Understanding Phase Transformations in Lithium Battery Materials by Transmission Electron Microscopy

Abstract

16.1. INTRODUCTION Downsizing of electronic components demands energy storage systems with high energy and power densities, and thus continuously drives battery research and development efforts. The most commonly used batteries in portable computers and cellular phones are lithium rechargeable batteries 1 . In these batteries, chemical energy stored in the positive electrode is released and converted into electrical energy during discharge through an intercalation (insertion) process by which lithium ions are incorporated within the host structure of the positive electrode material. During charge, the process is reversed (de-intercalation) and electric al energy is applied to remove lithium ions from the positive electrode. Lithium transition metal oxides that host mobile lithium ions on the interstitial sites have been studied as positive electrode materials and the intercalation and de-intercalation processes of lithium ions are accompanied by redox of transition metal ions. The energy output of lithium rechargeable batteries is dependent on the voltage upon which lithium ions are inserted and the number of interstitial sites that can accommodate lithium ions in the host structure. Ideally , intercala tion and de-intercalation of lithium ions and redox of transition metal ions, should leave the host structure intact. In practice, however, variation in lithium content in the host structure leads to lattice expansion or contraction, migration of transition metal ions and local or global lattice distortion as a result of ordering of lithium and vacancies or Jahn-Teller distortion of transition metal ions. These structural changes can 1) induce stresses and strains within lithium transition metal oxide crystals, 2) affect the electronic conductivities of lithium battery materials, and 3) lead to irreversible phase changes and thus decrease in energy outputs of lithium batteries. Therefore, much research has been focused on characterizing structural changes of lithium transition metal