Pressure-induced irreversible conductivity enhancement in lithium acetylide - Drs. Lijuan Wang and Xiao Dong

AUGUST 27， 2017

Lithium acetylide, as a widely used electrode material for Li-ion battery has the highest theoretical capacity while with low conductivity for applications. New work from a HPSTAR team increased the conductivity of lithium acetylide by 9 orders of magnitude in the use of high pressure method. And this pressure-enhanced conductivity could be kept to ambient conditions. Their discoveries are published by the Journal of Physical Chemistry Letters (DOI: 10.1021/acs.jpclett.7b01779).

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of stimulus of polymerization. Pressure is a straightforward and clean external stimulus for polymerization, which is always referred as pressure-induced polymerization. Except for chemical reactions, pressure may also tune other physical properties.

At ambient conditions, lithium acetylide (Li2C2) is electrode material which is mostly used for lithium extraction in lithium batteries. While the low conductivity at ambient conditions confine lithium acetylide’s potential applications.

The HPSTAR team including Drs. Lijuan Wang, Xiao Dong, Yajie Wang, Haiyan Zheng, Kuo Li, Yufei Meng, and Ho-Kwang used high pressure to tune the electrical properties of lithium acetylide.

They found that the conductivity was enhanced by nine orders compared to that uncompressed.

More importantly, the heavily enhanced conductivity could be kept to room pressure. This indicates its more practical applications in Li-ion batteries.

To find the behind reasons, the team performed the following high-pressure x-ray diffractions, Raman, IR etc supportive measurements.

The C22- anions in lithium acetylide approached to each other and polymerized to ribbon structure from chains upon compression. “So the compression of the distances between should be the main reason for the enhancement of the conductivity and the pressure induced C22- anions polymerization is responsible for the irreversibility”, explained Dr. Lijuan Wang, the lead author.

The polymerization will disproportionate with pressure released to ambient conditions. “This disproportionation implies that the carbon skeletal is highly electrochemically active and potentially useful in designing electrode materials”, added Dr. Xiao Dong, the co-lead author.

This work demonstrated that applying pressure is an effective method to prepare novel Li−C frameworks, and hence shed light on the search for novel carbon-based electrode materials.

Capition: Pressure induced polymerization and disproportionation of Li2C2.

碳化锂(Li2C2)是一种重要的锂电极材料，但其低的电导率限制了碳化锂的进一步应用。在新的研究中，王丽娟等人使用高压的方法使碳化锂的电导率提高了9 个数量级，并且增强后的电导率可以保留到常压。一系列原位测试表明碳化锂中的C22-离子会在高压下发生聚合反应。因此碳化锂电导率的增加应该是由于在压缩下C22-离子间间距缩短导致，而聚合反应帮助了增强的电导率可以被保留到常压。随着压力的卸载，高压下形成的聚合物会分解，表明碳骨架的电化学活性很高。