Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

Overpotentials and solid electrolyte interphase formation at porous graphite electrodes in mixed ethylene carbonate-propylene carbonate electrolyte systems

The first electrochemical lithium insertion was characterized for several graphite materials with high degree of crystallinity, different particle size distributions and surface morphologies in an ethylene carbonate (EC)/propylene carbonate (PC) electrolyte. For coarser graphite materials and graphites with a low superficial defect concentration, an irreversible process was observed which correlated with the electrochemical exfoliation of graphite. Different natural and synthetic graphites with similar particle size distribution and active surface area showed differences in the passivation behavior during the first electrochemical reduction. The fraction of graphite particles exfoliating during the first galvanostatic lithium insertion linearly increased with length of the irreversible plateau, which concomitantly moved to more positive potentials. This behavior can be rationalized when considering, besides the surface structure, local overpotentials for the solid electrolyte interphase formation process, and especially the overpotential distribution through the graphite electrode. These overpotentials cause a distribution of the local current density attributed to the passivation process. Optimizing the particle contacts in the electrode by applying mechanical pressure or by selecting the proper binder decreased the overpotentials and suppressed the graphite exfoliation in the EC/PC electrolyte. Therefore, both graphite surface structure and electrode engineering aspects have to be considered for successful passivation against exfoliation.     

The influence of the local current density on the electrochemical exfoliation of graphite in lithium-ion battery negative electrodes

The local current density related to the exfoliation process of graphite negative electrodes in mixed ethylene carbonate/propylene carbonate electrolytes was controlled by a variation of the current applied to lithium half-cells containing either single type graphite electrodes or electrodes consisting of mixtures of an exfoliating and a non-exfoliating graphite. The partial local current density attributed to graphite passivation and its distribution within the volume of the electrode was found to be a key parameter to explain differences in the exfoliation behaviour observed for graphite electrodes. The local current density is related with a local overpotential which may suppress one of several possible electrochemical processes. In a negative electrode consisting of a mixture of a non-exfoliating and an exfoliating graphite component, the exfoliation of the exfoliation-sensitive graphite component can be completely suppressed when increasing the partial local current density attributed to the graphite exfoliation process above a certain threshold, by either decreasing the amount of exfoliating graphite particles in the electrode or by increasing the total current density, i.e., the specific current. The consideration of the local current density distribution for the electrochemical processes throughout the electrode leads to a more general concept for the graphite passivation behaviour during the first lithium insertion in lithium-ion batteries.     

Solvents effects on electrochemical characteristics of graphite fluoride—lithium batteries

A study was made of the electrochemical characteristics of graphite fluoride—lithium batteries in various non-aqueous solvents. Two types of graphite fluorides, (C₂F)_n and (CF)_n were used as cathode materials. The discharge characteristics of graphite fluorides were better in dimethylsulfoxide, γ-butyrolactone, propylene carbonate and sulfolane in that order. The relation between electrode potential of graphite fluoride and solvation energy of lithium ion with each solvent indicates that solvated lithium ion is intercalated into traphite fluoride layers by the electrode reaction. Both the difference in the overpotentials and in the rates of OCV recovery among these solvents further supports the proposed reaction mechanism.     

锂离子电池首次充、放电时石墨负电极与电解液界面所发生的反应

采用恒电流充、放电——原位XRD法对锂离子电池（LIB）首次充、放电过程进行了研究。实验结果表明，LIB首次充电时电解液于石墨负电极的界面处发生还原反应，生成了电子不可导而锂离子可导的固体电解质中介相（SEI）薄膜。FTIR分析结果证明SEI膜系由无定形碳酸锂和烷基碳酸锂组成。恒电流充、放电实验和循环伏安实验结果表明，如果所选择的电解液（例如EC基电解液）在石墨负电极表面的还原反应很缓和，反应中所产生气体的量和速率很小，则在石墨负电极表面将形成薄而致密的SEI膜。薄而致密的SEI膜所消耗的Li^＋量小，可以降低首次充电时的不可逆容量，同时减小Li^＋对石墨进行插层和脱层时的阻力，增大LIB的充、放电容量，提高充、放电效率。     

Influence of graphite surface properties on the first electrochemical lithium intercalation

The electrochemical insertion of lithium ions into graphite materials having different surface chemistry and defect concentration was studied during the first cycle in half-cell containing 1 M LiPF₆ in an electrolytic solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The graphite surface properties were varied by thermal treatments in either hydrogen, oxygen, or nitrogen oxide or chemical treatment in boiling nitric acid. The influence of the surface modifications on the course of the first electrolyte reduction was investigated. The surface group chemistry was analyzed by temperature-programmed desorption coupled with mass spectrometry. The surface defect concentration was determined in terms of the active surface area (ASA) measured by oxygen chemisorption and a subsequent temperature-programmed desorption. The experimental results showed that the ASA parameter governs the exfoliation tendency of the graphite negative electrode material with the existence of a critical value below which the graphite systematically exfoliates. The specific charge loss during the first electrochemical insertion of lithium and the exfoliation behavior of the graphite negative electrode material are not influenced by the type and amount of oxygen surface groups. But hydrogen present on the graphite surface increased the graphite exfoliation tendency even for graphite materials with an ASA above the critical value.     

Measurement of transference numbers for lithium ion electrolytes via four different methods, a comparative study

We report here on comparative measurements of cationic transference numbers of some lithium battery related electrolytes including lithium tetrafluoroborate in propylene carbonate, lithium hexafluorophosphate in blends of ethylene carbonate/diethyl carbonate and ethylene carbonate/propylene carbonate/dimethyl carbonate, as well as lithium difluoromono (oxalate) borate in an ethylene carbonate/diethyl carbonate blend via four different methods. Whereas three electrochemical methods yield transference numbers decreasing with concentration in accordance with electrostatic theories, valid for low to intermediate concentrations of the electrolyte, nuclear magnetic resonance spectroscopy measurements show increasing transference numbers with increasing concentration. The discrepancy is attributed to effects of ion–ion and ion–solvent interaction.     

Electrochemical behaviour of some transition metal oxides in molten dimethylsulphone at 150°C

In view of the possible application in non-aqueous líthium cells operating at relatively high temperatures, molten dimethylsulphone (DMSO₂) has been used as the electrolyte solvent in lithium cells at 150°C. The stability of lithium in molten DMSO₂ has been found to be good as compared with that observed in organic solvents such as propylene carbonate, thus indicating that the Li⁺/Li system can be used as a suitable reference electrode in this medium.The electrochemical behaviour of some transition metal oxides has been investigated in LIClO₄ solutions in molten DMSO₂. The results obtained from voltammetric and chronopotentiometric measurements have shown a satisfactory behaviour for all the cathodic materials tested. Moreover, electrochemical insertion of Li⁺ ions into the crystal lattice of these oxides is a very fast process. Thus molten DMSO₂ appears to be a very interesting organic solvent usable in high energy density non-aqueous lithium cells.     

Improving electrochemical performance of graphitic carbon in PC-based electrolyte by nano-TiO2 coating

Graphitic carbon was coated with nano-TiO₂ by a simple mechanical process. X-ray diffraction and scanning electron microscopy were used to measure the crystal structure and surface morphology of the coated composite. Tests of galvanostatic discharge and charge and cyclic voltammograms suggest that the decomposition of propylene carbonate and the exfoliation of graphite are greatly suppressed. Lithium ions can reversibly intercalate into and deintercalate from the TiO₂-coated graphite, and quite stable cycling behavior in propylene carbonate-based electrolyte is achieved.     

Electrochemical characterization of polymer precoated lithium electrodes

The electrochemical behaviour of lithium electrodes in contact with solutions of propylene carbonate is investigated as a function of different polymer precoating materials (poly-(2-vinylpyridine) and poly-(ethylene oxide)). Impedance and polarization measurements show that the electrochemical behaviour of the surface is influenced markedly by precoating due to a modification of the passivating layer. Such information is important for lithium electrodes in batteries with organic electrolytes.     

Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li-C₆ graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.     

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn₂O₄ in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm⁻³ LiClO₄ electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li⁺ followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.     

Organic electrolytes for corrosion testing of aluminium and Al-Li binary alloys

The electrochemical behavior of high purity aluminium and Al-Li alloy has been investigated in various organic environments. Various lithium- and aluminium-compatible solvents were considered before selecting propylene carbonate (PC) and tetrahydrofuran (THF) with addition of either chloride or perchlorate ions. THF finally proved to be the most suitable solvent for these tests. The aluminium dissolution reaction in a THF environment is fast and reversible. The Al/Al(III) couple can therefore be used as a reference electrode. Aluminium salts can be reduced by lithium, making the Li/Li⁺ couple unsuitable for the reference electrode. The voltammograms obtained show voltage characteristics (protective potentialE _p and breakdown potentialE _b) similar to those found in aqueous environments. The voltage variations recorded by potentiometric measurements could be associated with changes in the alloy surface state.     

In situ FTIR spectra at the Cu electrode/propylene carbonate solution interface

In situ Fourier transform infrared spectroscopy (FTIR) spectra measurements were obtained for a Cu electrode/solution of lithium perchlorate in propylene carbonate (PC). The dependence of potential on the concentration of PC in the vicinity of the electrode was investigated. The bands due to free PC and PC solvated to lithium ions in the solution were distinguished by the single reflection attenuated total reflection (ATR) spectra. In the FTIR spectra, the reversible change in the concentration of free PC and solvated PC in the diffuse double layer was observed to be accompanied by a change in potential. As the potential decreased, the free PC concentration increased, while the concentration of the PC solvated to lithium ions decreased. Thus, it can be concluded that the equilibrium shifts from Li⁺(PC)₄ to Li⁺(PC)₃ + PC as the potential decreases. Furthermore, Li⁺(PC)₃ orientates itself so that it is parallel to the electrode surface.     

Novel SEI formation of maleimide-based additives and its improvement of capability and cyclicability in lithium ion batteries

This investigation elucidates three maleimide (MI)-based aromatic molecules as additives in electrolyte that is used in lithium ion batteries. The 1.1 M LiPF₆ in ethylene carbonate (EC):propylene carbonate (PC):diethylene carbonate (DEC) (3:2:5 in volume) containing MI-based additives can prompt the formation of a solid electrolyte interface (SEI); and inhibit the entering into the irreversible state during lithium intercalation and co-intercalation. The reduction potential is 0.71-0.98 V versus Li/Li⁺ as determined by cyclic voltammetry (CV). The morphology and element analysis of the positive and negative electrode after the 100th charge-discharge cycle are examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Moreover, the MI was used in lithium ion batteries and provided 4.9% capacity increase and 16.7% capacity retention increase when cycled at 1C/1C. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries. MI is decomposed electrochemically to form a long winding narrow SEI strip on the graphite surface. This novel SEI strip not only prevents exfoliation on the graphite electrode but also stabilizes the electrolyte. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries.     

An investigation of the electrochemical reduction of pentachlorophenol with analysis by HPLC

A study has been made of the electrochemical reduction of pentachlorophenol (PCP) on a lead electrode in both water and propylene carbonate. A combination of voltammetric measurements and analysis of reaction products by high performance liquid chromatography (HPLC) have established that the reduction of PCP in propylene carbonate occurs by two different pathways, depending upon the electrode potential. At -1.9V vs SCE, reduction is mediated by adsorbed hydrogen, whereas at -2.3V vs SCE direct reduction of PCP also occurs. A similar approach has established that the reduction of PCP in water follows only one reaction pathway, namely, direct electron transfer from lead to PCP.     

Behavior of lithiated graphite electrodes comprising silica based binder

Graphite electrodes comprising silica binder were tested in ethylene carbonate–dimethyl carbonate (EC–DMC), propylene carbonate and tetrahydrofuran solutions. The electrochemical behavior of these electrodes was analyzed using chronopotentiometry, slow-scan rate cyclic voltammetry (SSCV), impedance spectroscopy and potentiostatic intermittent titration technique (PITT). The electrode morphology and integrity were studied by scanning electron microscopy (SEM). Using silica binder, graphite particles are usually embedded in the current collector in an unoriented form. Thus, the electroanalytical study of these electrodes and the comparison of their response with that of highly oriented PVDF based graphite electrodes, provided insight into the effect of particle orientation on the general electrochemical behavior of lithiated graphite anodes. In general, the higher the particle orientation and compact packing in the electrodes' active mass, the better the performance o f the Li–graphite electrodes, as the surface films developed are better passivating and the interparticle electronic contact is also better. The silica binder may have advantages over other binders such as PVDF in its ability to better retain the electrode integrity upon cycling. However, the practical use of such electrodes requires further optimization, especially in connection with particle orientation and compact packing.     

Electrochemical lithiation and compatibility of graphite anode using glutaronitrile/dimethyl carbonate mixtures containing LiTFSI as electrolyte

The compatibility of glutaronitrile (GLN) and its mixtures with dimethyl carbonate (DMC) containing lithium bis-(trifluoromethane sulfonyl) imide (LiTFSI) with graphite negative electrode was investigated. GLN/DMC/LiTFSI electrolytes’ mixtures were characterized in terms of their ionic conductivities and viscosities. Cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy were performed in order to study the performances of the graphite anode in the GLN-based electrolytes. Results clearly indicate that no significant Li intercalation occurs in graphite in pure GLN, but when GLN/DMC (1:1 and 1:3 w/w) mixtures were used, the cycling ability of the electrode was improved as the coulombic efficiency reaches 98 and 99 %, respectively. Moreover, SEM images of the graphite anode indicate that after being cycled in GLN-based electrolytes, the electrode surface was homogenously covered by a Solid Layer Interface which insures a reversible lithiation of graphite anode.     

Electrochemical properties of graphite electrode in propylene carbonate-based electrolytes containing lithium and calcium ions

Electrochemical properties of graphite electrode are studied in propylene carbonate (PC) electrolytes containing both LiN(SO₂CF₃)₂ and Ca(N(SO₂CF₃)₂)₂ salts, and the influence of the salt concentrations on the intercalation/de-intercalation properties of graphite electrode is clarified. In the higher concentration electrolytes, reversible lithium-ion intercalation/de-intercalation at graphite electrode takes place. In contrast, only the exfoliation of graphite occurs in the lower concentration electrolytes. The effect of the salt concentrations on the electrochemical properties of graphite is discussed.     

纳米Cu电极的制备及其在电催化还原CO_2反应中的应用

Cu电极较之玻碳(GC)电极,更有利于CO2的电还原反应,而电沉积制备的纳米Cu电极可以进一步提高其电催化还原性能。本文考察了电沉积条件,包括沉积时间、H2SO4浓度、沉积电位、Cu2+浓度等对所制备的纳米Cu电极对CO2电还原反应催化性能的影响。针对该反应优化了纳米Cu电极的制备条件,并用SEM表征了其微观形貌。最后还将制备的纳米Cu电极运用于CO2与环氧丙烷的电合成反应,提高了碳酸丙烯酯的反应产率。