Phase Transformation of Doped LiCoPO4 during Galvanostatic Cycling.

Wen Zhu,Catherine Gagnon,Dongqiang Liu,Karim Zaghib,Vincent Gariépy,Michel L Trudeau,Ashok Vijh

doi:10.3390/ma13173810

Abstract

In situ X-ray diffraction was employed to investigate the crystal structure changes in Cr/Si co-doped Li(Co,Fe)PO4 cathode material during a galvanostatic charge/discharge process at a slow rate of C/30. The evolution of the X-ray patterns revealed that the phase transformation between the Cr/Si-Li(Co,Fe)PO4 and Cr/Si-(Co,Fe)PO4 is a two-step process, which involves the formation of an intermediate compound of Cr/Si-Li0.62(Co,Fe)PO4 upon the extraction of Li ions from the pristine phase. Different from the previously reported two biphasic transition steps, the phase transformation of the Cr/Si-Li(Co,Fe)PO4 followed a solid solution and a biphasic reaction pathway at different stages of the delithiation/lithiation process, respectively.

Highlights

LiFePO4 (LFP) is a cathode material characterized by structural and thermal stability, low cost, and high safety, as well as a low operating voltage (3.5 V) and low conductivity [1,2,3]
A two-phase reaction mechanism was first proposed for LFP, owing to the narrow single-phase ranges near the stoichiometry compositions of LFP and FePO4 (FP)
Later studies [7,20,21,22] introduced a stepwise phase separation mechanism by monitoring the phase evolution during cycling via the in situ synchrotron and in situ neutron diffraction techniques. This mechanism involved an intermediate phase of Li0.6–0.7 CoPO4, and the transformation went through two biphasic regions: (1) LiCoPO4 ↔ Li0.6 CoPO4 + 0.4Li+ + 0.4 e− ; (2) Li0.6 CoPO4 ↔ CoPO4

Summary

Introduction

LiFePO4 (LFP) is a cathode material characterized by structural and thermal stability, low cost, and high safety, as well as a low operating voltage (3.5 V) and low conductivity [1,2,3]. LiCoPO4 (LCP) is of great interest, owing to its large theoretical specific capacity (167 mAh g−1 ) and high operating voltage 4.8 V (vs Li/Li+ ) These properties result in a specific energy of ∼800 Wh kg−1 (i.e., ~25% higher than that of conventional cathodes in Li-ion batteries [5]). Later studies [7,20,21,22] introduced a stepwise phase separation mechanism by monitoring the phase evolution during cycling via the in situ synchrotron and in situ neutron diffraction techniques This mechanism involved an intermediate phase of Li0.6–0.7 CoPO4 , and the transformation went through two biphasic regions: (1) LiCoPO4 ↔ Li0.6 CoPO4 + 0.4Li+ + 0.4 e− ; (2) Li0.6 CoPO4 ↔ CoPO4. The results confirmed the appearance of an intermediate phase, Cr/Si-Li0.62 (Co,Fe)PO4 , and revealed the concurrence of the solid solution and two-phase pathways during the charge/discharge process, for the first time

Experimental

Discussion

Refined

Variation

Conclusions