High Energy Density Calendered Si Alloy/Graphite Anodes

Abstract

The effect of calendering and adding graphites of different particle sizes to composite electrodes comprising Si alloy particles was evaluated. It was found that calendered alloy coatings containing graphite results in increased cycling performance, reduced volume expansion and increased energy density compared to a pure alloy coating. Such high energy density, low volume expansion coatings are expected to be practical for implementation in high energy density Li-ion cells.The development of high energy Li-ion cells is of great technological importance. One method to increase Li-ion cell energy is to use active alloys in the negative electrode. Si-based alloys are promising candidates for high energy density anode materials due to the low average voltage and high volumetric capacity 2194 Ah/L (corresponding to Li 15 Si 4 ) of Si. 1 However, the lithiation of Si is associated with a large volume expansion of up to 280%. 2 This expansion can lead to structural degradation of the electrode and the disconnection of active regions from each other and the current collector. The corresponding capacity loss is a major detriment to the application of pure Si as an anode material in Li-ion cells. One way to circumvent this problem is the use of active/inactive alloys. 3 The design principles of active/inactive alloys have been described in Reference 2. The idea of this approach is to dilute the expansion of the active phase during lithiation with an inactive matrix. In this way alloys with lower volume expansion and with good cycle life have been achieved. 4 The performance of composite coatings containing alloy particles is also highly dependent on binder chemistry and electrode processing conditions. Although alloys generally cycle poorly in poly(vinylidene fluoride) (PVDF) binder, Li et al. reported that composite silicon coatings with PVDF binder have significantly better cycle life after a heat-treatment above the PVDF melting point, such that the PVDF forms a continuous film on the Si surface. 5 Better cycling performance can be achieved when the binder forms strong bonds with the alloy particle surface in addition to forming a continuous coating. Hochgatterer et al. investigated Si composite coatings with sodium carboxymethyl cellulose (CMC) binder and concluded that the carboxylic acid sites formation of a covalent bond between the carboxylic acid sites of CMC and Si surface effectively improved the cycling stability. 6 Cycling can be further improved with binders that contain a higher concentration of carboxylic acid sites than CMC, such as lithium polyacrylate (LiPAA), 7 polyacrilic acid (PAA) 8 or alginate. 9 Komaba et al. showed that in addition to forming strong bonds at the alloy surface, PAA binder continuously coats the alloy surface as well. 10 These studies and others 11,12 indicate that good binders for Si alloys should be capable of forming a continuous coating on alloy particles that binds strongly to the alloy surface.After coating, composite electrodes are highly porous. In order to achieve a high vo...

NixSi1-xAlloys Prepared by Mechanical Milling as Negative Electrode Materials for Lithium Ion Batteries

“…9 Such coatings are In a previous study of sputtered Ni-Si thin films, we showed that the lithiation voltage is depressed with increasing Ni content, while the delithiation voltage remains unchanged.…”

“…2,7 In a recently published article, it was shown that the formation of Li 15 Si 4 is avoided during cycling of 3 M V6 active/inactive Si alloy, even at lower cutoff voltages as low as a 5 mV. 8,9 The absence of Li 15 Si 4 formation has been shown to result in improved cycling performance in alloys. 3 Si-TM alloys (TM = a transition metal) have been extensively studied as negative electrodes.…”

Differentiating the Degradation Phenomena in Silicon-Graphite Electrodes for Lithium-Ion Batteries

“…1,2 In contrast to conventional intercalation anode materials, such as graphite (LiC 6 , 372 mAh g −1 , 890 Ah L −1 ), the specific capacity of silicon alloy electrodes is significantly higher (Li 15 Si 4 , 3579 mAh g −1 , 2194 Ah L −1 ). 3 Nonetheless, commercialization of silicon-based electrodes is still hampered because of two major challenges: 4 (i) Large volume expansions up to 280% upon repeated (de-)lithiation of silicon particles deteriorate the electrode integrity, thus causing isolation of active material.…”

“…8 In the course of this, SEI-forming additives in the electrolyte, e.g., FEC, are depleted, which was shown to result in a significant increase in cell polarization and a concomitant rapid capacity drop. 8,11 Various strategies have been proposed to overcome the detrimental effects associated with the volume expansion during (de-)lithiation of silicon and to reduce concomitant irreversible capacity losses, including preparation of silicon thin-films with a significantly reduced silicon/electrolyte interface, 6,[12][13][14] Si-Al-Fe active/inactive alloy electrodes that reduce the volume expansion of the active phase, 5,15,16 and design of nanostructured silicon materials with carbonaceous compounds, such as graphite, to improve the electrical conductivity within the electrode and to better accommodate the volume expansion of silicon. [17][18][19][20] Although the surface area per capacity usually increases for nanostructured silicon materials with decreasing diameter, 21 which leads to a higher first cycle irreversible capacity loss, silicon nanowires offer the advantage of a smaller relative surface area change upon (de-)lithiation and in addition usually reveal less morphological changes, due to a reduced mechanical stress within the materials.…”

Li15Si4Formation in Silicon Thin Film Negative Electrodes

“…2 To overcome the issues associated with Li 15 Si 4 phase formation and volume expansion during cycling, researchers have demonstrated high cycle life in Li cells by using Si in the form of nanoparticles, 3,4 nanowires, 5,6 nanopillars, 7,8 thin films, 9,10 and in alloys with inactive or active phases. 11,12 These studies have reported various threshold sizes for nanoparticles (150 nm), 3,4 nanowires (300 nm), 13 and amorphous thin films (2.5 μm), 14 below which Li 15 Si 4 formation does not occur. These results imply that the Li 15 Si 4 phase may be a consequence of particle aggregation; however, the variance in threshold size among Si morphologies raises questions.…”