[中文]1.引言
现今的锂离子电池通常包括一个嵌锂碳负极,有机电解液和锂过渡金属氧化物阴极。这种结构提供比镍镉和铅酸结构具有明显的优势,即优越的能量密度,低毒性,但仍存在具有安全问题的碳阳极。一个可能更安全的替代方法是使用固态无机氧化物材料同时作为正负电极
1994年,Thackeray等证明尖晶石Li4Ti5O12可作为一种可充电锂电池的电极材料[1]。它的性能稳定且能循环使用,但是,它作为负极使用时电压比参比电极Li/Li+高1.5V。现在研究的目的是探讨根据取代原理用其他3d过渡金属取代一些Ti4+后,Li4Ti5O12的电化学效应:3M3+ 2Ti4++Li+,M3+=Fe3+,Ni3+,Cr3+.特别地,它是希望对与参比电极Li/Li+的工作电势能降低。Blasse首次报道了Fe的固溶体晶体化学,虽然这些电化学行为并不广为人知。
Fe由于储量丰富及低毒性的特点,比其他过渡金属如Co ,V更可取。一个可能的不足之处就是,包含有尖晶石的Fe3+在性能方面有点反常,也就是说一些Fe3+离子占据四面体8a位置以及八面体16d的位置。人们普遍认为,在尖晶石中过渡金属和锂离子共享四面体的位置,抑制锂离子在嵌入的过程中在晶格中的扩散并导致反复充放电循环的性能下降。镍和铬作为可选的参杂剂,其与Ti4+相似的离子半径也使得他们在八面体中配位更协调。尖晶石LiCrTiO4已经为人们所知。这样的实验结果来自于对其电化学及结构的研究,其中包括中子衍射,三种固溶体的形式都是显示出来了。
2.实验
起始试剂Li2CO3, Fe2O3, NiO, Cr2O3 and TiO2 (all Aldrich, >99%)须在实验前干燥好以备使用,然后称量,和乙醇混合,并且把混合物在室温下研磨30分钟,使其充分混合。这种球形研磨可以缩短反应时间,降低烧结温度,从而使锂的损耗降到最低。然后将粉末在600-700℃的温度下干燥并烧结几个小时,重新研磨,再把粉末置于900-1000℃的温度下烧结1-2小时使其充分反应。最后吧样品再次球形研磨30分钟。
试验中,使用基于Cu Kα辐射制造的飞利浦PW1830X射线衍射仪来确定相。实验数据是在角度在10°≤2θ≤70°的范围内测定的。由于晶格参数的原因,Si的参数被作为了标准,数据是在2θ在90°范围内几个小时测定的。
粉末种子衍射数据是用位于英国,牛津郡抵得渴特美国钢铁协会的阿普尔顿卢瑟福实验室中的高分辨率粉末衍射仪得到的。实验在一个充满标准钒的容器中进行,持续大概4小时(800 μA h)。飞行时间在2000- 19550 μs.范围内测定。用Rietveld方法的GSAS程序使结构精细、精纯。数据中大于11000μs的要冲最终的提纯数据中剔除,因为它们包含了最强的反射信息,会影响最终结果。
在电化学测试中,复合电极是在一个易变的溶剂中加入75 wt.%悬浮活性物质,10 wt.%活性极好的活性碳(提高电子电导),和15 wt.%聚乙烯(亚乙烯基氟化物)粘合剂[化学试剂公司],然后把悬浮液置于铜箔上,在真空中75℃干燥一夜。脉冲横流测量方法是用两个电极单元和一个电脑辅助识别电池测试系统来进行测试的。锂被用来制相反电极,含1molal 锂 PF6[森田]的EC:DMC(1:2) [Grant Chemicals]做电解液。电势一般在0.00–2.00 V之间,电流密度是0.1mA •cm−2,大约C/10.。
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[外文]1. Introduction
Present day lithium-ion cells typically comprise a lithiated carbon anode, organic liquid electrolyte and a lithium transition metal oxide cathode. These cells offer obvious advantages over nickel-cadmium and lead-acid cells i.e. superior energy density and low toxicity; however, there are still safety concerns regarding the carbon anode. A possibly safer alternative is to use solid state inorganic oxides as both the negative and positive electrode materials.
In 1994, Thackeray et al. demonstrated Li4Ti5O12 spinel could be used as an electrode material in rechargeable lithium batteries [1]. The capacity is very stable with cycling; however, the voltage is rather high for utilisation as the negative electrode at more than 1.5 V vs. Li/Li+. The aim of the present study was to investigate the effect on the electrochemistry of Li4Ti5O12 (≡Li1.33Ti1.67O4) of replacing some of the Ti4+ by other 3d transition metals according to the substitution mechanism: 3M3+ 2Ti4++Li+ where M3+=Fe3+, Ni3+, Cr3+. In particular, it was hoped that the working potential of 1.55 V vs. Li/Li+ could be lowered. Blasse first reported the crystal chemistry of the Fe solid solution ,although the electrochemical behaviour is not well known .Fe is preferable over other transition metals such as Co and V due to its abundance and low toxicity. One possible disadvantage is the tendency for Fe3+-containing spinels to be slightly inverse in character, i.e., some of the Fe3+ ions occupy the tetrahedral 8a sites as well as the 16d octahedral sites. It is generally believed that transition metals sharing the tetrahedral sites with the lithium ions in spinel oxides inhibit the lithium ions’ diffusion through the lattice during intercalation and cause a decline in performance with repeated charge–discharge cycling. Ni and Cr were chosen as alternative dopants for this work given their similar ionic radii to Ti4+ and preference for octahedral coordination. The spinel LiCrTiO4 is already known .
Results obtained from electrochemical and structural studies, including neutron diffraction, on all three solid solutions are presented.
2. Experimental
The starting reagents, Li2CO3, Fe2O3, NiO, Cr2O3 and TiO2 (all Aldrich, >99%) were dried overnight prior to use then weighed, mixed with ethanol, and ball-milled at room temperature for 30 min to ensure intimate mixing. Ball milling was also found to shorten reaction times and reduce firing temperatures, thereby minimising the risk of lithia loss. The powders were then dried and heated at 600–700°C for several hours and reground before firing at 900–1000°C for 1–2 h to complete reaction. Finally, samples were ball-milled again for a further 30 min.
For phase identification, a Philips PW1830 X-ray diffractometer with Cu Kα radiation was used. Data were collected over the range 10°≤2θ≤70°. For lattice parameter determination, Si was used as an internal standard and data were collected up to 90° 2θ over several hours.
Powder neutron diffraction data were collected using the high resolution powder diffractometer (HRPD) situated at the ISIS neutron facility, Rutherford Appleton Laboratories, Didcot, Oxon, UK. Experiments were run at room temperature in a standard vanadium can for about 4h (800 μA h). Time-of-flight data were collected between 2000 and 19550 μs. Structural refinements were performed by the Rietveld method using the GSAS programme. Data greater than 11000 μs were excluded from the final refinement as they contained information from the most intense reflection and were found to falsely skew the result.
For electrochemical testing, composite electrodes were prepared by suspending 75 wt.% active material, 10 wt.% super S carbon (to enhance electronic conductivity) and 15 wt.% poly(vinylidene fluoride) binder [Aldrich] in a fugitive solvent, such as cyclopentanone. The slurry was cast onto Cu foil and vacuum dried overnight at 75°C. Galvanostatic measurements were made using two electrode cells and a Macpile computerised battery test system. Li metal was used as the counter electrode and 1 molal Li PF6 [Morita] in EC:DMC (1:2) [Grant Chemicals] as the electrolyte. The potential range was generally 0.00–2.00 V and the current density 0.1 mA cm−2, equivalent to about C/10.
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