Negative Electrode For Li-ion Batteries Research Articles

Exploration of the electrochemical properties for Zn3Mo2O9 with different morphologies as novel negative electrodes of Li-ion batteries

Exploration of the electrochemical properties for Zn3Mo2O9 with different morphologies as novel negative electrodes of Li-ion batteries

(Invited) Advanced Photoelectron Spectroscopy on Battery Materials: Lessons Learnt on Silicon- Containing Electrodes

While Silicon (Si) is being incorporated more and more as an active material in commercial lithium-ion batteries, it is still a topic of intense research and the motivation for these research efforts remains unchanged: Silicon’s large volume expansion during cycling leads rapid capacity fading largely associated with changes in the Si surface chemistry and unstable solid electrolyte interphase (SEI). Using advanced photoelectron spectroscopy (PES) characterization, we have investigated crucial interfaces on Si model systems and Si-containing negative electrodes for Li-ion batteries. Here, we focus on two approaches to influence the SEI composition intrinsically: Binders and electrolytes.Starting with pristine Si powders, we studied the effect of carboxylic acids as electrode slurry additives on the Si surface chemistry before and after electrochemical cycling [1] using synchrotron PES depth profiling. Further, we used photoelectron spectroscopy to confirm cross-linking in poly-acrylic acid-based binder for Si electrodes and showed that the cross-linking agent pentaerythritol had no negative effect on the SEI thus attributing improved cycling performance to increased binder cross-linking [2].In silicon/graphite negative electrodes, we used PES to elucidate the influence of fluoroethylene carbonate (FEC) on the degradation phenomena in electrodes with varying silicon content [3]. Finally, photoelectron spectroscopy provided insights into what happens to the SEI on Si-containing electrodes if we replace the classical electrolyte solvent ethylene carbonate with either FEC or glyoxylic acetal-based alternatives [4, 5].[1] F. Jeschull, H.Q. Pham, A. Ghamlouche, P.K. Thakur, S. Trabesinger, J. Maibach, J. Phys. Energy 2023, 5 (2), 025002. https://doi.org/10.1088/2515-7655/acbbee.[2] J. He, C. Das, F. Yang, J. Maibach, Electrochimica Acta 2022, 411, 140038. https://doi.org/10.1016/j.electacta.2022.140038.[3] A. Ghamlouche, M. Müller, F. Jeschull, J. Maibach, J. Electrochem. Soc. 2022. https://doi.org/10.1149/1945-7111/ac4cd3.[4] L. Gehrlein, C. Njel, F. Jeschull, J. Maibach, ACS Appl. Energy Mater. 2022, 5 (9), 10710–10720. https://doi.org/10.1021/acsaem.2c01454.[5] L. Gehrlein, C. Leibing, K. Pfeifer, F. Jeschull, A. Balducci, J. Maibach, Electrochimica Acta 2022, 140642–140642. https://doi.org/10.1016/j.electacta.2022.140642.

Morphology Control of Ball Milled Si-Ti Alloys and a Comparison of Their Surface Reactivity with SiOx

Si-based anode materials have been widely studied due to their higher volumetric capacity compared to graphite.1 However, they can suffer from poor cycling due to severe volume changes, crystallization effects (formation of Li15Si4), an unstable solid electrolyte interphase (SEI) and surface erosion. Alloying Si with an electrochemically inactive material can decrease the volume expansion and lead to better cycling performance. If nanostructured or amorphous Si-based alloys are used, the phase transition due to the crystallization effect is suppressed.1,2 Considering all improvements, Si-based anode materials still experience large surface erosion that continuously increases the surface area of active material particles during cycling. Different studies show that the main causes of surface erosion are the volume changes during cycling and the reaction between the active material and electrolyte.3,4 It is known that higher active material surface area leads to a higher reaction rate with electrolyte. This leads to a larger lithium inventory loss and a decrease in coulombic efficiency (CE), which is undesirable.5 Mechanical milling has long been used to create metal alloys. Cao et al showed that preparing Si-based alloys with a two-step milling method, in which Si is first milled alone with the metal added in the second step, produces Si-M alloys having an amorphous active Si-phase and a reduced surface area compared to conventional one-step milling.5 SiOx has also been extensively studied as an anode material because of reduced volume expansion, less side reactions with electrolyte, and high capacity retention during cycling.6 Despite decades of research to our knowledge, it has yet to be determined how the surface erosion or reactivity of SiOx compares with Si-based alloys with a similar surface area. This fundamental question needs to be answered if Si-based alloys are to be considered as candidates for practical application as negative electrode materials. A barrier to answering this question has been the very different surface areas of Si-based alloys (typically high surface area materials prepared by ball milling) and SiOx, which has a low surface area. Increasing the surface area of SiOx and reducing the surface area of the Si-based alloys using the two-step milling method can provide the opportunity to compare their surface reactivity.In this study, SixTi100-x (x= 85,70, 60) alloys were prepared by ball milling using the one-step and two-step methods, allowing low and high surface area alloys to be synthesized. The results show that reducing alloy surface area results in an increased CE, however, the difference is not pronounced. The surface erosion or reactivity of low surface area Si0.85Ti0.15 was compared with SiOx with a similar surface area. Cycling results show that Si0.85Ti0.15 has a higher CE than SiOx in initial cycles, but SiOx has a markedly higher CE after longer term cycling. These results are verified by direct observations of reduced surface erosion on cycled SiOx alloy particles compared to Si0.85Ti0.15 particles. Figure 1(a) and (b) shows the cross-section SEM images of low surface area Si0.85Ti0.15 and SiOx with a similar surface area after 50 cycles, respectively. The SiOx particles are almost pristine, while the Si0.85Ti0.15 particles are severely eroded. These results imply that the chemistry of SiOx plays a more important role in its good cycling performance than does its low surface area. References. Obrovac, M. N. & Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chem Rev 114, 11444–11502 (2014).Iaboni, D. S. M. & Obrovac, M. N. Li15Si4 Formation in Silicon Thin Film Negative Electrodes. J Electrochem Soc 163, A255 (2016).Chevrier, V. L., Aiken, C. & Krause, L. J. Impact of Electrolyte on the Cycling of Si-Based Materials. in 228th ECS Meeting – Phoenix (2015).4.Krause, L. J., Brandt, T., Chevrier, V. L. & Jensen, L. D. Surface Area Increase of Silicon Alloys in Li-Ion Full Cells Measured by Isothermal Heat Flow Calorimetry. J Electrochem Soc 164, A2277 (2017).Cao, S., Tahmasebi, M. H., Gracious, S., Bennett, J. C. & Obrovac, M. N. Preparation of Low Surface Area Si-Alloy Anodes for Li-Ion Cells by Ball Milling. J Electrochem Soc 169, 060540 (2022).Liu, Y., Charlton, M., Wang, J., Bennett, J. C. & Obrovac, M. N. Si85Fe15Ox Alloy Anode Materials with High Thermal Stability for Lithium Ion Batteries. J Electrochem Soc 168, 110521 (2021). Figure 1

Si1-xGex alloys as negative electrode for Li-ion batteries: Impact of morphology in the Li+ diffusion, performance and mechanism

Si1-xGex alloys present interesting cycling performance as Li-ion anodes, owing to the synergetic effect from silicon's high capacity and germanium's electrical conductivity and Li+ diffusion. Various morphologies of Si1-xGex powders were obtained, micron-sized by ball-Milling (BM) and nano-sized by laser pyrolysis (LP) to study the effect of the morphology and composition on the electrochemical behavior. The electrical conductivity was measured and shows an increase with the Ge content, from 4.58E-03 for Si to 0.11 S m−1 for Si0.5Ge0.5. Half-cells were cycled at C/5, LP samples showed a better capacity retention than their BM counterparts (88% vs 72 % at the 35th cycle for Si0.5Ge0.5). To rationalize these trends, Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) were used to determine the apparent Li+Diffusion in the Si1-xGex series. It was shown that the apparent Li+Diffusion is dependent to the state of charge. Moreover, it gave insight about phase transformations during cycling. During lithiation, similar values (10−11cm2 s−1) were obtained for BM and LP Si0.5Ge0.5. However, a big variation (10−13 – 10−10cm2 s−1) was found for the BM sample delithiation, which is attributed to the delithiation of c-Li15(Si0.5Ge0.5)4 phase into amorphous Lix(Si0.5Ge0.5) (x<3.75). Asymmetric C-rate tests evidenced lithiation as the limiting mechanism for the Si1-xGex negative electrodes.

Using Aquatic Plant-Derived Biochars as Carbon Materials for the Negative Electrodes of Li-Ion Batteries

The current study focuses on the production of biochars derived from aquatic plants, specifically red seaweed Ahnfeltia and seagrass Zostera and Ruppia, found in brackish lagoons in the Sea of Okhotsk, Sakhalin Island. These biochars were obtained through a stepwise pyrolysis process conducted at temperatures of 500 and 700 °C. The characteristics of the biochars, including their elemental composition, specific surface area, and particle size distribution, were found to be influenced by both the type of biomass used and the pyrolysis temperature. The primary objective of this research was to investigate the potential of these biochars to be used as negative electrodes for lithium ion batteries. Among the various samples we tested, the biochar derived from the macroalgae Ahnfeltia tobuchiensis, produced at 700 °C, exhibited the highest carbon content (70 at%) and nitrogen content (&gt;5 at%). The reversible capacity of this particular biochar was measured to be 391 mAh g−1 during the initial cycles and remained relatively stable at around 300 mAh g−1 after 25 cycles. These findings suggest that biochars derived from aquatic plants have the potential to be utilized as effective electrode materials in lithium ion batteries. The specific properties of the biochar, such as its elemental composition and surface area, play a significant role in determining its electrochemical performance. Further research and optimization of the pyrolysis conditions may lead to the development of biochar-based electrodes with improved capacity and cycling stability, thereby contributing to the advancement of sustainable and environmentally friendly energy storage systems.

Machine learning-based high-throughput screening of Mg-containing alloys for hydrogen storage and energy conversion applications

The development of novel materials for hydrogen storage and conversion applications is expected to facilitate the transition to clean energy. In particular, near-ambient hydrogen storage, thermal energy storage, and lithium conversion electrodes are selected in this study as the applications for which the development of novel Mg-containing materials is of great importance. We utilize a machine learning model, based on the graph neural network, developed for predicting hydride formation enthalpy in intermetallic compounds, to perform high-throughput screening based on the atomic composition and crystal structure of the starting intermetallic compounds. Trends and structure-property relations are discussed, as well as the possibilities for tailoring the stability of Mg-containing hydrides by alloying. For 636 compounds identified as stable by DFT calculations, we predict hydride formation enthalpy and equilibrium potential of metal hydride conversion electrode for Li-ion batteries. Based on the predicted enthalpy of hydride formation, 32 intermetallics are identified as suitable for near-ambient hydrogen storage applications. Among them, MgBe13, seen as a promising material to achieve a high gravimetric density of hydrogen, is additionally studied using DFT. Further investigation of the Na-Mg-Al alloys is proposed as a good route in the search for new thermal energy storage materials. Binary Mg-containing intermetallics are discussed as conversion-type negative electrodes in Li-ion batteries.

Highly Reversible Ti/Sn Oxide Nanocomposite Electrodes for Lithium Ion Batteries Obtained by Oxidation of Ti3 Al(1-x) Snx C2 Phases.

Among the materials for the negative electrodes in Li-ion batteries, oxides capable of reacting with Li+ via intercalation/conversion/alloying are extremely interesting due to their high specific capacities but suffer from poor mechanical stability. A new way to design nanocomposites based on the (Ti/Sn)O2 system is the partial oxidation of the tin-containing MAX phase of Ti3 Al(1-x) Snx O2 composition. Exploiting this strategy, this work develops composite electrodes of (Ti/Sn)O2 and MAX phase capable of withstanding over 600 cycles in half cells with charge efficiencies higher than 99.5% and specific capacities comparable to those of graphite and higher than lithium titanate (Li4 Ti5 O12 ) or MXenes electrodes. These unprecedented electrochemical performances are also demonstrated at full cell level in the presence of a low cobalt content layered oxide and explained through an accurate chemical, morphological, and structural investigation which reveals the intimate contact between the MAX phase and the oxide particles. During the oxidation process, electroactive nanoparticles of TiO2 and Ti(1-y) Sny O2 nucleate on the surface of the unreacted MAX phase which therefore acts both as a conductive agent and as a buffer to preserve the mechanical integrity of the oxide during the lithiation and delithiation cycles.

Photolithographic Structuring of Ordered Silicon Micropillar Electrodes for Lithium-Ion Batteries and Electrochemical Performance

Ordered and vertically oriented silicon (Si) micropillar structures, when optimally designed, can accommodate the lithiation-induced volumetric expansion that is limiting the implementation of Si as negative electrodes in Li-ion batteries. As there is not much work in this area, this study is focused on evaluating systematically the potential of Si micropillar electrodes prepared by photolithographic structuring of Si. The specific objective is to determine the effect of micropillar length and the free space around the pillars on the electrochemical performance of the electrode. The Si micropillar electrode with the pillar length of 14 μm showed a high specific capacity (2600 mAh/g) and a high areal capacity (3 mAh/cm2), which were stable for over 100 cycles. However, there was significant capacity degradation in 32 and 48 μm-long Si pillar electrodes due to cracking and electrode damage during cycling. In the 32 μm Si micropillar electrode, a small change in porosity from 69 to 86% caused a substantial increase in specific capacity from 1400 to 2000 mAh/g over 100 cycles, with the areal capacity decreasing from 3 to 2 mAh/cm2. The findings also show the directions for further work for the optimization of Si micropillar electrodes.

Defective graphene decorated with TiO2 nanoparticles as negative electrode in Li-ion batteries

In this work, the performance of novel negative electrodes for Li-ion batteries based on defective graphene synthesized via a scalable thermal exfoliation of graphite oxide and decorated with TiO2 nanoparticles is investigated. Titania polymorphs are interesting as battery electrode materials, owing to their high cycle stability, safety, abundance and negligible solid electrolyte interphase formation and volume changes upon cycling. Defective graphene, on the other hand, can embed TiO2 nanoparticles, forming a conductive hybrid nanocomposite anode material for Li-ion batteries, with improved Li-ion and electron transport, optimising power density. Here we propose two different synthetic approaches for the decoration of graphene with TiO2 nanoparticles: I) a novel chemical route where TiO2 nanoparticles, mainly anatase, were grown on graphene in hydrothermal mild conditions and II) a physical solid-state approach where hydrothermal TiO2 nanoparticles and graphene were mixed together via high energy ball-milling. The synthesized materials were analysed via powder X-ray diffraction, micro-Raman spectroscopy and HR-TEM, while the electrodes were electrochemically tested. Operando synchrotron diffraction was conducted on half-cell to investigate phase transitions in the electrode materials. Even in presence of small amount of graphene, significant improvement in capacity, reversibility, and high-rate capability were observed. In particular, the sample obtained through the addition of 1 wt% of graphene displayed a reversible capacity of more than 180 mA h/g after prolonged and wearing cycling. This result outperforms by 327 % the reversible capacity of pure TiO2 electrode.

Nanoarchitecture factors of solid electrolyte interphase formation via 3D nano-rheology microscopy and surface force-distance spectroscopy

The solid electrolyte interphase in rechargeable Li-ion batteries, its dynamics and, significantly, its nanoscale structure and composition, hold clues to high-performing and safe energy storage. Unfortunately, knowledge of solid electrolyte interphase formation is limited due to the lack of in situ nano-characterization tools for probing solid-liquid interfaces. Here, we link electrochemical atomic force microscopy, three-dimensional nano-rheology microscopy and surface force-distance spectroscopy, to study, in situ and operando, the dynamic formation of the solid electrolyte interphase starting from a few 0.1 nm thick electrical double layer to the full three-dimensional nanostructured solid electrolyte interphase on the typical graphite basal and edge planes in a Li-ion battery negative electrode. By probing the arrangement of solvent molecules and ions within the electric double layer and quantifying the three-dimensional mechanical property distribution of organic and inorganic components in the as-formed solid electrolyte interphase layer, we reveal the nanoarchitecture factors and atomistic picture of initial solid electrolyte interphase formation on graphite-based negative electrodes in strongly and weakly solvating electrolytes.

MnP4 electrode for Na-ion batteries: a complex and effective electrochemical mechanism

MnP4 has recently been identified as a possible negative electrode for Li-ion batteries.

Heterostructured and Mesoporous Nb2O5@TiO2 Core-Shell Spheres as the Negative Electrode in Li-Ion Batteries

Niobium pentoxides have received considerable attention and are promising anode materials for lithium-ion batteries (LIBs), due to their fast Li storage kinetics and high capacity. However, their cycling stability and rate performance are still limited owing to their intrinsic insulating properties and structural degradation during charging and discharging. Herein, a series of mesoporous Nb2O5@TiO2 core-shell spherical heterostructures have been prepared for the first time by a sol-gel method and investigated as anode materials in LIBs. Mesoporosity can provide numerous open and short pathways for Li+ diffusion; meanwhile, heterostructures can simultaneously enhance the electronic conductivity and thus improve the rate capability. The TiO2 coating layer shows robust crystalline skeletons during repeated lithium insertion and extraction processes, retaining high structural integrity and, thereby, enhancing cycling stability. The electrochemical behavior is strongly dependent on the thickness of the TiO2 layer. After optimization, a mesoporous Nb2O5@TiO2 core-shell structure with a ∼13 nm thick TiO2 layer delivers a high specific capacity of 136 mA h g-1 at 5 A g-1 and exceptional cycling stability (88.3% retention over 1000 cycles at 0.5 A g-1). This work provides a facile strategy to obtain mesoporous Nb2O5@TiO2 core-shell spherical structures and underlines the importance of structural engineering for improving the performance of battery materials.

Boron-Doped Methylated Amorphous Silicon for Negative Electrodes in Li-Ion Batteries

In spite of its outstanding capacity for alloying with lithium, silicon cannot be practically used as a negative electrode for Li-ion batteries: its large volume expansion upon lithiation leads to a poor capacity retention [1]. Promising results have been obtained by incorporating methyl groups in amorphous silicon (methylated amorphous silicon). This material exhibits an improved stability upon electrochemical cycling while keeping a capacity close to that of pure silicon [2]. However, the conductivity of methylated amorphous silicon may be a strong limitation, especially at high methyl content: for example, 10% methylated amorphous silicon is 10000 more resistive than pure amorphous silicon.Doping is a well-known method to enhance the electronic conductivity of semiconductors, even if the dopant activity is lower in amorphous semiconductors than in crystalline ones. 2% boron doping increases the conductivity of 10% methylated amorphous silicon by five orders of magnitude compared to the undoped material.Boron doped methylated silicon thin films (100nm thick) with various methyl content were cycled in the range 0.025V – 1V at C/2 rate (electrolyte: LP30 with 5%FEC). 10% methylated amorphous silicon with 2% boron doping exhibits a capacity retention of 70% after 500 cycles of full lithiation/delithiation, an improved performance as compared to the undoped material (see Figure 1a). Interestingly, boron doping allows for using higher methyl content without demanding pre-conditioning procedures for the electrochemical cycling of the material. The stability upon cycling is found to be further increased for 15% and 20% methylated electrodes, with a capacity retention exceeding 80% over 1000 cycles of full lithiation/delithiation (Figure 1b). This figure comes at the expense of a decreased total capacity (which remains 3 to 4 times larger than that of the current carbon electrodes). The SEI evolution and structural changes are currently investigated using operando ATR FTIR and ex-situ Raman spectroscopies, in order to rationalize the factors limiting the Coulombic efficiency to 99.7%.

Accelerated Solid Electrolyte Interface Quality Assessment of Lithium - Ion Batteries By Mediator-Enhanced Coulometry

Li-ion batteries (LIBs) offer particularly high performance among rechargeable batteries and are used in a variety of industrial domains. They were primarily used as a power supply for portable devices in the past. In recent years their applications have expanded to encompass stationary energy storage systems and electric vehicles (EVs), driving demand for lower-cost LIBs with even higher performance. Demand for LIBs for use in EVs is growing particularly rapidly as governments around the world fast-track measures to promote automobile electrification1 , 2.The quality of the Solid Electrolyte Interphase (SEI) on the negative electrode of Li-ion batteries has a strong impact on the cycle life so that clear determination of their properties is of essential importance to achieve progress in manufacturing processes3. Many kinds of measurements and tests are necessary at each state of the battery manufacturing process to assure the quality of each process. In addition, measurements and testing are essential in a variety of settings, during not only manufacturing, but also for battery research and finished-product inspections.In this work, a new, cheap and easily-implementable methodology to estimate the electrically-insulating quality of the SEI in LIBs is proposed. First, a redox-mediator is added in the electrolyte after the SEI formation cycle, and the redox mediator leads to an internal self-discharge process that is inversely proportional to the electrically-insulating character of the SEI. Second, a few charge and discharge cycles are applied to the battery and the presence of the redox-mediator provokes a shuttle effect enables by the lack of electrically protecting character of the SEI which consumes charges decreasing the coulombic efficiencies, enhancing the sensibility to the SEI protecting nature.The proposed methodology is probed on lithium iron phosphate batteries in pouch cell configuration with two redox mediators (ferrocene (FC) and methyl phthalimide (PHT)) by evaluating the influence of vinylene carbonate as electrolyte additive in the resulting SEI.We believe that the findings based on the application of this mediator-enhanced coulometry can be used to accurately predict the cyclic behavior of LIBs under extended operating conditions, which is especially relevant for a better comprehension of future industrial needs for battery R&D in cell components and production fields. Acknowledgment The authors acknowledge the financial support by the NanoBat project. NanoBat has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement no. 861962 References S. Manzetti and F. Mariasiu, Renewable & sustainable energy reviews, 51, 1004–1012 (2015).E. Fan et al., Chemical Reviews (2020).E. Ventosa, Current Opinion in Electrochemistry, 25, 100635 (2021) https://www.sciencedirect.com/science/article/pii/S2451910320301708. Figure 1

In-Operando Neutron Diffraction Investigation of Structural Transitions during Lithiation of Si Electrode in Li-Ion Battery

A major challenge for Si negative electrodes in Li-ion batteries is how to accommodate the large lithiation-induced volume expansion and prevent electrode fragmentation, such that impressive Li storage capacity of Si can be exploited in practice. Electrochemically etched Si mesoporous electrodes have significant potential in this context. This research is focused on an optimum mesoporous Si electrode structure that shows a very high energy storage density, electrochemically cycling well without cracking or fragmentation. To explore the factors causing the superior performance, this study performed in-operando neutron diffraction experiments on optimized electrode during lithiation-delithiation cycles in a simple in-situ electrochemical cell. It is shown that an unusual diffraction phenomenon arises from lithiation-induced expansion of Si leading to the development of mosaic structure in Si. This new phenomenon appears to arise from the increased contribution of kinematic diffraction from the lithiated Si, relative to the conventional dynamic diffraction. This is also supported by changes in diffraction intensities directly synchronizing with the volume changes in Si during lithiation-delithiation cycle. The in-operando experiments explain why the optimum mesoporous Si electrode possesses a high specific capacity without electrode fragmentation. The present findings can help to improve Si electrode designs for high energy density Li-ion batteries.

Entropy Profiling for the Diagnosis of NCA/Gr-SiOx Li-Ion Battery Health

Graphite-silicon (Gr-Si) blends have become common in commercial Li-ion battery negative electrodes, offering increased capacity over pure graphite. Lithiation/delithiation of the silicon particles results in volume changes, which may be associated with increased hysteresis of the open circuit potential (OCP). The OCP is a function of both concentration and temperature. Entropy change measurement—which probes the response of the OCP to temperature—offers a unique battery diagnostics tool. While entropy change measurements have previously been applied to study degradation, the implications of Si additives on the entropy profiles of commercial cells have not been explored. Here, we use entropy profiling to track ageing markers in the same way as differential voltage analysis. In addition to lithiation/delithiation hysteresis in the OCP of Gr-Si blends, cells with Gr-Si anodes also exhibit differences in entropy profile depending on cycling direction, reflecting degradation-related morphological changes. For cycled cells, entropy change decreased during discharge, likely corresponding to graphite particles breaking and cracking. However, entropy change during charge increased with cycling, likely due to the volume change of silicon. Over a broad voltage range, these combined effects led to the observed rise in entropy hysteresis with age. Conversely, for calendar aged cells entropy hysteresis remained stable.

New Technique for Probing the Protecting Character of the Solid Electrolyte Interphase as a Critical but Elusive Property for Pursuing Long Cycle Life Lithium-Ion Batteries.

The formation of a protecting nanolayer, so-called solid electrolyte interphase (SEI), on the negative electrode of Li-ion batteries (LIBs) from product precipitation of the cathodic decomposition of the electrolyte is a blessing since the electrically insulating nature of this nanolayer protects the electrode surface, preventing continuous electrolyte decomposition and enabling the large nominal cell voltage of LIBs, e.g., 3.3–3.8 V. Thus, the protection performance of the nanolayer SEI is essential for LIBs to achieve a long cycle life. Unfortunately, the evaluation of this critical property of the SEI is not trivial. Herein, a new, cheap, and easily implementable methodology, the redox-mediated enhanced coulometry, is presented to estimate the protecting quality of the SEI. The key element of the methodology is the addition of a redox mediator in the electrolyte during the degassing step (after the SEI formation cycle). The redox mediator leads to an internal self-discharge process that is inversely proportional to the protecting character of the SEI. Also, the self-discharge process results in an easily measurable decrease in Coulombic efficiency. The influence of vinylene carbonate as an electrolyte additive in the resulting SEI is used as a case study to showcase the potential of the proposed methodology.

The Role of Silicon in Silicon-Graphite Composite Electrodes Regarding Specific Capacity, Cycle Stability, and Expansion

Silicon-based electrodes are promising candidates to enable the next generation of Lithium-ion batteries with high energy densities on the cell level beyond 350 Wh kg−1 [1, 2]. Nevertheless, the commercialization of silicon-based electrodes is still hindered due to the large volume expansion during cycling of up to 300% [3]. This volume expansion upon repeated (de-)lithiation of silicon particles worsens the electrode integrity by causing isolation of active material components [4-6]. Continuous electrolyte decomposition at the silicon/electrolyte interface, caused by the repeated volume expansion and contraction, results in a gradual loss of active lithium [7].While X-ray diffraction (XRD) as well as theoretical methods provide atomic scale information, dilatometry is an appropriate tool for the in-situ investigation of the volumetric work of battery composite electrodes on the macroscopic level. Therefore, the technique itself as well as the obtained data have a high potential for their industrial implementation [8].In this work, the expansion of a single-side-coated silicon-based anode is investigated with a commercial, high-precision dilatometer, which detects the thickness change perpendicular to the surface of the coating on the electrode level during cycling against a lithium metal counter electrode. The measured samples consist of 95 wt.% active material (0 - 20 wt.% of silicon in combination with 95 - 75 wt.% of graphite, respectively) and 5 wt.% of non-active components.The investigated samples in this work differ in their silicon and graphite content. Moreover, the samples were also investigated electrochemically via conventional coin cells. The comparable electrochemical behavior of the electrodes in both experiment methods is proof of the plausibility of the dilatometry results. Increasing the silicon content led to higher specific capacities of the SiG electrodes. The relative capacity loss and the thickness change increased linearly with the silicon content as well. Based on the characteristic thickness change for the SiGs, we concluded that silicon predominantly lithiated before graphite, and delithiated after graphite.The used non-commercial electrodes were designed and produced within the scope of the project “HighSafeII”, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0306B. Literature [1] D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, and B. Stiaszny, “Future generations of cathode materials: an automotive industry perspective”, J. Mater. Chem. A, 3 (13), 6709 (2015).[2] O. Gröger, H. A. Gasteiger, and J.-P. Suchsland, “Review—Electromobility: Batteries or Fuel Cells?”, J. Electrochem. Soc., 162 (14), A2605 (2015).[3] D. Ma, Z. Cao, and A. Hu, “Si-Based Anode Materials for Li-Ion Batteries: A Mini Review” Nano-Micro Lett, 6(4), 347 (2014).[4] M. N. Obrovac, L. Christensen, D. B. Le, and J. R. Dahn, “Alloy Design for Lithium-Ion Battery Anodes”, J. Electrochem. Soc., 154 (9), A849 (2007).[5] D. S. M. Iaboni and M. N. Obrovac, “Li15Si4 Formation in Silicon Thin Film Negative Electrodes”, J. Electrochem. Soc., 163 (2), 255 (2016).[6] M. N. Obrovac and V. L. Chevrier, “Alloy Negative Electrodes for Li-Ion Batteries”, Chem. Rev., 114, 11444 (2014).[7] R. Petibon, V. Chevrier, C. P. Aiken, D. S. Hall, S. Hyatt, R. Shunmugasundaram, and J. R. Dahn, “Studies of the Capacity Fade Mechanisms of LiCoO2/Si-Alloy: Graphite Cells”, J. Electrochem. Soc., 163 (7), 1146 (2016).[8] Daniel Sauerteig, Svetlozar Ivanov, Holger Reinshagen, Andreas Bund, „Reversible and irreversible dilation of lithium-ion battery electrodes investigated by in-situ dilatometry“, J. Power Sources, 342, 939-946 (2017).

(Digital Presentation) Do Silicon-Based Composite Electrodes for Li-Ion Batteries Need High Molecular Weight Polymeric Binder to Well Perform?

Among all potential active materials for the anodes of Li-ion batteries, silicon is considered as one of the most promising candidate because of its high specific and volumetric capacities, about 3600 mAh.g-1 and 2200 mAh.cm-3 respectively,1 as well as its low operation voltage (0.4 versus Li/Li+), abundant resources (the second largest in the earth’s crust), and environmental benignity (non-toxic). However, during the lithiation (formation of LixSi, x ~ 3.75) the silicon undergoes an expansion of about 280% of its initial volume2 which induces numerous damages to the electrode. The silicon micrometer particles tend to be pulverized when the size of the particles are upper than 150 nm.3 However, nanosizing the materials leads to more electrolyte degradation due to a higher specific surface developed and requires more binder and eventually more conducting additive in the electrode formulation. The pulverized Si particles are scattered and become electrically isolated and the adhesion of the Si electrode to the current collector are also damaged by these volume variations4. This effects drastically reduces the electrochemical performance and lifetime of the Si electrode. As a result, these challenges have seriously restricted the commercialization of Si anodes in high energy LIBs.5 To deal with these problems many researches are developing advanced binders. This later has a crucial role since it reinforces the mechanical strength of the electrode and thus helps to preserve the electrode architecture upon cycling against the large Si volume change.6 It is generally accepted that binders of very high molar masses (polymers) are necessary to allow cycling of silicon-based electrodes, as they favor the formation of more robust molecular bridges and therefore are a priori more capable of maintaining particle-to-particle contacts and therefore cohesion in the electrode. Quite surprisingly, we have discovered that it is possible to obtain good cyclability of silicon-based electrodes by using an organic binder of low molar mass (molecule). This calls into question the understanding of the mechanism by which the binder operates in silicon-based electrodes.This low molecular weight binder is a natural polyphenol, namely tannic acid. Here we will highlight that Silicon-based composite electrodes of high areal capacity (~7 mAh.cm-2) when prepared with tannic acid as small binder show a stable cycling like the one obtained with one state-of-the-art binder such as carboxymethyl cellulose. We will report in-depth characterization of the interactions between tannic acid and the silicon particles surface as well as of the rheological behavior of the electrode slurries and the electrodes properties and electrochemical performances.References(1) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev. 2014, 114 (23), 11444–11502.(2) Jung, S. C.; Choi, J. W.; Han, Y.-K. Anisotropic Volume Expansion of Crystalline Silicon during Electrochemical Lithium Insertion: An Atomic Level Rationale. Nano Lett. 2012, 12 (10), 5342–5347.(3) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6 (2), 1522–1531.(4) Hernandez, C. R.; Etiemble, A.; Douillard, T.; Mazouzi, D.; Karkar, Z.; Maire, E.; Guyomard, D.; Lestriez, B.; Roué, L. A Facile and Very Effective Method to Enhance the Mechanical Strength and the Cyclability of Si-Based Electrodes for Li-Ion Batteries. Adv. Energy Mater. 2018, 8 (6), 1701787.(5) Ko, M.; Chae, S.; Cho, J. Challenges in Accommodating Volume Change of Si Anodes for Li-Ion Batteries. ChemElectroChem 2015, 2 (11), 1645–1651.(6) Eshetu, G. G.; Figgemeier, E. Confronting the Challenges of Next-Generation Silicon Anode-Based Lithium-Ion Batteries: Role of Designer Electrolyte Additives and Polymeric Binders. ChemSusChem 2019, 12 (12), 2515–2539.

Phase Diagram of Li-Graphite Intercalation Compound Formed by the Charge/Discharge Reaction in Li-Ion Battery

The graphite negative electrode for Li-ion batteries was subjected to precision charge/discharge measurements at low current loads. The derivatives of the potential vs capacity curve and the potential vs C/Li composition curve were used to analyze elementary charge/discharge reactions. The number of peaks in the potential-capacity derivative curve increased with decreasing current load, and the elementary reactions, based on the formation of an unconfirmed superlattice structure, could be clearly separated and analyzed. By synchronizing these results with operando synchrotron X-ray diffraction data, a phase diagram of the Li-graphite intercalation compound formed by the charge/discharge reaction in Li-ion battery was established.