Electronic Environments and Electrochemical Properties of Lithium Storage Materials

The local electronic environments and energy storage properties of lithium electrodes are investigated through inelastic electron scattering and electrochemical measurements. Experimental and computational methods are developed to characterize the electronic structure of lithiated compounds during electrochemical cycling. An electrochemical investigation of new lithium alloys has led to a better understanding of the thermodynamics, kinetics, and mechanical properties of nanostructured materials. These studies have also inspired the development of new anode materials for rechargeable lithium batteries. One of the large controversies regarding lithium cathodes concerns the arrangement of the local electronic environments in the host material and how these environments are affected by lithium intercalation. To investigate this issue, the core edges of the 3d transitionmetal oxides were studied using electron energy-loss spectrometry. A number of techniques were developed to better understand how characteristics of the electronic structure are reflected in the core edge and near-edge structure of metal oxides. An empirical relationship is established between the transition-metal L2,3 white line intensity and the transition-metal 3d occupancy. In addition, the near-edge structure of the oxygen K-edge was used to investigate the 2p electron density about the oxygen ions. The results of these investigations were used to study charge compensation in lithiated transition-metal oxides (e.g., LiCoO2 and LiNi0.8Co0.2O2) during electrochemical cycling. These results show a large increase in state occupancy of the oxygen 2p band during lithiation, suggesting that much of the lithium 2s electron is accommodated by the anion. Ab initio calculations of the oxygen 2p partial density of states curves confirm the increase in unoccupied states that accompany lithium extraction. In contrast with the large changes observed in the oxygen K-edge, much smaller changes were observed in the transition-metal L2,3 white lines. Surprisingly, for layered LiCoO2 and Li(Ni, Co)O2, the transition-metal valence changes little during the charge compensation accompanying lithiation. These results have led to a better understanding of intercalation hosts and the role of oxygen in these layered structures. Recent demand for alternatives to graphitic carbon for lithium anodes motivated an investigation into novel binary lithium alloys. The large volume expansions associated with lithium insertion is known to generate tremendous microstructural damage, making most alloys unsuitable for rechargeable lithium batteries. Electrodes of nanostructured lithium alloys were prepared in an attempt to mitigate the particle decrepitation that occurs during cycling and to shorten diffusion times for lithium. Anodes of silicon and germanium were prepared in thin film form as nanocrystalline particles (10 nm mean diameter) and as continuous amorphous thin films (60–250 nm thick). These nanostructured materials