微波辅助固相法合成锂离子电池正极复合材料Li2FeSiO4/C

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曹雁冰, 段建国, 胡国荣, 姜锋, 彭忠东, 杜柯. 微波辅助固相法合成锂离子电池正极复合材料Li₂FeSiO₄/C . 无机材料学报, 2012, 27(10): 1023-1029
CAO Yan-Bing, DUAN Jian-Guo, HU Guo-Rong, JIANG Feng, PENG Zhong-Dong, DU Ke. Synthesis of Li₂FeSiO₄/C Composite as Cathode Materials for Lithium Ion Battery by Microwave Assisted Solid Reaction Method . Journal of Inorganic Materials, 2012, 27(10): 1023-1029 复制到剪切板

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微波辅助固相法合成锂离子电池正极复合材料Li₂FeSiO₄/C

曹雁冰^1,², 段建国¹, 胡国荣¹, 姜锋², 彭忠东¹, 杜柯¹

1. 中南大学冶金科学与工程学院, 长沙 410083

2. 中南大学材料科学与工程学院, 长沙 410083

曹雁冰(1982-), 男, 博士, 讲师. E-mail:cybcsu@csu.edu.cn

基金:中央高校基本科研业务费项目(2010Q22D0101, 2012QNZT018); 国家科技支撑计划项目(2007BAE12B01)

摘要

以Na₂SiO₃·9H₂O和FeCl₂·4H₂O为原料, 采用低热固相反应获得了分散均匀的β-FeOOH/SiO₂前驱体; 再以Li₂CO₃为锂源、聚乙烯醇和超导电炭黑为复合碳源, 通过微波辅助固相法合成了Li₂FeSiO₄/C材料. 通过X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)和恒电流充放电测试等方法对材料的结构、微观形貌及电化学性能进行表征. 650℃下微波处理12 min可获得结晶好、晶粒细小均匀的Li₂FeSiO₄/C材料; 在选用的微波合成体系下, 超导碳和聚乙烯醇热分解的无定形碳不仅利于合成反应的顺利进行, 而且提高了Li₂FeSiO₄的整体导电性能. 制备的复合正极材料在60 ℃下0.05C倍率首次放电容量为129.6 mAh/g, 0.5C倍率下为107.5 mAh/g, 0.5C下15次循环后保持为104.8 mAh/g, 具有较好的放电比容量和良好的循环稳定性能. 结果表明, 微波辅助固相合成工艺是制备Li₂FeSiO₄/C复合材料的一种很有前景的方法.

关键词: 正极材料; Li₂FeSiO₄; 微波辅助固相合成; 碳包覆

中图分类号:TM912 文献标志码:A 文章编号:1000-324X(2012)10-1023-07

Synthesis of Li₂FeSiO₄/C Composite as Cathode Materials for Lithium Ion Battery by Microwave Assisted Solid Reaction Method

CAO Yan-Bing^1,², DUAN Jian-Guo¹, HU Guo-Rong¹, JIANG Feng², PENG Zhong-Dong¹, DU Ke¹

1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

2. School of Materials Science and Engineering, Central South University, Changsha 410083, China

Fund:Fundamental Research Funds for the Central Universities of China (2010Q22D0101, 2012QNZT018); National Key Technology Research and Development Program of China (2007BAE12B01)

Abstract

The homogeneous distribution of FeOOH/SiO₂ precursor material was prepared from FeCl₂·4H₂O and Na₂SiO₃·9H₂O by low-heating solid-state reaction. Then Li₂FeSiO₄/C composites were prepared by microwave assisted solid reaction method, with prepared precursor FeOOH/SiO₂ and Li₂CO₃as the starting materials, PVA and super-P carbon as carbon sources. The prepared samples were characterized by X-ray diffractometry, SEM, TEM, and galvanostatic charge-discharge test. Highly pure Li₂FeSiO₄/C material with uniform and fine particle size was obtained at 650℃ in 12 min. Under the selective microwave synthesis system, super-P carbon powder and pyrolyzed amorphous carbon not only effectively provided the high temperature to induce the complete reaction but also formed the conductive network to enhance the electronic conductivity. The optimum Li₂FeSiO₄/C composite displayed discharge capacity of 129.6 mAh/g at 0.05C rate and 107.5 mAh/g at 0.5C rate at 60℃. After 15 cycles the discharge capacity maintained 104.8 mAh/g at 0.5C, indicating good electrochemical capacity and cycling stability. Consequently, the results show that microwave assisted solid synthesis process is a promising method for preparing Li₂FeSiO₄/C composite.

Keyword: cathode materials; Li₂FeSiO₄; microwave assisted solid synthesis; carbon coating

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从资源、环保和安全性能方面出发, 开发成本低廉、环境友好的新型锂离子电池正极材料一直是人们研究的重点. 自从Goodenough 实验室报道LiFePO₄以来^{[ 1]}, 铁系聚阴离子型化合物作为有潜力的锂离子电池正极材料受到了极大的关注. 而硅酸盐系列材料因具有矿藏丰富和环境相容性好等特点也颇具吸引力^{[ 2, 3, 4, 5]}. Li₂FeSiO₄具有与LiFePO₄相似的化学结构和化学稳定性能. 由于具有很强的Si-O键结合力, Li₂FeSiO₄形成了稳定的晶体框架结构, 可使锂离子的脱出和嵌入电位保持稳定, 在锂离子嵌脱过程中正极材料结构能较好地保持, 具有高的安全性能^{[ 6, 7, 8, 9]}, 成为很有发展前景的锂离子电池正极材料.

但是铁系材料的本身结构决定了其电导率低, 电化学活性差, 限制了其实际应用. 室温下Li₂FeSiO₄的电子电导率远低于其它正极材料, 大约为6×10^-14S/cm, 而且在充放电过程中, Li⁺在Li₂FeSiO₄和LiFeSiO₄两相中的扩散系数很小, 导致材料的可逆容量较低和倍率性能差, 特别是室温下的电化学性能更差^{[ 7, 9, 10]}, 即使以小电流充放电其放电比容量也很难达到理论比容量. 因此对于Li₂FeSiO₄的制备与改性来说提高电子-离子的导电率十分关键. 合成颗粒尺寸细小的材料并与导电相材料(如碳)良好复合, 形成有效的导电网络可以提高电子-离子的导电率. 目前报道材料大多含有杂相, 而且合成工艺都需要很长的处理时间. 在改善Li₂FeSiO₄材料电导率方面, 大多是通过基于有机盐体系的溶胶-凝胶法^{[ 11, 12, 13]}来控制颗粒尺寸大小以制备较高纯度和较好循环性能的硅酸盐复合正极材料, 但其合成过程较复杂、流程时间较长且成本偏高. 因此寻找工艺简单、节能高效的新合成方法对硅酸盐正极材料的研究具有重要意义. 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[ 14, 15, 16, 17, 18, 19]}.

本工作首先通过低热固相法^{[ 16, 20]}获得均匀细小的前驱体, 再采用低温预烧-微波合成法制备Li₂FeSiO₄/C正极材料. 在合成过程中加入超导电炭黑(super-P carbon, SP), 一方面超导电炭黑具有较好的吸波能力, 可提高材料反应体系的整体热导率和吸波能力; 另一方面添加导电能力较强的超导电炭黑分散在Li₂FeSiO₄颗粒中间, 能更好地提高Li₂FeSiO₄/C整体电导率. 本工作选择聚乙烯醇和超导电炭黑为复合碳源以期控制产物尺寸, 使得到的Li₂FeSiO₄/C材料形成有效的碳导电网络, 提高正极材料电化学性能.

1 实验部分

以预先磨细的Na₂SiO₃·9H₂O和FeCl₂·4H₂O为原料, 在无水乙醇溶剂中研磨30 min后用去离子水和无水乙醇反复洗涤数次, 过滤, 烘干, 得橙黄色的前驱体样品. 将获得的前驱体与化学计量的碳酸锂和适量的聚乙烯醇粉末(PVA)充分混匀. 首先在450℃预烧6 h使有机碳源分解和水合氧化物脱水, 然后与2wt%的超导电炭黑混匀后压片, 再采用氩气保护进行微波固相反应, 选择热处理温度为650℃, 在不同辐射时间下合成Li₂FeSiO₄/C材料. 微波合成设备为长沙隆泰科技公司设计, 可精确地进行温度控制, 主频率为2.45 GHz.

采用Shimadzu-FTIR-8400红外光谱仪测试低热固相法合成的前驱体结构. 使用的X射线粉末衍射仪为日本理学Rigaku D/max 2550VB⁺ 18 kW转靶X射线衍射仪: CuKα辐射, 40 kV, 100 mA, 扫描速度0.5°/min, 扫描角度2 θ为10~70°, 步长为0.02°. 用日本JEOL公司JSM-6360LV型扫描电子显微镜对样品的形貌进行观察, 采用TecnaiG²20型透射电子显微镜观察微观形貌.

以N-甲基-2-吡咯烷酮(NMP)为溶剂, 按质量比75 : 15 : 10分别称取活性物质、乙炔黑和PVDF, 混合均匀后, 涂在预处理过的铝箔上, 放入真空干燥箱中在120℃下干燥得到正极片. 在充满氩气的手套箱中, 以金属锂片为负极, 0.8 mol/L双草酸硼酸锂(LiBOB)/碳酸乙烯酯(EC)+碳酸丙烯酯(PC)+碳酸甲乙酯(EMC)(1:1:3体积比)为电解液, Celgard2400多孔聚丙烯膜为隔膜, 组装扣式电池. 用LAND电池测试系统对模型电池进行恒流充放电循环测试, 在60℃环境下, 电压范围为2.0~4.0 V, 如没有特别说明, 电化学测试都在60℃下进行. 在室温25℃下测试时, 电压范围为1.8~4.5 V. 电流密度1 C = 160 mA/g.

2 结果与讨论

对Na₂SiO₃·9H₂O和FeCl₂·4H₂O经过低热固相反应获得的前驱体进行XRD分析, 如图1所示. 对照JCPDS标准数据卡75-1594可知, 获得的产物中含有β-FeOOH, 为晶态的四方相结构. 前驱体样品衍射峰不尖锐, 呈现明显宽化, 而且基线不平滑、背底很强, 说明样品结晶性不好或者颗粒细小. 在谱图中没有发现SiO₂的衍射峰, 生成的SiO₂以无定形存在.

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	图1 低热固相反应制备前驱体的XRD图谱Fig. 1 XRD pattern of the precursor prepared by low-heating solid-state reaction method

图2给出了前驱体的红外光谱图, 如图所示, 在890与700 cm^-1左右均有特征吸收, 与文献[21]报道的β-FeOOH的红外光谱图相一致. 而其中1097.38与475.03 cm^-1的吸收峰是SiO₂的特征吸收, 其中1097.4 cm^-1处的吸收峰对应于Si-O键的伸缩振动, 475.03 cm^-1处的吸收峰对应于Si-O-Si 键的弯曲振动^{[ 22]}, 结合XRD分析结果说明前驱物中的SiO₂以无定形存在.

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	图2 低热固相化学反应制备前驱体的红外光谱Fig. 2 FTIR spectrum of the precursor prepared by low-heating solid-state reaction method

图3为低热固相反应法制备前驱体的SEM照片, 如图所示颗粒细小, 粒子形状规则, 微粒与微粒之间分布均匀, 利于缩短在固相合成过程中原子的扩散路程, 可减少合成均相材料所需的反应时间.

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	图3 低热固相化学反应制备前驱体的SEM照片Fig. 3 SEM image of the precursor prepared by low-heating solid-state reaction method

图4为微波热处理不同时间合成的Li₂FeSiO₄/C样品的XRD图谱, 从图中可以看出, 微波处理8 min已经出现了Li₂FeSiO₄的衍射峰, 但衍射峰明显的宽化, 而且存有杂相峰. 这是由于合成时间短, 反应不充分, 导致晶体结晶度不高. 当加热时间增加到12 min时, 杂质峰已完全消失, 样品的衍射峰尖锐, 结晶良好, 强衍射峰与已报道的图谱十分吻合^{[ 2]}, 属于正交晶系, 空间群为Pmn2₁. 继续延长反应时间, 衍射峰强度增加, 晶型变得更加完整, 但处理时间较长会导致产物颗粒尺寸生长过大. 未发现碳对应的衍射峰, 估计残留的热解碳以无定形存在. 由此可见, 采用微波辅助固相工艺可在较短的时间里得到结晶良好的高纯度Li₂FeSiO₄材料, 与文献报道相比^{[ 11, 23]}, 微波固相法合成的材料XRD中并未出现Li₂SiO₃、Fe₃O₄等杂质相的衍射峰. 这主要由于低热固相法获得了粒径细小、分散均匀的前驱体, 利于缩短固相中离子迁移路径, 减少反应时间; 分散在反应物颗粒之间的超导碳和聚乙烯醇热分解的无定形碳是良好的微波耦合剂, 它们能促进微波与反应原料相互作用, 提高反应体系的热导率^{[ 19, 24]}. 在微波场作用下整个反应体系能够获得快速均匀的加热, 使离子扩散和碳热还原反应同时进行, 从而可在较短的处理时间内制备出高纯度、结晶良好的Li₂FeSiO₄材料.

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	图4 微波热处理不同时间制备的 Li₂FeSiO₄/C材料的XRD 图谱Fig. 4 XRD patterns of Li₂FeSiO₄/C material prepared by microwave assisted low-heating solid-state reaction for different time

图5为微波热处理不同时间合成产物的SEM照片. 随着加热时间的增加, 样品颗粒尺寸有明显增大的趋势. 微波辐射8 min制备的样品颗粒细小, 但颗粒形状较不规则, 颗粒间的分界不明显, 颗粒还未生长完整; 微波辐射延长至12 min时, 样品颗粒尺寸没有发生明显变化, 形态生长完整, 形成规则、细小、分布均匀的颗粒; 处理时间为16 min时, 粒度有所增大, 颗粒生长较不规则, 出现较多团聚颗粒; 时间增加到20 min后, 颗粒长大而且颗粒间团聚严重, 颗粒异常长大将不利于Li⁺在Li₂FeSiO₄粒子中的嵌入和脱出. 通过选择合适的微波处理时间可获得颗粒尺寸较小, 均一性良好的Li₂FeSiO₄/C材料. 微波合成体系中受热体均匀地被加热, 具有很快的加热速率可促进反应快速地进行, 利于目标产物晶核的形成, 在较短的处理时间内促使细小颗粒的形成. 另外合成过程中颗粒间碳的存在能够阻碍晶粒的异常生长, 可获得分布均匀的材料.

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	图5 微波热处理不同时间制备的Li₂FeSiO₄/C材料的SEM照片Fig. 5 SEM images of Li₂FeSiO₄/C material prepared by microwave assisted low-heating solid-state reaction for different time(a) 8 min; (b) 12 min; (c) 16 min; (d) 20 min

图6是微波热处理不同时间合成的Li₂FeSiO₄/C材料0.05 C充放电倍率的首次充放电曲线. 从图6可以看出, 处理时间8 min时, 样品的首次放电比容量为115 mAh/g, 微波热处理12 min合成的样品电压平台较平坦且范围变宽, 具有最好的电化学性能, 首次放电比容量为129.6 mAh/g, 随着加热时间的继续延长容量有所下降, 微波热处理16 min样品首次放电比容量为121.7 mAh/g, 20 min样品电压平台倾斜, 颗粒变大导致的电极材料极化增大, 首次放电比容量降至106.5 mAh/g. 聚阴离子型正硅酸盐材料可逆脱嵌锂的动力学性能受到电子和锂离子扩散过程的控制. 分布均匀、尺寸细小的颗粒可以降低锂离子的固相扩散距离, 增强活性颗粒中的锂离子扩散能力, 提高其可逆比容量. 从样品的XRD图谱和SEM照片可知, 微波热处理8 min合成的材料结晶性能不完善而且有杂相导致放电比容量偏低; 微波热处理16和20 min合成的样品因为颗粒开始团聚, 使电极材料与电解液的有效接触面积减少, 颗粒较大使锂离子扩散路径增长, 可逆比容量下降快^{[ 6, 9]}. 微波热处理12 min合成的样品结晶良好, 颗粒细小均匀, 故其具有较好的可逆比容量和平台稳定性.

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	图6 不同时间制备的Li₂FeSiO₄/C材料0.05 C倍率下的首次充放电曲线Fig. 6 First charge-discharge curves of Li₂FeSiO₄/C materials prepared by microwave assisted low-heating solid-state reaction for different time at 0.05 C rate

采用微波辅助固相法在650℃下微波处理 12 min的样品具有较好的电化学性能. 进一步分析其微观形貌、倍率充放电以及循环性能. 图7给出了此条件下制备的Li₂FeSiO₄/C材料的TEM照片, 如图所示, 产物的一次颗粒近似球形, 碳在Li₂FeSiO₄颗粒之间形成连续的连接, 在材料晶粒表面和颗粒间构成有效的导电网络, 增加Li₂FeSiO₄颗粒之间的导电性, 整体导电性能得到改善; 同时复合的碳增加了正极材料的比表面积, 使材料与电解液充分接触, 给电子、离子交换提供了较好的通道^{[ 25, 26]}, 利于电子和锂离子在电极/电解液界面的传递, 减小极化, 可提高电极反应动力学性能.

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	图7 微波辅助固相合成的Li₂FeSiO₄/C材料的TEM照片Fig.7 TEM image of Li₂FeSiO₄/C material prepared by microwave assisted low-heating solid-state reaction

制备的Li₂FeSiO₄/C材料0.05 C倍率时的前2次和第5次充放电曲线如图8所示, Li₂FeSiO₄材料首次充电过程中发生了相变, 形成更稳定的结构^{[ 2, 6]}. 电压平台从首次充放电的3.1 V左右转到了后续的充电过程中的2.8 V, 放电电压平台没有发生偏移. 第2次与第5次的充放电曲线较好的重叠, 放电比容量分别为129.8和129.4 mAh/g, 放电效率分别为99 %和99.1 %, 说明Li₂FeSiO₄的相转变在首次充放电过程中基本完成, 在以后循环中Li₂FeSiO₄具有良好的可逆性和循环稳定性.

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	图8 制备的Li₂FeSiO₄/C材料在0.05 C下前2次和第5次充放电曲线Fig. 8 The first, second and fifth charge-discharge curves of the as-prepared Li₂FeSiO₄/C material at 0.05 C rate

图9(A)为制备的正极材料在0.1 C、0.2 C、0.5 C不同倍率下的第二次充放曲线和倍率循环性能曲线. 所合成的Li₂FeSiO₄/C材料在0.1 C、0.2 C、0.5 C倍率时的放电比容量分别为124.5、117.2和107.5 mAh/g, 保持了0.05 C容量的95.9%、90.3%和82.8%. 低倍率下放电平台保持平坦, 随着放电倍率的增加, 电极极化增大, 较高倍率下平台下降明显, 可逆性变差. 图9(B)给出了Li₂FeSiO₄/C材料的不同倍率下的循环性能. 0.1 C和0.2 C倍率下10次循环后比容量分别为123.5 mAh/g和115.9 mAh/g, 0.5 C倍率下15次循环后比容量保持为104.8 mAh/g, 0.1 C、0.2 C、0.5 C倍率下循环的容量保持率分别为99.4%、99%和97.7%, 表明合成的Li₂FeSiO₄材料具有较好的循环性能.

	Figure Option View Download New Window
	图9 Li₂FeSiO₄/C材料在不同倍率下第二次充放电曲线(A)和不同倍率循环曲线(B)Fig. 9 Second charge-discharge curves (A) and cycle capability (B) of Li₂FeSiO₄/C at different rates

进一步考察了正极材料的室温性能, 图10(A)为合成的正极材料在1.8~4.5 V区间内0.05 C充放电倍率下前二次充放电曲线. 如图10(A)所示, 前二次容量分别为130.2和129.9 mAh/g. 充放电循环过程中, 由于Li₂FeSiO₄材料室温下极低的电导率, 电极极化偏大, Li₂FeSiO₄材料的放电平台保持较差. Li₂FeSiO₄材料的电子电导率较低, 受环境温度影响较明显, 相对较低的温度对聚阴离子型材料的离子扩散和电子迁移速率都不利, 从而导致锂离子在固相中的扩散速度以及电荷转移速度很慢, 表现为室温倍率性能和循环性能较差, 如图10(B)所示为Li₂FeSiO₄/C材料的0.05 C和0.2 C倍率下的循环性能. 0.05 C倍率下经过10次循环后放电比容量为128.4 mAh/g, 0.2 C倍率下从107.3 mAh/g经10次循环降到103.4 mAh/g.

	Figure Option View Download New Window
	图10 Li₂FeSiO₄/C材料室温下前2次充放电曲线(A)和循环性能图(B)Fig. 10 Initial and second charge-discharge curves(A) and cycle capability(B) of Li₂FeSiO₄/C cycled at room temperature

采用微波辅助固相法结合碳包覆改性改善了Li₂FeSiO₄的电化学性能, 获得了具有良好充放电平台的正硅酸盐材料. 这主要在于一是制备了纯度高、颗粒尺寸细小的正极材料; 二是聚乙烯醇热解碳包覆在颗粒表面, 实现了颗粒之间有效的导电接触, 同时超导电碳粉为颗粒之间提供了附加的导电通路, 增强了颗粒与颗粒之间的电子传输, 提高了正极材料的整体电导率.

3 结论

1)以Na₂SiO₃·9H₂O和FeCl₂·4H₂O为原料, 通过低热固相反应, 获得了β-FeOOH/SiO₂前驱体. 以制备的FeOOH/SiO₂前驱物和Li₂CO₃为原料, 聚乙烯醇和超导碳为碳源, 通过低温预烧-微波辅助固相法合成了Li₂FeSiO₄/C正极材料.

2)在650℃下微波热处理12 min合成了结晶好、细小均匀、高纯度的Li₂FeSiO₄/C材料. 制备的材料具有较好的电化学性能, 在60℃下0.05 C倍率首次放电容量为129.6 mAh/g, 0.5 C倍率下有107.5 mAh/g, 15次循环后保持为104.8 mAh/g;

3)室温下0.05 C倍率下首次放电容量为130.2 mAh/g, 循环10次后为128.4 mAh/g, 衰减较快. Li₂FeSiO₄材料具有较低的电子电导率和离子电导率, 相对较低的温度对聚阴离子型Li₂FeSiO₄材料的电化学性能不利.

参考文献

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[17]	DENG Fei, ZENG Xie-Rong, ZOU Ji-Zhao, et al. Effects of preparation temperature on pyrolytic carbon coated LiFePO₄/vapor- grown carbon fiber (PCLFP/VGCF) composite cathode material. Journal of Inorganic Materials, 2011, 26(11): 1141-1146. [本文引用:1] [JCR: 0.531] [CJCR: 1.106]
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1997

2.588

0.0

... 自从Goodenough 实验室报道LiFePO₄以来^[1], 铁系聚阴离子型化合物作为有潜力的锂离子电池正极材料受到了极大的关注 ...

2005

4.425

0.0

Electrochem. Commun.. 2005, 7(2):156-160 DOI:10.1016/j.elecom.2004.11.008

Electrochemical performance of Li₂FeSiO₄ as a new Li-battery cathode material

Abstract

Phase-pure lithium iron silicate (Li₂FeSiO₄) has been prepared successfully. Its ambient temperature structure has been determined by X-ray diffraction and its electrochemical performance characterised at 60 °C. The resulting cyclic voltammogram suggests a phase transition to a more stable structure after the first cycle. This could involve a structural ordering process from a solid-solution to a long-range-ordered structure. The initial charge capacity of 165 mAh/g (99% of the theoretical value) stabilises after a few cycles to around 140 mAh/g (84% of the theoretical value).

... 而硅酸盐系列材料因具有矿藏丰富和环境相容性好等特点也颇具吸引力^[2,3,4,5] ...

... 当加热时间增加到12 min时, 杂质峰已完全消失, 样品的衍射峰尖锐, 结晶良好, 强衍射峰与已报道的图谱十分吻合^[2], 属于正交晶系, 空间群为Pmn2₁ ...

... 05C倍率时的前2次和第5次充放电曲线如图8所示, Li₂FeSiO₄材料首次充电过程中发生了相变, 形成更稳定的结构^[2,6] ...

2010

0.531

1.106

Journal of Inorganic Materials. 2010, 25(3):327-331 DOI:10.3724/SP.J.1077.2009.09472

Effect of heat-treatment temperature on the electrochemical performances of the Li₂MnSiO₄/C composite prepared through polyol process

1．School of Material Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China；2.School of Science, Xi’an University of Technology, Xi’an 710048, China

The Li₂MnSiO₄/C composite was synthesized through a polyol process following by heattreatment at intermediate temperatures. The precursors were precipitated from Si(OC₂H₅)₄, LiAc·2H₂O and Mn(Ac)₂·4H₂O in ethylene glycol by refluxing at 196℃ for 16h, the dried powders were milled, pressed and heat-treated at 500℃, 600℃ and 700℃ for 10h. The obtained samples were characterized by X-ray diffraction, automatic elemental analyzer, field emission scanning electron microscope (FESEM), energy dispersive spectroscope (EDS), and high resolution transmission electron microscope (HRTEM). As a result of the Rietveld refinement, two possible Li₂MnSiO₄ forms crystallizing in Pmn21 and P121/n1 space groups, respectively, are considered as the major phases, and Li2SiO3 is identified as the impurity phase. According to the FESEM observation, the mean particle size increases from 5-20nm to 20-40nm as the heattreatment temperature increasing from 500℃ to 700℃. Carbon is homogenously distributed on the prepared Li₂MnSiO₄/C composites. The HRTEM examination confirms that the Li₂MnSiO₄ particles are surrounded by a very thin amorphous carbon layer. The electrochemical experiments reveal that the Li₂MnSiO₄/C composite heat-treated at 600℃ for 10h has the higher electrochemical performance with an initial discharge capacity of 132.4mAh/g and a capacity retention ratio of 80% at the tenth cycle.

采用多元醇法合成Li₂MnSiO₄/C复合材料:将Si(OC₂H₅)₄、LiAc·2H₂O、Mn(Ac)₂·4H₂O在乙二醇中196℃回流16h，沉淀物过滤、干燥以后与蔗糖混合并在500、600和700℃分别加热10h,自然冷却到室温.利用X射线衍射、元素分析、场发射扫描电镜、高分辨透射电镜等手段对合成样品进行表征.Rietveld精修表明，合成样品中存在两种Li₂MnSiO₄相，空间群分别为Pmn21和P121/n1，还有Li₂SiO₃杂相.FESEM观察显示当热处理温度从500℃升高至700℃时，样品的颗粒粒径从5~20nm提高到20~40nm.Li₂MnSiO₄/C复合材料中的碳分布是均匀的.HRTEM分析表明在Li₂MnSiO₄颗粒上包覆有一层无定型的碳薄膜.电化学性能测试表明，在600℃保温10h得到的样品具有最佳的电化学性能，首次放电容量达到132.4mAh/g，10次循环后容量保持率为80%.

... 而硅酸盐系列材料因具有矿藏丰富和环境相容性好等特点也颇具吸引力^[2,3,4,5] ...

2006

4.425

0.0

Electrochem. Commun.. 2006, 8(5):797-800 DOI:10.1016/j.elecom.2006.03.012

An ab initio study of the Li-ion battery cathode material Li₂FeSiO₄

Abstract

A density functional theory (DFT) calculation is reported for the novel Li-ion battery cathode material lithium iron silicate (Li₂FeSiO₄) and for three possible Li arrangements in the delithiated structure (LiFeSiO₄). Relevant battery-related properties have been derived: average voltage (2.77 V vs. Li/Li⁺), energy density (1200 Wh/l) and specific energy (440 Wh/kg). Lattice constants and atomic fractional coordinates are also given for each case. The calculated values are in good agreement with recent experimental values (A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun., 7 (2005) 156). Voltages were calculated (again vs. Li/Li⁺) for the three different Li arrangements in LiFeSiO₄; these differed by 0.22 V – a difference which could perhaps be related to the experimentally observed 0.30 V drop in voltage between the first and subsequent charge cycles.

... 而硅酸盐系列材料因具有矿藏丰富和环境相容性好等特点也颇具吸引力^[2,3,4,5] ...

2007

8.238

0.0

... 而硅酸盐系列材料因具有矿藏丰富和环境相容性好等特点也颇具吸引力^[2,3,4,5] ...

2006

5.968

0.0

... 由于具有很强的Si-O键结合力, Li₂FeSiO₄形成了稳定的晶体框架结构, 可使锂离子的脱出和嵌入电位保持稳定, 在锂离子嵌脱过程中正极材料结构能较好地保持, 具有高的安全性能^[6,7,8,9], 成为很有发展前景的锂离子电池正极材料 ...

... 微波热处理16和20 min合成的样品因为颗粒开始团聚, 使电极材料与电解液的有效接触面积减少, 颗粒较大使锂离子扩散路径增长, 可逆比容量下降快^[6,9] ...

... 05C倍率时的前2次和第5次充放电曲线如图8所示, Li₂FeSiO₄材料首次充电过程中发生了相变, 形成更稳定的结构^[2,6] ...

2006

5.968

0.0

... 10^-14S/cm, 而且在充放电过程中, Li⁺在Li₂FeSiO₄和LiFeSiO₄两相中的扩散系数很小, 导致材料的可逆容量较低和倍率性能差, 特别是室温下的电化学性能更差^[7,9,10], 即使以小电流充放电其放电比容量也很难达到理论比容量 ...

2006

0.0

2011

5.968

0.0

2006

4.425

0.0

Electrochem. Commun.. 2006, 8(2):217-222 DOI:10.1016/j.elecom.2005.11.010

Structure and electrochemical performance of Li₂MnSiO₄ and Li₂FeSiO₄ as potential Li-battery cathode materials

Abstract

Recently, preparation and preliminary testing of Li₂FeSiO₄, a representative of a new class of Li storage materials, has been reported [A. Nyten, A. Abouimrane, M. Armand, T. Gustaffson, J.O. Thomas, Electrochem. Commun. 7 (2005) 156]. In the present paper, we report preparation of another material from this class: Li₂MnSiO₄. To the best of our knowledge, the existence of this compound has not been reported so far. Similarly as in the case of the LiMPO₄ materials family (M = Fe, Mn), the Mn analogue shows considerably poorer electrochemical performance. Quite unexpectedly, however, the Mn analogue shows a better stability, especially under harsh conditions. This property appears to be crucial for determination of detailed structural features of this class of materials. The obtained structure reveals partial occupation of alternate tetrahedral sites by Li and Mn, thus implying a high ionic conductivity of these materials. The poor electrochemical performance is most likely due to poor electron wiring.

2008

4.675

0.0

... 在改善Li₂FeSiO₄材料电导率方面, 大多是通过基于有机盐体系的溶胶-凝胶法^[11,12,13]来控制颗粒尺寸大小以制备较高纯度和较好循环性能的硅酸盐复合正极材料, 但其合成过程较复杂、流程时间较长且成本偏高 ...

... 由此可见, 采用微波辅助固相工艺可在较短的时间里得到结晶良好的高纯度Li₂FeSiO₄材料, 与文献报道相比^[11,23], 微波固相法合成的材料XRD中并未出现Li₂SiO₃、Fe₃O₄等杂质相的衍射峰 ...

2008

1.766

0.0

2009

1.766

0.0

2003

0.0

... 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[14,15,16,17,18,19]} ...

2005

0.72

0.914

Chinese J. Inorg. Chem.. 2005, 21(2):249-252

Synthesis of cathode material "carbon-included" LiFePO₄ by microwave heating

Cathode material LiFePO₄ of lithium-ion battery was synthesized by microwave heating. The “carbon-included” LiFePO₄ with improved conductivity was synthesized by the addition of graphite. And the influence of microwave-heating time on structure, morphology and charge/discharge performance of the products was discussed. The results of XRD, SEM, XPS, CV and charge/discharge testing measurements showed that the LiFePO₄ product after 9 min in microwave oven had more advantages than other products.

... 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[14,15,16,17,18,19]} ...

2007

3.777

0.0

Electrochimica Acta. 2007, 52(24):6778-6783 DOI:10.1016/j.electacta.2007.04.104

Preparation and characterization of nano-sized LiFePO₄ by low heating solid-state coordination method and microwave heating

Abstract

The precursors of LiFePO₄ were prepared by low heating solid-state coordination method using lithium acetate, ammonium dihydric phosphate, ferrous oxalate and citric acid as raw materials. Olivine phase LiFePO₄ as a cathode material for lithium-ion batteries was successfully synthesized by microwave heating in a few minutes. X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to characterize its structure and morphology. Cyclic voltammetry (CV) and charge–discharge cycling performance were used to characterize its electrochemical properties. The results showed that the grain size of the optimal sample was about 40–50 nm, and the as-prepared particles were homogeneous. The nano-sized LiFePO₄ obtained has a high electrochemical capacity (125 mAh g⁻¹) and stable cycle ability.

... 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[14,15,16,17,18,19]} ...

... 本工作首先通过低热固相法^[16,20]获得均匀细小的前驱体, 再采用低温预烧-微波合成法制备Li₂FeSiO₄/C正极材料 ...

2011

0.531

1.106

Journal of Inorganic Materials. 2011, 26(11):1141-1146 DOI:10.3724/SP.J.1077.2011.01141

Effects of preparation temperature on pyrolytic carbon coated LiFePO₄/vapor- grown carbon fiber (PCLFP/VGCF) composite cathode material

(1. School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China; 2. College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China; 3. Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, China)

Pyrolytic carbon coated LiFePO4/vapor-grown carbon fiber (PCLFP/VGCF) composite cathode materials were prepared by microwave chemical vapor deposition (MCVD) process. Effects of preparation temperature on microstructures and electrochemical properties of the composites were investigated. The results show that with preparation temperature increasing from 500℃ to 600℃, the discharge capacities of the composite electrodes are improved because of the gradual formation of conductive network of VGCF. However, it decreases when the preparation temperature is up to 700℃, which may be attributed to the exaggerated grain growth of LiFePO₄ and the reduction of VGCF network. In addition, the composite cathode materials prepared at 600℃ show better electrochemical properties, with the discharge capacity as high as 163, 159, 153 and 143mAh/g at rate of 0.2C, 0.5C, 1C, 3C, respectively (25℃).

采用微波化学气相沉积法一步合成了热解炭包覆磷酸铁锂/气相生长炭纤维复合正极材料. 借助X射线衍射仪、场发射扫描电子显微镜、高分辨透射电镜和电化学测试仪等测试手段研究了不同制备温度对材料晶体结构、显微形貌和电化学性能的影响. 结果表明, 当制备温度由500℃升至600℃时, 磷酸铁锂主晶相的颗粒尺寸没有发生明显变化, 而原位VGCF的网络程度却明显增加, 材料的放电比容量随之提高; 当制备温度进一步升高到700℃时, 磷酸铁锂颗粒异常生长现象加剧, VGCF直径较大且粗细不均, 材料的电化学性能变差. 研究发现, 当温度为600℃时, 材料表现出较优的电化学性能, 25℃在0.2C、0.5C、1C和3C倍率下的放电比容量分别可达163、159、153和143mAh/g.

... 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[14,15,16,17,18,19]} ...

2004

0.0

... 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[14,15,16,17,18,19]} ...

2004

0.0

... 近几年微波合成技术Re成为快速制备高性能新材料的有效方法, 应用于锂离子电池正极材料的制备^{[14,15,16,17,18,19]} ...

... 分散在反应物颗粒之间的超导碳和聚乙烯醇热分解的无定形碳是良好的微波耦合剂, 它们能促进微波与反应原料相互作用, 提高反应体系的热导率^[19,24] ...

2003

0.531

1.106

Journal of Inorganic Materials. 2003, 18(2):313-319

Application of the low-heating solid-state reaction method in preparation of multi-metal oxides composite#cod#x02014;synthesis, structure and electrochemical properties of rhombohedral LiMnO₂ cathode material for lithium-ion batteries

1.College of Chemistry and Chemical Engineering; Hunan University; Changsha 410082; China; 2.Institute of Applied Chemistry; Xinjiang University; Wulumuqi 820046; China

LiMnO₂ compounds with rhombohedral structure were prepared by the low-heating solid-state reaction method at the
temperature range of 350～700℃. The powder X-ray diffraction technique was applied to investigate the structures of the obtained samples.
XRD result shows that the structures of these samples are the same as that of LiCoO₂. Compared with LiCoO₂ and LiCo_0.8Ni_0.2O₂ prepared by
the same method, the lattice parameter of r-LiMnO₂, c/a, is relatively smaller due to the Jahn-Teller deformation. Selected-area electron diffraction
and XRD results indicate the existence of the cubic spinel impurities in the r-LiMnO₂ samples sintered at 600℃ and 700℃. The tests of
charge/discharge show that the samples prepared above 600℃ have two voltage plateaus of 3V and 4V, of which the plateau at 4V is related to the
cubic spinel impurities. Whereas, the samples obtained below 500℃ present only one plateau of 3V. Especially, the sample obtained at 500℃
exhibits good structural stability in lithium-ion intercalation/deintercalation processes.

以氢氧化锂,醋酸锰和草酸为原料,采用低热固相反应法于350～700℃下直接通过热处理制备了锂离子电池正极材料r-LiMnO₂粉体样品.X射线衍射分析表明,采用该法得到的样品与LiCoO₂具有类似的晶型.由于Mn³⁺的Jahn-Teller效应使r-LiMnO₂与同样方法合成的LiCoO₂及LiCo_0.8Ni_0.2O₂相比,晶胞形状变得更加扁平,晶胞体积增大.选区电子衍射研究表明高于600℃焙烧温度所得的r-LiMnO₂样品中含立方尖晶石结构杂质相. 2.5～4.3V之间的充放电测试结果表明,样品的充放电容量随焙烧温度的升高而增大,但高于600℃的样品具有3V和4V两个充放电平台,而350～500℃的样品只有一个3V充放电平台,并且循环过程中结构非常稳定.

... 本工作首先通过低热固相法^[16,20]获得均匀细小的前驱体, 再采用低温预烧-微波合成法制备Li₂FeSiO₄/C正极材料 ...

1998

0.0

1986

1.597

0.0

... 03 cm^-1处的吸收峰对应于Si-O-Si 键的弯曲振动^[22], 结合XRD分析结果说明前驱物中的SiO₂以无定形存在 ...

2008

0.0

0.588

2007

0.0

2008

4.675

0.0

... 同时复合的碳增加了正极材料的比表面积, 使材料与电解液充分接触, 给电子、离子交换提供了较好的通道^[25,26], 利于电子和锂离子在电极/电解液界面的传递, 减小极化, 可提高电极反应动力学性能 ...

2011

0.531

1.106

Journal of Inorganic Materials. 2011, 26(3):265-270 DOI:10.3724/SP.J.1077.2011.00265

Property and structure of carbon-coated LiFePO₄

1. School of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China; 2. School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China; 3. School of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, China

LiFePO₄/Ccomposite cathode materials containing different amounts of carbon were fabricated by mechanical activation–high temperature solid statereaction using citric acid as a carbon source, with the aim to study the effectof carbon-coating on LiFePO₄internal structure. The crystal structure, surface morphology and electrochemical properties of the obtained composites coated with various carbon contents were investigated by the techniques of XRD, SEM, BET, HRTEM, SAED, AC Impedance spectra, and constant current charge-discharge testing. And the structural changes originating from the carbon-coatingwere also analyzed. Under high temperature, citric acid was decomposed into network-like structure amorphous carbon, and coated on the surface of LiFePO₄ particles, which could inhibit the grain growth and cause the smaller sizes.The crystal growth direction and micro-structure of LiFePO₄/C were significantly influenced by carbon-coating that resulted in growing along direction preferentially.According to AC Impedance spectra measurements, all the impedances of carboncoated LiFePO₄ decreased evidently, and the diffusion coefficient of Li ions increased by up to two orders of magnitude compared with uncoated LiFePO₄, indicating the ion and electric conductivity,discharge capacity and cycling performance were greatly improved.

为了研究碳包覆对LiFePO₄结构的影响,以柠檬酸为碳源, 采用机械活化-高温固相法, 合成了不同碳包覆量的LiFePO₄/C复合正极材料. 通过XRD、SEM、BET、HRTEM、选区电子衍射(SAED)、交流阻抗谱(ACI)和恒电流充放电等现代分析方法, 全面研究了碳包覆量不同时,LiFePO₄/C复合正极材料的结构、形貌和电化学性能, 并对C包覆对结构影响的成因进行了分析.结果表明, 柠檬酸高温分解后生成无定形碳非晶物质,在LiFePO₄颗粒表面包覆形成一种网络结构,抑制了颗粒的生长; C包覆影响了晶体的生长方向和微观结构,LiFePO₄/C的优势生长为方向; 交流阻抗分析表明包覆后锂离子扩散系数比未改性的LiFePO₄提高了两个数量级,且各项阻抗值均降低, 从而提高了材料的离子及电子电导性、放电性能和循环性能. "