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==Types of anodes==
==Types of anodes==
There are several approaches used to overcome the problem of the loss of electrical contact due to volume expansion. There are several different methods currently being study to allow the use of silicon in lithium ion batteries. Regardless of method all of the approaches aim to maintain capacity during cycling well increasing the specific capacity of the battery.
There are several approaches used to overcome the problem of the loss of electrical contact due to volume expansion. There are several different methods currently being study to allow the use of silicon in lithium ion batteries. Regardless of method all of the approaches aim to maintain capacity of the battery during cycling as well as increasing the specific capacity of the battery over tradtional carbon anoded batteries.


===Amorphous Film Silicon===
===Amorphous Film Silicon===

Revision as of 04:10, 4 December 2009

Template:MECH370




Introduction

With increasing amount of mobile and high energy demand technology there is a need for high density, low weight and small size energy storage system. To accomplish this researchers are looking into the use of silicon as an anode material in lithium ion batteries to improve there energy storage capacity. Silicon is being studied for this use because of the high amount of lithium ions that can be diffused into the metal. This type of high denstiy energy stoarge is being looked at to be used in next generation of electric cars and as a stroage medium for wind trubines and solar cycles during calm wind conditions and low light conditions. This type of battery could also be used to power moblie devices for long peroids of time with a smaller battery require less material to manufacture.

History

The search for a high density energy storage device began in the 1970's. This was triggered by the oil crisis during this time.[1]. This led to the creation of primary Lithium ion batteries using lithium metal as an anode. This was done because of lithium’s properties being ideal for this use. The problem was that during charging and discharge cycles was that dendrites would grow on the surface of the anode and eventually contact the cathode causing a short circuit which would heat the battery and cause it to overheat which lead to explosions and fire [1]. The break though came in the early 90’s when the first secondary lithium ion batteries that used a carbon anode that lithium could diffuse into which eliminated dendrite growth and made the batteries safe to use [1]. This type of lithium battery has reached its maximum potential for energy storage so now a new material is need to increase the energy capacity. This high energy density is needed to power the abundance of energy hungry mobile devices with a light weight power supply and energy storage for generating sources such as solar cells and wind turbines that due to weather conditions cannot produce power all of the time.

Existing Technology

The majority of existing lithium ion batteries are based on a carbon anode with a Lithium X Oxide metal alloy cathode [2]. The technology used today is very similar to the original technology when secondary lithium ion batteries were introduced. There have been some improvements to the batteries such as surface treatments and different cathode materials but none have really dramatically increased the energy density of the batteries which is around 350 mAh/g this is still uch less than its theortical storage capacity [1]. The value of 350mAh/g is for the total weight of the anode material when the weight of the rest of the battery components is taken into account the energy denstiy goes down to 52 mAh/g.[3]

A lithium ion battery works by exchanging Li ions between the anode to cathode during discharge and the reverse during charging the reaction is as follows [1].

Current ollector in lithium ion cell

[2]





6C + LiCoO2 ↔ Li1-xCoO2 + LixC6 [1]






Cut away of lithium ion cell construction

[2]

The two types of cell construction are cylindrical cells and prismatic cells. This is only the shape of the cell the internal components are exactly the same [2].


The internal components consist of a cathode that is usually made of aluminum as the current collector with a coating of the lithium cobalt oxide. The anode is traditionally made out of carbon usually in the form of graphite which is on a copper current collector [2]. The anode and cathode are separated by a separator that is made out of polyethylene or polypropylene film that is porous to allow for particle diffusion [2]. The electrolyte used is a organic solvent. All of these components are arranged inside of the cell casing made of steel or aluminum [2]. There are multipliable layers of anodes and cathodes in each cell.



Properties of Silicon

Advantages

Silicon is being looked at as a material to replace carbon as anodes in lithium ion batteries because of the potential of vastly increasing the specific capacity that the batteries could archive. Silicon has the highest theoretical specific capacity of all metals at 4200mAh/g [4]. This is compared to regular carbon anodes with a specific capacity of carbon which is 372mAh/g [1].

Problems

Depiction of fracturing of bulk silicon anode

[5]

The largest problem to overcome when using silicon is the strains that are associated with the huge volume expansion that occurs during the lithiation of the silicon. The silicon can increase in volume by up to 400%[6]. The volume change causes fracturing of the lithium and after several cycles the lithium losses electrical contact with the current collector [5].



Types of anodes

There are several approaches used to overcome the problem of the loss of electrical contact due to volume expansion. There are several different methods currently being study to allow the use of silicon in lithium ion batteries. Regardless of method all of the approaches aim to maintain capacity of the battery during cycling as well as increasing the specific capacity of the battery over tradtional carbon anoded batteries.

Amorphous Film Silicon

To counter the problem of fracturing of the anode amorphous silicon could be used. Amorphous silicon has shown that it remains stable even after repeated lithium ion insertion.[7] The amorphous silicon stays stable because of its lack of crystal structure. Since it has no crystal structure the metal is not fractured when the lithium is inserted. Using a pure silicon in a bulk form has the advantage of having a larger amount of silicon per unit volume.


There are several methods to create amorphous silicon. A common method is to use DC Magnatron Sputtering. This process is a phsyical vapour deposition process used to deposit flims on to substrats. During the process a target is bombarded by energenic ions which dislodge atoms on the target material. Under vacumn pressures the atoms then fly around unit they stick to the substrate.[8] The atoms that are dislodge are usally ions so they can be diercted with electric fields onto the subsrate.


The amorphous silicon anodes show a very good charge capacity of 3000mAh/g. This is about 10 times larger than that of current carbon anodes. This is not as high as for crystiline silicon due to the different structure. The amorphous silicon also out proforms bulk silicon during repeted charge and discharge cycles with the capacity showing minamial fading during cycling out to a useful number of cycles.[7]

Silicon Carbon Composites

One way of improving the cycle performance of silicon is to reduce the size of the particles that are used in the anode and coat them in

Image of Si-C composite

[9] carbon. The size reduction helps to control the volume change and stresses in the Si. The carbon coating on the silicon acts like a electrical path way so that even when there is a volume change contact is not lost with the current collector. [10]. The size of silicon particales used are in the range fo several micrometers to 50 nanometers.


The Si-C composite can be made using thermal vapour deposition (TVD) or carbon netting of silicon [9]. With each process the starting material is silicon less then 100nm in diameter. These fine particles are achieved though mechanical milling. During the TVD process benzene or toluene and silicon is heated up to 1000 ⁰C the vapours of the coating collects on the silicon and decomposition of the benzene or toluene occurs on the surface of the silicon [9]. The carbon netting method is slightly more involved. The silicon is first dispersed in a solution of sucrose with sulphuric acid being added to it. The solution is then dehydrated well it is being mechanically agitated [9]. Regardless of preparationmethod the carbon contains the silicon and keeps the integrity of the active particles [9].


The Si-C composites do out preform traditonal carbon anodes in charge capacity with a capacity typically around 1000mAh/g to 1800mAh/g. This is dependent on the weight precent of silicon to carbon with more silicon rasing the capacity. The trade off is the more silicon in the mix the larger the particale size and the poorer the cycling preformance.[10]

Etched Surface Si-C Anodes

To help improve the preformace of Si-C composites by increasing the weight % of carbon the surface of the current collector can be modified. With a etched surface the Si-C anode has more surface area to bond to and resists delaimanting from the surface of the current collector during repeated cycling.[11] The modified surface can be accomplished by chemical etching and electroplating.


To chemically etch the surface of a copper current collector a corrsive solution is used to dissovle some of the surface. This etching results in a modertly roughed surface. The roughest of the surfaces used are nodule type foil. To create this foil a specialized electro plating process is used. The final surface texture looks like small pryimad structures that have large bumps coating each side of the pyramid.[11]


Etching the surface of the current collector offers a large advantage when using a Si-C composite vs a flat current collector. When the capacity is compared during charge and discharge cycles of Si-C composite with the same capacity, surface etching shows an improvment over a flat current collector. The nodule foil type current collectors achive much better cycle performance over enough cycles to make them useful. The chemically etched type current collectors only show stable cycling for a small number of cycles. The improvment in cycling preformance is due to the much higher surface area helping to hold the anode on the current collector.[11]


Silicon Nanowires

To also counter the problems caused by the volume changes in silicon is to use nanowires. The wires are directly on the stainless steel

Silicon nanowires on stainless steel substrate

[5] current collector. This means that every wire is connected directly to the current collector and all wires contribute to the capacity. The wires also have more efficient 1D electronic pathways [5].


The nanowires can be grown using VLS (vapour-liquid-solid) deposition process. The substrate is first coated in Au which is the catalyst for the nanowire growth. The substrate is the heated to melting point of the catalyst. An atmosphere of silicon vapour and inert gas is pumped into the chamber and the silicon becomes super saturated in the catalysis and participates out of solution and grows the wire [6].


The nanowire anodes have shown a very high capicity around 3500mAh/g. In some experiments on the first charge cycle the silicon nanowire anode showed a capcity matching its theortical capcity.[5]. After the first cycle however the capacity dropped down to 3541mAh/g and this stayed stable during more cycles and little fading happened. With the energy capictiy staying stable during repeated cycles shows that the nanowires are resisting the volume change, not fracturing into particles, and staying firmly attached to the current collector[5].

Implementation

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Fu. L. J, Liu. H, Li. C, Wu. Y. P, Rahm. E, Holze. R, Wu. H. Q. (2006). Surface modifications of electrode materials for lithium ion batteries. Solid State Sciences, 8, 113-128
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Lithium Ion Technical Manual. (n. d). Retrieved from http://www.tayloredge.com/reference/Batteries/Li-Ion_TechnicalManual.pdf
  3. The High Power Lithium Ion.(2006). Retrieved from http://www.batteryuniversity.com/partone-5A.htm
  4. Xu. Y. H, Yin. G. P, Zuo. P. J. (2008). Geometric and electronic studies of Li15Si4 for silicon anode. Electrochimica Acta, 54, 341-345.
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Chan C. K, Peng H, Liu G, McIlwarth K, Zhang X. F, Huggins R. A, Cui Y. (2008). High-Performance lithium battery anodes using silicon nanowires. Nature Nanotechnology, 3, 31- 35
  6. 6.0 6.1 Ruffo R, Hong S. S, Chan C. K, Huggins R. A, Cui Yi. (2009). Impedance Analysis of Silicon Nanowires Lithium Ion Battery Anodes. J. Phys. Chem, 113, 11390-11398.
  7. 7.0 7.1 Baranchugov V, Markevich E, Pollak E, Salitra G, Aurbach D. (2007). Amorphous silicon thin flims as a high capacity anodes for Li-ion batteries in ionic liquid electrolytes. Electrochemistry Communications, 9, 796-800.
  8. Magnetron Sputtering.(2005). Retrieved from http://www.pvd-coatings.co.uk/theory-of-pvd-coatings-magnetron-sputtering.htm
  9. 9.0 9.1 9.2 9.3 9.4 Yang X, Wen Z, Zhu X, Huang S. (2005). Preparation and Electrochemical Properties of Silicon/Carbon Composite Electrodes. Electrochemical and Solid State Letters, 8, A481-A483.
  10. 10.0 10.1 Qin S, Hanai K, Imanishi N, Kubo M, Hirano A, Takeda Y, Yamamoto O. (2009). Highly reversible carbon-nano-silicon composite anodes for lithium rechargeable batteries. Journal of Power Sources, 189, 761-765. Cite error: Invalid <ref> tag; name "[6]" defined multiple times with different content
  11. 11.0 11.1 11.2 Kim Y. L, Sun Y. K, Lee S. M. (2008). Enhanced electrochemical performance of silicon-based anode material by using current collector with modified surface morphology. Electrochimica Acta, 53, 4500-4504.
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