The increasing demand for lithium-ion batteries (LIBs) in electric vehicles and grid-scale energy storage necessitates the development of sustainable and cost-effective cathode materials. High-nickel nickel-manganese-cobalt (NMC) cathodes, particularly those with high Ni content, offer high energy density but suffer from several drawbacks, including surface reconstruction, oxygen release, transition metal (TM) dissolution, and structural degradation during cycling. The presence of cobalt (Co) in NMC cathodes, while enhancing structural stability and cycling performance, raises significant concerns due to its high cost, environmental impact, and geopolitical supply chain risks. Therefore, the development of cobalt-free alternatives with comparable or superior performance is a critical research priority. The current research focuses on addressing this challenge by investigating the impact of lithium stoichiometry on the synthesis and electrochemical performance of cobalt-free LiNi_xMn_yO₂ (NMC) cathode materials. Previous studies have explored Li-excess and Li-stoichiometric compositions, but the Li-deficient regime remains relatively understudied. The authors hypothesize that controlling the lithium stoichiometry during synthesis can significantly influence the structural and morphological properties of the resulting material, leading to improved electrochemical performance and enhanced stability. The purpose of this study is to systematically investigate the effects of lithium deficiency on the synthesis, structure, morphology, and electrochemical behavior of cobalt-free NMC cathodes. The successful development of a high-performing cobalt-free cathode with controlled lithium stoichiometry would have a significant impact on the sustainability and cost-effectiveness of LIB technologies, paving the way for wider adoption of electric vehicles and renewable energy storage solutions.

High-nickel NMC cathode materials, while possessing high energy density, suffer from issues like surface reconstruction, oxygen release, transition metal dissolution, and structural degradation. Cobalt plays a crucial role in mitigating these issues by facilitating Li/Ni ordering during calcination, enhancing structural stability and cycling performance. However, the high cost and environmental concerns associated with cobalt necessitate the development of cobalt-free alternatives. Studies on nickel-manganese-based cobalt-free cathodes (LiNiMnO₂) have shown promising high capacity, but they suffer from Li/Ni disordering and cycling instability due to the introduction of Ni²⁺ to maintain charge neutrality. Li/Ni mixing is unavoidable and worsens with higher Mn content, leading to capacity decay and voltage fade. Existing strategies to improve the performance of cobalt-free layered cathodes include surface modifications, doping, and structural engineering. However, the need for a cost-effective and efficient strategy that addresses the inherent challenges of cobalt-free cathodes remains. This paper explores lithium stoichiometry control as a novel approach to achieve this goal.

The researchers synthesized Li_xNi_0.95Mn_0.05O₂ (NM9505) cathode materials with varying lithium stoichiometry (x) ranging from 0.9 to 1.1. The synthesis involved a two-step high-temperature calcination process of a Ni_0.95Mn_0.05(OH)₂ precursor with LiOH·H₂O at different molar ratios. The chemical composition of the synthesized materials was determined using inductively coupled plasma mass spectrometry (ICP-MS). The particle morphology and microstructure were characterized using scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The crystal structure and phase composition were analyzed using synchrotron X-ray diffraction (XRD), including in situ XRD to monitor the phase transformation during calcination. Three-dimensional transmission X-ray microscopy (TXM) combined with X-ray absorption near-edge structure (XANES) spectroscopy was employed to analyze the local distribution of nickel valence states. Electrochemical performance, including first-cycle Coulombic efficiency (CE), capacity retention, and voltage fade, was evaluated using coin-type cells with lithium metal counter electrodes. Computational modeling was used to investigate the phase transformation and particle growth mechanisms during calcination. Specifically, the synthesis involved using a Taylor Vortex Reactor for the hydroxide coprecipitation of the Ni_0.95Mn_0.05(OH)₂ precursor. This precursor was then mixed with LiOH·H₂O at different Li/TM ratios and calcined in a tube furnace under oxygen flow. The electrochemical tests involved assembling coin cells with the synthesized NM9505 as the cathode, lithium metal as the anode, and an electrolyte solution of LiPF₆ in EC/DEC. The cells were cycled at various rates (0.1C and 0.5C) within a voltage window of 2.7-4.4 V. The computational modeling involved mesoscale simulations to study the calcination process and understand the influence of Li stoichiometry on particle growth and phase transformation.

The study revealed a strong dependence of the structure and morphology of the NM9505 cathode material on the lithium stoichiometry. Li-deficient samples (Li/TM < 1) exhibited a composite structure consisting of intergrown layered and rocksalt phases, with smaller primary particle sizes (below 100 nm) compared to Li-excess samples (Li/TM > 1), which showed a larger, mainly layered structure with particle sizes up to a few hundred nanometers. In situ XRD studies showed that Li deficiency led to slower phase transformation kinetics and crystal growth during calcination, resulting in the retention of the rocksalt phase. This is attributed to the difference in the mechanisms of particle growth in the Li-deficient and Li-excess regime. In the Li-deficient regime, lithiation induced crystallization dominated, while in the Li-excess regime, liquid phase sintering played a key role. HAADF-STEM and 3D TXM/XANES imaging confirmed the intimate mixing of the layered and rocksalt phases in the Li-deficient composite structure. Electrochemically, the Li-deficient NM9505 (0.95 Li) exhibited superior performance compared to the Li-excess counterpart (1.05 Li). It displayed a high first-cycle Coulombic efficiency (>90%), a high discharge capacity (226 mAh/g), and excellent capacity retention (90% after 100 cycles at 0.5C) with negligible voltage fade. The improved electrochemical performance is attributed to the smaller particle size and the composite layered-rocksalt structure, which minimizes the anisotropic strain during cycling and enhances structural stability. The authors also found that increasing Mn substitution beyond 5% (in NM9010) resulted in lower initial capacity despite showing better capacity retention. Therefore, their focus remained on the NM9505 composition with varied Li stoichiometry.

The findings demonstrate that controlling the lithium stoichiometry during the synthesis of cobalt-free NMC cathodes is a highly effective strategy for enhancing their electrochemical performance and structural stability. The Li-deficient composite structure, with its smaller particle size and intimate mixing of layered and rocksalt phases, effectively mitigates the structural strain induced during cycling, leading to improved capacity retention and reduced voltage fade. The observed slower phase transformation kinetics in Li-deficient samples suggests a more controlled and uniform crystal growth, which contributes to the enhanced electrochemical properties. The results challenge the conventional approach of using Li-excess to compensate for Li loss during calcination, highlighting the advantages of employing a Li-deficient approach to synthesize high-performance cobalt-free cathodes. This approach offers a cost-effective alternative to other methods, such as surface coatings or dopants, for enhancing the stability of NMC cathodes. The mechanistic insights provided by the multiscale modeling support the experimental observations and provide a clearer understanding of the influence of lithium stoichiometry on the synthesis process and resulting material properties.

This study successfully demonstrates a new route to synthesize high-performance cobalt-free cathode materials for lithium-ion batteries by precisely controlling lithium stoichiometry. The Li-deficient LiNi_0.95Mn_0.05O₂ cathode with a unique layered-rocksalt composite structure exhibits remarkable electrochemical performance, surpassing its Li-excess counterparts in terms of capacity retention, Coulombic efficiency, and voltage stability. The findings underscore the importance of lithium stoichiometry in governing the phase transformation kinetics and crystal growth during calcination, offering a cost-effective and efficient pathway for developing sustainable and high-performance cobalt-free cathodes for next-generation LIBs. Future research could explore the optimization of other compositional parameters and synthesis conditions to further enhance the performance of these promising cobalt-free cathodes. Investigating the long-term cycling performance at higher C-rates and under different operating conditions is also warranted.

While the study provides compelling evidence for the benefits of Li-deficient synthesis, the long-term cycling stability beyond 100 cycles needs further investigation under more stringent conditions such as higher charge/discharge rates and wider temperature ranges. The computational modeling was limited to mesoscale simulations; atomistic simulations could provide more detailed insights into the atomic-scale interactions and mechanisms governing phase transformation and crystal growth. The study primarily focused on one specific composition (LiNi_0.95Mn_0.05O₂); broader exploration of other compositions within the Li-Ni-Mn ternary system would be beneficial to further optimize the material properties.