PO₄³⁻ polyanion-doping for stabilizing Li-rich layered oxides as cathode materials for advanced lithium-ion batteries

Abstract

PO43− polyanion-doped Li-rich layered oxides offer excellent energy density retention during long cycling due to the stronger anion bonding of PO43− polyanions to transition metal cations.

Surface Structural Transition Induced by Gradient Polyanion‐Doping in Li‐Rich Layered Oxides: Implications for Enhanced Electrochemical Performance

“…This can be ascribed to the PO 4 3− polyanions, with higher electronegativity, that alter the local electronic structure of the layered oxides. [30] Moreover, at a concentration of 7 at% PO 4 3− , an impurity phase of Li 3 PO 4 appears (Figure 1a), which is consistent with observations in a previous report. [30] The electrochemical behavior of the materials with and without PO 4 3− doping was evaluated using galvanostatic charge-discharge testing.…”

“…[30] Moreover, at a concentration of 7 at% PO 4 3− , an impurity phase of Li 3 PO 4 appears (Figure 1a), which is consistent with observations in a previous report. [30] The electrochemical behavior of the materials with and without PO 4 3− doping was evaluated using galvanostatic charge-discharge testing. Figure 2a compares the initial charge-discharge curves of the pristine and PO 4 3− -doped LLO materials at a current density of 0.1 C (25 mA g −1 ) from 2.0 to 4.8 V at room temperature.…”

“…Incorporation of phosphate leads to slight shifts to higher binding energies for the Mn/Co cations, suggesting that the average valence of Mn/Co cations increases in order to balance the negatively charged PO 4 3− . [30] For the PO 4 3− -doped material, the P 2p peak is detected at 133.7 eV ( Figure S5, Supporting Information), corresponding to the presence of phosphate polyanions. [42] To confirm that gradient polyanion-doping occurs in the material surface, XPS depth profiles were collected (Figure 6d-f Depletion of Mn is associated with the enrichment of Ni and P and is evidence of doping in the surface region (as marked by the dotted box) for the PO 4 3− -doped samples (Figure 6d-f and Table S5, Supporting Information).…”

“…It is apparent, in the PO 4 3− -doped materials, that both lattice parameters a and c increase steadily as the doping level is raised. [30] As shown in Figure 1c, additional XRD peaks in the 2-theta range of 20-22° are considered as superlattice reflections of the Li 2 MnO 3 -like phase. As the PO 4 3− doping level reaches 5 at% or higher, sharper and stronger (020) peaks are observed when compared with the pristine material ( Figure 1c).…”

“…[19,20] The cationic doping with other metallic cations (such as Mg, [21] Al, [22] Ti, [23] Sn, [24] Ru, [25] Y, [26] Zn, [27] etc.) and polyanion doping based on nonmetal elements, such as BO 4 5− , [28] SiO 4 4− , [29] PO 4 3-, [30] etc., have been employed to improve the cyclic durability by weakening the TM-O covalency in the oxygen closepacked structure. In addition, surface coatings using metal oxides, [31][32][33][34] fluorides and phosphates, [35][36][37] LiNiPO 4 and Li 3 VO 4 , [38][39][40] have been applied to protect the surface structure from side reactions with the electrolyte under high voltage and to restrain the layered-to-spinel transformation which occurs preferentially on the crystal surface and leads to capacity fading of LLO materials.…”