Behavior Investigation of Oxide Electrodes for Li Ion Batteries Viewing from Charge Compensation

Abstract

Charge compensation resulted defect reactions have played important role on affecting electrical and structural properties of oxides. Depending upon the types of ions available, ionic defects and/or electronic defects are created. For instance, ionic defects tend to enhance the migration of cations/anions. Electronic defects including electrons or hole defects may enhance the electronic conductivity of oxides. For Li batteries, the lithiation of transition metal oxides may be viewed as substitution of Li for transition metal ions. As a result, the lithiation induces the oxidation of Ni, Co ions. Further lithiation may also cause the structural transformation from rock-salt oxide to layered oxides. During a charging/delithiation process at cathode, electron extraction and Li removal from the Li sublattice may be viewed as the creation of electronic/ionic defects. On the contrary, opposite defect reactions including electron injection (or reduction) and lithiation may be observed from the anode. For a high-capacity cathode, Li-rich layered oxides formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) show a unique charging plateau around 4.5V. The extraction of electrons is given by rearranged oxygen ions during 1st charging process. In the following discharging as seen in Fig. 1, reduction of Mn+4 from accepting e’ gave a reversible capacity as high as 250 mAh/g. Interestingly, a backward defect reaction was observed in Li4Ti5O12 (LTO) spinel anode during the charging/lithiation process. A self-improving behavior may be explained through accompanying defect reaction. During low-pO2 synthesis, lattice oxygen near surface of LTO tends to be removed with the introduction of Ti+3. As a result, the electron conduction and electrochemical properties of LTO were significantly improved. Finally, the structure transformation of cycled Li-rich oxides from Layered to Spinel structure may be illustrated due to the formation high concentration electronic defects.