BU-808b: What Causes Li-ion to Die?

Learn what‘s behind the aging process of Li-ion

The ultimate focus of maximizing the energy density of Li-ion shifted in 2006 when Li-ion unexpectedly disassembled in consumer products and millions of packs were recalled. Safety gained attention and batteries became safer. With the advent of the electric vehicle (EV), longevity is moving to the forefront and experts have begun exploring why batteries fail.

While a 3-year battery life with 500 cycles is acceptable for laptops and mobile phones, the mandated 8-year life of an EV battery seems long at first. However, it still makes an EV buyer cringe when learning that a replacement battery carries the price of a compact car with an internal combustion engine. If the life of the battery could be extended to, say, 20 years, then driving an EV would be justified even if the initial investment is high.

Manufacturers of electric vehicles choose battery systems that are optimized for longevity rather than high specific energy. These batteries are normally larger and heavier than those used in consumer goods.

Batteries chosen for an electric powertrain go through strenuous life cycle testing and Nissan selected a manganese-based Li-ion for the Leaf EV because of solid performance. To beat the clock, the test protocol mandated a rapid charge of 1.5C (less than 1 hour) and a discharge of 2.5C (20 minutes) under a temperature of 60°C (140°F). Under these harsh conditions, a heavy-duty battery is expected to lose 10 percent after 500 cycles, which represents 1–2 years of driving. This emulates driving an EV through the heat of a biblical hell, leaving rubber marks from aggressive driving, and still coming out with a battery that boasts 90 percent capacity.

In spite of the careful selection and extensive testing, the owners of the Nissan Leaf realized a capacity loss of 27.5 percent after 1–2 years of ownership, and this without aggressive driving. Why then would the Leaf under sheltered conditions drop the capacity by so much?

To get a better understanding of what causes irreversible capacity loss in Li-ion batteries, the Center for Automotive Research at the Ohio State University in collaboration with Oak Ridge National Laboratory and the National Institute of Standards and Technology performed forensic tests by dissecting failed batteries to find suspected problem areas on the electrodes.

Unrolling a 1.5-meter-long (5 feet) strip of metal tape representing the anode and cathode coated with oxide revealed that the finely structured nanomaterials had coarsened. Further studies revealed that the lithium ions responsible for shuttling electric charge between the electrodes had diminished on the cathode and had permanently lodged on the anode. This resulted in the cathode having a lower lithium concentration than a new cell, a phenomenon that is irreversible.


Coulombic Efficiency

Professor Jeff Dahn and his team at Dalhousie University in Halifax studied the longevity of Li-ion by examining coulombic efficiency (CE). CE defines the completeness by which electrons are transferred in an electrochemical system during charge and discharge. The higher the efficiency, the less stress there is on the battery and the longer it should live.

During charge, lithium gravitates to the graphite anode (negative electrode) and the voltage potential changes. Removing the lithium again during discharge does not reset the battery fully. A film called solid electrolyte interface (SEI) consisting of lithium atoms forms on the surface of the anode. Composed of lithium oxide and lithium carbonate, the SEI layer grows as the battery cycles. The film gets thicker and eventually forms a barrier that obstructs interaction with graphite. (See BU-701 How to Prime Batteries)

The cathode (positive electrode) develops a similar restrictive layer known as electrolyte oxidation. Dr. Dahn stresses that a voltage above 4.10V/cell at elevated temperature causes this, a demise that can be more harmful than cycling a battery. The longer the battery stays in a high voltage, the faster the degradation occurs.

The buildup can result in a sudden capacity loss that is difficult to predict by testing the duration of a battery through cycling alone. This phenomenon had been known for some years and measuring the coulombic efficiency can verify these effects in a more scientific and systematic manner than mere cycling.

Similar to an EV, Li-ion in satellites must also endure a lifespan of 8 years and more. To achieve this, the cells are charged to only 3.90V/cell and lower. An interesting discovery was made by NASA in that Li-ion dwelling above 4.10V/cell tend to decompose due to electrolyte oxidation on the cathode, while those charged to lower voltages lose capacity due to the SEI buildup on the anode.

NASA reports that once Li-ion passes the 8 year mark after having delivered about 40,000 cycles in a satellite, cell deterioration caused by this phenomenon progresses quickly. Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity, but this reduces the capacity to only about 60 percent. (See BU-808: How to Prolong Lithium-based Batteries)

Coulombic efficiency is capable of measuring both changes: the lithium lost due to SEI growth on the anode and electrolyte oxidation at the cathode. The results can be used to rank the life expectancy of a battery by quantifying the parasitic reaction.

The CE of a perfect battery would be 1.000,000. If this were the case, Dr. Dahn says, the Li-ion battery would last for ever. An excellent coulombic efficiency is 0.9999, a level that some lithium cobalt oxides (LCO) reach. By far the best Li-ion in terms of CE is lithium titanate (LTO); it has a potential to deliver 10,000 cycles. The negatives are high cost and relatively low specific energy. (See BU-205: Types of Lithium-ion.)

The coulombic efficiency readings vary with temperature and charge rate, also known as C-rate. As the cycle time gets longer, self-discharge comes into play and CE drops (gets worse). Electrolyte oxidation at the cathode, in part, causes this self-discharge. Li-ion loses about 2 percent per month at 0ºC (32ºF) with a state-of-charge of 50 percent and up to 35 percent at 60ºC (140ºF) when fully charged.

Table 1 provides data for the most common Li-ion systems. For simplicity reasons, CE is described as excellent, good, moderate and poor taken at 30°C (86°F).
 

Chemical name Material Coulombic efficiency1 Notes
Lithium Cobalt Oxide2 (LCO) LiCoO2
(60% Co)
Good, only slight drop at 50–60°C High capacity, limited power; fragile. Mobile phone, laptop
Lithium Manganese Oxide2 (LMO) LiMn2O4 Poor, CE is low, drops further at 40°C

High capacity, high power, tolerant to abuse.


Power tools, e-bikes, EV, medical, UPS

Lithium Iron Phosphate2 (LFP) LiFePO4 Moderate, CE drops at 50–60°C
Lithium Nickel Manganese Cobalt Oxide2 NMC LiNiMnCoO2
(10–20% Co)
Good, small drop at 60°C
Lithium Nickel Cobalt Aluminum Oxide2 (NCA) LiNiCoAlO2
(9% Co)
N/A Electric powertrain (Tesla Model S), grid storage
Lithium Titanate3  (LTO) Li4Ti5O12 Excellent Very durable but expensive and low specific energy

Table 1: Most commonly used Li-ion with coulombic efficiency rated as excellent, good, moderate and poor. Battery manufacturers may one day specify CE in a number.

1 Taken at C/20 (0.05C) and 30°C (86°F). (20h charge & discharge); 2 Cathode material; 3 Anode material
 

Additives and the effects on Coulombic Efficiency

Lithium-ion has improved and much credit goes to electrolyte additives. Each cell has several additives and manufacturers keep the combinations a secret. Additives lower internal resistance by reducing corrosion, decreasing gassing, speeding up manufacturing by fine-tuning the wetting process, and improving low and high temperature performance. Adding 1–2 percent vinylene carbonate improves SEI on the anode, limits electrolyte oxidation at the cathode and enhances the CE readings. (See also BU-307: Electrolyte)

Additives make up less than 10 percent of the electrolyte and the chemicals are consumed in the formation of the SEI layer. Folks ask, “Can additives interact with each other?” The answer is, “Absolutely.” A battery behaves like a living organism and, as a patient taking multiple medications must inform the doctor before additional pills can be prescribed, similar conditions exist with a battery. Using coulombic efficiency allows the discovery of possible interferences in weeks rather than having to wait for years for symptoms to develop.

To examine the correlation between CE and longevity, Dalhousie University worked with battery manufacturers, including E-One Moli. While a university can carefully document ingredients, cell manufacturers keep these as top secret. The test bed consisted of 160 cells, four of each type. E-One Moli provided 80 cells with their own secret sauce; Dalhousie specified the other 80 electrolyte samples.

Dalhousie identified five batteries of interest, each with its own architecture and additives. Figure 2 shows the coulombic efficiency of these five samples with values ranging from 0.9960 to 0.9995. Figure 3 demonstrates the test results when cycled to death. To Dalhousie’s anticipation and satisfaction, CE harmonized well with the cycle count. Batteries with high CE lasted the longest; those with low CE values were the first to die.

Figure 2: Coulombic efficiency. Five experimental batteries were tested for coulombic efficiency. A higher CE provides a longer life.
Courtesy of the Dalhousie University
 
Figure 3: Relationship of coulombic efficiency and cycle life. High CE values live the longest; low values die first.

Courtesy of the Dalhousie University

Battery wear and tear also includes structural degradation that can be captured with traditional cycle testing. Dr. Dahn calls this type of testing the “sausage machine.” While measuring coulombic efficiency assists in battery development by giving a snapshot assessment of additives; the old sausage machine does the verification thereafter.

Figure 4 demonstrates capacity loss caused by the structural degradation of an older Li-ion when cycled at a 1C, 2C and 3C. The elevated capacity loss at higher C-rates may be lithium plating at the anode caused by rapid charging. [See BU-401a: Fast and Ultra-fast chargers]

 

Figure 4: Cycle performance of Li-ion with 1C, 2C and 3C charge and discharge.

Moderate charge and discharge currents reduce structural degradation. This applies to most battery chemistries.


Energy Efficiency

While the coulomb efficiency of lithium-ion is normally better than 99 percent, the energy efficiency of the same battery has a lower number and relates to the charge and discharge C-rate. With the 20-hour charge rate of 0.05C, the energy efficiency is a high 99 percent. This drops to about 97 percent at 0.5C and decreases further at 1C. In the real world, the Tesla Roadster is said to have an energy efficiency of 86 percent. Ultra-fast charging on newer EV will have a negative effect on energy efficiency (and the battery).

Parasitic reaction that occurs within the electrochemistry of the cell prevents the coulombic efficiency from reaching 100 percent; heat during discharge decreases the energy efficiency. Energy efficiency relates to losses in the battery and is calculated by comparing the average charge and discharge voltages. Ultra-fast charging and heavy loading reduces the energy efficiency and also promotes battery strain.


Capacity degradation in Electro Powertrains

When choosing batteries for the powertrains, manufacturers of electric vehicles come to different conclusions. Tesla cars use the 18650 cell because the cell is readily available and has a low price. This was a strange choice for the Tesla Roadster, the first EV by Tesla, as the cell was designed for portable devices such as laptops and medical and military devices. Perhaps unknown to Elon Musk, the founder of Tesla Motors, cobalt-blended lithium-ion has a high CE reading that adds to longevity in the way the battery is being used in that application.

The newer Tesla models use the same concept and to reduce stress, Tesla “super-sizes” the pack. The battery is so large that it operates at a C-rate of only 0.25C (C/4), even at highway speed. This allows Tesla to focus on high energy density for maximum runtime; power density becomes less important. The negative of super-sizing is increased energy consumption due to a heavier vehicle and a higher battery price. (For more information on EV battery choices see BU-1003: Electric Vehicle.) 
 

Summary

The manganese-based Li-ion batteries chosen for the Nissan Leaf and other EVs have excellent lab results. What may have been overlooked in the Nissan Leaf test is the damage that is being done when keeping the battery at high voltage and elevated temperature. As the CE tests reveal, these two conditions can cause more damage than cycling.

The four suspected renegades responsible for capacity loss and the eventual end-of-life of the Li-ion battery are:

  1. Mechanical degradation of electrodes or loss of stack pressure in pouch-type cells. Careful cell design and correct electrolyte additives minimize this cause. (See Figure 4.)
  2. Growth of solid electrolyte interface (SEI) on the anode. A barrier forms that obstructs the interaction with graphite, resulting in an increase of internal resistance. SEI is seen as a cause for capacity loss in most graphite-based Li-ion when keeping the charge voltage below 3.92V/cell. Electrolyte additives reduce some of the effect.
  3. Formation of electrolyte oxidation at the cathode that may lead to a sudden capacity loss. Keeping the cells at a voltage above 4.10V/cell and at an elevated temperature promotes this phenomenon.
  4. Lithium-plating on the surface of the anode caused by high charging rates. (Elevated capacity loss at higher C-rates in Figure 4 might be caused by this.)

SEI EO

Figure 5: A cell voltage of 3.92V appears neutral; lower voltages add to SEI, higher to EO.

Last Updated 2017-05-09


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Comments

On August 26, 2014 at 6:41pm
Wallace Fiala wrote:

I believe I have the perfect environment for my 2012 Leaf.  It’s parked inside an attached garage out of the heat in summer and no lower than 40 degrees in the winter.

On August 26, 2014 at 7:13pm
HZC wrote:

Three questions:
1. How can CE seperate the parasitic reactions of the cathode and the anode?
2. If the electrolyte oxidation at the cathode is ” more harmful than cycling”, then why change the anode material to LTO can lead to excellent CE as in table 1?
3. Is there an exact quantitative relationship between CE and cycle life?
Thank you!

On August 27, 2014 at 10:43am
Finn Hansen wrote:

Interesting article. Why is CE for Tesla Model = N/A?

Thank you in advance!

On August 27, 2014 at 12:45pm
J Austin wrote:

I need a more complete description of abbreviations:

  What is “C”, what is the 18650?

  Which formula is used in the Boeing 787?

On August 27, 2014 at 6:46pm
suzan phillips wrote:

Brilliant!  You should be very proud of yourselves…

On September 29, 2014 at 11:29am
MTDSG wrote:

^
Same question as Finn Hansen.

Good article. Thanks.

On April 20, 2015 at 10:50am
HLewis wrote:

C refers to the rate of charge or discharge and is dependent on the Ah rating of the battery itself.  For instance:

A 25Ah cell will have a 1C of 25 Amps.  Meaning it takes one hour to go from 0%SOC to 100%SOC.  The same cell will have a C/2 of 12.5.

Likewise a 10Ah cell has a 1C of 10 Amps, a 2C of 20 Amps, and a C/2 of 5 Amps. 

18650 refers to a specific cell form factor, which has an industry standard size.  These particular cells are cylindrical hard cased cells with the anode, cathode, and separator rolled in a “jelly-roll” and put into the 18650 sized casing.

On November 22, 2015 at 7:03am
Jerry Jorgenson wrote:

The 360 Watt hours per mile for the Telsa seems high. At 61,000 miles my lifetime average is 248 Watt hours per mile. I’d suggest the fleet average would be more like 300.

On May 4, 2016 at 12:49pm
David Hale wrote:

Battery University has a wealth of information here. THANK YOU VERY MUCH !!
I have researching what the extra (smaller) contacts are for batteries used in laptops and drills with no luck.  I presume they may be used for sensing battery conditions during charging and/or during battery drain. 

I am thinking of using my (drill)  LBXR12 Lithium-ion 12V battery for a robot since it is compact and I have the recommended chargers for it.  This battery has 3 extra contacts. Only one of the three extra contacts mate with the drill.  All 3 extra contacts mate in both chargers that I have.  The battery is now charged (12.3 V).

Checking the two extra terminals used only for the chargers and referencing the negative terminal. They read 4.11 and 8.21 Volts.  (OK. They are simply taps to each cell.)

Checking the extra terminal for the drill. - With a meter connected to the negative side of the battery measures 0.0 Volts.  Resistance to negative terminal is ~11.4K Ohms.  Any guess to this pin’s function ?

Thank you very much again.

On June 22, 2016 at 5:10pm
ian Louden wrote:

laptop batteries have internal monitoring electronics that check cell voltages do not exceed 4.2V per cell when charging, this is usually done as most laptops have a 2 wire external mains adapter with no intelligence for charging, Drills on the other hand always have a dedicated charger, the cell voltages are measured by the charger hence you have 4,8 and 12V (3 cell in series), the additional pin with 11.4kohms is likely to be a temperature sensor pin to prevent charging if it is to hot, again this is internal in a laptop battery. usage of drill batteries varies wildly from laptop batteries and can have much shorter life expectancy, keeping the electronics in the charger reduces the cost of the battery pack, even more if you have 2 or 3 on the go!

Great article BTW.

On July 20, 2016 at 2:48pm
Rean Bootsma wrote:

“The elevated capacity loss at higher C-rates may be lithium planting at the anode caused by rapid charging.”

Should be “plating”.

On July 24, 2016 at 3:24pm
David Kelly wrote:

Great Amount of Lithium Battery Information, As I Am Always Looking For Info Regarding Extending The Life Of Power Tool Batt.s

On August 7, 2016 at 3:42pm
Conor Fenlon wrote:

Hello. If I have a 5v solar panel system that can output (almost) 1Amp of current in ideal conditions, would I damage my 2200mah 18650 batteries if it can’t supply the necessary current for the CC phase of charging? I use a simple USB 2 x 18650 charger with the panel, which works. But I’m just curious, am I actually damaging my cells by supplying inadequate current if the sun goes behind a cloud? Any info would be appreciated. Thank you.

On February 10, 2017 at 5:51am
Jason Hamilton wrote:

Conor,
According to the data in this article you are plating your cathode with Li at a 5v charge. The charge seemingly needs to be regulated to 3.92Vdc to prevent this from happening based on the information provided here.

You could place a standard duty LED inline with your charging circuit that would drop 0.7 Vdc from your USB feed, but further regilation would be required to get below the overcharge condition that comes from utilizing a 5V nominal charge.

This article also talks of concerns that arise from allowing currents to flow into the cell beyond a drop out threshold. There are many different manufacture suggested dropout thesholds. This article suggests a dropout of 97% to avoid damaging heating effects and SEI effects. This means you need to monitor the current flow into the cell and keep it from dropping too low.

Manufacturer data sheets typically identify this charging amperage. 

I really like this article for bringing longevity considerations into the performance discussion of this chemistry.