Multielectron Redox and Insulator-to-Metal Transition upon Lithium Insertion in the Fast-Charging, Wadsley-Roth Phase PNb₉O₂₅

Abstract

PNb 9 O 25 , a Wadsley-Roth compound whose structure is obtained by appropriate crystallographic shear of the ReO 3 structure, is a high-power electrode material that can reach 85 % of the equilibrium capacity in 30 minutes and 67% in 6 minutes. Here we show that multielectron redox, as observed through X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, and an insulator-to-metal transition upon lithium insertion, as suggested by a number of complementary techniques, contribute to the impressive performance. Chemically tuning the tetrahedral site between phosphorus and vanadium leads to significant changes in the electrochemistry and kinetics of lithium insertion in the structure, pointing to larger implications for the use of crystallographic shear phases as fast-charging electrode materials.

High-Capacity Li+ Storage through Multielectron Redox in the Fast-Charging Wadsley–Roth Phase (W0.2V0.8)3O7

“…This has also been noted in other shear phases, for example in Li x PNb 9 O 25 where the initial doped electrons localize (albeit magnetically in that case) on the central corner-connected octahedron. 40 As stated above, W/V substitution is likely to further hinder electron transport due to disorder and potentially yield some local moment behavior in the magnetism.…”

“…[26][27][28][29][30] The utility of TiNb 2 O 7 was rediscovered by Goodenough and coworkers. 31,32 Recent studies have demonstrated that large micronsized particles of the shear structured phases Nb 12 WO 33 , 33 H-Nb 2 O 5 , 34,35 39 and PNb 9 O 25 , 40 are all capable of impressive lithium storage capacity at high cycling rates.…”

Wadsley–Roth Crystallographic Shear Structure Niobium‐Based Oxides: Promising Anode Materials for High‐Safety Lithium‐Ion Batteries

“…As shown in Figure 9c, the crystalline structure of PNb 9 O 25 is built up from nine NbO 6 octahedra corner‐share to form a 3 × 3 block in the ab plane, and these blocks are held together by corner‐sharing PO 4 tetrahedra. [ 12,52 ] It is interesting that the P atoms in PO 4 tetrahedra can also be substituted by redox‐active V to obtain VNb 9 O 25 with a similar crystal structure. [ 53 ] The lithiation mechanism of PNb 9 O 25 is a combination of three solid–solution regions (the sloping voltage profile between 3.0 and 1.62 V, 1.54 and 1.11 V, and 1.08 and 1.0 V) and two biphasic regions (two plateaus around 1.62 and 1.11 V), determined by in situ XRD (Figure 9d).…”

Fast lithium-ion conductivity in the ‘empty-perovskite’ n = 2 Ruddlesden–Popper-type oxysulphide Y2Ti2S2O5

“…Materials with Wadsley-Roth-type structures typically display an insulator-to-metal transition at low levels of lithiation, due to delocalised electrons in the conduction band, which is composed of relatively diffuse Nb 4d orbitals. 18,29,30,53,70 Y 2 Ti 2 S 2 O 5 is a medium-gap semiconductor 33 with a bandgap of $2 eV, and it has been applied as a photocatalyst for overall water-splitting under visible light. 52 Upon intercalation of Li + ions, Li x Y 2 Ti 2 S 2 O 5 is reported to behave as a semiconductor at x ¼ 0.3, with some degree of Curie contribution to the magnetic susceptibility, indicating the presence of localised S ¼ 1/2 magnetic moments, attributed to localised electrons on Ti 3+ ions.…”