Interfacial Effects in ε-Li_xVOPO₄ and Evolution of the Electronic Structure

Abstract

The epsilon polymorph of vanadyl phosphate ε-VOPO 4 is a promising cathode material for high-capacity Li ion batteries, owing to its demonstrated ability to reversibly incorporate two lithium ions per redox center. As lithium is inserted into the nanosized particles within the cathode, the electrochemical reaction can be largely affected by the interfacial chemistry at the nanoparticle surface. We performed X-ray photoelectron spectroscopy using both soft (XPS) and hard (HAXPES) X-rays to chemically distinguish and depth-resolve the interfacial phase transitions in ε-VOPO 4 electrodes as a function of electrochemical discharge. Our analysis shows that the second lithium reaction begins before the full incorporation of the first lithium. This results in a pronounced lithium gradient within the nanoparticles, with the ε-Li 2 VOPO 4 phase only forming near the surface. These results indicate that a disruption of the kinetics are limiting the realized capacity in our hydrothermally synthesized ε-VOPO 4. Moreover, from inspection of the valence band region, we were able to monitor the evolution of ε-VOPO 4 to ε-Li 2 VOPO 4 at the surface of our nanoparticles. These assignments are confirmed by hybrid density functional theory of the three end phases.

Bilayered VOPO4⋅2H2O Nanosheets with High‐Concentration Oxygen Vacancies for High‐Performance Aqueous Zinc‐Ion Batteries

“…For V 2p spectrum of bilayer-VOP nanosheets sample (Figure 5d), the characteristic peak located at 518.3 eV in 2p 3/2 region is assigned to V 5+ , while the nearby split peak at 517.1 eV is the evidence of formation of V 4+ . [11,41] The same resulted can be also observed from the V 2p 1/2 region. While for bulk-VOPO 4 sample, only the characteristic peaks attributed to V 5+ can be detected.…”

“…Considering the various valence change of V in VOPO 4 framework during the electrochemical reaction, the theoretical specific capacity of layered VOPO 4 ⋅2H 2 O should be higher than 300 mAh g −1 . [11,12] Nevertheless, up to date, the reported optimal capacity of VOPO 4 ⋅2H 2 O cathode in aqueous ZIBs is less than 170 mAh g −1 . The main reason should be attributed to the sluggish diffusion of Zn 2+ and strong electrostatic interactions between Zn 2+ and VOPO 4 ⋅2H 2 O host.…”

Vanadyl Phosphates AxVOPO4 (A = Li, Na, K) as Multielectron Cathodes for Alkali‐Ion Batteries

“…showed that the VOPO 4 ↔ LiVOPO 4 reaction is plagued with side reactions, possibly due to the formation of a semireversible SEI. [ 12,40 ] This is shown in Figure 12 , where the measured capacity (red squares) is much larger than the capacity due to V redox (red diamonds). They also mentioned that these side reactions are not observed in nanosized samples which did not undergo HEBM.…”

“…It was shown that due to slow Li intercalation kinetics, the second Li starts to intercalate before the intercalation of the first Li is complete, creating a Li concentration gradient within ε‐VOPO 4 nanoparticles. [ 12 ] We will show later, that even in the optimized nanocrystalline ε‐VOPO 4 /graphene material, the rate performance is limited by the slower high‐voltage process, which remains one of the major limitations of this material. [ 13 ]…”

Electrochemical Lithium and Sodium Insertion Studies on Tungsten Oxyphosphate

“…These phases demonstrate the ability to insert/de‐insert significant amount of Li/Na ions. The oxy polyanionic framework type compounds such as Li x VOPO 4 and Li x TiOXO 4 , (X= S, P and Si) have been also reported for Li insertion and de‐insertion studies . Monoclinic ϵ‐VOPO 4 phase shows insertion of two Li ions with redox couple of V 5+ /V 4+ and V 4+ /V 3+ vs. Li + /Li in the voltage window of 4.5‐2.0 V .…”