The electrolyte additive fluoroethylene carbonate (FEC) is known to significantly improve the lifetime of Li-ion batteries with silicon anodes. In this work, we show that FEC can indeed improve the lifetime of silicon-carbon composite anodes but is continuously consumed during electrochemical cycling. By the use of 19 F-NMR spectroscopy and charge/discharge cycling we demonstrate that FEC is only capable to stabilize the cell performance as long as FEC is still remaining in the cell. Its total consumption causes a significant increase of the cell polarization leading to a rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of μmol FEC /mg electrode ) in the cell. The latter quantity therefore determines the lifetime of silicon anodes rather than the concentration of FEC in the electrolyte. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in literature, a new reductive decomposition mechanism for FEC is proposed yielding CO 2 , LiF, In the emerging market of electric vehicles (EVs), the development of batteries with higher energy density and improved cycle-life is essential.1 However, their penetration of the mass market significantly depends on cost and the available driving range.2 The US Advanced Battery Consortium (USABC) defined the target value of 235 Wh/kg (at a C/3 rate) on a battery level until 2020.3 As outlined in the recent review by Andre et al., 4 reaching this goal requires an increase of the energy density of today's batteries by a factor of roughly 2 to 2.5 and can only be achieved by the development and integration of novel anode and cathode active materials. A critical element to reach this goal is the implementation of anode active materials with much higher specific capacity than currently used graphite anodes (372 mAh/g 1,5,6 ), with silicon being considered as the most likely next generation anode material due to its high natural abundance and very high theoretical specific capacity of roughly 3600 mAh/g (corresponding to the Li 15 Si 4 phase 7 ). The alloying of silicon with lithium is accompanied by large structural changes, resulting in a volume increase by 310% upon full lithiation.5,7-11 These huge volumetric changes upon lithium insertion and extraction are responsible for the generally shorter cycle-life of silicon electrode materials comp...

The anodic oxidation stability of battery components like the conductive carbon black (Super C65) and the co-solvent ethylene carbonate (EC) is of great relevance, especially with regards to high-voltage cathode materials. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) to deconvolute the CO and CO 2 evolution from the anodic oxidation of carbon and electrolyte by using a fully 13 C-isotope labeled electrolyte based on ethylene carbonate with 2 M LiClO 4 . We present a newly developed two-compartment cell, which provides a tight seal between anode and cathode compartment via a solid Li + -ion conducting separator, and which thus allows us to examine the effect of trace amounts of water on the anodic oxidation of carbon ( 12 C) and ethylene carbonate ( 13 C) at high potentials (> 4.5 V) and 10 to 60 • C. Moreover, we report on the temperature dependence of the water-driven hydrolysis of ethylene carbonate accompanied by CO 2 evolution. Finally, by quantifying the evolution rates of 12 CO/ 12 CO 2 and 13 CO/ 13 CO 2 at 5.0 V, we demonstrate that the anodic oxidation of carbon and electrolyte can be substantial, especially at high temperature and in the presence of trace water, posing significant challenges for the implementation of 5 V cathode materials. The requirements placed on Li-ion battery technology have changed from powering small portable electronics to applications demanding high energy and high power density, such as hybrid and plug-in electric vehicles.1,2 As a result, high-voltage cathode materials have been developed that raise the cell voltage from 3.7 V in the case of a traditional LiMO 2 (M = Co, Ni, Mn) cathode to 4.8 V in new cathodes such as the high-voltage spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) or LiCoPO 4 (LCP).3-6 The state-of-the-art electrolyte, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), dimethyl carbonate (DMC), and/or ethyl methyl carbonate (EMC) with dissolved LiPF 6 salt, tends to decompose on the surface of the delithiated cathode at potentials higher than 4.5 V vs. Li/Li + , especially at high temperature.7-12 Thus, the practical application of these high-voltage materials remains hindered by several obstacles: [13][14][15][16][17][18] (i) the limited anodic stability of electrolyte solvents and salts as well as of binders, (ii) the loss of active Li + -ions, (iii) the corrosion of the aluminum current collector, and (iv) the instability of conductive carbon additives due to anion intercalation and/or carbon oxidation.While the corrosion of conductive carbons is suggested by the frequently observed increase of the electrical resistivity of long-term cycled high-voltage cathodes, 15 quantitative measurement on the anodic decomposition of carbons have only been made in the context of Li-air battery research, making use of isotopically labeled battery components to decouple electrode and electrolyte related CO 2 evolution. For example, Thotiyl et al. used a 13 C carbon cathode in DMSO and tetraglyme-based electrolyte to study CO 2 evolution from...

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Carbon coatings on cathode materials with low electrical conductivity like phospho-olivines LiMPO 4 (M = 3d-transition metal) are known to improve their performance in Li-ion batteries. However, at high potentials and in the presence of water, the stability of carbon coatings on high-voltage materials (e.g., LiCoPO 4 ) may be limited due to the anodic oxidation of carbon. In this work, we describe the synthesis of LiFePO 4 (LFP) with an isotopically labeled 13 C carbon coating (characterized by Raman spectroscopy, electrical conductivity, and charge/discharge rate capability tests) as a model compound to study the anodic stability of carbon coated cathode materials in ethylene carbonate-based electrolytes. We characterize the degradation of the 13 C carbon coating by On-line Electrochemical Mass Spectrometry (OEMS) through the 13 CO 2 and 13 CO signals in order to differentiate the anodic oxidation of the coating ( 13 C) from the oxidation of electrolyte, conductive carbon, and binder (all 12 C) in the electrode. The oxidation of the carbon coating takes place at potentials ≥ 4.75 V for electrolyte without H 2 O (< 20 ppm) and ≥ 4.5 V for electrolyte with 4000 ppm H 2 O, and it is strongly enhanced for H 2 O-containing electrolyte. The extent of carbon coating oxidation over 100 h at 4.8 and 5.0 V vs. Li/Li + (25 • C) is projected on the basis of our OEMS data, suggesting that carbon coatings have insufficient stability at such high cathodic potentials. Furthermore, our results prove the in situ formation of H 2 O during the anodic decomposition of ethylene carbonate-containing electrolyte. The H 2 O formation is monitored via the detection of gaseous POF 3 , which is formed from the reaction of Li-ion batteries are extensively investigated as energy storage devices for electric vehicles (EVs) due to their high energy density and reasonable life time.1 In order to make EVs competitive with gasoline or diesel cars, and to eventually reduce CO 2 emissions by the electrification of personal mobility, many fundamental challenges still need to be overcome. In contrast to NMC and other layered oxides, phospho-olivines like LFP and LCP suffer from very poor electrical conductivity which limits their rate capability, i.e., their performance at high charge/discharge rates. 7 In the case of LFP, its poor electrical conductivity can be overcome by using small primary particles (0.1-0.5 μm) in combination with an electrically conductive very thin carbon coating (thickness 1-2 nm) 8 applied to the primary particles using different kinds of precursors.7,9 While uncoated LFP samples either show very poor rate capability or low capacity at rates as low as 0.1 C, 10-12 Lou et al. 13 showed that carbon coated LFP can reach 116 mAh/g LFP at high rates of 10 C (theoretical capacity: 170 mAh/g LFP ).3 Even when the current is increased to 30 C, the discharge capacity can still reach 75 mAh/g LFP , and in the 100 th cycle the capacity loss is only 2.3%. 14 presented a carbon coated LFP which is able to reach discharge capacities of 1...

† Electronic Supplementary Information (ESI) available: State of the art of HT, ST, SCF, MWHT, and MWST synthesis of LCP; PXRD measurement of an empty capillary; PXRD of Li2SO4 • H2O obtained from the reaction solution; Rietveld fit of LCP-MW-w; cell parameters, atomic coordinates, thermal displacement parameters, and selected interatomic distances obtained from all Rietveld refinements; TGA/DSC and temperature-dependent in situ PXRD data of LCP-MW; FTIR and Raman spectra; additional SEM images; additional electrochemical measurements of LCP-MW materials with varying amounts of Li2SO4 and of electrodes with high loading. See Olivine-type LiCoPO4 is considered a promising high-voltage cathode material for next-generation lithium-ion batteries. However, preparing high-performance LiCoPO4 by a simple approach has been challenging. Herein, we present a facile and rapid (30 min) one-step microwave-assisted solvothermal synthesis route using a 1:1 (v/v) water/ethylene glycol (EG) binary solvent mixture and a temperature of 250 °C. The technique delivers high-performance LiCoPO4 nanoparticles without additional post-annealing or carbon coating steps. The as-prepared powder consists of single crystalline LiCoPO4 and features a hexagonal platelet-like morphology with dimensions of 700-800 nm × 400-600 nm × 100-220 nm. Selected area electron diffraction (SAED) experiments reveal that the platelets show the smallest dimension along [010], which is the direction of the lithium diffusion pathways in the olivine crystal structure. Furthermore, the results indicate that the EG co-solvent plays an important role in tailoring the particle size, morphology, and crystal orientation of the material. Co L-edge soft X-ray absorption spectroscopy (XAS) of LiCoPO4 are presented for the first time and confirm that the material only consists of Co 2+ . Benefiting from the unique morphology, which facilitates Li-ion conduction, electrochemical measurements deliver an initial discharge capacity of 137 mAh/g at 0.1 C, a remarkably stable capacity retention of 68% after 100 cycles at 0.5 C, and a specific energy density of 658 Wh/kg based on its capacity and voltage, which is the best performance of LiCoPO4 obtained from microwave-assisted solvothermal synthesis to date.