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.