Study of structural and composition redesign to enhance the thermostability and electrochemical performance of a Co-less Ni-rich LiNi0.92Co0.04Mn0.04O2 layered cathode through transition-metal concentration gradient strategies

Abstract

Among the commercialized cathodes, Co-less Ni-rich layered cathode materials (Ni > 90%) have been reported as promising materials to achieve high energy density and economically benign cathodes for Li-ion batteries (LIBs). However, Co-less Ni-rich layered cathodes (Ni > 90%) suffer from intrinsic issues, such as phase transition, transition metal dissolution, and microcrack generation, leading to poor cycling performance and capacity degradation. In this study, full-concentration gradient strategies were introduced for studying LiNi_0.92Co_0.04Mn_0.04O₂ cathode materials (denoted as FG-NCM92) synthesized via a facile co-precipitation approach using a novel scalable Taylor-vortex reactor (TVR) and solid-state reaction method. In the novel structural design, the relatively Mn-rich outer part reinforced the surface, thermal, and structural stability, while the Ni-rich core part imparted high energy density to the cathode materials. The successful synthesis of FG-NCM92 was strongly confirmed by cross-sectional energy dispersive spectroscopy line scanning. The effects of the concentration gradient strategies on the physical, chemical, and electrochemical performance of FG-NCM92 were studied in detail. Further, the self-discharge mechanism, transition-metal dissolution, and thermal behavior of FG-NCM92 and non-gradient NCM92 (denoted as NG-NCM92) cells were investigated and compared. Importantly, the electrochemical performance of the FG-NCM92 material was improved compared to that of the NG-NCM92 cell. The charge–discharge test on a coin-type half-cell exhibited that FG-NCM92 delivered an initial discharge capacity of 214.3 mA h g⁻¹ at 0.1C and 167.7 mA h g⁻¹ at 10C with a capacity retention of 84% at 4.3 V, 76% at 4.5 V, 63% at 45 °C after 100 cycles at 1C, and 82% at 4.3 V after 50 cycles at 10C. In contrast, the non-gradient NCM92 sample delivered a low discharge capacity of 211.1 mA h g⁻¹ at 0.1C and 151.2 mA h g⁻¹ at 10C with a capacity retention of only 65% at 4.3 V, 61% at 4.5 V, 58% at 45 °C after 100 cycles at 1C, and 65% at 4.3 V after 50 cycles at 10C. Moreover, FG-NCM92//graphite pouch-type full-cells maintained a stable cycling performance of 88.8% over 300 cycles. Thus, our full-concentration gradient Ni-rich cathode materials offer an exciting strategy for improving electrochemical performance to facilitate the development of high-energy density, high-performance Li-ion batteries.