The surface triple-coupling on single crystalline cathode for lithium ion batteries

Abstract

Single crystalline (SC) cathode materials, which are less susceptible to micro/nano-cracks formation and offer better structure stability compared to the polycrystalline counterpart, have attained great attention. However, the parasitic side reactions at the cathode-electrolyte interface induces the loss of active species, which consequently leads to continual degradation of the electrochemical performances. Herein, a triple coupling of concentration-gradient Na + , F - co-doping and surface NaF coating are exploited for the first time on SC LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode by the hydrolysis of NaPF 6 . This process regulates the external structure of materials by constructing a “sandwich” configuration from surface to bulk: rock salt - mixing zone - layered phase. The detailed interface transformation mechanism is revealed by Neutron powder diffraction (NPD), spherical aberration corrected high-resolution scanning transmission electron microscopy (HR-STEM), electron energy loss spectroscopy (EELS), and Ar + sputtering assisted X-ray photoelectron spectroscopy (XPS). The synergistic effects endow the SC cathode with outstanding capacity retentions: 91.3% at 25 °C and 85% at 45 °C, after 500 cycles at 5 C between 3.0 and 4.5 V. In addition, a high full-cell reversible capacity of 168.9 mAh g −1 with a capacity retention of 92.4% is achieved after 300 cycles at 1 C. Multiple characterizations further indicate that these superior results are mainly ascribed to the overall structure integrity of SC material, the thin cathode electrolyte interface, high content of lithium fluoride, and the low solubility of transition metal ions. This work opens a new avenue to construct a benign interface towards high-performance lithium ion batteries. Stabilization of single-crystalline cathode offers a great potential to conquer the challenges presented in its polycrystalline counterpart. The triple-coupling of Na + and F - co-doping together with NaF coating is boosted on single-crystalline particles, which induces its surface phase reconstruction to endow the material with appealing electrochemical properties. • A surface triple-coupling is exploited on single crystalline LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode by the hydrolysis of NaPF 6 . • The modulation process constructs a “sandwich” configuration from surface to bulk: rock salt - mixing zone - layered phase. • The as-prepared sample exhibits a significantly improved cycle stability at 4.5 V and 45 °C.