A reflection on lithium-ion battery cathode chemistry

Abstract

Lithium-ion batteries have aided the portable electronics revolution for nearly three decades. They are now enabling vehicle electrification and beginning to enter the utility industry. The emergence and dominance of lithium-ion batteries are due to their higher energy density compared to other rechargeable battery systems, enabled by the design and development of high-energy density electrode materials. Basic science research, involving solid-state chemistry and physics, has been at the center of this endeavor, particularly during the 1970s and 1980s. With the award of the 2019 Nobel Prize in Chemistry to the development of lithium-ion batteries, it is enlightening to look back at the evolution of the cathode chemistry that made the modern lithium-ion technology feasible. This review article provides a reflection on how fundamental studies have facilitated the discovery, optimization, and rational design of three major categories of oxide cathodes for lithium-ion batteries, and a personal perspective on the future of this important area.

Fig. 1. Positions of the redox energies relative to the top of the anion: p bands.

The top of the S²⁻:3p band lying at a higher energy limits the cell voltage to <2.5 V with a sulfide cathode. In contrast, the top of the O²⁻:2p band lying at a lower energy enables access to lower-lying energy bands with higher oxidation states and increases the cell voltage substantially to ~4 V.

Fig. 2. Discovery of three classes of oxide cathodes in the 1980s.

Layered LiCoO₂ with octahedral-site lithium ions offered an increase in the cell voltage from <2.5 V in TiS₂ to ~4 V. Spinel LiMn₂O₄ with tetrahedral-site lithium ions offered an increase in cell voltage from 3 V for octahedral-site lithium ions with Mn^3+/4+ couple to ~4 V, with an accompanying cost reduction. Polyanion oxide Li_xFe₂(SO₄)₃ offered yet another way to increase the cell voltage through inductive effect from <2.5 V in a simple oxide like Fe₂O₃ to 3.6 V, with a further reduction in cost and improved thermal stability and safety. Oxford and UT Austin, refer, respectively, to the University of Oxford and the University of Texas at Austin.

Fig. 3. Lithium-diffusion pathways with lower energy barriers in close-packed oxides.

a Two-dimensional lithium diffusion from one octahedral site to another octahedral site in the lithium plane through a neighboring empty tetrahedral site in the O3 layered LiMO₂ cathodes. b Three-dimensional lithium diffusion from one 8a tetrahedral site to another 8a tetrahedral site through a neighboring empty 16c octahedral site in the spinel cathodes.

Fig. 4. Role of counter-cations in shifting the redox energies in polyanion oxides.

a Lowering of the redox energies of the Fe^2+/3+ couple and the consequent increase in cell voltage on going from a simple oxide Fe₂O₃ to a polyanion oxide Fe₂(MoO₄)₃ and then to another polyanion oxide Fe₂(SO₄)₃ with a more electronegative counter-cation S⁶⁺ vs. Mo⁶⁺, i.e., with a more covalent S–O bond than the Mo–O bond. b Molecular orbital energy diagram illustrating the lowering of the Fe^2+/3+ redox energy in Fe₂(SO₄)₃ compared to that in the isostructural Fe₂(MoO₄)₃, due to a weakening of the Fe–O covalence by a more covalent S–O bond than the Mo–O bond through inductive effect.

Fig. 5. Challenges associated with high-nickel layered oxide cathodes and the role of cation doping.

a Schematic illustration of the dissolution and migration of transition-metal ions from the cathode to the graphite anode and the consequent catalytic formation of thick SEI layers on the graphite anode. (b) Substitution of transition-metal ions with a small amount of inert ion like Al³⁺ that makes the lattice robust by perturbing the long-range metal-metal interaction and increasing the metal-oxygen bond strength and thereby suppressing metal-ion dissolution.