Aurora Gomez‐Martin

Electrolyte motion induced salt inhomogeneity (EMSI): during cycling, electrolyte moves into and out of the jelly roll, which leads to a lasting in-plane LiPF₆ concentration gradient, possibly causing secondary effects like localized lithium plating.

To achieve a broader public acceptance for electric vehicles based on lithium-ion battery (LIB) technology, long driving ranges, low cost, and high safety are needed. A promising pathway to address these key parameters lies in the further improvement of Ni-rich cathode materials for LIB cells. Despite the higher achieved capacities and thus energy densities, there are major drawbacks in terms of capacity retention and thermal stability (of the charged cathode) which are crucial for customer acceptance and can be mitigated by protecting cathode particles. We studied the impact of surface modifications on cycle life and thermal stability of LiNi_0.90Co_0.05Mn_0.05O₂ layered oxide cathodes with WO₃ by a simple sol–gel coating process. Several advanced analytical techniques such as low-energy ion scattering, differential scanning calorimetry, and high-temperature synchrotron X-ray powder diffraction of delithiated cathode materials, as well as charge/discharge cycling give significant insights into the impact of surface coverage of the coatings on mitigating degradation mechanisms. The results show that successful surface modifications of WO₃ with a surface coverage of only 20% can prolong the cycle life of an LIB cell and play a crucial role in improving the thermal stability and, hence, the safety of LIBs.

Invited for this issue's Front Cover is the group of Prof. Dr. Martin Winter from the MEET Battery Research Center (University of Münster, Germany). The cover picture shows two battery‐shaped coffee cups filled with coffee. In addition, the scientists (represented by gloved hands) have added both pure iron and an iron solution to the cups. The left cup filled with iron is highly energetic and strikes sparks, indicating better electrochemical performance and crystallinity than the right cup filled with the iron solution. The impact of activator addition prior to ‘catalytic’ graphitization and its effect on material and electrochemical properties is systematically investigated. Read the full text of the Research Article at 10.1002/celc.202201073.

Silicon, as potential next‐generation anode material for high‐energy lithium‐ion batteries (LIBs), suffers from substantial volume changes during (dis)charging, resulting in continuous breakage and (re‐)formation of the solid electrolyte interphase (SEI), as well as from consumption of electrolyte and active lithium, which negatively impacts long‐term performance and prevents silicon‐rich anodes from practical application. In this work, fluorinated phosphazene compounds are investigated as electrolyte additives concerning their SEI‐forming ability for boosting the performance of silicon oxide (SiO_x)‐based LIB cells. In detail, the electrochemical performance of NCM523 || SiO_x/C pouch cells is studied, in combination with analyses regarding gas evolution properties, post‐mortem morphological changes of the anode electrode and the SEI, as well as possible electrolyte degradation. Introducing the dual‐additive approach in state‐of‐the‐art electrolytes leads to synergistic effects between fluoroethylene carbonate and hexafluorocyclotriphosphazene‐derivatives (HFPN), as well as enhanced electrochemical performance. The formation of a more effective SEI and increased electrolyte stabilization improves lifetime and results in an overall lower cell impedance. Furthermore, gas chromatography‐mass spectrometry measurements of the aged electrolyte with HFPN‐derivatives as an additive compound show suppressed ethylene carbonate and ethyl methyl carbonate decomposition, as well as reduced trans‐esterification and oligomerization products in the aged electrolyte.

Aqueous processing of Ni‐rich layered oxide cathode materials is a promising approach to simultaneously decrease electrode manufacturing costs, while bringing environmental benefits by substituting the state‐of‐the‐art (often toxic and costly) organic processing solvents. However, an aqueous environment remains challenging due to the high reactivity of Ni‐rich layered oxides towards moisture, leading to lithium leaching and Al current collector corrosion because of the resulting high pH value of the aqueous electrode paste. Herein, a facile method was developed to enable aqueous processing of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) by the addition of lithium sulfate (Li₂SO₄) during electrode paste dispersion. The aqueously processed electrodes retained 80 % of their initial capacity after 400 cycles in NCM811||graphite full cells, while electrodes processed without the addition of Li₂SO₄ reached 80 % of their capacity after only 200 cycles. Furthermore, with regard to electrochemical performance, aqueously processed electrodes using carbon‐coated Al current collector outperformed reference electrodes based on state‐of‐the‐art production processes involving N‐methyl‐2‐pyrrolidone as processing solvent and fluorinated binders. The positive impact on cycle life by the addition of Li₂SO₄ stemmed from a formed sulfate coating as well as different surface species, protecting the NCM811 surface against degradation. Results reported herein open a new avenue for the processing of Ni‐rich NCM electrodes using more sustainable aqueous routes.

Lithium ion batteries (LIBs) using silicon as anode material are endowed with much higher energy density than state‐of‐the‐art graphite‐based LIBs. However, challenges of volume expansion and related dynamic surfaces lead to continuous (re‐)formation of the solid electrolyte interphase, active lithium losses, and rapid capacity fading. Cell failure can be further accelerated when Si is paired with high‐capacity, but also rather reactive Ni‐rich cathodes, such as LiNi_0.8Co_0.1Mn_0.1O₂ (NCM‐811). Here, the practical applicability of thermal evaporation of Li metal is evaluated as a prelithiation technique on micrometer‐sized Si (µ‐Si) electrodes in addressing such challenges. NCM‐811 || “prelithiated µ‐Si” full‐cells (25% degree of prelithiation) can attain a higher initial discharge capacity of ≈192 mAh g_NCM‐811⁻¹ than the cells without prelithiation with only ≈160 mAh g_NCM‐811⁻¹. This study deeply discusses significant consequences of electrode capacity balancing (N:P ratio) with regard to prelithiation on the performance of full‐cells. The trade‐off between cell lifetime and energy density is also highlighted. It is essential to point out that the phenomena discussed here can further guide the direction of research in using the thermal evaporation of Li metal as a prelithiation technique toward its practical application on Si‐based LIBs.

Silicon (Si) holds great promise as an anode material for high energy density lithium ion batteries owing to its theoretical capacity of up to 3579 mAh g⁻¹. However, this potential comes at the expense of major challenges, because the solid electrolyte interphase (SEI) at Si anodes hardly provides long‐term protection due to severe volume expansion. Yet, when it comes to the SEI, the formation mechanism is not thoroughly understood. Here, thin AlF₃ coatings are deposited on Si thin film to stabilize the SEI. To evaluate the SEI, systematic observation utilizing X‐ray photoelectron spectroscopy is performed at different (de‐)lithiation states, allowing stage‐by‐stage analysis to reveal the role, mechanism, and impact of AlF₃ coating. Results show that the capacity retention is significantly improved for 90% after 100 cycles. The transformation of AlF₃ into Li‐Al‐F compounds, as confirmed by ion chromatography, is responsible for an enhanced performance due to its high ionic conductivity. Moreover, the SEI of coated Si thin films is rich in inorganic species (i.e., LiF) which is beneficial to prevent electrons to pass through. This work will deepen the understanding of SEI on Si anodes with respect to the coating approach, suggesting future directions to improve coating layers on Si.

We report the performance of a conversion-type magnetite-decorated partially reduced graphene oxide (Fe₃O₄@PrGO) negative electrode material in full-cell configuration with LiNi_0.8Co_0.15Al_0.05O₂ (NCA) positive electrodes. To enable practical implementation of the conversion-type negative electrodes in full cells, the beneficial impact of electrochemical prelithiation on mitigating active lithium losses and improving cycle life is shown here for the first time in the literature. The initial Coulombic efficiency (ICE) of the full cells is improved from 70.8 to 91.2% by prelithiation of the negative electrode to 35% of its specific delithiation capacity. The prelithiation is shown to improve the surface passivation of the Fe₃O₄@PrGO electrodes, leading to less electrolyte reduction on their surface which is prominent from the significantly lowered accumulated Coulombic inefficiency values, lower polarization growth, and doubled capacity retention by the 100th cycle. The reduced surface reactions of the negative electrode by prelithiation also aids in reducing the extent of aging of the NCA positive electrode. Overall, the prelithiation leads to a longer cycle life, where a retained capacity of 60.4% was achieved for the prelithiated cells by the end of long-term cycling, which is 3 times higher than the capacity retention of the non-prelithiated cells. Results reported herein indicate for the first time that the electrochemical prelithiation of the Fe₃O₄@PrGO electrode is a promising approach for making conversion negative electrode materials more applicable in lithium-ion batteries.

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 LiNi_0.6Mn_0.2Co_0.2O₂ (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.

Ni‐rich layered oxide cathodes are promising candidates to satisfy the increasing energy demand of lithium‐ion batteries for automotive applications. Aqueous processing of such materials, although desirable to reduce costs and improve sustainability, remains challenging due to the Li⁺/H⁺exchange upon contact with water, resulting in a pH increase and corrosion of the aluminum current collector. Herein, an example was given for tuning the properties of aqueous LiNi_0.83Co_0.12Mn_0.05O₂electrode pastes using a lithium polyacrylate‐based binder to find the “sweet spot” for processing parameters and electrochemical performance. Polyacrylic acid was partially neutralized to balance high initial capacity, good cycling stability, and the prevention of aluminum corrosion. Optimized LiOH/polyacrylic acid ratios in water were identified, showing comparable cycling performance to electrodes processed with polyvinylidene difluoride requiring toxicN‐methyl‐2‐pyrrolidone as solvent. This work gives an exemplary study for tuning aqueous electrode pastes properties aiming towards a more environmentally friendly processing of Ni‐rich cathodes.