基于二维导电网络的LiMnPO4体系@石墨烯材料的功能调控及其储锂特性

LiMnPO4 has a great potential application as cathode material due to its low cost and high safety. However, the low electronic conductivity and poor rate performance of the material limit the sufficient application in the field of lithium-ion battery. Our previous theoretical calculations show that the transport property of carrier is an important factor in restricting the rate performance of such material. In this project, the leading role of theoretical calculations is used to design the structure of LiMnPO4, and the scientific objective improving the rate performance is investigated by ameliorating the transport properties of carriers of LiMnPO4 materials. From the design of the materials, an intrinsic relation between the microscopic electronic structure and macroscopic properties is established by the first-principles calculations. A method is established to study conduction mechanism of LiMnPO4, and then the factors influencing the electronic transport property of carrier are revealed to provide a theoretical basis for the design and functional regulation of the material. Afterward, the two-dimensional conductive network LiMnPO4@graphene materials were controllably synthesized. The electrode reaction kinetics of the materials and electrolyte at the surface / interface is studied using in situ technology, and then seek an effective way to improve the material properties from the mesoscopic scale to the microscopic scale. The theoretical calculations, controllable synthesis and mechanism analysis are closely combined to achieve the functional design and preparation of the high power LiMnPO4 cathode material under the theoretical guidance.

低廉的价格和高安全性使LiMnPO4成为极有应用潜力的正极材料。但该材料的电子电导率低、倍率性能差，限制了它在锂离子电池领域的应用。我们前期的理论计算表明，载流子的输运性能是制约该类材料倍率性能的重要因素。项目拟利用理论计算的先导作用，对LiMnPO4的结构进行设计，并围绕改善其载流子输运性能，提高其倍率性能这一科学目标展开研究。从材料的设计环节出发，利用第一性原理计算建立其微观电子结构和宏观性质的内在联系，建立适于研究LiMnPO4导电机制的方法，揭示影响载流子输运性能的因素，为材料的设计和功能调控提供理论依据。然后，可控合成二维导电网络的LiMnPO4@石墨烯材料，利用原位技术研究材料与电解液表面/界面处的电极反应动力学，从介观到微观尺度上寻求改善材料性能的有效途径。项目将理论计算、可控合成、功能调控及机理分析紧密结合起来，实现理论指导下高功率型LiMnPO4材料的功能化设计与制备。

利用第一性原理计算LiMnPO4及MnPO4 的电子结构及其力学稳定性，揭示了微观电子结构与电池材料的宏观性质的内在联系。结果表明，O2p与P3s的轨道将发生有效重叠，并形成共价键，Mn3d和O2p之间能够有效地发生重叠并形成共价键。Li+的脱嵌削弱了Mn-O(I)键，导致材料的力学性能发生显著改变。由于Mn-O(I)主要分布在{101}和(1̅01)晶面上，这导致与上述两晶面相关滑移系统变得非常活跃。因此MnPO4容易产生剪应形变和位错。沿着(101)和(1̅01)晶面分布的位错使得紧邻的MnO6八面体以共享边的方式相连接，而最紧邻的两个PO4四面体则通过一个共享顶点(O原子)相连。每个Mn2P2O8 (2MnPO4)将产生一个多余的O原子，该氧原子将在界面区域释放。因此，脱Li后的MnPO4力学稳定性降低，因此MnPO4稳定性较差，LiMnPO4在力学上是稳定的。采用溶剂热法合成LiMn1-xFexPO4/C(0 ≤ x ≤ 0.5)，结果表明，当Fe2+量x<0.5时，材料为纳米片状，而LiMn0.5Fe0.5PO4为纳米棒状。LiMn1-xFexPO4/C(x=0, 0.1, 0.2, 0.3, 0.4, 0.5)在0.05C倍率下的首次容量依次为109.5、132.7、144.3、143.1、141.7和156.4 mAh·g-1。LiMn0.5Fe0.5PO4/C具有最佳的电化学性能。合成了纳米棒状的LiAlO2-LiMn0.5Fe0.5PO4/C(wt(LiAlO2) =0, 3%, 5%, 10%)在5C充放电下，首次放电容量分别为104、106.3、107和91.1 mAh·g-1，循环100次后，容量保持率分别为72.3%、88.9%、86.4%和87.8%。包覆适量的LiAlO2提高了LiMn0.5Fe0.5PO4的倍率性能和循环性能。合成了纳米棒状的Li0.3La0.56TiO3(LLTO)-LiMn0.5Fe0.5PO4/C。包覆量为3和5 wt.%时，材料的倍率性能和循环性能最佳。在大倍率5C充放电下，循环100次后，材料的放电比容量分别为78.2、106.4、106.2和97.7 mAh·g-1，容量保持率分别为67.2%、81.0%、82.5%和78.3%。EIS测试表明包覆LLTO可以减小材料的电化学阻抗Rct，有利于Li+的扩散。