Department of Chemistry, National Taiwan University , Taipei 106, Taiwan.
Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation , Lucas Heights, NSW 2234, Australia.
J Am Chem Soc. 2016 Jul 20;138(28):8824-33. doi: 10.1021/jacs.6b03932. Epub 2016 Jul 6.
The mechanism of capacity fade of the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) composite positive electrode within a full cell was investigated using a combination of operando neutron powder diffraction and transmission X-ray microscopy methods, enabling the phase, crystallographic, and morphological evolution of the material during electrochemical cycling to be understood. The electrode was shown to initially consist of 73(1) wt % R3̅m LiMO2 with the remaining 27(1) wt % C2/m Li2MnO3 likely existing as an intergrowth. Cracking in the Li2MnO3·LiMO2 electrode particle under operando microscopy observation was revealed to be initiated by the solid-solution reaction of the LiMO2 phase on charge to 4.55 V vs Li(+)/Li and intensified during further charge to 4.7 V vs Li(+)/Li during the concurrent two-phase reaction of the LiMO2 phase, involving the largest lattice change of any phase, and oxygen evolution from the Li2MnO3 phase. Notably, significant healing of the generated cracks in the Li2MnO3·LiMO2 electrode particle occurred during subsequent lithiation on discharge, with this rehealing being principally associated with the solid-solution reaction of the LiMO2 phase. This work reveals that while it is the reduction of lattice size of electrode phases during charge that results in cracking of the Li2MnO3·LiMO2 electrode particle, with the extent of cracking correlated to the magnitude of the size change, crack healing is possible in the reverse solid-solution reaction occurring during discharge. Importantly, it is the phase separation during the two-phase reaction of the LiMO2 phase that prevents the complete healing of the electrode particle, leading to pulverization over extended cycling. This work points to the minimization of behavior leading to phase separation, such as two-phase and oxygen evolution, as a key strategy in preventing capacity fade of the electrode.
采用原位中子粉末衍射和透射 X 射线显微镜方法相结合，研究了全电池中 Li2MnO3·LiMO2（M = Li、Ni、Co、Mn）复合正极容量衰减的机理，使人们能够了解材料在电化学循环过程中的相、晶体和形态演变。结果表明，该电极最初由 73(1)wt%R3̅m LiMO2组成，其余 27(1)wt%C2/m Li2MnO3可能以共生形式存在。在原位显微镜观察下，Li2MnO3·LiMO2电极颗粒的开裂被揭示是由 LiMO2 相在充电至 4.55 V（相对于 Li(+)/Li）时的固溶反应引发的，并在进一步充电至 4.7 V（相对于 Li(+)/Li）时加剧，在此过程中，涉及任何相的最大晶格变化的 LiMO2 相的两相反应和 Li2MnO3 相的氧释放同时发生。值得注意的是，在随后的放电过程中，Li2MnO3·LiMO2 电极颗粒中产生的裂纹发生了显著的愈合，这种再愈合主要与 LiMO2 相的固溶反应有关。这项工作揭示了尽管在充电过程中电极相的晶格尺寸减小导致了 Li2MnO3·LiMO2 电极颗粒的开裂，但开裂的程度与尺寸变化的幅度相关，在放电过程中发生的反向固溶反应中可以实现裂纹愈合。重要的是，LiMO2 相的两相反应中的相分离阻止了电极颗粒的完全愈合，导致在长时间循环过程中粉碎。这项工作表明，最小化导致相分离的行为，如两相反应和氧释放，是防止电极容量衰减的关键策略。
