Benchmarking operando neutron diffraction for high-power Li-ion batteries

Homogeneity in Li-ion battery electrodes is a crucial element in obtaining high performance and long cycle life. In this work inhomogeneities in lithium distribution in NMC cathodes and graphite anodes in cylindrical cells have been tracked in real time during cycling, using operando and ex situ high energy X-ray diffraction. A set of 18650 cells having undergone different cycling protocols and at different stages of degradation have been studied during operation at two different C-rates using synchrotron radiation, scanned along the height of the cells. Significant lithium gradients along the height of the cells have been found, with large changes in inhomogeneity over the course of up to 11 charge/discharge cycles performed operando. Lithium inhomogeneity has been found to be different in cathodes and anodes, and depend heavily on degradation state, state of charge, C-rate, and relaxation steps. Phase relaxation during OCV was quantified, such that after charge LiC6+LiC18+C→LiC12, and after discharge LiC12+C→LiC18.

Energy storage media based on different technologies have gained in importance for a wide field of applications ranging from supplying portable devices to large electric vehicles. In recent decades the energy storage technology, based on lithium ions, dominates to the market due to the best compromise between energy and power densities (both gravimetric and volumetric), low self-discharge when not in use, tiny memory effect etc. Despite the overall success of Li-ion technology, a further progress permanently demands for lower-cost, longer-life, higher energy/power density batteries resulting in active development and research to the field. Nowadays modern Li-ion batteries are sophisticated electrochemical devices, possessing numerous degrees of freedom (chemical, morphological, transport) along with complicated geometries of the electrode integration. This along with the need to minimize the risks for possible materials oxidation, electrolyte evaporation, cell charge changes etc. requires new dedicated experimental techniques capable to reveal “live” information about processes occurring inside the cell. In such instance neutron scattering is already a well-established experimental technique for the characterization of Li-ion batteries1. Neutron scattering methods when combined with electrochemical characterization undergo an increasing relevance for studies of lithium-ion based electrochemical energy storage systems on different length scales, e.g. neutron imaging, reflectometry, small-angle neutron scattering, quasielastic neutron scattering and powder diffraction. Nowadays in situ experiments with neutrons are performed on different self-developed/special test cells and commercial Li-ion cells of diverse designs depending on the research targets and needs. Simple in principle, but complicated in practice, designs of modern Li-ion batteries may result in spatial inhomogeneity of current, lithium or electrolyte distribution, which are often difficult to quantify, but they will surely affect performance, cycling stability and/or safety. Despite the increasing popularity of neutron scattering studies of batteries at their operating conditions the problem of cell non-uniformity (indirectly pointed by electrochemical measurements) and its effect is often not properly accounted in literature. Here a combination of three neutron-based experimental techniques, namely computed neutron tomography, high-resolution neutron powder diffraction and spatially-resolved neutron powder diffraction, applied in situ for studies on commercial Li-ion cells of the 18650-type will be presented along with results of electrochemical studies. The details of the cell organization on different length scales and its evolution on various factors like state-of-charge, temperature and fatigue will be presented in light of 3D lithium distribution in cylinder-type2-3 and prismatic4 Li-ion batteries. Reference s : [1] H. Ehrenberg, A. Senyshyn, M. Hinterstein, H. Fuess, in: E.J. Mittermeijer & U. Welzel (Eds.), Modern Diffraction Methods, Weinheim: Wiley-VCH, 2012, pp. 491-518. [2] A. Senyshyn, M. J. Mühlbauer, O. Dolotko, M. Hofmann, H. Ehrenberg, Sci. Rep. 5 (2015) 18380 [3] M.J. Mühlbauer, O. Dolotko, M.Hofmann, H.Ehrenberg, A.Senyshyn, J. Power Sources 348 (2017) 145-149 [4] M.J. Mühlbauer, A. Schökel, M. Etter, V. Baran, A. Senyshyn, J. Power Sources 403 (2018) 49-55

Performing in situ and operando measurements on electrode materials for Li-ion and Na-ion batteries is of importance for their understanding and improvement. Electrode materials need to be studied in their environment (in situ) and in real time while they function (operando), since they normally operate in non-equilibrium conditions. Real-time experiments upon charge/discharge of the electrodes (i.e. upon lithium or sodium extraction/insertion from/into the electrodes) unveil dynamics that are not accessible by other means and allow a more complete understanding of the electrodes’ functioning. The use of different probes is an important requirement for the study of such reactions. The combined use of X-Ray Powder Diffraction (XRPD), Synchrotron radiation XRPD and Neutrons Powder Diffraction (NPD) allows observing any atomic element in any crystalline electrode. However, custom setups are required to carry out operandodiffraction experiments on batteries. We will address the development of a sample environment suitable for both electrochemistry and neutron powder diffraction using an electrochemical cell featuring a (Ti,Zr) alloy and a deuterated version of the electrolyte [1]. The cell is able to function using massive electrodes (massive on the battery scale, but a small sample for neutrons) with good performances and able to give NPD patterns of high quality for data analysis. Importantly, we showed the possibility to succeed in reliable structural refinements (by the Rietveld method) and thus to observe structural modifications in details, from unit cell parameters to atomic coordinates and even site occupancy factors. We will discuss a few studies done with this setup, namely the observation of lithium extraction from different samples in the family of spinels Li1+xMn2-xO4. We performed NPD in real time on three samples (LiMn2O4, Li1.05Mn1.95O4 and Li1.10Mn1.90O4) and showed how the Li/Mn ratio influences the phase diagram of the material [2]. New insights obtained from high resolution – high flux Synchrotron X-Ray diffraction data will be presented, in particular the existence of a Li+ and Mn3+/Mn4+ ordering within the spinel-delithiated composition Li0.5Mn2O4[3]. Acknowledgements This research is performed in the frame of the French network RS2E (http://www.energie-rs2e.com) and of the European network ALISTORE-ERI (http://www.alistore.eu).

A robust 2D organic polysulfane nanosheet with grafted polycyclic sulfur for highly reversible and durable lithium-organosulfur batteries

Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still unknown. In this work, a combination of operando gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNixMnyCo1-x-yO2) and LCO (LiCoO2) electrodes. A partially delithiated LFP (LixFePO4) counter electrode was used to selectively identify the products formed through processes at the positive electrode. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (<80% state-of-charge, SOC), ethylene carbonate (EC) is dehydrogenated to form vinylene carbonate (VC) at the NMC surface, whereas at high potentials (>80% SOC), 1O2 released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to CO2, CO and H2O. The formation of water via this mechanism was confirmed by reacting 17O-labelled 1O2 with EC and characterising the reaction products via 1H and 17O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/LixFePO4 cells, which is ascribed to the consumption of water (from the reaction of 1O2 and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells.

Neutron diffraction study of [formula omitted], a nonstoichiometric ramsdellite and its transition to a double hexagonal close packed structure for x = 0

Rapid proton transport in solid-hosts promotes a new chemistry in achieving high-rate Faradaic electrodes. Exploring the possibility of hydronium intercalation is essential for advancing proton-based charge storage. Nevertheless, this is yet to be revealed. Herein, a new host is reported of hexagonal molybdates, (A2 O)x ·MoO3 ·(H2 O)y (A = Na+ , NH4 + ), and hydronium (de)intercalation is demonstrated with experiments. Hexagonal molybdates show a battery-type initial reduction followed by intercalation pseudocapacitance. Fast rate of 200 C (40 A g-1 ) and long lifespan of 30000 cycles are achieved in electrodes of monocrystals even over 200µm. Solid-state nuclear magnetic resonance confirms hydronium intercalations, and operando measurements using electrochemical quartz crystal microbalance and synchrotron X-ray diffraction disclose distinct intercalation behaviours in different electrolyte concentrations. Remarkably, characterizations of the cycled electrodes show nearly identical structures and suggest equilibrium products are minimally influenced by the extent of proton solvation. These results offer new insights into proton electrochemistry and will advance correlated high-power batteries and beyond.

Expanded graphite was prepared by the intercalation of natural graphite using ammonium peroxodisulfate as an oxidizing agent and a high purity of sulfuric acid as an intercalate. The amounts of oxidizing agent and intercalate were changed to determine the preparation conditions of expanded graphite as the anode material for high-power Li-ion batteries. The expanded graphite was tested as the anode material and further composited with the different amounts of petroleum pitch to improve the electrochemical properties. Although the expanded graphite anode showed the improved electrochemical properties such as initial reversible capacities of around 400 mAh/g and a charge capacity at 5 C-rate of 83 mAh/g as compared with those for the natural graphite anode of 378 and 19.4 mAh/g, respectively, it still had some weak points for a high-power anode material such as low initial efficiency and potential plateaus with the stage characteristic. The anode composites with high performance could be obtained by compositing the expanded graphite and the petroleum pitch at the ratio of 1:2, showing an improved initial efficiency of 78 % and decreased potential plateaus with 389 mAh/g of the initial reversible capacity.

Inhomogeneous distribution of lithium and electrolyte in aged Li-ion cylindrical cells

Atomic mechanism of the distribution and diffusion of lithium in a cracked Si anode

Distribution of lithium in doped nuclear pores of polyethylene terephthalate by neutron depth profiling

Developing high performance Li-ion batteries (LiBs) is of great interest to meet a demand for primary power sources that can operate portable electronic devices, hybrid or full-electric vehicles, and renewable energy storage grids. Among the various electrode materials available for LiBs, silicon has been recognised as the most promising anode material due to its extremely high lithiation capacity (theoretically, 4200 mAh/g for Li4.4Si alloy phase), which is over 10 times higher than that of graphite (372 mAh/g), the traditional anode material in LiBs. Nevertheless, the practical application of Si in the commercialized LiBs is still challenging due to its massive volume variation (approximately 300%) that occurs during electrochemical lithiation/delithiation. The volume variation can result in the fracture and pulverization of Si particles, which eventually causes a loss of an electrical contact between the electrode materials, then finally leading rapid capacity fading during further cycling. In order to address this issue, tremendous attempts have been devoted mostly through structural modification of Si materials, including synthesis of nano-structured Si (nanowires, nanotubes, yolk-shell, and nanoporous Si) and Si-based composite materials using nano-carbon matrix (CNTs, CNFs, and graphene). Owing to the efficient accommodation of the volume variation of Si, which result from the well-designed nanostructure or the buffering effect of the nano-carbon matrix, these Si-based electrodes exhibit the significant improvements in the cycling stability. However these strategies generally involve complicated synthetic steps or require additional materials, thereby their scalability and compatibility remains a challenge under the cost-related practical point of view. Furthermore, the structure-modified Si, electrode active material, cannot stand alone to construct electrode for conventional battery cells. To support the sufficient electrical conductivity and structural robustness into the battery electrodes, a large amount, typically over 30%, of inactive components including conductive agent and polymeric binder is required. However this portion of inactive components is relatively higher than that used for the practical electrode formulations (less than total 20%) and it may result in the decrease in the energy density of LiBs. Given these limitations, increasing attention has been focused on the advancement of the inactive components of battery electrodes. Traditionally, these inactive components possess their own independent functionality. The polymeric binder covers the active materials and conductive agent to mechanically combine together in the entire electrode. And the conductive agent creates the conducting bridges between active materials for their electrochemical utilization. Such a classic electrode system that consists of tri-components (active material, conductive agent, and polymeric binder) is very typical in the conventional LiBs, but it does not suitably work for the Si-based electrodes which have huge volume variation. This is attributed to the break of connections between the non-adhesive conductive agents and the active materials by continuous volume variation. To the end, bi-component electrode system based on dual-functional conducting polymer were further developed to enhance the electrochemical performance of Si-based electrodes. Here, the conducting polymer plays two roles simultaneously not only as a conductive agent but also as a binder, and it can replace the traditional two inactive components by its dual-functionality. This nature of conducting polymer allow to more effectively maintain the continuous electrical network even during huge volume variation of active Si. Additionally, the bi-component electrode that only consists of active Si material and conducting polymer can maximize the energy density of the LiB cells by minimizing portion of total inactive components, which might be great merit of this bi-component system. Based on the above concepts, herein we propose the advanced bi-component Si electrode by employing poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), one of the most common and the successful conducting polymer. PEDOT:PSS is a commercially acquirable polyelectrolyte with excellent physical properties including high electrical conductivity, chemical stability, and mechanically robust. Owing to these great advantages, there has been several effort directed toward developing battery electrodes by exploiting the intrinsic properties of PEDOT:PSS. Here, we report the Si/PEDOT:PSS bi-components electrode via a very simple strategy; the high solubility in water caused by PSS polyanion chains enable the PEDOT:PSS solution to be directly adopted into simple slurry casting technique, the most efficient way that was used in the practical battery electrode manufacturing process. This simple synthesis strategy should allow for the development of Si-electrode, and should also serve as a guide for engineers and scientists engaged in the rational design of electrode system for LIBs.

: The ultimate goal of this project is to develop a viable C-coating process of the current collector in order to improve the overall power performance of the electrode of Li-ion batteries. During this third-year period, the performance of different thickness of carbon coating on Al foil is compared. The result shows that only the samples with sufficient thickness to form conductive channel on the surface would give a positive effect on the performance. Scale up of this coating process is further developed to make 1.5 m long C-coated tape by a contentious process followed with a special batch thermal process called roll-calcination. The same positive effects seen on small samples have been reproduced.

Safety problems of high energy density and high-power Li-ion batteries (LIBs) associated with thermal runaway (TRA) have become a serious problem resulting in massive recalls by manufacturers, and at times even endangering the lives of the consumers. In general, the TRA consists of a battery’s rapid self-heating sourced from exothermic (electro-)chemical reactions and/or mechanical abuse. It is impossible to directly monitor the occurring events during the TRA in practical operating conditions (e.g., in cell phones or electric vehicles). However, the change in electrical parameters (pattern) during the TRA-like events could potentially indicate the existence of a failure, thus allowing to foresee the LIBs malfunction.In this contribution, we employ multi-physics modeling and machine learning (ML) to address this important TRA problem in LIBs. Specifically, we use ML techniques to learn and predict the likelihood of the TRA, using the data acquired from the multi-physics modeling of LIBs. The multi-physics modeling approach is comprised of a coupled thermal, electrochemical (P2D model) and degradation sub-models. In this work, a degradation phenomenon leading to the TRA is the solid electrolyte interface (SEI) formation/decomposition in the negative electrode. Subsequently, due to the time-varying characteristic of the variables affecting TRA (i.e, voltage, C-rate, state-of-charge, temperature, etc.), we propose three ML techniques, namely, Support Vector Machine, Deep Neural Network, and Recurrent Neural Network to be customized and applied for determining the TRA likelihood. For deep learning methods, we propose a new activation function, TK-MARS, based upon a partitioning based multivariate adaptive regression spline (MARS). Finally, to mitigate the uncertainty of the prediction results under different degradation conditions, we built an ensemble framework (i.e., a committee of single prediction models) incorporating the trained individual models, in order to maximize the robustness and generalization power of the prediction for a new unseen observation without prior knowledge on the degradation. Thus, the developed combined multi-physics and ML modeling approach establish a basis for accelerated or ‘on-the-fly’ prediction of the TRA as well as a framework for extending machine learning methodologies to broad applications in electrochemistry.

Highly catalytic electrodes play a vital role in exploiting the capability of vanadium redox flow batteries (VRFBs), but they suffer from a tedious synthesis process and ambiguous interaction mechanisms for catalytic sites. Herein, a facile urea pyrolysis process was applied to prepare graphitic carbon nitride-modified graphite felt (GF@CN), and by the virtue of a density functional theory-assisted calculation, the electron-rich pyridinic nitrogen atom of CN granules is demonstrated as the adsorption center for redox species and plays the key role in improving the performance of VRFBs, with 800 cycles and an energy efficiency of 75% at 150 mA cm-2. Such experimental and computational collaborative investigations guide a realizable and cost-effective solution for other high-power flow batteries.