Thermal behavior of the solid electrolyte interphase (SEI) on a silicon electrode for lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50–300 °C, LEDC decomposes to evolve CO2 and C2H4 gases leaving lithium propionate (CH3CH2CO2Li) and Li2CO3 as solid residues. The lithium propionate decomposes over the temperature range of 300–600 °C to evolve pentanone leaving Li2CO3 as a residual solid. Finally, the Li2CO3 decomposes over 600 °C to evolve CO2 leaving Li2O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of Li x PO y F z at 300 °C and the polyimide binder at 550 °C. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propionate and Li2CO3, resulting in increased complexity of the SEI.

The fluorinated phosphate lithium bis (2,2,2-trifluoroethyl) phosphate (LiBFEP) has been investigated as a film-forming additive employed to passivate the cathode and hinder continuous oxidation of the electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry coupled with online electrochemical mass spectrometry (LSV-OEMS) on a conductive carbon electrode (i.e., a C65/PVDF composite) showed that LiBFEP decreases electrolyte oxidation (CV and LSV) and LiPF 6 decomposition at high potentials. Incorporation of LiBFEP (0.1 and 0.5 wt%) into LiPF 6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 wt) results in improved coulombic efficiency and capacity retention for LNMO/graphite cells. Ex-situ surface analysis of the electrodes suggests that incorporation of LiBFEP results in the formation of a cathode electrolyte interface (CEI) and modification of the solid electrolyte interface (SEI) on the anode. The formation of the CEI mitigates electrolyte oxidation and prevents the decomposition of LiPF 6 , which in turn prevents HF-induced manganese dissolution from the cathode and destabilization of the SEI. The passivation of the cathode and stabilization of the SEI is responsible for the increased coulombic efficiency and capacity retention. Since their debut in 1991, lithium ion batteries (LIB) have become the universal power source for consumer electronics.1 Larger format LIBs such as those needed to power electric vehicles (EVs), an important future market, have amassed considerable interest; however higher specific energy densities are required for larger format LIBs.1,2 The practical way to increase energy density is to employ cathode materials with increased theoretical capacities and/or high discharge plateaus, and thus high energy (HE) or high voltage (HV) cathodes are required in order for LIBs to meet the demands of the EV market.3 While both HE and HV cathodes have been implemented, current research efforts are focused on overcoming the caveats associated with these materials. The oxidative instability of carbonate-based electrolytes is a central limitation for cells with various cathode chemistries operated above 4.4 V. [3][4][5][6][7] In addition to the instability of the electrolyte, cathodes such as nickel-rich layered oxides (LiNi x Mn y Co z O 2 ), lithium-rich layered oxides (0.6 Li 2 MnO 3 • 0.4 Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 ), and HV spinel (LiNi 0.5 Mn 1.5 O 4 ) (LNMO) all suffer from structural instability when operated at high potentials.5-11 While the layered oxides are capable of delivering higher practical energy densities, the lack of cobalt in LNMO alleviates the issues of cost and resource limitations.7 As the higher energy densities associated with HE materials can only be obtained at higher cutoff potentials, oxidation of the electrolyte is a universal problem to both HE and HV cathodes. This work focuses on improving the performance of LNMO/Graphite cells.The capacity fading observed in LNMO/Graphite cells is due to continuous oxidation of the electrolyte and transition...

The novel electrolyte additive lithium dimethyl phosphate (LiDMP) has been synthesized and characterized. Incorporation of LiDMP (0.1% wt) into LiPF 6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (3:7 wt) results in improved rate performance and reduced impedance for graphite / LiNi 1/3 Mn 1/3 Co 1/3 O 2 cells. Ex-situ surface analysis of the electrodes suggests that incorporation of LiDMP results in a modification of the solid electrolyte interphase (SEI) on the anode. A decrease in the concentration of lithium alkyl carbonates and an increase in the concentration of lithium fluoro phosphates are observed. The change in the anode SEI structure is responsible for the increased rate performance and decreased cell impedance. Lithium ion batteries (LIB) are currently the preferred source of power for consumer electronics such as mobile phones, computers, and cameras and are of interest for large-scale high-powered battery markets including aerospace, military, and electric vehicles. The reaction of non-aqueous electrolytes on the surface of the anode during the first few charging cycles results in the generation of a solid electrolyte interphase (SEI) which is critical to the performance of LIB.1 While the structure and function of the anode SEI is still poorly understood, lithium ion intercalation through the SEI and into the anode is one of the largest limitations for high rate performance. 2-5Electrolyte additives have been used to modify the structure of the SEI and improve the performance of LIB via decreasing the irreversible capacity during formation, lowering SEI resistance, or stabilizing cells against extreme conditions such as high temperature and high rate cycling.1,6-8 Vinylene carbonate (VC) is one of the most frequently investigated additives and has been used to generate a more stable SEI on graphite, but unfortunately the films are typically more resistive.9 Improving the kinetics of lithium ion batteries has been investigated via incorporation of alternative co-solvents to improve electrolyte conductivity 10 or incorporation of electrolyte additives, such as propane sultone (PS), to reduce the impedance of the SEI.11 Organophosphorus additives such as trimethyl phospshate and dimethylmethyl phosphonate have also been investigated as novel flame retarding additives.12-14 Recently, a novel phosphorus additive, lithium difluoro phosphate (F 2 PO 2 Li), has been reported to improve the interfacial kinetics of the anode SEI. 15 In this manuscript, we report on the development of a structurally related novel organophosphorous additive, lithium dimethyl phosphate (LiDMP), which has been found to function as an anode film-forming additive, which decreases cell impedance. ExperimentalMaterials.-All of the materials for the synthesis of LiDMP were purchased from Sigma Aldrich or Acros and used without further purification. Battery-grade ethylene carbonate (EC), ethyl methyl carbonate (EMC), and lithium hexafluorophosphate (LiPF 6 ) were provided by BASF, Germany, and used as received. LiDMP was...

Lithium titanate (LTO) has been investigated as one of the leading anode materials for lithium ion batteries in grid storage and automotive applications. However, one of the primary challenges is cell gassing which can significantly limit life of cells despite the excellent lifetime performance of LTO anodes. Gas evolution has been previously attributed to water impurities from the electrolyte, moisture trapped in the electrode, the breakdown of lithium salt forming hydrofluoric acid (HF), and/or solvent reactions with the surface of the electrode. The role of electrolyte in gas evolution has been investigated during formation, high temperature storage, and high temperature cycling. The effect of LiTFSI, LiFSI, EC-free formulations, and a novel LTO electrolyte additive (tris (trimethysilyl) borate) have been investigated. Incorporation of an EC free electrolyte or a novel electrolyte additive result in a significant reduction in gas generation. Analysis of the gas composition suggests that the majority of the gas results from solvent reactions with the LTO surface while the reactions of the residual water are a secondary source of gassing.

Batteries consisting of Li 4 Ti 5 O 12 (LTO) anodes do not require the formation of a solid electrolyte interface to deliver robust high-rate performance at room temperature, however performance suffers at elevated temperatures due to gas evolution. Research has linked gas evolution to the instability of the electrolyte on the surface of charged LTO at elevated temperatures. If this is the case, a passivation layer, which prevents the electrolyte from coming into contact with the charged surface of LTO, should inhibit gas evolution. Several classes of electrolyte additives have been investigated in Li 4 Ti 5 O 12 /LiMn 2 O 4 coin cells and pouch cells. ATR-IR and X-ray photoelectron spectroscopy has been used to gain an understanding of the surface films formed with different additives while in-situ gas measurements based on Archimedes' principle and gas chromatography have given insight into how the implementation of these additives affects gassing. The results from this study enable the selective design of surface films for LTO anodes, which reduces gassing at elevated temperatures without sacrificing performance.