Ni‐rich LiNi_1−x−yMn_xCo_yO₂ (NMC) layered oxides are promising cathode materials for high‐energy density lithium ion batteries but suffer from severe capacity fading upon cycling. Elemental substitution (= doping) with Mg has repeatedly attracted attention in NMC materials to overcome instability problems at reasonable cost, yet rational compositional tuning is needed to guarantee sufficient cycle life without compromising energy density on the material level. Herein, a series of Mg‐substituted NMC materials with 90 mol% Ni are investigated regarding key performance metrics in NMC || graphite full‐cells benchmarked against LiNi_0.80Mn_0.10Co_0.10O₂ and LiNi_0.90Mn_0.05Co_0.05O₂ synthetized using the same co‐precipitation route. A linear correlation between cycle life and attainable gravimetric capacities is demonstrated, which are directly influenced by the degree of Mg substitution and the amount of Li⁺ cycled upon (de‐)lithiation processes. A Mg content <2 mol% should be considered to take notable benefit from the increase in Ni content from 80 to 90 mol% to achieve a higher energy density. The present study highlights the importance of evaluating the true implications of elemental substitution on cell performance and is expected to be an insightful guideline for the future development of NMC‐type cathode materials in particular with high Ni and low Co content.

Despite the recent interest in catalytic graphitization to obtain graphite-like materials from hard-carbon sources, many aspects of its mechanism are still poorly unknown. We performed a series of in situ experiments to study phase transformations during graphitization of a hard-carbon precursor using an iron catalyst at temperatures up to 1100 °C and ex situ total scattering experiments up to 2000 °C to study the structural evolution of the resulting graphitized carbon. Our results show that upon heating and cooling, iron undergoes a series of reductions to form hematite, magnetite, and wüstite before forming a carbide that later decomposes into metallic iron and additional graphite and that the graphitization fraction increases with increasing peak temperature. Structural development with temperature results in decreasing sheet curvature and increased stacking, along with a decrease in turbostratic disorder up to 1600 °C. Higher graphitization temperatures result in larger graphitic domains without further ordering of the graphene sheets. Our results have implications for the synthesis of novel biomass-derived carbon materials with enhanced crystallinity.

Silicon (Si) is one of the most promising anode candidates to further push the energy density of lithium ion batteries. However, its practical usage is still hindered by parasitic side reactions including electrolyte decomposition and continuous breakage and (re‐)formation of the solid electrolyte interphase (SEI), leading to consumption of active lithium. Pre‐lithiation is considered a highly appealing technique to compensate for active lithium losses. A critical parameter for a successful pre‐lithiation strategy by means of Li metal is to achieve lithiation of the active material/composite anode at the most uniform lateral and in‐depth distribution possible. Despite extensive exploration of various pre‐lithiation techniques, controlling the lithium amount precisely while keeping a homogeneous lithium distribution remains challenging. Here, the thermal evaporation of Li metal as a novel pre‐lithiation technique for pure Si anodes that allows both, that is, precise control of the degree of pre‐lithiation and a homogeneous Li deposition at the surface is reported. Li nucleation, mechanical cracking, and the ongoing phase changes are thoroughly evaluated. The terms dry‐state and wet‐state pre‐lithiation (without/with electrolyte) are revisited. Finally, a series of electrochemical methods are performed to allow a direct correlation of pre‐SEI formation with the electrochemical performance of pre‐lithiated Si.