(Invited) The Challenge of High Energy Density in Li/Na-Ion Batteries

Abstract

Anionic redox in Li-rich and Na-rich transition metal oxides (A-rich-TMOs) has emerged as a new paradigm to increase the energy density of rechargeable batteries. [1,2] However, most A-rich-TMOs reported so far with high energy density due to anionic redox are prone to structural instabilities that prevent their use in practical applications. [3] Several alternatives have been proposed to limit or suppress these instabilities, among them the increase of M–O bond covalency, [4] the chemical substitution of M for d 0 metals, [5] the control of cationic migration, [6] or the use of cation-disordered rocksalt structures. [7] The unified picture developed in this introduces the number of holes per oxygen (h O) as another critical parameter to sustain a reversible anionic capacity. From a representative set of materials reported in the literature, h O = 1/3 seems to be the critical value to avoid O2 release and achieve fully reversible anionic redox. [8] Measurable quantities such as ΔCT, x stoichiometry and disorder are therefore sufficient to determine h O and to infer the electrochemical behaviour of A[AxM1−x]O2 electrodes. Within this general framework, the best candidates for high energy density are those exhibiting the an homogeneous O-network (no cation disorder), a (1-x)e- cationic capacity and an anionic capacity limited to h O ≤ 1/3 on each individual O-sublattice. Tridimensional structures should be preferred over layered structures to prevent structural instability at low A content and take advantage of the full theoretical capacity. Ordered structures should favour homogeneous O networks and prevent one O sublattice to uptake a critical h O > x. Very few TMOs should satisfy these requirements, in particular when they also have to meet industrial specifications such as cost, toxicity and natural abundance. Overall, strategies devoted to the activation of anionic redox as a lever to improve the energy density of electrode materials are risky, as they implicitly question the general chemical rules to guarantee structural stability of oxide-materials. The triptych of high potential, high capacity and high structural stability still appears out of our current reach and calls for trade-offs. Reference s : [1] Koga, H. et al. Reversible oxygen participation to the redox processes revealed for Li1.20Mn0.54Co0.13Ni0.13O2. J. Electrochem. Soc. 160, A786–A792 (2013).[2] Sathiya, M. et al. Reversible anionic redox chemistry in high-capacity layered-oxide electrodes. Nat. Mater. 12, 827–835 (2013).[3] Assat, G. et al. Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium-rich cathodes. Nat. Commun. 8, 2219 (2017).[4] Pearce, P. E. et al. Evidence for anionic redox activity in a tridimensional- ordered Li-rich positive electrode β-Li2IrO3. Nat. Mater. 16, 580–587 (2017).[5] Yabuuchi, N. et al. High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure. Proc. Natl Acad. Sci. USA 112, 7650–7655 (2015).[6] Gent, W. E. et al. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nat. Commun. 8, 2091-1–12 (2017).[7] Urban, A., Matts, I., Abdellahi, A. & Ceder, G. Computation design and preparation of cation-disordered oxides for high-energy-density Li-ion batteries. Adv. Energy Mater. 6, 1600488-1–1600488-8 (2016).[8] Ben Yahia, M.; Vergnet, J., Saubanère, M., Doublet, M.-L., Unified picture of anionic redox in Li/Na-ion batteries. Nat. Mat. 18, 496-502 (2019).