A Hybrid Ionic and Electronic Conductive Coating Layer for Enhanced Electrochemical Performance of 4.6 V LiCoO₂

Abstract

The LiCoO2 cathode undergoes undesirable electrochemical performance when cycled with a high cut-off voltage (≥4.5 V versus Li/Li+). The unstable interface with poor kinetics is one of the main contributors to the performance failure. Hence, a hybrid Li-ion conductor (Li1.5Al0.5Ge1.5P3O12) and electron conductor (Al-doped ZnO) coating layer was built on the LiCoO2 surface. Characterization studies prove that a thick and conductive layer is homogeneously covered on LiCoO2 particles. The coating layer can not only enhance the interfacial ionic and electronic transport kinetics but also act as a protective layer to suppress the side reactions between the cathode and electrolyte. The modified LiCoO2 (HC-LCO) achieves an excellent cycling stability (77.1% capacity retention after 350 cycles at 1C) and rate capability (139.8 mAh g–1 at 10C) at 3.0–4.6 V. Investigations show that the protective layer can inhibit the particle cracks and Co dissolution and stabilize the cathode electrolyte interface (CEI). Furthermore, the irreversible phase transformation is still observed on the HC-LCO surface, indicating the phase transformation of the LiCoO2 surface may not be the main factor for fast performance failure. This work provides new insight of interfacial design for cathodes operating with a high cut-off voltage.

Conformal Coating of a High-Voltage Spinel to Stabilize LiCoO2 at 4.6 V

“…46 The intensity of the lattice oxygen peak can somehow reflect the thickness of the CEI at the cathode surface; that is, the thicker the CEI, the lower the intensity of the lattice oxygen peak. 32 As shown in Figure 4e, the lattice oxygen contents of B-LCO and C-LCO1 after two cycles are actually similar, close to 20%. However, after 100 cycles, the lattice oxygen content of B-LCO drops to 14% owing to the continuous growth of the CEI, while C-LCO1 nearly remains unchanged, which suggests the formation of a stable CEI on C-LCO1 during long-term cycling.…”

“…The peak located at 529.5 eV represents the lattice oxygen in the LCO structure (Figure a–d) . The intensity of the lattice oxygen peak can somehow reflect the thickness of the CEI at the cathode surface; that is, the thicker the CEI, the lower the intensity of the lattice oxygen peak . As shown in Figure e, the lattice oxygen contents of B-LCO and C-LCO1 after two cycles are actually similar, close to 20%.…”

“…The above results indicate that the LCS coating constructs a stable surface that can reduce the occurrence of side reactions with the electrolyte, inhibiting the continuous growth of the CEI. 32,48,49 Accompanied by the decomposition of the electrolyte, the crack of LCO and generation of an electrochemically inert Co 3 O 4 phase inside the bulk of LCO causes the loss of electrochemical activity. Apart from the changes of CEI, TEM and Raman spectroscopy were used to analyze the structural changes of the LCO after cycles.…”

“…Besides, the coating material is supposed to own appropriate ionic and electronic conductances, which is beneficial to the interface electronic and ionic transport kinetics of LCO. 32 For this purpose, the cathode materials that can work stably under high voltage are suitable choices (e.g., spinel LiNi 0.5 Mn 1.5 O 4 , olivine LiCoPO 4 , etc.). 33−35 Pang et al reported the synthesis of LiNi 0.5 Mn 1.5 O 4 -coated LCO through a ball-milling method.…”

“…Therefore, it is desirable to construct a continuous coating layer with a proper thickness to shield the electrolyte from oxidative LCO and also with high structural and chemical stability to resist oxygen loss, so as to address the surface issues of LCO at high-voltage charging. Besides, the coating material is supposed to own appropriate ionic and electronic conductances, which is beneficial to the interface electronic and ionic transport kinetics of LCO . For this purpose, the cathode materials that can work stably under high voltage are suitable choices (e.g., spinel LiNi 0.5 Mn 1.5 O 4 , olivine LiCoPO 4 , etc.…”