Fluorine and Copper Codoping for High Performance Li₂O-Based Cathode Utilizing Solid-State Oxygen Redox

Abstract

Transition-metal-doped Li2O cathodes using redox reaction of solid-state oxygen are candidates as high capacity cathode materials for lithium-ion batteries. In our previously reported study, copper-doped Li2O (CuDL) exhibited a high charge–discharge capacity of 300 mAh g–1. However, the developed cathode not only exhibited poor cyclability but also decomposed during charge–discharge cycles. In this study, fluorine and copper were codoped into the Li2O structure by stepwise mechanochemical reaction to create a high performance cathode with high cyclability. Fluorine-, copper-doped Li2O (F–CuDL) was prepared by the mechanochemical reaction of Li2O with LiF, followed by a reaction with CuO. The F–CuDL cathode exhibited a good cycle performance (300 mAh g–1, 30 cycles at a constant current of 50 mA g–1). X-ray diffraction (XRD) and Cu K-edge X-ray absorption near edge structure (XANES) analyses revealed that F–CuDL does not undergo decomposition even after charge–discharge cycles, revealing that doping with fluorine leads to the stabilization of the CuDL crystal structure.

The Intriguing Nature of Fluorine Doping on Li2CuO2 and the Reasons Behind the Inhibition of Oxygen Evolution

“…Given that the molar quantities of CuF 2 added in the synthesis are relatively low, 19 F very fast magic angle spinning NMR (VFMAS-NMR) was carried out to verify the presence of fluoride in the F-doped samples. 19 F VFMAS-NMR of LiF and CuF 2 was first obtained, (Figure S2), and the LiF sample exhibited a peak centered at −211.79 ppm, consistent with what has been reported by Shimada et al 25 Whereas, in the 19 F NMR spectrum of CuF 2 , two signals are observed at −212.08 ppm, associated with HF adsorbed on the surface, and at −125 ppm, attributed to impurities in the raw material (Figure S2 (a)). Further, given the paramagnetic nature of copper(II), a signal associated with F bonded to Cu in CuF 2 is not expected.…”

Cation-Disorder-Assisted Reversible Topotactic Phase Transition between Antifluorite and Rocksalt Toward High-Capacity Lithium-Ion Batteries

“…These prominent peaks are attributable to 111, 220, and 311 reflection peaks of the cubic antifluorite phase, with a lattice parameter of 4.64 Å. It may be suggested that Li 6 CoO 4 decomposes to Li 2 O and CoO; however, this XRD evidence confirms the absence of Li 2 O, given that the lattice parameter of ball-milled Li 2 O is reported to be about 4.615 Å . Co K -edge X-ray absorption near edge structure (XANES) spectra also rule out the decomposition (see Figure d,e).…”

“…It may be suggested that Li 6 CoO 4 decomposes to Li 2 O and CoO; however, this XRD evidence confirms the absence of Li 2 O, given that the lattice parameter of ball-milled Li 2 O is reported to be about 4.615 Å. 53 Co K-edge X-ray absorption near edge structure (XANES) spectra also rule out the decomposition (see Figure 1d,e). Both in the pre-edge region and in the white line of the spectrum, the trend exhibited by BM-LC matches well that of P-LC and is observed to be quite different from that of CoO.…”

“…Antifluorite structure-based oxides such as Li 5 FeO 4 and Li 6 CoO 4 also have much potential as high-capacity cathode materials because of their rich lithium content. Their cathode performances were first reported in 1999, and subsequently, the electrochemical performances of these types of species and the analyses of the relevant redox reactions have been widely reported. − In Li 5 FeO 4 , the transformation to disordered rocksalt Li 3 FeO 3.5 proceeds, whereas in Li 6 CoO 4 , the transformations to Li 4 CoO 4 , or CoO 2 are feasible, by the extraction of two Li ions. However, in each case, reversibility is poor, resulting in discharge capacity contributed by a one-electron reaction, which is due to only TM redox.…”

Universal solid-state oxygen redox in antifluorite lithium oxidesviatransition metal doping

“…Recently, the cyclability of CuDL has been improved using fluorine doping. 57 Doping other elements such as fluorine in cationic and anionic sites potentially can enhance the cathode performances of TMDLs. In addition, the surface modification of CoDL by vinylene carbonate or fluoroethylene carbonate shows excellent capacity improvement from 270 to 450 mA h g À1 while retaining its cyclability.…”