Billy Wu

David S Eastwood

Diana

Emanuel Peled

Farid Tariq

Golodnitsky

Gregory Offer

Kathrin Freedman

Moshiel Biton

Nigel Brandon

Peter D Lee

Vladimir Yufit

Yu Merla

Zhangwei Chen

CiteSeerX

 1
In-operando X-ray tomography study of lithiation induced 1 
delamination of Si based anodes for lithium-ion batteries 2 
Farid Tariq1*, Vladimir Yufit1, David S. Eastwood2,3, Yu Merla4, Moshiel Biton1, Billy Wu1, 3 
Zhangwei Chen1, Kathrin Freedman5, Gregory Offer4, Emanuel Peled5, Peter D. Lee2,3, Diana 4 
Golodnitsky5 and Nigel Brandon1 5 
1. Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK  6 
2. School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK 7 
3. Research Complex at Harwell, Harwell Oxford, Didcot, Oxfordshire, OX11 OFA, UK 8 
4. Department of Mechanical Engineering, Imperial College London,SW7 2AZ, UK   9 
5. Research School of Chemistry, Tel Aviv University, 4300 Israel  10 
(*corresponding author e-mail: farid.tarid02@imperial.ac.uk) 11 
 12 
Keywords: Lithium-ion, Tomography, Silicon Anode 13 
Abstract 14 
Silicon-Lithium based rechargeable batteries offer high gravimetric capacity. However 15 
cycle life and electrode microstructure failure mechanisms remain poorly understood. Here 16 
we present an x-ray tomography method to investigate in-operando lithiation induced stress 17 
cracking leading to the delamination of a composite Si based electrode. Simultaneous voltage 18 
measurements show increased cell resistance correlating with severe delamination and 19 
microstructural changes. 3D analysis revealed 44.1% loss of the initial electrode-current 20 
collector area after 1 hour of operation at 2.4mA/cm2 and a 21.2% increase in new anode 21 
surface area. The work represents a new basis for future investigation of Si-Li anodes. 22 
 2
1.0 Introduction 23 
Lithium-ion batteries provide the highest energy and power density amongst existing 24 
secondary batteries, and are often favoured in energy storage applications. However, many 25 
applications require energy and capacity beyond current battery technologies. 26 
 27 
Silicon anodes are an attractive alternative to carbon based anodes due to their high specific 28 
capacity (4200 mAh/g corresponding to Li22Si5 versus only 372 mAh/g for LiC6) (1, 2). Despite 29 
this advantage, silicon based anodes suffer from up to 400% volume expansion resulting in 30 
poor cycle performance due to the loss of electroactive material upon continuous solid 31 
electrolyte interphase (SEI) growth and isolated island formation (3). To alleviate these 32 
adverse phenomena nano-stuctured silicon is usually mixed/coated with carbon and polymer 33 
binders (4, 5). Depending on the preparation route, significant improvement in performance 34 
and cycle life can be achieved (6). For example double-walled Si-C nanotubes and yolk-shell 35 
Si-C nanoparticles have been proposed as the most successful designs to diminish the 36 
negative effects of silicon expansion/contraction on charge/discharge (7).  37 
 38 
The performance of composite silicon anodes is a strong function of a micro/nanostructure; 39 
therefore it is important to understand the ageing and failure mechanisms at these structural 40 
length scales in three dimensions, which remain comparatively poorly understood. X-ray 41 
radiography and tomography provides direct non-destructive imaging of microstructural 42 
evolutionary changes at length scales appropriate to the study of Si and SnO based anodes 43 
(3, 8, 9). Here, for the first time known to the authors, the development and implementation of 44 
a Si based battery anode imaging method is presented that enables in-operando tracking and 45 
3D imaging of electrode microstructure. It is possible to explore evolutionary processes and 46 
 3
quantify them at fine length scales, in real time, and furthermore to study induced failure 47 
mechanisms as they occur. The work provides a platform for future studies to explore the 48 
effects of key factors such as preparation routes, composition, charge/discharge conditions, 49 
etc. on composite silicon anodes.  50 
2.0 Experimental 51 
A customised Si-Li half-cell was assembled in an argon filled glove box (Fig.1a).  A lithium 52 
metal anode and a silicon-carbon composite cathode were used. The copper current collector 53 
rod was machined into a conical tip on the silicon side for X-ray imaging purposes. The final 54 
cathode composition was 70 wt.% carbon-coated Si, 20 wt.% Shawinigan Black Carbon 55 
(Chevron Chemicals), and 10 wt.% sodium carboxymethyl cellulose (Sigma-Aldrich). The 56 
carbon-coated Si power was prepared via pyrolysis of 60 wt.% Si 30nm nanoparticles 57 
(Umicore), 10 wt.% 50nm MWCNT (Skyspring Nanomaterials) and 30 wt.% of sugar(10). The 58 
current collector was dipcoated into Si/C slurry and vacuum dried in a furnace. Ionic liquid 59 
made from 1M LiPF6 EC:DEC 1:1 v/v and 2 wt.% of VC electrolyte (Solvionic) was the 60 
electrolyte. The active materials were enclosed by a Kapton® polyimide tube and sealed 61 
air-tight using Torr seal® epoxy (Goodfellow). The Si based anode (0.1-0.2mg) provides an 62 
estimated 200–400µAh capacity. The final design was 6mm in diameter and 53mm in length.  63 
 64 
The electrochemical cell was mounted in an X-ray Microtomography (XMT) system (Phoenix 65 
v|tome|x, GE, USA) for direct imaging/tomography operated at 70 kV. Electrical wires were 66 
attached to the cell to lithiate the silicon anode and current control was maintained through an 67 
Ivium VERTEX (Ivium, Eindhoven, Netherlands) potentiostat. A galvanostatic charge current 68 
of 100μA was applied to the cell for 1 hour whilst monitoring voltage and simultaneously 69 
imaging the battery anode using X-rays (Fig.1a). After 1 hour the electrode was partially 70 
 4
lithiated and delamination was observed; then the current was stopped, electrical connectors 71 
removed and an X-ray tomography scan performed by rotating the sample. The 3D structure 72 
of the electrode, electrolyte and current collector was reconstructed using 602 projections 73 
captured over a 360° range and a filtered back projection algorithm; producing a total volume 74 
of 8mm3 at a final voxel size of 9.3µm. Segmentation was carried out manually using Avizo 75 
Fire (FEI, Bordeux, France) to separate features of interest on both radiographic and 76 
tomographic datasets.  77 
2.1 Modeling 78 
An analogous 2D finite element model of the electrode was setup using Abaqus CAE to 79 
simulate stress distributions in the anode through lithiation induced volume expansion. The 80 
model was run in standard FE solver mode, with the assumption that 100 µA for 3500s would 81 
lead to a calculated 63 % volume expansion, and the silicon anode was isotropic, linear and 82 
elastic. An elastic modulus of 100 GPa and Poisson’s ratio of 0.26 were used (11, 12). 83 
Several simulations were run with the anode circular interface fixed at angles from 240°, 180°, 84 
120° and 60° respectively. The remaining part of the anode was allowed to move away from 85 
the current collector on volume expansion to follow the observed direction and effect of 86 
delamination (Fig.2).  87 
 88 
3.0 Results and Discussion 89 
 90 
This study successfully demonstrates a novel methodology for capturing valuable information 91 
in-operando of silicon based anode failure mechanisms. Previous in-situ x-ray methods have 92 
focused mostly upon SnO particles (9, 13), but not silicon based anodes which present 93 
difficulties in acquisition and data analysis due to low x-ray attenuation. The cone-tip design 94 
 5
employed here ensured that the entire surface of the current collector was covered in a low 95 
attenuation X-ray observable silicon anode layer. This provides large areas of the electrode 96 
that can be imaged for detail, and therefore maximised the likelihood of capturing observed 97 
failure mechanisms. The compact cell design also minimises the object-source distance, 98 
improving the X-ray resolution.  99 
 100 
The results show a clear difference between two regions of the silicon electrode. The 101 
electrode region in contact with an argon bubble (white in Fig.1c and supplementary material) 102 
showed neither lithiation nor delamination, whereas the region in contact with the electrolyte 103 
upon lithiation lifted off and delaminated in a concave ‘bow shape’ away from the current 104 
collector. The inert bubble was present from battery manufacture. However, from these 105 
results it is evident that lithiation results in anode expansion, that causes severe delamination. 106 
Upon volumetric expansion, stresses are generated which are accommodated through the 107 
creation of new surfaces and this results in the bowing of the anode away from the current 108 
collector (Fig.1c-Fig.2). Interestingly, in a few locations the anode maintains good adhesion 109 
with the current collector allowing the anode to continue to function; however, in-operando 110 
imaging also shows that, during lithiation, delamination eventually results in areas of crack 111 
propagation through the electrode thickness that reach the electrolyte. This is evident through 112 
the 3D reconstruction (Fig.1b) where it is possible to see delamination leading to regions 113 
where cracks propagate from the electrode-current collector interface through to the surface 114 
of the electrode. The local electrode-current collector adhesion would influence this 115 
behaviour. Over an operational lifetime, coalescence of cracks would therefore result in loss 116 
of active material and hence capacity loss. The 3D data also shows the presence of intrinsic 117 
porosity within the silicon electrode, although this accounts for < 1 vol %, these pores are 118 
 6
ca.30-50 µm wide and may facilitate crack propagation, and will be investigated in future 119 
studies. 120 
 121 
Radiograph images during lithiation, at 100μA (current density of 2.4 mA/cm2) show the 122 
delamination of the silicon electrode from the copper current collector (Fig.1c-Fig.2a). The 123 
height of the delamination continues to increase with charging time, as does the width but at a 124 
reducing rate. From the measured voltage data a rapid decrease in the cell potential is 125 
observed, suggesting that the electrode-current collector contact resistance is increasing due 126 
to the volumetric expansion caused by lithiation. After 500s the potential of the cell slightly 127 
decreases until reaching 1500s. An increase in the available active surface area caused by 128 
delamination may be responsible for this. Though 3D data analysis the electrode-current 129 
collector contact area decreases as a result of delamination by 1.84 mm2 from an original 130 
4.17mm2 which is equivalent to a 44.1% loss of contact area (Fig.1c). Concurrently, the 131 
interfacial area between the delamination induced void and silicon anode increases to 2.13 132 
mm2, which corresponds to 21.2% of the total silicon anode area following lithiation. 133 
Consequently, the ability to track this is significant for cells since this fresh surface would 134 
result in capacity degradation through the consumption of lithium in SEI layer formation. 135 
 136 
From the imaging results it is clear that lithiation induces electrode delamination. Simulations 137 
reveal that regions of the electrode fixed in contact with the current collector experience 138 
greater stresses (Fig.2b). The peak stresses occur at the beginning of the delamination 139 
void-current collector interface for all simulated angular positions. These positions correspond 140 
to the delamination crack tip in the imaging experiments, and delamination would therefore 141 
facilitate reducing stresses at the electrode-current collector interface. The imaging data 142 
suggests electrode delamination is not linear with time and interestingly, the modeling shows 143 
 7
that peak stresses decrease with decreasing angular position. A decrease in peak stresses 144 
could result in lower delamination rates with time. The present simulation is over-constrained, 145 
since stress relaxation simulations are beyond the scope of present work. Once these are 146 
considered then lower predicted peak stresses would be expected; however current values 147 
are of the same order of magnitude as those in literature (12), and most importantly similar 148 
trends are still expected to occur.  149 
4.0 Conclusions 150 
Here we present an X-ray radiographic and tomographic imaging methodology to study in-151 
operando degradation mechanisms in a Si anode based battery. The lithiation induced stress 152 
cracking was sequentially followed by electrode-current collector delamination which led to an 153 
increase in contact resistance of the cell. Simulation results suggest peak stresses occur at 154 
the delamination location between the current collector and electrode. Quantitative analysis 155 
shows that 44.1% of the initial electrode-current collector area was lost within just 1 hour of 156 
operation at 100μA (2.4mA/cm2). The volume expansion caused by the lithiation of the silicon 157 
anode resulted in 21.2 % of fresh electrode area being revealed.  158 
 159 
The ability to quantify in 3D, the detailed microstructural evolutionary changes in a Si based 160 
anode as it operates provides opportunities to understand sources of battery degradation and 161 
failure. Furthermore, the approach is not limited to Si based anodes but can be applied to 162 
other chemistries, and offers insights into evolutionary microstructural changes and observed 163 
battery performance/lifetime.  164 
 165 
Acknowledgements 166 
 8
The authors would like to thank: EPSRC Structural evolution across multiple time and length 167 
scales project (EP/I02249X/1), EPSRC Energy Storage for Low Carbon Grids project 168 
(EP/K002252/1), the US Office of Naval Research, the Materials for Aging Resistant Li-ion 169 
High Energy Storage for Electric Vehicle Project, the Diamond-Manchester Collaboration and 170 
the Research Complex at Harwell, Climate KIC, G.Cui and S.J.Cooper for assistance.  171 
 9
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Figure 1. a) Si based anode tomography cell construction and X-ray direction, b) 3D reconstruction after lithiation with 205 
delamination and, c) radiography during lithiation and delamination with voltage response. 206 
 207 
Figure 2. a) Radiography based anode delamination and, b) 2D simulation of anode expansion with varying fixed boundary 208 
positions showing decreasing peak stresses with decreasing contact angle corresponding to increased delamination. 209 
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In-operando X-ray tomography study of lithiation induced delamination of Si based anodes for lithium-ion batteries

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In-operando X-ray tomography study of lithiation induced delamination of Si based anodes for lithium-ion batteries

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