Effects of lithium phosphorous oxynitride film coating on electrochemical performance and thermal stability of graphite anodes

In this study, we investigated the effects of lithium phosphorus oxynitride (LiPON) solid electrolyte thin-film deposition on the electrochemical performance and thermal stability of pristine graphite and carbon-coated graphite composite anodes. The LiPON film was deposited by radio frequency (rf) magnetron sputtering. We studied the thermal stability of the lithiated electrodes when immersed in the presence of a liquid electrolyte by differential scanning calorimetry (DSC).The LiPON thin-film coating suppressed the impedance growth during the cycling process and inhibited the reaction between the lithiated electrode and the electrolyte, thus improving the cycle performance and thermal stability of the graphite electrode. However, for the carbon-coated graphite electrode, the heat evolution below 250 °C decreased, whereas that below 300 °C increased. We attributed this phenomenon to the low thermal stability of the LiPON thin-film coating owing to an exothermic reaction between the LiPON film and the electrolyte that occurs at approximately 290 °C.     

锂电池中负极表面固体电解质膜的SERS研究

本文对锂电池中负极材料表面固体电解质膜 (SEI膜 )的SERS谱进行了研究 :极化低电位下对贵重金属的研究表明 ,非水电解质溶液中痕量水的存在将对SEI膜产生重要影响。在微量水存在的情况下 ,RCOCO2 Li不是SEI膜的稳定成份 ,Li2 CO3、LiF、LiOH或LiOH·H2 O等物种才是其稳定组成。进一步的研究表明 :SEI膜的某些谱带具有不同的光吸收特性 ;它是对外界条件非常敏感的一种表面膜。通过对比分析 ,SEI膜的特征谱带得到了进一步归属 ,并对其形成机理做了讨论。     

TG-MS analysis on thermal decomposable components in the SEI film on Cr2O3 powder anode in Li-ion batteries

The thermal decomposable species in the solid electrolyte interphase (SEI) film on Cr₂O₃ powder anode at different lithiated and delithiated states in the first cycle were analyzed by thermogravimetry and mass spectrometry (TG-MS) technique. The weight loss ratio in a fully lithiated Cr₂O₃ electrode during TG measurement at 50–500 °C is 8.9 wt%, which is decreased to 1.5 wt% for a fully delithiated Cr₂O₃ electrode. This indicates that the SEI film on Cr₂O₃ powder anode is decomposed electrochemically upon delithiation. The main gas products are CH₂=CH₂, CO₂, and CH₃-containing volatile species in thermal reaction. They are released step-by-step in four characteristic temperature regions, which were originated mainly from oligomer and polyethylene-oxide-like species, partly from ROCO₂Li. It is also observed that the amount of thermal decomposable components in the SEI film on the fully lithiated Cr₂O₃ powder electrode is much higher than that on graphite and hard carbon anodes, indicating different SEI features of transition metal oxide anodes.     

Negative electrodes in rechargeable lithium ion batteries — Influence of graphite surface modification on the formation of the solid electrolyte interphase

Rechargeable lithium ion cells operate at voltages of ∼4.5 V, which is far beyond the thermodynamic stability window of the battery electrolyte. Strong electrolyte reduction and corrosion of the negative electrode has to be anticipated, which leads to irreversible loss of electroactive material and electrolyte, and thus strongly deteriorates cell performance. To minimize these reactions, negative electrode and electrolyte components have to be combined bringing about the electrolyte reduction products to form an effectively protecting film at the anode/electrolyte interface. This film hinders further electrolyte decomposition reactions and acts as membrane for the lithium cations, i.e., behaves as asolidelectrolytei2nt erphase (SEI). The present paper gives a review of our recent work in the field of negative electrodes in lithium ion batteries. The effects of the graphite anode surface and graphite anode surface modification on the formation of the SEI are discussed in detail by using the example: modification with carbon dioxide. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Effect of nickel coating on electrochemical performance of graphite anodes for lithium ion batteries

Among the different materials often studied and proposed as negative electrodes for lithium-ion batteries, graphite anodes are the most used in commercial batteries. For this study, synthetic graphite was tested. During the first discharge 0.2 Li ions were consumed for the formation of the SEI film and the capacity reaches about 387 mAh/g. But at the end of the first charge only 72% of the initial charge was recovered (the reversible capacity is about 279 mAh/g). In order to improve this performance we have deposited metallic nickel on graphite with the intention to obtain a homogeneous thin layer able to modify the nature of the SEI film, to allow the diffusion of lithium ions through the protective layer, and also to increase the performance of graphite electrodes. The results show a decrease of the irreversible capacity loss (16% instead of 28% for pure graphite electrodes) as well as better cycleability for a nickel-deposited graphite electrode with only 11% weight ratio of nickel. On the other hand, an increase of the nickel content decreases this performance.     

Impacts of lithium tetrafluoroborate and lithium difluoro(oxalate)borate as additives on the storage life of Li-ion battery at elevated temperature

The impacts of boron-based Li salt additives including lithium tetrafluoroborate (LiBF₄) and lithium difluoro(oxalate)borate (LDFOB) on the storage life of Li-ion battery at elevated temperature are investigated. Adding 1 wt% additives in the electrolyte significantly affects the storage life of the LiNi_0.8Co_0.15Al_0.05O₂/graphite full cell at 55 °C. The anode solid electrolyte interphase (SEI), preventing the loss of Li⁺ and e^? in anode, is the key factor affecting the storage life. The formation and aging of SEI on the graphite anode with and without additives are investigated. It is found that the SEI formed with the addition of LiBF₄ is thick and loose due to LiF crystals produced by the decomposition of LiBF₄ and the SEI cannot prevent the Li⁺ and e^? loss in anode and the decomposition of the electrolyte solvent, resulting in shorter storage life of the battery. On the contrary, the SEI formed with the addition of LDFOB is thick and compact due to formation of the lithium oxalate in the SEI, produced by the decomposition of LDFOB. The SEI efficiently inhibits decomposition of the electrolyte solvent on anode and makes a longer storage life of the battery.     

Identification of solid electrolyte interphase formed on graphite electrode cycled in trifluoroethyl aliphatic carboxylate-based electrolytes for low-temperature lithium-ion batteries

Trifluoroethyl aliphatic carboxylates with different length of carbon-chain in acyl groups have been introduced into carbonate-based electrolyte as co-solvents to improve the low-temperature performance of lithium-ion batteries, both in capacity retention and lowering polarization of graphite electrode. To identify the further influence of trifluoroethyl aliphatic carboxylates on graphite electrode, the components and properties of the surface film on graphite electrode cycled in different electrolytes are investigated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The IR and XPS results show that the chemical species of the solid electrolyte interphase (SEI) on graphite electrode strongly depend on the selection of co-solvent. For instance, among those species, the content of RCOOLi increases with an increasing number of carbon atoms in RCOOCH₂CF₃ molecule, wherein R was an alkyl with 1, 3, or 5 carbon atoms. We suggest that the thickness and components of the SEI film play a crucial role on the enhanced low-temperature performance of the lithium-ion batteries.     

XPS, XRD and SEM characterization of a thin ceria layer deposited onto graphite electrode for application in lithium-ion batteries

Thin ceria layer deposited by electro-precipitation onto graphite was synthesised and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electro-precipitated ceria has a cubic structure with nanocrystallites of about 6 nm. The SEM analyses shows that the ceria layer reflects the morphology of the graphite electrode, exhibits small cracks usually found on the electro-precipitated films but covers almost completely the surface of the graphite. The ceria layer is composed of 75% Ce(IV) and 25% Ce(III) oxides as indicated by the XPS analyses. Cyclic voltammetry and galvanostatic charge-discharge tests in ethylene carbonate/dimethyl carbonate (1/1) (wt/wt) in the presence of 1 M LiPF₆ show that reversible lithium insertion and deinsertion occurs in the graphite/ceria electrode and that the ceria layer on the graphite electrode prevents from the loss of capacity during the first four cycles. The reduction of the electrolyte occurs at about 0.7 V vs Li/Li⁺ on both electrodes but XPS and SEM analyses show that the SEI layer is thin and not as homogenous on the graphite as on the graphite/ceria electrode. The composition of the SEI layer on the graphite/ceria electrode, mainly composed of Li₂CO₃, ROCO₂Li, R-CH₂OLi and LiF, is different than those obtained on the graphite.     

Influence of the lithium salt nature over the surface film formation on a graphite electrode in Li-ion batteries: An XPS study

The formation of a passivation film (solid electrolyte interphase, SEI) at the surface of the negative electrode of full LiCoO₂/graphite lithium-ion cells using different salts (LiBF₄, LiPF₆, LiTFSI, LiBETI) in carbonate solvents as electrolyte was investigated by X-ray photoelectron spectroscopy (XPS). The analyzes were carried out at different potential stages of the first cycle, showing the potential-dependent character of the surface film species formation and the specificity of each salt. At 3.8 V, for all salts, we have mainly identified carbonated species. Beyond this potential, the specific behavior of LiPF₆ was identified with a high LiF deposit, whereas for other salts, the formation process of the SEI appears controlled by the solvent decomposition of the electrolyte.     

Trimethoxymethylsilane as a solid-electrolyte interphases improver for graphite anode

To improve the cycling performance of graphite anode materials, we propose a functional electrolyte additive, trimethoxymethylsilane (TMSi), which contains a silyl ether functional group as part of its molecular structure. First principal calculation studies, in addition to ex situ analyses, demonstrated that electrochemical reduction of ethylene carbonate (EC) gives an anionic reduced EC product. Subsequent chemical reaction with TMSi then generates solid-electrolyte interphase (SEI) layers of Si–O and Si–C functionalized carbonate on the surface of the graphite anode, which prolongs and stabilizes the cycling performance of the cells. As a result, the cell cycled with TMSi-controlled electrolyte exhibits a cycling retention of 89.5%, whereas the cell cycled with standard electrolyte suffers from poor cycling retention (84.3%) after 100 cycles.     

Design of ionic liquid-based hybrid electrolytes with additive for lithium insertion in graphite effectively and their effects on interfacial properties

To address the challenge of the IL-based electrolyte cannot be effectively intercalated in graphite anode, and especially the urgent needs for the compatibility between high performance and high security, the IL-based hybrid electrolyte systems with ethylene carbonate/propylene carbonate (EC/PC) as a co-solvent and vinylene carbonate (VC) as an additive were designed. The high dielectric constant of EC/PC significantly increased the ionic conductivity and lithium ion migration of the electrolyte system. Meanwhile, the presence of VC can form SEI preventing EC and PYR₁₄⁺ reductive decomposition on the electrode interface, and at the same moment, the SEI promotes effective Li cation insertion into the graphene interlayer. The Li/C half-cells showed high reversible capacity, cycling efficiency, and good cycle stability with the IL-based hybrid electrolyte. It is worth to highlight the better performance, in terms of the excellent thermal stability and high safety. Thus, the IL-based hybrid electrolyte combined with good electrochemical performance holds substantial promise for lithium-ion battery, and should have broad application prospects in the high energy density, especially high-security requirements, of the new lithium-ion battery.     

Forming solid electrolyte interphase in situ in an ionic conducting Li_(1.5)Al_(0.5)Ge_(1.5)(PO_4)_3-polypropylene(PP) based separator for Li-ion batteries

A new concept of forming solid electrolyte interphases(SEI) in situ in an ionic conducting Li_(1.5)Al_(0.5)Ge_(1.5)(PO_4)_3-polypropylene(LAGP-PP) based separator during charging and discharging is proposed and demonstrated. This unique structure shows a high ionic conductivity, low interface resistance with electrode, and can suppress the growth of lithium dendrite. The features of forming the SEI in situ are investigated by scanning electron microscopy(SEM) and x-ray photoelectron spectroscopy(XPS). The results confirm that SEI films mainly consist of lithium fluoride and carbonates with various alkyl contents. The cell assembled by using the LAGP-coated separator demonstrates a good cycling performance even at high charging rates, and the lithium dendrites were not observed on the lithium metal electrode. Therefore, the SEI-LAGP-PP separator can be used as a promising flexible solid electrolyte for solid state lithium batteries.     

锂离子电池相关材料的Raman光谱学研究

锂离子电池是目前综合性能最好的可充电池。本文总结我们实验室用Raman光谱学研究锂离子电池相关材料的一些结果 ,包括聚合物电解质的微结构和离子输运机制 ,低温热解碳负极材料的结构表征和锂离子在其中的嵌入 /脱出机理 ,元素替代引起正极材料LiMn2 O4的结构变化以及在充放电过程中电极 /电解质界面形成的钝化层的性质及其对电池性能的影响     

Hydroxyl terminated Poly(dimethylsiloxane) as an electrolyte additive to enhance the cycle performance of lithium-ion batteries

Hydroxyl terminated poly(dimethylsiloxane) (PDMS-HT) is used as an electrolyte additive in electrolyte systems containing 1 M LiPF₆ in EC:DMC (ratios 1:9; 3:7; 4:6 and 1:1 v/v) to enhance the cycle performance of lithium-ion batteries. Adding a small amount of PDMS-HT to the standard LIB electrolyte leads to improved specific capacity as well as improved capacity retention over prolonged cycles. There is also a slight increase in Li⁺ ion conductivity when PDMS-HT is added. Also, the PDMS-HT additive allows the formation of a more stable solid electrolyte interface (SEI) layer that enables the LIB cells to be cycled for longer cycles with minimal capacity fading. This combination of improved ionic conductivity and stable SEI layer formation due to the PDMS-HT additive, makes it an excellent candidate for an electrolyte additive for lithium ion batteries.     

Investigation of electronic and ionic transport properties in α-MoO3 cathode material by electrochemical impedance spectroscopy

Electrochemical impedance spectra (EIS) for lithium ion insertion and extraction in α-MoO₃ cathode material were obtained at different potentials during initial discharge–charge cycle. A significant “three semicircles” were obtained at 0.5 V in the Nyquist diagram, and were assigned to lithium ion migration through solid electrolyte interphase (SEI) film, the electronic properties of the material as well as charge transfer step, respectively. An equivalent circuit that includes elements related to the electronic and ionic transport, in addition to the charge transfer process, is proposed to simulate the experimental EIS data. The variations of the resistance of SEI film, the electronic conductivity of the material and the resistance of charge transfer along with the increase and decrease of electrode polarization potential were quantitatively analyzed, and the reasonable explanation is given. Furthermore, the chemical diffusion coefficients of lithium ion in α-MoO₃ cathode material were calculated.     

Tea polyphenols as a novel reaction-type electrolyte additive in lithium-ion batteries

In this study, we reported tea polyphenols (TP) as a novel, cheap, environment-friendly and easy dissolution in common electrolytes reaction-type electrolyte additive for the graphite anode of the lithium-ion batteries. The TP can capture less stable radical anions that are harmful to oxidation stability of ethylene carbonate (EC) to form stable polymer. To a certain extent, it improved the electrochemical performance of the graphite electrode such as reversible capacity and cyclic stability by charge-discharge test, cyclic voltammetry (CV), scanning electron microscope (SEM), and electrochemical impedance microscope (EIS). The first charge capacities of the graphite electrodes in electrolytes without and with TP were 327.1 and 349.1 mAh g^?1, respectively. The charge capacities were 306.8 and 344.2 mAh g^?1 after 100 cycles and the capacity retention were 93.79 and 98.60%, respectively. The improvement was benefited from the effective scavenging the less stable radical anions and improvement the oxidation stability of EC and formation of a stable, compact and thin solid electrolyte interface (SEI) film with lower resistance.     

Electrochemical intercalation of lithium into graphitized carbons

The change of the carbon structure with electrochemical intercalation of lithium has been investigated by X-ray diffraction (XRD) method. Graphitized carbons showed the first and the second stage structures clearly during the intercalation process. However, the layer spacing corresponding to the 1st stage structure of graphitized carbon was smaller than that of graphite. This is because the first stage structure of graphitized carbon is the mixed structure of lithiated graphite crystallites and lithiated turbostratic disordered layers. The lithium is mainly intercalated into turbostratic disordered layers above 0.1 V versus Li/Li⁺, and intercalated into graphite crystallites rather than turbostratic disordered layers below 0.1 V versus Li/Li⁺.     

Lithium-enriched polypyrrole as a prospective cathode material for Li-ion cells

Lithium substitution in polypyrrole can be accomplished by a variety of approaches and the present work introduces one of the cost-effective techniques using a relatively less expensive lithium salt, n-butyllithium in hexanes (n-BuLi), as the dopant. Chemical oxidative polymerization method is employed to synthesize polypyrrole (PPy) using anhydrous ferric chloride as the oxidant and it is dedoped using NH₄OH solution in the fully reduced state. The dedoped polypyrrole is treated with n-butyllithium in hexanes (n-BuLi) in an argon-filled glove box to get the lithiated form of polypyrrole (PPyL) and the concentration of n-BuLi is varied to improve metalation. The lithiated PPy is characterized by FTIR spectroscopy, XRD, FESEM, and TEM techniques to understand the structural and the morphological details. The lithium content in the lithiated samples is estimated using ICP-AES analysis. The thermal studies using the TGA technique show that the lithiated polypyrrole has good thermal stability. Coin cells are assembled in the argon-filled glove box using Li-substituted polypyrrole as the cathode, lithium metal foil as the anode, and lithium hexafluorophosphate (LiPF₆) as the electrolyte. The assembled cells are electrochemically characterized using cyclic voltammetry and charge–discharge cycling techniques and it is seen that the Li-substituted polypyrrole-based Li-ion cells are electrochemically active.     

Poly[lithium methacrylate-co-oligo(oxyethylene)methacrylate] as a solid electrolyte with high ionic conductivity

Poly[lithium methacrylate-co-oligo(oxyethylene)methacrylate] film was prepared as a polymeric solid electrolyte which showed lithium ionic conductivity of 2×10^?7(S/cm). This film contained no organic plasticizer nor low molecular weight lithium salts and shown to be a single-ion conductor in solid state. Li⁺ ionic conductivity was deeply influenced by the glass transition temperature and lithium methacrylate content of this film. A rechargeable battery composed of metallic lithium/this film/graphite showed better characteristics than any previously reported systems using polymeric solid electrolytes.     

Effect of SO<Subscript>2</Subscript> and CO<Subscript>2</Subscript> additives on the cycle performances of commercial lithium-ion batteries

The effects of SO₂ and CO₂ additives in electrolytes on the cycle properties of liquid-state Al-plastic film lithium-ion batteries were first investigated. The experimental electrolytes were added with different amounts of SO₂ and CO₂. The baseline electrolyte was 1 mol L⁻¹ LiPF₆ in ethylene carbonate/dimethylcarbonate/ethyl-methyl carbonate (1:1:1, by volume), and graphite was used as anode. The main analysis tools were cycling test, rate capability, internal resistance test, low-temperature performance, and thermal stability. The results showed that both of the additives could promote to form an excellent solid electrolyte interface film on the surface of graphite anode, leading to excellent cycle performances, the capacity retentions of CO₂ and S5 were 94% and 97% after 400 cycles, respectively. Besides, the results also exhibited that the electrochemical performances of internal resistance, rate capability, low-temperature performance, and thermal stability were not changed significantly by the use of SO₂ and CO₂ as electrolyte additives.