Hwang Jaeseong, Myeong Seungjun, Jin Wooyoung, Jang Haeseong, Nam Gyutae, Yoon Moonsu, Kim Su Hwan, Joo Se Hun, Kwak Sang Kyu, Kim Min Gyu, Cho Jaephil
Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.
Beamline Research Division, Pohang Accelerator Laboratory (PAL), Pohang, 37673, Republic of Korea.
Adv Mater. 2020 Aug;32(34):e2001944. doi: 10.1002/adma.202001944. Epub 2020 Jul 12.
Li- and Mn-rich layered oxides (LMRs) have emerged as practically feasible cathode materials for high-energy-density Li-ion batteries due to their extra anionic redox behavior and market competitiveness. However, sluggish kinetics regions (<3.5 V vs Li/Li ) associated with anionic redox chemistry engender LMRs with chemical irreversibility (first-cycle irreversibility, poor rate properties, voltage fading), which limits their practical use. Herein, the structural origin of this chemical irreversibility is revealed through a comparative study involving Li Mn Co Ni O with relatively localized and delocalized excess-Li in its lattice system. Operando fine-interval X-ray absorption spectroscopy is used to simultaneously observe the interplay between transition-metal-oxygen (TM-O) redox chemistry and TM migration behavior in real time. Density functional theory calculations show that excess-Li localization in the LMR structure attenuates TM-O covalency and stability, leading to overall chemical irreversibility. Hence, the delocalized excess-Li system is proposed as an alternative design for practically feasible LMR cathodes with restrained TM migration and sustainable O-redox chemistry.
富锂锰基层状氧化物（LMRs）因其额外的阴离子氧化还原行为和市场竞争力，已成为用于高能量密度锂离子电池的切实可行的阴极材料。然而，与阴离子氧化还原化学相关的缓慢动力学区域（相对于Li/Li为<3.5 V）导致LMRs具有化学不可逆性（首次循环不可逆性、倍率性能差、电压衰减），这限制了它们的实际应用。在此，通过对晶格系统中具有相对局域化和离域化过量锂的LiMnCoNiO进行比较研究，揭示了这种化学不可逆性的结构起源。采用原位精细间隔X射线吸收光谱法实时同步观察过渡金属-氧（TM-O）氧化还原化学与TM迁移行为之间的相互作用。密度泛函理论计算表明，LMR结构中过量锂的局域化减弱了TM-O的共价性和稳定性，导致整体化学不可逆性。因此，提出离域化过量锂体系作为一种替代设计，用于具有受限TM迁移和可持续O-氧化还原化学的切实可行的LMR阴极。
