Quenching-Etched Surface Spinel to Passivate Layered Cathode Materials from Structural Degradation at High Potentials

The layer-structured cathode materials with high specific capacities are highly required to meet the demands of high-energy-density Li-ion batteries. However, the oxygen loss and the transition metal migration at the deep delithiation state lead to the generation of the surface spinels, which was believed to be responsible for the fade of the reversible capacity and the discharge potential. Different from the conventional "additive" surface modification such as surface coating and surface doping, we hereby apply a "subtraction" strategy to reconstruct the surface of LiCoO2 by quenching it in an aqueous solution of KAl(SO4)2. Such salt-solution quenching merges the Li+/H+ ion-exchange in the acidic solution and the Al3+ doping in molten salt to form a Co3O4-like ((Co,Al)3O4) spinel on the LiCoO2 surface. A combined investigation of the crystalline and electronic structural characterizations and density functional theory calculations correlates the presence of the "artificial" (Co,Al)3O4 surface spinel and the suppressed structural degradation of LiCoO2 at high potentials (4.6 V vs Li+/Li). The LiCo2O4-like spinel observed on naked Li1–xCoO2 is energetically apt to transform to Co3O4 spinel. In contrast, the specifically fabricated spinel hinders the Co migration and the subsequent formation of the LiCo2O4-like spinel, while the high binding energy of oxygen in the LiCoO2/(Co,Al)3O4 heterojunction prevents the oxygen escape. Therefore, the presence of the LiCo2O4-like spinel as an electrochemical intermediate facilitates the structural degradation from LiCoO2 to Co3O4 while the artificial spinel (Co,Al)3O4 passivates the LiCoO2 surface by preventing the formation of the LiCo2O4-like spinel intermediate. These findings clarify the role of the surface spinels generated by different ways and their impacts on the evolution of the structure and electrochemical behaviors of the layer-structured cathode materials and inspire new strategies on improving the performances of the cathode materials.