriate electrolyte additives for practical pouch batteries.

High density stacking faults of ■ compression twin in magnesium alloys

{1011} compression twins with high density stacking faults were studied at atomic scale using Cscorrection transmission electron microscopy. On one side of the {1011} twin boundary, there were many steps arranged alternately with the coherent twin boundaries. Most of the steps were linked with stacking faults inside twins. Burgers vector of twinning dislocations and the mismatch strain at steps were characterized. Due to the compressive mismatch strain at steps, the high density stacking faults inside twins were formed at twin tips during twinning process. The localized strain at the steps would be related to the crack nucleation in magnesium alloys.

Formation mechanism of hydride precipitation in commercially pure titanium

Since titanium has high affinity for hydrogen and reacts reversibly with hydrogen,the precipitation of titanium hydrides in titanium and its alloys cannot be ignored.Two most common hydride precipitates in α-Ti matrix areγ-hydride and δ-hydride,however their mechanisms for precipitation are still unclear.In the present study,we find that both γ-hydride and δ-hydride phases with different specific orientations were randomly precipitated in the as-received hot forged commercially pure Ti.In addition,a large amount of the titanium hydrides can be introduced into Ti matrix with selective precipitation by using electrochemical treatment.Cs-corrected scanning transmission electron microscopy is used to study the precipitation mechanisms of the two hydrides.It is revealed that the γ-hydride and δ-hydride precipitations are both formed through slip+shuffle mechanisms involving a unit of two layers of titanium atoms,but the difference is that the γ-hydride is formed by prismatic slip corresponding to hydrogen occupying the octahedral sites of α-Ti,while the δ-hydride is formed by basal slip corresponding to hydrogen occupying the tetrahedral sites ofα-Ti.

Formation Mechanism of Lamellar M_(23)C_6 Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000℃

The precipitation of the lamellar-shaped M23C6 carbide within the dendritic matrix of a cobalt-base superalloy during thermal exposure at 1000 ℃ has been investigated. Such a precipitation is not commonly observed in cobalt-base superalloys. It is found that M23C6 particles nucleate preferentially at stacking faults （SFs） in the dendritic matrix and grow along the SFs to develop a lamellar character. Additionally, a Cr depletion zone is observed in the vicinity of the lamellar M23C6 carbide, which strongly supports the presence of Suzuki segregation.

Carrier mobility tuning of MoS_(2) by strain engineering in CVD growth process

Strain engineering is proposed to be an effective technology to tune the properties of two-dimensional(2D)transition metal dichalcogenides(TMDCs).Conventional strain engineering techniques(e.g.,mechanical bending,heating)cannot conserve strain due to their dependence on external action,which thereby limits the application in electronics.In addition,the theoretically predicted strain-induced tuning of electrical performance of TMDCs has not been experimentally proved yet.Here,a facile but effective approach is proposed to retain and tune the biaxial tensile strain in monolayer MoS_(2) by adjusting the process of the chemical vapor deposition(CVD).To prove the feasibility of this method,the strain formation model of CVD grown MoS_(2) is proposed which is supported by the calculated strain dependence of band gap via the density functional theory(DFT).Next,the electrical properties tuning of strained monolayer MoS_(2) is demonstrated in experiment,where the carrier mobility of MoS_(2) was increased by two orders(~0.15 to~23 cm^(2)·V^(−1)·s^(−1)).The proposed pathway of strain preservation and regulation will open up the optics application of strain engineering and the fabrication of high performance electronic devices in 2D materials.

Synergistic tuning of electrochemical surface area and surface Co^(3+)by oxygen plasma enhances the capacities of Co_(3)O_(4)lithium-oxygen battery cathodes

Modifying electrochemical surface area(ECSA)and surface chemistry are promising approaches to enhance the capacities of oxygen cathodes for lithium-oxygen(Li-O_(2))batteries.Although various chemical approaches have been successfully used to tune the cathode surface,versatile physical techniques including plasma etching etc.could be more effortless and effective than arduous chemical treatments.Herein,for the first time,we propose a facile oxygen plasma treatment to simultaneously etch and modify the surface of Co_(3)O_(4)nanosheet arrays(NAs)cathode for Li-O_(2)batteries.The oxygen plasma not only etches Co_(3)O_(4)nanosheets to enhance the ECSA but also lowers the oxygen vacancy concentration to enable a Co^(3+)-rich surface.In addition,the NA architecture enables the full exposure of oxygen vacancies and surface Co^(3+)that function as the catalytically active sites.Thus,the synergistic effects of enhanced ECSA,modest oxygen vacancy and high surface Co^(3+)achieve a significantly enhanced reversible capacity of 3.45 mAh/cm^(2)for Co_(3)O_(4)NAs.This work not only develops a promising high-capacity cathode for Li-O_(2)batteries,but also provides a facile physical method to simultaneously tune the nanostructure and surface chemistry of energy storage materials.

面心立方金属中非共格孪晶界原子结构的孪晶厚度效应研究

非共格孪晶界(ITB)能显著影响金属力学性能,受到了研究者极大关注.理论模型预测ITB是由系列偏位错组成,ITB两侧的晶面没有沿[111]方向的相对位移.本文利用分子静力学模拟研究了Ag,Cu,Ni和Al中ITB的结构,发现ITB的结构具有显著的尺寸效应.对于较薄的ITB,界面两侧的晶面处于同一水平面,没有沿[111]方向的相对位移,与理论模型结果一致.而对于较厚的ITB,界面两侧的晶面有明显的[111]方向的相对位移,且相对位移随着ITB厚度的增加而增加.此外,研究揭示出ITB两侧晶面沿[111]方向的位移与金属的层错能直接相关.该研究表明ITB的结构并不唯一,不同的厚度、层错能对应不同的ITB结构.本研究为理解孪晶厚度和层错能对金属力学性能的影响提供了新见解.