Li_4Ti_5O_（12）电极材料的二步煅烧固相反应法合成及炭包覆改

1、Li_4Ti_5O_（12）电极材料的二步煅烧固相反应法合成及炭包覆改        【中文摘要】锂离子电池具有高能量密度、高工作电压、无记忆效应等优点,有看成为电动汽车的主要动力来源之一。尖晶石结构的钛酸锂(Li_4Ti_5O_(12)是一种“零应变”材料,理论嵌锂电位为1.55V(vs.Li+/Li),理论比容量为175mAh·g(-1),充放电过程中体积结构几乎不发生变化,具有充放电性能好、循环性能优良、充放电电压平台稳定等优点。同时钛资源丰富、价格低廉。这些优点使其成为锂离子电池极具发展远景的电极

2、材料,有着巨大的研究价值和贸易应用价值。然而,Li_4Ti_5O_(12)材料的本征电子导电能力(电导率10(-13)S·cm(-1)和离子导电能力(锂离子扩散系数约为2×10(-8)cm2·s(-1)不高,因此Li_4Ti_5O_(12)在大电流充放电时容量衰减快、倍任性能较差,限制了其作为高倍率负极材料在动力锂离子电池中的应用。本文从有利于推进Li_4Ti_5O_(12)实际应用出发,针对钛酸锂材料电子导电率低,传统固相法烧结能耗高、纯度低的题目,以TiO_2和Li_2CO_3为原料采用二步煅烧固相反应法合成了纯相Li_4Ti_5O_(12),并以糖类为炭源导

3、电先驱体对Li_4Ti_5O_(12)进行炭包覆改性,获得了Li_4Ti_5O_(12)/C复合材料,改善了材料的导电性能,进步了材料的大电流充放电倍任性能,不仅丰富了Li_4Ti_5O_(12)材料的研究内涵,而且促进了Li_4Ti_5O_(12)材料在锂离子动力电池中的实际的应用。其中获得的主要研究成果与结论如下:(1)研究二步煅烧固相反应法合成Li_4Ti_5O_(12)的煅烧制度。结果表明,以Li_2CO_3与TiO_2为合成原料,当锂过量3%时,原料经650二步煅烧8h再于800保温6h的二步煅烧制度能得到纯相的尖晶石结构Li_4Ti_5O_(12),首次放电比容量为168.7mAh

4、·g(-1)，0.5C充放电循环首次放电比容量为120.6mAh·g(-1)，1C循环首次放电比容量为80.6mAh·g(-1),相对于一步煅烧的含杂质相产物,倍任性能明显进步。(2)以葡萄糖为炭源对二步固相法合成的Li_4Ti_5O_(12)产物进行炭改性研究。研究结果表明,适当地进步炭含量与延长炭化时间都有利于倍任性能的改善。Li_4Ti_5O_(12)/C复合材料目标炭含量为2%在Li_4Ti_5O_(12)掺杂葡萄糖,N_2中800炭化4h所得产物电化学性能较优,1C循环容量保持率比未改性样品进步近40%。在确定较优炭含量和热处理时间等改性工艺条件的基础上

5、,研究了葡萄糖、蔗糖、淀粉三种不同炭源对Li_4Ti_5O_(12)/C复合材料电化学性能的影响。分析结果表明,以淀粉为导电先驱体对Li_4Ti_5O_(12)倍任性能的改善效果较优,蔗糖次之,葡萄糖的改性效果相对较差。其中以淀粉为炭源得到的Li_4Ti_5O_(12)/C复合材料首次放电比容量为159mAh·g(-1)，2C循环时首次放电比容量为125mAh·g(-1)。(3)研究中间相球磨炭包覆Li_4Ti_5O_(12)改性。原料经650二步煅烧后得到中间相,加进葡萄糖、蔗糖、淀粉等炭源与中间相球磨混合,然后再经800高温处理,合成Li_4Ti_5O_(12)/C复合

6、材料。XRD衍射结果分析表明Li_4Ti_5O_(12)/C复合材料中Li_4Ti_5O_(12)纯度高,解决了原料中加进碳源合成Li_4Ti_5O_(12)/C复合材料反应不完全杂质相种类多含量高的题目。电化学性能检测显示以淀粉为导电先驱体Li_4Ti_5O_(12)倍任性能改善效果最好,首次放电比容量为162 mAh·g(-1)，1C循环首次放电比容量为146.5 mAh·g(-1)，容量保持率约为未改性Li_4Ti_5O_(12)的2倍,2C循环时放电比容量为136mAh·g(-1)，容量保持85%。');【Abstract】 Lithium ion

7、 battery may be one of the most promising power source of electric vehicle as its high energy density, high potential and nonmemeory effect as well. Li-ion intercalated compound Li_4Ti_5O_(12) has been demonstrated to be one of the most promising electrode materials for such applications, because it

8、 has a flat voltage range, high reversible capacity(175mAh·g(-1), and especially the long cycling performance due to no structural change zero-strain insertion material during charge-discharge cycling, besides of these, titanium is inexpensive and abundantly existing in nature.However, the draw

9、back of its poor electrical conductivity(10(-13) S·cm(-1) and low Li-ion diffuse coefficient (2×10(-13)cm2·s(-1) caused its poor rate capability. Considering of promoting the industry producing of Li_4Ti_5O_(12), it's synthesized by solid reaction, and the usual saccharide was use

10、d to improve its rate capability. The main research results and conclusion are as follows:(1) The calcined system of synthesizing Li_4Ti_5O_(12) was investigated through solid reation at high temperature. The result showed that the pure phase spinel Li_4Ti_5O_(12) can be obtained when two-step calci

11、nation system at low temperature was used. The detailed process was: The raw materials of Li_2CO_3 and TiO_2 with a ratio of Li/Ti was excessive by 8% were pre-calcined at 650for 8h then continued were treated at 800for 6h. The product has a first discharge specific capacity at 168.7mAh·g(-1),

12、and a discharge-capacity of 120.6mAh·g(-1) at 0.5C charge-discharge rate while the first discharge-capacity was 80.6mAh·g(-1) at 1C charge-discharge cycle which has an improved rate capability compared with impurity containing product.(2) The carbon modification using glucose as carbon sou

13、rce was researched. The effect of carbon content and carbonized time on the Li_4Ti_5O_(12)/C composite material structure and electrochemical properties was investigated. The sample with 2% carbon(mass, aim content in Li_4Ti_5O_(12)/C) which was kept at 800for 4 hours in N_2 gas displayed the best e

14、lectrical performance, a 154.3 mAh·g(-1) of initial specific capacity was obtained at 0.1C, while 148. 8 mAh·g(-1) at 0.2C and the specific capacity keep rate improved by 40% compared with the raw Li_4Ti_5O_(12). Based on the result, the effect of the different carbon sources such as gluco

15、se, saccharose and starch on the carbon modification was studied. The result indicated that starch has the best effect to improve Li_4Ti_5O_(12) eletrochemical propeties at high current followed by saccharose. When starch was used as carbon source, the composite material of Li_4Ti_5O_(12)/C has an i

16、nitial discharge rate capacity of 159mAh·g(-1) and even a specific capacity of 125mAh·g(-1) at 2C.(3) The Li_4Ti_5O_(12)/C composite materials was synthesized by intermediate ball-milled method whose carbon sources were glucose, saccharose and starch. XRD results showed that these Li_4Ti_5

17、O_(12)/C composite materials had a high purity of Li_4Ti_5O_(12), which meant that the problem that the products contained many impurities gotten through primary pre-calcined system was been resolved. The results showed that the Li_4Ti_5O_(12)/C composite materials using starch as carbon source has an excellent eletrochemical p