1. Lithium Metal Anodes for Rechargeable Batteries
Group 16
Jeremy Parulian
Ashton Pearce
Katelyn Pearson
Monica Pfeffer
Main Article: “Lithium Metal Anodes for Rechargeable Batteries” 2014
Supporting Articles: “High Rate and Stable Cycling of Lithium Metal Anode” 2015
“The Li-Ion Rechargeable Battery: A Perspective” 2013
“Nanomaterials for Rechargeable Lithium Batteries” 2008
“The Impact of Nanomaterials on Lithium Ion Rechargeable Batteries” 2007

2. Current Status of Rechargeable Batteries
Currently, lithium batteries include different lithium chemistries in the anode material.
Almost all differ in the composition of the cathode material.
These batteries are rechargeable but only reach nominal voltages.
Many are researching solutions to improve the voltage and life of these batteries.
These batteries started being produced commercially in 1989 in the “AA” size.
The “AAA” size followed in 2004.
Currently, these batteries have a life of up to 10 years, significantly reducing waste from used batteries across all consumers.
These batteries work in everyday electronics, but only certain models are suitable for extreme temperatures and energy demanding devices.
The energizer batteries shown to the right have a cathode composed of iron disulfide on an aluminum foil substrate.
The anode is pure lithium metal.
The electrolyte is composed of lithium salt and organic solvent.
The battery contains a vent ball, which is a safety mechanism that provides internal pressure release.
A plastic film label covers the battery as well to electrically insulate it.
The jellyroll construction produces a high surface area for high power by spirally wounding layers of anode, separator, and cathode material.

3. Why Use Lithium (Li) as Anode Material?
Lithium anodes have a very high theoretical specific capacity.
Lithium is the lowest of all materials on the graph (on the right).
Anode capacity of 3,860 mAh/g
Lithium has a low density of g/cm^3.
Lithium has the lowest negative electrochemical potential.
Lithium will transport electrons from anode to cathode quickly.
Lithium’s theoretical capacity is 10 times that of graphite, the current material of the lithium ion battery anodes.
Historically, Sony and Asahi Kasei replaced Li with graphite in anode (example shown in image to the left).
Most commercial Li-ion batteries use carbon in the anode material with battery electrolyte concentrations of ~1 M.
Ionic conductivity peaks at this concentration, and the energy density of these batteries is reaching its limit.
A new anode material is needed to operate at greater electrolyte concentrations.

4. Complications with Lithium Anode
Lithium is thermodynamically unstable with any kinds of organic solvents.
Electrolyte components and Li metal have significant side reactions that lead to low Coulombic efficiency and consume Li metal.
The graphic shows images a-h which correspond to different times as the battery cycles.
The composition of the anode material changes throughout cycling.
The structure of the anode changes immediately from image a to b, and well-defined plates can be seen in h image.
These plates indicate the formation of a solid-electrolyte interphase (SEI) film.
This film leads to high impedance failure of battery.
The graph corresponding to the pictures shows that the voltage across the battery drops immediately after the first cycle.
The graph proves that the SEI film interferes with the battery potential, causing it to fail.
Since lithium has already been proven to be the best anode material, another solution must be found to prevent side reactions in the battery.
The research paper found targets the solvent in the battery, proposing a different solvent that will not result in harmful side reactions.

5. Ether Solvents to Improve Performance of Lithium Anode
Carbonate-based electrolytes (LiPF6-PC) were the electrolytes used previously, participating in the many side reactions that formed the SEI layer.
Ether-based electrolytes (LiFSI-DME) were proposed to have more stable side reactions, avoiding interference with the performance of the battery.
Ether-based (LiFSI-DME) electrolytes were compared to carbonate-based (LiPF6-PC) electrolytes in Cu|Li cells.
The graphic at the right shows the anode after several cycles of the battery.
Scanning electron microscopy was used to analyze the anode as well.
After cycling, the carbonate-based electrolyte (left) formed needle-like structures that will penetrate most separators.
These needle-like structures are formed from lithium side reactions, and they make up a thick SEI layer that is commonly known as Elton’s Grey Layer.
On the right, the ether-based electrolytes maintained their tubular structure without any interference from the copper cathode.
The anode did not change color to dark grey as well.
This experiment showed the ether-based solvents maintained a pristine lithium anode with no side reactions, proving the efficiency of ether-based solvents.

6. Concentration of Ether Solvent
In comparing different concentrations of ether solvents, greater concentrations of solvent performed better.
The graphs show 2 M LiFSI DME, 3 M LiFSI DME, and 5 M LiFSI DME.
As the concentration increases, the current of each cycle becomes more consistent. In 2 M LiFSI DME, cycle 1 jumped much higher in current than the other cycles.
The current efficiency decreased as cycling continued in the 2 and 3 M LiFSI DME.
The reason for that highly concentrated ether solvents perform better is that there is a reduced availability of reactive solvent present.
Less side reactions occur, preventing a harmful SEI layer from forming.
Increasing the lithium ion concentration in the ether solvent enables high-rate lithium plating and stripping, avoiding accumulation of material on the anode.
The consistency of plating and stripping on the surface of the lithium anode allows the cycles to have a more consistent current, proving that higher concentrations of solvent perform better.
Anions are less prone to reduction in side reactions and stabilize the charge density on the lithium metal surface.
The image on the right shows 1-M LiFSI-DME electrolyte in (a) and 4-M LiFSI-DME electrolyte in (b).
The reactive DME solvent molecules are colored light grey, while the anions are colored.
The vivid color in the more highly concentrated solvent shows that the higher concentration reduces the reactivity of the solvent with the lithium anode.

7. Long-Term Cycling Stability
Since we’ve concluded that highly concentrated ether solvents work best with lithium metal anodes, the long-term cycling stability of the 4 M LiFSI-DME electrolyte was tested with the lithium metal electrode.
A high current density of 10.0 mA cm^-2 was used to compare the cycling stability in this test to the cycling stability of the carbonate solvent (shown in slide 4).
The battery potential cycled consistently for over 600 hours. In this time frame, over 6000 charging and discharging cycles were completed successfully.
The cycling stability of these new batteries can now compete with the commercial batteries produced, lasting 10 years.
The battery was found to have a coulombic efficiency of 98% during the cycling test, indicating that only 2% charge was directed toward side reactions to form the SEI layer.

8. Impedance Performance
Impedance is the circuit’s resistance to alternating current.
The image on the left shows three graphs with the impedance spectra of Li|Li cells.
(a) is the impedance performance of 1 M LiPF6-PC, a carbonate-based solvent.
(b) is 1 M LiFSI-DME.
(c) is 4-M LiFSI-DME.
The carbonate-based solvent’s impedance varied drastically as the electrolytes were exposed to the lithium metal.
Both concentrations of the LiFSI-DME solvents showed consistent impedance spectra over long periods of time.
The image on the right is the impedance spectra of a Li|Cu cell with 4 M LiFSI-DME electrolyte.
The spectra was taken before cycling the cell and after various cycles with 1 mA cm^-2 current.
The impedance for the Li|Cu cell decreases upon cycling.
The Li|Li cell with the carbonate cell formed an even more resistive interface with Li metal, proven by the increase in impedance.
The lower impedance for the highly concentrated ether solvent in the Cu|Li cell shows that this solvent works more effectively.
There is less resistance from charging to discharging in this battery, leading to a more consistent performance.

9. Why So Stable? Stable SEI Layer
After running the long term cycling test, investigation was made as to why the battery could cycle for so long without interference.
The reason can be seen in the image to the right.
No porous, needle-like lithium metal can be seen in the electron microscope image as was seen in the SEI layer previously.
Although a SEI layer still formed, the layer is compact and did not impact the performance of the battery.
The layer formed only on the surface of the lithium anode.
Column three in the image shows that the surface has plated but the material underneath is unaffected.
Additionally, no corrosion was observed in the lithium metal, although it can be seen in the copper metal.
The compactness of the SEI layer prevents further lithium corrosion, optimizing the life of the lithium metal in the anode.
This layer forms quickly after the battery begins cycling and then maintains consistency of cycling as time passes.
SEI layer

10. Composition of SEI Layer
Composition of SEI Layer
The SEI layer forms as a result of slow anion degradation.
These anions are mostly inorganic components.
The table at the right shows x-ray photoelectron spectroscopy (XPS) analysis for the composition of the SEI layer.
The concentration of each anion deposited onto the layer stabilizes as cycles increase.
Stabilizes after initial cycles
More highly concentrated ether solvents have more anions present, forming the composition of the SEI layer. This explains why higher concentrations consistently perform better.
The graphs on the left demonstrate the high conductivity of the SEI layer.
The more highly concentrated 4 M LiFSI-DME could produce greater battery potentials at a larger capacity over the 1 M LiFSI-DME solvent.
Additionally, the 4 M LiFSI-DME solvent produced these results over longer cycles.
The increase in potential and current capacity can be described by the SEI layer because it forms for greater concentrated ether solvents.
Additionally, the SEI layer led to 100% efficiency for at least 400 cycles for all currents that were tested. The lower currents could last for more than 1000 cycles.

11. Rechargeable Batteries: The Basics
The figure to the left portrays how a rechargeable battery regains its charge from an external charger.
The switch in the external circuit is disconnected, and a charger is connected to complete the circuit between the cathode and anode.
The secondary cells of rechargeable batteries produce energy through current in the same way as regular batteries.
However, unlike that of a regular battery, the reaction in a rechargeable battery is reversible.
The battery stores energy produced from the electrochemical reaction in the anode and cathode, and the electrolyte that separates the two electrodes then forces the electric energy to the circuit outside of the cell, where a current is then produced
Through applying an external voltage and current from the battery charger, the electron flow from the anode to the cathode that occurs when the battery is being use is reversed.
This electron flow reversal is depicted in the figure to the right.
As electrons are cycled back to the anode due to the current of the charger, the battery’s charge is restored.
The cathode now has a positive charge, and the anode is negative as they were before the reaction within the cell had occurred.
The figure above shows how rechargeable batteries connected to a battery charger.
The batteries are attached to the charger at their positive and negative terminal ends, allowing the current from the charger to run through them and reverse the electrochemical reaction in the secondary cell.

12. Electrode Chemical Reactions
The two types of reversible chemical reactions that can occur at solid electrolytes in reversible batteries are displacement and insertion reactions.
While insertion reactions may occur in solid cathodes and anodes, solid anodes primarily undergo displacement reactions.
The general mechanism for a displacement reaction is depicted in the figure below.
Lithium reacts with one component of the alloy in the anode, replacing the other component.
The reaction for this process is: Li + xMNy → LiMx + xyN , where Li displaces N at the anode
Displacement reaction anodes allow the rechargeable battery to have a higher capacity and to charge safer and faster.
The above figure shows the process that occurs at cathodes and anodes for insertion reactions.
In these types of reversible reactions, Li is added to the crystal structure of the electrode during the discharging phase.
Li ions from the cathode are then removed and inserted into a discharged anode during the charging process.
Insertion reactions at electrodes minimize volume expansion during a reaction, which improves the overall life cycle and maximizes energy density of rechargeable batteries.
Dendrite formation can also be prevented through reversible Li insertion.

13. The Self-Healing Electrostatic Shield Mechanism
The self-healing electrostatic shield mechanism is strongly influential in completely eliminating dendrite formation on a Li metal anode.
The basic process for this mechanism is depicted in the figure to the left.
Image (a) shows the initial absorption of both Li and non-lithium cations into the substrate surface
In (b), an Li protuberant tip begins to form on the surface because the substrate surface is uneven
As shown in (c) and (d), the high electron-charge density gathering at this tip attracts the non-lithium cations.
The non-lithium cations cluster around the Li tip in (d) act as an electrostatic shield, protecting the tip from additional Li ions adsorbing into it, which would lead to dendrite formation.
Unlike mechanical blocking as a method to prevent dendrite growth, this shield mechanism does not depend on the protective strength of a blocking layer, but it uses an electrostatic shield to drive away Li cations.
The image below shows the effect of this self-healing electrostatic shield mechanism within the battery cell at three different concentrations of the non-lithium cation.
Even at the lowest concentration of .005 M of this cation in (b), there is a significant decrease dendrite growth on the anode compared that of (a) when the cation concentration was zero.
At a concentration of .05 M in (c), no noticeable dendrite formation is observed.
From its effectiveness at completely eliminating dendrite growth at Li metal anodes, this electrostatic shield mechanism proves to be a significant breakthrough in improving rechargeable Li batteries.

14. Li Battery Energy Capacity
The figure to the left shows the capacity of a particular rechargeable battery at two different depths of discharge (DOD) as a function of the number of discharging and charging cycles it has gone through.
The depth of discharge of a battery is the amount of energy the battery has that is discharged for a cycle before recharging.
For the battery that was completely discharged, its capacity significantly decreases as it undergoes more charging and discharging cycles.
This is due to the Li dendrite growth caused by deep cycling and quick charging, which is known to lower battery capacity and potentially short circuit the battery cell.
The battery with a 50% DOD has a long lifecycle and consistent capacity throughout due to slow charging of the battery in testing and a low depth of discharge.
The irreversible loss of capacity shown by batteries in this graph can also be attributed to changes in electrode volume or composition and chemical side-reactions between electrodes and the electrolyte during discharge or charging.
The figure to the right depicts a problem with a common type of rechargeable battery, the Li ion battery that affects the battery’s capacity.
During the cycling process, a permeable solid electrolyte interface (SEI) layer of lithium ions will form along the surface of the anode, lowering the capacity of the battery cell as well as its overall life cycle length.
However, in the case of Li metal rechargeable batteries, the presence of an SEI layer on the Li metal anode helps to increase the battery’s life cycle and capacity.

15. Energy Storage of Rechargeable Batteries
The figure to the right explains the voltage of a rechargeable battery as it relates to its energy.
The energy gap between the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbital for an electrolyte is called its energy window.
This window is expressed as Eg in the figure.
The voltage of a cell (Voc in the figure) is limited by this electrolyte window and is calculated as the difference between the potentials of the anode (μa) and cathode (μc).
The cycle life for a rechargeable battery with a given voltage is determined by the electrolyte window.
Of all the commonly used secondary cell types in rechargeable batteries, Li batteries have the highest mass and volume energy densities as shown in the figure below.
This is because Li rechargeable batteries use organic electrolytes with a larger LUMO and HOMO difference, which increases the stored energy density for a given time frame in the battery.

16. Other Types of Rechargeable Batteries
The general structure of a Li-S cell is shown in the top left image, and the Li ion battery cell structure and mechanism explained previously is displayed in the top right image.
Unlike Li ion batteries, Li-S batteries do not have lithium ions intercalated into the electrodes.
Instead, in Li-S cells, the lithium ion is released from the anode during discharging.
The energy density of the Li-S battery is dramatically greater than the Li ion battery, making it more efficient and practical to use in application.
Of the rechargeable battery types currently in development, the Li ion battery has been the most successful.
Significant improvements to the Li-S battery in recent years make it a possible contender to the Li ion battery.
•Depicted in the graph to the left is the relationship between the energy density of the battery to its total life cycle.

17. Driving with Rechargeable Li Batteries
The portable rechargeable Li battery is considered a possibility to replace the internal combustion engine to power vehicles.
Rechargeable batteries are currently being used with some car batteries as shown in the bottom left image.
Because of the limited energy capacity of rechargeable Li batteries, thousands of cells like the one in the bottom right image must be used to power one car.
Ways to increase the energy density of rechargeable batteries so that it can mimic the energy production and power output of the internal combustion engine are being sought out.
Because the cathode voltage is currently limited to 5 V, increased energy density to power an electric vehicle would require a significant increase in cathode voltage capacity.
The above figure compares cars that run on rechargeable Li batteries to those with internal combustion engines.
The primary downsides of the rechargeable battery are the time it takes to charge and the low mile range.
Improving the energy capacity of the battery could increase its range
Rapid charging time significantly contributes to dendrite formation, so methods to eliminate dendrite formation in these batteries must be used to decrease charging time.

18. Mechanical Blocking and Additives
A commonly used technique to suppress dendrite growth is by mechanically blocking the dendrites with a strong electrolyte layer.
In the mechanical blocking process, a blocking material may be added to the solid electrode or the electrolyte to protect the Li from dendrite formation.
Polymer electrolytes, especially for polymers of high molecular weight, are also commonly used in blocking dendrite growth.
The figure to the right shows how polymer backbones in the battery electrolyte works in the battery during charging and discharging.
To further prevent dendrite formation, additionally additives may be introduced to the electrolyte.
Because these electrolyte additives have high reduction voltages, they promote the formation of SEI films on the Li metal anode surface, hindering dendrite growth.
The figure to the left displays this phenomenon as additive (VC) is added to the electrolyte (EC).
This additive VC rapidly reacts with the Li anode, forming a SEI film layer to block dendrite growth faster than the electrolyte alone could.

19. Complications of Development
One primary drawback to developing and commercializing effective rechargeable Li metal batteries is the growth of dendrites on the anodes.
As depicted in the figure to the right (c), dendrite formation on Li metal anodes may cause safety hazards and impede the usefulness of these batteries.
Growth of dendrites on an anode can cause the battery to short circuit, increasing the current density passing through a high enough value to melt the electrode.
Image (c) also addresses the relatively low Columbic efficiency of Li metal rechargeable batteries, which causes them to have a decreased cycle life and low energy density.
Unlike Li metal batteries, Li ion batteries do not experience the same dendrite deformation as depicted in the differences between (a) and (b) in the figure.
Dendrites are formed after repeated charging and discharging of the battery.
The figure to the left shows the morphology of the rechargeable battery that occurs at various positions of the Li metal anode after one charge due to dendrite formation
Dendrite growth affects the battery at all points, particularly at each electrode, and it is strongly connected to overall battery failure and inefficiency
The surface morphology characterization in this image observes the changes in Li electrodes that take place during cycling
In the figure, images (b) and (c) show the rapid deterioration of the interface on the Li surface caused by dendrite formation, which also results in decreased battery capacity.
Attempts to suppress dendrite growth require a deeper understanding of the relationship between growth rate and applied current density. The growth rate of dendrites increases with increasing current density.
Increasing the shear modulus of the electrolyte to about twice the value of the Li anode can also reduce dendrite growth.
Research found that pulse charging a Li metal battery, rather than using a galvanostatic charge style, decreases dendrite growth by up to 96%

20. Polymer Electrolytes A way to prevent dendrites from forming
Cross linked block copolymers consisting of soft polymer blocks and hard polymer blocks blocks can reduced dendrite growth.
The rigid network has a high shear modulus which provides mechanical strength against the dendrite growth while the soft block of the polymer forms lithium conducting nanotube channels.
The dendrites are typically larger than ten nanometers.
This causes the dendrites to be blocked from passing through the soft block of the polymer.
The figure shows the nanotubes formed by the polymer that allow for the condition of lithium ions.
Drylyte is a nano-structured polymer electrolyte containing cross linked soft block copolymers of polyethylene oxide for the formation of lithium ion nanochannels with a width of about ten nanometers.
The drylyte batteries can safely use higher density energy for a longer time span than regular lithium batteries because of the nanotube channels preventing dendrite penetration.
This is shown in the graph to the right.
The application temperature for this method of dendrite prevention is degrees Centigrade which restricts its practicality to stationary use.

21. Surface Coating
One form of dendrite growth can be prevented by exposing the lithium electrode to tetraethoxysilane which creates an artificial solid electrolyte interphase layer.
This layer is meant to mechanically block the growth of the dendrites on the lithium metal surface while also allowing the lithium ions to redeposit on the lithium metal electrode.
Exposing lithium metal to tetraethoxysilane forms a thin silica film on the lithium metal surface.
The silica acts as a surface coating on the lithium metal surface.
The film acts as a protective layer that allows lithium ions to redeposit while preventing dendrite growth through mechanical resistance.
The figure shows cross section scanning electron microscope micrographs of the treated lithium electrodes.
(a) is the lithium before treatment.
The label “Face” is representing the coated lithium metal surface.
The coating separates the lithium ions from the lithium metal face and acts as a protective interface.
(b) is the lithium with a higher magnification show the coating from a closer perspective allowing for a more noticable difference in the scanned images from the bottom left to the bottom right.
(c) is the lithium after 1000 seconds of stripping of the lithium metal adjacent to the surface coating.
The gap created is indicated with a black arrow and a dark area between the lithium metal and the tetraethoxysilane coating.
A label of “Removed Li” also indicated the stripping of the lithium metal.
(d) is the lithium after 1000 seconds of stripping the lithium surface adjacent to the coating and second of depositing of lithium ions back onto the lithium metal surface adjacent to the surface coating.
The image shows that it is pure lithium ions that are deposited back onto the lithium metal surface, no dendrites are noticeably visible, even with the extra magnification that carries throughout the images after the first.

22. Polymeric Single Ion Conductors
To have maximum electrochemical performance for lithium metal anodes, the transference number of the lithium ion need to approach unity.
This can be accomplished by adding nano-particle fillers.
Tin oxide (TiO2) and silicon oxide (SiO2) can increase the lithium transference number from to 0.6.
Though the change in transference number is small, the difference in capacity is significant, as shown in the graph.
The graph on the right shows the charge and discharge capacities of lithium particles with and without the tin oxide.
Even after 20 cycles, the tin oxide nanoparticle fillers increase the charge and discharge capacities of the battery.
The figure below show the tin oxide as deposited vs the heat treated tin oxide
Although the appearance of the heat treated is rougher, the specific capacity discharge is increased.

23. Inorganic Hybrid Electrolytes
Another way to delay the onset of dendrite formation would be to add inorganic fillers, acid-treated silica particles, to the electrolyte of the lithium metal anode rechargeable battery..
This is due to the lower interfacial resistance and increased conductivity from acid-treated silica nanoparticles.
The aggregation of the nanoparticles forms a cross linked structure which, in turn, increases the elastic modulus while also maintaining high conductivity of lithium ions.
The high elastic modulus provides a strong mechanical resistance to prevent the onset of dendrite growth or to keep the dendrite growth to a minimum.
The chemical structure of the single ion triblock copolymer P(STFSILi)-b-PEO-b-P(STFSILi) is shown to the left of the graph.
The graph shows the drastic increase in the elastic modulus for the single ion triblock copolymer P(STFSILi)-b-PEO-b-P(STFSILi) compared to PS–PEO–PS.
Adding inorganic fillers to liquid electrolytes also suppresses dendrite growth.
Nano SiO2 fillers form a mechanically strong interface that stunts the dendrites, forming many small mushroom shaped dendrites instead of fewer, larger dendrites which are more detrimental to lasting battery efficiency.

24. Inorganic Solid State Li-ion Conductors
Thin films of solid state lithium ion conductors are good materials to prevent dendrite formation because of the high mechanical strength and chemical performance.
The pink strip on the figure represents where the thin film would be located (labeled “Interlayer”).
The most widely used inorganic thin film ion conductor is nitrogen doped lithium ion phosphate film (LiPON) that was developed by Bates an Dudney.
The conductivity is 2x10^(-6) S/cm at 25 degrees Centigrade.
The shear modulus is approximately 77GPa.
The mechanical strength of nitrogen doped lithium ion phosphate is independent of substrate type as well as thickness which allows for thin film application.
The graph shows the difference in specific capacitance of batteries with and without the nitrogen doped lithium ion phosphate film.

25. Effect of Substrate Area
Although decreasing the current density is proven to delay the onset of dendrite growth, it is not practical to keep up with the demand for high powered rechargeable lithium metal anode batteries.
However, another effective method of suppressing dendrite growth is increasing the substrate area of the electrode.
Using a graphene nanosheet as for the anode matrix with large surface area would not only delay the onset of dendrite growth, but would also decrease the effective current density which prolongs the life of the rechargeable battery by keeping the efficiency of the battery steady for a larger number of cycle.
The increase in surface area of the graphene matrix is demonstrated by the figure below: the mesh on either side of the blue and green figure act as the graphene nanosheet matrix which noticeably increases the surface area of the figure as a whole.
However, one problem with this method is that the Coulombic efficiency for this electrolyte is less than 80% which is not as efficient as it possibly could be for this type of battery, but it does prolong the life of the lithium metal anode rechargeable battery.
This means that the consumption of lithium ions from the electrolyte would be extremely high for long term operation of the rechargeable battery which would require a large excess of lithium metal used for the battery.

26. CO2/SO2 Cycling
Having carbon dioxide present in the electrolyte can significantly increase the cycling of lithium ions in the battery for every stripping and redepositing cycle of the charging and discharging of the battery.
The addition of CO2 to lithium ions forms Li2CO3 which acts as a good passivating agent because the Li2CO3 acts as a protectant of the lithium ions.
The interfacial impedance in electrolyte containing CO2 is lower than electrolyte without the CO2.
Any dendrites that would be formed are suppressed by the formation of LiCO3.
The figure to the right show the stable form of the lithium ions in contact with the carbon dioxide.
The most stable form of this combination is the center form which “protects” the lithium ions and in turn prevents dendrite formation.
Similarly to the CO2, the addition of SO2 also reacts with the lithium to form a protective film on the lithium electrode.
There a a couple differences in the addition of SO2 compared to CO2.
One difference between the addition of the SO2 versus the addition of the CO2 to the electrolyte is that the SO2 is not as efficient at protecting the lithium from forming dendrites.
Another difference is that the SO2 has a dramatically positive effect on cycling efficiency of the lithium metal anode rechargeable battery.
However, the toxicity of SO2 in its gaseous form prevents its use for many applications and practical purposes.
The toxic form of SO2 is shown in the image to the left.

27. Metal Ions
Another method to suppress the onset of dendrite formation and to prevent dendrite growth once started is to added metal ions to the electrolyte.
It was found that the lithium electrodes had a greatly increased coulombic efficiency in electrolyte solutions containing metal ions with a higher reduction potential than lithium.
Examples of these ions include tin, aluminum, gallium, and bismuth.
The inorganic metal cations electrostatically and chemically form thin layers of deposits on the lithium metal anode and the lithium metal electrode.
This depositing mostly happens on the active points on the lithium metal anode surface which caused a more uniform surface.
The dendrite prevention comes from the uniform surface of the lithium metal anode: which uniform surface coating, the dendrites have fewer opportunity to attach to an active site to begin to form.
In preventing the dendrite growth, the Coulombic efficiency is greatly increased throughout many cycles of depositing and strippingof the lithium ions.
The graph on the right shows the lasting capacity of the thin film with Sn compared to a film without Sn.
The Thin film runs out of maximum capacitance very quickly as compared to the Sn coated film.
One of the problems with using the metal ion in the electrolyte method to suppress dendrite formation is that only certain metal cations are effective in the electrolyte at suppressing dendrite formation while keeping high coulombic efficiency throughout many cycles of the battery.
Alkaline or alkaline earth metals must be reduced before use in the electrolyte. These metal ions may react with the lithium to for alloys that may increase dendrite growth.

28. Effect of Lithium Salts
The surface of the lithium metal anodes are strongly affected by reducing salt anions in the nonaqueous electrolyte.
The salt will react with the lithium metal anode and form a thin protective coating on the lithium metal anode surface.
The thin layer of film that is formed will be uniform across the surface of the anode: this makes it more difficult for the dendrites to find an active site in which to attach.
This is provide mechanical resistance to the formation of dendrites which will result in the delay of dendrite growth onset and stunt the growth of dendrites that have already formed.
The delay of dendrite growth will increase the efficiency of the lithium metal anode rechargeable battery because the battery will be able to withstand a higher number of cycles before short circuiting and becoming ineffective.
The top graph shows the capacitance of the battery for different electrolyte salt combinations during the discharge stag.
The most effective salts are LiBOB and LiDFOB
The other salt are slightly less efficient at capacitance discharge the top two, but are still acceptably effective.
There are four important attributes of a good salt for the electrolyte of a lithium metal anode rechargeable battery:
The salt must have high chemical stability and also be compatible with the lithium metal so it can react and form the thin film that acts as a barrier to dendrite growth.
The salt must also have a high electrochemical window.
For safety of the battery, the salt must also have high safety and low toxicity when reacting with the lithium metal and ions.
The salt must also have high ionic conductivity in the electrolyte solution: the Coulombic efficiency must remain high for the rechargeable battery to remain effective.
Examples of efficient salts for the electrolyte solution that will prevent dendrite growth while maintaining Coulombic efficiency would include LiBF4, LiPF4, LiSO3CF3, and LiTFSI.
These salts are very reactive with lithium and will result in a thin film.
Salts that are less reactive result in a thicker film that consequently results in higher resistance to conductivity.
The bottom graph shows the capacitance of the lithium battery for different salt combinations in the electrolyte during the charging stage.
The most effiecent salts for the charging stage fall in the mid-range for the discharge stage: LiDFOB and LiBF4.

29. Effect of Lithium Salts
The figure to the left shows the difference between electrolytes with salt and electrolytes without salt.
The top pathway demonstrates the pathway with a low salt content in the electrolyte.
The uneven blue part of the top figure represents the formation of dendrites because of the drastically non- uniform surface.
The salt and lithium reaction did not create a high enough concentration for form a uniform thin film on the lithium metal anode or cathode.
The top two pictures on the far right of the figure are actual images of the destruction caused by the dendrites on the battery: the buildup of dendrites is clearly shown in both images.
The bottom pathway demonstrates a high concentration of salt relative to the solvent.
The blue side of the figure is visibly smoother than in the top pathway.
The lithium and salt reaction resulted in a high enough concentration to fully cover the surface of the lithium metal in a thin protective film.
The dendrite formation is drastically reduced .
The two bottom images on the far right of the image are much smoother compared to the top pathway.
The smoother appearance of the lithium metal prolongs the life of the battery and the efficiency of the battery in each cycle.
Effect of Lithium Salts

30. Nanomaterials in Lithium Batteries
First generation lithium-ion batteries were composed of powders which contained millimeter-sized particles for both electrodes.
However, limits existed because of diffusitivity (10^-8 cm^2/s), which limits charge and discharge rate.
Using nanomaterials with reduced dimensions would have a far higher intercalation/disintercalation rate, where intercalation is the reversible insertion of molecule/ion into compounds with layered structures.
The advantages of nanomaterials in lithium-ion batteries include:
Nanomaterials enable electrode reactions to occur that cannot take place in materials with micrometer-sized particles.
Nanomaterials, having reduced dimensions, significantly increase the rate of lithium-ion insertion and removal.
With reduced dimensions, there is a higher surface area, which permits a high contact area with the electrolyte and hence a high lithium-ion flux across the interface.
The disadvantages of nanomaterials in lithium-ion batteries include:
Nanomaterials are more difficult to synthesize, and it is more difficult to control their dimensions.
While higher surface area leads to higher contact area, it may also lead to significant side reactions with the electrolyte.
The density of a nanopowder is generally less than the same material in the form of micrometer-sized particles.

31. Positive Electrode: Nanoparticles
Nanoparticles can be prepared by grinding, synthesis from solution, or sol- gel approaches.
The rate of lithium intercalation/deintercalation (and therefore charge and discharge rate) is increased bc of shorter diffusion lengths and higher contact area.
However, some problems exist, including:
instability
maintaining good electronic contact
Certain examples include nanoparticulate LiFePO4 , which has the following properties:
low cost
high thermal stability
high chemical stability
lower voltage (3.4 V)
higher electrochemical stability

32. Positive Electrode: Nanodomain Structures
For LiMnO2, removal of 50% of lithium induces conversion to a spinel structure.
It is possible for manganese and lithium ions to occupy CCP (cubic close-packed) oxide in 2 ways: lithium 8a and manganese 16d, or lithium 8b manganese 16c.
This corresponds to spinel structure, leading to nucleation and growth of spinel nanodomains within the micrometer-sized particles.
Without nanodomain structure, LiMn2O4, the nucleation and growth of the distorted phase on cycling lithium results in poor reversibility.
However, with a nanodomain structure, entire domains are free to spontaneously switch between cubic and tetragonal structures during lithium insertion and removal.
This leads to a dramatic improvement in the retention of capacity on cycling.

33. Positive Electrode: Ordered Mesoporous Materials
Mesoporous materials are micrometer-sized particles containing pores of 2-50 nm diameter, which are identical in size and ordered such that the thickness of the walls between the pores is the same throughout.
Ordered mesoporous materials exhibit similar packing to that of conventional powders.
Internal pores can be flooded with electrolyte, ensuring a high surface area in contact with the electrode, and thus a high flux of Li across the interface.
The thin walls of equal dimensions throughout the material ensures that a shorter diffusion path of lithium ion intercalataion and deintercalation will occur, which leads to high rates of transport throughout the material.
The following graph below shows the charge storage for lithium as a function of cycle number for the latter mesoporous material.
Usually, they are solids based on silicas. However, recently transition-metal oxides have been developed, some of which are mesoporous Co3O4 and LT-LiCoO2 . which are shown in the figure to the right.

34. Negative Electrode: Nanotubes/wires
Carbon nanotubes exhibit twice Li storage compared with graphite.
However, concerns exist with surface-layer formations, and safety.
Carbon nanotubes may be low cost, low toxicity, protected against lithium deposition and fabricated to deliver fast lithium insertion and removal, the attention has been shifted to Li4Ti5O12.
The variation of charge stored in Li4Ti5O12 is shown as a function of cycling rate to the left.
This presents a very feasible option, exhibiting same properties as carbon nanotubes but being significantly safer and alleviating problems of lithium deposition.
The best option so far is TiO2- (B), the 5th polymorph of titanium dioxide. Its structure is shown above. It exhibits the following properties:
Low cost
Low toxicity
High safety
Eliminates Lithium Plating
Increased amount of Li able to be stored

35. Negative Electrode: Nanotubes/wires
The graph below portrays variation of charge of nanoparticulate Li4Ti5O12 in place of graphite.
The capacity to store Li is only half that of graphite (150 m A h g-1 vs. 300 m A h g-1).
The reduced cell voltage of 0 V, compared to 1.5 V, is a result of increased potential of the negative electrode.
Based on the above two facts, there is reduced energy density.
In searching for alternatives that can combine all of the positive aspects of carbon nanotubes without the harmful side effects, Li3Ti5O12 was a convincing candidate.
The graph above portrays variation of charge of Li4Ti5O12 , which is a defect spinel used in intercalation as a host for lithium.
It is non-toxic, and when fabricated as a nanoparticle gives high rates lithium insertion and removal.
Intercalation occurs at potential of 1.5 V: therefore, the lithium deposition problem avoided.

36. Negative Electrode: Nanoalloys (Tin)
Nanoalloys are simply metal particles reduced to nano dimensions.
They are highly sought after because of their ability to store large amounts of lithium.
Different synthetic routes have been used to create nanostructured metals, including sol-gel, ball- mining and electrodeposition.
However, nanoalloys are usually accomplished through electrodeposition.
The figure below depicts electrochemical behavior of nanoalloy tin in lithium cells.
Although nanoalloys can cycle Li better than bulk materials, they are unable to sustain hundreds of cycles necessary for a rechargeable battery, as seen in the falling off of discharge capacity in the graph below.
The figure above shows tin electrodeposited on a copper foil substrate under different conditions.
By selecting a morphology suited for a specific application, electrodes may be enhanced in comparison to conventional, bulk materials.

37. Negative Electrode: Nanoalloys (Nickel)
The figures below shows a Ni3Sn4 alloy before and after cycling.
Before, the nanoparticles are uniformly deposited without any coalescence between them.
The figures above show the voltage profiles of the first two cycles and the capacity delivered of Ni3Sn4 as negative electrode of lithium cell, respectively.
There are no significant signs of decay.
The volume changes exceed %, and reduction of the particle size alone is insufficient.

38. Electrolytes: Amorphous Polymer Electrolytes
Progress in lithium battery technology is dependent on replacement of conventional liquid electrolyte, which may be achieved through lithium-conducting polymers.
Solid-polymer electrolytes have certain drawbacks, including high conductivity only in specific temperature ranges and conductivity due mainly to motion of anion.
Nanocomposite Polymer Electrolytes (NCPE) have been implemented in order to account for these drawbacks.
The figure below shows the Arrhenius plots of an electrolyte containing S-ZrO2 filler, and the same without the filler, demonstrating the increased conductivity affiliated with the filler.
The figure above shows the Arrhenius plots of an electrolyte containing S-ZrO2 nanofiller, and the same without the filler.
This filler optimizes a higher cycling capacity, a lower capacity decay upon cycling, and a more stable charge-discharge efficiency. a
This proves that NCPEs are a less reactive interface of lithium-electrolyte interfaces.

39. Electrolytes: Crystalline Polymer Electrolytes
Salts dissolved in solid polymers form crystalline complexes that can support ionic conductivity.
The previously established view was that crystalline polymer electrolytes were insulators, and conduction only occurs in the amorphous state.
The figure below shows the crystal structure of short, polyethylene oxide chains which form tunnels within which Lithium ions may migrate.
The figure to the top right shows the effects of conductivity as a function of molecular weight.
The figure to the bottom right shows that ionic conductivity is higher in polydisperse chains than monodisperse chains.

40. Suggestions for Future Work and Research
In order to make Li metal anode rechargeable batteries more effective and able to be used in a real-world context, some future work and research is necessary.
Currently, one of the main issues with these types of rechargeable batteries is the dendrite formation as depicted in the figure to the left that occurs during battery cycling.
In studying dendrite formation, it is important to consider how the interactions of dendrites with each other in a Li battery affects their growth as previous tests on this subject were conducted in isolation.
Further research should also be conducted to identify how dendrite growth is affected by the current density applied.
The problem addressed in the image to the right should also be addressed and fixed in further research.
Over time, rechargeable batteries lose their charging capacity.
Thus, there is a need to improve the stored energy density for more practical and efficient application in cars as a replacement for the internal combustion engine, and in long-lasting electronic devices.
Research to improve the cathode voltage of a rechargeable battery cell without sacrificing the battery’s capacity.