Comprehensive Insights into the Porosity of Lithium-Ion Battery Electrodes: A Comparative Study on Positive Electrodes Based on LiNi0.6Mn0.2Co0.2O2 (NMC622)

Porosity is frequently specified as only a value to describe the microstructure of a battery electrode. However, porosity is a key parameter for the battery electrode performance and mechanical properties such as adhesion and structural electrode integrity during charge/discharge cycling. This study illustrates the importance of using more than one method to describe the electrode microstructure of LiNi0.6Mn0.2Co0.2O2 (NMC622)-based positive electrodes. A correlative approach, from simple thickness measurements to tomography and segmentation, allowed deciphering the true porous electrode structure and to comprehend the advantages and inaccuracies of each of the analytical techniques. Herein, positive electrodes were calendered from a porosity of 44–18% to cover a wide range of electrode microstructures in state-of-the-art lithium-ion batteries. Especially highly densified electrodes cannot simply be described by a close packing of active and inactive material components, since a considerable amount of active material particles crack due to the intense calendering process. Therefore, a digital 3D model was created based on tomography data and simulation of the inactive material, which allowed the investigation of the complete pore network. For lithium-ion batteries, the results of the mercury intrusion experiments in combination with gas physisorption/pycnometry experiments provide comprehensive insight into the microstructure of positive electrodes

Comprehensive Insights into the Porosity of Lithium-Ion Battery Electrodes: A Comparative Study on Positive Electrodes Based on LiNi0.6Mn0.2Co0.2O2 (NMC622)

Porosity is frequently specified as only a value to describe the microstructure of a battery electrode. However, porosity is a key parameter for the battery electrode performance and mechanical properties such as adhesion and structural electrode integrity during charge/discharge cycling. This study illustrates the importance of using more than one method to describe the electrode microstructure of LiNi0.6Mn0.2Co0.2O2 (NMC622)-based positive electrodes. A correlative approach, from simple thickness measurements to tomography and segmentation, allowed deciphering the true porous electrode structure and to comprehend the advantages and inaccuracies of each of the analytical techniques. Herein, positive electrodes were calendered from a porosity of 44–18% to cover a wide range of electrode microstructures in state-of-the-art lithium-ion batteries. Especially highly densified electrodes cannot simply be described by a close packing of active and inactive material components, since a considerable amount of active material particles crack due to the intense calendering process. Therefore, a digital 3D model was created based on tomography data and simulation of the inactive material, which allowed the investigation of the complete pore network. For lithium-ion batteries, the results of the mercury intrusion experiments in combination with gas physisorption/pycnometry experiments provide comprehensive insight into the microstructure of positive electrodes

Finding the sweet spot: Li/Mn-rich cathode materials with fine-tuned core–shell particle design for high-energy lithium ion batteries

Among current cathode materials, particular attention to Li/Mn-rich layered transition-metal oxides (LMR-NCM) emerged, due to their high energy content accompanied by concurrently low raw material cost. However, until today the step toward a successful market implementation is still impeded by substantial capacity and voltage fade phenomena upon cycling. Herein, we demonstrate a comprehensive structural and morphological approach to increase the long-term stability behavior of LMR-NCM materials within a lithium ion cell. Therefore, a recently introduced core–shell particle design concept was applied, which involves a Co-free and Mn-rich particle core and a low Co-containing shell. The resulting lower anionic redox activity of the shell is key to improve the electrochemical performance. With the aid of a Couette Taylor Flow Reactor, spherical secondary particles with high tap density and narrow particle size distribution are co-precipitated, leading to a valuable hierarchical morphology with superior electrochemical long-term behavior. Thereby, excellent initial Coulombic efficiencies of 90 – 95 % are attained. Finally, another main focus of this work concentrates on the impact of effective performance-improving shell thickness and, thus, provides further insights into the intrinsic nature of the carbonate-derived integrated LMR-NCM active materials

Quantification of aging mechanisms of carbon-coated and uncoated silicon thin film anodes in lithium metal and lithium ion cells

In this work, a comprehensive investigation of the effect of carbon-coating on the aging mechanism of silicon thin films in lithium metal and lithium ion cells is presented. In Li||Si cells with sufficient lithium excess, 92% of the total capacity loss of the silicon film was attributed to a loss of active material and 8% was attributed to an increased cell resistance. Carbon-coating reduces the loss of active material by improving the mechanical integrity of the silicon thin film, leading to a 67% reduction of capacity loss per cycle. In Si||LiFePO4 lithium ion cells, 86% of the total capacity loss was attributed to a loss of lithium inventory and 14% attributed to an increase in cell resistance. Furthermore, a loss of 69% of silicon active material was observed. Carbon-coating reduces the capacity loss per cycle by 28%. After aging of the lithium ion cells, the negative electrode of the carbon-coated silicon retained double the capacity compared to the uncoated silicon electrode. Hence, carbon-coating is an effective measure to improve mechanical stability of silicon thin film electrodes, but it must be coupled with additional strategies to reduce lithium consumption in order to increase the overall effectiveness of the coating in lithium ion cells

Exploiting the Degradation Mechanism of NCM523Graphite Lithium‐Ion Full Cells Operated at High Voltage

Layered oxides, particularly including Li[NixCoyMnz]O2 (NCMxyz) materials, such as NCM523, are the most promising cathode materials for high‐energy lithium‐ion batteries (LIBs). One major strategy to increase the energy density of LIBs is to expand the cell voltage (>4.3 V). However, high‐voltage NCMurn:x-wiley:18645631:media:cssc202002113:cssc202002113-math-0002 graphite full cells typically suffer from drastic capacity fading, often referred to as “rollover” failure. In this study, the underlying degradation mechanisms responsible for failure of NCM523urn:x-wiley:18645631:media:cssc202002113:cssc202002113-math-0003 graphite full cells operated at 4.5 V are unraveled by a comprehensive study including the variation of different electrode and cell parameters. It is found that the “rollover” failure after around 50 cycles can be attributed to severe solid electrolyte interphase growth, owing to formation of thick deposits at the graphite anode surface through deposition of transition metals migrating from the cathode to the anode. These deposits induce the formation of Li metal dendrites, which, in the worst cases, result in a “rollover” failure owing to the generation of (micro‐) short circuits. Finally, approaches to overcome this dramatic failure mechanism are presented, for example, by use of single‐crystal NCM523 materials, showing no “rollover” failure even after 200 cycles. The suppression of cross‐talk phenomena in high‐voltage LIB cells is of utmost importance for achieving high cycling stability

Direct investigation of the interparticle-based state-of-charge distribution of polycrystalline NMC532 in lithium ion batteries by classification-single-particle-ICP-OES

The presented case study provides mesoscopic insights into the state-of-charge (SOC) distribution of battery electrodes containing layered transition metal oxides with Li(Ni0.5Mn0.3Co0.2)O2 (NMC532). The application of classification-single-particle inductively coupled plasma optical emission spectroscopy (CL-SP-ICP-OES) enables the rapid screening of the lithium content of individual cathode active material (CAM) particles achieving a statistically viable elucidation of the mesoscale SOC distribution between different particles of the electrode. The results reveal the evolution of a persistent mesoscale SOC heterogeneity of the electrode upon delithiation at slow rates and extensive relaxation times as confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The implications of local chemical and structural ramifications of the investigated NMC532 for heterogeneous active material utilization are thoroughly discussed. Furthermore, it is found that the evolved SOC heterogeneity of the electrode is strongly dependent on the current density. The correlation to the decreased capacity utilization is further investigated with a straightforward quantification approach revealing a considerable contribution to capacity fading by persistently inactive lithium in the CAM. The results highlight the importance of the analysis of persistent mesoscale SOC heterogeneity as a potential capacity fade mechanism in layered lithium transition metal oxide-based battery electrodes

Al2O3 protective coating on silicon thin film electrodes and its effect on the aging mechanisms of lithium metal and lithium ion cells

In this work, an investigation of the effect of Al2O3-coating on the aging mechanisms of silicon anode thin films in lithium metal and lithium ion cells is presented. Aging mechanisms, namely: loss of lithium inventory, loss of silicon active material and loss of utilizable capacity due to an increase of cell resistance were determined for both, Li||Si and Si||LiFePO4 cells. Al2O3-coating was shown to be an effective strategy to reduce the loss of lithium inventory, while having a marginal effect on decreasing the loss of silicon active material. Indeed, in case of Si||LiFePO4 cells, where fading is governed by loss of lithium inventory, a 5 nm Al2O3-coating leads to a significant reduction (-64%) of the capacity fade per cycle. On the contrary, in case of Li||Si, where the aging mechanism is governed by the loss of active material, Al2O3-coated and uncoated silicon showed comparable tendencies regarding the capacity fade per cycle. It emerges, also, that loss of silicon active material and loss of lithium inventory are independent of each other. This indicates that the main contribution of loss of lithium inventory is not the lithium trapped in electrically insulated silicon, but rather lithium consumed in the ongoing SEI formation. Al2O3-coating could reduce the latter due the insulating nature of the coating. Ex situ investigations of the SEI by means of X-ray photoelectron spectroscopy confirmed a decrease in solvent decomposition in presence of the Al2O3-coating