Elucidating the Origins of Hysteresis and Reaction Mechanisms of Electrode Materials for Li and Na Batteries.

Abstract

Voltage hysteresis and irreversibility upon electrochemical cycling of promising electrode chemistries are major impediments to increasing the energy density of Li ion batteries. To understand the origins of hysteresis and irreversibility, we performed a first principles study of the electronic, thermodynamic and kinetic properties of various anode materials undergoing three different types of reactions: intercalation, conversion (displacement), and alloying.

We investigate the origins of phase transformation hysteresis in electrodes of Li-ion batteries, focusing on the alloying reaction of Li with Sb. To this end, we perform a first-principles calculation of the thermodynamic and kinetic properties of Sb, Li2Sb and Li3Sb, three phases that can coexist as Li reacts with Sb. We identify a lever effect in the driving force for nucleation at high overpotentials that favors phases with large changes in Li concentration over phases that are closer in composition along the equilibrium charge or discharge path.

Electrode materials undergoing displacement and conversion reactions can achieve very high capacities, but also suffer from a variety of limitations that need to be overcome. These include a poor reversibility and large capacity losses during charge and discharge. One exception to this trend is Cu2Sb, a candidate anode material that undergoes a displacement reaction with Li, but exhibits only a limited degree of hysteresis in the voltage profile between charge and discharge. We perform a comprehensive first-principles study of Cu2Sb to elucidate the intrinsic properties of the various reaction compounds that facilitate reversibility and minimize hysteresis between discharge and charge.

We also considered the origins of irreversibility of TiO2(B), a promising intercalation compound. We found that a phase transformation from LiTi2O4(B) to anatase based LiTi2O4 is energetically possible and occurs along a strain invariant plane common to both anatase LiTi2O4 and LiTi2O4(B).

In addition to investigating the origins of hysteresis, we also studied the electrochemistry of novel intercalation host materials such as CaTi5O11 and Na3TiP3O9N for Li-ion and Na-ion battery applications, respectively.