Abstract
Li-rich rocksalt oxides are promising candidates as high-energy density cathode materials for next-generation Li-ion batteries because they present extremely diverse structures and compositions. Most reported materials in this family contain as many cations as anions, a characteristic of the ideal cubic closed-packed rocksalt composition. In this work, a new rocksalt-derived structure type is stabilized by selecting divalent Cu and pentavalent Sb cations to favor the formation of oxygen vacancies during synthesis. The structure and composition of the oxygen-deficient Li4CuSbO5.5□0.5 phase is characterized by combining X-ray and neutron diffraction, ICP-OES, XAS, and magnetometry measurements. The ordering of cations and oxygen vacancies is discussed in comparison with the related Li2CuO2□1 and Li5SbO5□1 phases. The electrochemical properties of this material are presented, with only 0.55 Li+ extracted upon oxidation, corresponding to a limited utilization of cationic and/or anionic redox, whereas more than 2 Li+ ions can be reversibly inserted upon reduction to 1 V vs Li+/Li, a large capacity attributed to a conversion reaction and the reduction of Cu2+ to Cu0. Control of the formation of oxygen vacancies in Li-rich rocksalt oxides by selecting appropriate cations and synthesis conditions affords a new route for tuning the electrochemical properties of cathode materials for Li-ion batteries. Furthermore, the development of material models of the required level of detail to predict phase diagrams and electrochemical properties, including oxygen release in Li-rich rocksalt oxides, still relies on the accurate prediction of crystal structures. Experimental identification of new accessible structure types stabilized by oxygen vacancies represents a valuable step forward in the development of predictive models.
Highlights
The development of high-energy density cathode materials for Li-ion batteries has opened a large field of investigation for solidstate chemists to propose new materials and new energy storage concepts
Li4CuSbO5.5 can be prepared by a simple ceramic method starting from Li2CO3, CuO and Sb2O3 in proportions Li:Cu:Sb = 4.4:1:1 and heating the hand-ground mixture at 1100 °C for 12 h under air (Figure 1a, orange)
The final cation stoichiometry was confirmed by inductively coupled plasma optical emission spectroscopy (ICP-OES) to be
Summary
The development of high-energy density cathode materials for Li-ion batteries has opened a large field of investigation for solidstate chemists to propose new materials and new energy storage concepts. The discovery of the reversible redox activity of oxygen anions in the so-called “Li-rich” rocksalt oxides with formula Li1+xM1−xO2 (M = transition metal) represents an important paradigm shift in that field.[1,2] The possibility to extract electrons from oxygen in addition to the transition metals gives rise to a large increase in the specific capacity of these materials compared with stoichiometric LiMO2 layered oxides such as LiCoO2 or the NMC phases Li(Ni1−x−yMnxCoy)O2. This increase in capacity is often accompanied by several shortcomings linked to stabilization mechanisms for oxide anions upon their oxidation. Research efforts are focused on understanding how the redox activity of the anion sublattice can be controlled and finely tuned through appropriate choice of cations, including redox active transition metals and redox inactive elements (alkali, alkaline earth, early transition metals with no d electrons or p-block elements)
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