High‐Voltage Layered Ternary Oxide Cathode Materials: Failure Mechanisms and Modification Methods †

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

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 (Li_1.5Al_0.5Ge_1.5P₃O₁₂) 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.

Realization of a High-Voltage and High-Rate Nickel-Rich NCM Cathode Material for LIBs by Co and Ti Dual Modification

Nickel-rich Li(Ni_xCo_yMn_1-x-yO₂) (x ≥ 0.6) is considered to be a predominant cathode for next-generation lithium-ion batteries (LIBs) due to its towering specific energy density. Unfortunately, serious structural degradation causes rapid capacity degradation with the increase in nickel content. Herein, a Co and Ti co-modified LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) cathode ameliorates the reversible capacity together with the rate capability by obviously alleviating the lattice structure degradation and microscopic intergranular cracks. Further studies show that the titanium doping effectively reduces the cation mixing and also stabilizes the crystal structure, while the spinel phase formed at the surface by a cobalt oxide coating is much stable than the layered phase at high voltage, which can alleviate the generation of micro-cracks. After 0.5% Co oxide coating and 1% Ti doping (T1Co0.5-NCM), a superior rate capability (121.75 mA h g^-1 at 20 C between 2.7 and 4.5 V) and predominant capacity retention (74.2%) are observed compared with the pristine NCM-811 (59.5%) after 400 cycles between 2.7 and 4.7 V. This work supplies an eminent design of high-voltage and high-rate layered cathode materials and has a huge application prospect in the next generation of high-energy LIBs.