高能量密度、多锂离子嵌脱的硅酸盐正极材料系统研究

Lithium-ion batteries are being seriously considered for the efficient storage and utilization of intermittent renewable energies like solar and wind. They are also being intensively pursued for vehicle applications including hybrid electric vehicles and pure electric vehicles. To obtain high energy density lithium-ion batteries, it is vital to exploit the cathode materials that can extract/insert more than one mole of lithium ions per mole of compound. Orthosilicates Li2MSiO4 (M = Fe, Mn, Co, Ni), with two lithium ions in a formula unit, can in principle reach this "multiple lithium ions extraction/insertion" goal. Recently, a totally new and simple way to synthesize polyoxyanion compounds was found in our lab, and Li2MSiO4/C (M = Fe, Mn) composites synthesized by this method can reversibly extract/insert more than one lithium ion per formula unit easily, with the discharge capacity reaching 230 mAh/g at current density of 0.1 C (1 C = 166 mA g-1) at room temperature. Discharge capacities of 185, 150, and 120 mAh/g are obtained at high rates of 1 C, 5 C, and 10 C, respectively. The M?ssbauer spectra for the electrode materials at different charging stages show that both Fe2+/Fe3+ and Fe3+/Fe4+ redox couples are related to these high capacities. But until now the detailed extraction/insertion mechanisms of these lithium ions in the electrode are still not clear. Here we propose to investigate these mechanisms by following the whole charge-discharge process for several cycles using a wide range of facilities including in-situ XRD, SEM, TEM, HRTEM, Raman, FTIR, XPS and the M?ssbauer spectra. To adjust the crystal structures and to control the electrochemical properties of cathode materials (such as the electronic conductivity, the charge/discharge voltage plateau, etc.), we will synthesize different solid-solutions of the transition metal silicates such as Li2M1-xNxSiO4 (M, N = Fe, Mn, Co, Ni) and Li2M1-x-yNxN'ySiO4 (M, N, N' = Fe, Mn, Co, Ni). To increase the amount of lithium ions that can reversibly extract/insert from/into the electrode materials and to increase the diffuse rate of the lithium ions in the materials, we will fabricate Li2+xMSi1-xAlxO4 and Li2-xMSi1-xPxO4 by substituting the Si by elements with different valents, respectively. To increase the direct contact area of the electrode materials with the electrolyte, we will fabricate the abovementioned cathode materials with mesoporous structures and explore their electrochemical properties. Due to the specificity of our synthesis methods, the abovementioned cathode materials are easy to obtain. The systematic and fundamental research we propose for the high energy density, multiple lithium ions extraction/insertion cathode materials will help our country to get the Chinese intellectual property rights of the innovative and high technology about the next generation of lithium batteries.

作为新型电动汽车的动力电池，锂离子电池的重要性日益明显。由于单个分子中存在两个锂离子，硅酸盐正极材料的理论比容量为磷酸盐的理论比容量的两倍。但长期以来，单个硅酸盐分子中稳定地嵌脱多个锂离子的愿望一直没能实现。最近申请人在新合成出来的硅酸铁锂中实现了多个锂离子的稳定嵌脱：室温下单个硅酸铁锂分子能长期可逆嵌脱1.4个锂离子。材料的大比容量及其穆斯堡尔谱证明了其中四价铁离子的形成与长期存在。这种高价态铁离子的安全性、稳定性、氧化还原循环过程、相关的晶体结构变化等是本申请项目的主要研究内容。为了有利于多锂离子的嵌脱,项目的研究内容还包括：合成新型的硅酸盐固溶体及其晶体结构和性能的调控、用不同价态的元素对晶体中的硅进行部分取代，以调节材料嵌脱锂离子的电极电位，并产生间隙锂离子或锂离子空位等。这些关于多锂离子嵌脱的硅酸盐正极材料的基础研究有助于我国获得下一代高比能的锂离子电池高新技术。

为了提高锂离子电池正极材料的储能密度和充放电循环性能，突破过渡金属氧化物和磷酸盐的极限，实现单分子多锂离子嵌脱，本项目以过渡金属硅酸盐为研究对象，做了如下研究：1）合成了碳包覆单斜相Li2FeSiO4纳米颗粒和纳米蠕虫，并在此基础上分别引入二维石墨烯薄膜和大孔三维石墨烯，合成了石墨烯-硅酸铁锂复合材料Li2FeSiO4/C/G和三维石墨烯-硅酸铁锂复合材料3D-G/Li2FeSiO4/C，分析了各种具特定结构与形貌的Li2FeSiO4的形成机理，检测了其作为锂电正极材料的电化学性能。上述Li2FeSiO4都显示出了超高的比容量和优良的循环稳定性，如Li2FeSiO4/C/G在0.1C、1C、2C、5C、10C、30C和50C倍率下的放电比容量分别可达310、280、240、200、168、113和53 mAh/g，其中在0.1C、1C、2C和5C倍率下的比容量相当于每个Li2FeSiO4分子中分别可逆嵌入1.86、1.65、1.44和1.20个Li+离子，该比容量不仅高于迄今为止所有文献报道中的数据，而且表明Li2FeSiO4不仅在低倍率或高温条件下可以实现，在室温且高倍率条件下同样能实现单个Li2FeSiO4分子多个Li+的嵌脱。具纳米结构的Li2FeSiO4在充放电过程中的多平台、大放电比容量和高倍率性能表明材料在充放电过程中涉及多个Li+离子的嵌脱与Fe3+/Fe4+的氧化还原。在不同的充放电状态下对电极材料进行了穆斯堡尔谱测量，谱图证明Li2FeSiO4在充放电过程中先后经历了Fe2+/Fe3+和Fe3+/Fe4+的氧化还原过程，在首轮充电至4.8V的过程中，Fe2+/Fe3+与 Fe3+/Fe4+的转化同时进行；非原位 XRD则表明该过程之后材料会发生相变。2）在Li2FeSiO4的Fe位掺杂V时合成了一系列Li2FeSiO4-Li3PO4共生物；在Li2FeSiO4的 Si位掺杂不同比例的 P，合成了一系列 Li2-yFeSi1-yPyO4/C化合物，研究了Si位掺杂P对Li2FeSiO4电化学性能的影响。3）研究了Li2MnSiO4、镍钴锰三元、层状-尖晶石集成结构Li2MnO3、具多级结构和暴露特定晶面的富锂富钠材料的电化学性能。这些关于多锂离子嵌脱的硅酸盐正极材料和其他二次电池正极材料的基础研究有助于我国获得下一代高比能的锂离子电池高新技术。