Li Huang,Jian Gao,Zhijie Bi,Ning Zhao,Jipeng Wu,Qiu Fang,Xuefeng Wang,Yong Wan,Xiangxin Guo
The cubic garnet Li7La3Zr2O12(c-LLZO) is one of the most promising solid electrolytes due to its high ionic conductivity and large electrochemical window. However, the critical issue of Li2CO3formation on the c-LLZO surface when exposed to air is problematic, which is detrimental to the ionic conductivity and storage. Herein, comparative studies were carried out on the air stability of Al-doped Li7La3Zr2O12(Al-LLZO), Al-Ta-doped Li7La3Zr2O12(Al-LLZTO), and Al-Nb-doped Li7La3Zr2O12(Al-LLZNO). It was found that Al-LLZTO and Al-LLZNO are less reactive with air than Al-LLZO. The morphology of Li2CO3on Al-LLZTO micro-sized powders after air exposure was island-like with ~1.5 μm in thickness. The interfacial resistance of Li/Al-LLZTO was also a factor of ~3 smaller than that of Li/Al-LLZO, leading to the improved cycle stability of Li/Al-LLZTO/Li symmetric cells. The first-principles calculations based on density functional theory (DFT) verified that the decomposition energy of Al-LLZTO was larger than that of Al-LLZO, inhibiting the reaction product of Li2O and, thus, the next step product of Li2CO3following the reactions of Li2O + H2O → LiOH and LiOH + CO2→ Li2CO3.
To improve the capacity at a high rate and cycle stability of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), a simple strategy is developed to synthesize the tungsten/niobium (W/Nb) co-doped cathode material. W/Nb co-doping remarkably reduced the system energy of single W- or single Nb-doping. The co-doping makes the material have a more stable structure, and inhibits the anisotropic volume change caused by H2↔H3 phase transition, thus maintaining the integrity of particles and preventing the formation of microcracks. Moreover, the passivation layer formed by co-doping prevents the occurrence of side reactions as well as the formation of microcracks. The synergistic effect of W and Nb is also conducive to inhibiting the formation of internal oxygen vacancies and surface Li 2 CO 3 . For the W/Nb co-doped sample, the capacity retention of 54.9% after 400 cycles and 52.3% after 500 cycles are maintained even at high currents of 5.0 and 10.0 C, respectively. The strategy of synergistic effect on enhancing co-doping with high valence cations can be spread as a simple and practical method to improve the electrochemical performance of nickel-rich layered cathode materials for lithium-ion batteries at high cut-off voltage.
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