Computer simulation of negative electrode operation in lithium—ion battery: Galvanostatic discharg active intercalating agent grains,role of diffusion limitations

Computer simulation of the structure and methods of operation (galvanostatic discharge) of the negative electrode of a lithium-ion battery is performed. Two possible models of the active anode layer were compared. 1. The model of porous active layer (mixture of active substance grains with grains of electrolyte). Here, the electrochemical process occurs within a porous active layer. 2. The film model (constant-thickness layer) of pure active substance (intercalating agent) grains without admixture of grains of electrolyte. In this case, the electrochemical reaction occurs only on the planar active electrode layer/interelectrode space interface. In both cases, the optimum working parameters of anode active layers were calculated: porous active layer thickness (in the film model, this was the calculation parameter), duration of full anode discharge, specific electric capacitance and finite difference between the intercalating agent/electrolyte potentials at the active anode layer/interelectrode space interface. It is found that each of these two models has its advantages and faults. Specific electric capacitance C cannot exceed the values of the order of magnitude of 10 C/cm² when a porous active layer is used. Whereas in the film model, much higher values of C may be obtained: tens and even hundreds of C/cm². On the other hand, in the case of anode discharge, the reasonable discharge current density value, its maximum value, at which practically full recovery of lithium atoms from active intercalating agent grains is still possible, proves to be by orders of magnitude higher in the case of an anode with a porous active layer, as compared with a film-type anode. Thus, in the case of development of electrode active layers of lithium-ion batteries, there is a possibility of choosing from two variants. There is the variant of an active film-type layer providing high capacitance values, but low discharge current density. Or there is another variant: a porous active layer with limited capacitance but then much higher values of discharge current density.     

Computer simulation of negative electrode operation in lithium—ion battery: Galvanostatic discharge,porous electrode model and film model

Computer simulation of the structure and methods of operation (galvanostatic discharge) of the negative electrode of a lithium-ion battery is performed. Two possible models of the active anode layer were compared. 1. The model of porous active layer (mixture of active substance grains with grains of electrolyte). Here, the electrochemical process occurs within a porous active layer. 2. The film model (constant-thickness layer) of pure active substance (intercalating agent) grains without admixture of grains of electrolyte. In this case, the electrochemical reaction occurs only on the planar active electrode layer/interelectrode space interface. In both cases, the optimum working parameters of anode active layers were calculated: porous active layer thickness (in the film model, this was the calculation parameter), duration of full anode discharge, specific electric capacitance and finite difference between the intercalating agent/electrolyte potentials at the active anode layer/interelectrode space interface. It is found that each of these two models has its advantages and faults. Specific electric capacitance C cannot exceed the values of the order of magnitude of 10 C/cm² when a porous active layer is used. Whereas in the film model, much higher values of C may be obtained: tens and even hundreds of C/cm². On the other hand, in the case of anode discharge, the reasonable discharge current density value, its maximum value, at which practically full recovery of lithium atoms from active intercalating agent grains is still possible, proves to be by orders of magnitude higher in the case of an anode with a porous active layer, as compared with a film-type anode. Thus, in the case of development of electrode active layers of lithium-ion batteries, there is a possibility of choosing from two variants. There is the variant of an active film-type layer providing high capacitance values, but low discharge current density. Or there is another variant: a porous active layer with limited capacitance but then much higher values of discharge current density.     

Computer modeling of negative electrode operation in lithium-ion battery: Model of equal-sized grains,galvanostatic discharge mode,calculation of characteristic parameters

A computer simulation of the negative electrode (anode) operation in a lithium-ion battery is performed. A complete research program is carried out in accordance with the recommendations of the theory of porous electrodes: the “model of equal-sized grains of two types” was studied, percolation properties of the anode active layer were researched, values of effective coefficients were calculated for charge transfer and mass transport, a complete system of equations describing operation of the anode is presented. Two specific cases of galvanostatic mode of anode discharge are considered in detail: an “ideal” anode and anode with nanosize particles. Working anode parameters are calculated: optimum bulk concentration of graphite in the active layer, active layer thickness, time of complete anode discharge, its specific electric capacitance and final potential on the active/layer interelectrode space interface. Advisability of working with anodes with nanosize grains and electrolyte with enhanced specific conductivity is shown.     

Computer modeling of positive electrode operation in lithium-ion battery: Model of equal-sized grains,percolation calculations

A computer simulation of the negative electrode (anode) operation in a lithium-ion battery is performed. A complete research program is carried out in accordance with the recommendations of the theory of porous electrodes: the “model of equal-sized grains of two types” was studied, percolation properties of the anode active layer were researched, values of effective coefficients were calculated for charge transfer and mass transport, a complete system of equations describing operation of the anode is presented. Two specific cases of galvanostatic mode of anode discharge are considered in detail: an “ideal” anode and anode with nanosize particles. Working anode parameters are calculated: optimum bulk concentration of graphite in the active layer, active layer thickness, time of complete anode discharge, its specific electric capacitance and final potential on the active/layer interelectrode space interface. Advisability of working with anodes with nanosize grains and electrolyte with enhanced specific conductivity is shown.     

Computer simulation of positive electrode operation in lithium-ion battery: Optimization of active mass composition

The work of the positive electrode (cathode) of a lithium-ion battery is simulated. The model of equally sized grains of three types: the intercalating agent grains with a volume fraction g, the electrolyte grains with a volume fraction g _i, and the carbon black grains with a volume fraction g _e is studied. The optimal composition of cathode active mass providing maximum specific capacity of cathode is determined. It is shown that a fraction of carbon black grains should be as small as possible: g _e = 0.35. The variation in the fraction of intercalating agent grains within the allowable limits (0 ?? g ?? 0.3) changes the main parameters of cathode active mass: a fraction of electrochemically active intercalating agent grains g* (g* < g); a specific surface area S, on which the electrochemical process proceeds; and the conductivity k* by lithium ions in the ionic percolation cluster, which forms in the cathode active mass. The parameters g* and S decrease and parameter k* steeply increases with decreasing g. Therefore, in the range of possible values of g, specific capacity of cathode reaches the maximum value at g = g _opt. The value of g _opt is determined under the galvanostatic mode of cathode discharge. The cathode working parameters: the active layer thickness, discharge time, specific capacity, and potential at the cathode active layer/interelectrode space interface at the instant of discharge completion are calculated in relation to a fraction of intercalating agent grains g.     

Development and experimental verification of a mathematical model of lithium ion battery

A model of the lithium ion battery is developed which takes into account intercalation and extraction of lithium ions in the active mass of negative and positive electrodes, the dependences of equilibrium electrode potentials on the concentration of intercalated lithium, the ion transfer in pores of electrodes and the separator, the kinetics of electrode reactions, and the electric double layer charging. As the active material for the negative electrode, UAMS graphite material is used. Lithium-nickel-cobalt oxide serves as the positive electrode. The porous structure of electrodes is studied by the method of standard contact porosimetry. Sufficiently high porosity values found for both electrodes (50% for anode and 27% for cathode) made it possible to consider the interface as regards the internal pore surface found from porosimetry data rather than as regards their external surface as in the previous studies. A comparison of calculated and experimental discharge curves demonstrates their closeness, which points to the correctness of the model. By the fitting procedure, the coefficients of solid-state diffusion of lithium ions and the rate constants for reactions on both electrodes are found.     

Impedance spectroscopy studies of changes in the properties of lithium-sulfur cells in the course of cycling

Changes in the properties of lithium-sulfur cells during cycling were studied by impedance spectroscopy. The electric conductivity of the electrolyte changed during the charging and discharging of the lithium-sulfur cells as a result of the dissolution of lithium polysulfides formed in electrochemical reactions. The maximum resistance of the electrolyte and the surface layers on the sulfur and lithium electrodes was achieved in the region of the transition between the low- and high-voltage areas on the charge and discharge curves of the cells. This region corresponded to the highest concentration of lithium polysulfides in the electrolyte. For nearly charged or discharged lithium-sulfur cells, the impedance spectra contained linear segments which could be attributed to diffusion limitations at low frequencies. An analysis of the results of impedance studies suggested that the electrochemical processes in lithium-sulfur cells were controlled by diffusion in the surface layer on the sulfur electrode at high degrees of charge or discharge and by the transport properties of the electrolytic system at moderate degrees of charging.     

Measurement of lithium diffusion coefficient in thin metallic multilayer by neutron depth profiling

Diffusion of Li ions in thin sandwich films with copper or lead encompassing layers (obtained by ion beam sputtering deposition technique) has been studied. These metals are promising candidates for electrodes in lithium-ion batteries. It is because they exhibit an ability to store and release Li ions during charging and discharging processes. Lithium diffusion was induced in samples by thermal annealing cycles. The lithium depth profile was measured using a nondestructive neutron depth profiling technique after each thermal annealing step. The analysis of experimental data allowed to evaluate the lithium depth profiles and directly calculate the diffusion coefficients.     

Characterization of bi-material electrodes for electrochemical hybrid energy storage devices

The hybridization of an electrochemical double layer capacitor and a lithium-ion battery at the electrode level can be realized by combining lithium insertion materials and capacitive materials in bi-material electrodes. A bi-material electrode based on activated carbon and LiMn₂O₄ has been prepared and characterized in the present work. An experimental setup was developed in order to measure the current sharing between the two different active materials in a single segmented bi-material electrode. This setup allows distinguishing the contribution of each material to the overall electrode performance. The characterization consisted of cyclic voltammetry and galvanostatic charge discharge cycling. The behavior of the bi-material electrode is essentially a linear combination of the behaviors of the two materials.     

In Situ Polymerized 1,3-Dioxolane Electrolyte for Integrated Solid-State Lithium Batteries

Solid-state lithium batteries are promising and safe energy storage devices for mobile electronics and electric vehicles. In this work, we report a facile in situ polymerization of 1,3-dioxolane electrolytes to fabricate integrated solid-state lithium batteries. The in situ polymerization and formation of solid-state dioxolane electrolytes on interconnected carbon nanotubes (CNTs) and active materials is the key to realizing a high-performance battery with excellent interfacial contact among CNTs, active materials and electrolytes. Therefore, the electrodes could be tightly integrated into batteries through the CNTs and electrolyte. Electrons/ions enable full access to active materials in the whole electrode. Electrodes with a low resistance of 4.5 Ω □⁻¹ and high lithium-ion diffusion efficiency of 2.5×10⁻¹¹ cm² s⁻¹ can significantly improve the electrochemical kinetics. Subsequently, the batteries demonstrated high energy density, amazing charge/discharge rate and long cycle life.     

Tin oxalate as a precursor of tin dioxide and electrode materials for lithium-ion batteries

Tin(II) oxalate was studied as a novel precursor for active electrode materials in lithium-ion batteries. The discharge of lithium cells using tin oxalate electrodes takes place by three irreversible steps: tin reduction, forming a lithium oxalate matrix; solvent decomposition to form a passivating layer; and oxalate reduction in a two-electron process. These are followed by reversible alloying of tin with lithium, leading to a maximum discharge of 11 F/mol. Cycling of the cells showed reversible capacities higher than 600 mAh/g during the first five cycles and ca. 200 mAh/g after 50 cycles. Tin oxalate was converted to tin dioxide by thermal decomposition at 450 °C and also by a chemical method by dissolving tin oxalate powder in 33% v/v hydrogen peroxide at room temperature. The ultrafine nature of the tin dioxide powders obtained by this procedure allow their use as electrodes in lithium cells. The best capacity retention during the first five cycles was achieved for a sample heat treated to 250 °C to eliminate surface water. Electronic Publication     

金属锂负极失效机制及其先进表征技术

尽管传统的石墨负极在商业化锂离子电池中取得了成功,但其理论容量低(372 mAh·g^?1)、本身不含锂的先天缺陷限制了其在下一代高比能量锂电池体系中的应用,特别是在需要锂源的锂-硫和锂-空气电池体系中。金属锂因其极高的理论比容量(3860 mAh·g^?1)和低氧化还原电势(相对于标准氢电极为?3.040 V),被认为是下一代锂电池负极材料的最佳选择之一。但是,金属锂负极存在库伦效率低、循环性能差、安全性差等一系列瓶颈问题亟待解决,而循环过程中锂枝晶的生长、巨大的体积变化、以及电极界面不稳定等是导致这些问题的关键因素。本文综述了近年来关于金属锂负极瓶颈问题及其机理,包括金属锂电极表面固态电解质界面膜的形成,锂枝晶的生长行为,以及惰性死锂的形成。同时,本文还介绍了目前用于研究金属锂负极的先进表征技术,这些技术为研究人员深入认识金属锂负极的失效机制提供了重要信息。     

Models of lithium transport as applied to determination of diffusion characteristics of intercalation electrodes

In order to elucidate the mechanism of lithium transport in intercalation electrodes based on solid lithium-accumulating compounds and determine its parameters, the kinetic models are used which allow the combined analysis of electrode impedance spectroscopy, cyclic voltammetry, pulse chronoampero- and chronopotentiometry data to be carried out. The models describe the stages of consecutive lithium transport in the surface layer and bulk of electrode-material particles, including the accumulation of species in the bulk. The lithium transport stages that occur in the surface layer of an intercalation-material particle and in its bulk are both of the diffusion nature but substantially differ as regards their characteristic times and diffusion coefficients D. Taking account of this peculiarity and assessing adequately the geometrical configuration of intercalation system allow the diffusion parameters of lithium transport to be correctly determined.     

Combined Method for Reducing Irreversible Capacity of the Negative Graphite Electrode in Lithium-Ion Battery

A method for the complete removal of the irreversible capacity of the negative electrode of lithium-ion batteries is suggested. The method consists of bringing the graphite electrode in contact with metallic lithium in the electrolyte. The distinguishing feature of the modified method is that all operations (assembling and filling cells, storing electrodes) are performed in a carbon dioxide atmosphere. Performing all these operations in a carbon dioxide atmosphere improves the reproducibility of the method. The balance between the weight of lithium and graphite, which is optimum for the complete removal of the irreversible capacity, is calculated. It is shown that applying the combined method does not lead to a decrease in the reversible capacity of the negative electrode.     

Intermetallic insertion electrodes derived from NiAs-, Ni2In-, and Li2CuSn-type structures for lithium-ion batteries

The implications of designing intermetallic insertion electrodes for lithium-ion cells are discussed in terms of materials with the NiAs-, Ni₂In-, and Li₂CuSn-type structures. Specific reference is made to a recent announcement that lithium can be inserted topotactically into η-Cu₆Sn₅ at approximately 400 mV above the potential of metallic lithium. These materials hold promise for developing a new family of electrode structures to replace carbon as the negative electrode in state-of-the-art lithium-ion cells.     

An unsymmetrical lithium-ion pathway between charge and discharge processes in a two-phase stage of Li4Ti5O12

In this work, we investigated lithium-ion diffusion in spinel Li(4)Ti(5)O(12) nano-particles with carbon coating by electrochemical impedance spectroscopy (EIS), and proposed a hybrid model of the unsymmetrical lithium-ion pathway between charge and discharge processes. In this hybrid model, the charge process still follows the core-shell model, but in the discharge process, the phase transition evolves by growth of a few nuclei on the surface. And this hybrid model is possibly attributed to the nonuniform electron conductivity inside the Li(4)Ti(5)O(12) particles. Additionally, the relaxation process and the particle morphology are also carefully discussed in the experiment to show that this hybrid model is quite practical. Thereby, this investigation presents an unsymmetrical lithium-ion pathway in Li(4)Ti(5)O(12) particles, which could be extended to other active materials in lithium ion batteries.     

锂离子固相扩散控制下的材料放电过程

从理论上分析了在锂离子相扩散控制条件下,电极材料的恒流放电过程,数值计算的结果表明,Q值（放电时率和扩散时间常数之比）对材料的放电容量有非常重要的影响,模拟了LiMn2O4正极材料和石墨负极材料的恒流放电曲线,分析了颗粒粒径对这两种材料放电容量的影响。     

Effects of solid polymer electrolyte coating on the composition and morphology of the solid electrolyte interphase on Sn anodes

In order to discuss the effect of polymer coating layer on the Sn anode, the composition and morphology of the solid electrolyte interphase (SEI) film on the surface of Sn and Sn@PEO anode materials have been investigated. Compared with the bare cycled Sn electrode, the SEI on the surface of cycled Sn@PEO electrode is thinner, smoother, and more stable. Therefore, the Sn@PEO nanoparticles can basically keep the original appearance during cycling. Based on the results obtained from X-ray photoelectron spectroscopy (XPS), the SEI formed on the Sn@PEO electrode is characterized by inorganic components (Li₂CO₃)-rich outer layer and organic components-rich inner which could make the SEI more stable and inhibit the electrolyte immerging into the active materials. In particular, the elastic ion-conductive polyethylene oxide (PEO) coating could increase the toughness of SEI and allow the SEI to endure the stress variation in repetitive lithium insertion and extraction process. As a result, the Sn@PEO electrodes show significantly better capacity retention than bare Sn electrodes. The findings can serve as the theoretical foundation for the design of lithium-ion battery electrode with high energy density and long cycle life.