Dr. Aurora Gómez Martín

The electrification of the transport sector places ever-increasing demands on the energy density, fast-charging performance, and lifetime of lithium-ion cells. In this study, we investigate fast-charging of high energy density (∼ 800 W h L−1) prototype cylindrical 4695 lithium-ion cells with two different (“low” and “high”) electrolyte amounts. Using pore volume calculations, computer tomography and moment of inertia measurements, we find that the volume change of the active material causes electrolyte motion into and out of the jelly roll upon cycling in the high electrolyte cells, while no electrolyte motion occurs in the low electrolyte cells. At the same time, the high electrolyte cells show a significantly worse capacity retention during fast charge cycling compared to the low electrolyte cells (24% vs. 5% capacity loss after 130 cycles). We demonstrate that a coupling of in-plane electrolyte motion with the through-plane LiPF6 concentration gradient rapidly causes a strong LiPF6 concentration gradient along the jelly roll height (= in-plane) on a cm-scale: after only 18 cycles, LiPF6 concentrations at the jelly roll edges and center (as determined by ion-chromatography) deviate by more than ± 50% in comparison to the initial average value. We term this hitherto unknown effect “electrolyte motion induced salt inhomogeneity” (EMSI). This long-scale salt concentration gradient causes a loss of cell capacity, an increase in resistance, and eventually highly localized lithium plating. Finally, we discuss the far-reaching implications of the EMSI effect on cell design and testing, not only for cylindrical cells but any large-format lithium-ion cell under high compression. © 2024 The Royal Society of Chemistry.

Silicon (Si)-based negative electrodes have attracted much attention to increase the energy density of lithium ion batteries (LIBs) but suffer from severe volume changes, leading to continuous re-formation of the solid electrolyte interphase and consumption of active lithium. The pre-lithiation approach with the help of positive electrode additives has emerged as a highly appealing strategy to decrease the loss of active lithium in Si-based LIB full-cells and enable their practical implementation. Here, the use of lithium squarate (Li2C4O4) as low-cost and air-stable pre-lithiation additive for a LiNi0.6Mn0.2Co0.2O2 (NMC622)-based positive electrode is investigated. The effect of additive oxidation on the electrode morphology and cell electrochemical properties is systematically evaluated. An increase in cycle life of NMC622||Si/graphite full-cells is reported, which grows linearly with the initial amount of Li2C4O4, due to the extra Li+ ions provided by the additive in the first charge. Post mortem investigations of the cathode electrolyte interphase also reveal significant compositional changes and an increased occurrence of carbonates and oxidized carbon species. This study not only demonstrates the advantages of this pre-lithiation approach but also features potential limitations for its practical application arising from the emerging porosity and gas development during decomposition of the pre-lithiation additive. © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.

Ni-rich LiNi1−x−yMnxCoyO2 (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 LiNi0.80Mn0.10Co0.10O2 and LiNi0.90Mn0.05Co0.05O2 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. © 2022 The Authors.

Hard carbons are the material of choice as negative electrode in sodium ion batteries. Despite being extensively studied, there is still debate regarding the mechanisms responsible for storage in low- and high-potential regions. This work presents a comprehensive approach to elucidate the involved storage mechanisms when Na ions insert into such disordered structures. Synchrotron X-ray total scattering experiments were performed to access quantitative information on atomic ordering in these materials at the nanoscale. Results prove that hard carbons undergo an atomic rearrangement as the graphene layers cross-link at intermediate temperatures (1200–1600 °C), resulting in an increase of the average interplanar distance up to 1400 °C, followed by a progressive decrease. This increase correlates with the positive trend in the reversible capacity of biomass-derived carbons when processed up to 1200–1600 °C due to an increased capacity at low potential (≤0.1 V vs Na/Na+). A decrease in achievable sloping capacity with increasing heat-treatment temperature arises from larger crystalline domains and a lower concentration of defects. The observed correlation between structural parameters and electrochemical properties clearly supports that the main storage of Na ions into a hard-carbon structure is based on an adsorption–intercalation mechanism.

Graphitized carbon materials from biomass resources were successfully synthesized with an iron catalyst, and their electrochemical performance as anode materials for lithium-ion batteries (LIBs) was investigated. Peak pyrolysis temperatures between 850 and 2000 °C were covered to study the effect of crystallinity and microstructural parameters on the anodic behavior, with a focus on the first-cycle Coulombic efficiency, reversible specific capacity, and rate performance. In terms of capacity, results at the highest temperatures are comparable to those of commercially used synthetic graphite derived from a petroleum coke precursor at higher temperatures, and up to twice as much as that of uncatalyzed biomass-derived carbons. The opportunity to graphitize low-cost biomass resources at moderate temperatures through this one-step environmentally friendly process, and the positive effects on the specific capacity, make it interesting to develop more sustainable graphite-based anodes for LIBs.