Amorphous Columnar Silicon Anodes for Advanced High Voltage Lithium Ion Full Cells: Dominant Factors Governing Cycling Performance

In the present study, we explore the use of lithium oxalate as a "sacrificial salt" in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO 2 (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li + . LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to ∼11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (∼1%) compared to cells without lithium oxalate. When CO 2 from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO 2 in the cells. Further analysis (e.g., FEC consumption by 19 F-NMR) indicate that CO 2 is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO 2 has a synergistic effect on the lifetime of full-cells with SiG anodes. Lithium nickel manganese spinel (LiNi 0.5 Mn 1.5 O 4 , LNMO) is a promising cathode material for high energy lithium ion batteries due to its high operating potential around 4.7 V vs. Li/Li + , its high rate capability, structural stability and the absence of cobalt. However, its lower specific capacity (146 mAh/g LNMO ) compared to layered oxide materials (e.g. lithium nickel manganese cobalt oxide (NMC), specific capacity 150-250 mAh/g NMC ) 1 is regarded as a major drawback. In full-cells, the practically achievable capacity of LNMO is even lower, as the formation of the solid-electrolyte interphase (SEI) on the graphite anode consumes active lithium. For many layered oxide cathodes, however, the first cycle irreversible capacity of the cathode is similar to the capacity needed for SEI formation (∼20 mAh/g NMC ), and hence the practical discharge capacity of the cathode and the remaining active lithium are more or less balanced again. 2,3 In contrast, the first cycle irreversible capacity of LNMO (∼6 mAh/g LNMO ) is much lower than the capacity needed for SEI formation. This leads to a mismatch between active lithium and practical cathode capacity, i.e., there is not enough active lithium available to fully discharge the LNMO cathode during subsequent cycling.In cells with silicon-based anodes, active lithium losses on the anode are even higher compared to graphite, as the expansion of the silicon particles during the first lithiation leads to a continuous exposure of fresh, unpassivated silicon surface. 4 On this new surface, electrolyte reduction occurs instantaneously, which reduces the total lithium reservoir in the cell. Therefore, different ideas to increase the amount of active lithium in lithium ion full-cells have been suggested, ...

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

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Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Solchenbach

Wetjen

Pritzl

et al. 2018

“…[32][33][34][35][36] In Ref. 37, it was shown that for Li-Si film electrodes, more negative potentials promoted a thinner and stiffer SEI that yielded a higher current efficiency.…”

Section: Discussion Of Resultsmentioning

confidence: 99%

Fabrication and Characterization of Lithium-Silicon Thick-Film Electrodes for High-Energy-Density Batteries

Xiao

Balogh

Raghunathan

et al. 2016

We have developed a method to operate lithium-silicon (Li-Si) thick-film electrodes in a manner consistent with traction applications. Key to the operating strategy is the voltage control of the electrode. It is expected that strong reducing environments, created by operating the electrode at low potentials (near that of Li), reduce battery life. We show that operating Li-Si at higher potentials is also damaging, and this is counterintuitive in that common negative electrodes (e.g., graphites and titanates) do not suffer from this limitation. Arguments based on measured Coulombic efficiencies, cycle life tests, in-situ stress measurements, and high-resolution microscopy resolve the otherwise anomalous findings. We show that promising half-cell ( Silicon film electrodes have been shown to be useful for characterization purposes insofar as one need not treat binders, various particle geometries, conductive diluents, and other complications inherent in the construction of porous electrodes. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In this work, we focus on the potential utility of Si thick-film electrodes formed on roughened copper current collectors. If sufficient capacity can be obtained, such a geometry might simplify electrode fabrication, as the electrode host material would consist of nothing more than Si deposited on a current collector, and high specific energies (Wh/L) and energy densities (Wh/kg) could result.Four topics are examined in this endeavor. First, by controlling the potential limits over which we operate a Li-Si electrode, can we obtain sufficient stability for relatively thick-film electrodes (i.e., yielding more than 2 mAh/cm 2 )? Second, how does the electrochemical performance correlate with stress in the electrode over the potential window of operation? Third, because the electrode consists of only Li and Si, we can isolate equilibrium hysteresis behavior associated with this alloy, and we derive and implement a simple model to represent the voltage-composition data that may be useful in subsequent engineering analyses of electrodes with a significant Li-Si contribution. Last, we examine full cells (Li-Si vs. an NMC622 positive electrode, corresponding to Ni 0.6 Mn 0.2 Co 0.2 O 2 ) in the context of capacity, cycle life, cycle efficiency, microstructure, and elemental analysis. A cell modeling tool 45 is used to examine the efficacy of the Si film electrodes for actual battery electric vehicle applications based on specific energy and energy density calculations.

“…The 31 P- 19 F coupling constant is approximately 942 Hz. The PF 6 − anions show as a septet in 19 F NMR and as a doublet in 31 P NMR ( Figures S1 and S2) demonstrated in the supporting information. Figures 7c and 7d show that no LiPO 2 F 2 is detected in watercontaining electrolyte plus graphite or LCO electrodes.…”

Section: Resultsmentioning

confidence: 98%

Some Effects of Intentionally Added Water on LiCoO₂/Graphite Pouch Cells

Xiong

Petibon

Madec

et al. 2016

LiCoO 2 /graphite pouch cells with 1 M LiPF 6 in EC:DEC (1:2 v/v) (control electrolyte) containing selected additives and different amounts of intentionally added water were studied. Archimedes principle was used to track gas evolution while an automated storage system, electrochemical impedance spectroscopy and cell cycling experiments were used to study electrochemical properties imparted by the addition of water. Water was added from 125 to 2000 ppm to control electrolyte and to electrolyte with 2% vinylene carbonate (VC) and/or 2% lithium bis(trifluoro methane sulfonyl imide (LiTFSI). The addition of water to either VC or VC + LiTFSI-containing electrolyte resulted in minimal or no changes in voltage drop and charge transfer resistance during storage while both added water and LiTFSI actually decreased swelling during storage. Added water to control electrolyte or VC-containing electrolyte did not impair cell cycling performance at 55 • C. Solution nuclear magnetic resonance (SNMR), in-situ gas measurements, gas chromatography coupled with a thermal conductivity detector (GC-TCD), and X-ray photoelectron spectroscopy (XPS) were employed to understand why added water did not affect cell performance. Lithium-ion batteries are generally used in power tools and portable electronics applications. Li-ion battery packs for electrified vehicles and grid energy storage need even longer lived and, preferably, less expensive cells. Water is commonly regarded as being detrimental to cell performance, because HF generated from water and LiPF 6 (widely used in lithium-ion batteries) can react with SEI components and corrode positive materials causing transition metal dissolution.1,2 Therefore, Li-ion cell manufacturers generally lower water contents in electrolyte solvents and electrolytes to less than 50 ppm. 3Stringent water content specifications lead to increased manufacturing cost. Recently, Burns et al. [4][5][6] found that water (up to 1000 ppm) added intentionally to the electrolyte of Li-ion prismatic cells with LiCoO 2 (LCO) or LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) positive electrodes and graphite or LTO negative electrodes did not have a detrimental effect on cell performance. In some cases, the addition of water was even beneficial in terms of coulombic efficiency, cell impedance and voltage drop during storage. They found that the added water caused cell swelling during the first cycle and generally the more water that was added the larger expansion the cells experienced. The swelling of cells can lead to mechanical stress of the devices containing the cells. Pouch cells are widely used in the Li-ion industry and are an excellent cell format to study gas evolution due to their flexible packaging. 7,8 In addition, pouch cells are degassed after the first formation step so any initial gases produced by the presence of water can be removed.In this study, pouch cells were chosen to study the impact of intentionally added water on their electrochemical properties and gas evolution. Various experimental techniques were ...