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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.

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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...

metal oxides. Li-ion battery cathodes are composed of lithium metal oxides with varying lithium contents (e.g., Li x CoO 2 ), which are generally synthesized by calcination. [1,2] The Ni-rich layered oxide (LiNi 1-x-y Co x Mn y O 2 , NRNCM) is one of the leading cathode materials for next-generation Li-ion batteries with high energy/ power densities for electric vehicle applications. [3][4][5][6] NRNCMs, the calcination process is the key step in enabling the lithium source to completely react with the hydroxide precursor Ni x Co y Mn z (OH) 2 , thereby yielding particles with a uniform chemical composition. The reaction of metal hydroxide with ambient oxygen and solid-state lithium sources drives a series of heterogeneous phase transitions with gas evolution upon increasing the temperature. The impurities and structural heterogeneity resulting from such solid-state synthesis deteriorate the cell capacity and the cycling stability of the NRNCM cathode. [6][7][8][9][10][11][12][13][14][15] Recently, the subtle alteration of calcination intermediate was confirmed to greatly affect the NRNCM performance. [14] While the precise control of the calcination reactions is critical for achieving an optimal battery performance, the reaction pathway heterogeneity stemming from complex mass transport and During solid-state calcination, with increasing temperature, materials undergo complex phase transitions with heterogeneous solid-state reactions and mass transport. Precise control of the calcination chemistry is therefore crucial for synthesizing state-of-the-art Ni-rich layered oxides (LiNi 1-x-y Co x Mn y O 2 , NRNCM) as cathode materials for lithium-ion batteries. Although the battery performance depends on the chemical heterogeneity during NRNCM calcination, it has not yet been elucidated. Herein, through synchrotron-based X-ray, mass spectrometry microscopy, and structural analyses, it is revealed that the temperature-dependent reaction kinetics, the diffusivity of solid-state lithium sources, and the ambient oxygen control the local chemical compositions of the reaction intermediates within a calcined particle. Additionally, it is found that the variations in the reducing power of the transition metals (i.e., Ni, Co, and Mn) determine the local structures at the nanoscale. The investigation of the reaction mechanism via imaging analysis provides valuable information for tuning the calcination chemistry and developing high-energy/power density lithium-ion batteries.

Silicon is an attractive anode material for all-solid-state batteries (ASSBs) because it has a high energy density and is safer than metallic lithium. Conventional silicon powder composite electrodes have significant internal voids and detrimental interfaces that suppress the lithium transport and lifetime. Here, we demonstrate that surface-treated thin silicon wafers could serve as monolithic additive-free, electrolyte-free, and void-free electrodes that can achieve high areal capacity at room temperature (∼25 °C). A dense solid electrolyte interface could effectively suppress the cracks and pulverization found in the liquid electrolyte. We demonstrated that the grooved <110> wafer exhibited reversible (de)lithiation owing to fast lithium distribution along the <110> thickness direction. The surface groove could effectively penetrate the electrolyte layer, yielding a stable interfacial resistance and homogeneous alloying/dealloying processes during cycling. Our silicon wafer electrode achieved an areal capacity of 10 mAh cm–2 at room temperature, which can be improved by further optimization.

The all-solid-state battery (ASSB) has become one of the most promising next-generation battery systems, since the aspect of safety has emerged as a crucial criterion for new large-scale applications such as in electric vehicles. Despite the recent remarkable progress in the performance enhancement, the real-world implementation of the ASSB still requires full comprehension/evaluation of its properties and performance under various practical operational conditions. Unlike batteries employed in conventional electronic devices, those in electric vehicles—the major application that the ASSB is expected to be employed—would be exposed to wide temperature variations (−20 to ∼70 °C) at various states of charges due to their outdoor storage and irregular discharge/rest/charge conditions depending on vehicle drivers' usage patterns. Herein, we investigate the reliability of a Li6PS5Cl-based ASSB system in practically harsh but plausible storage conditions and reveal that it is vulnerable to elevated-temperature storage as low as 70 °C, which, in contrast to the common belief, causes significant degradation of the electrolyte and consequently irreversible buildup of the cell resistance. It is unraveled that this storage condition induces the decomposition of Li6PS5Cl in contact with the cathode material, involving the SOx gas evolution particularly at charged states, which creates a detrimental porous cathode/electrolyte interface, thereby leading to the large interfacial resistance. Our findings indicate that the stability of the solid electrolyte, which has been believed to be failsafe, needs to be carefully revisited at various practical operational conditions for actual applications in ASSBs.

Elucidating high-rate cycling-induced nonequilibrium electrode reactions is crucial for developing extreme fast charging (XFC) batteries. Herein, we unveiled the distinct rate capabilities of a series of Ni-rich layered oxide (NRLO) cathodes by quantitatively establishing their dynamic structure−kinetics relationships. Contrary to conventional views, we discovered electrode kinetic properties obtained ex-situ near equilibrium states failed to assess the effective rate capability of NRLOs at ultrafast C rates. Further, the kinetic phase heterogeneity, characterized by the dynamic separations in in-situ X-ray diffraction patterns and deviations in NRLO c-axis lattice parameters, exclusively correlated with the capacity reduction under XFC and became an effective indicator of the NRLO rate capability. Enhancing the cycling temperature boosted the rate capability of studied NRLOs by ∼10%, which was further verified to mitigate the kinetic phase heterogeneity during XFC. Overall, this study lays the groundwork for tuning the kinetic phase heterogeneity of electrodes to develop ultrafast batteries.