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 LiCoO 2 by quenching it in an aqueous solution of KAl(SO 4 ) 2 . Such salt-solution quenching merges the Li + /H + ion-exchange in the acidic solution and the Al 3+ doping in molten salt to form a Co 3 O 4 -like ((Co,Al) 3 O 4 ) spinel on the LiCoO 2 surface. A combined investigation of the crystalline and electronic structural characterizations and density functional theory calculations correlates the presence of the “artificial” (Co,Al) 3 O 4 surface spinel and the suppressed structural degradation of LiCoO 2 at high potentials (4.6 V vs Li + /Li). The LiCo 2 O 4 -like spinel observed on naked Li 1– x CoO 2 is energetically apt to transform to Co 3 O 4 spinel. In contrast, the specifically fabricated spinel hinders the Co migration and the subsequent formation of the LiCo 2 O 4 -like spinel, while the high binding energy of oxygen in the LiCoO 2 /(Co,Al) 3 O 4 heterojunction prevents the oxygen escape. Therefore, the presence of the LiCo 2 O 4 -like spinel as an electrochemical intermediate facilitates the structural degradation from LiCoO 2 to Co 3 O 4 while the artificial spinel (Co,Al) 3 O 4 passivates the LiCoO 2 surface by preventing the formation of the LiCo 2 O 4 -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.