Week 1 Understanding Different Battery Chemistry

                                        WEEK-1

1.Prepare a table which includes materials & chemical reactions occurring at the anode and cathode of LCO, LMO, NCA, NMC, LFP and LTO type of lithium-ion cells. Give your detailed explanation on it

Introduction to lithium-ion cell and its types: -

Lithium-ion is named for its active materials; the words are either written in full or shortened by their chemical symbols. A series of letters and numbers strung together can be hard to remember and even harder to pronounce, and battery chemistries are also identified in abbreviated letters.

For example, lithium cobalt oxide, one of the most common Li-ions, has the chemical symbols LiCoO2 and the abbreviation LCO. For reasons of simplicity, the short form Li-cobalt can also be used for this battery. Cobalt is the main active material that gives this battery character. Other Li-ion chemistries are given similar short-form names. This section lists six of the most common Li-ions. All readings are average estimates at time of writing.

Lithium-ion batteries can be designed for optimal capacity with the drawback of limited loading, slow charging and reduced longevity. An industrial battery may have a moderate Ah rating but the focus in on durability. Specific energy only provides part of battery performance.

Lithium Cobalt Oxide(LiCoO2) — LCO

Its high specific energy makes Li-cobalt the popular choice for mobile phones, laptops and digital cameras. The battery consists of a cobalt oxide cathode and a graphite carbon anode. The cathode has a layered structure and during discharge, lithium ions move from the anode to the cathode. The flow reverses on charge. The drawback of Li-cobalt is a relatively short life span, low thermal stability and limited load capabilities (specific power). The cathode has a layered structure. During discharge the lithium ions move from the anode to the cathode; on charge the flow is from cathode to anode.

 

Lithium Manganese Oxide (LiMn2O4) — LMO

Li-ion with manganese spinel was first published in the Materials Research Bulletin in 1983. In 1996, Moil Energy commercialized a Li-ion cell with lithium manganese oxide as cathode material. The architecture forms a three-dimensional spinel structure that improves ion flow on the electrode, which results in lower internal resistance and improved current handling. A further advantage of spinel is high thermal stability and enhanced safety, but the cycle and calendar life are limited. Low internal cell resistance enables fast charging and high-current discharging. In an 18650 package, Li-manganese can be discharged at currents of 20–30A with moderate heat build-up. It is also possible to apply one-second load pulses of up to 50A. A continuous high load at this current would cause heat build-up and the cell temperature cannot exceed 80°C (176°F). Li-manganese is used for power tools, medical instruments, as well as hybrid and electric vehicles.

 

Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) — NMC

One of the most successful Li-ion systems is a cathode combination of nickel-manganese-cobalt (NMC). Similar to Li-manganese, these systems can be tailored to serve as Energy cells or Power cells. For example, NMC in an 18650 cell for moderate load condition has a capacity of about 2,800mAh and can deliver 4A to 5A; NMC in the same cell optimized for specific power has a capacity of only about 2,000mAh but delivers a continuous discharge current of 20A. A silicon-based anode will go to 4,000mAh and higher but at reduced loading capability and shorter cycle life. Silicon added to graphite has the drawback that the anode grows and shrinks with charge and discharge, making the cell mechanically unstable.

The secret of NMC lies in combining nickel and manganese. An analogy of this is table salt in which the main ingredients, sodium and chloride, are toxic on their own but mixing them serves as seasoning salt and food preserver. Nickel is known for its high specific energy but poor stability; manganese has the benefit of forming a spinel structure to achieve low internal resistance but offers a low specific energy. Combining the metals enhances each other strengths. NMC is the battery of choice for power tools, e-bikes and other electric powertrains. The cathode combination is typically one-third nickel, one-third manganese and one-third cobalt, also known as 1-1-1. Cobalt is expensive and in limited supply. Battery manufacturers are reducing the cobalt content with some compromise in performance. A successful combination is NCM532 with 5 parts nickel, 3 parts cobalt and 2 parts manganese. Other combinations are NMC622 and NMC811. Cobalt stabilizes nickel, a high energy active material.

Lithium Iron Phosphate (LiFePO4) — LFP

In 1996, the University of Texas (and other contributors) discovered phosphate as cathode material for rechargeable lithium batteries. Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are high current rating and long cycle life, besides good thermal stability, enhanced safety and tolerance if abused. Li-phosphate is more tolerant to full charge conditions and is less stressed than other lithium-ion systems if kept at high voltage for a prolonged time. As a trade-off, its lower nominal voltage of 3.2V/cell reduces the specific energy below that of cobalt-blended lithium-ion. With most batteries, cold temperature reduces performance and elevated storage temperature shortens the service life, and Li-phosphate is no exception. Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. This can be mitigated by buying high quality cells and/or using sophisticated control electronics, both of which increase the cost of the pack. Cleanliness in manufacturing is of importance for longevity. There is no tolerance for moisture, lest the battery will only deliver 50 cycles. Figure 9 summarizes the attributes of Li-phosphate.

Li-phosphate is often used to replace the lead acid starter battery. Four cells in series produce 12.80V, a similar voltage to six 2V lead acid cells in series. Vehicles charge lead acid to 14.40V (2.40V/cell) and maintain a topping charge. Topping charge is applied to maintain full charge level and prevent sulfation on lead acid batteries. With four Li-phosphate cells in series, each cell tops at 3.60V, which is the correct full-charge voltage. At this point, the charge should be disconnected but the topping charge continues while driving. Li-phosphate is tolerant to some overcharge; however, keeping the voltage at 14.40V for a prolonged time, as most vehicles do on a long road trip, could stress Li-phosphate. Time will tell how durable Li-Phosphate will be as a lead acid replacement with a regular vehicle charging system. Cold temperature also reduces performance of Li-ion and this could affect the cranking ability in extreme cases.

Lithium Nickel Cobalt Aluminium Oxide (LiNiCoAlO2) — NCA

Lithium nickel cobalt aluminium oxide battery, or NCA, has been around since 1999 for special applications. It shares similarities with NMC by offering high specific energy, reasonably good specific power and a long-life span. Less flattering are safety and cost. Figure 11 summarizes the six key characteristics. NCA is a further development of lithium nickel oxide; adding aluminium gives the chemistry greater stability.

Lithium Titanate (Li2TiO3) — LTO

Batteries with lithium titanate anodes have been known since the 1980s. Li-titanate replaces the graphite in the anode of a typical lithium-ion battery and the material forms into a spinel structure. The cathode can be lithium manganese oxide or NMC. Li-titanate has a nominal cell voltage of 2.40V, can be fast charged and delivers a high discharge current of 10C, or 10 times the rated capacity. The cycle count is said to be higher than that of a regular Li-ion. Li-titanate is safe, has excellent low-temperature discharge characteristics and obtains a capacity of 80 percent at –30°C (–22°F).

LTO (commonly Li4Ti5O12) has advantages over the conventional cobalt-blended Li-ion with graphite anode by attaining zero-strain property, no SEI film formation and no lithium plating when fast charging and charging at low temperature. Thermal stability under high temperature is also better than other Li-ion systems; however, the battery is expensive. At only 65Wh/kg, the specific energy is low, rivalling that of NiCad. Li-titanate charges to 2.80V/cell, and the end of discharge is 1.80V/cell.

S.no	Battery	Cathode	Anode	Electrolyte	Reaction to Anode	Reaction to Cathode
1.	LCO	LiCoO2	Lic6	LiPF6	LixC6→xLi++xe-+C6	Li1-xCoO2+xLi++xe-→LiCoO2
2.	LMO	LiMn2O4	Lic6	LiPF6	LixC6→xLi++xe-+C6	Li1-xMn2O4+xLi++xe-→LiMn2O4
3.	NCA	LiNiCoAlO2	Lic6	LiPF6	LixC6→xLi++xe-+C6	Li1-xNiCoAlO2+xLi++xe-→LiNiCoAlO2
4.	NMC	LiNi0.8Mn0.1Co0.1O2	Lic6	LiPF6	LixC6→xLi++xe-+C6	Li1-xNi0.8Mn0.1Co0.1O2+xLi++xe-→LiNi0.8Mn0.1Co0.1O2
5.	LFP	LiFePO4	Lic6	LiPF6	LixC6→xLi++xe-+C6	Li1-xFePO4+xLi++xe-→LiFePO4
6.	LTO	LiMn2O4	Lic6	LiPF6	Li4Ti5O12→Ti5O12+4Li++4e	4Li1-xMn2O4+4Li++4e-→4LiMn2O4

2. Compare the differences between each type of Li+ion batteries based on their characteristics

Answer: 

As discussed in problem statement-1, each of the major 6 types of Lithium-ion batteries was discussed briefly. Now, the main differences between their characteristics can be best found out when choosing a battery type that can be best suited for your requirements or which does the job for you more efficiently than the others. In that context, there are certain factors for which the 6 types can be differentiated from one another and can be given in below: 

• Nominal Voltage: It is the system voltage at which the device is designed to operate. The rated voltage is usually higher than the nominal voltage to ensure the safe operation of the device.
• Cycle: The charge and discharge when took place each for a single time, makes a Cycle. The higher the cycles can take place, the higher is the lifespan.
• The current rate (C): This is used to determine the fast-charging capability and short-circuit discharge capability of a battery.
• Thermal Runaway: In simplest terms, thermal runaway begins when the heat generated within a battery exceeds the amount of heat that is dissipated to its surroundings. If the cause of excessive heat creation is not remedied, the condition will worsen.  Internal battery temperature will continue to rise, causing battery current to rise, creating a domino effect.  The rise in temperature in a single battery will begin to affect other batteries nearby, and the pattern will continue, thus the term “runaway".

Specific Energy: It is the amount of energy stored per unit mass (Wh/Kg). This is a very important deciding factor that decides how much energy or power we are going to get from different batteries with the same amount of weight.

The main differences between the 6 types of Li+ ion batteries can be differentiated based on their characteristics:

Nominal Voltage:  Most of them are very close when it comes to this characteristic. LMO(3.7) has the highest range and LTO(2.4) has the lowest range in volts.

Cycle Life: They differ by a significant amount from each other mostly. LTO battery has the longest life cycle that's why it can last for a longer period of time and it is exactly the opposite for the LMO type batteries.

Charge Current Rate: Higher the charge current, the less time the battery will take to get fully charged. LFP and LTO are highest, NCA comes close to its contenders but finishes last in this case.

Discharge Current Rate: In severe cases, you can cause an internal short circuit and/or even an explosion. Most consumer applications require only 1C from their batteries. If you need higher discharge rates and faster charging times, you should definitely choose a high-rate battery instead. LTO has a discharge rate 10 times higher than others, that is why it is one of the safest batteries out there.

Thermal Runaway: It is discussed before, a Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback. We know that the higher the temperature at which the thermal runaway will occur, the safer the battery will be. LTO has the highest temperature rating in this and NCA & LCO has the lowest.

Specific Energy: The Specific energy density can be calculated by integrating from 0 to tcutoff (time to reach the cut off potential) the expression V(t)*I*A dt and then dividing for (3600*Mw). In this way, you can obtain the specific energy density in Wh kg-1. Lithium-ion with cobalt cathodes(NCA-type batteries) offer the highest energy densities. Typical applications are cell phones, laptops, and digital cameras. Power Density (W/kg) indicates how much power a battery can deliver on-demand. For ease of choosing a battery from a layman's perspective, the 6 key buying point are being considered and these 6 key features are given a value within 0-5 range for all the 6 types of batteries and then all of them are given with the same parameters which are considered in case of "5 Point Hexagonal Analysis" of the batteries. The 6 factors are as follows:

• Specific Energy
• Specific Power
• Safety
• Performance
• Life Span
• Cost