Gaofeng Teng

Sn(II,IV) steric and electronic structure effects enable self-selective doping on Fe/Si-sites of Li2FeSiO4 nanocrystals for high performance lithium ion batteries

We report Sn(II) and Sn(IV) self-selective dual-doping, respectively, on Fe and Si sites of Li2FeSiO4 nanocrystals due to the steric and electronic structure effects of Sn(II,IV). Combined with experimental studies and theoretical calculations, we investigate the structure–property relationship of tin doped Li2FeSiO4 as the cathode material for high performance Li-ion batteries, in which the dual-doping enhances the electronic conductivity and lithium-ion diffusion coefficient. The doped sample with 5% Sn(IV) source shows the best electrochemical performance due to the improved electronic conductivity and Li-ion diffusivity. Density functional theory (DFT) calculations also reveal that tin dual-doped Li2FeSiO4 has better electronic conductivity and lower voltage of delithiation than that of the undoped Li2FeSiO4, which is in accordance with our experimental results.

Role of Superexchange Interaction on Tuning of Ni/Li Disordering in Layered Li(NixMnyCoz)O2

Ni/Li exchange (disordering) usually happens in layered Li(NixMnyCoz)O2 (NMC) materials and affects the performance of the material in lithium-ion batteries. Most of previous studies attributed this phenomenon to the similar size of Ni2+ and Li+, which implies that Ni2+ should be more favorable than Ni3+ to be located at Li 3b sites in the Li slab. However, this theory cannot explain why in Ni-rich NMC materials where most Ni cations are Ni3+, Ni/Li exchange happens even more frequently. Using extensive ab initio calculations combined with experiments, here we report that a superexchange interaction between transition metals plays a dominating role in tuning the Ni/Li disordering in NMC materials. Under this scheme, we also propose a new charge compensation mechanism that describes that after Ni3+/Li exchange the nearest Co3+ transforms to Co4+ in Ni-rich NMC materials. On the basis of this theory, the existence of Co4+ in the initial Ni-rich NMC samples was predicted for the first time, which was further confirmed by our synchrotron-based soft X-ray absorption spectroscopy.

Cationic Ordering Coupled to Reconstruction of Basic Building Units during Synthesis of High-Ni Layered Oxides

Metal (M) oxides are one of the most interesting and widely used solids, and many of their properties can be directly correlated to the local structural ordering within basic building units (BBUs). One particular example is the high-Ni transition metal layered oxides, potential cathode materials for Li-ion batteries whose electrochemical activity is largely determined by the cationic ordering in octahedra (e.g., the BBUs in such systems). Yet to be firmly established is how the BBUs are inherited from precursors and subsequently evolve into the desired ordering during synthesis. Herein, a multimodal in situ X-ray characterization approach is employed to investigate the synthesis process in preparing LiNi0.77Mn0.13Co0.10O2 from its hydroxide counterpart, at scales varying from the long-range to local individual octahedral units. Real-time observation corroborated by first-principles calculations reveals a topotactic transformation throughout the entire process, during which the layered framework is retained; however, due to preferential oxidation of Co and Mn over Ni, significant changes happen locally within NiO6 octahedra. Specifically, oxygen loss and the associated symmetry breaking occur in NiO6; as a consequence, Ni2+ ions become highly mobile and tend to mix with Li, causing high cationic disordering upon formation of the layered oxides. Only through high-temperature heat treatment, Ni is further oxidized, thereby inducing symmetry reconstruction and, concomitantly, cationic ordering within NiO6 octahedra. Findings from this study shed light on designing high-Ni layered oxide cathodes and, more broadly, various functional materials through synthetic control of the constituent BBUs.

Mechanism of Exact Transition between Cationic and Anionic Redox Activities in Cathode Material Li2FeSiO4

The discovery of anion redox activity is promising for boosting the capacity of lithium ion battery (LIB) cathodes. However, fundamental understanding of the mechanisms that trigger the anionic redox is still lacking. Here, using hybrid density functional study combined with experimental soft X-ray absorption spectroscopy (sXAS) measurements, we unambiguously proved that Li(2- x)FeSiO4 performs sequent cationic and anionic redox activity through delithiation. Specifically, Fe2+ is oxidized to Fe3+ during the first Li ion extraction per formula unit (f.u.), while the second Li ion extraction triggered the oxygen redox exclusively. Cationic and anionic redox result in electron and hole polaron states, respectively, explaining the poor conductivity of Li(2- x)FeSiO4 noted by previous experiments. In contrast, other cathode materials in this family exhibit diversity of the redox process. Li2MnSiO4 shows double cationic redox (Mn2+-Mn4+) during the whole delithiation, while Li2CoSiO4 shows simultaneous cationic and anionic redox. The present finding not only provides new insights into the oxygen redox activity in polyanionic compounds for rechargeable batteries but also sheds light on the future design of high-capacity rechargeable batteries.