Unexpectedly Large Contribution of Oxygen to Charge Compensation Triggered by Structural Disordering: Detailed Experimental and Theoretical Study on a Li₃NbO₄–NiO Binary System

Abstract

Dependence on lithium-ion batteries for automobile applications is rapidly increasing. The emerging use of anionic redox can boost the energy density of batteries, but the fundamental origin of anionic redox is still under debate. Moreover, to realize anionic redox, many reported electrode materials rely on manganese ions through π-type interactions with oxygen. Here, through a systematic experimental and theoretical study on a binary system of Li 3 NbO 4 −NiO, we demonstrate for the first time the unexpectedly large contribution of oxygen to charge compensation for electrochemical oxidation in Ni-based materials. In general, for Ni-based materials, e.g., LiNiO 2 , charge compensation is achieved mainly by Ni oxidation, with a lower contribution from oxygen. In contrast, for Li 3 NbO 4 −NiO, oxygen-based charge compensation is triggered by structural disordering and σ-type interactions with nickel ions, which are associated with a unique environment for oxygen, i.e., a linear Ni−O−Ni configuration in the disordered system. Reversible anionic redox with a small hysteretic behavior was achieved for LiNi 2/3 Nb 1/3 O 2 with a cation-disordered Li/Ni arrangement. Further Li enrichment in the structure destabilizes anionic redox and leads to irreversible oxygen loss due to the disappearance of the linear Ni−O−Ni configuration and the formation of unstable Ni ions with high oxidation states. On the basis of these results, we discuss the possibility of using σ-type interactions for anionic redox to design advanced electrode materials for highenergy lithium-ion batteries.

Partially Reversible Anionic Redox for Lithium-Excess Cobalt Oxides with Cation-Disordered Rocksalt Structure

“…A similar trend for structural changes, the loss of Nb ion ordering by Nb/Li enrichment, is also observed for the Li 3 NbO 4 −NiO binary system. 28 Similar primary particle morphology can be noted from SEM images in Figure 1b for all three samples with a relatively small diameter (<1 μm) without crystal facets. Because the Co 2+ /Co 3+ redox reaction results in complicated changes in spin configurations as discussed in a later section, a member of the Li 2 TiO 3 − LiCoO 2 binary system with Co 3+ ions, Li 6/5 Co 2/5 Ti 2/5 O 2 , is also synthesized for comparison.…”

“…First-principles calculations were also performed for Li 21 Co 8 Nb 7 O 36 (Li 7/6 Co 4/9 Nb 7/18 O 2 ) with the DRS structure, in which cation arrangements were optimized by a genetic algorithm as previously reported for Li 18 Ni 12 Nb 6 O 36 with the DRS structure. 28 5c). On the other hand, π-type bond formation is indicated between Co t 2g and O 2p orbitals for down-spin states (Figure 5d).…”

“…In contrast, a clear difference is noted in O K-edge spectra for SrCoO 3 and LaCoO 3 , and a pre-edge peak centered at 527 eV is observed for SrCoO 3 , which is found at a 2 eV lower energy region for LaCoO 3 . Similarly, for SrFeO 3 with ligand holes, the energy position of the O K-edge spectrum at a pre-edge region is lower (2.6 eV) than that of LaFeO 3 without ligand holes . However, the intensity of the spectrum at ca .…”

“…Because strong interaction with oxygen is expected for σ-type bonding formation, the pre-edge peak at 527 eV probably corresponds to holes stabilized by σ-type interaction in SrCoO 3 . First-principles calculations were also performed for Li 21 Co 8 Nb 7 O 36 (Li 7/6 Co 4/9 Nb 7/18 O 2 ) with the DRS structure, in which cation arrangements were optimized by a genetic algorithm as previously reported for Li 18 Ni 12 Nb 6 O 36 with the DRS structure Figure displays the calculated projected density of states (PDOS) for (a) Li 7/6– y Co 4/9 Nb 7/18 O 2 and (b) a specific oxygen atom in a linear Co–O–Co coordination with a bonding angle of 180°.…”

“…Similar to F – ions, the P–O bonds in the structure suppress the excessive oxidation of oxygen because of their strong covalent bond characteristic compared with Li–O and Mn–O bonds. More recently, we reported a new type of anionic redox mechanism created by a linear Ni–O–Ni configuration in LiNi 2/3 Nb 1/3 O 2 ( x = 0.33 in the x Li 3 NbO 4 –(1 – x )­NiO binary system), which is stabilized through σ-type interaction of the O 2p orbital with the Ni e g orbital . Interestingly, the Li-excess environment here, namely the enrichment of the Li–O–Li configuration, does not show good reversibility for anionic redox because of the increase of unstable Ni ions in the crystal structure upon delithiation, and oxygen loss is triggered by electron back-donation from oxygen to unstable Ni ions .…”