Li-Ion Batteries

Energy Storage Systems  


There are many types of energy storage devices including batteries, electrochemical capacitors, and fuel cells. A common method of comparing energy storage devices is to create a Ragone plot, which is a plot of the energy storage capacity as a function of the power output. An example of a typical Ragone plot is shown in Figure 1, and we can see where each type of energy storage device has its advantages and disadvantages. 



Figure 1:  Ragone plot showing power and energy densities of various energy storage devices.


Electrochemical Capacitors


Electrochemical capacitors (also known as ultracapacitors or supercapacitors) are energy storage devices that have higher specific power and longer cycle lives than batteries, however they have less energy density. Our detailed description of ultracapacitors can be found here. However it should be noted that ultracapacitors are best suited for applications which require power densities greater than 500 W/kg because at this power rating their energy density exceeds that of many battery technologies.


Primary Batteries


Primary batteries are non-rechargeable batteries. Classic primary batteries function due to a difference in electrochemical potential that exists between the anode and cathode. For example, the commercially successful alkaline battery can be described by the following oxidation and reduction reactions:


                                            Anode:           Zn + 2 OH- → ZnO + H2O + 2 e-

                                            Cathode:        2 MnO2 + H2O + 2 e- → Mn2O3 + 2 OH-


The overall reaction is a result of the difference in electrochemical potentials:


                                            Zn + 2MnO2 → ZnO + Mn2O3             E0 = 1.5 V


These batteries consume their stored energies by depleting the chemicals such as the MnO2. Since this reaction involves a geochemically unfavorable restructuring in order to reverse the reaction, these batteries cannot be recharged and thus are called primary batteries. Alkaline batteries are one of the most common consumer batteries and are used in many consumer applications.


Secondary Batteries


Unlike primary batteries, “secondary” batteries can be recharged many times. One of the most prevalent and popular of these secondary batteries is the lithium-ion battery. Mechanistically, these batteries, like the alkaline battery above, take advantage of differences in oxidation and reduction potentials for energy storage. This is illustrated below:


                                            Cathode:        LiCoO2 ↔ Li1-xCoO2 + xLi+  +  xe-

                                            Anode:           C + xLi+ ↔ CLix


In these batteries, lithium in the cathode is ionized and moves to the carbon anode during the charging cycle as shown in Figure 2.



Figure 2:  Depiction of the reaction of a Li-Ion battery


Therefore, the lithium in a fully charged lithium-ion battery is primarily in the carbon. At full charge, the potential of this battery is approximately 4 V. During discharge, this lithium moves back into the cathode where it is reduced. This is a reversible battery because lithium can be inserted easily into both the anode and cathode without a sizable change in structure. This switching of Li ions back and forth between the two solids has resulted in this battery’s nickname--the “rocking chair” battery.  Li-ion batteries have become very common as the main energy source in high powered consumer electronics like portable computers and mobile telephones.


SolRayo’s Li-Ion Battery Research


One of the main problems with Li-Ion batteries is that each time you charge and discharge them, they lose some of their energy storage capability. Typically Li-Ion batteries can undergo 500 charge/discharge cycles before they have dramatic energy loss. Anyone who has had a mobile phone or laptop for over two years has seen this affect because the batteries do not last as long after all of those cycles. Here at SolRayo, we are using nanotechnology to improve the cycle life of Li-Ion batteries so they can retain their energy storage capacity longer.