Lithium Iron Phosphate Electrodes Research Articles

Dislocation based stresses during electrochemical cycling and phase transformation in lithium-ion batteries

Lithium iron phosphate (LiFePO4) contains defects that play an important role in structural degradation of batteries. Linear defects called dislocations are introduced inside the lithium ion battery material during fabrication process accompanied by distortion produced stress fields around them. The present study deals with these mechanical stresses around dislocations in lithium iron phosphate and the change in stress field during phase transformation of lithium iron phosphate electrode material. A model consisting of multiple dislocations inside a lithium iron phosphate material incorporating anisotropic material properties is used to calculate stress fields using linear elastic theory and the superposition method. The stress fields around dislocations during phase transformation of lithium-iron phosphate are numerically calculated by incorporating the anisotropic properties of the material. The change in electrochemical behaviour of material due to change in stress field during phase transformation is also studied, where a modified electrochemical kinetics equation (i.e., Butler Volmer equation) is derived and used to account for dislocation induced stresses during the reversible cyclic voltammatery of the lithium iron phosphate. The results shows the stress inside material does not remain constant during phase transformation and its variations are dislocation orientation dependent. In addition, the result shows that the presence of stress fields around dislocations changes the electrochemical behaviour of the material as suggested by the shift in the cyclic voltammograms. The effect of increasing scan rate on cyclic voltammogram is also studied for lithium iron phosphate. The results show that the increase in current at peaks is independent of the orientation of dislocations studied. Moreover, the decrease in current corresponding to a particular overvoltage value before anodic peak and increase in current after the anodic peak is found to be somehow proportional to the scan rate. Increased scan rates show increased deviation of current from a cyclic voltammogram for material in which there is no phase transformation. The results provide an insight into how presence of defects and phase transformation changes the electrochemical behaviour of the material. It is concluded that the combined effect of the stresses induced around dislocations during phase transformation and high scan rate can be used for modifying battery materials for various applications by changing electrochemistry of electrodes. The present study incorporates electrochemistry, defects and phase transformation into one battery chemistry and thus is important in our understanding of the Li-ion batteries.

Can Energy Storage Pack Hybridization Improve the Best Theoretically-Achievable Battery State Estimation Accuracy?

This article examines the degree to which the creation of a series hybrid ultracapacitor-battery string can improve achievable battery state estimation accuracy. The article is motivated by the existence of fundamental information-theoretic limits on the degree to which one can use input/output current/voltage data to estimate a battery's internal states (e.g., state of charge, state of health, etc.). Previous literature quantifies these limits rigorously using Fisher information theory [1] – [5]. One conclusion of this quantification is that it is particularly difficult to obtain accurate battery state estimates during phase transition processes, such as the phase transition that occurs in lithium iron phosphate electrodes during charge/discharge. The problematic effect, in this context, is the very small dV/dQ slope, or effective electrical elastance, of the battery. Poor battery elastance complicates battery state estimation accuracy by directly contributing to the worsening of the best-achievable battery state estimation accuracy. The literature already shows this through the use of Fisher information analysis which shows the dependence of the best-achievable battery state estimation accuracy and the effective battery elastance. Building on the above literature, the key conjecture motivating this article is that the creation of hybrid series ultracapacitor-battery strings can improve battery state estimation accuracy substantially by furnishing a significant dV/dQ system behavior. We derive this conclusion analytically, using Fisher information analysis. Then consider estimating the lithium-ion battery state of charge using input current and output battery voltage, where the current measurement is biased and the voltage sensor has zero-mean, random, Gaussian noise. By comparison between the Cramer-Rao lower bound of both the battery-only and ultracapacitor-battery hybrid, we demonstrate improvements in theoretically-achievable initial charge accuracy as well as current bias, and in turn improve the battery SOC estimation. This conclusion is validated using a Monte Carlo simulation study. Performing least square method to estimate both initial charge in the battery and current sensor bias to achieve point clouds with and without an ultracapacitor in series, as shown in Figure 1a. One can conclude that there is: (i) a dependence between the estimated current bias and initial charge; (ii) a large condition number of the Fisher information metric which is reflected in the thin shape of the cloud ellipse; (iii) significant decrease in the estimation error after adding a series ultracapacitor into the system. Finally, the article presents an experimental ultracapacitor-battery validation study, as shown in Figure 1b. The full experiment setup includes a system of three parallel 400F ultracapacitors in series with a single 0.6Ah LFP cell. Experimental data is used to solve the optimization problem of minimizing the mean squared error of physical and simulation system outputs with respect to estimated initial charge and current bias. Results show clear improvement on the estimation accuracy. Figure 1. (a) Monte Carlo simulation results comparison (above) (b) Experiment setup structure (below) This article answers the title question by providing quantifiable evidence of improved state estimation through theory, simulation and experiment. Figure 1

In Situ Chemo-Mechanical Responses of Lithium Iron Phosphate Electrodes during Electrochemical Cycling

Rechargeable Li-ion batteries (LIBs) have been successfully commercialized for portable electronics in the last two decades. Although there has been a great interest to use LIBs in more demanding applications such as electric vehicles, adoption of LIBs has been hindered by capacity loss and poor performance. The current stage of the cathode performance is a limiting factor for the development of higher performance LIBs. Lithium intercalation induces considerable volumetric changes with associated stress generation in the electrode. Even small strain can cause particle fractures in brittle cathode materials. The formation of a fracture particles leads to the capacity fade and limits the lifetime of batteries. To investigate chemo-mechanical response of the cathodes, we previously calculated electrochemical stiffness changes cathodes by coordinating in situ stress and strain measurements1. The electrochemical stiffness response of lithium manganese oxide revealed that the underlying mechanisms governing stress and strain are intrinsically different. In this study, our objective is to evaluate chemo-mechanical responses of lithium iron phosphate (LFP) cathode. In situ stress measurements were carried out with the beam curvature method whereas digital image correlation (DIC) is utilized to measure in situ strain generation. Electrodes are constrained on the cantilever for stress measurement, whereas free-standing electrodes were fabricated for strain measurements. LFP electrodes were fabricated with a composition of 8:1:1 weight ratio of LFP, carboxymethyl cellulose binder and carbon black. Electrodes are cycled with cyclic voltammetry technique at different scan rates (between 10 and 250μV/s) in various electrolytes. Stress and strain derivatives are calculated with respect to applied potential. Derivatives present crucial information to probe potential-dependent surface stress evolutions and structural changes in the electrode. As expected, strain derivatives are well-correlated with the changes in the crystal structure during delithiation and lithiation. Interestingly, stress derivatives reveal that electrolyte solution has a role on the evolution of surface stress. Changes in stress and strain during electrochemical cycling reveal underlying mechanisms governing chemo-mechanical responses in the LFP electrode. References 1- Ö. Ö. Çapraz, K. L. Bassett, A. A. Gewirth, and N. R. Sottos, Adv. Energy Mater., 1601778–7 (2017). Acknowledgement This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.”

Temperature Effects on the Behavior of Lithium Iron Phosphate Electrodes

The systematic study of the effect of temperature (in the range from −45 to +60°C) on the process of lithium extraction from LiFePO4 and its insertion into FePO4 is carried out. At a current of about C/1.5, with decreasing temperature, the capacity decreases, the polarization increases, the range of compositions corresponding to nonequilibrium solid solutions widens, and the slope of the linear section of the galvanostatic curves corresponding to the two-phase system increases. The decrease in the capacity with decreasing temperature is not described by the simple Arrhenius equation. It is assumed that the process on the lithium iron phosphate electrodes has a mixed diffusion-activation nature. The polarization of the anodic and cathodic processes increases with decreasing temperature in a complicated way, and the polarization of the anodic process exceeds that of the cathodic process appreciably.

Glyoxal-Based Solvents for Electrochemical Energy-Storage Devices.

In this work an investigation about the use of the solvents 1,1,2,2-tetramethoxyethane, also called tetramethoxy glyoxal (TMG), and 1,1,2,2-tetraethoxyethane [also called tetraethoxy glyoxal (TEG)], which belong to the chemical family of carbonyl derivatives, as electrolyte components for electrical double layer capacitors (EDLCs) and lithium-ion batteries (LIBs) is reported for the first time. TEG and TMG are commercial solvents displaying a good set of properties, a low toxicity, and a low price. Although for EDLCs the use of these solvents does appear particularly appealing, their use in LIBs is certainly interesting. The preliminary results reported in this study indicate that the performance of lithium iron phosphate electrodes in LIBs using electrolytes based on TEG and TMG is promising in terms of capacity, capacity retention at high C rates, and cycling stability.

In Situ Measurement of Strain and Stress Evolution in Lithium Iron Phosphate Electrodes during Electrochemical Cycling

Rechargeable lithium-ion batteries have been widely commercialized in portable electronics. However, adoption of lithium-ion batteries in more demanding applications such as electric vehicles and renewable energy storage has been hindered by capacity loss and poor performance. Although there has been enormous progress on the advancement of high capacity anode materials such as silicon, the current stage of the cathode performance is a limiting factor for the development of higher performance Li-ion batteries. Lithium iron phosphate, LiFePO4, (LFP) is a promising candidate material because it is inexpensive, environmentally friendly and has a high theoretical capacity (170 mAh g-1). Unfortunately, LFP suffers from mechanical degredations and surface instabilities that lead to capacity fade. In this work, we investigate in situ stress and strain generation in LFP composite electrodes during battery charging – discharging. Repeated lithiation and delithiation cause continuous stress and strain evolution in the electrode due to lithium intercalation and the interaction between electrode and electrolyte species. Electrodes are constrained on substrate for stress measurements, whereas free-standing electrodes are fabricated for strain measurements. Digital image correlation (DIC) is utilized to measure in situ strain generation, whereas beam curvature technique is employed to monitor in situ stress evolution in the electrode. Electrodes are cycled between 2.6 to 4.4 V at different scan rates in various electrolytes. Strain and stress derivatives are calculated with respect to applied potential. The derivatives provide remarkable information about the evolution of surface stress and structural changes in the electrode due to phase transformations. Electrochemical stiffness is calculated by combining stress and strain. Changes in stiffness during electrochemical cycling reveal underlying mechanisms governing stress and strain evolution in the LFP electrode. Acknowledgement This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.”

Lithium iron phosphate electrode semi-empirical performance model

The galvanostatic performance of a pristine lithium iron phosphate (LFP) electrode is investigated. Based on the poor intrinsic electronic conductivity features of LFP, an empirical variable resistance approach is proposed for the single particle model (SPM). The increasing resistance behavior observed at the end of discharge process of LFP batteries can be justified by the increased ohmic resistance, a resistive-reactant feature of LFP as the positive electrode active materials. The model is validated for two different laboratory made Li/LFP coin cells: a high-energy and a high-power configuration. Comparisons between the experimental results and the model predictions reveal that a variable resistance is successful to tackle the increasing overpotential. Schematic of the coated LFP active material particles in (a) beginning of discharge with well-connected particles, (b) end of discharge with poor-connected particles

Method and Measurement Setup for Battery State Determination Using Optical Effects in the Electrode Material

In this paper, a novel measuring method for the battery state is presented to supplement the usual methods on the basis of electrical measurements and computational battery models. Measurement setup and procedure for observing the change in the electrode material during the storage and removal of lithium ions are presented in detail. The ion intercalation of the electrochemical process can be made visible by means of a test cell with a transparent window and continuous image recording and processing. The change in reflectance and color on the surface of a lithium iron phosphate electrode was found to closely correlate with the charge state calculated from cell current. Furthermore, we discovered that an electrochromic marker introduced into the electrode material significantly enhances the optical effect. Observing the electrode allows for the analysis of the ion intercalation within the electrode. The spatially resolved information could be used to determine the parameters of the reaction kinetics and material characteristics. Image recording on optical test cells was used for comparative materials testing in various geometries. The aim is to develop a sensor that can be used in commercial battery cells. For this purpose, preliminary experiments with optical fibers have been conducted. The optical fibers are to be prepared so that the transmitted light interacts with the electrode material, and inserted into the cell structure. This fiber sensor could provide a complementary battery management function to calculate the state-of-charge and as a redundant safety level to avoid critical battery conditions.

Identifying compatibility of lithium salts with LiFePO4 cathode using a symmetric cell

The electrochemical performance of lithium-ion batteries is dominated by the interphase electrochemistry between the electrolyte and electrode materials. A multitude of efforts have been dedicated to the solid electrolyte interphase (SEI) formed on the anode. However, the interphase on the cathode, namely the cathode electrolyte interphase (CEI), is left aside, partially due to the fact that it is hard to single out the CEI considering the complicated anode-cathode inter-talk. Herein, a partially delithiated lithium iron phosphate (Li0.25FePO4) electrode is used as the anode. Owing to a high voltage plateau (≈3.45 V vs. Li/Li+), negligible reduction reactions of electrolyte occur on the L0.25FePO4 anode. Therefore, the CEI can be investigated exclusively. Using a LiFePO4|Li0.25FePO4 symmetric cell configuration, we scrutinize the compatibility of the electrolytes containing a wide spectrum of lithium salts, Li[(FSO2)(Cm F2m+1SO2)N] (m = 0, 1, 2, 4), with the LiFePO4, in both cycling and calendar tests. It is found that the Li[(FSO2)(n-C4F9SO2)N] (LiFNFSI)-based electrolyte exhibits the highest compatibility with LiFePO4.

In situ multi-length scale approach to understand the mechanics of soft and rigid binder in composite lithium ion battery electrodes

Intercalation-induced dimensional changes of composite battery electrodes containing either a stiff or a soft polymeric binder is one of the many factors determining the cycling performance and ageing. Herein, we report dimensional changes in bulk composite electrodes by in situ electrochemical dilatometry (eD) combined with electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D). The latter tracks the mechanical properties on the level of the electrode particle size. Lithium iron phosphate (LiFePO4, LFP) electrodes with a stiff binder (PVdF) and a soft binder (NaCMC) were investigated by cycling in lithium sulfate (Li2SO4) aqueous solution. The electrochemical and mechanical electrode performances depend on the electrode cycling history. Based on combined eD and EQCM-D measurements we provide evidence which properties are preferred for a binder used for a composite Li-ion battery electrode.

A MODELLING APPROACH TO UNDERSTAND CHARGE DISCHARGE DIFFERENCES IN THERMAL BEHAVIOUR IN LITHIUM IRON PHOSPHATE – GRAPHITE BATTERY

Lithium iron phosphate is a promising positive electrode material. It shows apparent asymmetry between charge and discharge affecting not only the electrochemical but also the thermal behaviour. Physics based models for batteries are usually parameterized for discharge behaviour, which can lead to inaccuracies in prediction of battery behaviour during charging or dynamic conditions, especially in the case of this battery chemistry. This has consequences for battery modelling and safety. A pseudo 2D electrochemical – 3D thermal model is parameterized for charging using experimental data. Substantial differences in parameters between charge and discharge are seen in the validated model. The model is used to quantify the thermal differences during charging and discharging and separate the contribution of the different battery layers. Reversible heat losses are seen to be the main cause for the difference between charge and discharge while graphite electrode are seen to have a much higher heat contribution compared to lithium iron phosphate electrode.

A Semi-Empirical Aging Model for Lithium Iron Phosphate Electrode

First, galvanostatic performance of a pristine lithium iron phosphate (LFP) electrode is studied by introducing a variable resistance single particle model (SPM) that is verified by experimental data from a Li/LFP coin cell. The empirical variable resistance is coupled with SPM to account for the poor ionic and electrical conductivity features of LFP. This variable resistance represents two features of LFP active material: 1. The low ionic conductivity of LFP active material results in increasing the diffusion overpotential especially at the end of discharge where the larger particles participate in the intercalation/deintercalation of ions. 2. The resistive-reactant feature of this material increases the ohmic resistance where poorly coated particles (intraparticle resistance) and poorly connected particles to the matrix (interparticle resistance) play important roles at the end of discharge process. Based on an inverse method, a Parameter Estimation (PE) process is conducted to provide the most influential electrochemical parameters of the LFP positive electrode. These parameters are the solid diffusion coefficient (Ds,p), the intercalation/deintercalation reaction-rate constant (Kp), the total electroactive area of particles (Sp), and the unknown coefficients in the cell resistance equation. In this regard, a least square function and the Genetic Algorithm (GA) are employed as the objective function and the optimizer of the inverse method, respectively. After finding all unknown parameters for a pristine Li/LFP half-cell, the most important parameters, which change by aging, are detected from the analysis of experimental data. These data are extracted from a high-power Li/LFP coin cell built at the Laboratoire d'électrochimie interfaciale et appliquée (LÉIA) of Université de Sherbrooke. The experimental data consist of galvanostatic 1C charge/discharge curves and electrode impedance spectroscopy (EIS) of the cathode versus Li foil as the counter and reference electrode. Based on EIS, we conclude that the charge transfer and the electrolyte resistances both increase with cycling. In fact, damage made in the conductive coating around active material particles results in both decreasing reaction sites and losing active materials. Consequently, charge transfer resistance increases. A variable total electroactive area, representing the reduction in the reaction sites and the increase of the charge transfer resistance, is considered as the main parameter changed by aging. The increase of the electrolyte resistances, on the other hand, is addressed by the variable resistance, which also introduced to consider diffusion overpotential and ohmic resistance at the end of discharge process. Comparisons between the experimental results and the model predictions show that the variable resistance SPM is able to predict the performance of LFP positive electrode. Decreasing total electroactive surface area and increasing resistivity of LFP active material are found to be the most important parameters to simulate aging phenomena in this active electrode material.

Polymeric Gel Electrolyte Consisting of Network Polymer Using Tri-Functional Glycerol Ester for Li-Ion Batteries

Development of a safer electrolyte has become a key issue for realizing larger sizes of lithium-ion batteries (LIBs) for energy storage and transportation uses. Flame-retardant additives and/or ionic liquids are considered to be effective components that form safer electrolyte solutions. However, those systems still have risks coming from electrolyte leakage due to their liquid states. Solidification of the electrolyte is one of the effective ways to improve the safety of the battery system. In order to ensure sufficient ionic conductivity, a number of polymeric gel electrolyte systems have so far been proposed. We have investigated in this paper a novel polymeric gel electrolyte based on a network polymer having tri-functional glycerol ester for lithium-ion batteries. A triply branched monomer possessing a terminal olefin unit was newly synthesized from glycerol ethoxylate (M n~1,000) and 10-undecenoly chloride (3 eq.) (Fig. 1 (a)). The electrolyte consisting of 1 M LiPF6/ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by vol.) was added to a mixture of trimethylolpropane tris(3-mercaptopropionate) (Fig.1 (b)) and the triply branched monomer with a smaller amount of benzophenone sensitizer (1:1:0.05 by molar ratio). The resulting mixed solution was applied to a glass plate, and was broaden. Finally, the monomer-coated glass plate was exposed to UV irradiation to get a gel film. A transparent polymeric gel electrolyte was obtained as a self-standing film. The ionic conductivity of the gel electrolyte was measure by an AC method and found to be 5.2, 3.4, and 2.8 mS cm-1 (at 297 K) for the films containing 10, 20, and 30 wt% of polymer matrix, respectively. The electrochemical stability toward anodic oxidation was investigated for the gel electrolyte by linear sweep voltammetry (LSV). The anodic oxidation current was observed as a current peak at 4.2 V. This was attributed to the oxidative decomposition of the thiol group in the polymer matrix1). Thus, the present polymeric gel electrolyte can be used in a potential range up to 4.2 V. The application to the graphite negative and lithium iron phosphate (LiFePO4) electrodes will be reported in the presentation. Reference 1) M.-H. Ryou, et al., Electrochim. Acta, 60, 23 (2012). Figure 1

Optimization of Inactive Material Content in Lithium Iron Phosphate Electrodes for High Power Applications

The electrochemical performance of lithium iron phosphate (LiFePO4) electrodes has been studied to find the optimum content of inactive materials (carbon black+polyvinylidene difluoride [PVDF] polymer binder) and to better understand electrode performance with variation in electrode composition. Trade-offs between inactive material content and electrochemical performance have been characterized in terms of electrical resistance, rate-capability, area-specific impedance (ASI), pulse-power characterization, and energy density calculations. The ASI and electrical conductivity were found to correlate well with ohmic polarization. The results showed that a 80:10:10 (active material: binder: carbon agents) electrode had a higher pulse-power density and energy density at rates above 1C as compared to 90:5:5, 86:7:7 and 70:15:15 formulations, while the 70:15:15 electrode had the highest electrical conductivity of 0.79Scm−1. A CB/PVDF ratio of ca. 1.22 was found to be the optimum formulation of inactive material when the LiFePO4 composition was 80wt%.

Calendering effect on the electrochemical performances of the thick Li-ion battery electrodes using a three dimensional Ni alloy foam current collector

High surface area and a three dimensional NiCrAl alloy foam current collector was used for two kinds of thick lithium iron phosphate electrodes. One kind of electrodes were compressed after the slurry of active material in the metal foam was dried and then annealed at 140°C for half a day whereas the other kind of electrodes were prepared without pressing. When the addition of carbon black was 4wt% for the two kinds of electrodes, a charge-discharge test revealed that the capacity of the cell using the pressed electrode faded much more although the voltage-drop was much smaller at the plateau region. For example, the capacity of the pressed electrode exhibited 85mAhg−1, while it was 135mAhg−1 for the unpressed electrode although the voltage-drop at the plateau region was 250mV higher at 0.5C-rate for the unpressed electrode. The AC impedance analysis showed that the charge transfer resistance of the pressed electrode was only 15Ω whereas it was 4 times higher for the unpressed electrode. The results illustrated that the effective redox area was much larger for the unpressed electrode since the cell using the unpressed electrode exhibited much higher capacity even at the condition of poor electronic conductivity. To solve the low electronic conductivity issue for the unpressed electrode, the addition of carbon black was further increased to 14 wt% and as a result, there was almost no difference in voltage drop at plateau region or charge transfer resistance between the two kinds of electrodes. Obviously, the capacity of unpressed electrode exhibited much higher at higher current rate due to the larger effective redox area.

Judicious design of lithium iron phosphate electrodes using poly(3,4-ethylenedioxythiophene) for high performance batteries

Designing of lithium iron phosphate electrodes using poly(3,4-ethylenedioxythiophene) for high performance batteries.

Simulation of lithium iron phosphate lithiation/delithiation: Limitations of the core–shell model

In this work the limitations in using the core–shell models to simulate the operation of lithium iron phosphate electrodes are discussed and analyzed. The complications arising from the stepwise nature of the core–shell model and erroneous predictions of this model for special conditions are detailed.

Physical properties of a new Deep Eutectic Solvent based on lithium bis[(trifluoromethyl)sulfonyl]imide and N-methylacetamide as superionic suitable electrolyte for lithium ion batteries and electric double layer capacitors

Herein we present a study on the physical/chemical properties of a new Deep Eutectic Solvent (DES) based on N-methylacetamide (MAc) and lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI). Due to its interesting properties, such as wide liquid-phase range from −60°C to 280°C, low vapor pressure, and high ionic conductivity up to 28.4mScm−1 at 150°C and at xLiTFSI=1/4, this solution can be practically used as electrolyte for electrochemical storage systems such as electric double-layer capacitors (EDLCs) and/or lithium ion batteries (LiBs). Firstly, relationships between its transport properties (conductivity and viscosity) as a function of composition and temperature were discussed through Arrhenius’ Law and Vogel–Tamman–Fulcher (VTF) equations, as well as by using the Walden classification. From this investigation, it appears that this complex electrolyte possesses a number of excellent transport properties, like a superionic character for example. Based on which, we then evaluated its electrochemical performances as electrolyte for EDLCs and LiBs applications by using activated carbon (AC) and lithium iron phosphate (LiFePO4) electrodes, respectively. These results demonstrate that this electrolyte has a good compatibility with both electrodes (AC and LiFePO4) in each testing cell driven also by excellent electrochemical properties in specific capacitance, rate and cycling performances, indicating that the LiTFSI/MAc DES can be a promising electrolyte for EDLCs and LiBs applications especially for those requiring high safety and stability.

Comparing anode and cathode electrode/electrolyte interface composition and morphology using soft and hard X-ray photoelectron spectroscopy

Electrode/electrolyte interface depth profiling was performed on lithiated graphite and delithiated lithium iron phosphate electrodes after electrochemical cycling in a balanced full cell configuration containing a carbonate based LiPF6 electrolyte. The profiling was performed by synchrotron radiation based hard X-ray photoelectron spectroscopy, HAXPES, and soft X-ray photoelectron spectroscopy, SOXPES. In this way, the probing depth was varied over a wide range in the order of 2–50nm. Both more surface and more bulk sensitive investigations than possible using traditional in-house X-ray photoelectron spectroscopy (XPS) could thus be performed. The composition and morphology of the lithiated graphite anode/electrolyte interface (solid electrolyte interphase, SEI) and the delithiated lithium iron phosphate cathode/electrolyte interface (solid permeable interface, SPI) were compared. In the vicinity of the highly reductive graphite active material in the SEI, low binding energy components like Li2O were found while no obvious composition gradients were observed in the SPI. Both in the cathode SPI and the anode SEI, significant amounts of C–O and P–F containing compounds were found to deposit during cycling. Evidence for mixing of the porous binder and other SEI/SPI components was observed in both the anode and cathode electrode/electrolyte interfaces. The lithiated graphite SEI was estimated to be of the order of two tens of nanometers, while the cathode SPI thickness was estimated to a few nanometers only.

Interactions between organic additives and active powders in water-based lithium iron phosphate electrode slurries

The interactions of organic additives with active powders are investigated and are found to have great influence on the determination of the mixing process for preparing electrode slurries with good dispersion and electrochemical properties of lithium iron phosphate (LiFePO4) electrodes. Based on the analyses of zeta potential, sedimentation, and rheology, it is shown that LiFePO4 prefers to interact with styrene–butadiene rubber (SBR) relative to other organic additives such as sodium carboxymethyl cellulose (SCMC), and thus shows preferential adsorption by SBR, whereas SBR has much lower efficiency than SCMC in dispersing LiFePO4. Therefore, for SCMC to interact with and disperse LiFePO4 before the interaction of LiFePO4 with SBR, it is suggested to mix SCMC with LiFePO4 prior to the addition of SBR during the slurry preparation process. For the electrode prepared via the suggested process, i.e., the sequenced adding process in which SCMC is mixed with active powders prior to the addition of SBR, a much better electrochemical performance is obtained than that of the one prepared via the process referred as the simultaneous adding process, in which mixing of SCMC and SBR with active powders in simultaneous.