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Journal Club Theme of December 2015: Mechanics of Lithium Ion Battery Electrodes

Mechanics of Lithium Ion Battery Electrodes – Insights from Multi-Scale Simulations

Huck Beng Chew, Haoran Wang

Department of Aerospace Engineering, University of Illinois at Urbana-Champaign

 

1.  Introduction

Lithium ion batteries are high energy density systems that store energy by insertion of lithium ions into solid electrodes. Silicon is one of the most promising electrode materials for high performance lithium ion batteries, since it possesses the highest known specific capacity of 4200 mAh/g, which is an order of magnitude higher than conventional graphite electrodes. During lithiation, the silicon electrodes form LixSi compounds, and undergo huge volume expansion of about 300% since one silicon atom can theoretically bond with a maximum of x = 3.75 lithium atoms. When attached to a metal current collector, such as copper, the massive and inhomogeneous volume changes during repeated lithiation and delithiation charge cycles lead to colossal cracking of the silicon electrode.(1-4) Recent studies have shown that silicon electrodes of small feature sizes, such as nanowires, nanoparticles, porous structures, and thin films, display significantly higher reversible charge capacities and longer cycle life.(5-8) In fact, a critical feature size of these nanostructured silicon electrodes exists, below which fracture would be completely mitigated. It is believed that the improved fracture resistance originates from the ability of the nanoscale structure to accommodate the lithiation-induced strain by plastic deformation, resulting in lower stresses present during volume changes.(9,10) However, the delamination of crack-free nanostructured silicon electrodes from current collectors after a critical number of charge cycles has been widely reported, resulting in the loss of electrical contact and consequent capacity fade.(1,6,11,12)

What gives rise to the size-dependent cracking of lithiated silicon? 

Why does the lithiated silicon thin film electrodes delaminate from the current collector after a critical number of charge cycles? 

Why does lithiated silicon undergo plastic deformation, given that pure silicon is inherently brittle?

In this month’s iMechanica Journal Club, we would like to provide an overview of recent efforts to answer these questions through modeling and simulations at various length-scales. We hope this brief overview will initiate further discussions on this research front.  

2.  Cracking mechanism in LixSi thin film electrodes

Silicon thin films are a particularly useful model system to study the size-dependent fracture of silicon electrodes. Li et al. examined the cracking patterns generated during electrochemical cycling of amorphous silicon thin film electrodes, and found that decreasing film thickness significantly increased the density of cracks, resulting in island diameters that are smaller but nearly uniform (Fig. 1A).(13) Once formed, the individual silicon islands remain crack-free under subsequent lithiation and delithiation cycles. This suggests that a critical island diameter exists for a fixed film thickness, below which fracture of the electrode would be mitigated.(7,13) Therefore, electrode cracking may be mitigated by patterning silicon islands on a substrate. 

 

Fig. 1: Cracking patterns in LixSi thin film electrodes.(16) (A) SEM images of cracking patterns formed on amorphous silicon (a-Si) thin films.(13) (B) Schematic showing the development of a high tensile stress zone caused by bending of the lithium-silicon subsurface layer near the film edges. A micro-crack develops in the high tension zone at a critical distance dc from the edges. (C) Contour maps of in-plane stresses during sequential cracking, showing (i) the development of high tensile stresses dc from the edges, (ii) nucleation of a micro-crack which allows the inner section of the film to bend, and (iii) development of a new high tension zone dc from the crack after continued lithiation. (D) Comparison of the critical island size for different film thickness h predicted by FEM simulations versus experiments.

The crack patterns that form in silicon thin film electrodes closely resemble through-thickness crack networks that form in drying media, leading many to conclude that the fracture of the silicon electrodes occurs during delithiation.(13-15) Instead, finite element analyses demonstrate that the fracture of lithiated silicon films occurs by a sequential cracking mechanism during lithation (Fig. 1B).(16) The expansion of the lithium-silicon subsurface layer during early-stage lithiation bends the film near the edges, and generates a high tensile stress zone at a critical distance away within the lithium-free silicon. Fracture initiates at this high tension zone and creates a new film edge, which bends and generates high tensile stresses a further critical distance away (Fig. 1C). Under repeated cycling, this sequential cracking creates silicon islands of uniform diameter, which scales with the film thickness. The predicted island sizes, as well as the abrupt mitigation of fracture below a critical film thickness, due to the diminishing tensile stress zone, is quantitatively in good agreement with experiments(1,13,17) (Fig. 1D).

3.  Sliding and delamination of a LixSi thin film electrode from a Cu current collector

Even though the cracking of LixSi thin films can be mitigated through patterning individual silicon islands,(14) the uncracked electrode still delaminates from the current collector after a critical number of charge cycles.(1) To date, much is still unknown about the interface bonding the silicon electrode and a metal current collector, such as copper. Studies have suggested that sliding readily occurs along the silicon-copper interface to accommodate the massive volume changes in lithiated silicon during charge cycling.(14) However, understanding the mechanisms of interface sliding and delamination is complicated by significant intermixing of Cu, Si, and Li atoms at the interface between a lithiated-silicon film and the copper substrate.(1,18-20)

 

Fig. 2: Sliding and delamination of LixSi thin film electrodes from the Cu current collector.(21) (A) Atomic structure of the interdiffused Li-Si-Cu interphase between a LixSi electrode and a Cu current collector. (B) Interface sliding facilitated by the formation of well-delineated and weakly bonded Si-Cu and Li-Cu crystalline atomic layers within this interphase structure. (C) Shear stress versus shear strain response demonstrating distinct regions of stress build-up and release leading to interface sliding, and stress accumulation leading to interface delamination. 

Using first principle calculations, we recreate model structures of the interdiffused Li-Si-Cu interphase (Fig. 2A), and show that the interdiffusion among Li, Si, and Cu atoms leads to the formation of well-delineated, crystalline Si−Cu and Li−Cu atomic layers at intermediate lithium concentrations (Fig. 2B).(21) These atomic layers are weakly bonded in shear, and readily slide to relieve the interfacial stresses during lithiation processes. Ideally, interface sliding between the silicon electrode and the copper current collector will help limit film stresses introduced by the lithiation process. However, sliding between the Si−Cu and Li−Cu atomic layers cannot occur indefinitely. The formation of pinning defects in the form of LiSi3 compounds along the interface can eventually inhibit sliding (Fig. 2C). The consequential buildup of interfacial stresses leads to delamination failure of the silicon electrode from the copper current collector. Understanding the atomic-scale mechanisms that promote or impede sliding provides the critical first steps toward designing silicon-copper interface structures to mitigate electrode failure.

4.  Brittle-to-ductile transition of LixSi electrodes

In contrast to pure silicon which is inherently brittle, LixSi alloys are able to undergo significant plastic deformation.(9,22,23) Using first principle calculations, we show that LixSi electrodes exhibit a sharp transition from brittle to ductile behavior with increasing lithium content (Fig. 3A).(24) The brittle behavior of pure Si and LiSi2 is associated with the interconnected network of strong Si-Si covalent bonds which have low bond stretchability of ~5%; breaking of these Si-Si bonds lead to sudden catastrophic failure (Fig. 3B – top). In contrast, the increased density of weaker Li-Li bonds which can be stretched to ~25% in LiSi and Li15Si4 causes the transition to ductile behavior (Fig. 3B – bottom). 

 

Fig. 3: Plasticity of LixSi electrodes.(24) (A) Stress-strain response of four LixSi alloys subjected to uniaxial straining. (B) Evolution of damage within LiSi2 (top) and Li15Si4 (bottom) supercells with applied strain; brown lines represent Si–Si bonds, blue spheres represent the nanoporous regions.

5.  References

[1]  Maranchi, J.P., Hepp, A.F., Evans, A.G., Nuhfer, N.T., Kumta, P.N., J. Electrochem. Soc. 2006, 153, A1246−A1253.

[2]  Zhao, K.J., Wang, W.L., Gregoire, J., Pharr, M., Suo, Z.G., Vlassak, J.J., Kaxiras, E., Nano Lett. 2011, 11, 2962−2967.

[3]  Rohrer, J., Albe, K., J. Phys. Chem. C 2013, 117, 18796−18803.

[4]  Cubuk, E.D., Kadras, E., Nano Lett. 2014, 14, 4065−4070.

[5]  Lee, S.W., McDowell, M.T., Beria, L.A., Nix, W.D., Cui, Y., PNAS 2012, 109, 4080–4085.

[6]  Liu, X.H., Zheng, H., Zhong, L., Huang, S., Karki, K., Zhang, L.Q., Liu, Y., Kushima, A., Liang, W.T.,Wang, J.W., Cho, J.-H., Epstein, E., Sayeh, S.A., Picraux, S.T., Zhu, T., Li, J., Sullivan, J.P., Cumings, J.,Wang, C., Mao, S.X., Ye, Z.Z., Zhang, S., Huang, J.Y., Nano Lett. 2011, 11, 3312–3318.

[7]  Takamura, T., Ohara, S., Uehara, M., Suzuki, J., Sekine, K., J. Power Sources 2004, 129,96–100.

[8]  Graetz, J., Ahn, C.C., Yazami, R., Fultz, B., Electrochem. Solid-State Lett. 2003, 6, A194–A197.

[9]  Sethuraman, V.A., Srinivasan, V., Bower, A.F., Guduru, P.R., J. Electrochem. Soc. Commun. 2010, 12,1614–1617.

[10]Zhao, K., Pharr, M., Cai, S., Vlassak, J.J., Suo, Z., J. Am. Ceram. Soc. 2011, 94, S226–S235.

[11]Huggins, R.A., Nix, W.D., Ionics 2000, 6, 57−63.

[12]Neudecker, B.J., Dudney, N.J., Bates, J.B., J. Electrochem. Soc. 2000, 147, 517−523.

[13]Li, J., Dozier, A.K., Li, Y., Yang, F., Cheng, Y.-T., J. Electrochem. Soc. 2011, 158, A689–A694.

[14]Xiao, X., Liu, P., Verbrugge, M.W., Haftbaradaran, H., Gao, H., J. Power Sources 2011,196, 1409-1416.

[15]Beaulieu, L.Y., Eberman, K.W., Turner, R.L., Krause, L.J., Dahn, J.R., Electrochem. Solid-State Lett. 2001, 4, A137–A140.

[16]Chew, H.B., Hou, B., Wang, X., Xia, S., Int. J. Solids Struct. 2014, 51, 4176−4187.

[17]Moon, T., Kim, C., Park, B., J. Power Sources 2006, 155, 391-394.

[18]Stournara, M.E., Xiao, X.C., Qi, Y., Johari, P., Lu, P., Sheldon, B.W., Gao, H.J., Shenoy, V.B., Nano Lett.2013, 13, 4759−4768.

[19]Santhanagopalan, D., Qian, D., McGilvray, T., Wang, Z., Wang, F., Camino, F., Graetz,J., Dudney, N.,Meng, Y.S., J. Phys. Chem. Lett. 2014, 5, 298−303.

[20]Fister, T.T., Long, B.R., Gewirth, A.A., Shi, B., Assoufid, L., Lee, S.S., Fenter, P., J. Phys. Chem. C 2012,116, 22341−22345.

[21]Wang, H., Hou, B., Wang, X., Xia, S., Chew, H.B., Nano Lett. 2015, 15, 1716-1721.

[22]Sethuraman, V.A., Srinivasan, V., Bower, A.F., Guduru, P.R., J. Electrochem. Soc. 2010, 157, A1253-A1261. 

[23]Chon, M.J., Sethuraman, V.A., McCormick, A., Srinivasan, V., Guduru, P.R., Phys. Rev. Lett. 2011, 107, 045503.

[24]Wang, H., Wang, X., Xia, S., Chew, H.B., J. Chem. Phys. 2015, 143, 104703.

 

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Comments

Dibakar Datta's picture

Dear Prof. Huck Beng Chew and Haoran,

Thank you for your wornderful post. I would like to draw attention of iMechanicians to our paper : 

Atomistic Mechanism of Phase Boundary Evolution during Initial Lithiation of Crystaline Silicon

 

Abstract Image

In lithium-ion batteries, the electrochemical reaction between Li and Si causes structural changes in the negative electrode. The dynamics of lithiation of Si can be further complicated by the crystalline-to-amorphous phase transition. In situ TEM experiments show that a sharp interface, known as phase boundary, is formed in between c-Si and a-LixSi during initial lithiation. Despite intensive study of the mixing mechanism during lithiation of Si negative electrode, the atomistic investigation of the formation and propagation of phase boundary for different orientation of Si remains unclear. We, therefore, performed molecular dynamics simulations to characterize the structural evolution of the phase boundary with a newly developed reactive force field (ReaxFF) potential for Li–Si. Our results confirm the phase boundary formation in between c-Si and a-LixSi. Structure and dynamics of the phase boundary depend on the crystalline phase of the Si. In particular, the location of the (111) plane plays a key role in crystal-to-amorphous phase transformation. A relatively thick phase boundary is developed at the (100) surface, while an atomically sharp interface of negligible thickness is formed at the (111) surface. An amorphous phase of lithiated Si is developed beyond the phase boundary, in which the ratio of lithium to silicon atoms is steady at 0.8. Partial RDF studies revealed that the structures of the phase boundary and the lithiated Si region are c-LiSi and a-Li15Si4, respectively.

 

Hi Dibarkar,

Thank you for sharing your research with us. I think I have seen at least 3 papers where they developed REAXff potential for LiSi system. I compared 2 of them and find their parameters are quite different. Did you compare your potential with them? Besides, how is the reaction/diffusion rate and mechanical propoerties compare with real situation? 

Thanks,

Haoran

Dibakar Datta's picture

Dear Prof. Huck Beng Chew and Haoran,

Thank you for your wornderful post. I would like to draw attention of iMechanicians to our paper : 

Enhanced Lithiation in Defective Graphene

 

Bonding charge density for Li for (a) pristine, (b, c) Stone–Wales and (d, e) ...

We performed first-principle calculations based on density functional theory (DFT) to investigate adsorption of lithium (Li) on graphene with divacancy and Stone–Wales defects. Our results confirm that lithiation is not possible in pristine graphene. However, enhanced Li adsorption is observed on defective graphene because of the increased charge transfer between adatom and underlying defective sheet. Because of increased adsorption, the specific capacity is also increased with the increase in defect densities. For the maximum possible divacancy defect density, Li storage capacities of up to ∼1675 mAh/g can be achieved. While for Stone–Wales defects, we find that a maximum capacity of up to ∼1100 mAh/g is possible. Our results provide deeper understanding of Li-defect interactions and will help to create better high-capacity anode materials for Li-ion batteries.

RongXu's picture

Dear Prof. Huck Beng Chew and Haoran,

Thank you for your nice post. We have a recent paper: 

Rong Xu, Kejie Zhao. Mechanical interactions regulated kinetics and morphology of composite electrodes in Li-ion batteries. Extreme Mechanics Letters (2015).

Most of prior studies have been extensively focused on single particles or mono-phase materials. The mechanics and mechanical interactions regulated kinetics and morphological evolution in three-dimensional composite electrodes are much less exploited. In this paper, we model the chemo-mechanical behaviors of a cluster of active materials in a mechanically confined medium, using a finite element program that integrates a continuum theory of coupled diffusion and large elasto-plastic deformation. Here are a few findings:

  • Li profiles and stresses in multiple particles constrained by a matrix are significantly different from that of a free-standing configuration.
  • Capacity of electrodes is not linearly scaled with the mass of the active materials but is limited by the inhomogeneous storage of Li.
  • Capacity and stresses of batteries can be tuned by optimizing the composition, architecture, and porosity of the electrodes.
  • For multiple Si nanowires, the mechanical stress transforms the anisotropic deformation to an isotropic behavior that may increase the fracture resistance as is recently found in an experimental study.

Hope you find this paper of interest, and look forward to your comments!

 

Best,

Rong Xu

Hi Rong,

Thank you for posting this interesting paper. I have seen some other groups in our department doing experimental studies on composite electrode system. For example, this one (http://www.sciencedirect.com/science/article/pii/S0378775315005091). While the problem of pulverization of Si (or other electrode material) can be solved by using nano-size structure, the interface delamination (between Si and the matrix) will be a big challenge. 

Thanks,

Haoran

RongXu's picture

Hi Haoran,

Thanks for your information. Nanostructure Si electrode like Si nanowire has a better fracture resistance during lithiation (C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, Y. Cui. High Performance Lithium Battery Anodes Using Silicon Nanowires). You can image that there is more space between Si nanowires for their swelling. However, as you said, the deformation mismatch and stress concentration at the interface between Si and matrix will induce the delamination. It is interesting to design the 3D structures to avoid this delamination. 

Best,

Rong

Kejie Zhao's picture

Dear Huck Beng,

Thank you for bringing up this interesting topic. There were a few loops of discussion on mechanics in batteries and relevant energy materials, just for anyone who is interested in more background:

Mechanics Issues in Nanocapacitors and Ramifications for Energy Storage

Lithium batteries--When mechanics meets chemistry

Journal Club December 2010: Mechanics of Energy Storage

Journal Club Theme of February 2014: Energy Challenges and Mechanics

Indeed Li-ion batteries are an important technology that enables the age of portable information. It is also a model system for the study of fundamental aspects of coupling between mechanics and electrochemistry. The kinetics of diffusive and interfacial reactions in the electrodes induces a wealth of intriguing phenomena, such as cavitation, reactive plasticity, and corrosive fracture.  The electrochemical processes modulate large deformation and stress generation in the electrodes. Meanwhile, mechanical stresses influence the thermodynamics and kinetics of lithiation reactions, ion diffusion, and phase transitions. I wish to follow this thread to describe a few examples that highlight the coupling between the multiphysics - there have been a really large volume of literature on batteries and relevant materials, please bear with me for not citing the very important works in this field.

1. Diffusion-induced stresses

http://imechanica.org/files/Picture1_0.pngDeformation and stresses of host materials induced by diffusion of guest species are well known, and theoretical framework that accounts for the stress effect on the thermodynamics of solid solution is traced back at least to the work of Larche, Cahn, and Li. In the above figure, the inhomogeneous distribution of Li causes a field of stress, the compressive stress in the Li-rich region would retard the diffusion while the tensile stress facilitates the diffusion. 

While much of early work and lot of recent research have used this diffusion-stress coupling for different types of battery materials (intercalation type, conversion reaction type, solid solution type, etc), I am not aware of any experimental evidence on the stress effect on diffusion in battery materials, or to be specific, in materials that the diffusing species form relatively strong bonds with the host atoms. It seems easier to imagine the stress effect on solid solution, but I am puzzled on if mechanical stress is large enough to drive the species migration in an ionic/covalent network.  Any comments are appreciated.

2. Stress-regulated interfacial reaction

http://imechanica.org/files/Picture2_0.png

Interfacial reaction is the rate-limiting process in many electrochemical systems. For example, during lithiation of crystalline Si, the crystalline phase is amorphized and it forms a core-shell structure.  The interface separating the crystalline core and amorphous shell is atomically sharp, with the thickness of 1~2 nanometers.  The kinetics of lithiation is limited by the short-range process of bond breaking and re-forming at the reaction front, rather than by the long-range diffusion of Li through the amorphous phase. A stress field is associated with the sharp interface, and the stress field influences the thermodynamic driving force for the propagation of the reaction front. At extreme occasions, the mechanical stress even counterbalances the chemical driving force and causes staganation of the reaction front, as shown in the right panel of the figure. The following paper describes a simplfied model on this coupling effect:

Kejie Zhao, Matt Pharr, Qiang Wan, Efthimios Kaxiras, Joost J. Vlassak and Zhigang Suo. Concurrent reaction and plasticity during lithiation of crystalline silicon in lithium-ion batteries. Journal of the Electrochemical Society. 159, A238-A243 (2012)

3. Reactive plasticity

http://imechanica.org/files/Picture3_0.png

Plastic flow limits the magnitude of stresses and accommodates the large deformation in high-capacity electrodes. In the electrochemical process of Li insertion and extraction, the chemical reaction and plastic flow are concurrent, and may not be distinguishable at the atomistic level. The chemical reaction may promote plasticity by decreasing the stress needed to initiate the plastic flow. The above figure shows a basic picture, for a material network under the electrochemical attack, the host atom bonds are weekened and broken, and the valence state of matter is under dynamic change, this process will facilate the plasticity in that the shear stress needed to maintain certain atomic displacement will be lower than that without reaction. At the continuum level, we may need to treat both the diffusion/reaction and plasticity as non-equilibrium processes. The following papers describe the modeling and theoretical work:

Kejie Zhao, Georgios Tritsaris, Matt Pharr, Wei L. Wang, Onyekwelu Okeke, Zhigang Suo, Joost J. Vlassak, Efthimios Kaxiras. Reactive flow in silicon electrodes assisted by the insertion of lithium. Nano Letters. 12, 4397-4403 (2012)

Laurence Brassart, Zhigang Suo. Reactive flow in solids. Journal of the Mechanics and Physics of Solids 61, 6177 (2013).

4. Stress-regulated electrochemical growth

http://imechanica.org/files/Picture4.png

Another observation that exemplifies the stress-regulated chemical reaction is the inhomogeneous growth of lithiated phase for materials with geometric curvatures. The above figure shows the schematics of a structure with perturbed surface of small amplitude. It grows into a wavy morphology during lithiation reactions. The inhomogeneous growth may be attributed to the stress effect. The stress field is modified by the geometry factor of surface curvature. The lithiated phase of convex curvature develops a field of tensile stresses, facilitating Li transport through the lithiated material and promoting the interfacial reaction at the phase boundary. On the contrary, a material element in the lithiated phase of concave curvature is under a field of compressive stresses, retarding the electrochemical growth of the layer. The following paper describes some experimental observations:

Yuefei Zhang, Yujie Li, Zhenyu Wang, Kejie Zhao. Lithiation of SiO2 in Li-ion Batteries: in-situ Transmission Electron Microscopy Experiments and Theoretical Studies. Nano Letters. 14, 7161 (2014)

5. Stress-induced solid-state reaction instability

http://imechanica.org/files/Picture5.png

The above figure shows the schematic of the morphology of the solid state reaction front (SSRF) during lithiation of a nanowire electrode. In a typical setup of a single nanowire in contact with the counter electrode and solid electrolyte Li/Li2O on the left hand, Li ions diffuse quickly on the nanowire surface and form a thin wetting layer with typical thickness of a few nanometers. The nanowire is first lithiated radially and the extent of lithiation is limited by the amount of Li transport through the thin wetting layer. The SSRF primarily propagates along the longitudinal direction of the nanowire away from the electrolyte. The radial lithiation forms a structure of lithiated shell and unlithiated core. A field of stress is associated with the core-shell geometry. While the tensile stress in the lithiated shell usually induces discrete surface cracks and facilitates Li transport through the crack surfaces, the compressive stress in the pristine core diminishes the thermodynamic driving force of the electrochemical reaction and impedes the local lithiation. The asymmetry of the characteristic stresses breaks the planar SSRF into a curved interface. Such an effect is similar to the Mullins-Sekerka interfacial instability during solidification of liquids.

 

Yuefei Zhang, Zhenyu Wang, Yujie Li, Kejie Zhao. Lithiation of ZnO nanowires studied by in-situ transmission electron microscopy and theoretical analysis. Mechanics of Materials. 91, 313 (2015)

 

6. Electrochemically driven mechanical energy harvester

http://imechanica.org/files/Picture6_0.pnghttp://imechanica.org/files/Picture7_0.png

The colleagues at MIT cleverly devised a class of mechanical energy harvesters via stress-voltage coupling in electrochemically alloyed electrodes. The device consists of two identical Li-alloyed Si as electrodes, separated by electrolyte-soaked polymer membranes. Bending induced asymmetric stresses generate chemical potential difference, driving lithium ion flux from the compressed to the tensed electrode to generate electrical current. Removing the bending reverses ion flux and electrical current. The device can sustain thousands of cycles and demonstrates a practical use of stress-composition-voltage coupling in electrochemically active alloys to harvest low-grade mechanical energies from various low-frequency motions, such as everyday human activities.

Sangtae Kim, Soon Ju Choi, Kejie Zhao, Hui Yang, Giorgia Gobbi, Sulin Zhang, Ju Li. Electrochemically driven mechanical energy harvesting. Nature Communications. (2015).

This post has been long, thank you for reading up to this point. I look forward to your and others' comments.

-Kejie

 

chenlei08's picture

Hi Kejie

I have posted a messy comment (http://imechanica.org/comment/27929#comment-27929). I found you are one of the moderators. Could you please help me to delete this messy comment?

Many thanks in advance!

Best regards

 

Lei

Kejie Zhao's picture

Lei, I think the post has been deleted.  -Kejie

hbchew's picture

Hi Kejie

 

Thanks for sharing your work with the battery community. Nice post!

Zheng Jia's picture

Dear Huck Beng and Haoran,

Thank you for sharing your excellent work on Lithium-ion batteries. Si-based anodes hold the promise to revolutionize the industry of lithium-ion batteries, as it possesses the highest known theoretical capacity stemming from the large amount of Li atoms that hosted by Si. However, Si’s superb capability of hosting Li atoms seems to be a double-edged sword: insertion of Li atoms cause excessive volume change and thus induce large mechanical stresses which significantly affect the performance of battery through many aspects including mechanical integrity, deformation behavior and lithiation kinetics. I would like to discuss some of our past work on these aspects. Your comments will be greatly appreciated.

1.  Mechanical Integrity: lithiaion-induced fracture and design of crack-free anodes

To maintain electrical connectivity in Si anodes, there is substantial interest in incorporating low dimensional carbon nanomaterials such as CNTs in the design of Si anodes. However, native adhesion between Si and sp2 carbon layers has proven to be inherently weak and interfacial delamination remains inevitable during lithiation. After debonded from CNT, Si anodes lose mechanical support and tend to crack across the axial direction, as shown in the left panel of the above figure. To avoid easy detachment between Si and CNT, strong interface can be achieved by functionalizing CNT surface by carboxylic functional groups. This treatment allows the growth of discrete amorphous silicon beads symmetrically threaded along the CNT (as shown in the right panel). In the absence of delamination, the tensile stress in the Si anode is effectively mitigated due to the strong mechanical constraint of the chemically tailored Si-C interface. As a result, the axial tensile stress in the a-Si bead is never high enough to initiate cracks in the unlithiated Si, leading to a crack-free anode design.

C.F. Sun, K. Karki, Z. Jia, H.W. Liao, Y. Zhang, J. Cummings, T. Li, Y. Qi, Y.H. Wang, A beaded-string silicon anodesACS Nano, 7(3), 2717-2724 (2013) 

2.  Deformation Behavior: wrinkling-enabled robust anode design

Similar to lithiation of silicon, sodiation of tin anode in sodium-ion batteries occurs in a similar fashion. A composite anode of tin film coated on a cellulose fiber is made, as shown in the above plot. As the system is charged, Na ions insert into Sn film and induce compressive stresses in the film. The compressive stress increases with the degree of sodiation. Given the large aspect ratio of the Sn film and large Sn/wood fiber stiffness ratio, the initially smooth morphology of the thin Sn film becomes unstable and wrinkles when the compressive film stress is sufficiently high. The formed wrinkling pattern can effectively release the sodiation/desodiation induced mechanical stresses and prevents pulverization of tin anode, which enables a robust anode design for sodium-ion battery.

H.L. Zhu, Z. Jia, Y.C. Chen, J.Y. Wan, N.J. Weadock, Y.Y. Li, O. Vaaland, X.G. Han, T. Li, L.B. Hu, Tin anode for sodium-ion batteries using natural wood fiber as a mechanical buffer and electrolyte reservoirNano Letters, 13(7), 3093-3100 (2013) 

3. Lithiation-kinetics: hollow silicon anode with enhanced lithiation kinetics

Lithiation-induced stress field could affect the driving force of lithiation: Evolving stress field across the reaction front acts as an energy barrier and retards the lithiation reaction. Therefore, it is expected that the contribution of stresses to the driving force for Li-Si reaction results in the observed slowing of reaction front in solid particle silicon anodes. Intriguingly, it is shown that energy barrier due to stresses in hollow silicon particle anodes is much lower than that in solid ones, which may act as a possible explanation of enhanced lithiation kinetics in hollow silicon anodes. The reduced energy barrier arises from the stress fields dictated by the presence of inner surface.

Z. Jia, T. Li, Stress-modulated driving force for lithiation reaction in hollow nano-anodesJournal of Power Sources, 275, 866–876, (2015)

chenlei08's picture

Dear Huck Beng, Kejie, Haoran and Jia

Thank you very much for posting all of your wonderful works. I have learned a lot. Here, I would like to discuss a topic that is more electrochemical, i.e., lithium dendritic growth. Below please see the brief introduction of our recent work  

L. Chen, H.W. Zhang, L.Y. Liang, Z. Liu, Y. Qi, P. Lu, J. Chen, and L.Q. Chen, “Modulation of dendritic patterns during electrodeposition: A nonlinear phase-field model”, Journal of Power Sources, vol. 300, pp. 376-385, 2015 

Electrodeposition has been widely observed in numbers of applications such as electroplating, electroforming, electrocorrosion and battery charging. However, dendrites characterized as multilevel branching usually occur at the electrode-electrolyte interface during electrodeposition processes if they are not carefully controlled. Such dendrites generated far from equilibrium have also fascinated scientists for decades due to their important effects on physical and chemical properties of the electrodeposition systems and the performance of electrochemical devices.  For example, Lithium (Li) electrodeposition on a Li-metal electrode often takes place in high capacity Li-O2 (lithium-oxygen) and Li-S (lithium-sulfur) batteries. These newly developed high capacity lithium batteries, however, still suffer  from unexpectedly failure by short-circuiting via the dendrites that grow even across electrodes upon recharging.

A nonlinear phase-field model, accounting for the Butler-Volmer electrochemical reaction kinetics, is developed to investigate the dendritic patterns during an electrodeposition process. Using lithium electrodeposition as an example, the proposed model is first verified by comparison with the Nernst equation in a 1D equilibrium system. The nonlinear electrochemical kinetics is also confirmed at non-equilibrium condition. The dendritic patterns are examined as a function of applied voltage and initial electrode surface morphology. A design map is proposed to tailor the electrode surface morphology and the applied voltage to avoid undesired dendritic patterns.

                                                                            

hbchew's picture

Very beautiful patterns. Congrads on this nice work.

Kejie Zhao's picture

Lei,  Thank you for introducing your nice work!  Could you please lecture us the mechanism of dendrite growth? why is it particular for Li metals? and how to implement the growth mechanism into the modeling?  Thank you!

-Kejie

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