Carbon dioxide is shown to be an effective additive to standard Li-ion electrolyte for extending the cycle life of full pouch cells containing an engineered silicon alloy. CO 2 was introduced to pouch cells by adding a few milligrams of dry ice to cells before sealing. The cells contained composite negative electrodes formulated with 15 to 17 wt% of an engineered silicon alloy and a LiCoO 2 positive electrode. Parasitic electrolyte reactions were measured in-situ by isothermal micro-calorimetry and high precision coulometry and compared to cells containing 1-fluoro ethylene carbonate (FEC). Extended cycling of cells containing CO 2 were compared to cells containing FEC. Cell gas generation and gas consumption were measured by applying the Archimedes principle. A new approach using small tubes in pouch cells to differentiate volume changes from gassing and solid expansion is introduced. Cells with CO 2 showed significantly lower parasitic thermal power, improved coulombic efficiency and better capacity retention compared to cells containing electrolytes with FEC. The gas generation/consumption experiments showed that Si alloy reacts with CO 2 during cycling until it is fully consumed. Combining FEC and CO 2 reduces the consumption rate of CO 2 . Microscopy of cross-sectioned cycled electrodes showed a thin SEI layer and minimal silicon alloy erosion. The combined work establishes CO 2 as a powerful precursor to an effective SEI layer on silicon alloys. Finally, CO 2 is shown to be an effective SEI former for graphite in EC-free electrolytes. The desire to increase energy density in Li-ion cells has fueled the research and development of silicon and silicon-based materials as a component of Li-ion negative electrodes. In most cases, however, only small amounts of silicon or silicon alloy are used largely because of the generally inferior cycle life of silicon compared to graphite. While there can be several reasons for the inferior cycle life of silicon and silicon alloys one important contributor to poor cycle life is the erosion of the silicon particle by parasitic electrolyte reactions. [1][2][3] Because of the volume changes Si experiences during cycling, continuous passivation is required at the surface and the demands on the electrolyte system are greater than for graphite.Carbon dioxide (CO 2 ) has been described as an effective electrolyte additive in metallic Li cells many years ago. 4 In that work it was shown that cycling efficiencies of the Li electrode were greatly increased in the presence of CO 2 . With the development of Li-ion cell chemistry the use of CO 2 was also explored early on. [5][6][7] In a few papers on the subject it was shown that the CO 2 generators, known as pyrocarbonates, were effective in forming SEI layers on graphite. 8,9 More recently a patent by workers at Sanyo has described the effect of electrolytes containing CO 2 on the cycle life of sintered negative electrodes containing silicon. 10In the course of our work on the erosion of silicon alloys our interest was drawn to the ...

Li-ion pouch cells utilizing a negative electrode formulated with 15 wt% of an engineered Si alloy in a graphite composite electrode were cycled in an isothermal heat flow calorimeter against a LiCoO 2 positive electrode. Two different electrolytes were investigated: a blend of ethylene carbonate and ethyl methyl carbonate (3EC:7EMC) and a blend of ethylene carbonate, ethyl methyl carbonate and 1-fluoro ethylene carbonate (27EC:63EMC:10FEC). Both electrolytes were 1 M in LiPF 6 salt. The parasitic thermal power and coulombic efficiency was derived from isothermal heat flow measurements and high precision current-source meters. Cells without FEC showed high parasitic thermal power which increased with cycle number indicative of a surface area increase which was confirmed by post-cycling scanning electron micrographs and surface area measurements. Cells with FEC showed relatively stable parasitic thermal power. These measurements demonstrate the surprising function of FEC in controlling or attenuating the evolution of surface area in Si alloys. Vinylene carbonate was also found to be effective at controlling the increase in alloy surface area. Meeting the promise of silicon or Si alloys to increase the energy density of Li-Ion cells has been the subject of a great deal of research and development. To date the full commercial impact of this materials technology has not been met owing to the difficulties found with silicon. [1][2][3][4][5][6] In general the high capacity fade often found in Li-ion cells containing Si has 3 main causes: 1) mechanical or electrical disconnect of the alloy particles in the composite electrode owing to large volume changes 1-4 2) crystallization effects, mainly the formation of the Li 15 Si 4 phase, which shows high irreversibility 5-7 and 3) unusual electrolyte reactivity. 3,6,8,9 It is the latter failure mechanism which this paper addresses.It has been recognized and shown by many that the electrolyte composition can be crucial for cycling capacity in cells containing silicon or Si alloys.9,10 Recent work by Petibon et al. on pouch cells using a negative electrode composed of 72.3 wt% graphite, 15 wt% Si alloy, 10 wt% KS6 and 2.7 wt% carboxymethyl cellulose-butadiene styrene binder (CMC:SBR) showed approximately 300 cycles to 80% of the original capacity at a C/2 rate. In that work the base electrolyte was 1 M LiPF 6 in a blend of EC, EMC and FEC in the weight ratios of 27:63:10 respectively. The capacity fade was linear until about 250 cycles at which point the capacity fade rate accelerated significantly. 4 The authors showed that the additive FEC was consumed during cycling and the acceleration of the fade rate occurred when the FEC was consumed. Analysis of the gas produced during the first cycle was shown to be mainly CO 2 with small amounts of H 2 and CO. The authors attributed the gas formed as products from the reductive formation of SEI layers at the graphite and alloy surfaces. They argued that the absence of ethylene suggests that FEC inhibits the reduction of EC and noted ...

In this study, a tunable Helmholtz resonator is proposed for active noise cancelation in a primary acoustic system. In the tunable Helmholtz resonator, the resonator’s top wall is replaced by a flexible membrane and four actuators are mounted on the side of the resonator. These actuators are connected to the membrane in order to tune its radial force (tension) by pulling or releasing the membrane during the operation. This causes the resonant frequency of the modified Helmholtz resonator to change due to the change in the membrane tension. The maximum noise attenuation is achieved when the the resonant frequency of the active resonator matches with the noise frequency in the primary system. In this paper, first mathematical modeling is used to derive nonlinear coupled differential equations for the tunable Helmholtz resonator with the membrane. The differential equations were linearized to obtain an analytical formulation for the resonant frequency of the tunable resonator in terms of the membrane tension. The analytical formulation for the resonant frequency was verified via simulation of the original nonlinear differential equations. Finally, to demonstrate the validity of the mathematical modeling of the tunable resonator, experimental results are provided.

LiMn1.5Ni0.5O4 (LMNO) spinel has recently been the subject of intense research as a cathode material because it is cheap, cobalt-free, and has a high discharge voltage (4.7 V). However, the decomposition of conventional liquid electrolytes on the cathode surface at this high oxidation state and the dissolution of Mn2+ have hindered its practical utility. We report here that simply ball-mill coating LMNO using flame-made nanopowder (NPs, 5–20 wt %, e.g., LiAlO2, LATSP, LLZO) electrolytes generates coated composites that mitigate these well-recognized issues. As-synthesized composite cathodes maintain a single P4332 cubic spinel phase. Transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS) show island-type NP coatings on LMNO surfaces. Different NPs show various effects on LMNO composite cathode performance compared to pristine LMNO (120 mAh g–1, 93% capacity retention after 50 cycles at C/3, ∼67 mAh g–1 at 8C, and ∼540 Wh kg–1 energy density). For example, the LMNO + 20 wt % LiAlO2 composite cathodes exhibit Li+ diffusivities improved by two orders of magnitude over pristine LMNO and discharge capacities up to ∼136 mAh g–1 after 100 cycles at C/3 (98% retention), while 10 wt % LiAlO2 shows ∼110 mAh g–1 at 10C and an average discharge energy density of ∼640 Wh kg–1. Detailed postmortem analyses on cycled composite electrodes demonstrate that NP coatings form protective layers. In addition, preliminary studies suggest potential utility in all-solid-state batteries (ASSBs).

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