Investigation of the relationship between the cycle performance and the electronic structure in LiAl_xMn_2−xO₄ (x = 0 and 0.2) using soft X-ray spectroscopy

Abstract

Al doping into LiMnO is one of the well-known methods to improve the cycle performance of the LiMnO cathode. We carried out soft X-ray emission spectroscopy (XES) for LiMnO and LiAlMnO to elucidate the relationship between the Mn 3d electronic structures and cycle performances. After the first cycle, the XES spectra of LiAlMnO are almost unchanged compared to the initial state. In contrast, charge-transfer excitation for the XES of LiMnO is significantly reduced, indicating that the Mn 3d-O 2p hybridization in LiMnO should be easily weakened by charge-discharge. In LiAlMnO, the Mn-O bond becomes more stable due to the decrease of Mn ions with Jahn-Teller distortion by Al doping, resulting in the improved cycle performance.

Large Charge‐Transfer Energy in LiFePO4 Revealed by Full‐Multiplet Calculation for the Fe L3‐edge Soft X‐ray Emission Spectra

“…As we have reported, [23] Mn L 3 -edge resonant XES spectra for LiMn 2 O 4 (LMO)h ave large CT excitations reflecting the CT effect for the total of Mn 3 + and Mn 4 + states. The CT excitations considerably decreased after the first cycle, suggesting that the strength of hybridization between the Mn 3d and O2po rbitals would be weakened due to lithiation/delithiation.T he large change of CT after cycling could causeamechanical stress on the MnÀOb ond, corresponding to the not-so-high cyclability of LMO.…”

“…Here, we compare the properties of LFP with TM‐oxide cathode materials for which covalent character should be higher. As we have reported, Mn L 3 ‐edge resonant XES spectra for LiMn 2 O 4 (LMO) have large CT excitations reflecting the CT effect for the total of Mn 3+ and Mn 4+ states. The CT excitations considerably decreased after the first cycle, suggesting that the strength of hybridization between the Mn 3d and O 2p orbitals would be weakened due to lithiation/delithiation.…”

Understanding the Effect of Al Doping on the Electrochemical Performance Improvement of the LiMn2O4 Cathode Material

“…Among the above elements, Al has been reported to be one of the most favorable elements as it is nontoxic, abundant, less expensive, and lighter than other TM elements. − Also, the chemical bond of Al–O (512 kJ mol –1 ) is stronger than that of Mn–O (402 kJ mol –1 ), so Al substitution will lead to the lattice parameter decrease and further increase the stability of octahedral sites by inhibiting the Jahn–Teller distortion . Based on the above two aspects, Al doping may be beneficial to the cycling and rate performance improvement of the LiMn 2 O 4 cathode material. − Recently, a phase transition from spinel LiMn 2 O 4 to layered Li­(Al x Mn y )­O 2 was directly identified after Al 2 O 3 coating, which can stabilize the surface structure of LiMn 2 O 4 by preventing dissolution of Mn ions and mitigating cathode–electrolyte solid electrolyte interface film reactions . However, investigation into the effect of Al doping on the evolution of the surface and bulk microstructure of the LiMn 2 O 4 spinel at the atomic scale is deficient.…”

Operando resonant soft X-ray emission spectroscopy of the LiMn2O4 cathode using an aqueous electrolyte solution

“…some changes in the electronic structure even after the first cycle although the composition of SEI is dependent on the electrolyte. 31,42 This behavior is significant for the LiMn 2 O 4 powder, compared to the LiMn 2 O 4 thin film which exhibited high reproducibility on the RXES spectra after the first cycle. 22 The LiMn 2 O 4 particles in the powder should be greatly affected by the SEI because the interfacial area to the electrolyte should be larger than that for the thin film sample.…”