New Electron-Doped Superconducting Cuprate Li_x_Sr_2_CuO_2_Br_2_

基于第二代高温超导带材的高载流超导导体研究进展王银顺【摘要】由于其高临界电流密度以及优越的机械性能和电磁特性,第二代高温超导带材(也叫涂层导体)在高温低场的电力传输和低温高场下的磁体应用具有广阔的应用前景.在电力传输的低场应用中,高温超导导体在低电压大容量场合需要几千安培甚至上万安培的传输电流.在大型高场磁体应用方面,为了避免由于过高电感在磁体失超和快速关断过程中的感应高压问题,大载流容量、高电流密度高温超导导体在运行于4.2K及以下温度的大型高场超导磁体方面具有很好的应用前景.近年来,基于第二代高温超导带材,国际上相继提出了几种高载流容量的高温超导导体,本文介绍几种高温超导导体的结构及研发现状和进展,并对其结构、性能和工艺进行简单的比较和评述.%The second-generation high-temperature superconductors (HTS) tapes,so called REBCO coated conductor (CC),are attractive for power transmission at liquid nitrogen temperature and high-field application at low temperatures because of their high critical current density and the excellent mechanical performance as well as electro-magnetic characteristics.For the former application,high-current cables or conductors made from HTS tapes with capacity of tens kA up to more than 100kA are desirable.In the latter application,high-current HTS conductors are essential for larger high-field magnets operated at 4.2K or lower to avoid high inductances which would cause high-voltage breakdown in case of quench or fast shutdown.In recent years,several prototypes of HTS conductors,consisting of 2G HTS tapes,were successively proposed internationally.This paper presents an overview of such configurations aswell as their progress and status,and briefly reviews their geometrical structure and performance as well as processing technology.【期刊名称】《电工电能新技术》【年(卷),期】2017(036)011【总页数】15页(P21-35)【关键词】第二代高温超导带材;电缆;涂层导体;导体;股线;管内电缆导体;卢瑟福电缆【作者】王银顺【作者单位】新能源电力系统国家重点实验室,华北电力大学,北京102206;高压与电磁兼容北京市重点实验室,华北电力大学,北京102206【正文语种】中文【中图分类】TM26在过去二十几年中，实用高温超导材料的研究取得了很大进展。

第 44 卷第 9 期2023年 9 月Vol.44 No.9Sept., 2023发光学报CHINESE JOURNAL OF LUMINESCENCE双钙钛矿Ba2LuNbO6∶Tb3+闪烁体多模式X射线探测岳杨1，郭龙超1，刘昊哲2，卜卫芳1，徐旭辉3，谢俊奎4，仇广宇4，王婷2*，余雪1*（1. 成都大学机械工程学院，四川成都　610106；　2.成都理工大学材料与化学化工学院，四川成都　610059；3. 昆明理工大学材料科学与工程学院，云南省新材料制备与加工重点实验室，云南昆明　650093；4. 交通运输部南海航海保障中心北海航标处，广西北海　536000）摘要：采用高温固相法制备了双钙钛矿结构的Ba2LuNbO6∶x Tb3+（x = 0.01，0.02，0.05，0.10，0.20）闪烁体材料，并系统地研究了其晶体结构、形貌和X射线激发的光学性能。

研究表明，在X射线激发下，Ba2LuNbO6∶Tb3+的发射光谱主要由Tb3+的特征发射组成，其中最强发射峰位于545 nm处。

X射线发射（RL）强度随Tb3+浓度的增加逐渐增大，当x = 0.1时发射强度达到最大值。

此外，X射线辐照5 min后的热释光（TL）曲线显示该样品存在位于T1（377 K）和T2（460 K）的两个陷阱。

其陷阱深度分别为0.754 eV和0.920 eV，这表明该材料具有潜在的X射线信息存储性能。

因此，我们可通过加热或者980 nm激光二极管激发，有效诱导读出存储在深陷阱中的载流子，实现高亮度光激励发光（PSL）和热刺激发光（TSL）。

基于此，由Ba2LuNbO6∶Tb3+与聚二甲基硅氧烷（PDMS）所制备的柔性闪烁体薄膜，在低X射线剂量辐照下表现出优异的X射线成像分辨率（12.5 lp/mm）以及延时成像特性。

以上结果表明，所制备的Ba2LuNbO6∶0.1Tb3+在X射线探测和X射线信息存储方面具有潜在的应用前景。

第44卷第6期2021年6月核技术NUCLEAR TECHNIQUESV ol.44，No.6June2021水系锌离子电池正极材料碲化铋层间质子可逆输运的原位观测彭磊1,2,3王娟1,2,3何燕1,2,3杨科1,2,31（中国科学院上海应用物理研究所上海201800）2（中国科学院大学北京100049）3（中国科学院上海高等研究院上海同步辐射光源上海201204）摘要可充电水系锌离子电池因具备低成本、高安全性、无毒环保等优点备受关注，具有高比容量和工作电压的新型正极材料则是水系锌离子电池研究的热点之一，而碲化铋正是这样一种新兴材料。

本工作采用一步水热法剥离碲化铋（Bi2Te3）粉末获得稳定的碲化铋纳米结构材料，并首次探索将其作为正极材料应用于水系锌离子电池中。

扫描电镜和原子力显微镜测试结果均表明合成的碲化铋具有纳米片形貌，厚度仅为3~5nm。

为了进一步深入研究其反应机理，利用同步辐射原位X射线衍射技术，实时表征碲化铋纳米片正极在电池充放电过程中的微观结构变化，并实时观察到碲化铋纳米片反应过程中高度可逆的质子插层现象，证实了质子在碲化铋正极材料中的可逆输运特性。

关键词水系锌离子电池，碲化铋纳米片，原位同步辐射X射线衍射，可逆质子插层中图分类号TL99DOI:10.11889/j.0253-3219.2021.hjs.44.060103In-situ observation of reversible proton transport through Bi2Te3anodeof aqueous zinc-ion batteryPENG Lei1,2,3WANG Juan1,2,3HE Yan1,2,3YANG Ke1,2,31(Shanghai Institute of Applied Physics,Chinese Academy of Sciences,Shanghai201800,China)2(University of Chinese Academy of Sciences,Beijing100049,China)3(Shanghai Advanced Research Institute,Shanghai Synchrotron Radiation Facility,Chinese Academy of Sciences,Shanghai201204,China)Abstract[background]The rechargeable aqueous zinc-ion batteries have attracted increasing attention due to their low cost,high safety,non-toxicity and environmental protection.The exploitation of new anode materials with high specific capacity and working voltage is one of research hotspots of aqueous zinc-ion battery whilst bismuthtelluride is such an emerging materials.[Purpose]This study aims to observe the reversible proton transport in Bi2Te3anode of aqueous zinc-ion battery.[Method]First of all,bismuth telluride nanostructure was obtained by one-step hydrothermal exfoliating of bismuth telluride powder,and the bismuth telluride nanostructure was applied first time as anode material to aqueous zinc-ion battery.Then,both the scanning electron microscopy(SEM)and atomic force microscopy(AFM)were employed to oberve the nanosheets morphology and measure the thickness of synthesized中国科学挑战专项(No.TZ2018001)资助第一作者：彭磊，男，1995年出生，2018年毕业于南华大学，现为硕士研究生，研究领域为水系锌离子电池正极材料研究通信作者：王娟，E-mail：收稿日期：2021-02-25，修回日期：2021-03-23Supported by Science Challenge Project of China(No.TZ2018001)First author:PENG Lei,male,born in1995,graduated from University of South China in2018,master student,focusing on cathode materials of aqueous zinc-ion batteryCorresponding author:WANG Juan,E-mail:Received date:2021-02-25,revised date:2021-03-23核技术2021,44:060103Bi 2Te 3nanosheets.Finally,in-situ synchrotron radiation X-ray diffraction (XRD)technique was used to track the changes of structure of Bi 2Te 3nanosheets during the charge-discharge process of the battery.[Result]The thickness of synthesized Bi 2Te 3nanosheets measured by AFM is about 3~5nm,and a highly reversible proton intercalation disclosed by in-situ synchrotron radiation based XRD during the reaction process is responsible for the practical battery operation.[Conclusions]This study confirms the reversible transport properties of protons in bismuth telluride cathode materials.Key wordsAqueous zinc-ion battery,Bismuth telluride nanosheets,In-situ synchrotron radiation X-ray diffraction,Reversible proton intercalation可再生能源的开发是当今社会一直备受关注的热点问题之一。

Electrochimica Acta 92 (2013) 248–256Contents lists available at SciVerse ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taGel-combustion synthesis of LiFePO 4/C composite with improved capacity retention in aerated aqueous electrolyte solutionMilica Vujkovi´c a ,Ivana Stojkovi´c a ,Nikola Cvjeti´canin a ,Slavko Mentus a ,b ,∗,1a University of Belgrade,Faculty of Physical Chemistry,P.O.Box 137,Studentski trg 12-16,11158Belgrade,Serbia bThe Serbian Academy of Sciences and Arts,Kenz Mihajlova 35,11158Belgrade,Serbiaa r t i c l ei n f oArticle history:Received 2October 2012Received in revised form 3January 2013Accepted 5January 2013Available online 11 January 2013Keywords:Aqueous rechargeable Li-ion battery Galvanostatic cycling Gel-combustion Olivine LiFePO 4LiFePeO 4/C compositea b s t r a c tThe LiFePO 4/C composite containing 13.4wt.%of carbon was synthesized by combustion of a metal salt–(glycine +malonic acid)gel,followed by an isothermal heat-treatment of combustion product at 750◦C in reducing atmosphere.By a brief test in 1M LiClO 4–propylene carbonate solution at a rate of C/10,the discharge capacity was proven to be equal to the theoretical one.In aqueous LiNO 3solu-tion equilibrated with air,at a rate C/3,initial discharge capacity of 106mAh g −1was measured,being among the highest ones observed for various Li-ion intercalation materials in aqueous solutions.In addition,significant prolongation of cycle life was achieved,illustrated by the fact that upon 120charg-ing/discharging cycles at various rates,the capacity remained as high as 80%of initial value.The chemical diffusion coefficient of lithium in this composite was measured by cyclic voltammetry.The obtained val-ues were compared to the existing literature data,and the reasons of high scatter of reported values were considered.© 2013 Elsevier Ltd. All rights reserved.1.IntroductionThanks to its high theoretical Coulombic capacity (170mAh g −1)and environmental friendliness,LiFePO 4olivine became a desir-able cathodic material of Li-ion batteries [1,2],competitive to other commercially used cathodic materials (LiMnO 4,LiCoO 2).As evidenced in non-aqueous electrolyte solutions,a small vol-ume change (6.81%)that accompanies the phase transition LiFePO 4 FePO 4enables Li +ion insertion/deinsertion reactions to be quite reversible [1–3].The problem of low rate capability,caused by low electronic conductivity [4,5],was shown to be solv-able to some extent by reduction of mean particle size [6].Further improvements in both conductivity and electrochemical perform-ances were achieved by forming composite LiFePO 4/C,where in situ produced carbon served as an electronically conducting con-stituent [5,7–27].Ordinarily,both in situ formed carbon and carbon black additive,became unavoidable constituent of the LiFePO 4-based electrode materials [28–37].Zhao et al.[27]reported that Fe 2P may arise as an undesirable product during the synthesis of LiFePO 4/C composite under reducing conditions,however,other authors found later that this compound may contribute positively∗Corresponding author at:University of Belgrade,Faculty of Physical Chemistry,P.O.Box 137,Studentski trg 12-16,11158Belgrade,Serbia.Tel.:+381112187133;fax:+381112187133.E-mail address:slavko@ffh.bg.ac.rs (S.Mentus).1ISE member.to the electronic conductivity and improve the electrochemical per-formance of the composite [28–30].Severe improvement in rate capability and capacity retention was achieved by partial replace-ment of iron by metals supervalent relative to lithium [31–37].Thus one may conclude that the main aspects of practical applica-bility of LiFePO 4in Li-ion batteries with organic electrolytes were successively resolved.After the pioneering studies by Li and Dahn [38,39],recharge-able Li-ion batteries with aqueous electrolytes (ARLB)attracted considerable attention [40–50].The first versions of ARLB’s,suf-fered of very low Coulombic utilization and significantly more pronounced capacity fade relative to the batteries with organic electrolyte,regardless on the type of electrode materials [43].For the first time,LiFePO 4was considered as a cathode material in ARLB’s by Manickam et al.in 2006[44].He et al.[46],in an aqueous 0.5M Li 2SO 4solution,found that LiFePO 4displayed both a surprisingly high initial capacity of 140mAh g −1at a rate 1C and recognizable voltage plateau at a rate as high as 20C,which was superior relative to the other electrode materials in ARLB’s.Recently,the same authors reported a high capacity decay in aer-ated electrolyte solution,amounting to 37%after only 10cycles [48].In the same study,they demonstrated qualitatively by a brief cyclovoltammetric test,that a carbon layer deposited from a vapor phase over LiFePO 4particles,suppressed the capacity fade [48].Inspired by the recent discoveries about excellent rate capa-bility [46]but short cycle life [48]of LiFePO 4in aerated aqueous solution,we attempted to prolong the cycle life by means of protecting carbon layer over the LiFePO 4particles.Therefore we0013-4686/$–see front matter © 2013 Elsevier Ltd. All rights reserved./10.1016/j.electacta.2013.01.030M.Vujkovi´c et al./Electrochimica Acta92 (2013) 248–256249synthesized LiFePO4/C composite by a fast and simple glycine-nitrate gel-combustion technique.This method,although simpler than a classic solid state reaction method combined with ball milling[44,48],was rarely used for LiFePO4synthesis[19,27].It yielded a porous,foamy LiFePO4/C composite,easily accessible to the electrolyte.Upon the fair charging/discharging performance was confirmed by a brief test in organic electrolyte,we examined in detail the electrochemical behavior of this material in aqueous electrolyte,by cyclic voltammetry,complex impedance and cyclic galvanostatic charging/discharging methods.In comparison to pure LiFePO4studied in Ref.[48],this composite displayed markedly longer cycle life in aerated aqueous solutions.The chemical dif-fusion coefficient of lithium was also determined,and the reasons of its remarkable scatter in the existing literature were considered.2.ExperimentalThe LiFePO4/C composite was synthesized using lithium nitrate, ammonium dihydrogen phosphate(Merck)and iron(II)oxalate dihydrate(synthesized according to the procedure described else-where[51])as raw materials.Our group acquired the experience in this synthesis technique on the examples of spinels LiMn2O4 [52]and LiCr0.15Mn1.85O4[53],where glycine served as both fuel and complexing/gelling agent to the metal ions.A stoichiometric amount of each material was dissolved in deionized water and mixed at80◦C using a magnetic stirrer.Then,first glycine was added into the reaction mixture to provide the mole ratio of glycine: nitrate of2:1,and additionally,malonic acid(Merck)was added in an amount of60wt.%of the expected mass of LiFePO4.The role of malonic acid was to decelerate combustion and provide con-trollable excess of carbon[14].After removing majority of water by evaporation,the gelled precursor was heated to initiate the auto-combustion,resulting in aflocculent product.The combustion product was heated in a quartz tube furnacefirst at400◦C for3h in Ar stream,and then at750◦C for6h,under a stream of5vol.%H2in Ar.This treatment consolidated the olivine structure and enabled to complete the carbonization of residual organic matter.The VO2powder prepared by hydrothermal method was used as an active component of the counter electrode in the galvanostatic experiments in aqueous electrolyte solution.The details of the syn-thesis and electrochemical behavior of VO2are described elsewhere [54,55].The considerable stoichiometric excess of VO2was used,to provide that the LiFePO4/C composite only presents the main resis-tive element,i.e.,determines the behavior of the assembled cell on the whole.The XRD experiment was performed using Philips1050diffrac-tometer.The Cu K␣1,2radiation in15–70◦2Ârange,with0.05◦C step and2s exposition time was used.The carbon content in the composite was determined by its com-bustion in theflowing air atmosphere,by means of thermobalance TA SDT Model2090,at a heating rate of10◦C min−1.The morphology of the synthesized compounds was observed using the scanning electron microscope JSM-6610LV.For electrochemical investigations,the working electrode was made from LiFePO4/C composite(75%),carbon black-Vulcan XC72 (Cabot Corp.)(20%),poly(vinylidenefluoride)(PVDF)binder(5%) and a N-methyl-2-pyrrolidone solvent.The resulting suspension was homogenized in an ultrasonic bath and deposited on electron-ically conducting support.The electrode was dried at120◦C for 4h.Somewhat modified weight ratio,85:10:5,and the same drying procedure,were used to prepare VO2electrode.The non-aqueous electrolyte was1M LiClO4(Lithium Corpo-ration of America)dissolved in propylene carbonate(PC)(Fluka). Before than dissolved,LiClO4was dried over night at140◦C under vacuum.The aqueous electrolyte solution was saturated LiNO3solution.The cyclic voltammetry and complex impedance experiments were carried out only for aqueous electrolyte solutions,by means of the device Gamry PCI4/300Potentiostat/Galvanostat.The three electrode cell consisted of a working electrode,a wide platinum foil as a counter electrode,and a saturated calomel electrode(SCE) as a reference one.The experiments were carried out in air atmo-sphere.The impedance was measured in open-circuit conditions, at various stages of charging and discharging,within the frequency range10−2−105Hz,with7points per decade.Galvanostatic charging/discharging experiments were carried out in a two-electrode arrangement,by means of the battery testing device Arbin BT-2042,with two-terminal connectors only.In the galvanostatic tests in non-aqueous solution,working electrode was a2×2cm2platinum foil carrying2.3mg of compos-ite electrode material(1.5mg of olivine),while counter electrode was a2×2cm2lithium foil.The cell was assembled in an argon-filled glove box and cycled galvanostatically within a voltage range 2.1–4.2V.The galvanostatic tests in the aqueous electrolyte solution were carried out in a two-electrode arrangement,involving3mg of cathodic material,as a working electrode,and VO2in a multi-ple stoichiometric excess,as a counter electrode.According to its reversible potential of lithiation/delithiation reaction[55],VO2per-formed as an anode in this cell.The4cm2stainless steel plates were used as the current collectors for both positive and negative electrode.The cell was assembled in room atmosphere,and cycled within the voltage window between0.01and1.4V.3.Result and discussion3.1.The XRD,SEM and TG analysis of the LiFePO4/C compositeFig.1shows the XRD patterns of the composite LiFePO4/C pre-pared according to the procedure described in the Experimental Section.As visible,the diffractogram agrees completely with the one of pure LiFePO4olivine,found in the JCPDS card No.725-19. The narrow diffraction lines indicate complete crystallization and relatively large particle dimensions.On the basis of absence of diffraction lines of carbon,we may conclude that the carbonized product was amorphous one.Fig.2shows the SEM images of the LiFePO4/C composite at two different magnifications.Theflaky agglomerates,Fig.2left,with apparently smooth surface and low tap density,are due to a partial liquefaction and evolution of gas bubbles during gel-combustion procedure.These agglomerates consist of small LiFePO4/CFig.1.XRD patterns of LiFePO4/C composite in comparison to standard crystallo-graphic data.250M.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256Fig.2.SEM images of LiFePO 4/C composite at two different magnification,20000×and 100000×.composite particles visible better at higher magnification,Fig.2,ly at the magnification of 100,000×,one may see that the size of majority of composite particles was in the range 50–100nm.The mean particle diameter,2r,as per SEM microphotograph amounted to 75nm.This analysis evidences that the gel-combustion method may provide nanodisprsed particles,desirable from the point of view of rate capability.For instance,Fey et al.[16]demonstrated that particle size reduction from 476to 205nm improved the rate capa-bility of LiFePO 4/C composite in organic electrolyte,illustrated by the increase of discharge capacity from 80mAh g −1to 140mAh g −1at discharging rate 1C.Also,carbon matrix prevented particles from agglomeration providing narrow size distribution,contrary to often used solid state reaction method of synthesis,when sintering of ini-tially nanometer sized particles caused the appearance of micron sized agglomerates [22].The SEM microphotograph (Fig.2)alone did not permit to rec-ognize carbon constituent of the LiFePO 4/C composite.However,carbonized product was evidenced,and its content measured,by means of thermogravimetry,as described elsewhere [9].The dia-gram of simultaneous thermogravimetry and differential thermal analysis (TG/DTA)of the LiFePO 4/C composite performed in air is presented in Fig.3.The process of moisture release,causing a slight mass loss of 1%,terminated at 150◦C.In the temperature range 350–500◦C carbon combustion took place,visible as a drop of the TG curve and an accompanying exothermic peak of the DTA curve.However,the early stage of olivine oxidation merged to some extent with the late stage of carbon combustion,and therefore,the minimum of the TG curve,appearing at nearly 500◦C,was not so low as to enable to read directly the carbon content.Fortunately,as proven by XRD analysis,the oxidation of LiFePO 4at tempera-ture exceeding 600◦C,yielded only Li 3Fe 2(PO 4)3and Fe 2O 3,whatFig.3.TGA/DTA curve of LiFePO 4/C under air flow at heating rate of 10C min−1.corresponded to the relative gain in mass of exactly 5.07%[9].Therefore,the weight percentage of carbonaceous fraction in the LiFePO 4/C composite was determined as equal to the difference between the TG plateaus at temperatures 300and 650◦C,aug-mented for 5.07%.According to this calculation the carbon fraction amounted to 13.4wt.%,and by means of this value,the electro-chemical parameters discussed in the next sections were correlated to pure LiFePO 4.Specific surface area of LiFePO 4,required for the measurement of diffusion constant,was determined from SEM image (Fig.2).Assuming a spherical particle shape and accepting mean particle radius r =37.5nm,the specific surface area was estimated on the basis of equation [17,22,45,46]:S =3rd(1)where the bulk density d =3.6g cm −3was used .This calculation resulted in the value S =22.2m 2g −1.In this calculation the contri-bution of carbon to the mean particle radius was ignored,however the error introduced in such way is more acceptable than the error which may arise if standard BET method were applied to the com-posite with significant carbon ly,due to a usually very developed surface area of carbon,the measured specific sur-face may exceed many times the actual surface area of LiFePO 4.3.2.Electrochemical measurements3.2.1.Non-aqueous electrolyte solutionIn order to compare the behavior of the synthesized LiFePO 4/C composite to the existing literature data,available predominantly for non-aqueous solutions,a brief test was performed in non-aqueous 1M LiClO 4+propylene carbonate solution by galvano-static experiments only.The results for the rates C/10,C/3and C,within the voltage limits 2.1–4.2V,were presented in Fig.4.The polarizability of the lithium electrode was estimated on the basis of the study by Churikov [56–67],who measured the current–voltage curves of pure lithium electrode in LiClO 4/propylene carbon-ate solutions at various temperatures.To the highest rate of 1C =170mA g −1in nonaqueous electrolyte,the corresponding cur-rent amounted to 0.25mA,which was equal to the current density of 0.064mA cm −2through the Li counter electrode.According to Fig.2in Ref.[67],for room temperature,the corresponding over-voltage amounted to only 6mV.Since lithium electrode is thus practically non-polarizable in this system,the voltages presented on the ordinate of the left diagram are the potentials of the olivine electrode expressed versus Li/Li +reference electrode.The clear charge and discharge plateaus at about 3.49V and 3.40V,respec-tively,correspond to the LiFePO 4 FePO 4phase equilibria [5].At discharging rate of C/10,the initial discharge capacity,within the limits of experimental error,was close to a full theoreticalM.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256251Fig.4.The initial charge/discharge curves (a)and cyclic performance (b)of LiFePO 4/C composite in 1M LiClO 4+PC at different rates within a common cut-off voltage of2.1–4.2V.Fig.5.Charge/discharge profile and corresponding cyclic behavior of LiFePO 4/C in 1M LiClO 4+PC at the rate of 1C.capacity of LiFePO 4(170mAh g −1).This value is higher than that for LiFePO 4/C composite obtained by glycine [19],malonic acid [14]and adipic acid/ball milling [15]assisted methods.As usual,the discharge capacity decreased with increasing discharging rate (Fig.4b),and amounted to 127mAh g −1at C/3,and 109mAh g −1at 1C.For practical application of Li-ion batteries,a satisfactory rate capability and long cycle life are of primary importance.The charge/discharge profiles and dependence of capacity on the cycle number at the rate 1C are presented in Fig.5.The capacity was almost independent on the number of cycles,similarly to theearlier reports by Fey et al.[37–39].For comparison,Kalaiselvi et al.[19],by a glycine assisted gel-combustion procedure,with an additional amount (2wt.%)of carbon black,produced a similar nanoporous LiFePO 4/C composite displaying somewhat poorer per-formance,i.e.,smaller discharge capacity of 160mAh g −1at smaller discharging rate of C/20.On the other hand,better rate capability of LiFePO 4/C com-posite,containing only 1.1–1.8wt.%of carbon,in a non-aqueous solution,was reported by Liu et al.[21].For instance they mea-sured 160mAh g −1at the rate 1C,and 110at even 30C [21].This may be due to a thinner carbon layer around the LiFePO 4olivine particles.However the advantage of here applied thicker carbon layer exposed itself in aqueous electrolyte solutions,as described in the next section.3.2.2.Aqueous electrolyte solution3.2.2.1.Cyclic voltammetry.By the cyclic voltammetry method (CV)the electrochemical behavior of LiFePO 4/C composite in satu-rated aqueous LiNO 3solution was preliminary tested in the voltage range 0.4–1V versus SCE.The cyclic voltammograms are pre-sented in Fig.6.The highest scan rate of 100mV s −1,tolerated by this material,was much higher than the ones (0.01–5mV s −1)used in previous studies in both organic [13,24,25]and aqueous electrolyte solutions [47,48].Since one deals here with the thin layer solid redox electrode,limited in both charge consumption and diffusion length,the voltammogram is more complicated for interpretation comparing with the classic case of electroactive species in a liquid solution.A sharp,almost linear rise of current upon achieving reversible potential,with overlapped rising parts at various scan rates,similar to ones reported elsewhere [21,25],resembles closely the voltammogram of anodic dissolution ofaFig.6.Cyclic voltammograms of LiFePO 4/C in saturated LiNO 3aqueous electrolyte with a scan rate of 1mV s −1(left)and at various scan rates in the range 1–100mV s −1.252M.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256Fig.7.Anodic and cathodic peak current versus square root of scan rate forLiFePO 4/C composite in aqueous LiNO 3electrolyte solution.thin metal layer [56],which proceeds under constant reactant activity.Since the solid/solid phase transitions LiFePO 4 FePO 4accompanies the redox processes in this system [5,8,57,58],the positive scan of the voltammograms depict the phase transition of LiFePO 4to FePO 4,while the negative scan depicts the phase transi-tion FePO 4to LiFePO 4.As shown by Srinivasan et al.[5],LiFePO 4may be exhausted by Li not more than 5mol.%before to trans-form into FePO 4,while FePO 4may consume no more than 5%Li before to transform into LiFePO 4,i.e.cyclic voltammetry exper-iments proceeds under condition of almost constant activity of the electroactive species.Although these aspects of the Li inser-tion/deinsertion process do not fit the processes at metal/liquid electrolyte boundary implied by Randles–Sevcik equation:i p =0.4463F RT1/2C v 1/2AD 1/2(2)this equation was frequently used to estimate apparent diffusion coefficient in Li insertion processes [5,17,21,46,59].To obtain peak current,i p ,in amperes,the concentration of lithium,C =C Li ,should be in mol cm −3,the real surface area exposed to the electrolyte in cm 2,chemical diffusion coefficient of lithium through the solid phase,D =D Li ,in cm 2s −1,and sweep rate,v ,in V s −1.The Eq.(2)pre-dicts the dependence of the peak height on the square root of sweep rate to be linear,as found often in Li-ion intercalation processes [17,21,25,59,60].This condition is fulfilled in this case too,as shown in Fig.7.The average value of C Li may be estimated as a reciprocal value of molar volume of LiFePO 4(V M =44.11cm 3mol −1),hence C Li =2.27×10−2mol cm −3.The determination of the actual surface area of olivine is a more difficult task,due to the presence of carbon in the LiFePO 4/C ly,classical BET method of sur-face area measurement may lead to a significantly overestimated value,since carbon surface may be very developed and participate predominantly in the measured value [15].Thus the authors in this field usually calculated specific surface area by means of Eq.(1),using mean particle radius determined by means of electron microscopy [17,22,45,46].Using S =22.2m 2g −1determined by means of Eq.(1),and an actual mass of the electroactive substance applied to the elec-trode surface (0.001305g),the actual electrode surface area was calculated to amount to A =290cm 2.This value introduced in Randles–Sevcik equation yielded D Li ∼0.8×10−14cm 2s −1.From the aspect of capacity retention,the insolubility of olivine in aqueous solutions is advantageous compared to the vanadia-based Li-ion intercalation materials,such as Li 1.2V 3O 8[61],LiV 3O 8[62]and V 2O 5[63],the solubility of which in LiNO 3solution was perceivable through the yellowish solutioncoloration.Fig.8.The Nyquist plots of LiFePO 4/C composite in aqueous LiNO 3solution at var-ious stages of delithiation;inset:enlarged high-frequency region.3.2.2.2.Impedance measurements.Figs.8and 9present the Nyquist plots of the LiFePO 4/C composite in aqueous LiNO 3solution at various open circuit potentials (OCV),during delithiation (anodic sweep,Fig.8)and during lithiation (cathodic sweep,Fig.9).The delithiated phase,observed at OCV =1V,as well as the lithi-ated phase,observed at OCV =0V,in the low-frequency region (f <100Hz)tend to behave like a capacitor,characteristic of a surface thin-layered redox material with reflective phase bound-ary conditions [64].At the OCV not too far from the reversible one (0.42V during delithiation,0.308V during lithiation),where both LiFePO 4and FePO 4phase may be present,within the whole 10−2–105Hz frequency range,the reaction behaves as a reversible one (i.e.shows the impedance of almost purely Warburg type).The insets in Figs.8and 9present the enlarged parts of the impedance diagram in the region of high frequencies,where one may observe a semicircle,the diameter of which corresponds theoretically to the charge transfer resistance.As visible,the change of open circuit potential between 0and 1V,in spite of the phase transition,does not cause significant change in charge transfer resistance.The small charge transfer resistance obtained with the carbon participation of 13.4%,being less than 1 ,is the smallest one reported thus far for olivine based materials.This finding agrees with the trend found by Zhao et al.[27],that the charge transfer resistance scaleddownFig.9.The Nyquist plots of LiFePO 4/C composite in aqueous LiNO 3solution at var-ious stages of lithiation;inset:enlarged high-frequency region.M.Vujkovi´c et al./Electrochimica Acta 92 (2013) 248–256253Fig.10.The dependence Z Re vs.ω−1/2during lithiation at 0.308V (top)and delithi-ation at 0.42V (down)in the frequency range 72–2.68Hz.to 1000,400and 150 when the amount of in situ formed carbon in the LiFePO 4/C composite increased in the range 1,2.8and 4.8%.For OCV corresponding to the cathodic (0.42V)and anodic (0.308V)peak maxima,the Warburg constant W was calculated from the dependence [21]:Z Re =R e +R ct + W ω−1/2(3)In the frequency range 2.7–72Hz,almost purely Warburg impedance was found to hold (i.e.the slope of the Nyquist plot very close to 45degrees was found).At the potential of cathodic current maximum (0.42V),from Fig.10, W was determined to amount to 7.96 s −1/2.At the potential of anodic maxima,0.308V, W was determined to amount to 9.07 s −1/2.In the published literature,for the determination of diffusion coefficient on the basis of impedance measurements,the following equation was often used [66,68,69]:D =0.5V M AF W ıE ıx2(4)where V M is molar volume of olivine,44.1cm 3, W is Warburg con-stant and ıE /ıx is the slope of the dependence of electrode potential on the molar fraction of Li (x )for given value of x .However,the potentials of CV maxima in the here studied case correspond to the x range of two-phase equilibrium,where for an accurate deter-mination of ıE /ıx a strong control of perturbed region of sample particles is required [69],and thus the determination of diffusion coefficients was omitted.3.2.2.3.Galvanostatic measurements.The galvanostatic measure-ments of LiFePO 4/C in saturated LiNO 3aqueous solution were performed in a two-electrode arrangement using hydrother-mally synthesized VO 2[55]as the active material of thecounterFig.11.Capacity versus cycle number and charge/discharge profiles (inset)for thecell consisting of LiFePO 4/C composite as cathode,and VO 2in large excess as anode,in saturated LiNO 3aqueous electrolyte observed at rate C/3.electrode.Preliminary cyclovoltammetric tests of VO 2in saturated LiNO 3solution at the sweep rate 10mV s −1,evidenced excellent cyclability and stable capacity of about 160mAh g −1during at least 50cycles.The voltage applied to the two-electrode cell was cycled within the limits 0and 1.4V.Due to a significant stoichiometric excess of VO 2over LiFePO 4/C composite (5:1)the actual voltage may be considered to be the potential versus reference VO 2/Li x VO 2electrode.Fig.11shows the dependence of the discharging Coulombic capacity of the LiFePO 4/C composite on the number of galvano-static cycles at discharging rate C/3,as well as (in the inset)the voltage vs.charging/discharging degree for 1st,2nd and 50th cycle.The charge/discharge curves do not change substantially in shape upon cycling,indicating stable capacity.For an aqueous solution,a surprisingly high initial discharge capacity of 106mAh g −1and low capacity fade of only 6%after 50charge/discharge cycles were evidenced.This behavior is admirable in comparison to other elec-trode materials in aqueous media reported in literature (LiTi 2(PO 4)3[42],LiV 3O 8[57]),and probably enabled by a higher thermody-namic stability of olivine structure [1].Fig.12presents the results of cyclic galvanostatic investigations of LiFePO 4/C composite in aqueous LiNO 3solution at various dis-charging rates.The charging/discharging rate was initially C/3for 80cycles and then was increased stepwise up to 3C.ThecapacityFig.12.Cyclic performance of LiFePO 4/C in saturated LiNO 3aqueous electrolyte at different charging/discharging rates.。

a r X i v :c o n d -m a t /0507564v 1 [c o n d -m a t .s u p r -c o n ] 25 J u l 2005Typeset with jpsj2.cls <ver.1.2>LetterKink Structure in the Quasiparticle Band of Doped Hubbard SystemsYoshiro Kakehashi 1∗and Peter Fulde 2†1Department of Physics and Earth Sciences,Faculty of Science,University of Ryukyus,1Senbaru,Nishihara,Okinawa903-0213,Japan2Max-Planck-Institut f¨u r Physik komplexer Systeme,N¨o thnitzer Str.38,D-01187Dresden,GermanyBy making use of the self-consistent projection operator method with high-momentum and high-energy resolutions,we find a kink structure in the quasiparticle excitation spectrum of the two-dimensional Hubbard model in the underdoped regime.The kink is caused by a mixing between the quasiparticle state and excitations with short-range antiferromagnetic order.We suggest that this might be the origin of the strong concentration dependence of the ’kink’found in La 2−x Sr x CuO 4(x =0.03−0.07).KEYWORDS:kink,quasiparticle excitations,Hubbard model,ARPES,cuprate,LSCORecent high-resolution photoemission experiments show that there is a well-defined ’kink ’in the quasipar-ticle band dispersion of high-T c cuprates,whose energy scale is ωkink =60−70meV in both the normal and superconducting states.1–3The kink along the nodal di-rection was found to have a universal feature,2i.e.,the Fermi velocity v F in the low-energy region (|ω|<ωkink )is not sensitive to the type of cuprates,doping concen-tration,and isotope substitution,4although v F in the high-energy regime (|ω|>ωkink )strongly depends on the latter.Various theoretical explanations for the kink have been attempted from two different points of view.One type of theory relies on the coupling of an electronic quasiparticle to the spin fluctuation resonance mode ob-served in inelastic neutron scattering experiments.5,6An-other relies on a coupling to phonons,particularly to the longitudinal optical phonon mode found in neutron ex-periments.2,3The latter approach raised again the fun-damental question on the mechanism underlying high-T c superconductivity in cuprates,i.e.,electron-or phonon-mediated.Although the reliability of these theories is un-der debate and the improvements of these theories are in progress,7,8it has not yet been seriously studied whether or not the kink in the quasiparticle state can be solely of electronic origin.We deal with this problem in the present letter and report the appearance of a kink due to long-range electron correlations in the two-dimensional (2D)Hubbard model for small doping concentrations.The difficulty in the present problem is that the per-turbation approach is not applicable to cuprates be-cause of strong electron correlations.Therefore,ad-vanced theories such as the Lanczos method,9the quan-tum Monte-Carlo (QMC)method,10–12and dynamical cluster approximation (DCA),13,14have been applied to the cuprate system.They clarified the global structure of the single-particle excitation spectrum in the 2D Hub-bard model.A detailed structure of the low-energy ex-citations at low temperatures,however,has not been derived because of the limited resolutions in both mo-mentum and energy and the limited range of intersitez −ǫk −Λk (z ).(1)Here,z =ω+iδ,where δis a positive infinitesimal number,and ǫk is the Hartree-Fock one-electron energy dispersion measured from Fermi energy.In the SCPM,the momentum-dependent self-energy Λk (z )is calcu-lated from the Fourier transform of nonlocal memory functions M ij asΛk (z )=U2jM j 0(z )exp(i k ·R j ).(2)Note that R j is the position vector of site j .High-momentum and high-energy resolutions are achieved bytaking into account the off-diagonal terms M ij (z )up to infinity.We calculate M ij (z )by means of an incremental cluster expansion in an effective medium with a co-herent potential ˜Σ(z ).Within the two-site approxima-tion,the M ij (z )are given by M ii (z )=M (i )ii (z )+ l =i M (il )ii (z )−M (i )ii (z ) and M i =j (z )=M (ij )i =j (z ).M (i )ii (z )and M (ij )i =j (z )are the matrix elements of the clus-ter memory matrices defined by M (c )lm (z )= ˆM(c ) 1−L (c )·ˆM(c )−1lm(c =i,ij ).Here,L (c )(z )is a 1×1-15-10-5051015ΓXMΓωkδh = 0.14 U=8Fig.1.Excitation spectra along high symmetry line calculated at hole concentration δh =0.14,U =8,and T =0.Γ=(0,0),X =(0,π),and M =(π,π)in units of the lattice constant.The energy unit is chosen so that the nearest-neighbor transfer integral is one.Open circles with error bars are the results obtained by the QMC method 12at T =0.33.The dashed curve shows the Hartree-Fock contribution ǫk .(for c =i )or (2×2)(c =(ij ))cluster Liouvillean,whose diagonal matrix elements are given by L (i )(z )=U (1−2 n i −σ )/[ n i −σ (1− n i −σ )]using an average elec-tron number n iσ with spin σon site i .The screenedmemory matrix element ˆM (c )ij(z )is obtained using renor-malized perturbation theory 15asˆM (c )ij(z )=A ij dǫdǫ′dǫ′′˜ρ(c )ij (ǫ)˜ρ(c )ij (ǫ′)˜ρ(c )ji (ǫ′′)χ(ǫ,ǫ′,ǫ′′)vFδhFig.4.Fermi velocity along nodal direction as function of hole concentration.Closed circles denote the velocity in the marginal Fermi liquid state,while open circles indicate the velocity in the normal Fermi liquid state.The dashed line is the result for a noninteracting system.range magnetic order(i.e.,a precursor of the gap for-mation due to antiferromagnetic correlations).11,13The kink atωkink=−0.8|t|is caused by a mixing between the quasiparticle state and magnetic excitations.Re-cently,we have reported that a marginal Fermi liquid17 (MFL)-like behavior is found in the underdoped region (δh<∼0.03)away from half-filling because of a pinning of Fermi energy to the van Hove anomaly due to a trans-fer of spectral weight from the lower Hubbard band to the upper one.18Antiferromagnetic correlations should be enhanced in this region because of nesting.The kink behavior appears in this regionδh<∼0.03.On the other hand,the lower Hubbard band collapses whenδh>∼0.03. The MFL-like state with antiferromagnetic correlations changes to a normal Fermi liquid state and the kink dis-appears,as noted in Fig.3.The calculated Fermi velocity along theΓ-M line is presented in Fig.4.The velocity shows a weak concentration dependence,and the renor-malization factor of the Fermi velocity is about1.8from the underdoped region to the overdoped one.We have also examined in detail excitations alongΓ-X-M.Because of the van Hove singularity,the quasipar-ticle band around the X point is quiteflat.We do not find a kink behavior in thisflat band region.The mix-ing between the quasiparticle band and the band of the magnetic excitations takes place away from the linear dispersion regime near the Fermi level,i.e.,(|k|,ω)= (0.5π,−1.2)on theΓ-X line.Because both bands are flat,it is not clear whether this region contains a kink. The present model is too simple for attempting a comparison with the experimental data.Nevertheless, it is plausible that the kink is enhanced with decreas-ing doping concentration because of the development of short-range antiferromagnetic order.We speculate that the strong concentration dependence of the kink in La2−x Sr x CuO4(LSCO)3,19may be caused by the present mechanism.In fact,we obtain for the charac-teristic kink energyωkink=70meV when we choose the transfer integral so that the calculated Fermi veloc-ity along the nodal direction agrees with the observed one(1.8eV·˚A).The value forωkink agrees well with the experimental one(60-70meV).19In summary,we have investigated the quasiparticle band in the doped2D Hubbard model on the basis of the SCPM with high-momentum and high-energy resolutions.Wefind a kink along the nodal direction (0,0)−(π,π)in the range of doping concentrations 0<δh<∼0.03where the MFL behavior persists.It is caused by a mixing between the quasiparticle excitations and magnetic excitations with short-range antiferromag-netic order.The kink decays rapidly with the decrease in antiferromagnetic correlations.We speculate that the kink in the underdoped regime of LSCO may be due to the present mechanism.AcknowledgementsThe authors would like to thank Drs.J.Fink and O. 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LiTi 2(PO 4)3/C 复合材料的制备及电化学性能袁铮崔永丽沈明芳强颖怀庄全超*(中国矿业大学材料科学与工程学院,锂离子电池实验室,江苏徐州221116)摘要:采用聚乙烯醇(PVA)辅助溶胶-凝胶法合成了具有Na +超离子导体(NASICON)结构的LiTi 2(PO 4)3/C 复合材料.运用X 射线衍射(XRD)、扫描电子显微镜(SEM)、充放电测试、循环伏安(CV)、电化学阻抗谱(EIS)等对其结构形貌和电化学性能进行表征.实验结果表明:合成的LiTi 2(PO 4)3/C 具有良好的NASICON 结构,首次放电容量为144mAh ·g -1.电化学阻抗谱测试结果显示,LiTi 2(PO 4)3/C 复合材料电极在首次嵌锂过程中分别出现了代表固体电解质相界面(SEI)膜及接触阻抗、电荷传递阻抗和相变阻抗的圆弧，并详细分析了它们的变化规律.计算了Li +在LiTi 2(PO 4)3中嵌入/脱出时的扩散系数,分别为2.40×10-5和1.07×10-5cm 2·s -1.关键词:LiTi 2(PO 4)3/C 复合材料;电化学阻抗谱;固体电解质界面膜;接触阻抗;相变;扩散系数中图分类号:O646Preparation and Electrochemical Performance of LiTi 2(PO 4)3/CComposite Cathode for Lithium Ion BatteriesYUAN ZhengCUI Yong-LiSHEN Ming-FangQIANG Ying-HuaiZHUANG Quan-Chao *(Li-Ion Batteries Laboratory,School of Materials Science and Engineering,China University of Mining and Technology,Xuzhou 221116,Jiangsu Province,P .R.China )Abstract:LiTi 2(PO 4)3/C composite with a Na +superionic conductor (NASICON)-type structure was prepared by a sol-gel method.The LiTi 2(PO 4)3/C composite had a good NASICON structure and good electrochemical properties as revealed by X-ray diffraction (XRD),scanning electron microscopy (SEM),charging/discharging tests,cyclic voltammetry (CV),and electrochemical impedance spectroscopy (EIS).The first discharge capacity was 144mAh ·g -1.The EIS results indicated that there appeared semicircles respectively representing the solid electrolyte interface (SEI)film as well as the contact resistance,charge transfer resistance,and phase transformation resistance in the initial lithiation process of LiTi 2(PO 4)3/C composite electrode.The chemical diffusion coefficients of intercalation and de-intercalation of Li +in the LiTi 2(PO 4)3cathode material were calculated to be 2.40×10-5and 1.07×10-5cm 2·s -1,respectively.Key Words:LiTi 2(PO 4)3/C composite material;Electrochemical impedance spectroscopy;Solidelectrolyte interface film;Contact resistance;Phase transformation;Diffusion coefficient[Article]d oi:10.3866/PKU.WHXB201203012物理化学学报(Wuli Huaxue Xuebao )Acta Phys.-Chim.Sin .2012,28(5),1169-1176May Received:November 18,2011;Revised:February 20,2012;Published on Web:March 1,2012.∗Corresponding author.Email:zhuangquanchao@;Tel:+86-136********.The project was supported by the Fundamental Research Funds for the Central Universities,China (2010LKHX03,2010QNB04,2010QNB05)and Innovation and Ability Enhancement Funds for Fostering Subject of China University of Mining and Technology (2011XK07).中央高校基本科研业务费专项资金(2010LKHX03,2010QNB04,2010QNB05)和中国矿业大学培育学科创新能力提升基金(2011XK07)资助项目ⒸEditorial office of Acta Physico-Chimica Sinica1引言自20世纪90年代以来,锂离子电池以其较高的工作电压、较大的比容量以及绿色安全等优点引起了企业和研究机构的广泛关注.锂离子电池正极材料占锂离子电池总成本的40%左右,也是制约其容量的主要因素,因此开发理想的正极材料,是今后锂离子电池研究的热点和发展电动交通工具的关键.11169Acta Phys.-Chim.Sin.2012Vol.28磷基材料以其较高的能量密度和良好的散热性能受到了研究学者的广泛关注.2-7在过去的研究中,人们把注意力大都集中在橄榄石型结构的LiMPO4(M=Mn,Fe,Co,Ni)系列材料上,特别是具有较高的理论容量(170mAh·g-1),适中的电位(Li/ Li+为3.4V),以及对环境友好等特点的LiFePO4材料.2近年来,除了传统的橄榄石型的磷酸盐外,其他晶体结构的磷酸盐也逐渐受到重视.LiM2(PO4)3 (M=Fe,V,Ti)是一类Na+超离子导体(NASICON)结构的材料,不仅可以用于固体电解质,也可用于锂离子电池电极材料.8-11NASICON结构是带负电的三维骨架,结构式为Ti2P3O12,是由PO4四面体和TiO6八面体相连接,每个TiO6八面体与6个PO4四面体相连接,Li+在晶体结构的三维通道中迁移.12LiTi2(PO4)3属于NASICON 型超离子导体材料,有两种不同的Li+位(M1和M2), M1被完全填充,而M2都是空位.在放电过程中,有两个Li+嵌入LiTi2(PO4)3中形成Li3Ti2(PO4)3,充电的过程两个Li+从Li3Ti2(PO4)3中脱出形成LiTi2(PO4)3.13,14目前关于用LiTi2(PO4)3作为锂离子电池电极材料的研究报道并不多,而且LiTi2(PO4)3多采用高温固相法制得.本文采用溶胶-凝胶法替代传统的高温固相法制备了LiTi2(PO4)3/C复合材料,并研究其电化学性能.2实验部分聚乙烯醇(PV A,含量高于97%)、NH4H2PO4(含量高于99%,分析纯)均为天津福晨化学试剂厂产品,Li2CO3(含量高于98%,分析纯)、TiO2(含量高于98%,化学纯)均为国药集团化学试剂公司产品.采用溶胶-凝胶法制备LiTi2(PO4)3/C.具体过程为:先将PV A溶于去离子水中,然后加入摩尔计量比的Li2CO3、NH4H2PO4、TiO2,使其在去离子水中充分溶解.混合物在80°C水浴搅拌器内搅干形成固体,然后在鼓风干燥箱内80°C下干燥24h,研磨后得到前驱体.将前驱体移入瓷舟内并放入管式炉内,在Ar气氛保护下,以10°C·min-1升温速率加热到900°C,保温12h,即得到LiTi2(PO4)3/C样品.反应方程式为:0.5Li2CO3+2TiO2+3(NH4)H2PO4→LiTi2(PO4)3+3NH3+4.5H2O+0.5CO2(1)物相测试在D/Max-3B型X射线衍射仪(日本理学Rigaku)上完成.测量条件为Cu靶,Kα射线,石墨单色器,管电压35kV,管电流30mA,扫描速率为3 (°)·min-1,采样间隔为0.02°.样品形貌用美国FEI 公司生产的Quanta250环境扫描电子显微镜进行观察.LiTi2(PO4)3/C复合材料电极按70%(w)的活性材料,20%(w)的聚偏二氟乙烯-六氟丙烯(PVDF-HFP)粘合剂(99%,Kynar HSV910,Elf-atochem,USA), 10%(w)的炭黑(99%,上海杉杉科技有限公司)组成,以铝箔作为集流体.电解液为1mol·L-1LiPF6-碳酸乙烯酯(EC)/碳酸二乙酯(DEC)/碳酸二甲酯(DMC) (体积比1:1:1)(张家港国泰华荣化工新材料公司).所有电化学实验均采用2032扣式电池体系,金属锂片为对电极.充放电实验在2XZ-2B电池检测系统(深圳新威电子仪器公司)上完成.充放电电压范围为1.50-3.50V,充放电倍率为0.1C(1C=138 mAh·g-1).循环伏安(CV)及电化学阻抗谱(EIS)测试在CHI660D电化学工作站(上海辰华仪器有限公司)上完成.CV的扫描速率为10-4-10-3V·s-1不等,电压范围为1.50-3.50V.EIS测试频率范围为10-2-105Hz,交流信号振幅为5mV,测试中获得的阻抗数据用Zview软件进行拟合.3结果与讨论3.1XRD及SEM结果图1为LiTi2(PO4)3/C复合材料的XRD图谱.对照JCPDS(#35-0754)卡片可知,各个衍射峰与标准卡片上一致,图谱衍射峰尖锐,峰强较高,说明材料具有良好的NASICON结构晶型.11此外,XRD图谱中出现3个低强度P2O5的杂质衍射峰,很可能在合成过程中由于未反应的(NH4)H2PO4在高温时分解引入的.图2为LiTi2(PO4)3/C复合材料的SEM图像.从图1LiTi2(PO4)3/C复合材料XRD的图谱Fig.1XRD pattern of LiTi2(PO4)3/C compositematerial1170袁铮等:LiTi 2(PO 4)3/C 复合材料的制备及电化学性能No.5图中可以看出,LiTi 2(PO 4)3/C 复合材料的颗粒比较大,在10μm 左右,这可能是因为在烧结的过程中,温度过高发生颗粒的团聚造成的.在LiTi 2(PO 4)3/C 复合材料中,颗粒的表面存在一些孔洞,这些孔洞的存在有利于Li +的脱出和嵌入,可提高活性物质的利用率.3.2CV 及充放电实验结果图3为LiTi 2(PO 4)3/C 电极的循环伏安曲线.可以看出,在1.50-3.50V 的充放电电压范围内,LiTi 2(PO 4)3/C.只存在一对氧化还原峰,氧化脱锂峰在2.60V 附近,还原嵌锂峰在2.40V 附近,说明NASICON 结构的LiTi 2(PO 4)3中的锂离子脱嵌是一步进行的.氧化还原电位差ΔV 为0.20V ,略大于充放电平台差(ΔV =0.05V),这是因为循环伏安模拟电池充放电过程时会存在极化现象,使ΔV 略有增加.从图中可以发现,第一周扫描过程中,还原嵌锂峰的峰值电流I p1为0.69mA,氧化脱锂峰值电流I p2为0.47mA,说明首次脱/嵌锂时的电化学动力学存在差异.在以后的几周中,还原峰值和氧化峰值基本一致,重合性也较第一周好,说明材料在经过第一周充放电后,结构的稳定性有所增加.15图4(a)为LiTi 2(PO 4)3/C 电极在第1和2周的充放电曲线,其中放电平台为2.45V ,充电平台为2.50V ,与理论充放电平台一致,16且平台较平,说明材料具有良好的电化学性能.第一周充放电容量分别为137和144mAh ·g -1.Wang 等11指出,NASICON 结构的LiTi 2(PO 4)3在锂离子嵌入/脱出时,除了Ti 4+/Ti 3+得到/失去电子外,[PO 4]3-也可以得失电子,这样理论上每1mol LiTi 2(PO 4)3反应会有5.2mol 的Li +嵌入/脱出.而在本研究中,1mol LiTi 2(PO 4)3中大约有2图2LiTi 2(PO 4)3/C 复合材料不同放大倍数的SEM 图像Fig.2SEM photographs of LiTi 2(PO 4)3/C compositematerial with differentmagnifications图3LiTi 2(PO 4)3/C 电极循环伏安曲线Fig.3Cyclic voltammetry curves of LiTi 2(PO 4)3/Celectrode图4(a)LiTi 2(PO 4)3/C 电极充放电曲线(第1、2周);(b)LiTi 2(PO 4)3/C 电极循环效率曲线Fig.4(a)Charge/discharge curves of LiTi 2(PO 4)3/C electrode (1st,2nd cycles);(b)cycling property curves ofLiTi 2(PO 4)3/C electroderange of voltage:1.50-3.50V1171Acta Phys.-Chim.Sin.2012Vol.28mol 的Li +嵌入/脱出(138mAh ·g -1),这说明在此电压范围内,锂离子的嵌入/脱出是由Ti 4+/Ti 3+的氧化还原造成的,并没有PO 3-4参加反应.LiTi 2(PO 4)3/C 电极的循环性能曲线如图4(b)所示,除首次充放电过程中存在一定的不可逆容量衰减外,充放电容量基本保持一致,充放电效率基本上保持在100%.但是材料容量的衰减比较快,循环20周后充放电容量分别为109和110mAh ·g -1,其容量保持率分别为79.56%和76.39%.容量衰减较快可能是因为材料在反复脱嵌锂的过程中体积有所变化,使活性物质与导电剂接触不理想,复合材料的导电性能下降造成的.3.3EIS 研究3.3.1LiTi 2(PO 4)3/C 电极在首次放电过程中的EIS谱基本特征图5为LiTi 2(PO 4)3/C 电极首次放电过程中的阻抗谱变化图.从图中可以看出,在开路电位2.90V 时,LiTi 2(PO 4)3/C 电极的Nyquist 图在整个测试频率范围内主要由三个部分组成,即高频区域的一个半圆、中频区的一个半圆及低频区域的一段斜线.依据经典的锂离子嵌入脱出机制模型,17,18高频区半圆(HFS)是与SEI 膜相关的半圆,但是考虑高频区的半圆在开路电位下(2.90V)就存在,且阻值很大,因而高频区的半圆除了与SEI 膜有关外,可能也与接触阻抗有关.中频区域半圆(MFS)是与电荷传递过程相关的半圆,而低频区部分的斜线(LFL)则反映了锂离子在电极材料固体中的扩散过程.在电极极化电位降低至2.40V 的过程中,Nyquist 图的基本特征与开路电位时相似.2.30V 时,Nyquist 图的一个重要特征为低频区域的斜线演变为一段斜线和一段圆弧,此时Nyquist 图由四部分组成,即高频区域与SEI 膜和接触阻抗相关的半圆,中频区域与电荷传递过程相关的半圆,低频区域与固态扩散相关的斜线以及更低频区域的一段圆弧.根据Barsoukov 等19的观点,更低频区域的半圆是与锂离子嵌入过程中材料本体发生相变有关的半圆,即LiTi 2(PO 4)3在反应电位时,锂离子大量嵌入,LiTi 2(PO 4)3转变为Li 3Ti 2(PO 4)3,并由于体积的膨胀,产生了一个新的相界面.新的相界面使锂离子在两相中传输速率发生变化,在Nyquist 图上表现为一个新的半圆.先前我们运用EIS 研究Cu 6Sn 5合金的嵌锂过程时,也观察到了类似的现象.20-22根据EIS 谱的基本特征,本文选取的等效电路如图6所示,其中R s 代表溶液电阻,R 1、R 2、R 3分别代表高频半圆、中频区半圆和相变相关的电阻,高频半圆电容、中频半圆电容、低频扩散阻抗和相变电容分别用恒相角元件(CPE)Q 1、Q 2、Q 3、Q 4表示,CPE 的导纳响应表达式如下:Y =Y 0ωn cos(n π2)+j Y 0ωn sin(n π2)(2)其中ω为角频率,j 为虚数单位-1.当n =0时,CPE相当于一个电阻;n =1,CPE 相当于一个电容;n =0.5,图5LiTi 2(PO 4)3/C 电极首次放电过程中EIS 随电极极化电位的变化Fig.5Variations of EIS with the electrode polarization potentials for LiTi 2(PO 4)3/C electrode during the first discharge processHFS:high frequency semicircle;MFS:middle frequency semicircle;LFL:low frequencyline1172袁铮等:LiTi 2(PO 4)3/C 复合材料的制备及电化学性能No.5CPE 相当于Warburg 阻抗.根据该等效电路对实验结果进行拟合.在拟合过程中,根据EIS 低频区域是否存在与扩散相关的斜线和与相变相关的圆弧,通过添减Q 3和R 3/Q 4等效电路元件实现对不同电位下EIS 的拟合.拟合结果见图7.由此可见图6所示的等效电路能满意地拟合不同电位下的EIS 实验数据,实验数据曲线与拟合曲线实现很好的重叠,各等效电路参数拟合误差小于15%.3.3.2各频率区间内数值变化及分析图8为LiTi 2(PO 4)3/C 电极在首次放电过程中R 1随电极电位变化关系.可以看出在放电过程中,2.90-2.50V 之间,R 1随电极极化电位的降低缓慢增大,表明LiTi 2(PO 4)3/C 电极的SEI 膜随电极极化电位的降低缓慢增厚,同时接触阻抗逐渐增大.2.50-2.40V 之间,R 1随电极极化电位的降低迅速增大.图9为Li 1+x Ti 2(PO 4)3首次放电过程中嵌锂度x 随电极极化电位的变化.可以看出首次放电过程中,随着电极极化电位的下降,嵌锂度不断增加.当电极极化电位降低到2.50V 时,锂离子的嵌入量为1.96%.从2.50V 到2.40V 时,嵌锂量突增到60.47%,说明此过程中有大量锂离子的嵌入.锂离子的大量嵌入会使颗粒体积膨胀,导致颗粒内部的应力增大.考虑到本研究中所制备的活性材料颗粒较大,颗粒内部的应力增大,会引起活性材料颗粒的破碎,导致活性材料表面积增大,进而使SEI 膜阻抗增大.同时颗粒的破碎也会使材料的接触变差,致使接触阻抗增大,这与文献报道15的一致.2.40V 以下,R 1随电极极化电位的降低缓慢增大,与2.90-2.50V 范围内R 1的变化规律类似.图6LiTi 2(PO 4)3/C 电极的EIS 拟合等效电路Fig.6Equivalent circuit proposed for EIS fitting of LiTi 2(PO 4)3/C electrodeR s :solution resistance;R 1:resistance of high frequency semicircle;R 2:resistance of middle frequency semicircle;R 3:resistance of phase transformation;Q 1:capacitance of high frequency semicircle;Q 2:capacitance of middle frequency semicircle;Q 4:capacitance of phase transformation;Q 3:impedance ofdiffusion图7LiTi 2(PO 4)3/C 电极首次放电过程中EIS 的模拟结果Fig.7Simulating results of EIS for LiTi 2(PO 4)3/C electrode in initial dischargeprocess图8LiTi 2(PO 4)3/C 电极在首次放电过程中R 1随电极电位变化的数值分析结果Fig.8Variations of R 1obtained from fitting the experimental impedance spectra of LiTi 2(PO 4)3/C electrode during the first dischargeprocessActa Phys.-Chim.Sin.2012Vol.28图10为LiTi 2(PO 4)3/C 在首次放电过程中电荷传递电阻R 2随电极电位变化的关系.可以看出,R 2值随着电极电位的变化先减小后增大.如果假设不存在嵌入电极的锂离子之间和锂离子与嵌锂空位之间的相互作用,电荷传递电阻R ct 与嵌锂度x 满足如下关系:23R ct =1fFAk s x 0.5(1-x )0.5(3)其中f ＝F /RT (F 为法拉第常数,R 为气体常数,T 为热力学温度),k s 为标准速率交换常数,A 为电极表面积.可以得出:当x →0或1时,R ct 快速增大,当x →0.5时,R ct 减小,即在放电的过程中,R ct 随电极电位的关系表现为先减小后增大.这与R 2随电极电位变化规律一致,证实中频区的半圆是与电荷传递过程相关的半圆.在我们前期研究石墨电极的电化学阻抗谱中,得出当嵌锂度很小(x →0)时,ln R ct 和E 满足公式:24ln R ct =ln RT n 2e F 2c max k 0(M Li +)(1-α)-αF (E -E 0)RT (4)其中n e 是反应过程中电子的转移数目,α为电化学反应的对称因子,E 和E 0分别代表电极的实际和标准电位,c max 为电极的最大嵌锂度,k 0为标准反应速率常数,M Li +为电极表面溶液中的锂离子浓度.从等式(4)可以得出,当x →0时,ln R ct 和电极极化电位呈线性变化关系.从图11可以看出,LiTi 2(PO 4)3/C 电极首次放电过程中ln R 2和电极电位E 成线性关系,这和式(4)是相吻合的,进一步说明中频区的半圆是与电荷传递相关的半圆.图12为LiTi 2(PO 4)3/C 电极在首次放电过程中相变电阻R 3随电极极化电位变化的关系.从图中可以看出,R 3的值随电极电位的降低逐渐增大,这是因为图10LiTi 2(PO 4)3/C 电极在首次放电过程中R 2随电极电位变化的数值分析结果Fig.10Variations of R 2obtained from fitting the experimental impedance spectra of LiTi 2(PO 4)3/C electrode during the first dischargeprocess 11LiTi 2(PO 4)3/C 电极在首次放电过程中ln R 2随电极电位的变化Fig.11Variations of the logarithm of R 2of LiTi 2(PO 4)3/Celectrode during the first dischargeprocess图12LiTi 2(PO 4)3/C 电极在首次放电过程中R 3随电极电位变化的数值分析结果Fig.12Variations of R 3obtained from fitting the experimental impedance spectra of LiTi 2(PO 4)3/C electrodeduring the first dischargeprocess9Li 1+x Ti 2(PO 4)3电极首次放电过程中嵌锂度x 随电位的变化Fig.9Potential in Li 1+x Ti 2(PO 4)3electrode as a function ofthe stoichiometry x in the first dischargeprocess袁铮等:LiTi 2(PO 4)3/C 复合材料的制备及电化学性能No.5随着电极电位的不断降低,LiTi 2(PO 4)3相转变为Li 3Ti 2(PO 4)3相的过程变得困难,当电极电位降到1.50V 时,相变电阻R 3值比在2.30V 时大了一个数量级,这与Barsoukov 等19采用电化学阻抗谱研究LiCoO 2相变过程的结果相似.此外,较大程度的相变会减少活性材料颗粒与颗粒之间及颗粒与整体电极之间的电接触,致使电极循环性能下降.3.4扩散系数的测定图13为LiTi 2(PO 4)3/C 电极在不同扫描速率下的循环伏安曲线.随着扫描速率的上升,还原峰向低电位转移,氧化峰向高电位转移,其间距ΔV 增大,这是因为LiTi 2(PO 4)3的电导率太低,在扫描速率较快时,发生较大极化造成的.同时,峰值电流(I p )随着扫描速率的增加而增加.峰值电流(I p )和扫描速率的平方根存在一个线性关系,如图14所示,这是典型的扩散控制过程,说明LiTi 2(PO 4)3中的两相转变动力学可以近似地认为是一个扩散过程.化学扩散系数可以通过Randles-Sevcik 方程25计算出:I p =0.4463n 32F 32CAR -1/2T -1/2D 12CV v12(5)其中,n 是反应过程中转移的电子数量,C 是反应物体相浓度,D CV 是由CV 确定出的化学扩散系数,v 是扫描速率.扩散系数可以通过式(5)计算得出.(5)式可以化简为:I p =2.72×105n 32CAD 12CV v12(6)计算出LiTi 2(PO 4)3的嵌锂/脱锂的扩散系数分别为2.40×10-5和1.07×10-5cm 2·s -1,这与文献11报道的一致.LiTi 2(PO 4)3的扩散系数远大于其他嵌锂材料,这是因为NASICON 结构中的传输通道和间隙对锂离子的扩散起到促进作用.4结论采用聚乙烯醇(PV A)辅助溶胶-凝胶法代替传统的高温固相法制备了LiTi 2(PO 4)3/C 复合材料,并运用XRD 、SEM 、充放电测试、循环伏安、电化学阻抗谱对其结构形貌和电化学性能进行了表征.XRD 研究结果表明,所制备的LiTi 2(PO 4)3/C 复合材料具有良好的NASICON 结构晶型.充放电结果显示,其首次放电容量为144mAh ·g -1.采用EIS 对LiTi 2(PO 4)3/C 复合材料首次嵌锂过程进行研究,发现当锂离子大量嵌入时,颗粒内应力增大,体积膨胀,体积的膨胀会造成颗粒的破碎和相变的产生,这对其电化学性能具有重要影响.并计算出Li +在LiTi 2(PO 4)3中嵌入/脱出时的扩散系数,分别为2.40×10-5和1.07×10-5cm 2·s -1.References(1)Wakihara,M.Mater.Sci.Eng .2001,33,109.(2)Padhi,A.K.;Nanjundaswamy,K.S.;Goodenough,J.B.J .Electrochem .Soc.1997,144(4),1188.(3)Padhi,A.K.;Nanjundaswamy,K.S.;Masquelier,C.;Okada,S.;Goodenough,J.B.J 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第42卷第2期2021年4月Vol.42No.2Apr.2021 Journal of C eramicsDOI:ki.tcxb.2021.02.005NaBr界面修饰S11O2基钙钛矿太阳能电池的研究骆鹏辉，江和栋，李家科，范学运，郭平春，黄丽群，孙健，朱华，王艳香(景德镇陶瓷大学，江西景德镇333403)摘要：采用一步法制备平面结构的钙钛矿太阳能电池(Perovskite Solar Cells,PSCs)。

采用旋涂法在SnO2电子传输层(Electron Transport Layers,ETLs)和钙钛矿层之间插入漠化钠(N^r)界面修饰层，主要研究了NaBr溶液的浓度对PSCs的影响，并探索了NaBr的对电池性能的影响机理。

通过XRD、SEM、ATM、XPS、PL、UV-Vis及J-V等对样品的形貌、结构、吸光度及光电性能等参数进行系统研究。

结果表明：NaBr能够增强钙钛矿的结晶性能和光吸收，增强SnO?ETLs 和钙钛矿层之间的界面结合，有效提升电池效率。

当NaBr浓度为0.2mol/L时，器件的光电性能最佳，其光电转换效率(Photoelectric Conversion Efficiency,PCE)为16.21%,开路电压(Open-circuit Voltage,Voc)为 1.07V,短路电流密度(Short-circuit Current Density,Jsc)为20.22mA/cm2,填充因子(Fill Factor,FF)为75.13%。

关键词：钙钛矿太阳能电池；SnO2电子传输层；NaBr；界面修饰中图分类号：TQ174.75文献标志码：A文章编号：1000-2278(2021)02-0271-08 NaBr Interface Modification on the Performances of SnO2-basedPerovskite Solar CellsLUO Penghui,JIANG Hedong,LI J iake,FANXueyun,GUO Pingchun,HUANG Liqun,SUNJian,ZHUHua,WANG Yanxiang(Jingdezhen Ceramic Institute,Jingdezhen333403,Jiangxi,China)Abstract:In this study,a one-step method was used to prepare perovskite solar cells(PSCs)with planar structure. Sodium bromide(NaBr)was inserted in between the SnO2electron transport layers(ETLs)and the perovskite layer as the interface modification layer by using spin coating.The effect of concentration of the NaBr solution on performance of the PSCs was studied,while the effect mechanism of NaBr was explored.With XRD,SEM,AFM,XPS,PL,UV-Vis and J-V,morphology, structure,absorbance and photoelectric properties of the samples were systematically studied.It is found that the modification of NaBr can improve the crystallization behavior and light absorption of the perovskite,enhance the interface bonding between the SnO2ETLs and the perovskite layers and effectively improve the efficiency of PSCs.When the concentration of NaBr was0.2 mol/L,the photoelectric performance of the device was optimized,with photoelectric conversion efficiency(PCE)of16.21%, open-circuit voltage(V O c)of1.07V,short-circuit current density(J sc)of20.22mA/cm2and fill factor(FF)of75.13%.Key words:perovskite solar cells;electron transport layer;NaBr;interface modification0引言近年来，有机-无机杂化钙钛矿太阳能电池(Perovskite Solar cells,PSCs)以其高效率和低成本的制备工艺引起了广泛的关注，其光电转换效率(Photoelectric Conversion Efficiency,PCE)从2009年的3.8%[1】提升到25.5%[2]o在PSCs的结构中，电子传输层(Electron Transport Layer,ETLs)是最重要的组成部分之一，其作用是与钙钛矿吸收层形成电子选择性接触，提高光生电子的抽取效率，并阻挡空穴向阴极方向迁移已勺。