In Situ Studies of Li/Cu‐Doped Layered P2 Na x MnO 2 Electrodes for Sodium‐Ion 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 Na 0.7 Li 0.1 Mn 0.9 O 2 and Na 2/3 Cu 1/3 Mn 2/3 O 2 using high‐resolution in situ synchrotron X‐ray diffraction experiments is reported and the evolution relative to the parent P2 Na 0.7 MnO 2 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 Na 0.7 Li 0.1 Mn 0.9 O 2 and P2 Na 2/3 Cu 1/3 Mn 2/3 O 2 show volume changes of 1.909(1) and 1.13(3) Å 3 of the major phase, respectively, during charge/discharge while the parent P2 Na 0.7 MnO 2 shows an overall volume change of 0.67(0) Å 3 for P2 and 61.9(1) Å 3 for orthorhombic phase. The maximum volume for P2 Na 0.7 Li 0.1 Mn 0.9 O 2 of 81.094(6) Å 3 is observed at 2.35 V during discharge while for P2 Na 2/3 Cu 1/3 Mn 2/3 O 2 of 81.753(6) Å 3 is observed at the discharged state (1.5 V). Compared to the P2 Na 0.7 MnO 2 the number of phase transitions experienced by the P2 Na 0.7 Li 0.1 Mn 0.9 O 2 and P2 Na 2/3 Cu 1/3 Mn 2/3 O 2 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 Na 0.7 MnO 2 during cycling.