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SiO x (x ≈ 1) is one of the most promising anode materials for application in secondary lithium-ion batteries because of its high theoretical capacity. Despite this merit, SiO x has a poor initial Coulombic efficiency, which impedes its widespread use. To overcome this limitation, in this work, we successfully demonstrate a novel synthesis of Mg-doped SiO x via a mass-producible physical vapor deposition method. The solid-state reaction between Mg and SiO x produces Si and electrochemically inert magnesium silicate, thus increasing the initial Coulombic efficiency. The Mg doping concentration determines the phase of the magnesium silicate domains, the size of the Si domains, and the heterogeneity of these two domains. Detailed electron microscopy and synchrotron-based analysis revealed that the nanoscale homogeneity of magnesium silicates driven by cycling significantly affected the lifetime. We found that 8 wt % Mg is the most optimized concentration for enhanced cyclability because MgSiO3, which is the dominant magnesium silicate composition, can be homogeneously mixed with silicon clusters, preventing their aggregation during cycling and suppressing void formation.

though new energy storage devices such as lithium-sulfur batteries [2] and lithium-air batteries [3] have shown great promise due to large theoretical capacity, lithium ion batteries (LIBs) are still dominating in portable electronic devices, prevailing in electric vehicles, and gradually entering grid-energy storage markets. [4] The unsatisfactory energy density of cathodes is widely recognized as the critical bottleneck for higher-performance LIBs. [5] Among various cathodes, Ni-rich layered lithium transition-metal oxides possessing a larger reversible capacity (>180 mAh g −1) than LiCoO 2 (140 mAh g −1), LiNi 1/3 Co 1/3 Mn 1/3 O 2 (160 mAh g −1), LiMn 2 O 4 (120 mAh g −1), and LiFePO 4 (160 mAh g −1), are regarded as one of the most promising cathodes for the next-generation LIBs. [6] In 2016, American electric vehicle company Tesla launched Model 3 with LiNi 0.85 Co 0.10 Al 0.05 O 2 as the cathode for its electric vehicle battery, a testament to the huge potential of Ni-rich layered oxides (NRLOs) (Scheme 1). NRLO cathodes generally consist of two main categories-LiNi 1−x−y Co x Mn y O 2 (NMC) and LiNi 1−x−y Co x Al y O 2 (NCA). The evolution from LiCoO 2 /LiNiO 2 / LiMnO 2 to LiNi 1−x−y Co x Mn y O 2 has been introduced in previous reviews. [7] Briefly, synthesizing LiCoO 2 with the low-defect density was relatively accessible due to the large difference in ionic radius between Li + and Co 3+ , but the irreversible structural change takes place when more than half a fraction of Li + is extracted from its lattice, restricting the capacity. Stoichiometric LiNiO 2 is difficult to prepare due to the instability of trivalent Ni at high temperatures, and the cation mixing between Li and Ni weakens the cycling stability of LiNiO 2. [8] Synthesis of the layered LiMnO 2 phase is not straightforward either, and the capacity fades rapidly because the structural transformation from layered to spinel phase is inevitable upon cycling. [9] LiNi 1−x−y Co x Mn y O 2 combines the strengths of nickel (high capacity), cobalt (good rate capability), and manganese (benign stability). [7] The redox couples of Ni 2+ /Ni 3+ /Ni 4+ and Co 3+ /Co 4+ contribute to the majority of the capacity. The existence of cobalt suppresses the cation mixing during the synthesis of stoichiometric compounds, while manganese helps stabilize the Ni-rich layered oxides (NRLO) are widely considered among the most promising cathode materials for high energy-density lithium ion batteries. However, the high proportion of Ni content accelerates the cycling degradation that restricts their large-scale applications. The origins of degradation are indeed heterogeneous and thus there are tremendous efforts devoted to understanding the underlying mechanisms at multi-length scales spanning atom/lattice, particle, porous electrode, solid-electrolyte interface, and cell levels and mitigating the degradation of the NRLO. This review combines various advanced in situ/ex situ analysis techniques developed for resolving NRLO degradation at multi-length scales and aims...

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cathode materials and improve their stability and energy density. [1,2] In particular, Ni-rich NCM (LiNi x Co y Mn 1−x−y O 2 , x > 0.5) is a promising cathode material with high reversible capacity and has been successfully implemented in commercial energy storage systems, such as mobile devices and electric vehicles. [1] For Ni-rich layered oxides, the whole reaction pathway has been described as proceeding through a series of isostructural hexagonal phases, conventionally labeled as H1, H2, and H3 phases depending on the depth of charge. [3,4] The fundamental difference between H1 and H2 is the Li-content-dependent in-plane structure in the Li layer. [3,5] Because of the structural similarities of these evolving phases during cycling, the solid solution reaction has been predicted to be a thermodynamically favorable reaction pathway by first-principle studies [6] and was also observed by operando X-ray diffraction (XRD) experiments at moderate cycling rates in Ni-rich layered oxides, such as LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) [7] and LiNi 0.8 Co 0.1 Mn 0.1 O 2 . [3,4] Homogeneous Li transport, characterized by solid solution behaviors, is considered to be advantageous for obtaining a long cycle lifetime. [8] However, in dynamic situations such as fast cycling, limited Li diffusivity can induce a heterogeneous Li distribution within Understanding the cycling rate-dependent kinetics is crucial for managing the performance of batteries in high-power applications. Although high cycling rates may induce reaction heterogeneity and affect battery lifetime and capacity utilization, such phase transformation dynamics are poorly understood and uncontrollable. In this study, synchrotron-based operando X-ray diffraction is performed to monitor the high-current-induced phase transformation kinetics of LiNi 0.6 Co 0.2 Mn 0.2 O 2 . The sluggish Li diffusion at high Li content induces different phase transformations during charging and discharging, with strong phase separation and homogeneous phase transformation during charging and discharging, respectively. Moreover, by exploiting the dependence of Li diffusivity on the Li content and electrochemically tuning the initial Li content and distribution, phase separation pathway can be redirected to solid solution kinetics at a high charging rate of 7 C. Finite element analysis further elucidates the effect of the Li-content-dependent diffusion kinetics on the phase transformation pathway. The findings suggest a new direction for optimizing fast-cycling protocols based on the intrinsic properties of the materials.