Researchers from China, Taiwan and the US report developing a 2D black phosphorus composite anode that supports a high charging rate without sacrificing capacity and cycling stability. Their paper is published in the journal Science.

To be competitive with 5-min refuel time for conventional combustion engine vehicles, all-electric road vehicles require LIB cells that reach a full charge of 350 W·hour kg⁻¹ after a comparable duration. Achieving these goals requires anode materials that can be charged to a specific capacity of 350 to 400 mA·hour g⁻¹ at a charge–discharge current density of >5 A g⁻¹. To this end, it is essential to develop an electrode material simultaneously featuring high theoretical capacity along with the excellent electron conductivity and Li⁺ diffusivity that are necessary for rapid charge.

Layered black phosphorus (BP) exhibits several attractive features for high-rate, high-capacity Li storage. Through a three-electron alloying reaction with Li⁺, BP can theoretically deliver a gravimetric capacity of 2596 mA·hour g⁻¹, which is only bettered by Si (4200 mA·hour g⁻¹) and Li metal (3860 mA·hour g⁻¹).

… Studies to date show a faster Li⁺ diffusion in bulk BP than in silicon or other conventional anode materials. However, an edge atom reconstruction near the zigzag diffusion channel of BP nanoflakes hinders the kinetics of Li⁺ transfer across the surface (14). Additionally, the volume change of BP during charge–discharge cycles renders the solid–electrolyte interphase (SEI) unstable, leading to poor cycling performance.

We present a BP-graphite (BP-G) hybrid with a covalently bonded BP-G interface, to prevent edge reconstruction and ensure efficient Li⁺ insertion and diffusion, and a thin polyaniline (PANI) polymer gel coating swollen by electrolytes to prevent the continued formation and buildup of less-conductive Li fluorides and carbonates, leading to a stable SEI that is more conductive for Li⁺. The designed (BP-G)/PANI composite with optimized interface for Li⁺ conduction delivers a combination of high rate and high capacity with robust cycling stability.

—Jin et al.

In their study, the researchers found that the formation of covalent bonds with graphitic carbon restrains edge reconstruction in layered BP particles to ensure open edges for fast Li⁺ entry; the coating of the covalently bonded BP-graphite particles with electrolyte-swollen polyaniline yields a stable solid–electrolyte interphase and inhibits the continuous growth of poorly conducting Li fluorides and carbonates to ensure efficient Li+ transport.

Schematics of the (BP-G)/PANI electrode, in which the PANI coating helps retain a stable SEI (I) to prevent continuous formation of poorly conducting Li fluorides and carbonates deep into the BP-G particles (J). Jin et al.

Resources

• Hongchang Jin, Sen Xin, Chenghao Chuang, Wangda Li, Haiyun Wang, Jian Zhu, Huanyu Xie, Taiming Zhang, Yangyang Wan, Zhikai Qi, Wensheng Yan, Ying-Rui Lu, Ting-Shan Chan, Xiaojun Wu, John B. Goodenough, Hengxing Ji, Xiangfeng Duan (2020) “Black phosphorus composites with engineered interfaces for high-rate high-capacity lithium storage” Science doi: 10.1126/science.aav5842