高能量密度镁电池正极材料的研究

With the increasing demand for green energy, lithium ion batteries (LIBs) are becoming the dominant batteries for mobile electronic devices and electric vehicles. With the decade’s development, the energy densities of LIBs are approaching the limitation of intercalation chemistry. Its capacities and energy density are hardly to be greatly improved. Rechargeable metal batteries, which pair metal anodes with various cathode materials, offer great promise for further improving battery energy density, because metal anodes provide much higher capacity and lower reduction potential than intercalation anodes. Among all metal anodes, the high capacity (especially volumetric capacity of 3833 mA h/cm3 for Mg vs. 2046 mA h/cm3 for Li), low reduction potential (-2.4 V vs. SHE) and most importantly, dendrite-free deposition with 100% coulombic efficiency in some electrolytes make Mg metal an ideal anode. For this reason, there is a growing interest to develop rechargeable Mg batteries during the past decade. However, due to its bivalent nature, the diffusion of Mg2+ in solid state cathode materials is much sluggish than monovalent cations like Li+ etc., which leads to large voltage hysteresis and low magnesiation degree for most materials. For this reason, finding cathode materials with acceptable kinetics and high energy density has become the major challenges for the development of rechargeable Mg batteries.

随着社会对绿色能源的需求，锂离子电池逐渐成为移动电子设备和电动汽车的主流电源。经过多年的发展，锂离子电池的能量密度逐渐接近嵌入式化学的理论极限，其容量和能量密度很难再获得大的提升。而金属二次电池可以把金属负极和正极材料结合起来，得益于金属负极的高容量和低电位，可以进一步提高电池的能量密度。而在所有的金属负极中，镁金属具有高容量（特别是体积容量为3833 mA h/cm3，对比锂金属的2046 mA h/cm3），低还原电位（-2.4 V vs. SHE）的优点。更重要的是，镁金属负极可以实现库伦效率100%的沉积和溶解，且没有枝晶产生，安全性能很高。但是由于镁离子带有两个单位的正电荷，与正极材料有强烈的相互作用，导致扩散缓慢，动力学性能差。因此很多镁电池正极材料的过电位比较大，镁离子的嵌入程度低。找到具有快速动力学、高能量密度的正极材料成为镁电池发展的主要挑战。

在后锂离子电池时代，镁金属电池有望取得更高的能量密度，被认为是最有潜力的新一代电池体系之一。目前镁电池的发展主要受限于缺乏合适的正极材料。本项目以获得高能量密度镁电池正极材料为目标，按照研究计划如期完成了以下研究内容：（1）基于取代反应机理，开发了高容量转化型硫化铜正极材料，其能量密度可与目前商用的锂离子电池体系相比。（2）鉴于传统的硫化物只基于过渡金属元素变价，每个过渡金属元素最多对应一个电子转移，容量受限。本项目创新性地提出在镁电池正极材料中引入阴阳离子共变价体系，进一步提升硫化物正极材料的容量。（3）本项目通过调节阴阳离子的能带结构，设计出一系列非晶阴阳离子共变价正极材料，取得260 Wh/kg的能量密度。通过非晶化打破原有晶格束缚，提高离子扩散速率，并结合过渡金属富硫的思路提供更多嵌镁位点，有助于局域多电子的电荷补偿过程。本项目共发表SCI论文6篇，申请中国发明专利5项，完成了项目预期目标。本项目的实施将为发展高比能、长寿命的镁电池新体系提供实验借鉴和理论依据。