Silicon-Lithium based rechargeable batteries offer high gravimetric capacity. However cycle life and electrode microstructure failure mechanisms remain poorly understood. Here we present an X-ray tomography method to investigate in-operando lithiation induced stress cracking leading to the delamination of a composite Si based electrode. Simultaneous voltage measurements show increased cell resistance correlating with severe delamination and microstructural changes. 3D analysis revealed 44.1% loss of the initial electrode-current collector area after 1 hour of operation at 2.4 mA/cm 2 and a 21.2% increase in new anode surface area. The work represents a new basis for future investigation of Si based anodes. © 2014 The Electrochemical Society

Biton, M

Brandon, N

Chen, Z

Eastwood, DS

Freedman, K

Golodnitsky, D

Lee, PD

Merla, Y

Offer, G

Peled, E

Tariq, F

Wu, B

Yufit, V

ECS Electrochemistry Letters

Tariq, Farid

Yufit, Vladimir

Eastwood, David S.

Merla, Yu

Biton, Moshiel

Wu, Billy

Chen, Zhangwei

Freedman, Kathrin

Offer, Gregory

Peled, Emanuel

Lee, Peter D.

Golodnitsky, Diana

Brandon, Nigel

The University of Manchester - Institutional Repository

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

28.4.15 KB. Ok to add accepted version to spiralSilicon-Lithium based rechargeable batteries offer high gravimetric capacity. However cycle life and electrode microstructure failure mechanisms remain poorly understood. Here we present an X-ray tomography method to investigate in-operando lithiation induced stress cracking leading to the delamination of a composite Si based electrode. Simultaneous voltage measurements show increased cell resistance correlating with severe delamination and microstructural changes. 3D analysis revealed 44.1% loss of the initial electrode-current collector area after 1 hour of operation at 2.4 mA/cm2 and a 21.2% increase in new anode surface area. The work represents a new basis for future investigation of Si based anodes

Offer, GJ

Brandon, NP

Spiral - Imperial College Digital Repository

 1In-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  21.0 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  3quantify 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  4lithiated 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  5employed 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  6ca.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  7that 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  8The 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  9References 172 1. H. Jung, J Power Sources, 115, 346 (2003). 173 2. K. Tokumitsu, H. Fujimoto, A. Mabuchi and T. Kasuh, 37, 1599 (1999). 174 3. U. Kasavajjula, C. Wang and a. J. Appleby, J Power Sources, 163, 1003 (2007). 175 4. Y. Liu, K. Hanai, J. Yang, N. Imanishi, A. Hirano and Y. 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Guduru, Journal 191 of the Mechanics and Physics of Solids, 62, 276 (2014). 192 13. M. Ebner, F. Marone, M. Stampanoni and V. Wood, Science, 342, 716 (2013). 193  194  195  196  197  198  199  200  201  202  203  204  10Figure 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  210 

In-Operando X-ray Tomography Study of Lithiation Induced Delamination of Si Based Anodes for Lithium-Ion Batteries

In-Operando X-ray Tomography Study of Lithiation Induced Delamination of Si Based Anodes for Lithium-Ion Batteries

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