电化学超级电容器负极材料Li₄Ti₅O₁₂的研究

【作者】 程亮；

【导师】 夏永姚；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 博士

【摘要】 近年来,伴随着电动汽车市场的蓬勃发展,由超级电容器作为辅助能源的二次电池以及燃料电池体系被认为是最有应用前景的一类车用储能体系。目前有许多研究工作者都致力于改善超级电容器体系的能量密度以及功率密度,并希望在降低体系成本的同时,采用一些环境友好的电极材料。不对称电化学超级电容器体系是较受关注的一种新型储能器件,该体系由活性炭电极以及电池材料电极组成,兼具一定的功率密度和能量密度。其中最有代表性的是2001年被提出的Li₄Ti₅O₁₂/活性炭不对称电容器体系,该体系由于其较高的功率密度,较高的能量密度以及出色的循环性能,受到研究者的关注。在该体系中,由于采用的Li₄Ti₅O₁₂材料嵌入电位较低,且容量较大,因此该不对称体系的能量密度是一般双电层电容器的数倍。但同时,该体系的功率密度也受到Li₄Ti₅O₁₂嵌入电极的制约。改善Li₄Ti₅O₁₂材料倍率特性的方法主要有如下两点:1.制备Li₄Ti₅O₁₂纳米材料,以缩短锂离子扩散的路径,从而达到提高材料倍率特性的目的;2.提高Li₄Ti₅O₁₂材料的电导率,减小电极在充放电过程中的极化。因此,本文的研究内容主要集中在改进材料的制备方法以制备纳米颗粒;通过一定的改性修饰方法提高材料的电导率;同时将此类高导电性的Li₄Ti₅O₁₂纳米材料运用到组装不对称电化学电容器中。1.CVD炭包覆方法提高Li₄Ti₅O₁₂材料电导率在这部分工作中,我们将常用的化学气相沉积技术应用于Li₄Ti₅O₁₂材料的改性。以甲苯蒸气作为炭源,N₂作为载气,在本体Li₄Ti₅O₁₂材料表面包覆一层具有一定石墨化程度的导电炭,使得整个材料颗粒表面获得电子的活性点位大大增加,从而在降低复合电极中导电炭使用量的同时,电极同样可以满足大电流条件下工作的要求。通过考测包覆温度对复合材料导电性的影响,我们发现在800℃下包覆处理2 hr得到的Li₄Ti₅O₁₂/C复合材料,其电导率可以上升至2.05 S/cm,大大优于本体材料的电导率。通过TEM表征发现,该包覆炭材料的厚度在5 nm左右,且表面有较多缺陷,并不妨碍锂离子在其间的迁移,且炭层与本体Li₄Ti₅O₁₂之间紧密结合、结构稳定,不易从本体剥离,使得该特性与材料本身的长循环寿命得以匹配。通过对材料在充放电过程中的阻抗分析可以发现,该导电炭层不但可以降低整个电极的内阻,并且降低了电极在充放电过程中的传荷阻抗。将该材料和传统Li₄Ti₅O₁₂材料分别组装不对称电容器进行的倍率测试表明,在使用同样导电剂用量（5%）的实验条件下,该材料的倍率特性更加出色,在24 C倍率下仍能保有49%的初始容量,而此时对比样仅能保有29%的初始容量。2.采用熔融盐方法制备Li₄Ti₅O₁₂纳米颗粒。本工作创新性地将熔融盐引入到合成工艺之中,借助于低温熔融的LiCl为熔盐,在反应过程中提供一个液相反应环境,使得TiO₂原料和反应锂盐得以充分接触,促进了反应的进行,缩短反应所需时间。而且在整个反应过程中LiCl熔盐为反应惰性,并不参与化学嵌锂反应,因此可以确保最终产物为化学计量比的Li₄Ti₅O₁₂。经过一系列比对实验,我们发现,随着LiCl熔盐比例（LiCl/TiO₂）的升高,最终产物的粒度随之下降,当LiCl比例为16:1时,最终产物的粒度可以控制在100 nm左右。我们在最优化合成条件（LiCl/TiO₂=16/1,煅烧温度750℃,处理时间1 hr）下得到的Li₄Ti₅O₁₂材料,其颗粒在100 nm左右,容量约为159 mAh/g。将此材料与活性炭配对组装成不对称电容器进行测试,其倍率性能远远优于应用传统方法合成的大颗粒Li₄Ti₅O₁₂所组装的电容器体系,其在130 C大电流倍率下放电,仍保有50%左右的初始容量,而此以大颗粒Li₄Ti₅O₁₂为负极的电容器,在50 C时的容量维持率已经低于50%。3.炭包覆纳米Li₄Ti₅O₁₂材料。我们率先提出此改良的固相合成方法。具体过程如下:1.通过炭包覆方法预处理TiO₂原料,使其表面均匀包覆一层导电炭;2.将该材料混合以化学计量比的锂盐,在惰性气氛下高温固相煅烧制备得到样品。该方法的优点有如下几点:1.由于炭层在惰性气氛下的稳定性,将反应原料颗粒互相隔开,避免材料由于高温处理而烧结导致颗粒增大。最终得到的产物,其粒度在几十纳米左右;2.由于炭层的高导电性,最终得到的Li₄Zi₅O₁₂材料也具备较高的电子导电性;3.由于该方法的可操作性与简便性,具备一定的量产前景。我们将该材料与传统材料分别组装实验电池,以金属锂为对电极,测试材料的大电流特性,发现该材料在25 C（3.75 A/g）放电时,其容量维持率仍可达70.6%,而此时传统的Li₄Ti₅O₁₂已经无法正常工作。该方法为我们提供了一种全新的合成制备途径,并可以延伸应用于其他一些锂离子嵌入化合物的制备合成过程之中。4.基于改良固相合成方法制备特殊形貌Li₄Ti₅O₁₂我们在此运用改良的固相合成方法,采用不同的TiO₂前驱体,制备得到了各种不同形貌的Li₄Ti₅O₁₂纳米材料,譬如,我们运用聚苯乙烯（PS）小球为模板,合成得到TiO₂空心球,并制备得到了Li₄Ti₅O₁₂纳米空心球;运用TiO₂微米球为前驱体,制备得到了Li₄Ti₅O₁₂微米球,完整的保持了前驱体的形貌特征。以Li₄Ti₅O₁₂纳米棒为例,我们运用水热合成方法制备得到TiO₂棒前驱体,通过气相包覆方法预处理,而后混合计量比的Li₂CO₃,经过800℃煅烧9 hr得到产物。最终产物半径为50-80 nm,都能基本保持前驱体的形貌,且整个材料表面均匀包覆炭层。以制备得到的Li₄Ti₅O₁₂纳米棒作为电极材料,以金属锂作为负极进行电化学倍率测试,该材料表现出优良的倍率特性,在20 C倍率下放电的容量维持率仍可达到79%。5.使用纳米Li₄Ti₅O₁₂为负极的不对称电化学电容器的组装与测试在这部分工作中,我们将自制的炭包覆纳米Li₄Ti₅O₁₂作为负极,与商用活性炭配对组装成不对称电化学电容器。我们同时使用商品Li₄Ti₅O₁₂材料与活性炭组装成电容器,作为比较。组装得到的AAA电容器体系,其能量密度可以达到6 Wh/kg,两倍于目前的活性碳/活性炭双电层电容器（EDLC）。将使用不同Li₄Ti₅O₁₂为负极的不对称电容器进行倍率测试,发现在高倍率测试条件下,采用纳米炭包覆Li₄Ti₅O₁₂的电容器,其倍率特性更为出色,在40 C时的容量维持率仍有62%,而采用商品化材料为电极的电容器,其容量维持率仅有48%。6.FeOOH/活性炭不对称电容器体系的研究在本部分工作中,我们采用铁盐水解的方法,制备了beta-FeOOH化合物,该化合物在工作区间1.5-3.3 V（vs.Li⁺/Li）表现出良好的循环特性,并且其容量可以达到200 mAh/g,比较适合作为电化学不对称电容器的负极。将FeOOH和商用活性炭配对组装的不对称电容器,其工作区间在0-3 V,比容量为30 mAh/g,其实际能量密度可以达到炭/炭双电层电容器的2-3倍。对该体系进行倍率测试,发现其具有较好的倍率特性,在10 C电流下工作,仍有80%的容量维持率,经过800次循环,基本无衰减。更多还原

【Abstract】 Supercapacitors coupled with batteries and fuel cells are considered promising mid-term and long-term solutions for low- and zero-emission transport vehicles by providing the power peaks for start-stop,acceleration and recovering the breaking energy.Nowadays,many researchers on the electrochemical capacitors aim to increase power and energy density as well as lower fabrication costs while using environmental friendly materials.The most useful approach is to develop hybrid systems that typically consist of an electrochemical double-layer capacitor（EDLC） electrode and a battery electrode.In the year 2001,an AC/Li₄Ti₅O₁₂ hybrid supercapacitor was reported.In the hybrid system,both increase of the working voltage and high energy density of the negative result in a significant increase of the overall energy density of the capacitors.But the power density depends on the rate capability of the intercalated compound Li₄Ti₅O₁₂.In order to obtain a high rate capability,two typical approaches have been developed to overcome this problem. One is to develop the nano-sized Li₄Ti₅O₁₂,and the other way is to reduce the electrode polarization by improving its electronic conductivity.My study mainly focused on the preparation of the nano sized Li₄Ti₅O₁₂ with high electronic conductivity.The electrochemical performance of the nano sized Li₄Ti₅O₁₂ and the hybrid supercapacitors consisting of Li₄Ti₅O₁₂ and AC were also studied in detailed.1.Carbon coated Li₄Ti₅O₁₂ by chemical vapour decomposition（CVD） method:the graphitized-carbon was coated on the surface of Li₄Ti₅O₁₂ through a CVD process. The electronic conductivity increased as coating temperature increased,but it results in poor electrochemical profile over 800℃.The optimal condition for Li₄Ti₅O₁₂ coating should be performed at 800℃.The graphitized carbon layer of Li₄Ti₅O₁₂/C obtained at optimal condition is about 5 nm thickness.It shows a much higher electronic conductivity of 2.05 S/cm than the raw one’s(＜10^-13 S cm^-1).In the rate capability test,the supercapacitor containing a carbon-coated Li₄Ti₅O₁₂ negative and activated carbon positive electrode keeps 50%of initial capacity at the rate of 24C （0.6 A/g） compared with 29%of the raw one’s.AC impedance tests reveal that the coated carbon layer can decrease the interracial charge-transfer resistance at some extent.The results demonstrated that the thermal vapor decomposition is a promising approach to improve the electronic conductivity of the Li₄Ti₅O₁₂. 2.Nano-sized Li₄Ti₅O₁₂ prepared by molten salt method:a nano-sized lithium intercalated compound Li₄Ti₅O₁₂ was prepared by using LiCl as a high temperature flux.Li₄Ti₅O₁₂ powders were easily obtained with homogeneity,regular morphology, and narrow particle-size distribution using molten LiCl as a high-temperature solvent. The flux produces a liquid/solid reaction interface,thus provides a large effective reaction area,and accelerates the Li₄Ti₅O₁₂ growth at a relatively short time.The particles size decreases and the distribution becomes narrow with the increasing flux content.Under the optimal synthetic condition（750℃for 1 h,N=16）,the average particle size is of 100nm and the sample has a discharge capacity of 159 mAh/g.The hybrid capacitor fabricated with this nano-sized sample and activated carbon show much better rate capability,even at 100 C discharge rate,the hybrid capacitor also keeps 60%of capacity compared with 3C discharge rate.3.Carbon coated Li₄Ti₅O₁₂ nano particles by carbon pre-coating process:In this synthesis process,the TiO₂ precursor is firstly coated with a carbon layer by a CVD method.After mixed with proper ratio of lithium salt,the carbon coated TiO₂ was then annealed at 800℃for 9 hrs.With the presence of carbon layer,the final product Li₄Ti₅O₁₂ is effectively prevented from agglomerating even under a high temperature solid state reaction.The carbon coated Li₄Ti₅O₁₂ nano particles has a partile size about 20-50 nm and has a capacity about 150 mAh/g at a current rate of 0.1 C（0.015 A/g）.The rate performance of the carbon-coated nano-sized Li₄Ti₅O₁₂ particles is outstanding than the Li₄Ti₅O₁₂ samples with bigger particles.When the rate is 25C （3.75 A/g）,the capacity can still retain about 110 mAh/g（70.6%）.The particle-size has a direct influence on the electrochemical rate performance of the cell.Smaller particles promote shorter pathways for solid-state diffusion of Li ions and result in better rate capability.And also the carbon layer can reduce electrode polarization by improving the electronic conductivity of Li₄Ti₅O₁₂.4.Li₄Ti₅O₁₂ materials with specialized morphologies obtained by the carbon pre-coating process:The carbon-coated nanostructure Li₄Ti₅O₁₂ products with various morphologies were achieved by a carbon pre-coating method from different TiO₂ precursors,for example Li₄Ti₅O₁₂ nano-rod,hollow sphere,nanoparticle,and microsphere.The pre-coated carbon layer can prevent the TiO₂ precursor particles from aggregation when reacted with lithium salt to form nanostructure Li₄Ti₅O₁₂,and keep the morphology of precursor.For instance,the Li₄Ti₅O₁₂ nano-rod has the diameter about 50-80 nm with a carbon layer wrapped around its surface.As a result of the specialized morphology and electronic conductive caobon layer,the Li₄Ti₅O₁₂ nano-rod can retain about 79%capacity even at the rate of 20 C.Moreover,the coated carbon layer can enhance the electronic conductivity of Li₄Ti₅O₁₂.As a result,the nanostructure Li₄Ti₅O₁₂ obtained by the technology described in the present work, shows high rate capability and high stability for lithium-ion intercalation,which is fundamental for materials of high power electrode.5.Assembly of Li₄Ti₅O₁₂/AC nonaqueous hybrid supercapacitors:By using the carbon coated Li₄Ti₅O₁₂ nano particles prepared by a carbon pre-coating process as the negative electrode and AC as the positive material,the AAA type supercapacitor was fabricated.The Li₄Ti₅O₁₂/AC hybrid supercapacitor has an energy density about 6 Wh/kg which is about 2 times higher than that of the commercialized EDLC.And also,the relationship between Li₄Ti₅O₁₂ characters and supercapacitor rate performance was investigated.A commercialized Li₄Ti₅O₁₂（200 nm） and carbon coated Li₄Ti₅O₁₂ nano particles have been both used to fabricate the Li₄Ti₅O₁₂/AC hybrid supercapacitor prototype for comparison.The capacitor using carbon coated Li₄Ti₅O₁₂ nano particles as negative showed much better rate performance.Even under the rate of 40 C,it can retain about 62%capacity comparing with that of commercialized material 48%.6.FeOOH used as negative material in hybrid supercapacitor:A nano-structural iron oxyhydroxide（FeOOH） was obtained via a hydrolyzing route under mild and facile synthesis condition.FeOOH delivered a capacity of 170 mAh/g in the voltage range from 1.6 to 3.3 V.The nanostructural FeOOH was used as a negative electrode for an asymmetric hybrid electrochemical supercapacitor combined with an activated carbon negative electrode in 1.0 M LiPF₆ ethylene carbonate/dimethyl carbonate（1:2 in volume） solution.The FeOOH/AC cell shows a capacity of 30 mAh/g base on the overall active materials,corresponding to the energy density of 67.5 Wh/kg,two time of DELC.I This system also shows a good performance of cycle life and rate test.It remains approximately 100%capacity after 1000 cycles.Even at 10C discharge rate, the capacitor also holds 80%of capacity at 1C discharge rate.更多还原