Gel Battery vs. Lithium-ion:
A Comparison of energy storage
There is a wide range of energy storage options when it comes to the stationary power market. Some of these include compressed air, capacitors, flywheels, rechargeable batteries, and compressed air. These different energy storage options have their own advantages, each depending on their use and application. Below, we are going to compare two different battery chemistries: lithium-ion and lead acid.
Figure 1: Variables in comparing battery types
Right now, the industry is dominated by nickel-based batteries and lead acid. However, nickel-based batteries are currently being phased out due to its negative effect on the environment and its high cost. This makes lead-acid the leading source of batteries, as it is more cost-effective and easier to manufacture. Another player entering the field is the lithium-ion, a technology that is more known for its portability and application to small-scale machinery. Through the years, lithium-ion is slowly climbing the ranks as one of the most efficient battery types, even in large-scale machinery. This is because although the initial cost is higher, it is more temperature sensitive, has better volume and weight, and is relatively cheaper to maintain.
Figure 2: Types of Rechargeable Batteries
In figure 2, you will see the placement of lithium ion and lead acid in the field of rechargeable batteries.
Basic Facts about Batteries
Lead Acid Batteries
Lead acid batteries have existed for a long period of time. In fact, this battery type has been around more than a hundred years ago. When lead acid batteries are fully charged, the cathode and anode house a 2V electric potential. The electrons will then pass through the load as it is being discharged. As this is happening, the internal chemical reactions at the boundary of the electrodes and electrolytes all work together in order to balance the charge equilibrium. In figure 3, you’ll see how the chemical state of lead acid batteries as it is fully charged, and what happens during discharge.
Figure 3: Charge States of Lead Acid
Lead acid batteries have two specific categories: the flooded, and the sealed/valve regulated. While the internal chemistry of these two categories are quite similar, they differ when it comes to design. Unlike sealed/valve regulated lead acid batteries, flood acid batteries require periodical electrolyte maintenance, a ventilated environment, and an upright orientation to prevent leakage.
These differences show that although flooded lead acid is more affordable, its complexities and secondary costs make sealed/valve regulated kind a more attractive alternative. There sealed/valve regulated batteries are further divided into two separate types: the Gel and Absorbed Glass Mat (AGM). In the former, the electrolyte is transformed from liquid to gel through the use of a thickening agent. In the latter, the liquid electrolyte is held together by a glass matrix. Basically, the difference between the two lies on their differences in electrolyte containment.
Both flooded and sealed/valve regulated lead acid batteries have shallow cycles and deep cycles. An example of a shallow cycle sealed/valve regulated battery is the light, ignition, and automotive start. This kind of battery requires the delivery of high power pulses for a short amount of time. The deep cycle, on the other hand, is more used in the stationary power market. Since deep cycle batteries have low discharge rates over a long period of time, it is the more convenient choice.
Lithium ion was first conceptualized in the 1970’s, but its widespread adoption did not start until the 1990’s. In this type of battery, the charged lithium-ion goes back and forth between the anode and the cathode. This process of shuttling back and forth happens during the process of charge and discharge. In figure 4, you will see a diagram of a lithium-ion reaction.
Figure 4: Lithium-ion Reaction
The packaging geometry, as well as the differences of the anode, cathode, and electrolyte when it comes to chemistry, both play a big part in the performance of the cell. Usually, the chemistry that is going to be altered the most is the cathode chemistry. It is reflected with terms like NCM, NCA, Cobalt, Manganese, and LPF. Overall, about 90% of anodes in lithium-ion batteries are made of graphite. Sometimes, silicon and titanium are also used in order to improve the power performance and lifespan of the battery. However, it would come at a more expensive rate compared to simpler versions.
Usually, in this kind of battery, the electrolytes are in liquid form. But for lithium polymer cells, the electrolytes will be absorbed in the polymer membrane of the battery. This is sometimes preferable, as this gives cell manufacturers the chance to use a pouch enclosure rather than a metal casing for liquid electrolyte. This difference has an impact on the performance of the lithium-ion cell.
As previously mentioned, lithium-ion batteries can have several chemical variations. However, these can generally be divided into two groups: the metal oxides (NCM, NCA, Cobalt, and Manganese) and the lithium iron phosphate (LFP, LiFePO4). The table below highlights the major differences between the two.
|VOLTAGE||3.3 V NOMINAL (2-3.6 V/CELL)||3.7 V NOMINAL (2.7-4.2 V/CELL)|
|ENERGY DENSITY||300 WH/L||735 WH/L|
|SPECIFIC ENERGY||128 WH/KG||256 WH/KG|
|POWER||1000 W/KG||512 W/KG|
|CYCLE LIFE||2,000 @ 100% DOD
3,000 @ 80% DOD
|750 @ 100% DOD
1,900 @ 80% DOD
|CALENDAR LIFE||6 YEARS||8 YEARS|
|COMMERCIAL SUPPLIERS||A123, VALENCE, BAK, BYD, K2, LISHEN, MANY CHINESE VENDORS||SANYO, PANASONIC, SAMSUNG, DOWKOKAM, SONY, LG CHEM, MOLI|
Table 1: Differences between Lithium-ion Phosphate and Metal Oxides
No matter the chemistry, all kinds of lithium-ion cells are deep cycle. This means that these batteries have the ability to be discharged and fully charged. Battery life in this type of cell will generally last longer if the rate of discharge does not exceed 80% of its capacity.