Use of Silicon Anodes in Lithium Ion Batteries

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[edit] 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 their 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 density energy storage is being looked at to be used in next generation of electric cars and as a storage medium for wind turbines, solar cells during calm wind conditions and low light conditions. This type of battery could also be used to power mobile devices for long periods of time with a smaller battery require less material to manufacture.

[edit] 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. 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. 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]. 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 much less than its theoretical storage capacity [2]

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 [3]




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. 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. 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. The electrolyte used is a organic solvent. All of these components are arranged inside of the cell casing made of steel or aluminum [4]. This is compared to regular carbon anodes with a specific capacity of carbon which is 372mAh/g [5]. The volume change causes fracturing of the lithium and after several cycles the lithium losses electrical contact with the current collector [6] 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 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 Magnetron Sputtering. This process is a physical vapor deposition process used to deposit films on to substrates. During the process a target is bombarded by energetic ions which dislodge atoms on the target material. Under vacuum pressures the atoms then fly around unit they stick to the substrate.Cite error: Invalid <ref> tag; invalid names, e.g. too many

[edit] 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 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. [7].


The Si-C composites do out perform traditional carbon anodes in charge capacity with a capacity typically around 1000mAh/g to 1800mAh/g. This is dependent on the weight percent of silicon to carbon with more silicon rasing the capacity. The trade off is the more silicon in the mix the larger the particle size and the poorer the cycling performance.[8] The modified surface can be accomplished by chemical etching and electroplating.


To chemically etch the surface of a copper current collector a corrosive solution is used to dissolve some of the surface. This etching results in a moderately 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 pryamid structures that have large bumps coating each side of the pyramid.[9]


[edit] 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 steel3:Silicon nanowires[10] ]. 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 [11].


The nanowire anodes have shown a very high capacity around 3500mAh/g. In some experiments on the first charge cycle the silicon nanowire anode showed a capcity matching its theoretical capacity.[12].

[edit] Energy Savings

The introduction of silicon anode battery could save energy throughout its life cycle in all aspects of its use. Energy saved can be in the production of the anode, the transport of the finished product, and during its use.

[edit] Carbon vs Silicon

Carbon is relatively common and easy to produce in low grades. The production of anode quality carbon requires high temperatures up to 2400 degrees celsius over several hour periods.[13]

[edit] Manufacturing

The manufacturing of silicon anode batteries cells will be more energy efficient. If a silicon anode material was being produced at a energy density of 3000 mAh/g it would take 8 times less silicon by mass to have the same battery power as carbon. With silicon being roughly twice as dense and 8 times less silicon is needed it would take 1/16 the volume of carbon assuming equivalent amounts could be attached to each current collector. So what this means is that a silicon anode battery of similar performance as a carbon anode battery would be apporximately 1/8 of the mass. This is an obviously going to create a material savings in the casing, electrolyte, and current collectors. The transport of complete batteries would be cheaper because more could fit on a truck or the truck could haul less weight and therefore need less energy to move the batteries.

[edit] Electric vehicles

This type of battery is being developed partly to fulfill the need for a lighter smaller battery for electric vehicles to become more practical. The energy savings would come in two ways when applied to vehicles. The first would be the reduced material and energy requirements to create the battery discussed above. The second is since the battery is lighter than a equivalent battery of another type, placed in the same car it would require less energy to accelerate the car and increase the battery range even further.


[edit] References

  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.

    [edit] Existing Technology

    The majority of existing lithium ion batteries are based on a carbon anode with a Lithium X Oxide metal alloy cathode

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Mat.png This page was developed as part of a project for MECH370, a Queen's University class on materials processing. It is now open edit.



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