Suppressing Vertical Displacement of Lithiated Silicon Particles in High Volumetric Capacity Battery Electrodes

Abstract

Silicon is ap otential high-capacity anode material for lithiumion batteries. However,l arge volume changesi nt he material remains ab ottleneck to its commercialization. Many works have been devoted to nanostructured composites with voids to accommodate the volumee xpansion. Yet, the full capability of silicon cannot be utilized, because these nanostructured electrodes have low volumetric capacities. Herein, we redesign dense silicon electrodes with three times the volumetric capacity of graphite. In situ electrochemical dilatometry reveals that the electrode thickness change is nonlineara safunction of state of chargea nd highly affected by the electrode composition. One key problemi st he large vertical displacement of the silicon particles duringl ithiation, which leads to irreversible particled etachment and electrode porosity increase. Better reversibility in electrode thickness changes can be achieved by using polyimide, with ah igher modulus and larger ultimate elongation, as the binder,leading to betterc ycle stability.Batteries have become key componentsi nm obile devices. The global market share of lithium-ion batteries was more than 10 billion US dollars in 2012, and is expected to increase significantly with the increasing demandi ne nergy storagef or renewable sourcess uch as wind farms, solar farms, and so forth.[1] Many researchers are developing new materials such as metal-oxide, tin-based, and silicon-based anodest oi ncrease the specific capacity and energy density of lithium-ion batteries.[2-8] However,t he bottleneck to battery development is not only the capacity,b ut also the reversibility of Li incorporation into an electrode without degradation. One of the main problems of next-generation, high-capacity anode materials is their large volumec hange during charge and discharge. For example, silicon (Si), with ac apacity of up to 4000 mAh g À1 ,h as at heoretical volume change of 311%.[9] Continual changes in materials tructure and mechanicalp roperties duringc ycling lead to fatigue, material cracking, loss of contact, and electrode delaminates that will decreaset he amount of usable materials and increase cell resistance, thereby decreasing the available capacity.Many researchers try to accommodate the volume change in Si electrodes by incorporating additional spacesw ithin the electrode with nanomaterials, graphene composites, and inactive phases. [10][11][12][13][14][15][16][17][18][19][20] Even though the cycle performance is, in general, improved, the volumetric capacity is sacrificed with the large number of voids.I nsome cases, the volumetric capacities of nanostructured silicon electrodes with packing densities less than 0.2 g Si cm À3 are even lower than that of ap ractical graphite electrode (ca. 560 mAh cm À3). For mobile applications, where volume matters, more dense Si electrodes are necessary to increase the overall volumetric capacity and energy density. The key question is, therefore, how to maintain mechanical reversibility in these packed electrodes.To understand and solve...

Impact of the Slurry pH on the Expansion/Contraction Behavior of Silicon/Carbon/Carboxymethylcellulose Electrodes for Li-Ion Batteries

“…This is a strong indication that the pH of the slurry has a major impact on the efficiency of the CMC binder to limit the cohesion loss of the electrode architecture with cycling. This is also supported by comparing the present work with the recent electrochemical dilatometry study of Yu et al 32 showing, under nearly similar experimental conditions, a total thickness expansion after full lithiation as high as 450% for a Si/C/CMC (60/20/20) composite electrode prepared at natural pH. By comparing Si-based electrodes prepared with different binders (PVdF, CMC and polyimide), they have shown that these have a major impact on the dilatometric behavior of the electrode.…”

“…[20][21][22] In recent years, several studies of Si-based anodes have integrated some electrochemical dilatometry results. 16,[23][24][25][26][27][28][29][30][31][32] However, the dilatometric responses are rarely analyzed in detail.The role of the binder is very critical for Si electrodes to maintain the electrode architecture despite the large Si volume change and thereby to achieve long cycle life.8 Although carboxymethyl cellulose * Electrochemical Society Member. z E-mail: roue@emt.inrs.ca (CMC) is not an elastomeric binder, it has been shown to significantly improve the cycling performance of Si electrodes.…”

Insights from Studying the Origins of Reversible and Irreversible Capacities on Silicon Electrodes

“…13 This further verifies that the amount of binder is a critical variable affecting electrode performance. These results suggest that to improve mechanical and electrochemical stability of a Si electrode, one should increase the amount of binder, choose a stronger binder and limit the discharge capacity.…”

“…Since the Si material used in both electrodes are the same, the difference in coulombic efficiency is not due to the material (surface or bulk), but to the strength of the binder, as PI is known to be stronger than CMC. 13,[27][28][29] Our results suggest that about 2-3% of the irreversibility of the Si-CMC electrodes comes from mechanical problems in the electrode. Figures 9c and 9d show the cycle performance of Si-CMC and Si-PI electrodes with 1000 mAh g −1 capacity limitation.…”

“…[3][4][5][6][7][8] Advanced imaging and characterizations such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), timeof-flight secondary ion mass spectrometry (TOF-SIMS) and in-situ dilatometry have helped scientists understand the underlying physical and mechanical interactions, as well as the chemical reactions in silicon electrodes during charge and discharge. [9][10][11][12][13] Though, eventually, it is the capacities that can be obtained from charge and discharge in each cycle, and how they can be sustained for a large number of cycles, that matters the most for battery applications. In this respect, there are hardly any studies that are able to tell how much of the measured capacity comes from different processes that are occurring in an electrode.…”