Formation Of Solid Electrolyte Interface Film Research Articles

LiF-induced in-situ engineering of a dense inorganic SEI for superior lithium storage in black phosphorus anode

Black phosphorus (BP) has been highly regarded as a favourable candidate for fast-charging anode applications owing to its high theoretical capacity and advantageous charge–discharge platform. However, BP faces challenges related to compromised electrochemical performance resulting from an unstable solid-electrolyte interface (SEI) and substantial volumetric expansion. This study proposes the engineering of an inorganic-dense SEI on the surface of BP-C through the strategic incorporation of lithium fluoride (LiF). The presence of LiF in the system preferentially promotes LiPF6 adsorption from the electrolyte, facilitating the in-situ formation of a lithium-enriched inorganic SEI film on the BP-C particulate surfaces. This strategic formation effectively mitigates subsequent electrolytic decomposition, accommodates volumetric expansion, and substantially improves the rate capability and cycling stability of the system. Consequently, the BP-LiF-C electrode demonstrates high initial coulombic efficiency of 86.8 % and maintains a steady capacity of 926.1 mAh g−1 over 700 cycles at 2000 mA g−1. Moreover, when paired with a LiFePO4 cathode, the full cell exhibits long cycling stability, retaining 98.6 % of its capacity after 500 cycles at 2000 mA g−1, and performs at high rate. Therefore, utilising LiF to modulate the interfacial architecture of BP-based electrode composites provide notable guidance to enhance energy storage systems.

Si/BiPO4 composite anode material for lithium ion batteries prepared by solvothermal method

Lithium ion batteries play an important role in various energy storage technologies due to their good safety performance. As an anode material, silicon has attracted attention for its higher theoretical capacity than commercial graphite. But large volume expansion and unstable solid electrolyte interface (SEI) during the cycling of silicon lead to rapid capacity decay, which limits the commercial application of silicon anode. In this article, Si/BiPO4 anode materials were prepared by solvothermal reaction. After morphology analysis and constant current charge discharge cycle analysis of Si/BiPO4 anode materials with different mass ratios, it was found that Si/BiPO4 anode materials with the mass ratio of 7:3 exhibited more excellent electrochemical performance. The conversion reaction of BiPO4 and Li generates Bi and Li3PO4, and the alloying reaction of Bi generates Li3Bi. Bi and Li3Bi reduce the internal resistance of the Si/BiPO4 composite, and Li3PO4 is distributed on the surface of Si material, participating in the formation of SEI film and improving the stability of the material. At a current density of 500 mA g−1, the first discharge specific capacity of the Si/BiPO4 anode is 2672.1 mA h g−1. After 200 cycles, the discharge specific capacity remains at 1308.9 mA h g−1. The electrochemical impedances of pure Si and Si/BiPO4 anode materials before and after cycling were analyzed. It was found that the resistance of the Si/BiPO4 anode before and after 100 cycles was lower than that of pure Si materials, which further proved that the addition of BiPO4 material helps to improve the charge transfer ability of pure silicon materials.

Design principles of heterointerfacial redox chemistry for highly reversible lithium metal anode

High electrochemical reversibility is required for the application of high-energy-density lithium (Li) metal batteries; however, inactive Li formation and SEI (solid electrolyte interface)-instability-induced electrolyte consumption cause low Coulombic efficiency (CE). The prior interfacial chemical designs in terms of alloying kinetics have been used to enhance the CE of Li metal anode; however, the role of its redox chemistry at heterointerfaces remains a mystery. Herein, the relationship between heterointerfacial redox chemistry and electrochemical transformation reversibility is investigated. It is demonstrated that the lower redox potential at heterointerface contributes to higher CE, and this enhancement in CE is primarily due to the regulation of redox chemistry to Li deposition behavior rather than the formation of SEI films. Low oxidation potential facilitates the formation of the surface with the highly electrochemical binding feature after Li stripping, and low reduction potential can maintain binding ability well during subsequent Li plating, both of which homogenize Li deposition and thus optimize CE. In particular, Mg hetero-metal with ultra-low redox potential enables Li metal anode with significantly improved CE (99.6%) and stable cycle life for 700 cycles at 3.0 mA cm-2. This work provides insight into the heterointerfacial design principle of next-generation negative electrodes for highly reversible metal batteries.

The optimization of the electrolyte for low temperature LiFePO4-graphite battery

The design of electrolyte is crucial to improve the low temperature electrochemical performance of the LiFePO4-graphite battery. In this paper, by optimizing the solvent composition and introducing film-forming additives in the electrolyte, the low temperature performance of the battery is significantly improved. In detail, the EP (ethyl propionate) solvent with low melting point and low viscosity is added to the original EC (ethylene carbonate)-EMC (ethyl methyl carbonate)-DMC (dimethyl carbonate) solvent systems, which improved the ionic conductivity of the electrolyte, especially at low temperature (−40 °C). On the other hand, the introduction of FEC (fluoroethylene carbonate)/DTD (ethylene sulfate) film-forming agents promotes the formation of SEI (solid electrolyte interface) film, and the SEI is thin and uniform. As a result, the LiFePO4-graphite battery exhibits high capacity and long cycle life under low temperature conditions.

Investigation and electrochemical analysis of SEI layer formation in natural graphite anode formation process in lithium-ion battery

The solid electrolyte interface (SEI) layer is a surface film formed by the decomposition of the electrolyte in a lithium-ion battery on the anode surface. A proper understanding of the SEI layer formation process in graphite anodes can provide a broader view of overcoming the challenges associated with these anodes, which will lead to better charge/discharge performance. In this research, cyclic voltammetry tests, electrochemical impedance spectroscopy and sequential charge/discharge tests have been used for cell to gain an accurate understanding of the SEI film formation process on the graphite anode surface. The results of the cyclic voltammetry test show that most of the SEI layer is formed in the first cycle and reaches stability with the continuation of the voltammetric cycles of the SEI layer. Also, the charge transfers resistance (Rct) has decreased from 123.5 Ω to 27.96 Ω after the electrochemical cycles, which indicates the facilitation of the charge transfer process after the formation of the SEI layer. The results of the consecutive charge/discharge test of anode graphite coin cell also show that the amount of non-reversible capacity in the first cycle is much higher than the amount of non-reversible capacity in subsequent cycles. This indicates the formation of a large part of the SEI layer in the first cycle. Considering that the SEI layer is almost stable after the second cycle, it can be said that the method of forming a lithium-ion cell with graphite anode can be done in two charge/discharge cycles.

Effect of crown ether additive on the compatibility of electrolyte and hard carbon anode in sodium ion battery

The addition of additives is an ideal method to form solid electrolyte interface (SEI) and improve electrolyte solvation structure of Sodium-ion batteries. 15-Crown-5 is effective in improving the ionic solvation structure owing to its high coordination ability with Na+. Through physical characterization, it was analyzed that the crown ether molecules reaching the surface of hard carbon anodes driven by the electric field force could reduce the side reaction products of the hard carbon anode surface, lower the degree of polarization of the hard carbon anode, participate in the formation of uniform and smooth SEI films, augment the content of inorganic components in the SEI, and thus improve the sodium conductivity of the SEI. The addition of 15-Crown-5 proved to improve the compatibility between electrolyte and the hard carbon anode 15-Crown-5 and the hard carbon demonstrated excellent cycling performance of 300 cycles on the basis of the electrolyte, achieving a capacity retention of 95.69 % and an 8.87 % higher discharge specific capacity at 50 mA g−1as compared to the cell without the additive.

Protecting Li-metal in O2 atmosphere by a sacrificial polymer additive in Li-O2 batteries.

Li-O2 batteries (LOBs) with Li-metal as the anode are characterized by their high theoretical energy density of 3500 W h kg-1 and are thus considered next-generation batteries with an unlimited potential. However, upon cycling in a harsh O2 atmosphere, the poor-quality solid electrolyte interphase (SEI) film formed on the surface of the Li-metal anode cannot effectively suppress the shuttle effect from O2, superoxide species, protons, and soluble side products. These issues lead to aggravated Li-metal corrosion and hinder the practical development of LOBs. In this work, a polyacrylamide-co-polymethyl acrylate (PAMMA) copolymer was innovatively introduced in an ether-based electrolyte as a sacrificial additive. PAMMA was found to preferentially decompose and promote the formation of a dense and Li3N-rich SEI film on the Li-metal surface, which could effectively prohibit the shuttle effect from a series of detrimental species in the Li-O2 cell during the discharge/charge process. Using PAMMA, well-protected Li-metal in a harsh O2 atmosphere and significantly enhanced cycling performance of the Li-O2 cell could be achieved. Thus, the use of a sacrificial polymer additive provides a promising strategy for the effective protection of Li-metal in Li-O2 cells in a severe O2 atmosphere during practical applications.

An accurate and computationally efficient method for battery capacity fade modeling

• An efficient and fast model is presented to capture capacity fade in lithium-ion batteries. • Efficiently model a comprehensive array of degradation physics. • Simulation times are reduced by an average of 91%. The industry demand for accurate and fast algorithms that model vital battery parameters, e.g., state-of-health, state-of-charge, pulse-power capability, is substantial. One of the most critical models is battery capacity fade. The key challenge with physics-based battery capacity fade modeling is the high numerical cost in solving complex models. In this study, an efficient and fast model is presented to capture capacity fade in lithium-ion batteries. Here, the high-order Chebyshev spectral method is employed to address the associated complexity with physics-based capacity fade models. Its many advantages, such as low computational memory, high accuracy, exponential convergence, and ease of implementation, allow us to efficiently model a comprehensive array of degradation physics such as solid electrolyte interface film formation, hydrogen evolution, manganese deposition, salt decomposition, manganese dissolution, and electrolyte oxidation. In this work, we developed a modeling framework that accurately and efficiently predicted degradation in a lithium-ion battery over extended cycles. For example, in long cycle battery operation, the implemented Chebyshev spectral method algorithm was found to be within 0.1358% – 0.28% of a high-fidelity model, while simulation times were reduced by an average of 91%. The developed Chebyshev spectral method algorithm shows great potential in advanced battery management systems, where maintaining accuracy and achieving a fast response is critical.

Predicting the impact of formation protocols on battery lifetime immediately after manufacturing

Predicting the impact of formation protocols on battery lifetime immediately after manufacturing

Control of SEI Formation for Stable Potassium-Ion Battery Anodes by Bi-MOF-Derived Nanocomposites.

Bismuth (Bi)-based electrodes are highly attractive for potassium-ion batteries (PIBs) while suffering from a short cycle life due to the larger diameter of K ion, leading to unstable solid electrolyte interface (SEI) films during continuous potassiation/depotassiation. Herein, we developed novel ultrathin carbon film@carbon nanorods@Bi nanoparticle (UCF@CNs@BiN) materials for the long cycle life anode of PIBs. Bi nanoparticles are uniformly distributed in carbon nanorods, which not only provides a high-speed channel for ion transport but also accommodates the volume change of Bi nanoparticles during continuous potassiation/depotassiation processes. The UCF@CN matrix can direct most SEI film formation on the surface of the carbon film, not on the surface of individual Bi nanoparticles, avoiding the fracture of the matrix. Benefiting from their unique structure, the UCF@CNs@BiN anodes exhibit an outstanding capacity of ∼425 mAh g-1 at 100 mA g-1 and a capacity decay of 0.038% per cycle over 600 cycles. Even at a higher current density of 1000 mA g-1, there is a capacity decay as low as 0.036% per cycle during 700 cycles. Meanwhile, this work provides a new way of utilizing the metal-organic framework structure and reveals a highly promising PIB anode.

Unraveling the Impact of Ether and Carbonate Electrolytes on the Solid-Electrolyte Interface and the Electrochemical Performances of ZnSe@C Core-Shell Composites as Anodes of Lithium-Ion Batteries.

The recognition of the solid electrolyte interface (SEI) between the electrode materials and electrolyte is limiting the selection of electrode materials, electrolytes, and further the electrochemical performance of batteries. Herein, we report ZnSe@C core-shell nanocomposites derived from ZIF-8 as anode materials of lithium-ion batteries, the electrochemical performances, and SEI films formed on ZnSe@C in both ether and carbonate electrolytes. It is found that ZnSe@C delivers a reversible capacity of 617.1 mA h·g-1 after 800 cycles at 1 A·g-1 in the ether electrolyte, much higher than that in the carbonate electrolyte. Both ex situ X-ray diffraction and X-ray photoelectron spectroscopies reveal that stable SEI films are formed on ZnSe@C in the ether electrolyte while selenium is involved in the formation of SEI films and further dissolved into the carbonate electrolyte because of the concurrent decomposition of electrolytes and insertion of Li+ into ZnSe, which differentiates between the cycling performances of ZnSe@C composites in ether and carbonate electrolytes.

Boehmite-coated microporous membrane for enhanced electrochemical performance and dimensional stability of lithium-ion batteries

A heat-resistant boehmite-coated polypropylene (PP) membrane has been successfully fabricated and its potential application as a promising separator in the lithium-ion battery was explored. The boehmite powders with average sizes of 0.78, 1.03, and 1.72 μm, respectively, were used to fabricate the coated membrane. It was demonstrated that the coated membrane prepared by boehmite with a 0.78-μm size showed superior heat tolerance and proper air permeability. As compared to the commercialized PP membrane, such coated membrane presented improved electrolyte uptake, better interface stability, and enhanced ionic conductivity. In addition, the lithium iron phosphate (LiFePO4)/Li cell using this composite membrane exhibited better rate capability and cycling retention than that using PP membrane owing to its facile ion transport and excellent interfacial compatibility. The coating layer showed an advantage on solid electrolyte interface film formation and greatly reduced charge transfer resistance. All these fascinating characteristics would boost the application of this composite membrane for high-performance lithium-ion battery.

SERS Analysis on Interfacial Reaction of Rechargeable Battery Electrode Using Plasmonic Sensor Element – SEI Formation at Graphite Electrode Surface

Surface enhanced Raman scattering (SERS) effect to magnificently intensify Raman signals, provided by nano-structured metal surface or plasmonic nano particles, is a powerful tool for analyzing interfacial phenomena on the electrodes [1,2]. To produce this SERS effect handily in practical analyses of solid-liquid interfacial reaction at the electrode, we have developed optical components made of transparent materials with nano particles of plasmon-active metals (Ag, Au, etc.), which we call as “SERS sensor” [3]. Just locating this sensor onto the electrode surface magnifies the Raman signals about chemicals at the electrode, providing the information to investigate the interfacial reaction phenomena. We employs this SERS measurement technique to reaction mechanism analyses for electrode surfaces. One of our present targets is the reaction behavior at electrodes in Li ion secondary battery (LIB), focusing on formation mechanism of solid-electrolyte interface (SEI) [4]. SEI formation is one of the critical factors to determine the performance of LIB [5,6], which could avoid farther decomposition of electrolyte, while permitting Li+ to penetrate. Such properties of SEI films strongly depend on the composition of the film itself, which is determined by unclear mechanism of SEI formation. To understand the SEI film formation mechanism in the LIB in detail, Raman spectroscopy including highly precise SERS measurements was performed. Here, Li2CO3 generation was particularly focused on, whose formation mechanism is not clear in spite of its critical roles in the SEI films. Two kinds of solvent were used as electrolytes: mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with 1:1 in volume, called as EC-EMC, and mixture of propylene carbonate (PC) and EMC with 1:1 (v/v), PC-EMC. This is because these two organic solvents, EC and PC, show clearly different performances on SEI formation. The graphite anode and the LiCoO2thin film cathode composed LIB model cell systems. SERS measurements using the SERS sensor revealed clear difference between EC-based and PC-based electrolytes for the SEI formation behavior. Formation of Li2CO3 in the SEI film was not detected in EC-EMC, while that appeared in the case of PC-EMC, which should be due to the difference in the electrolyte distribution; PC-based electrolytes locate relatively lower amount of PC at the graphite electrodes than the amount of EC at the electrode in the case of EC-EMC, which leads to 2-electron reduction mechanism of SEI formation, resulting in Li2CO3formation. Acknowledgements This research was partially supported by the “Development of Systems and Technology for Advanced Measurement and Analysis”, and the “Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING II)” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

High Performance Li-Ion Battery Capable of Near Zero Volt (~0V) Storage

State-of-the-art (SOA) Li-ion batteries (e.g., with graphite anode and LiCoO2 cathode) for consumer electronic devices (e.g., cell phones, laptops) are designed to operate at a certain voltage range, e.g., 3-4.2V [1]. When an electronic device, like a cell phone, indicates “low battery”, the battery is near its low operating voltage threshold, and soon the device may automatically shut down to prevent the battery from further use (discharge). If a SOA Li-ion battery is allowed to further discharge to a very low cell voltage, e.g., near 0V, the battery will deteriorate quickly, manifested with a reduced battery capacity (electric charge, Ah), i.e., reduced battery use time. This is because, at or near 0V, the anode potential can exceed the substrate (commonly Cu) dissolution potential, which results in corrosion of the Cu substrate (pittings, holes) leading to compromised battery performance [2]. A battery capable of 0V (or near 0V) storage is desirable as it overcomes irreversible battery performance losses from self-discharge (to a very low voltage). For this reason, SOA Li-ion batteries are maintained above a certain threshold, e.g. > 20-30% capacity. For large format Li-ion batteries/packs, the energy left inside batteries at storage can cause serious concerns regarding: (i) safe battery transport, (ii) high maintenance cost (monitoring and periodically charging the batteries) and (iii) safe battery storage. Li-ion battery safety is a real issue exemplified by the recent Boeing 787 battery [3].Prior art to remedy Li-ion battery at or near 0V storage includes the use of an anode substrate with a higher dissolution potential, e.g., Ti substrate [4]. However Ti current collectors possess shortcomings including high cost of the Ti current collector and poor electrode coating adhesion on Ti substrate. Moreover, fundamentally, for a graphite-based anode, a solid-electrolyte-interface (SEI) film formed on graphite anode (due to organic electrolyte reduction) can break down at or near 0V and adversely impact battery performance and life. Here we present a novel approach to develop a high performance Li-ion battery, capable of deep discharge and/or storage near 0V. Our anode active material selection criterions include: (i) amenable to substrates with high dissolution potentials, (ii) sufficient (electrical) potential to avoid SEI film formation and (iii) high specific capacity for battery high energy density. Our cathode active material selection criterions include high specific capacity and high voltage to achieve high battery specific energy. Other criterions common to the anode and the cathode include high rate capability and robust cycle life. Figure 1 shows cell capacity retention as a function of cycle number for ADA near 0V capable Li-ion cells. The cells were stored at ~ 0V for 2 days for every 5 cycles. The control cell was cycled without low voltage storage. After accumulative storage for 30 days at 50mV and 100mV, respectively, the ADA near 0V capable cells demonstrated little or no capacity fade, comparable to the control cell. We attribute the excellent cell deep discharge/storage resilience to our near 0V tolerant electrode material selections. For comparison purposes, we applied the identical test conditions to graphite based Li-ion cells as shown in Figure 2. The graphite based Li-ion cells experienced rapid capacity deterioration (i.e., capacity drop to zero) after a short period of ~ 0V storage. This is likely due to Cu current collector dissolution and dissolution/reformation of SEI films on graphite anode.Our study showed that this battery technology is a very promising high performance and safe power source capable of long-term ~ 0V deep discharge/storage. This battery technology is being developed for applications in the transportation, consumer electronics, military and space markets. References "Lithium Ion technical handbook". Gold Peak Industries Ltd. November 2003. Rui Guo, Languang Lu, Minggao Ouyang, and Xuning Feng, Scientific Report, 2016; 6: 30248. National Transportation Safety Board Report, Case No. DCA131A037, March 7, 2013. Hisashi Tsukamoto, Clay Kishiyama, Mikito Nagata, Hiroshi Nakahara, Tiehua Piao, US6596439. Figure 1

Understanding Solid Electrolyte Interfaces Film Formation on SnSb Anode Electrodes for Li-ion Batteries

The demand for higher energy and power density for Li-ion batteries (LIBs), which are made of transition metal oxide cathodes and graphite anode, motivates the research toward alternative electrode materials with larger specific charge capacities1. Among the several materials proposed to replace graphite (372 mAh.g-1) as an anode material, tin (Sn) and Sn-based anodes, such as SnSb, TiSnSb, CoSn, are very promising because of the superior theoretical capacity, e.g SnSb = 825 mAh.g-1 2. However, these materials suffer from the large volume change during the Li+ alloying/dealloying mechanism and formation of solid electrolyte interface (SEI) at the electrode surface3. The SEI film characteristics, such as film thickness, composition, and Li+ conductivity, dictate the electrochemical reaction occurring at the electrode surface, hence, the battery performance like first irreversible capacity loss, energy density, cycle stability, and cycle life performance4-5. Previous work shows in great detail the SEI formation mechanism for carbonaceous anodes5-6, while there are fewer studies for intermetallics materials. In this work, the SnSb anode electrode was fabricated using slurry cast method by mixing a SnSb powder, binding agent (PVDF), and conducting agent (Carbon Sp) with a ratio of 70:15:15. The electrochemical tests (GCPL, and PEIS) were performed using a standard three-electrode Swagelok cells assembled in a glove box filled with high purity argon. The half-cells consist SnSb as a working electrode, Li foil as the reference and counter electrode, and Whatman glass microfiber separator soaked in 1 M LiPF6 in EC:PC:3EMC with 1% VC,5 % FEC electrolyte solution. The PEIS were recorded by applying a dc bias and amplitude of 7 mV over the frequency range of 200 KHz to 10 mHz at various potential. The cell voltage at which PEIS measurement performed was achieved by galvanostatically cycling the cell at C/20. Fig1. shows the equivalent circuit used for analyzing the impedance spectra of SnSb/Li cell, where Re represents the total resistance of the electrode, electrolyte and separator; Rsei and Csei are the resistance and capacitance of the SEI film, respectively; Rct and Cdl are the charge-transfer resistance and double layer capacitance, respectively; and W is a Warburg impedance due to the semi-infinite diffusion of Li+ into SnSb electrode. Fig1. Equivalent circuit used for impedance analysis of SnSb/Li cell. Fig 2a & b, shows the impedance spectra for the first charge and discharge. At OCV before the first discharge, the impedance spectra has a semi-circle at high frequency (HF) due to the SEI film formation and a line at an angle close to 90 0 in lower frequency (LF) which is characteristics of capacitance, Fig 2a. This indicates that there is a formation of SEI film due to electrolyte decomposition by simple contact of the SnSb electrode surface with the electrolyte. The impedance spectra for the first alloying/de-alloying of Li+ into SnSb consists of two overlapped semi-circles at HF and medium frequency (MF) domain, which corresponds to the resistance of SEI and charge-transfer, followed by a spike at LF which is attributed to the diffusion of Li+. Fig 2. Typical Nyquist plot of SnSb electrode at various potentials for the (a) first discharge and (b) charge. During this work, the SEI formation on the surface of SnSb anode electrode for the first cycle will be discussed.

Density Functional Theory Research into the Reduction Mechanism for the Solvent/Additive in a Sodium-Ion Battery.

The solid-electrolyte interface (SEI) film in a sodium-ion battery is closely related to capacity fading and cycling stability of the battery. However, there are few studies on the SEI film of sodium-ion batteries and the mechanism of SEI film formation is unclear. The mechanism for the reduction of ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), ethylene sulfite (ES), 1,3-propylene sulfite (PS), and fluorinated ethylene carbonate (FEC) is studied by DFT. The reaction activation energies, Gibbs free energies, enthalpies, and structures of the transition states are calculated. It is indicated that VC, ES, and PS additives in the electrolyte are all easier to form organic components in the anode SEI film by one-electron reduction. The priority of one-electron reduction to produce organic SEI components is in the order of VC>PC>EC; two-electron reduction to produce the inorganic Na2 CO3 component is different and follows the order of EC>PC>VC. Two-electron reduction for sulfites ES and PS to form inorganic Na2 SO3 is harder than that of carbonate ester reduction. It is also suggested that the one- and two-electron reductive decomposition pathway for FEC is more feasible to produce inorganic NaF components.

Effects of lithium fluoride coating on the performance of nano-silicon as anode material for lithium-ion batteries

To overcome the capacity fading issue of Si particle upon repeated cycle process, in this study, lithium fluoride (LiF) is introduced to coat commercial nano silicon to form an artificial solid electrolyte interface (SEI) film. After a facile spray drying process, Si nanoparticles are aggregated to form spheres and the LiF is homogenously coated on the surface of Si nanoparticles. Compared with the bare nano-Si, Si/LiF composite shows a huge increase in capacity retention, increasing from 0.8% to 31.2% after 30 cycles. Such improvement results from artificial formation of a stable LiF-based SEI film on the Si surface, reducing the parasitic reaction and thus minimizing the capacity loss during the first lithiation process.

Influence of Fluoroethylene Carbonate Additive Concentration on Silicon/Graphene Composite Anode

Silicon has been promising anode material in lithium ion battery due to higher theoretical capacity of 4200 mAh/g and abundance (1, 2). However, volume expansion and contraction of silicon particle over cycling lead to electrode deformation and poor cyclic performance. Irreversible capacity loss from solid electrolyte interface (SEI) film formation is also major factor to degrade cyclic performance (3). Generally SEI film is getting thicker due to continuous consumption of Li salt and reduction of carbonate electrolyte. Previous studies has demonstrated the effect of additives such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), and other additives. FEC is found to be effective additive to improve the performance of silicon electrode (1, 4-6). In this study, we focused on influence of FEC additive concentration in carbonate electrolyte on electrochemical performance. Electrode morphology and surface chemistry of SEI was investigated with various FEC concentrations (3, 5, 10, 20 wt%) in 1.2M LiPF6 at EC/EMC (3/7, by wt) electrolyte. Electrodes were prepared by blending Si/graphene active material (XG sciences), graphene and poly acrylic acid (PAA) and casting the slurry on copper foil. In the presence of FEC additive, the better capacity retention with cycling, as shown in Figure 1, was obtained, which is consistent to previous report because of thinner and stable SEI film formation (5). FEC concentration effect on cycle performance of silicon electrodes can also be observed from Figure 1. Initially, cell containing 3wt% FEC has higher reversible capacity only until 20th cycle. The cells with other FEC concentration (5-20% FEC) have better capacity retention up to 69thcycles. Lower FEC concentration in Si/graphene electrode may not lead to enough SEI layer formation due to lack of the additive for longer cyclic life. In order to find out the FEC concentration related SEI properties, we conducted electrochemical impedance spectroscopy (EIS). Other surface analysis, such as SEM, EDX and Raman, will be reported. Optimum amount of FEC additive concentration will be discussed. Acknowledgement The silicon material is provided by XG sciences. The validation was conducted under Cell Analysis, Modeling, and Prototyping (CAMP) Facility at ANL. Support from David Howell and Peter Faguy of the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged.

Cycling Performance of Lithium-Ion Batteries with Non-Uniform Size Particles in the Carbon Anode

Lithium-ion batteries (LIBs) are being considered as portable power sources for modern electric and hybrid vehicles due to their high gravimetric and volumetric energy density. However, the discharge capacity of LIBs decreases with continuous cycling due to various associated side reactions. The intensity of degradation caused by side reactions might be reduced through control of design and operational parameters i.e. charge & discharge rate, operational voltage, electrode thickness and particle size. Mathematical modeling is an important tool to analyze the effect of these variables on the performance of LIBs, while measuring their influence through experiments is difficult and expensive. In a recent study1, a mathematical model was developed to quantify the solid-electrolyte interface (SEI) film thickness and gas volume in a carbon electrode for different discharge rates up to a 1000 cycles; however, it was assumed that the carbon electrode is composed of identical particles. In this work, the model developed in ref. [1] is being extended to analyze the effect of cycling lithium-ion cells with carbon negative electrodes having different particle configurations and to observe the impact of electrode morphology over cell performance. For this purpose, the carbon electrode will be split into three equal sub-parts such that within each layer particles of the same size exist. The radius of particles is either 0.7Rp, Rp or 1.3 Rp (so that the interlayer sizing is non-uniform), where Rp is radius of the reference case considered in ref. [1]. All possible combinations of the final electrode morphologies to be considered are shown in Fig. 1. One representation of fresh and cycled negative electrode is shown in Fig. 2, with smallest particle section sandwiched between medium and largest particle layers at the current collector and separator ends, respectively. As is given in Eq. (1), cycling of the cell results in the decomposition of the electrolyte at the negative electrode leading to SEI film formation and generation of ethylene gas as a by-product of the side reaction. The SEI film increases the resistance for lithium diffusion in the carbon active material and the gas generated occupies the space previously occupied by the pores of the electrode (as shown in Fig. 2) slowing the movement of Li-ions in the electrolyte, both leading to reduction in cell capacity. The governing equations of the pseudo-2D model2-3 along with SEI formation and gas evolution1 will be solved using COMSOL Multiphysics 4.3a for all morphologies of MCMB anode shown in Fig. 1 with LiMn2O4 cathode. The cell will be cycled till either of 80% loss (end of life) of rated capacity or 1000 cycles is reached. A comparative analysis of discharge capacity, cycling performance, thickness of SEI film and gas generation will be demonstrated. It is expected that this study will pave the way for design of carbon electrodes for achieving higher discharge capacity. From preliminary data results obtained so far, B-A-C or C-A-B based negative electrodes have shown much better performance than the remaining four combinations. In the latter cases, the battery loses 80% of the rated capacity before the completion of 1000 cycles, whereas a high discharge capacity, low film thickness and lesser gas is generated after the same number of cycles in the former cases.

Enhancement of the lithium cycling capability using Li–Zn alloy substrate for lithium metal batteries

The cycling performance of a Li metal electrode in rechargeable Li batteries is studied using a novelly formed Li–Zn alloy as a substrate. A Zn layer electrodeposited on a Cu disk with ultrasonic assistance is used as a substrate precursor. Li electrodeposition followed to form the Li–Zn alloy. The morphologies of the substrate before and after Li deposition and stripping are investigated by scanning electron microscopy (SEM), and the electrochemical properties of the substrate are investigated by galvanostatic charge-discharge and cyclic voltammetry (CV). The growth states of solid electrolyte interface (SEI) films of Li deposits on the Li–Zn alloy and Cu surfaces are compared by electrochemical impedance spectroscopy (EIS); exchange current densities of Li electrodeposition on Cu, Zn, and Li–Zn alloy substrates are also compared based on tests of constant current pulse deposition. The efficiency of Li deposition/stripping on the Li–Zn alloy substrate remains high at 96.7% after 400 cycles at a current density of 0.1mAcm−2 and 250 cycles at the current density of 0.2mAcm−2. These results can be attributed to the formation of a stable SEI film on the Li–Zn substrate and the high exchange current density of Li deposition and stripping on this substrate. The Li–Zn alloy proposed in this work may be a perfect substrate for enhancing the cycling capability of Li metal electrode.