In situ TEM Observation of the Electrochemical Progress of Black Phosphorus Anode for Lithium Ion Battery

Weiwei Xia, Feng Xu, Qiubo Zhang, Jing Chen, Chongyang Zhu

SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast University, Xuanwu District, Nanjing, China. [email protected]

Abstract Black phosphorus (BP) with special puckered double-layer structure, has drawn growing attention as the anode material for lithium ion batteries (LIBs) because of its high theoretical capacity. However, the fundamental mechanisms of BP LIBs are still unclear completely due to the lack of direct observation. Here a nano-battery is constructed inside the transmission electron microscopy (TEM) to in situ observe the electrochemistry behavior of BP electrode in first lithiation/delithiation process for the first time. Upon lithiation, BP shows obvious morphological change and anisotropic size expansions along two directions. The electrode is subject to phase change from orthorhombic BP to amorphous compounds LixPy. Unexpectedly, the product after discharging is polycrystalline Li3P rather than BP, resulting in the irreversible electrochemical process. Meanwhile, cracking, which will lead to the loss of electric contact and rapid capacity fading, appears in the electrode accompany with huge size expansion at the discharge state. Our experiment results afford profound insights into the very different lithiation/delithiation mechanism of BP anode during first cycle, and give direct evidence to interpret the underlying failure mechanism of BP LIBs.

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Figures

(a) (b) (c) (d) (i)

BP

Li/Li2O (e) (f) (g) (h) (j)

Figure 1. Time-resolved TEM images from video frames show the electrochemical behaviors of BP sheet during first charging/discharging process. (a) Pristine BP LIB device; (b)-(e) Charging process; (f)-(h) Discharging process. A constant negative/positive potential is applied to working electrode to initiate the charging/discharging process. The working electrode material-BP sheet suffers from huge anisotropic size expansion in first cycle. The existing of cracks at the discharge state would lead to the loss of electric contact and the resulting rapid capacity fading. (i) Electron diffraction pattern of the working electrode after lithiation process. Diffraction spots stand for the unlithiated BP (JCPDS no.74-1878), and the diffused rings indicate the product after lithiation process is amorphous LixPy. (j) Electron diffraction pattern of the working electrode after discharging process. The predominant diffraction rings can be indexed as the hexagonal Li3P (JCPDS no.74-1160) and the possible BP (JCPDS no.74-1878). The existing of BP after discharging may only come from the unlithiated BP as the lattice shrink caused by the delithiation behavior is scarcely possible to happen. The product after discharging is Li3P, rather than BP, would result in the highly irreversible of lithiation/delithiation process and extremely low first cycle coulomb efficiency. The electrochemical behaviors during first cycle are very different to the materials that have been reported previously, which give the direct evidences to explain the failure mechanism of BP LIBs.