In Situ Studies of Li/Cu-Doped Layered P2 NaxMnO2 Electrodes for Sodium-lon Batteries

By understanding how structural transitions affect the electrochemical performance of a battery, better electrode materials for reversible Na+ insertion/extraction can be developed. Here, the structural evolution of the recently synthesized P2 phases of Na0.7Li0.1Mn0.9O2 and Na2/3Cu1/3Mn2/3O2 using high-resolution in situ synchrotron X-ray diffraction experiments is reported and the evolution relative to the parent P2 Na0.7MnO2 is directly compared. All electrodes feature solid solution and two-phase reactions during charge/discharge/charge processes. Using a simplified single-phase evolution for comparative purposes, the P2 Na0.7Li0.1Mn0.9O2 and Na2/3Cu1/3Mn2/3O2 show volume changes of 1.909(1) and 1.13(3) angstrom(3) of the major phase, respectively, during charge/discharge while the parent P2 Na0.7MnO2 shows an overall volume change of 0.67(0) angstrom(3) for P2 and 61.9(1) angstrom(3) for orthorhombic phase. The maximum volume for P2 Na0.7Li0.1 Mn0.9O2 of 81.094(6) angstrom(3) is observed at 2.35 V during discharge while for P2 Na2/3Cu1/3Mn2/3O2 of 81.753(6) angstrom(3) is observed at the discharged state (1.5 V). Compared to the P2 Na0.7MnO2 the number of phase transitions experienced by the P2 Na0.7Li0.1Mn0.9O2 and P2 Na2/3Cu1/3Mn2/3O2 are significantly smaller. Li/Cu-doped samples generally show better cycle performance, and these results illustrate the structural response such substitutions have on the Na+ insertion extraction of P2 Na0.7MnO2 during cycling.