电荷有序层状结构MnO2的高压合成、液相剥离及超电容性能研究

Birnessite-type MnO2 with a layered structure is one of the important pseudocapacitive materials for supercapacitors. The theoretical value of specific capacitance of MnO2 can reach as high as 1380 F g-1, however, except for film samples, the low conductivity of MnO2 (10-5~10-6 S cm-1) has limits the practical specific capacitance of powder samples. To date, although there are lots of researches on applications of MnO2 nanomaterials for supercapacitors, there are few reports on the fundamental researches of enhancing the conductivity of MnO2 through modulations of its crystal structure and defects. This project is proposed to modulate the arrangement of Mn3+/Mn4+ and defects in birnessite through high pressure synthesis and subsequent exfoliation. Particularly, it is proposed to synthesize the sizable layered birnessite crystals with special Mn3+/Mn4+ ordering and chemical composition through high-pressure molten-salt technique. Birnessite nanosheets with different hole defects can be obtained through exfoliation of the birnessite crystals. Subsequently, birnessite nanosheets with high conductivity may be found for supercapacitors. Finally, it is expected that our results will put new insights into the rational synthesis of MnO2 electrode materials with high conductivity for supercapacitors.

层状结构MnO2（Birnessite）是一种重要的超级电容器用赝电容材料，其理论比容量可达1380 F g-1，然而，由于MnO2的电导率较低（10-5~10-6 S cm-1），除去一些薄膜材料，其粉体材料的实测比容量值一直不高。目前为止，虽然有较多MnO2纳米材料在超级电容器中的应用研究，但通过调制MnO2的晶体结构和微结构来提高其电导率的基础性研究仍少有报道。本项目拟通过高压合成和后期剥离对Birnessite中的Mn3+/Mn4+的排布和缺陷进行调控，即采用高压熔盐法合成特定结构和组分的大尺寸Mn3+/Mn4+离子排布有序的层状结构Birnessite晶体，进一步通过液相剥离法得到系列具有不同纳米孔洞缺陷的Birnessite纳米片，从中筛选出具有高电导率的超级电容器用Birnessite纳米片，以期对超级电容器用高电导率的MnO2电极材料的设计合成提供新的思路。

层状结构MnO2（Birnessite）是一种重要的超级电容器用赝电容材料，然而其粉体材料的实测比容量值一直不高且循环稳定性欠佳。本项目通过调制MnO2的晶体结构和微结构，即通过高压合成、高温合成和水热合成及后期剥离对Birnessite中的Mn3+/Mn4+的排布和缺陷进行调控。所得的结果是，调控温压条件，分别得到了正交相和单斜相Birnessite高质量毫米级的单晶体；通过调控压力可以对 Mn3+ /Mn4+ 的电荷比例和有序排列进行调控；Birnessite纳米片的长循环条件下发生缺陷诱导的重结晶导致容量部分衰减。关键数据包括，Birnessite单晶体最大边长在0.05-0.2 mm 之间；Birnessite单晶体的最佳合成条件为550-600℃，50-150MPa；通过压力瞬降控制birnessite从2×1型电荷有序正交相向2×2型电荷有序单斜相转化；水热合成的birnessite具有较好的比容量（150-200F g-1）。其科学意义有，解决了常压条件下得到的 Birnessite 晶体尺寸小（最大边长小于 0.05mm）且结晶度差，难以满足精细结构研究的需求的问题；进一步明确了压力对于晶体生长的作用；发现Birnessite超级电容器衰减的原因之一是循环过程中的重结晶，具有普遍意义。