1. Prepared By: Ryan Mulkey, Zack Montoux, Aaron Milhorn, Colin McDade
Nanoscale silicon as anode for Li-ion batteries: The fundamentals, promises, and challenges
Prepared By: Ryan Mulkey, Zack Montoux, Aaron Milhorn, Colin McDade
Group 13

2. Reasons for the Rise of Nanoscale Silicon Anode for Li-Ion Batteries
Increased demand for electric and hybrid cars
Reduced prices of such cars.
Global revenues from Lithium-ion batteries can rise to as high as 53.7 billion dollars.
Graphite cannot meet the high energy demands of the world.
One of the most abundant elements in the earth's crust.

3. Comparison of the different battery technologies in terms of volumetric and gravimetric energy density
Li is the most electropositive, smallest, and least dense of the metals
As such, storage cells with the highest energy density are Li-based
Lithium Ion batteries result in more than 63% of portable battery sales
Ni-Cd and lead Acid batteries are restricted to specialty uses
Ref [2]

4. Li Metal Battery Design and Problems
Cathode is composed of Li host material such as LiCoO2 or TiS2 with intercalated Li
Anode is Li metal
Advantages:
Most efficient anode selection because there is no host material ‘dead weight’
Li anodes are easy to produce
Ref [2]
Disadvantages:
As shown in the figure, repeated electroplating cycles from successive charging and discharging cause uneven dendritic growth on the surface of the Li anode
Dendritic growth leads to dangerous explosion hazards and increased risk of a short circuit

5. Li-ion Battery Design and Importance of Anode Selection
Conventional Design:
Anode is composed of another lithium hosting material, typically graphite
Cathode is composed of a lithium hosting metal oxide or LiCoO2
Conventional Design Problems:
Low Capacity
High Cost
Limited Power Output
Low natural abundance
Ref [2]
Improving the Design?
Increased demand for higher energy efficiency, power, and battery longevity
Efforts to improve the energy capacity of cathode materials have been relatively unsuccessful
Metal and metalloid anodes that can be electrochemically lithiated to form alloys present superior alternatives to graphite anodes

6. Importance of Anode Selection
Total Specific Capacity (mAh/g)
The specific energy capacity of a cell is given by:
QM is a function of cell dimensions and geometry
Ccathode is relatively fixed due to lack of suitable cathode alternative materials
Thus, the most reliable way to improve the capacity of the cell is by improving Li ion anode technology
The figure to the right shows effect of increasing anode specific capacity on the total cell capacity for particular cathode specific capacity values
At a critical anode capacity of about 1200 mAh/g, the total capacity is maximized
Ref [1]
Anode Specific Capacity (mAh/g)

7. Promise of Silicon as an Anode
High Li insertion capacity (as high as 4.4 mol Li/mol Si) for a possible storage capacity of 4200 mAh/g
10x that of graphite anode (372 mAh/g)
Si anode has a higher lithiation voltage plateau
Reduced potential for dendrite formation (still a possibility in Li-ion batteries)
Highly abundant in Earth’s crust
Besides portable electronics in general, high storage capacity materials are of particular interest for the development of hybrid/electric vehicles

8. Crystalline Silicon Diamond cubic close-packed crystal structure
Lattice constants of Angstrom
Atomic Radius 1.18 Angstrom
Atomic Density: 5.02 x 1022 cm-3
Density: 2328 kg/m3
Thermal Conductivity 0.46 W/cmK
Two interpenetrating face- centered cubic
Crystalline silicon is produced from naturally occurring silicon oxides through a series of reduction reaction
Outer shell of Si atoms has 4 valence electron
Ref [4]
The figure below shows the insertion mechanism of Li into crystalline silicon by peeling the atomic layers along the (111) plane
Ref [4]

9. Silicon-Lithium Phase Diagram
Crystalline Si-Li alloy phases can be formed formed by heat treatment.
LiSi, Li12Si7, Li13Si4, Li22Si5 and more
The phases above have lower Gibbs free energy than amorphous alternatives.
During real electrochemical lithiation, these phases do not form despite lower Gibbs free energy due to unique directional kinetics of the reaction
Ref [4]
Ref [4]

10. Electrochemical Lithiation of Crystalline Si Anode
Li is electrochemically inserted into crystalline Si to form an interfacial LixSi layer
The figure below shows the insertion of the Li and Si ions that is approximately 1 nm thick
Ref [4]
Because lithium inserts along the (111) crystallographic plane, irregularities of the crystallographic boundary of the Si material impede the formation of a crystalline LixSi layer
As lithiation reaches its critical value (x>3.75), the LixSi layer does reorient itself into a crystalline structure

11. Challenges of a Silicon Based Anode
Difficult to apply to an industrial scale due to the large volume change that occurs during the lithiation of Si (figure on the right)
The successive changes in volume from repeated charging/ discharging lead to:
Thickening of the SEI
Cracking of the anode material (see figure on the left)
Coalescence of lithiated Si particles
These factors contribute to the fading of the capacity of the battery system of the system with successive charging and discharging
Ref [4]
Ref [4]

12. Thickening of the SEI
Since the potential of the Si anode is typically below ∼1 V versus Li/Li+, the decomposition of the organic electrolyte at the electrode surface is thermodynamically favorable
The decomposition product forms a layer on the electrode material surface called the ‘‘solid-electrolyte interphase’’ (SEI).
This layer needs to be dense and stable, and it should be ionically conducting and electronically insulating in order to prevent further side chemical reactions from occurring.
The SEI stability at the interface between Si and the liquid electrolyte is a critical factor for obtaining long cycle life. However, the large volume change makes it very challenging to form a stable SEI.
The SEI formed in the lithiated (expanded) state can be broken as the particle shrinks during delithiation. This re-exposes the fresh Si surface to the electrolyte and the SEI forms again, resulting in thicker and thicker SEI upon charge/discharge cycling.
The thickening of the SEI consumes electrolyte and reduces the capacity.
Ref [3]

13. Pulverizing of the Si Domain
Lithiation causes anisotropic swelling in the amorphous domain of the Si anode
This swelling generates stress in the Si matrix leading to cracks
If the particle is large enough, it will shatter and explode, or pulverize, as shown in the image
The fracturing of the Si anode makes new surfaces upon which the SEI must form, consuming electrolyte and reducing capacity
Ref [4]

14. Effects of Anisotropic Swelling (cont.)
Smaller Si anode particles can reduce the cracking effect of anisotropic swelling
Volume expansion causes the boundaries of these particles to overlap with increasing lithiation
As these particles coalesce (shown in figure), the surface area ratio of the Si anode is reduced and storage density suffers.
Repeated anisotropic swelling from charging and discharging generally reduces the electronic contact in the LixSi layer, which causes Li particles to become increasingly trapped within the matrix, reducing storage density.
Ref [4]

15. Limiting Lithiation Due to Internal Stress
Experiments have been performed that show that the lithiation speed of Si nanostructures is not constant.
Generally, the lithiation is quick in the initial state, and slows down after
This decrease can be attributed to the internal stress caused by volume expansion
In the bottom figure, the majority of the lithiation occurs in the first 3600 s
Ref [4]

16. Lowering of Capacity due to Anisotropic Swelling
Due to the consequences of anisotropic swelling discussed, repeated. charging/discharging cycles reduce the storage capacity of the battery.
Si anode technology that decays in capacity at a slower rate is desired.
The figure hints at a possible solution: reducing the particle size of the Si anode material slows the decay in capacity.
Ref [1]

17. Differential capacity plot of Li/Si cell
The lithiation peaks are gradually shifted to the left .
Left shift is characteristic of increased polarization upon cycling.
The shifting of the peak causes the cells to obtain the cutoff potential earlier.
Cut off results in incomplete lithiation of the silicon anode with cycling.

18. Tracking the capacity of the battery with successive charging indicates that the loss in potential occurs with delithiation, which agrees with the mechanism for decay described.
Expansion of silicon electrode during lithiation results in large surface area change.
Electrolyte decomposition continues to occur on the exposed silicon surface.
Coulombic efficiency is poor because an increase in surface area is due to an increase in volume which results in cracking and worse performance.
Ref [1]

19. Stabilizing the SEI
One solution to the decay of Si anode capacity tackles the SEI thickening problem,
The resistance of the SEI layer is a critical factor impacting delithiation and cycleability.
The capacity retention of the silicon electrode is improved upon incorporation of the anode SEI stabilizing additive FEC.
The FEC has been reported to generate a more stable SEI on the surface of the silicon electrode following delithiation which suppresses further electrolyte decomposition.

20. Possible Solutions with Nanostructures
At a length scale of ~150 nm or less, the effects of anisotropic swelling are small enough to prevent pulverization of the Si anode particles and the corresponding decay in capacity.
These nanostructures can effectively withstand the stress induced by heterogeneous changes in the volume of Si anodes.
Various structural designs of Si materials have been proposed to overcome the challenges of a Silicon anode battery.

21. Solid Silicon Nanowires
The nanowire array provides sufficient empty space between adjacent nanowires to accommodate the volume change associated with alloying and de-alloying of Li.
Conductive metal current collector binds bases of wires, allowing for all nanowires to contribute to the capacity.
The Si nanowires feature continuous one-dimensional electronic pathways, allowing for efficient charge transport and making conductive carbon additives and polymer binders unnecessary. This is in contrast to the inefficient hopping of electrons between particles in traditional slurry-coated battery electrodes.
The figure above shows the nanowires bonded to the metal charge collector.
The figure below shows the high cycling stability of the capacity of the nanowire battery.
Capacity (mAh/g)
Cycle
Ref [3]

22.
Anisotropic expansion is attributed to the interfacial processes of accommodating large volumetric strains at the lithiation reaction front that depend sensitively on the orientation of the crystal.
This anisotropic swelling results in lithiated Si nanowires with a dumbbell-shaped cross section, which develops due to plastic flow and a resulting necking instability that is put on by the tensile stress buildup in the lithiated shell
The plasticity driven morphological instabilities often lead to fracture in lithiated nanowires

23. Hollow Silicon Nanostructures
Compared to solid structures, hollow structures provide empty interior space to accommodate the the volume expansion, resulting in much lower diffusion induced stress.
Lower stress values mean that the hollow nanostructures will fracture even less readily.
This leads to even better cycling stability than solid nanowires.
Hollow Si nanotubes increased the surface area accessible to the electrolyte by exposing both the interior and the exterior of the nanotubes to Li intercalation.
The nanotube electrodes demonstrate reversible charge capacities of ∼3200 mAh/g with capacity retention of 89% after 200 cycles at a rate of 1 C in practical lithium ion cells with improved rate.
Ref [3]

24. Clamped Hollow Silicon Nanostructures
Also known as double walled Silcion nanotubes
Similar to hollow nanostructures, but incorporates outside coating layer of mechanically strong SiOx through which Li can still pass
Outer layer prevents outward volume expansion of the nanowire, while the high aspect ratio of the interior electrolyte wetting
Net result is the stabilization of the SEI through successive charge cycling
Has similar nano-scale effect of reducing stress related to volume expansion
By preventing fracture AND stabilizing the SEI, these structures demonstrate unusually high capacity retention (88% after 600 cycles)
Ref [3]

25. Cycling Performance of Clamped Hollow Silicon Nanostructures
The highest reversible capacity and capacity retention were seen in the anode prepared at 1000 °C
When charged–discharged between 0.01 and 1.5 V, the 1000 °C anode showed an initial reversible capacity of 700 mAh g−1 and a reversible capacity close to 620 mAh g−1 in the 200th cycle
This is the longest cycling stability over time so far reported for powder-based Si anodes.
The good cycle performance was attributed to the structural stability of the electrode along with carbon deposition
Ref [1]

26. Additional Silicon Nanostructures for Anodes with High Capacity Retention
Si Nanoparticles:
Small enough to withstand stress from volume change
Challenging to bind to the current collector
Produce moderate charge capacities due to the requirement of binder material
Demonstrate stable cycling
Yolk-Shell Structures:
Rigid carbon coating surrounds Si Nanoparticles
Rigidity of the coating prevents volume changes and resulting thickening of SEI
Results in much higher and stabler charge capacities compared to uncoated nanoparticles
Ref [3]
Ref [3]

27. Native Oxide Layer
Si nanoparticles naturally have an amorphous SiOx layer at the surface that can block electronic contact necessary for the electrochemical lithiation
This causes the formation of Li2O crystalline regions at the interphase of the anode, as shown in the figure
Although gaps between the crystalline regions allow for electronic conductivity, the volume expansion and electron diffraction observed indicate far from full lithiation
Blocking of electronic contact, typically by the Si layer, thus reduces the storage capacity of the cell
Methods for improving the conductivity are thus desired
Ref [4]
Li2O crystalline regions

28. Improving Conductivity for Speed and Efficiency
Si has a valence of 4, and thus electrical conductivity can be improved by adding P-doping materials.
Increased electrical conductivity speeds up the reaction kinetics by several orders of magnitude.
In the figure to the right, P-doped Si nanowires undergo such fast lithiation that the resulting tensile stress from volume expansion causes the characteristic nanowire coiling observed.
Similar conductivity effects can be observed through carbon coating of nano-silicon or the incorporation of a conductive polymer.
Ref [4]

29. Additional Techniques for Preventing Stress Effects
Producing nano-structured silicon anode material is not the only way to prevent energy capacity decay due to stress from volumetric expansion, shown in the figure, and contraction
Four additional methods have been proposed for efficient reusable Si anodes
Si dispersed in an inactive matrix
Si dispersed in an active matrix
Si anodes with different binders
Si Thin Films
The majority of these techniques still involve nano- or micro- structured Si of some form.
All techniques seek to accommodate the volumetric expansion of larger Si materials.

30. Si Dispersed in an Inactive Matrix
Inactive material provides a support matrix for finely dispersed Si
Material acts as a buffering layer to offset the volume changes of the Si due to lithiation
Material should have high mechanical strength to accommodate the stress of Si volume change, as well as high conductivity to facilitate electron transfer throughout the matrix
Metallic compounds such as TiN and SiC have demonstrated suitable properties for this application
While the inactive matrix suppresses volumetric expansion effects, it also reduces the Li ion diffusivity, limiting retention of capacity through cycling
The figure below shows materials commonly used for the inactive matrix and highlights the much smaller Li ion diffusivity of this method
Ref [1]

31. Si Dispersed in an Active Matrix
Unlike the previous method, the an active Si dispersant matrix takes part in the lithiation reaction to form alloys of its own.
Active matrix dispersant solves the problem of low Li ion diffusivity observed with Si anodes dispersed in an inactive matrix.
It can be difficult to find an active dispersant material that exhibits good material properties.
Carbon is used as an effective active matrix over most metals due good Li ion diffusivity and favorable mechanical properties for volume expansion stress.
Nano-structured silicon dispersed on meso-carbon-microbeads through a complicated binding process demonstrate the best capacity retention performance of all Si anodes besides certain thin films (figure).
Ref [1]

32. Varying Si Powder Binder
Si powder binding material can be tailored to accommodate the volume swelling of Si micro- and nano- materials.
PVDF is a very frequently used binder for Si powder electrodes.
Crosslinking of the PVDF increases the mechanical strength and ability to contain volumetric expansion of the Si particles, improving capacity retention.
Investigation into electrically-conducting binder materials with superior mechanical properties (higher elongation-to-breakage ratio and lower modulus).
SBR binder material demonstrates these properties very finely, and the figure shows the drastic improvement in performance that results.
Ref [1]

33. Si Thin Films
Nano-thin Si films provide the nano- scale dimensions needed to reduce volumetric swelling effects while providing excellent contact with the current collecting material.
Very effective in enhancing capacity retention.
Mechanism behind lithiation and delithiation processes are poorly understood.
Rigidity of film suppresses internal volumetric expansion and thereby retards lithiation.
Finely tuning the properties of the film, such as thickness, drastically affects the cycle performance.
Figure shows the development of cracks and “islands” that diminish performance.
Despite this decomposition, Si thin films have been shown to reach unrivaled retention rates.
Ref [1]

34. Novel Self-Healing Polymer Approach
Si particles are surrounded by randomly branched amorphous polymer with strong H-bonding.
Volumetric expansion of the Si is buffered by the relaxation and reorientation of the H-bonds of the polymer around the expanding particle.
Accommodation of the anisotropic expansion substantially increases the life cycle of even millimeter-sized Si anode particles.
Scheme 1 in the figure shows the failure of typical binder material (blue) in preventing preventing surface increases from fracture and subsequent SEI formation
Scheme 2 shows the application of the self healing polymer in containing the fractured particles and maintaining surface area such that additional SEI formation does not occur.
Ref [4]

35. Novel Mesoporous Si Sponge Anode Material
Nano-structured mesoporous Si sponge material.
Increase in the volume from anisotropic swelling is offset by a reduction in the pore volume.
Overall volume expansion of the sponge material is reduced to only about 30%.
Volumetric expansion thus occurs within the material rather than outwards.
The overall effect of this phenomenon is to minimize the stress associated with volume expansion despite still using bulk Si material.
The mesoporous Si anode retains more than 80% of its moderate capacity after 1000 cycles - a huge improvement over conventional bulk Si anodes.
Ref [4]

36. Challenges Facing the Implementation of Nanoscale Li Ion Technology
While the ideal Si anode maximizes both energy storage capacity, in the figure, and retention of that capacity through successive charging/discharging cycles, implementation of the technology is constrained by three important factors
Scalability to industrial production
Many of the Si anodes discussed require many complicated and finely tuned steps in producing the highly ordered nanostructures required
Economic Feasibility
Many of complex specialty materials required to produce the highly functionalized Si anodes are too expensive for use outside of the laboratory
Environmental Harm
Fortunately, silicon nanomaterials are safe due to favorable biocompatibility, but many of the other materials described are not
Another challenge with nanoscale Li ion technology that has been indicated is the lack of understanding of many of the mechanisms behind it, leading to the potential underestimation or complete overlook of possible safety hazards
Ref [2]

37. Summary
Li-based battery technology is an important focus of research due to the potential for rechargeable batteries with critical energy storage density
Li-ion batteries (figure on the right) were developed using graphite anodes as an alternative to lithium metal (figure on the left) which forms dangerous dendritic growth
Alloy-forming metals and metalloids, particularly Si, can accommodate more Li particles per unit mass and thus present the opportunity for higher energy storage density than graphite anodes
Ref [2]
Ref [2]

38. Summary (cont.)
Unfortunately, electrochemical lithiation of Si causes anisotropic volumetric swelling, leading to:
Thickening of the SEI (figure on the left)
Cracking of the anode material (figure on the right)
Coalescence of lithiated Si particle
The combined effect of these phenomena leads to severe degradation in energy storage capacity of the Silicon anode Li-ion battery with successive charging/discharging cycles
Replacing the bulk Si anode with nano- and micro- structured Si and Si composite anode material can significantly reduce the detrimental effect of volume swelling
Ref [4]
Ref [3]

39. Summary (cont.)
Five types of methods have been proposed for reducing capacity degradation in efficient reusable Si anodes
Nano-scale Si anodes
Nanowires (figure), hollow nanowires, nanoparticles, etc.
Stress due to volumetric expansion minimized at nano-scale dimensions
Si dispersed in an inactive matrix
Inactive dispersant provides buffer for volumetric expansion, but also limits lithiation
Si dispersed in an active matrix
Less of a limit on lithiation, but demands very specific properties
Si anodes with different binders
Si powder binder with favorable mechanical properties is selected to buffer volumetric expansion
Si Thin Films
Capable of producing very high capacity and retention rates despite poor current understanding of the mechanisms
Ref [3]

40. Future Research Suggestions
Some further research topics that could be beneficial:
Li ion batteries in solar cells such as the scheme in the figure
Techniques to regenerate the decayed capacity of an old Si-based anode
Further research into the mechanisms of nano-scale Si lithiation, especially in Si thin film anodes
Suitable dispersant materials with superior mechanical properties
Ways to improve our research:
Investigating individual studies on nanoscale Si anodes rather than just relying on summaries given in review papers
Extend review of nanoscale Si technologies from academic journals to include patents as well

41. References
[1] U. Kasavajjula, C. Wang, A. J. Applby, "Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells", Journal of
Power Sources, vol. 163, pp , 2005
[2] J. M. Tarascon, M. Armand, “Issues and challenges facing rechargeable lithium batteries”, Nature, vol. 414, pp , 2016.
[3] H. Wu and Y. Cui, "Designing nanostructured Si anodes for high energy lithium ion batteries", Nanotoday, vol. 7, pp , 2012.
[4] M. Gu, Y. He, J. Zheng and C. Wang, "Nanoscale silicon as anode for Li-ion batteries: The fundamentals, promises, and challenges", Nano Energy, vol. 17, pp , 2015.