Composite Electrodes For Li-ion Batteries Research Articles

High energy density flexible and ecofriendly lithium-ion smart battery

The rapidly growing battery market demands both high energy density and waste-management solutions for the anticipated global annual battery waste of about two million metric tons. To address the energy-environment dilemma, we developed self-standing composite electrodes for Li-ion batteries without electrochemically inactive metal current collectors, additives, and binders, increasing energy density by up to 40%. The electrodes were prepared via in situ mixing of as-grown single-wall carbon nanotubes (SWNTs) with aerosolized electrode active materials, leading to adequate electrical conductivity and mechanical robustness. The SWNT scaffold exhibits a piezoresistive effect, with resistance depending on the voltage-weighted number of inter-nanotube contacts. We exploit this intrinsic piezoresistance of SWNT network for operando self-monitoring of battery structural health. The proposed solution-free battery fabrication technology and architecture eliminates environmentally harmful components and enables recycling by simple mechanical separation, aiming at safety and circular economy.

(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.

Silicon-based electrodes formulation in buffered solution for enhanced electrode-electrolyte interfaces

Silicon nanoparticles based composite electrodes for Li-ion batteries reach high specific capacities and enhance cyclability compared with larger silicon particles. Such electrodes are foreseen for the next generation Li-ion batteries (LiB). Playing on binder and conductive additives in the formulation of the Si based electrodes is a well-known strategy to enhance performance. Aqueous electrode formulations using carboxymethyl cellulose as environmentally friendly binder has already showed great cyclability enhancements, especially when employed in acidic conditions. In this study, pH = 1, 3 and 7 buffered solutions were studied as solvent to prepare the Si/C/CMC composite electrodes.The influence of the formulation pH on the electrode components interactions were followed through a combined ATR-FTIR/XPS experiment and discussed in relation with the electrode electrochemical performance. Interestingly, the pH = 7 buffered solution show increased capacity retention and coulombic efficiency during 100 cycles compared with the electrode obtained in acidic conditions. Post mortem analysis and EIS study highlighted differences of electrode/current collector (CC) adhesion and SEI deposition.

Characterizing the Surface Free Energy of Composite Electrodes for Li-Ion Batteries

One of the crucial steps in battery manufacturing is the electrolyte wetting step which typically requires a relatively long time to be completed. It is known that the relationship between polar and dispersive components of the electrolyte surface tension and the electrode surface free energy (SFE) plays a vital role in the extent of wetting and, thus, tuning the SFE can accelerate the wetting step in battery manufacturing. This work developed and validated a systematic approach to characterize surface free energy of composite electrodes for lithium-ion batteries, which has never been reported in literature. It’s demonstrated as shown in Figure 1 [1] that the SFE of LiNi0.5Mn0.3Co0.2O2 cathode was higher than the graphite anode. In addition, when switching manufacturing from conventional solvent based processing to aqueous processing, the polar component of SFE increases; it increases 74.1% from NMC532-NMP to NMC532-Water, and 48.2% from A12-NMP to A12-Water due to different binders. SFE decreases with reducing porosity. The results provide valuable guidance for our goal to optimize the electrolyte wetting process, helping lower costs and enable higher product quality. In addition, the method allows us to characterize the SFE of electrodes and optimize electrolyte wetting via tailored electrode manufacturing, including slurry and electrode formulation, mixing sequences, drying protocols and calendering. Figure 1 Surface energy of electrodes Reference: [1] A. Davoodabadi, J. Li, Y. Liang, R. Wang, H. Zhou, D.L. Wood III, T. J. Singler, and C. Jin, Journal of the Electrochemical Society, 165(11) (2018) A2493-A2501 Figure 1

Understanding of pre-lithiation of poly(acrylic acid) binder: Striking the balances between the cycling performance and slurry stability for silicon-graphite composite electrodes in Li-ion batteries

Poly(acrylic acid) (PAA) is widely used as a polymer binder for high capacity silicon (Si) anodes in Li-ion batteries. When used, the carboxyl (CO2H) groups of PAA can be partially lithiated (known as pre-lithiation treatment), which is believed to facilitates lamination process, especially for production on a large scale. However, such treatment impacts numerous physico-chemical properties of the PAA binder that affect the cycling performance of the electrode. Here we seek to quantify the pre-lithiation treatment effect on Li-ion cells containing Si-graphite composite electrodes. The electrochemical cycling results indicated that such pre-lithiation treatment of PAA can undermine the cycling performance, as more capacity loss was observed when pre-lithitated PAA binders were used. On the other hand, the same pre-lithiation practice is indeed beneficial for the lamination process, as it increases the viscosity of aqueous slurries at low shear rates (slows down sedimentation) and prompts the shear thinning (so that the slurries can be more easily mixed). Thus, there is an uneasy balance between the electrochemical performance that suffers from the pre-lithiation of PAA and the quality of slurry processing which benefits from the same practice. An alternative approach to slurry treatment would be desired to achieve better cycling performance without undermining the stability of slurry suspensions.

(Invited) Chemomechanical Behavior of Composite Electrodes in Li-Ion Batteries: Experiments and Modeling

The chemomechanical behavior of composite electrodes is largely different from that of single particles or mono-phase materials. The stress field, state of charge, and mechanical failure are strongly affected by the local details of the microstructure. On the experimental side, we study the structural disintegration of NMC cathode materials of a hierarchical structure by tracking the microstructural evolution of different marked regimes. Decohesion of primary particles constitutes the major mechanical degradation which results in the loss of connectivity of the conductive network and impedance increase. We employ operando nanoindentation to measure the evolution of elastic modulus, hardness, and interfacial strength of NMC cathodes at different states of charge and different cycles. We further use home-developed computational program to investigate the chemomechanical behavior of NMC composites in reconstructed 3D models. An ample space of design is open for the optimization of the capacity and mechanical performance of electrodes by tuning the size, shape, and pattern of active particles, as well as the properties of the inactive matrix.

Shortly Branched, Linear Dextrans as Efficient Binders for Silicon/Graphite Composite Electrodes in Li-Ion Batteries

Dextran was evaluated as a binder material for graphite electrode, silicon electrode, and Si/graphite composite electrode in Li-ion cells. The performance was also compared with poly(vinylidene fluoride) (PVDF) binder or lithium salt of poly(acrylic acid) (LiPAA) binder. A graphite-rich Si/graphite composite electrode shows a reversible capacity of about 525 mAh g–1 at a C/5 rate and has an excellent cycling stability. An interface study using impedance spectroscopy, X-ray photoelectron spectroscopy, and postmortem scanning electron microscopy analysis shows that the superior performance is due to the formation of a solid–electrolyte interface (SEI) layer that completely covers the surfaces of silicon and graphite. This could be attributed to a strong interaction between dextran through the shortly branched one-dimensional structure and silicon surface as evidenced by attenuated total reflection–infrared spectroscopy. This along with its low cost and environmentally friendly nature makes it an attractive binder material for Si/graphite composite electrodes in Li-ion batteries.

Charge-Transfer Perturbations Due to Interfaces on Electronic and Ionic Conductions within Composite Electrodes for Li-Ion Batteries

The understanding of electron and ion transfers in composite porous electrodes is essential to improve their electrochemical performance. Composite porous electrodes are generally made of an active material (AM), a carbon black conducting additive (CB) and a polymeric binder (B). At battery assembly, it is infiltrated by a liquid electrolyte (EL). An electrode has a double hierarchical structure: the first one consists of agglomerates of particles of AM and the second is of agglomerates of CB - B mixture. From the nanometric to the macroscopic level, this double structure is therefore a hierarchy of interfaces: AM/AM, CB/CB, CB/AM, EL/AM, EL/CB, electrode/current collector. The charge transports in composite electrodes are generally explained by considering the percolation and the tortuosity of the CB particles network for electrons on one hand, and of the pores network for ions on the other hand. The contributions of the different interfaces within composite electrodes are however scarcely studied. Indeed, this requires the simultaneous measurement of ionic and electronic conductivities. To do so, the development of an instrumentation that takes into account both the constraints imposed by the nature of the samples and the mobilities of mobile species (ions, electrons) is required. The in- and ex-situ dielectric spectroscopy enables this objective to be fulfilled over a wide frequency range from 40 Hz to 10 GHz between 200 and 300 K [1-5]. The nature and the quality of the interfaces depend on the shape and the dimensionality of the particles of the active material (for example, platelets, cuboids, etc.), which can strongly influence electronic transfers within the composite electrode [5]. Strong interactions between the liquid electrolyte and the active material, and between the electrolyte and the carbon black have been highlighted [3,4]. The binder (B) also affects the properties of these interfaces. Space charges, which are created on the surface of electronically conductive materials (AM and CB), generate electrical polarizations whose frequency responses are located in the range of radio frequencies and microwaves. The presence of an electrolyte modifies the intensities and dynamics of space charge polarizations because of ion-electron and dipole-electron Coulomb interactions [3,4]. On the other hand, ionic double-layer capacitances are created at the solid/liquid interfaces on the electrolyte side. The latter also generate electric polarizations at lower frequencies than those of electronic space charges because of the lower mobility of the ions. These different interactions modify the electronic mobility of the active material and the carbon black as well as the diffusion of the ions (cations and anions) of the confined electrolyte into the porous network of the electrode [4]. In this presentation, we will report our recent discoveries of the perturbations due to interfaces on electronic and ionic conductions within composite electrodes for Li-Ion batteries.

Crosslinked Chitosan Networks as Binders for Silicon/Graphite Composite Electrodes in Li-Ion Batteries

Silicon/graphite composites have the potential to improve the practical energy density of Li-ion batteries to enable mass-market penetration of electric vehicles. However, they require polymeric binders that are compatible with both silicon and graphite and can sustain alloying and intercalation reactions as well as the associated interfacial reactions. In this work, chitosan, a natural cellulose, is crosslinked with either molecular (citric acid) or polymeric (poly (acrylic acid), PAA) carboxylic acids to form networks with maximum interaction with the composite as evidenced by infrared spectroscopy. Li-ion half-cells of graphite-rich, silicon/graphite negative electrodes using crosslinked chitosan binders show higher initial Coulombic efficiency (ICE) and more stable cycling performance than pristine chitosan. This could be due to the polymeric network produced from the crosslinking reaction between chitosan and carboxyl acids as well as the strong interactions between polymeric network and surface of silicon and graphite as evidenced by adhesion tests. The crosslinked chitosan network can effectively accommodate large volume change of silicon particles and keep other electrode components connected during cycling as evidenced by scanning electron microscope (SEM) images leading to excellent cycling stability. This makes it very attractive for use as a binder in Si/graphite electrodes for Li-ion batteries.

A comparative study of polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) binders for Si-based electrodes

This work is a comparative study on two well-known binder systems for silicon-based negative composite electrodes for Li-ion batteries: polyacrylic acid (PAA) and carboxymethyl cellulose/citric acid (CMC/CA). The electrode slurry rheological properties and the dry electrode morphology and physical properties are studied as a function of the binder content. The electrochemical behavior of the electrodes is also studied as a function of the active mass loading (from 1 to 4.5 mg cm−2). Increasing the binder content from 4 to 12 wt% improves the 1st cycle efficiency, which is clearly related to its contribution as a pre-formed artificial solid electrolyte interphase (SEI) layer. The enhancement of the mechanical strength of the composites with increasing binder content is reflected in the better cyclability of the electrodes with active mass loading higher than 1 mg cm−2. CMC/CA is more efficient as a binder at low content while PAA is more efficient at high content, which could be related to their molecular structure. Finally, for both binders the influence of a post-processing treatment of the electrodes (called maturation) is shown to improve the electrochemical performance. This is however more significant with CMC/CA than with PAA.

Examining Inhomogeneous Degradation of Graphite/Carbon Black Composite Electrodes in Li-Ion Batteries by Lock-In Thermography

Understanding the degradation of graphite anodes plays a major role in the improvement of Li-ion batteries. Herein, we used the nondestructive character of lock-in thermography (LIT) to investigate the aging of graphite anodes in pouch cells. The aged cells show a typical ring pattern in lock-in thermography images, which can be explained by the help of Raman spectroscopy. We attribute this pattern to the different contact due to the formation of a gas during operation, which also leads to inhomogeneous degradation of the graphite electrode. Finally, we demonstrate the detection of two-dimensional variances on the sub-mm scale in the composition of the graphite anodes in pouch cells by LIT. For this purpose, we combined LIT measurements with Raman spectroscopy mapping to link differences in the heat emission to those in the structure of the graphite.

Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

In the emerging market of electric vehicles (EVs), the development of batteries with higher energy density and improved cycle-life is essential.1 Silicon-based anodes can significantly improve the energy density due to the huge specific capacity of ~3600 mAh/gSi, (corresponding to the Li15Si4 phase2) which is roughly ten times larger compared to conventional graphite electrodes (372 mAh/gC 3). However, the alloying of silicon with lithium is accompanied by large structural changes, resulting in a volume increase by 310% upon full lithiation.2 This causes fast degradation of the capacity due to particle cracking, loss of electric and ionic conductivity, increasing electrode polarization and continuous electrolyte consumption.4-6 In this work we will investigate the consumption of the electrolyte additive fluoroethylene carbonate (FEC) which is known to significantly improve the lifetime of Li-ion batteries with silicon anodes.7 Fig. 1 shows the cycling stability and coulombic efficiency of Si/C composites (40%wt nano-Si, 40%wt VGCF fibers, and 20%wt LiPAA binder; 1.2 mgSi/cm2) cycled in coin cells with metallic lithium counter electrodes and LP57 electrolyte containing various amounts of FEC. A striking feature is a rapid capacity drop which is delayed to higher and higher cycle numbers as more FEC is added to the electrolyte. By the use of 19F-NMR spectroscopy we demonstrate that the rapid capacity drop is due to the total consumptions of FEC. Its depletion causes a significant increase of the cell polarization, leading to the rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of µmolFEC/mgelectrode) in the cell. The lifetime of silicon anodes is therefore determined by the specific amount of FEC rather than by the FEC concentration. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in the literature, a new reductive decomposition mechanism for FEC is proposed yielding CO2, LiF, Li2O, Li2CO3, H2 and a partially cross-linked polymer. Fig. 1. (a) Coulombic efficiency and (b) specific lithiation capacity vs. cycle number of Si-Li coin cells with 75 µL LP57 electrolyte containing different amounts of FEC. The first three cycles are conducted at C/10 followed by cycling at C/3. The theoretical capacity is 1440 mAh/gelectrode and the specific amount of FEC (in µmolFEC/mgelectrode) is specified in the figure.

Computational analysis of chemomechanical behaviors of composite electrodes in Li-ion batteries

Abstract

Si Anode Diagnostic and Failure Mechanism in Full Li-Ion Cells Using NMR, STEM-Eels, XPS and FIB-TOF-SIMS Advanced Characterization Tools

Silicon-based electrodes are very attractive for lithium-ion batteries (LiB) compared to graphite due to their very high theoretical specific capacities (3572mAh g-1 versus 372mAh g-1 for graphite). Upon cycling, silicon electrodes are subject to capacity fading, due Si particle cracks and decrepitation, leading to electrical disconnection and creating new surfaces bound to react continuously with the electrolyte. Solving the large capacity fade observed during cycling of silicon electrodes is a complex issue. The behavior of silicon electrode in half cell set-up (i.e. cycled vs lithium metal) is now well understood and methods to optimize its performance have been extensively described. In this configuration, the lithium supply is not limited, therefore failure mechanisms can be completely different from those obtained in full cell (Li-ion) set-up. As of today, a much faster aging of silicon anode in full cell has been observed but the failure mechanism has not been understood. In this work, silicon electrodes are cycled in full cell set-up versus LiNi1/3Mn1/3Co1/3O2 as the positive electrode that initially contains the cyclable lithium. A combination of techniques, such as 7Li, 19F MAS NMR, XPS, TOF-SIMS and STEM-EELS, provides an in-depth characterization of the SEI forming on the surface of silicon particles as well as its evolution upon cycling. Synchrotron Reflectivity and TEM are used as well to probe the operando evolution of silicon anode upon cycling in half cells. Upon cycling, heterogeneous thick patches of LiF and carbonates appear at the surface of the Si particles. The development of the inorganic part of the SEI mostly occurs during the early stages of cycling, while an incessant degradation of the organic solvents of the electrolyte occurs continuously. After extended cycling, all the lithium from the positive electrode is consumed either trapped in an intermediate part of the SEI or in the electrolyte. While the cell cannot function properly anymore, degradation of the organic electrolyte solvents goes on, leading to the formation of Li-free organic degradation products thickening the SEI. Such comprehensive work allows for a better study of the failure mechanisms of Si-based composite electrodes for Li-ion batteries cycled in a Li-ion configuration. This also emphasizes the importance of achieving multiprobe and multiscale analysis by using various advanced characterization tools for this kind of study, in order to investigate various regions of an electrode at various scales, favoring a better understanding of the underlying mechanisms. From these multiprobe and multiscale characterization results, the mechanism of SEI formation will be described, and our comprehension of the origin of the much faster aging of silicon anode in full cell versus half cell will be disclosed.

Multiprobe and Multiscale Characterization of SEI Developing on Nano-Silicon Electrodes in a Full-Cell Setup By Using NMR, STEM-Eels, XPS and FIB-TOF-SIMS

Lithium-ion battery represents a quite promising energy storage technology addressing a large range of applications, including automotive applications. In order to meet their requirements, it is however necessary to find higher capacity electrode materials. Considering its specific capacity of 3579 mAh.g-1, silicon is one of the most promising materials to replace graphite as a negative electrode in this technology, opening the route to cheaper, though still safe, batteries. However Si-based electrodes suffer from poor cyclability due to the large volumetric expansion of Si particles upon cycling, as well as an unstable solid electrolyte interphase (SEI) related to a perpetual electrolyte degradation at the surface of the active material. In the end, this leads to Si particle pulverization (Si expansion) and loss of electrical contact within the electrode (Si expansion and SEI formation). The SEI dynamics of Si particles has been extensively studied with a large range of electrolytes in the literature [1-4]. However, all the studies describing the failure mechanisms of Si electrodes have been achieved in a half-cell setup so far. In such configuration, the Si electrode is cycled versus a lithium metal counter electrode: the supply of lithium is not limited. In the present work, an in-depth characterization of the SEI forming within Si electrodes cycled in a full-cell setup, with LiNi1/3Mn1/3Co1/3O2 as the positive electrode, has been achieved by using a combination of advanced characterization tools. 7Li/19F MAS NMR and STEM-EELS lead respectively to a global description of the SEI at the electrode scale, and to a local description of the SEI at the particle scale, while XPS allows to characterize the nature of the SEI developing at the electrode surface and FIB-TOF-SIMS brings complementary information concerning Li distribution throughout the electrode depth. Full batteries were prepared in a three electrodes configuration Swagelok cells. The latter were dismantled at different cycle numbers (1st, 10th and 100th at the end of lithiation and at the end of delithiation) in an argon glovebox before subsequent characterization. All electrochemical cycles were achieved in the electrolyte (1M LiPF6 in EC-DEC carbonates mixture) at a limited capacity of Si electrodes (1200mAh.g-1): this was necessary in order to keep the possibility of reaching a sufficiently high number of electrochemical cycles for the study. In a first phase, it was observed that Si electrodes simply soaked in the electrolyte enhances its degradation (this being highly favored by increasing the temperature) without the formation of a proper SEI at the surface of the silicon particles. This leads to a thin and heterogeneous interface layer, which remains quite stable after the first degradation step. Upon cycling, heterogeneous thick patches of LiF and carbonates appear at the surface of the Si particles: the development of the inorganic part of the SEI mostly occurs during the early stages of cycling, while an incessant degradation of the organic solvents of the electrolyte occurs endlessly. After extended cycling, all the lithium from the positive electrode is consumed, either trapped in an intermediate part of the SEI or in the electrolyte. While the cell cannot function properly anymore, degradation of the organic electrolyte solvents goes on, leading to the formation of Li-free organic degradation products thickening the SEI. Such comprehensive work allows for a better study of the failure mechanisms of Si-based composite electrodes for Li-ion batteries cycled in a full-cell setup. This also emphasizes the importance of achieving multiprobe and multiscale analysis by using various advanced characterization tools for this kind of study: such approach allows to investigate various regions of an electrode at various scales, favouring a better understanding of the underlying mechanisms. [1] M. N. Obrovac et al, J. Electrochem. Soc., 154 (2007) A103 [2] U. Kasavajjula et al, J. Power Sources, 163 (2007) 1003 [3] D. Mazouzi et al, Electrochem. Solid-State Lett., 12(11) (2009) A215 [4] W-J. Zhang, J. Power Sources, 196 (2011) 13

Grid indentation analysis of mechanical properties of composite electrodes in Li-ion batteries

Electrodes in commercial rechargeable batteries are microscopically heterogeneous materials. The constituent components, including active materials, polymeric binders, and porous conductive matrix, often have large variation in their mechanical properties, making the mechanical characterization of composite electrodes a challenging task. In a model system of LiNi0.5Mn0.3Co0.2O2 cathode, we employ the instrumented grid indentation to determine the elastic modulus and hardness of the constituent phases. The approach relies on a large array of nanoindentation experiments and statistical analysis of the resulting data provided that the maximum indentation depth is carefully chosen. The statistically extracted properties of the active particles and the surrounding medium are in good agreement with the tests of targeted indentation at selected sites. The combinatory technique of grid indentation and statistical deconvolution represents a fast and reliable route to quantify the mechanical properties of composite electrodes that feed the parametric input for the mechanics models.

Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

The electrolyte additive fluoroethylene carbonate (FEC) is known to significantly improve the lifetime of Li-ion batteries with silicon anodes. In this work, we show that FEC can indeed improve the lifetime of silicon-carbon composite anodes but is continuously consumed during electrochemical cycling. By the use of 19F-NMR spectroscopy and charge/discharge cycling we demonstrate that FEC is only capable to stabilize the cell performance as long as FEC is still remaining in the cell. Its total consumption causes a significant increase of the cell polarization leading to a rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of μmolFEC/mgelectrode) in the cell. The latter quantity therefore determines the lifetime of silicon anodes rather than the concentration of FEC in the electrolyte. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in literature, a new reductive decomposition mechanism for FEC is proposed yielding CO2, LiF, Li2O, Li2CO3, H2 and a partially cross-linked polymer.

Mechanical interactions regulated kinetics and morphology of composite electrodes in Li-ion batteries

Li-ion batteries are a system that features strong coupling between mechanical stresses and electrochemical reactions. Prior studies on stresses in electrodes are focused on single particles or mono-phase materials. The kinetics and morphology of composite electrodes regulated by the mechanical interactions are much less exploited. We integrate a continuum theory of coupled diffusion and large elasto-plastic deformation into a finite element program. Such a computational tool enables us to explore the intimate coupling between the lithiation kinetics and stresses in three-dimensional electrodes that are composed of multiple components. We find that Li profiles and stress states in multiple particles constrained by a matrix are significantly different from that in a free-standing configuration. The mechanical interactions regulate Li chemical potential in Si nanowires and transform the anisotropic deformation to an isotropic behavior and vice versa. The modeling is in good agreement with a recent experimental report.

Production of Sn–Cu/MWCNT composite electrodes for Li-ion batteries by using electroless tin coating

Cycling stability of pure tin electrodes were aimed to improve by using a suitable combination of copper and multiwalled carbon nanotubes (MWCNTs). For this purpose, firstly Sn–Cu composite powders were produced using an electroless process. Then, Sn–Cu/MWCNT composite electrodes were prepared with dispersing different amounts of MWCNT (10wt.%, 20wt.%, 40wt.%) by high energy mechanical milling method. The surface morphology of the produced Sn–Cu/MWCNT composite powders was characterized using scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) was used to determine the elemental surface composition of the composites. X-ray diffraction (XRD) analysis was performed to investigate the structure of the Sn–Cu/MWCNT composite powders. The electrochemical performance of Sn–Cu/MWCNT composite electrodes has been investigated by charge/discharge tests and cyclic voltammetry experiments. The cell discharge capacities were determined at a constant current in voltage range between 0.02V and 1.5V. AC Electrochemical Impedance Spectroscopy (EIS) analysis was also carried out to measure resistivity and Li-diffusion in the assembled cells. The amounts of MWCNTs were shown to be a crucial factor to improve Sn–Cu/MWCNT composite anodes for cyclability and reversible capacity.

Improvement cycleability of core–shell silicon/copper composite electrodes for Li-ion batteries by using electroless deposition of copper on silicon powders

In this work, core–shell Si/Cu composite powders were produced using an electroless process. The copper content on the surface of silicon powders was varied by using different concentrations of CuSO4 in the plating bath. The surface morphology of the produced Si/Cu composite powders was characterized using scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) was used to determine the elemental surface composition of the composites. X-ray diffraction (XRD) analysis was performed to investigate the structure of the Si/Cu composite powders. The discharge capacities and resistivity of the produced Si/Cu composite electrodes were studied using electrochemical impedance spectroscopy (EIS). The silicon/copper composite electrode produced using 40g/l CuSO4 exhibited the most stable capacity retention, and a discharge capacity of approximately 240mAh/g was obtained after 20cycles for this electrode. This study demonstrated that the conductivity of the electrodes was improved and the retention capacity of the Si/Cu composite electrodes was increased by increasing the copper content on the surface of silicon powders due to the buffering effect of copper against volume changes during the charging and discharging process.