Graphite-based Negative Electrodes Research Articles

Monitoring the State of Health (SOH) of Green Batteries (GreenBat)

Here we focus on the study of state of health monitoring of secondary batteries with advanced “green” formulations. European recycle map on battery energy storage plans to replace positive electrodes consisting of cobalt (Co) and fluorinated (F) graphite-based negative electrodes constituents with environmentally sustainable alternatives. Recently, "green formulations" was validated; in particular, recent trends suggest the use of electrodes based on silicon (Si), Co-free manganites (LiMnO2) aqueous soluble binders and fluorine-free electrolytes [1]. Our aims are to describe the health and the degradation of the cells with electrochemical techniques and to describe the chemical and morphological changes occurring during the battery cycling, through spectroscopies (Raman and IR) operating in parallel with impedance analysis. These experimental data will be integrated together in a multiblock dataset, which will constitute the basis for chemometric processing. The purpose of this chemometric modelling is to correlate SOH evolution to the chemical and physical state of the battery [1]. Here we illustrate the analysis of SOH of two different pouch cells formulation constituted by LTO (Lithium-Titanate) as anode, LFP (Lithium-Iron-Phosphate) as cathode and the LP30 electrolyte (EC:DMC 1:1 and LiPF6 1M); the second formulation is constituted by a thin film layer of commercial Li metal (purity of 99.7%) as anode, LFP as cathode and an ethers based formulation as electrolyte. After formation cycles (CC-CV), the cells have been submitted to aging tests. The different operating conditions are chosen with an Experimental Design (ED) and supported by a PCA analysis. The PCA analysis it was used to choose the operating parameters for the subsequent aging experiments. Is illustrated also the analysis of SOH of a coin cell formulation consisted by a LRLO (Lithium Rich Layer Oxide) as anode, commercial Li metal layer as cathode and LP30 additive with a ionic liquid (IL) as electrolyte. Turning to the chemometric modelling, as a starting point we verified the ability of PLS regression applied to the voltage profiles during charge cycles to estimate the SOH of a benchmark dataset by Lin et al. [1] References [1] M. Lin,D. Wu, Journal of Power Sources, 2022, 518, 230774. Figure 1

Identification of sodiation mechanisms in graphite-based negative electrodes by molecular dynamics simulations combined with potential of mean force

In this study, we presented a theoretical investigation focused on the free energy barriers associated with Na+ intercalation in various negative electrode morphologies based on graphite immersed in the [Pyr14][Tf2N] ionic liquid. Additionally, we explored the influence of triglyme on the co-intercalation process. Our findings reveal that larger interlayer distances effectively reduce the energy barrier for sodiation. However, this also promotes the intercalation of [Pyr14]+ ions and decreases the population of sodium-ions within the electrode. We observed the co-intercalation mechanism in all electrode morphologies, but the interlayer distance significantly impacts this behavior. Therefore, using electrode materials with interlayer distances slightly larger than those of graphite shows promise for enhancing Na+ intercalation. Nevertheless, when designing the electrolyte, careful consideration should be given to the size and shape of the cation in the ionic liquid to prevent unintended intercalation events.

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.

Lithiated Prussian blue analogues as positive electrode active materials for stable non-aqueous lithium-ion batteries

Prussian blue analogues (PBAs) are appealing active materials for post-lithium electrochemical energy storage. However, PBAs are not generally suitable for non-aqueous Li-ion storage due to their instability upon prolonged cycling. Herein, we assess the feasibility of PBAs with various lithium content for non-aqueous Li-ion storage. We determine the crystal structure of the lithiated PBAs via neutron powder diffraction measurements and investigate the influence of water on structural stability and Li-ion migration through operando X-ray diffraction measurements and bond valence simulations. Furthermore, we demonstrate that a positive electrode containing Li2-xFeFe(CN)6⋅nH2O (0 ≤ x ≤ 2) active material coupled with a Li metal electrode and a LiPF6-containing organic-based electrolyte in coin cell configuration delivers an initial discharge capacity of 142 mAh g−1 at 19 mA g−1 and a discharge capacity retention of 80.7% after 1000 cycles at 1.9 A g−1. By replacing the lithium metal with a graphite-based negative electrode, we also report a coin cell capable of cycling for more than 370 cycles at 190 mA g−1 with a stable discharge capacity of about 105 mAh g−1 and a discharge capacity retention of 98% at 25 °C.

Poly(hydroxybutyrate-co-hydroxyvalerate) as a biodegradable binder in a negative electrode material for lithium-ion batteries

In this work, graphite-based negative electrode for lithium-ion battery consisting a novel and biodegradable binder poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) is compared with standard graphite electrode with polyvinylidene fluoride (PVDF) as a binder. The rate and cycling performance of lithium ion insertion/extraction of electrodes with PHBV in a half-cell configuration are evaluated. Moreover, on the basis of the electrochemical tests it is concluded that the electrode with PHBV binder is characterized by similar specific capacity and diffusion coefficient of lithium ions as conventional graphite electrode with PVDF binder.

Effect of Temperature on the Deterioration of Graphite-Based Negative Electrodes during the Prolonged Cycling of Li-ion Batteries

In this paper, we report the effects of temperature on the deterioration of graphite-based negative electrodes during the long-term cycling of lithium-ion batteries (LIBs). After cycling 75 Ah pouch-type LIB full cells at temperatures of 45°C (45-Cell) and 25°C (25-Cell) until their end of life, we expected to observe changes in the negative electrode according to the temperature. The thickness of the negative electrode of the cell was greater after cycling; that of the electrode of 45-Cell (144 μm) was greater than that of the electrode of 25-Cell (109 μm). Cross-sectional scanning electron microscopy analysis confirmed that by-products caused this increase in the thickness of the negative electrode. The by-products that formed on the surface of the negative electrode during cycling increased the surface resistance and decreased the electrical conductivity. Voltage profiles showed that the negative electrode of 25-Cell exhibited an 84.7% retention of the initial capacity, whereas that of 45-Cell showed only a 70.3% retention. The results of this study are expected to be relevant to future analyses of the deterioration characteristics of the negative electrode and battery deterioration mechanisms, and are also expected to provide basic data for advanced battery design.

Research and Development of Thermally Durable Electrolyte for Lithium Ion Battery

For ensuring safety of lithium ion batteries (LIBs), we have extensively investigated the quasi-solid electrolyte where lithium ion conducive liquid is quasi-solidified at silica surfaces as thermally durable electrolyte, and applied it to high capacity and high energy density LIB. For the liquid phase, a solvate ionic liquid, which is an equimolar complex of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraethylene glycol dimethyl ether (G4), Li(G4)TFSA, was used. For enhancing discharge capability at a higher rate, Li(G4)TFSA was diluted by low viscos solvent such as propylene carbonate (PC). The developed electrolyte possessed a favorable volatilization temperature higher than 373 K. A 100-Wh-class laminated LIB with energy density of 363 Wh L−1 was fabricated by employing the electrolyte to graphite-LiNixCoyMnzO2 chemistry, and it generated neither fire nor smoke in a nail-penetration test. The result suggest that the developed LIB has high safety compared to a LIB comprised of a conventional organic liquid electrolyte. In addition, to enhance the cycle life of the LIB, the formation and growth mechanism of a solid-electrolyte interphase on a graphite-based negative electrode was investigated. Nuclear magnetic resonance and hard x-ray photoelectron spectroscopy revealed that the decompositions of LiTFSA, PC, and G4 contributed to the SEI formation at the initial charge, and that continuous decompositions of G4 and PC were a major reason for the SEI growth during charge-discharge cycles. Based on these analysis, we have substituted a highly concentrated sulfolane based liquid which exhibits a high Li ion conductivity with less amount of the low viscos solvent, for the G4 based liquid. The modification effectively improved the electrochemical durability of the electrolyte, leading to a higher capacity retention after charge-discharge cycle test.

Analysis of Solid-Electrolyte Interphase at the Interface between a Graphite Negative Electrode and a Diluted Solvate Ionic Liquid-Based Quasi-Solid-State Electrolyte

The formation and growth mechanism of a solid-electrolyte interphase (SEI) on a graphite-based negative electrode was investigated to enhance the cycle life of lithium ion batteries with a quasi-solid state electrolyte (QSE). In a QSE, liquid constituents including solvate ionic liquid (SIL), diluting solvent, and additives are quasi-solidified on surface of silica particles, which ensures the safety of a 100 Wh class laminated cell. For the SIL, an equimolar complex composed of tetraethylene glycol dimethyl ether (G4) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), was utilized. Propylene carbonate (PC) was used as diluting solvent to enhance the ionic conductivity of the SIL. Vinylene carbonate (VC) additive was introduced to form a robust SEI for inhibiting the reductive decomposition of G4 and PC. Nuclear magnetic resonance and hard X-ray photoelectron spectroscopy revealed that the decompositions of LiTFSA, PC, and G4 contributed to the SEI formation at the initial charge. During charge-discharge cycles, continuous decompositions of G4 and PC were a major reason for the SEI growth. To suppress the decomposition, charging at a low rate was introduced at beginning of the initial charge to enhance the VC decomposition and the robust SEI formation. Consequently, the decomposition of the QSE was inhibited, which enhanced its cycle life.

Cross Sectional Mapping of Nano-Mechanical Properties of Composite Electrodes for Lithium Ion Batteries Using Bimodal Mode Atomic Force Microscopy

Lithium-ion batteries (LIBs) have been widely used for various applications including portable devices and electric vehicles. Improvement of the cyclability and capacity is an important challenge. In the manufacturing process, active materials, conductive additives and binders are mixed in a solvent at a certain ratio and the slurry is casted, dried and pressed on a metallic foil current collector to yield a composite film. The composite films are assembled together with an electrolyte solution and a separator in coin or laminate cells. The packing density and mechanical strength of each material are important parameters for the cell performance because the volume expansion and resulting mechanical stress may cause a destruction of the electrical connections between binders, current collectors and the active materials during the charge/discharge processes. Young's modulus is one of the most important mechanical properties because it is an indicator of the mechanical strain in a material under a certain amount of stress. Nanoindentation has been used to measure the Young's moduli of various materials. However, it has a few restrictions as follows: (i) sample surface has to be destructed in nanoindentation and (ii) it has a limited spatial resolution, typically a few micrometers, due to the size of the disruption. Although recent atomic force microscopy (AFM) techniques allow non-destructive and high-resolution Young's modulus mapping by measuring force curves at each point using relatively weak force of<100 nN, this method is not applicable for stiff materials with modulus larger than 100 GPa because their sample deformation is too small to be detected by the cantilever deflection. Here, a newly developed technique, bimodal AFM that allows nanoscale mechanical mapping for a wide range of Young's modulus from 0.1 to 200 GPa in a non-destructive manner, was applied to LiCoO2-based positive and graphite-based negative composite electrodes before and after the charge-discharge cycles in a coin cell for a demonstration. Reference: H. Sakai, Y. Taniguchi, K. Uosaki, T. Masuda, “Quantitative cross-sectional mapping of nanomechanical properties of composite films for lithium ion batteries using bimodal mode atomic force microscopy”, 2019, 413, 29-33.

A comprehensive insight into the volumetric response of graphite electrodes upon sodium co-intercalation in ether-based electrolytes

In this study, the interplay between the electrochemical behavior, structural modifications and volumetric changes of graphite-based negative electrodes for sodium ion batteries is evaluated. The electrolyte solvents chosen for this study are ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TriGDME), and tetraethylene glycol dimethyl ether (TEGDME) while the salt is sodium trifluoromethanesulfonate (NaOTf). The volume changes undergone upon co-(de)intercalation of the [Na-solvent]+ complexes are systematically investigated by means of in situ electrochemical dilatometry and correlated to the structural modification of the graphite crystalline lattice as detected by in situ X-ray diffraction. The expected staging mechanism upon co-intercalation, leading to the formation of stage-I graphite intercalation compound, is observed with all electrolytes. However, the various solvents play a substantial role on the reversibility of such structural changes in the first cycle. The effect of temperature is also investigated showing that the most stable electrochemical performance is observed with the TEGDME at room temperature, but with TriGDME at higher temperatures. Nonetheless, post-mortem scanning electron microscopy suggests that the surface layer forming after the first sodiation may dissolve in the following de-sodiation.

Quantitative cross-sectional mapping of nanomechanical properties of composite films for lithium ion batteries using bimodal mode atomic force microscopy

Nanoscale Young's modulus mapping of the cross-section of electrode composite films used in lithium ion batteries was carried out using bimodal atomic force microscopy. Clear difference in Young's modulus was observed between the particles of active materials and matrix of conductive additives/binders in the composites of LiCoO2-based positive and graphite-based negative electrodes. Interestingly, there were a few particles showing significantly reduced Young's modulus in the 100% state-of-charge positive electrode although such particles were not present in the pristine electrode.

Pentafluorophenyl Isocyanate As Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium Ion Full Cells

Lithium ion batteries (LIBs) are considered as key technology for stationary energy storage systems and especially as power supply for electric vehicles (EVs). Even though LIBs are already used in EVs, there is a need of further improvements of the LIBs to achieve driving ranges of more than 500 km, what is considered to be a value for greater consumer acceptance of EVs. To reach this goal, the specific energy and energy density of LIBs need to be increased to approximately 350 Wh kg-1 and 750 Wh L-1 at cell level, respectively. The use of alternative anode materials to replace the state-of-the-art graphite anode, is considered as an efficient strategy to increase the energy density of LIBs. Silicon (Si) turns out to be the most promising material for advanced anodes in LIBs, as it offers a nearly 10 times higher specific capacity compared to graphite. However, the implementation of electrode materials containing contents of more than 5-10% Si in commercial LIBs is still hampered by huge volume changes leading to a continuous solid electrolyte interphase (SEI) re-formation, loss of active lithium and, therefore, to a poor capacity retention.[1] The application of nano-sized Si materials like nanoparticles, nanowires or thin films is reported to significantly improve the performance of Si based anodes, due to better accommodation of the huge volume changes upon lithiation/de-lithiation. Additionally, Si thin film electrodes exhibit improved specific capacities in comparison to typical composite-based electrodes, because of the absence of inactive components like binder and conductive additives.[2] An additional approach to improve the performance of Si based electrodes is the addition of additives to the electrolyte. Electrolyte additives like fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are commonly known to enhance the capacity retention of Si electrodes by forming a more stable SEI, thus, preventing ongoing electrolyte decomposition and continuous active lithium loss.[3] Isocyanate compounds are able to undergo reductive polymerization and, therefore, may be considered as effective electrolyte additives for Si anodes. Actually, several isocyanates were reported to function as effective film-forming additives for graphite-based negative electrodes.[4] Within this work, magnetron sputtering was utilized for the preparation of thin film Si anodes, which contain neither a binder nor a conductive agent. Therefore, the effect of the electrolyte additive can be directly related to the Si active material. Since it was recently reported, that lithium consumption related to SEI reformation is the main failure mechanism of lithium ion full cells containing a Si anode[5], the electrochemical performance of these Si thin film electrodes was investigated in Si/NMC-111 full cells using different electrolyte formulations. DFT calculations (HOMO/LUMO energies) were performed prior to electrochemical investigations for reductive and oxidative stability predictions of the electrolyte solvent and additive molecules. The addition of the pentafluorophenyl isocyanate electrolyte additive leads to an increased Coulombic efficiency and a significantly enhanced capacity retention of the LIB full cells during prolonged cycling, in comparison to the baseline electrolyte. Post-mortem investigations of the negative Si electrodes by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were performed to study the SEI layer, formed in the different electrolyte formulations. The enhanced cycling performance of the full cells can be correlated to an improved SEI formation. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] S. Dalavi, P. Guduru, B.L. Lucht, Performance Enhancing Electrolyte Additives for Lithium Ion Batteries with Silicon Anodes, Journal of The Electrochemical Society, 159 (2012) A642-A646. [4] C. Korepp, W. Kern, E.A. Lanzer, P.R. Raimann, J.O. Besenhard, M.H. Yang, K.C. Möller, D.T. Shieh, M. Winter, Isocyanate compounds as electrolyte additives for lithium-ion batteries, Journal of Power Sources, 174 (2007) 387-393. [5] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887.

Solid electrolyte interphase: Can faster formation at lower potentials yield better performance?

To make a Lithium Ion Battery (LIB) reliably rechargeable over many cycles, its graphite-based negative electrode requires the solid electrolyte interphase (SEI) as a protection layer. The SEI is formed through chemical and particularly electrochemical side reactions of electrolyte components in the first charging cycle(s) after manufacturing of a LIB. The SEI ideally serves two purposes: (i) act as a sieve permeable to Li ions but not to other electrolyte components and (ii) passivate the electrode against further electrolyte decomposition. Core element of conventional SEI formation is a lengthy, low-current galvanostatic charging step, which due to its time consumption contributes heavily to cell manufacturing costs. Here, we report on some non-conventional SEI formation protocols for composite carbon electrodes, inspired by recent experimental findings at smooth model electrodes. Acknowledging that the SEI forms in two main steps, taking place in a high-potential and a low-potential region, respectively, we demonstrate that less time spent in the high-potential region not only makes the process faster but even yields SEIs with superior kinetic properties. We tentatively explain this via basic rules of thin film growth and the role of grain boundaries for ion transport. We also report on the positive influence of multi-frequency potential modulations applied between high-potential and low-potential formation. Given that any new cell chemistry in principle requires its own tailor-made formation process, technologic success of future LIB cells will benefit from a systematic, well-understood toolbox of formation protocols. This paper is meant as a first step, highlighting potentially low-hanging fruits, but also flagging the demand for further systematic studies on model systems and on commercially manufactured cells.

(Invited) Electrode Crosstalk during Lithium-Ion Battery Aging

Lithium-ion cells containing layered-oxide based positive electrodes and graphite-based negative electrodes are being cycled at high voltages (> 4.30 V) to increase energy density. In this presentation we will detail performance changes in these cells during cycling, with a particular focus on the crosstalk between electrodes. We show that the dissolution of transition metal (TM) ions from the positive electrode, and their migration and incorporation into the solid electrolyte interphase (SEI) of the negative electrode, accelerates markedly as the upper cutoff voltage (UCV) exceeds 4.30 V. At voltages ≥ 4.40 V there is enhanced fracture of the oxide during cycling that creates new surfaces and causes increased solvent oxidation and TM dissolution. Despite this deterioration, cell capacity fade still mainly results from lithium loss in the negative electrode SEI. Among TMs, Mn content in the SEI shows a better correlation with cell capacity loss than Co and Ni contents. We estimate that each Mn ion deposited in the SEI causes trapping of ~100 additional Li+ ions thereby hastening the depletion of cyclable lithium ions. During the presentation we will highlight an electrocatalysis mechanism through which the deposited Mn ions accelerate electrolyte reduction and increase Li trapping in the negative electrode SEI. Methods to mitigate this cross-talk, through the use of coatings and additives, will also be discussed.

Impact of a Film Maturation Process on the Mechanical Stability of Silicon-Based Electrodes with High Areal Capacities

Over the last ten years, considerable efforts have been devoted to solve the problem of the low cycle life of Si-based electrodes for Li-ion batteries. This mainly originates from the huge volume variation (up to ~300%) of silicon during its lithiation/delithiation, which induces a rupture of the electrode conductive network and an instability of the solid electrolyte interphase (SEI). The use of nanosized Si materials (nanoparticles, nanowires, thin films…) able to better accommodate large strain without extensive electrode cracking has been successively developed for improving the Si electrode cycling stability. However, in most of these studies, the active mass loading is low, resulting in a low areal capacity, typically less than 1 mAh cm-². This is much lower than that of commercial graphite-based negative electrodes, which can reach up to 5 mAh cm-². Obtaining stable Si-based electrodes with high areal capacities is very challenging as the increase of the Si areal mass loading accentuates the mechanical strain generated by the Si volume change1. We have recently shown that the storing of a Si/C/CMC electrode in humid atmosphere for a few days before drying and cell assembling has a very positive impact on its cycling performance2. With such a ‘’maturated’’ electrode, an areal capacity higher than 4 mAh cm-2 can be achieved for more than 100 cycles compared to less than 3 cycles for a no matured electrode. The precise mechanism of this maturation process is still unclear Here, the impact of the maturation step on the mechanical properties of Si/C/CMC electrodes is investigated by means of indentation, peeling and scratch tests. They confirm the higher adhesion and cohesion strengths of the maturated electrode. Its more reversible expansion/contraction behavior upon cycling is also demonstrated from electrochemical dilatometry measurements and in-operando optical microscopy observations (Fig. 1). In addition, focused ion beam scanning electron microscopy (FIB-SEM) tomography shows a better preservation of the pore and Si particle connectivities with cycling for the matured electrode. Lastly, reflectance Fourier transform infrared spectroscopy ( ATR - FTIR) complemented by Nuclear Magnetic Resonance (NMR) analyses indicate that the nature and distribution of the Si-CMC bonds are modified by the maturation step. On the basis of these different analyses, a film maturation mechanism is proposed, which opens up new avenues for optimizing the manufacture process of high-performance Si-based electrodes. 1. Z. Karkar, D. Mazouzi, C. Reale Hernandez, D. Guyomard, L. Roué, B. Lestriez. Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive. Electrochim. Acta 215 (2016) 276-288. 2. C. Real Hernandez, Z. Karkar, D. Guyomard, B. Lestriez, L. Roué. A film maturation process for improving the cycle life of Si-based anodes for Li-ion batteries. Electrochem. Comm. 61 (2015) 102-105. Figure 1

Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells

Continuous operation of full cells with layered transition metal (TM) oxide positive electrodes (NCM523) leads to dissolution of TM ions and their migration and incorporation into the solid electrolyte interphase (SEI) of the graphite-based negative electrode. These processes correlate with cell capacity fade and accelerate markedly as the upper cutoff voltage (UCV) exceeds 4.30 V. At voltages ≥4.4 V there is enhanced fracture of the oxide during cycling that creates new surfaces and causes increased solvent oxidation and TM dissolution. Despite this deterioration, cell capacity fade still mainly results from lithium loss in the negative electrode SEI. Among TMs, Mn content in the SEI shows a better correlation with cell capacity loss than Co and Ni contents. As Mn ions become incorporated into the SEI, the kinetics of lithium trapping change from power to linear at the higher UCVs, indicating a large effect of these ions on SEI growth and implicating (electro)catalytic reactions. We estimate that each MnII ion deposited in the SEI causes trapping of ∼102 additional Li+ ions thereby hastening the depletion of cyclable lithium ions. Using these results, we sketch a mechanism for cell capacity fade, emphasizing the conceptual picture over the chemical detail.

Electrochemical Cross-Talk Leading to Gas Evolution and Capacity Fade in LiNi0.5Mn1.5O4/Graphite Full-Cells

Continuous destruction of the solid electrolyte interphase (SEI) on the graphite-based negative electrode during cycling operation is a significant degradation mechanism that raises safety concerns and limits the cycle life of LiNi0.5Mn1.5O4 (LNMO)/graphite full-cells. Herein, we report on gassing phenomena which are typically concomitant with SEI destruction processes. Abrupt H2 evolution is observed by differential electrochemical mass spectrometry and pressure measurements at the end of discharge. Using a lithium reference electrode reveals that the gassing, which intensifies with cycling, is caused by an increase in the anode potential. Lithium is irreversibly consumed upon SEI formation, but this loss is not compensated for by the intrinsic degradation of LNMO in the first cycle. When the potential of the anode on discharge increases above approximately 0.9 V, the SEI is instantly damaged, causing gas generation, and eventually capacity fade. We show that this (“mediator-free”) cross-talk phenomenon ...

Electrochemical Impedance Spectroscopy response study of a commercial graphite-based negative electrode for Li-ion batteries as function of the cell state of charge and ageing

The successful development of electrified vehicles is a key factor in the transition to a more environmentally friendly transportation sector. Li-ion batteries, which are today’s choice to power electrified vehicles, have to fulfill more stringent requirements in terms of ageing and need advanced tools to study the interfaces evolution upon cycling. This work is thus focused on understanding the impedance behavior of a commercial graphite-based negative electrode, which is used in a Li-ion battery designed for such vehicles. 3-electrode pouch cells were assembled with such negative electrode, a LMO-layered oxide-based positive electrode, a Celgard® type separator soaked with a carbonate solvents-LiPF6 mixture electrolyte and a LTO-based electrode as reference. Electrochemical Impedance Spectroscopy measurements were performed at different cell states of charge and ageing times. The impedance of the graphite-based anode is analyzed for first time with de Levie’s equation for porous electrodes. The analysis is supported by designed SEI layer formation experiments with vinylene carbonate and vinylene ethyl carbonate additives. The high frequency domain of the interfacial kinetic loop reflects porosity effects and the graphite particles–composite matrix electric tranfer. The SEI layer and charge transfer phenomena are reflected in the medium and medium to low frequency domains respectively, and their impedance contributions depend on the Li content of the graphite particles. Upon ageing, the interfacial impedance of the graphite-based electrode should increase due to SEI layer growing. However, from 100% to 80% of battery capacity retention, the impedance decreases. Our analysis backed by post-mortem characterizations allows to assign this unexpected behavior to porosity rise and slight Mn-contamination of the SEI layer.

Cycling Behavior of NCM523/Graphite Lithium-Ion Cells in the 3–4.4 V Range: Diagnostic Studies of Full Cells and Harvested Electrodes

Energy density of full cells containing layered-oxide positive electrodes can be increased by raising the upper cutoff voltage above the present 4.2 V limit. In this article we examine aging behavior of cells, containing LiNi0.5Co0.2Mn0.3O2 (NCM523)-based positive and graphite-based negative electrodes, which underwent up to ∼400 cycles in the 3–4.4 V range. Electrochemistry results from electrodes harvested from the cycled cells were obtained to identify causes of cell performance loss; these results were complemented with data from X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) measurements. Our experiments indicate that the full cell capacity fade increases linearly with cycle number and results from irreversible lithium loss in the negative electrode solid electrolyte interphase (SEI) layer. The accompanying electrode potential shift reduces utilization of active material in both electrodes and causes the positive electrode to cycle at higher states-of-charge. Full cell impedance rise on aging arises primarily at the positive electrode and results mainly from changes at the electrode-electrolyte interface; the small growth in negative electrode impedance reflects changes in the SEI layer. Our results indicate that cell performance loss could be mitigated by modifying the electrode-electrolyte interfaces through use of appropriate electrode coatings and/or electrolyte additives.

Diagnostic Examination of NCM523//Graphite Cells Cycled at High Voltages

High energy materials are needed to increase the energy density of Li-ion batteries for vehicular applications. Layered-oxides are promising high energy positive-electrode materials; however, the performance of cells containing these oxides degrades faster when cycled to high potentials (> 4.3 V). In order to understand the origin of this performance loss, we conducted extensive tests with 2032-type coin cells and reference electrode (RE) cells; the results of these tests will be reported during this presentation. The coin cells in this study contain 1.6 cm2 area Li1.03(Ni0.5Co0.2Mn0.3)0.97O2-based (NCM523) positive and a graphite-based negative electrode (Gr); the electrolyte is 1.2M LiPF6 in EC:EMC (3:7 w/w) electrolyte (Gen 2). The reference electrode cells contain 20.3 cm2 area NCM523-positive and Gr-negative electrodes, two Celgard 2325 separators enveloping a LixSn RE, and a Li-metal RE external to the electrode sandwich. Cell cycling includes formation cycles followed by aging cycles; pulse-power and AC impedance measurements are made periodically throughout the cycling.The Li-metal RE provides information on the positive and negative electrode potentials during full cell cycling; such information cannot be obtained from the coin cell data. For example, during cell formation, when the full cell is cycled between 3.0 and 4.4 V, the positive electrode cycles between 3.67 and 4.46 V vs. Li/Li+ and the negative electrode cycles between 0.67 and 0.06 V vs. Li/Li+. The electrode cycling potential ranges change during cell aging. Furthermore, electrode impedances measured with the LixSn RE also change during aging. We will highlight the reasons for these changes during the presentation. In addition to the electrochemistry results we will discuss data obtained by various diagnostic techniques that include the following: Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Our mechanistic understanding of the capacity fade and impedance rise of the NCM523//Gr cells will be discussed through these various data.