Gadolinium-Doped LiMn2O4Cathodes in Li Ion Batteries: Understanding the Stabilized Structure and Enhanced Electrochemical Kinetics

Gadolinium-doped LiGdxMn2−xO4 was synthesized by a sol-gel method and applied as cathode material in Li ion batteries. Gd-doping significantly changed the unit cell parameter and atomic arrangement. With an increase of x, a cell dimension contracts from 8.2419(2) to 8.2375(8) (x = 0.02), 8.2295(3) (x = 0.04), and 8.2120(6) Å (x = 0.08), with concomitant changes in bond length. The LiO4 tetrahedron expands to facilitate a fast electrochemical process and the MnO6 octahedron shrinks to provide structural integrity. The former contributes to a better high-rate performance and the latter leads to a variation in the cell dimensions during charge-discharge cycles that is less than that found in LiMn2O4 (structure-stabilization). The strong octahedral framework eventually results in minimized contraction of the unit cell after repeated cycles and in good capacity retention. A less capacity fading of LiGd0.02Mn2.98O4 can be correlated to a smaller dimensional variation between charged and discharged states (LiMn2O4: 8.1366 and 8.0460 Å (two phases, charged) and 8.2419 Å (discharged) vs. LiGd0.02Mn2.98O4: 8.1472 Å (single phase, charged) to 8.2375 Å (discharged)) and, thus, the less unit cell contraction for LiGd0.02Mn2.98O4 after repeated cycling (LiMn2O4: 8.2419 Å to 8.218 Å vs. LiGd0.02Mn2.98O4: 8.2375 Å to 8.229 Å).