Lithium Batteries

LithiumPeriodic Table position and atomic weight.

Simply, lithium batteries contain a cathode, an anode and lithium. The cathode and anode are separated by an organic liquid called an electrolyte and a porous material called the separator. Electrons which provide the battery current are stored in the porous lithium. They travel through the separator, within the liquid, between the Anode and Electrode.

The Lithium-ion battery is a generic term which describes a range of different batteries of this type. They differ from each other in the chemical composition of the anode and cathode which has an effect on their properties. The end use of the battery determines the choice of material composition. When maximum energy density and minimal weight is desired for smart phones and laptops then lithium cobalt is used. This battery chemistry has a limiting character because the end of life comes after 300 to 500 charging cycles. Probably, you have noticed that after a year the run time on your cell phone battery has diminished.

Other lithium batteries include: lithium iron phosphate (LiFePO4) which is very stable and does not have the occasional safety problems. Lithium titanate has a full range discharge whereas lithium cobalt graphite has an 80 % discharge. Lithium air (a super battery in development) has an experimental cathode using oxygen which can store several times more energy than today's batteries. There are many more lithium chemistries as well. Two research facilities working on lithium are Grenoble Institute of Technology and MIT. These are just two of the many around the world. Sony and Samsung produce premium cells. Lithium batteries are also produced in Shenzhen and Taiwan.

Lithium cobalt - graphite is ideal for portable electronics because of it's high charge density and slow self discharge when not in use. Combine these electrodes with a solid polymer composite instead of a liquid solvent and you have lithium-ion polymer. This evolution allowed the batteries to be made in a flexible wrapping instead of a solid case which is better suited for PDA's, mobile phones and Apple's design choice of embedding the battery inside the case. The polymer electrolyte also has a higher energy density which increases the operating time of devices like the iPad. The Lithium-ion polymer battery was commercially released in 1996 and has found many uses in most of our mobile electronics, electric cars (TESLA, Prius) and local storage for mircogrids. The Li-Ion battery is more difficult than other battery chemistries to charge and encapsulate. They do not respond well to overcharging. Constant current is used for 90+ percent of the fast charge, when the battery voltage has risen to about 4.0 volts, then the charge mode is switched to constant voltage for the last bit of electron packing with a cap voltage of 4.2 V. When they are overcharged, the cells expand warping or worst case - breaking the encapsulation. This is trouble if the lithium leaks out and catches and errant spark.

It is customary to stop the discharge of lithium cobalt (mobile batteries) at 3VDC so that there is a small amount of current remaining for the internal IC and it does not fall below a threshold where it will take a recharge.

The Gigafactory by Tesla will use 25 tons per year of lithium compounds of graphite, cobalt and lithium. The lithium batteries will be used not only in electric cars but also in homes this century. An analogy can be made that oil was to the 20th century as lithium batteries are to the 21st century. Both are energy storage. Using the sunlight to generate electricity stored in lithium batteries is a no brainer in terms of reducing CO2 emissions. These energy storage systems can deliver high power and energy density, long life, and excellent safety performance for the transportation, electric grid and commercial markets.

Lithium-ion batteries go through extensive testing before manufacturing. The likely-hood of lithium leakage is very low. Safety Standards and Testing Protocols for Lithium-Ion Cells include external short circuit, abnormal charge, forced discharge, crush, impact, shock, vibration, heating, temprature-cycling, continuous low rate charging, molded casting test, insulation resistance, internal resistance. FEMA is an acronym for failure mode and effects analysis before manufacturing. The failure probability can be achieved by understanding the failure mechanisms. You can see that the tests are quite extensive and the battery must pass these tests before manufacturing. Underwriter Labs (UL) has specific criteria for these tests.

Basics:

The electrodes - the Cathode and Anode - act like sponges which absorb electrons and then release them.

When the battery supplies power: electrons flow from the Cathode through the dielectric and then to the Anode, and then from the Anode into your device.

When charging the battery the process is reversed. Electrons are supplied to the ANODE and they flow back into the battery and pack on the sponge like holes in the Cathode. Mnemonic to help remember is ACID (Anode Current Into Device) See Michael Faraday below.

As the battery is packed with electrons, the internal resistance rises thus requiring a higher and higher voltage to push the electrons into the battery. By measuring this voltage you have the means of determining when to stop the charge 4.2V and also the state of charge.

Battery aging occurs with the cycles. When electrons flow back and forth between the anode and cathode filaments called dendrites begin to wick into the electrolyte. CAF is the term used in printed circuit board construction = Cathode to Anode Filaments. As the lithium disperses into the electrolyte, there is less space on the sponges to hold electrons. Different chemistries and battery cell manufacturers provide varying lifespan. A reasonable cycle count after which your battery will start to degrade is 200 cycles. Imagine the electrode as a parking garage with fewer places to park than it had when it was new - also known as original design capacity in milliampere hours.

Related digression: In printed circuit boards - some of the copper from the vias move into the insulating layers of the dielectric. FR-4 is one of the most commonly used board materials. It is a copper clad laminate between layers of woven glass and epoxy. The PCB and the lithium battery have a similar effect in that over time filaments grow in the insulating material. In the Madsonline circuit boards for the Cxx chargers, the dielectric constant of the circuit board material FR4 is 4.5 with a PrePeg thickness of 10.7 mil. The traces that run the data are differential pairs 17.25 mils wide with a 90 Ohm impedance.

The history of the Lithium battery is the exploration of the optimal materials to use for the Anode, Cathode and Electrolyte.

Michael Faraday

Michael Faraday 1791 - 1867 was born on September 22, 1791 in London. He was the son of a blacksmith and started as a book binder. After binding the books, he read them. Specifically, the Encyclopedia Britannica and the 600 page Conversations on Chemistry by Jane Marcet. He attended some public lectures on natural philosophy (physics) and had the good fortune of meeting Sir Humphrey Davy to whom he later became an apprentist. Faraday was a great experimenter and observer. Through experimentation came discovery. He was an empircist. Faraday discovered electromagnetism in which a varying magnetic field causes electricity to flow in an electric circuit. To put this to mathematical equations, James Clerk Maxwell took Faraday's observations provided them.

Faraday had many discoveries and contributions to advance science. He was a founder in the study chemical electrolysis which is about understanding what happens at the interface of an electrode with an ionic substance. Electrochemistry is the science that has produced the Li ion batteries.

He used the sunrise on the east to help his memory and naming. The Anode is the doorway where the current enters the electrolyte. The east being the Anode, the sun rising in the east traversing towards the West through the electrolyte. The positively charge cations move away from the anode to the Cathode during discharge.

Faraday discovered electromagnetism in which a varying magnetic field causes electricity to flow in an electric circuit. He is a major contributor to science in the 19th century.

There have been many different approaches to the composition of the Anode and Cathode sponges. The electrochemical properties of lithium intercalation in graphite were first discovered in 1980 by Rachid Yazam and colleagues at the Grenoble Institute of Technology and the French National Centre for Scientific Research. They demonstrated the reversible intercalation of lithium into graphite in a lithium/polymer electrolyte/graphite half cell. A long sentence to say ahmem...: it is the reversible nature that make batteries rechargeable. Their work was published in 1982 and 1983. It covered both the thermodynamics of the staging and the kinetics of the diffusion. Around 1991 Bell labs was experimenting on using graphite as the anode and manganese spinel for the cathode. In 1996, Goodenough, Akshaya Padhi and colleagues at the University of Texas in Austin identified lithium iron phosphate as cathode materials. In 2002, Yet-Ming Chiang and his group at MIT found a substantial improvement in the performance of lithium batteries by boosting the material's conductivity by doping it with aluminum, niobium and zirconium.

Then in 2004, Chiang again increased performance by utilizing iron-phosphate particles of less than 100 nanometers in diameter. This decreased the particle density by almost one hundredfold while increasing the cathode's surface area which improved both capacity and performance. Commercialization led to a patent infringement battle between Chiang and Goodenough.

Nanophosphate® technology developed at the Massachusetts Institute of Technology is built on novel nanoscale materials.