Incorporation of Aluminum into High-Nickel Layered Oxide Cathodes: An Interphase and Structural Study

Abstract

The rapidly growing energy market calls for advanced Li-ion batteries with high energy density and acceptable service life. Ni-rich layered oxides (LiNixM1 −xO2, x > 0.6, M = Mn, Ni, Co), especially those with ultrahigh Ni content (Ni > 0.9), are able to deliver a high capacity of > 200 mA h g−1 with a high average operating voltage of 3.8 V vs Li/Li+. They hold great promise to power a 300-mile electric vehicle and are being intensively studied in recent years. However, severe critical problems greatly hinder the widespread application of high-Ni layered oxides. First, during electrochemical cycling, the generation of monoclinic and hexagonal structures in the delithiated state induces large unit cell volume changes and anisotropic strains, which leads to microcrack formation and eventually secondary particle disintegration. Furthermore, this structural evolution process is worsened as Ni content increases due to the presence of the hexagonal 3 (H3) phase in the deeply delithiated state. Second, a high concentration of unstable Ni4+ in the highly delithiated high-nickel layered oxide tends to react aggressively with the carbonate electrolyte, resulting in the formation of cathode-electrolyte interphase (CEI). Furthermore, the unwanted reaction greatly expedites the irreversible transformation of the layered structure to a rock-salt (NiO) phase. Overall, these various degradation processes result in a high interfacial impedance and poor electrochemical performance. Herein, to tackle these issues, a trace amount of Al is substituted into a high-Ni layered oxide and the dual-function of Al cooperation is demonstrated. By substituting a small dose of Al (2 mol %) for Ni in LiNi0.92Co0.06Al0.02O2, the capacity retention of the full cells utilizing Al-doped LiNi0.92Co0.06Al0.02O2 cathode paired with graphite anode after 1000 cycles increases from 47% to 83% compared to that of Al-free LiNi0.94Co0.06O2. Through in-situ X-ray diffraction, we provide the operando evidence that Al dopant tunes the H2−H3 phase transition process from a two-phase reaction to a quasi-monophase reaction, minimizing the mechanical degradation. Furthermore, Al dopant facilities the formation of phosphate-enriched CEI on the cathode surface, which protects the cathode from severe electrolyte attack. Also, secondary-ion mass spectrometry reveals considerably suppressed transition-metal dissolution with Al doping, effectively preventing sustained parasitic reactions and active Li trapping due to chemical crossover on the graphite anodes. This work offers a viable approach for adopting high-Ni cathodes in Li-ion batteries. Figure 1