Composite Positive Electrode Research Articles

Preparation of Li2s-FeS x -C Composite Positive Electrode Materials and Their Electrochemical Properties with Pre-Cycling Treatment

1. Introduction Lithium sulfide (Li2S) is one of the promising cathode active materials for high-energy rechargeable lithium batteries, because it has a high theoretical capacity (ca. 1170 mAhg-1) and has an advantage that a variety of anode materials without lithium sources are applicable in the practical battery system [1]. However, this material is both electronically and ionically resistive, which gives rise to low active material utilization in the cells [2]. Several attempts have been performed to enhance the conductivity of Li2S, such as forming composites with several metals (Li2S-Fe, Li2S-Cu) [1,2] or carbon [3-5]. Recently, we have tried to prepare the composite with FeS2 (Li2S : FeS2 = 1 : 1 molar ratio) using the spark-plasma-sintering (SPS) process and found that the cells showed the initial charge capacity of ca. 330 mAhg-1 [6]. For improving the charge capacity, preparation of the composite with Li2S-rich composition is required. In the present work, we tried to prepare the Li2S-FeS x -C (x = 1, 2) composites with a Li2S : FeS x = 4 : 1 molar ratio using the combination processes of the SPS and the mechanical milling, and examined their electrochemical properties. 2. Experimental Li2S and FeS2 (or FeS) with a molar ratio of 4 : 1 were mixed thoroughly, and the mixture was then treated by the SPS process [6] at 600oC. The resulting pellet was ground and mixed with carbon (acetylene black) powder with a weight ratio of Li2S-FeS x : C = 9 : 1, and then it was mechanically milled for 8 h to yield the Li2S-FeS x -C composites. All the procedures were carried out in an argon atmosphere, because Li2S is very sensitive to atmospheric moisture. The obtained composite samples were characterized by XRD, as well as Fe and S K-edge XAFS measurements. Electrochemical lithium insertion / extraction reactions were carried out using lithium coin-type cells with 1M LiPF6 / (EC + DMC) electrolyte at a current density of 46.7 mAg-1 (corresponding to 0.04C) initially with charging. For improving the electrochemical performances, we applied pre-cycling treatment to the present Li2S-FeS x -C composite sample cells, whereby cycle capability of some oxide cathode active materials (such as Li(NiCoMn)O2) was successfully improved [7]. 3. Results and Discussion The obtained Li2S-FeS x -C composite samples were black in color, and their XRD patterns showed that they consisted of Li2S (antifluorite structure) with low crystallinity. The atomic ratio, estimated by ICP and atomic absorption spectroscopy, was Li : Fe : S = 7.8 : 1.0 : 4.8 for the Li2S-FeS x -C composite sample with Li2S : FeS = 1 : 4. Since the initial composition was Li : Fe : S = 8 : 1 : 5, this result indicates that negligibly slight amounts of Li and S (ca. 3 – 4%) were lost during the SPS and the following milling processes. The electrochemical tests for the Li2S-FeS x -C sample cells showed that the initial charge capacity was ca. 600 mAhg-1, which was much higher than the previously reported Li2S-FeS2 (1 : 1 molar ratio) sample cells (ca. 330 mAhg-1) [6]. The discharge capacity was ca. 330 mAhg-1 with the normal electrochemical cycling, and it was enlarged to ca. 730 mAhg-1 after the pre-cycling treatment, Fig. 1. This result indicates that the pre-cycling treatment was effective for improving the electrochemical performances of the Li2S-FeS x -C composite cells. Ex-situ XRD measurements showed that the peaks ascribed to Li2S disappeared after the normal electrochemical cycling, while they were still observable after the pre-cycling treatment. Also, ex-situ S K-XAFS measurements showed the reversible changes of the valence states of S atom with the pre-cycling treatment. Therefore, the pre-cycling treatment of the Li2S-FeS x -C sample was effective for maintaining the antifluorite structure, as well as for improving the reversibility of the valence states of S atom, which would be responsible for improving the electrochemical performances of the Li2S-FeS x -C composite sample cells.

Novel spectro-electrochemical cell for in situ/operando observation of common composite electrode with liquid electrolyte by X-ray absorption spectroscopy in the tender X-ray region

A novel spectro-electochemical cell for X-ray absorption spectroscopy in the tender X-ray region (TX-XAS) was designed and fabricated to investigate the electrochemical behavior of common battery materials with liquid electrolytes under in situ/operando conditions. The cell has several unique features, such as high X-ray transmittance, high signal to noise ratio, and high vacuum tightness. These features enable us quick and reliable XAS measurements. Operando P K-edge XAS measurements of an olivine-type LiFePO4 composite positive electrode were carried out to clarify its phosphorus environment during the electrochemical charging process. Results of spectral analysis show that there is no significant change in the oxidation state of phosphorus and in the coordination of the phosphate anions in the charging process, but a closer look of the consecutive XAS spectra suggests the shrinkage of the PO4 cage during the charging process, and the structural changes in a biphasic manner. These results demonstrate the usefulness of the cell for in situ/operando TX-XAS observations of light elements in practical batteries.

Analysis of aging of commercial composite metal oxide – Li4Ti5O12 battery cells

Commercial battery cells with Li4Ti5O12 negative electrode and composite metal oxide positive electrode have been analyzed with respect to aging mechanisms. Electrochemical impedance spectroscopy (EIS), differential capacity analysis (dQ/dV), differential voltage analysis (dV/dQ) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) were used to identify different ageing mechanisms such as lithium inventory loss, loss of active electrode material and surface film growth. The active material of the positive electrode was also examined by X-ray diffraction (XRD). Aging mechanisms were studied for both calendar-aged and cycle-aged cells. Data from half cells prepared from post mortem harvested electrode material, using lithium foil as negative electrode and pouch material as encapsulation, were used as reference to full cell data. Electrochemical analysis of full and half cells combined with material analysis showed to be a powerful method to identify aging mechanisms in this type of commercial cells. The calendar-aged cell showed insignificant aging while the cycle-aged cell showed noticeable loss of positive electrode active material and loss of cyclable lithium, but only minor loss of negative electrode active material. The results imply that Li4Ti5O12 negative electrode material is a good alternative to other materials if high energy density is not the primary goal.

Synthesis and Characterization of LiFeBO3/C Composite By Spray Pyrolysis with Heat Treatment

Synthesis and Characterization of LiFeBO3/C Composite By Spray Pyrolysis with Heat Treatment

Electrochemical reactivity of pyrolytic carbon film electrodes in organic carbonate electrolytes

The electrochemical reactivity of polyimide-derived pyrolytic carbon film electrodes in a standard organic carbonate lithium-ion battery electrolyte was studied and quantified. An oxidative heat treatment at 900°C under CO2 atmosphere was found to lower the reactivity of disordered carbons towards electrolyte oxidation. Cyclic voltammetry and potentiostatic measurements of the carbon film electrodes demonstrate the beneficial effect of the CO2 heat treatment in the potential range between 4.2 and 4.8V vs. Li/Li+ i.e., at potentials where high-energy Li-ion positive composite electrodes operate.

A lithium sulfur battery with high power density

The performance of all-solid-state lithium sulfur (Li/S) batteries is investigated for some types of conductive materials. The surface area of the conductive material is found to have a greater influence on the battery performance compared with its electronic conductivity. All-solid-state Li/S cells prepared using a positive activated-carbon-based composite electrode exhibit an excellent cycling performance with a reversible capacity of over 1600 mAh g−1 after 100 cycles at 1.3 mA cm−2 (1C) at 25 °C. Furthermore, the positive composite electrode exhibits a high power density of over 11000 W kg−1 at 50% state of charge.

Activation of sulfur active material in an all-solid-state lithium–sulfur battery

Charge–discharge performance of all-solid-state Li/S batteries using several solid electrolytes to enhance energy density is investigated at 25 °C. The sulfur content in the positive composite electrode is 50 wt%. A correlation between the P/S ratio in a solid electrolyte and the reactivity of sulfur is observed. The capacity of a positive composite electrode using a Li1.5PS3.3{60Li2S–40P2S5(mol%)} electrolyte is 1096 mAh g−1 under 6.4 mA cm−2 at 25 °C. This value is much higher than the capacity for a Li4.0PS4.5{80Li2S–20P2S5} electrolyte (565 mAh g−1), although the ionic conductivity of Li1.5PS3.3 was lower than Li4.0PS4.5 (2 × 10−5 S cm−1, 5 × 10−4 S cm−1, respectively).

A Li-Rich Layered Cathode Material with Enhanced Structural Stability and Rate Capability for Li-on Batteries

A new lithium rich composite positive electrode material of the composition 0.3Li2MnO3.0.7LiNi0.5Co0.5O2 (LLNC) was synthesized using the conventional co-precipitation method. Its crystal structure and electrochemistry in Li cells have been compared to that of the previously known material, 0.3Li2MnO3.0.7LiMn0.33Ni0.33Co0.33O2 (LLNMC). The removal of Mn from the LiMO2 (M = transition metal) segment of the composite cathode material allowed us to determine the location of the manganese oxide moiety in its structure that triggers the layered to spinel conversion during cycling. The new material resists the layered to spinel structural transformation under conditions in which LLNMC does. X-ray diffraction patterns revealed that both compounds, synthesized as approximately 300 nm crystals, have identical super lattice ordering attributed to Li2MnO3 existence. Using X-ray absorption spectroscopy we elucidated the oxidation states of the K edges of Ni and Mn in the two materials with respect to different charge and discharge states. The XAS data along with electrochemical results revealed that Mn atoms are not present in the LiMO2 structural segment of LLNC. Electrochemical cycling data from Li cells further revealed that the absence of Mn in the LiMO2 segment significantly improves the rate capabilities of LLNC with good capacity maintenance during long term cycling. Removing the Mn from the LiMO2 segment of lithium rich layered metal oxides appears to be a good strategy for improving the structural robustness and rate capabilities of these high capacity cathode materials for Li-ion batteries.

Composite positive electrode based on amorphous titanium polysulfide for application in all-solid-state lithium secondary batteries

Composite electrodes composed of amorphous titanium polysulfide (a-TiSx) and titanium disulfide (TiS2) were designed for application in all-solid-state batteries. The conductivity of the a-TiSx/TiS2-based working electrode was higher than that of the amorphous TiS4 (a-TiS4); the reversible capacity of the cell using a-TiSx/TiS2 electrode was higher than that of the cell using a-TiS4. Designing nanocomposite materials such as the a-TiSx/TiS2 described in this paper is an effective way to improve the performance of all-solid-state batteries.

Investigation of an Intermediate Temperature Molten Lithium Salt Based on Fluorosulfonyl(trifluoromethylsulfonyl)amide as a Solvent-Free Lithium Battery Electrolyte

The asymmetric amide anion, such as (fluorosulfonyl)(trifluoromethylsulfonyl)amide (fTfN–), formed intermediate temperature molten lithium salts at 100 °C, which was the lowest among the lithium single molten salts composed of conventional halogenide, nitrate, perchlorate, and perfluoro anions. The molten Li[fTfN] was highly viscous (17 Pa·s at 110 °C, however, it was classified as a good ionic liquid by Walden plot analysis, which indicates the good-dissociation between Li+ and fTfN–. Due to the high transport number of the lithium cation (0.94), the Li[fTfN] behaves as a single lithium-ion conducting material. The Li[fTfN] possesses an electrochemical window (5.0 V) due to the oxidative resistance of the perfluoro anion. The charge and discharge for a conventional composite positive electrode containing LiCoO2 or LiFePO4 was successful. In particular, LiFePO4 showed a high capacity and Coulombic efficiency at 0.1–5 C.

Effect of composition, sonication and pressure on the rate capability of 5 V-LiNi0.5Mn1.5O4 composite cathodes

Positive composite electrodes having LiNi0.5Mn1.5O4 spinel as active material, a blend of graphite and carbon black for increasing the electrode electrical conductivity and either polyvinyldenefluoride (PVDF) or a blend of PVDF with a small amount of Teflon® (1wt%) for building up the electrode. They have been processed by tape casting on an aluminum foil as current collector using the doctor blade technique. Additionally, the component blends were either sonicated or not, and the processed electrodes were compacted or not under subsequent cold pressing. Composites electrodes with high weight, up to 17mg/cm2, were prepared and studied as positive electrodes for lithium-ion batteries. The addition of Teflon® and the application of the sonication treatment lead to uniform electrodes that are well-adhered to the aluminum foil. Both parameters contribute to improve the capacity drained at high rates (5C). Additional compaction of the electrode/aluminum assemblies remarkably enhances the electrode rate capabilities. At 5C rate, remarkable capacity retentions between 80% and 90% are found for electrodes with weights in the range 3–17mg/cm2, having Teflon® in their formulation, prepared after sonication of their component blends and compacted under 2tonnes/cm2.

Characterization of Cycle-Life Aging in Automotive Lithium-Ion Pouch Cells

Lithium-ion battery electrodes utilizing mixtures of active materials have seen increased attention in recent years. These cells have merit for electric vehicle applications due to the potential for improved cycle life and increased performance. An aging campaign has been conducted with 15 Ah automotive pouch cells to quantify the relationship between a set of severity factors and the aging process for cells using composite electrodes, where aging is represented by capacity fade. A simplified electrochemical model which postulates that the materials within the composite electrode operate as parallel paths for current is used to interpret trends in the aging data. Using the model, loss of active material from individual components of the composite positive electrode can be simulated, as well as loss of cyclable lithium. Comparison with differential capacity data allows an assessment of the most likely degradation mechanism for the initial capacity fade.

Electrode contributions to the impedance of a high-energy density Li-ion cell designed for EV applications

Electrochemical Impedance Spectroscopy (EIS) measurements are performed on a high energy density Li-ion cell, which consists of a graphite-based composite negative electrode, a LiMn2O4-layered oxide blended composite positive electrode, and a carbonate solvents-based electrolyte. Signal analysis is applied to get information regarding cell internal components. The impedance behavior at different states of charge (SOC) is reproduced in an optimized laboratory scaled 3-electrode cell, which enables to: separate each electrode impedance contribution and understand the impedance variations when changing SOC. In the negative electrode, the impedance decreases when increasing SOC because the charge transfer process on the surface of graphite particles becomes less resistive with the lithium content. In the positive electrode, the impedance decreases rapidly at the beginning of charge because at low SOC the EIS response is governed by the layered oxide particles, whose electronic properties strongly depend on the lithium content. Finally, the negative electrode contribution to the total cell impedance varies between 60 and 74%; whereas the positive electrode contribution, being the complement, varies between 40 and 26%.

Improving high-capacity Li1.2Ni0.15Mn0.55Co0.1O2-based lithium-ion cells by modifiying the positive electrode with alumina

Practical high-capacity Li-ion cells containing Li1.2Ni0.15Mn0.55Co0.1O2-based positive and graphite-based negative electrodes show substantial capacity loss and impedance rise when repeatedly cycled to, or held for extended periods, at voltages exceeding 4.5 V. Their performance can be effectively improved by modifying the positive electrode. The positive composite electrodes are modified here in two different ways: by (i) alumina coatings of up to ∼4 nm applied via atomic layer deposition (ALD), and (ii) addition of nanoscale alumina powder. Thicknesses of the ALD coatings are estimated via X-ray photoelectron spectroscopy (XPS).Electrochemical cycling reveals that capacity retention is better, and impedance rise is smaller for cells containing ALD-coated electrodes. Cells with alumina-powder modified electrodes show also improved capacity retention, but without improvements in impedance. Improved capacity retention is primarily due to reduced Li trapping on the negative electrode. Lower impedance growth, in ALD modified cells, is attributed to improved electro-mechanical integrity and altered surface films inside the positive electrode. The alumina coating inhibits, but does not prevent, transition metal dissolution. The coating also reduces electrolyte oxidation. Significant accumulation of Al on the negative electrode indicates electrochemical crosstalk between the electrodes and chemical instability of the ALD coatings during extended cycling.

3D analysis of a LiCoO2–Li(Ni1/3Mn1/3Co1/3)O2 Li-ion battery positive electrode using x-ray nano-tomography

A full-field high-resolution x-ray nano-tomography technique, transmission x-ray microscopy (TXM), was used to reveal the 3D morphology of the lithium ion battery composite positive electrode: LiCoO2 (LCO)–Li(Ni1/3Mn1/3Co1/3)O2 (NMC). The TXM method allowed the unambiguous chemical identification of oxide particles by tuning the x-ray energy relative to the transition-metal absorption edges. The NMC particles have a much rougher surface compared to the LCO particles. Cracks due to processing exist in both LCO and NMC particles but the NMC particles exhibit more severe cracking and also tend to have internal pores in addition to radial cracks. Further, the carbon-based phases including the binder and the conductive carbon were identified using Zernike phase contrast imaging.

Evaluation of Commercial Lithium-Ion Cells Based on Composite Positive Electrode for Plug-In Hybrid Electric Vehicle Applications: III. Effect of Thermal Excursions without Prolonged Thermal Aging

Understanding the behavior of lithium-ion batteries exposed to thermal excursion is of great interest to plug-in hybrid electric vehicle (PHEV) applications, because vehicles often endure wide weather conditions in operation. Here we investigate a composite {LixMn2O4 + LixNi1/3Mn1/3Co1/3O2}-based commercial cell design to assess performance and degradation under thermal excursion from 25°C; through −20°C, −5°C, 10°C, 25°C, 40°C, and 60°C; to 25°C. In each isothermal regime, a reference performance test with charge and discharge cycles at C/25, C/5, C/2, 1C, and 2C is conducted to quantify cell capacity, rate capability, and other performance variations. The capacity fade caused by the thermal excursion is attributed to origins including loss of active material, degradation in reaction kinetics, and the ohmic resistance increases. Using electrochemical inference techniques, we found that thermal excursion in the range of −5°C to 40°C is benign to capacity fade. Exposure to −20°C and 60°C respectively leads to irreversible fade. The capacity fade at −20°C induced Li inventory loss and did not cause kinetic degradation, whereas the exposure at 60°C resulted in degradation in reaction kinetics. The evaluation protocols and results are helpful in assisting the study of path dependence of cell degradation in thermal aging.

Heterogeneous behaviour of the lithium battery composite electrode LiFePO4

To improve the performance of the positive electrode in batteries, it is important to thoroughly characterize these materials during battery cycling, beyond just electrochemical characterization. Among the very few techniques available for operando studies, X-ray Absorption Spectroscopy (XAS) appears very powerful, with high spatial and time resolutions provided by synchrotron radiation. Moreover XAS is well known for its capabilities in charge transfer and structure determinations, both being the major changes induced by electrochemical cycling of electrode materials. By an appropriate combination of three XAS beamlines using a specially designed electrochemical cell, we have studied composite positive electrodes made with LiFePO4 as the active material. We directly observed the heterogeneity of the electrode during operation, some parts being delayed and others advanced, compared to the mean charge state of the electrode. A mapping of this heterogeneity was made at different length scales.

A nonlinear equivalent circuit model for lithium ion cells

A nonlinear equivalent circuit model for lithium ion cells is developed where the circuit elements are based on electrochemical processes like charge transfer reaction, and variable resistances that are functions of the state variables. The potential drops are the natural state variables rather than cell state-of-charge (SOC). A commercial pouch cell consisting of composite positive and meso-carbon microbeads (MCMB) negative electrodes is analyzed. The state space model developed consists of the current balance which forms the state variable evolution equation and voltage balance which forms the output equation. The global set of parameters for a two pair resistor–capacitor circuit is calculated by solving the model equations consistently and minimizing the error with HPPC (Hybrid Pulse Power Characterization) pulse voltage at few SOCs. The entire HPPC protocol is predicted by the model accurately within an error of 50 mV. The model is validated using the full depth charge–discharge experiments. The model is used to resolve the overall cell potential into potential drops due to individual processes, yielding insights into controlling mechanisms. The variable resistor formalism enables cell voltage prediction without parameter re-estimation and hence can be developed as an on-board algorithm.

Characterization of LiNi1/3Mn1/3Co1/3O2/C Composite Positive Electrodes for Non-Aqueous Supercapacitors

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Evaluation of commercial lithium-ion cells based on composite positive electrode for plug-in hybrid electric vehicle applications. Part II. Degradation mechanism under 2 C cycle aging

Degradation phenomena and inference of their underlying mechanisms during 2 C cycle aging in a cell design comprising {LiMn 1/3Ni 1/3Co 1/3O 2 + LiMn 2O 4} composite positive electrode are studied and reported in this work. We describe how aging phenomena in the cells were studied and incremental capacity analysis applied to infer cell degradation mechanisms in the cycle aging process. Two stages of degradation were observed in the life cycle under this aging regime. In the first stage, we conclude that loss of lithium inventory was the cause of capacity fade. As a result of such parasitic loss, the cell further suffered from loss of active materials in the second stage, in which the positive electrode kinetics was hampered and the capacity loss accelerated.