(Invited) Atomistic Insights into Cathode Dissolution and Ion Transport in Li-Ion Batteries

Abstract

Fundamental understanding of inter-connected electrochemical phenomena, including chemical reactions, charge/ion transport, defect evolution, and solvation dynamics) is crucial to accelerate development of low-cost, safe, and high-performance Li-ion battery architectures. In particular, molecular mechanisms underlying (a) dissolution of cathode into liquid electrolyte; and (b) fast Li-ion transport in novel solid-electrolytes, need to be clearly identified to design Li-ion batteries with exceptional capacity, rate-capability, and cycle life; however, such knowledge remains in its infancy. In this talk, we will first address the key mechanisms governing dissolution of a commercially promising LiMn2O4 (LMO) cathode. Using a combination of ab initio molecular dynamics simulations (AIMD) and quantum-chemical calculations, we demonstrate that dissolved Mn ions exist in 2+ oxidation state in carbonate based electrolytes. This finding resolves the debate raised by recent electron paramagnetic resonance and X-ray absorption near edge structure experiments suggesting that Mn could preferentially exist in 3+ state within solution; and provides evidence supporting the long-standing belief that Hunter’s disproportionation mechanism (2 Mn3+ --> Mn4+ + Mn2+) precedes Mn dissolution. Furthermore, we employed AIMD and density functional theory (DFT) calculations to probe formation of Mn2+ on stable LMO (511) surfaces via surface Hunter charge disproportionation mechanisms, as well as chemical mechanisms involving hydrofluoric acid (HF) electrolyte impurities and oxidation of ethylene carbonate (EC) electrolyte molecules. These studies would guide future strategies to suppress Mn dissolution, and in turn, combat capacity fading in LiBs based on LMO cathodes. Next, we will demonstrate that strongly-correlated rare-earth perovskite nickelates exhibit fast Li-ion conduction with simultaneous suppression of electronic transport via Mott transition. The pronounced Li-ion conduction is facilitated by electron injection into Ni orbitals, and concomitant lattice expansion when the Li ions are inserted from a reservoir. These results will be discussed in the context of developing next-generation lithium ion batteries with exceptional performance.