Abstract
Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2−/Sen−) and (S2−/Sn−) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.
Highlights
Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning
As established by extensive theoretical work, the introduction of alkaline ions into the TM layer results in 2p lone pairs on the oxygen[5,6,7]. Their electronic states serve as a reservoir of electrons that can potentially participate in anionic redox processes and liberate additional capacity compared to conventional cathode materials based on cationic redox, provided that they are made accessible through TM(nd)/O(2p) hybridization owing to local distortions
In the majority of Li-rich materials, the formation of holes on oxygen leads to unstable electronic configurations, which is evidenced by structural rearrangements such as phase transitions, cationic migration, or oxygen release upon oxidation
Summary
Starting with the pristine samples, we could deduce from the variation of the XPS spectra (Supplementary Figure 15) a decrease of the binding energy difference ΔE(Ti–Ch) between the Ti 2p3/2 and the chalcogenide S 2p3/2 or Se 3p3/2 peaks with increasing x (Fig. 4a). Low-temperature (5 K) EPR measurements using echo detection reveal a strong isotropic signal centered at g = 1.98 and an anisotropic signal (gx,y = 1.97 and gz = 1.95) for both the delithiated and relithiated sample, indicating the presence of Ti3+ (Fig. 5c (inset left)), with a higher Ti3+ content for the relithiated sample, as expected To further interrogate this temperature-driven distortion deduced by EPR, we decided to complement our study by NMR, exploring the local 77Se and 7Li environments for the Li2TiSe3 sample in the temperature range from 292 to 118 K (see Supplementary information for details of fit).
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