Role of Metal-Lithium Oxide Interfaces in the Extra Lithium Capacity of Metal Oxide Lithium-Ion Battery Anode Materials

Abstract

Lithium storage capacities beyond the stoichiometric limit of conversion have been reported for the transition metal oxides MnO, CoO, NiO, CuO, and beyond the alloying limit for the main group metal oxide SnO. This work examines the molecular mechanisms responsible for this extra capacity using first principles plane wave density functional theory with periodic boundary conditions. Energies of the optimized structures were employed to construct first discharge curves. Analysis of lithiated structures of transition metal oxides with moderate lithium content revealed the formation of two different phases within the electrode material, namely bulk metal and lithium oxide, Li 2 O. At higher lithium content, the interfacial region between the two phases was found to expand, giving rise to "free volume" for additional lithium storage. MnO had the highest free volume in the interfacial region for lithium storage, followed by CoO, NiO, CuO, and SnO. Of particular interest was CuO, which upon lithiation, formed linear and branched chains of Cu atoms that percolated throughout a large portion of the system. This behavior suggests that CuO may better maintain conductivity through the anode material for more cycles than the other transition metal oxides examined. Lithium ion batteries (LiBs) are commonly used as sources of power in portable electronics because of their high gravimetric capacity and power density, owing to the lighter molecular weight of lithium.1 Commercially available LiBs typically use graphite as the anode, LiCoO 2 as the cathode, and LiPF 6 in alkyl carbonate solvents as the electrolyte.2 The choice of materials for different components of LiBs, such as electrodes, electrolyte, current collectors, and the interactions between them, determine the overall battery performance. Increased energy demands have intensified the efforts to improve commercially available LiBs, especially to power electric vehicles and for load leveling applications.3 Materials with higher capacities than that of graphite and LiCoO 2 have been studied as electrode materials to improve the energy density of LiBs. 4,5 Different classes of materials that have been synthesized and tested for their electrochemical performance as prospective anode materials can be broadly divided into three classes based on their reactivity with lithium: intercalation, alloying, and conversion materials.6 Apart from carbon-based compounds, oxides of titanium that store lithium by means of intercalation have been studied because of their stability, wide availability, low cost, and good reversible capacity at an operating potential of 1.5 V vs Li/Li + . 7 But their theoretical capacity, which is lower than that of graphite (372 mAh g -1 ), and poor electronic conductivity have limited their application in high energy density LiBs. Alloying anode materials such as Si, Sb, Sn, SiO, SnO 2 , and Ge have higher theoretical capacities than graphite, but are limited by high volume expansion and high irreversible capacity losses after first cycle. 8Research has be...

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