Effects of Ti Doping on the Structural Stability and Enhanced Electrochemical Performance of α‐LiVOPO₄

Abstract

Lithium vanadyl phosphate has seven different crystallographic phases. Among them, α-LiVOPO 4 is chosen because of its relatively high energy density and redox voltage of 3.9 V compared to the other phosphates. In this study, Ti 4+ -doped α-LiV 1−x Ti x OPO 4 (x = 0.00, 0.003, 0.005, 0.007, and 0.01) was prepared using a sol-gel method to increase the structure stability and electrochemical performance. The triclinic structure with the space group P-1 was confirmed by X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Inductively coupled plasma-optical emission spectroscopy was conducted to determine the precise state. The shape and size of the particles were observed by field-emission scanning electron microscopy. In situ X-ray absorption spectroscopy was performed to confirm the structural behavior during the electrochemical reaction. Electrochemical measurements such as cyclic voltammetry and galvanostatic charge-discharge were conducted. α-LiV 1−x Ti x OPO 4 (x = 0.003, 0.005, 0.007, and 0.01) showed structural stability during cycling as well as decreased polarization during charge and discharge with the increased diffusion coefficient of lithium ions. α-LiV 0.995 Ti 0.005 OPO 4 showed the best cycling stability and rate capability among all the samples examined because Ti doping retained its site to prevent structural collapse.

Structure, Composition, and Electrochemistry of Chromium-Substituted ε-LiVOPO4

“…29−32 substituted ε-LiVOPO 4 compared to the unsubstituted sample, while Xiong et al observed a reduction in the charge-transfer resistance for 4% Mn-substituted β-LiVOPO 4 . 29,30 An increased Li-ion diffusivity was observed for Zr-substituted β-LiVOPO 4 . 31 Despite the claimed success of substitution in these studies, the evidence is based only on X-ray photoelectron spectroscopy, which is surface sensitive, or inductively coupled plasma (ICP)-based spectroscopy, which measures all solid phases in the sample including amorphous and impurity phases.…”

“…Common strategies to improve the reaction kinetics and electrochemical stability at high voltages include the use of a conductive additive, surface coating, elemental substitution, and nanosizing. − ,− Among these approaches, aliovalent substitution has been shown to enhance the electrochemical performance of olivine-based phosphate cathodes by producing structural defects which alter their intrinsic properties for faster Li-ion diffusion. − Similarly, previous studies have reported improved electrochemical performance for cation-substituted LiVOPO 4 synthesized via a high-temperature solid-state method. − Lee and Ryu demonstrated a reduced voltage hysteresis and an improved retention rate for 0.5% Ti-substituted ε-LiVOPO 4 compared to the unsubstituted sample, while Xiong et al observed a reduction in the charge-transfer resistance for 4% Mn-substituted β-LiVOPO 4 . , An increased Li-ion diffusivity was observed for Zr-substituted β-LiVOPO 4 . Despite the claimed success of substitution in these studies, the evidence is based only on X-ray photoelectron spectroscopy, which is surface sensitive, or inductively coupled plasma (ICP)-based spectroscopy, which measures all solid phases in the sample including amorphous and impurity phases.…”

Probing how Ti- and Nb-substitution affect the stability and improve the electrochemical performance of β- and ε-LiVOPO4

“…Although there have been several reports on transition-metal substitution into (Li)VOPO 4 , 38–41 they all focus on intercalating only a single Li + ion, with the exception of Wen et al , who showed multi-Li cycling in Mo-substituted ε-VOPO 4 . 38 Additionally, all these studies focus solely on improving either β- or ε-LiVOPO 4 , but not both.…”

“…A separate study has also reported the use of Ti-substitution into LiVOPO 4 . 40 However, the study only focused on ε-LiVOPO 4 and did not take into consideration the effects of the large β-LiVOPO 4 impurity present nor the insertion of the second Li + ion to its electrochemical performance.…”

“…Thus, we propose the use of another technique to improve the electrochemical performance of LiVOPO 4 : transition metal substitution. [32][33][34][35][36][37][38] Although there have been several reports on transition-metal substitution into (Li)VOPO 4 , [38][39][40][41] they all focus on intercalating only a single Li + ion, with the exception of Wen et al, who showed multi-Li cycling in Mo-substituted 3-VOPO 4 . 38 Additionally, all these studies focus solely on improving either bor 3-LiVOPO 4 , but not both.…”