Lithium Cell Manufacturing

Smart Battery packs have very specialized hardware that make possible a battery to provide just the right power at just the right moment. This hardware includes the connector, the fuse, the charge and discharge FETs,  the cell pack, the sense resistor (RSENSE), the primary and secondary protection ICs,  the fuel-gauge IC, the thermistor,  the pc board, and the EEPROM or firmware for the fuel-gauge IC. One of the most critical component in this list is the cell pack.  The battery cell pack can be thought of as the holding area of the battery's chemical. The battery cell pack is critical to the overall capability of the smart battery. Cell packs have to be designed and integrated based upon the vitals of the battery including chemistry type (Li-ion, Li-po, NICD, NIMH, etc.) cycle life, storage-capacity loss, shelf life, impedance, capacity at different rates of discharge and temperature, and mechanical and environmental requirements. It is critical to say the least. But how do you make a battery cell pack? What are the manufacturing processes necessary to make lithium based cell?

Lithium cell manufacturing was first developed in Japan using heavy machinery and automated equipment to perform certain steps while using robots to transfer partially assembled materials from one step to another. Chinese companies developed a manual approach to take advantage of inexpensive labor.  This is not to say that it is 100% manual on the contrary it more correct to say that it is a semi-automatic production process of Li-ion cells using automated equipment in the most critical areas such as mixing of powder, coating and winding.

In this we are going to look at the critical processes involved in the manufacture of lithium-ion cells. What critical components are required for a lithium battery and how each component is made.

In this current on lithium cell manufacturing we are going to be looking at the processes that are used to construct a lithium battery cell. These processes are highly technical and require complete precision in order to make an individual battery cell function according to the specific power demands of the device that the cell will ultimately be used within.

On a macro level when we look at a battery we see completed unit (which we call a battery) however the battery is actually an assembled collection of material and hardware. The outside plastic cover is called the casing. The casing encloses and hermetically seals the battery cell and specialized hardware. Battery casing is manufactured in layers. The casing layers are developed from various raw materials and can include one or two, for example, of polyethylene terephthalate layers, a polymer layer, and a polypropylene layer. Another example may be a casing with layers of carbonized plastic.

Within the casing is the hardware and the battery cell. When we look inside a lithium based battery cell, for example, there are four main components and they include:

• The lithium (which acts as an anode)
• The metallic oxide cathode
• The electrolyte
• The metallic current collector

Lithium

As noted above lithium within the battery cell is used as the battery's anode. The anode is the part of the cell that acts an electrical conductor (electrode) through which electrical current flows into a polarized device. As current flows into the lithium, a chemical process called intercalation occurs. Intercalation is the joining of a molecule (or molecule group) between two other molecules (or groups). When it comes to charging your battery you are in effect pushing ions in and out of solid lithium compounds. These compounds have minuscule spaces between the crystallized planes for small ions, such as lithium, to insert themselves from a force of current. In effect ionizing the lithium loads the crystal planes to the point where they are forced into a current flow. The current flow is then channeled back and forth from anode to cathode and thereby creating an electrical flow to power on your device.

To create an electrical flow from lithium you have to move the lithium. To move the lithium chemistry (lithium-ion, lithium polymer, lithium iron phosphate, etc) you have to have a minimum voltage applied to the lithium. Most battery cells are charged up to 4.2 volts with relative safe workings at about 3.8 volts. Anything less than 3.3 volts will not be enough to charge or move the chemistry. One thing to note here is that volts are an algorithmic measurement of current. So in a sense to create current through your battery you have to introduce current into your battery's lithium.

The Metallic Oxide Cathode

The cathode is an electrode (electrical conductor) by which electrical current flows out of a polarized electrical device. The metallic oxide component of the cathode is the composition of the cathode.  Metal oxides are crystalline solids that contain a metal cation (an ion with more protons than electrons)and an oxide anion (an ion with more electrons than protons).

The Electrolyte

In the battery cell the electrolyte solution is the conducting medium in which the flow of electric current passes through between the electrodes. Electrolytes can be wet, solid, gel, or dry. Dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry. Dry polymers do not conduct electricity but allows for ion exchange.  The real benefit is the fact that the dry polymer design is only one millimeter (0.039 inches) thick. The drawback is that the dry polymer design suffers from poor conductivity. Today lithium hexafluorophosphate and tetrafluoroborate are the preferred electrolyte salts for lithium batteries.

The Metallic Current Collector

A current collector is an inert structure of high electrical conductivity used to conduct/transmit current from or to an electrode during discharge or charge. There a variety of metal based current collector from zinc to liquid metal that can provide a good conductive path between the electrodes.

Closing

From the above we can see that a battery cell, being just one component of a battery is a highly complex system.  As we move into the next portion of the series we will break down each of these 4 key components of  a battery cell and see how they are actually made.

Now we want to look at the processes of battery cell manufacturing more closely by breaking down how the four main components of a cell come together.

Lithium Ingots

Battery cell manufacturing processes begins with a lithium ingot. A lithium ingot is often times a cylindrical roll of lithium that weighs about 11 pounds on average. Special order ingots of course can be requested thereby changing the average weight.

Lithium ingots come from technical grade lithium carbonate which is a byproduct of lithium and a solution of lithium hydroxide. The conversion of lithium in the lithium hydroxide solution results in lithium carbonate as a fine white powder. This powder is placed into a billet container prior to being processed through the extrusion. The extruded billet may be solid or hollow in form, commonly cylindrical, used as the final length of material charged into the extrusion press cylinder. It is usually a cast product, but may be a wrought product or sintered from powder compact. This billet of lithium carbonate is the ingot.

As mentioned above the extrusion press – used to shape lithium by forcing it to flow through a shaped opening. The extruded lithium emerges as an elongated piece with the same profile as the opening. The shape is typically a thin piece of metal that stretches over 650 feet. Once the ingot is made the ingot is transformed by the extrusion press and accompanied roller system into a thin sheet of metal that is only 1/100th of an inch thick and 650 feet in length.  A laminator furthers the process by stretching the 655 foot lithium roll to about 1.25 miles of lithium used to make 210 lithium batteries. The battery cell is then tested to measure 3.6V. Volts – or V – are an electrical measure of energy potential. You can think of it as the pressure being exerted by all the electrons of a battery's negative terminal as they try to move to the positive terminal.

A punch machine is then used to cut the thin metal into the physical cell size requirements and a purification machine remove dirt and other unwanted particles.

We have looked at the battery cell on a macro level and then at the lithium ingot. Now we are looking at the cathode.

In every battery there must be present a cathode. The cathode is an electrode (electrical conductor) by which electrical current flows out of a polarized electrical device. A cathode can be either positively charged or negatively charged depending on the device type and operating mode.

There are various material compositions that cathodes can be made of including the common metallic oxide cathode. Metal oxides are crystalline solids that contain a metal cation (an ion with more protons than electrons) and an oxide anion (an ion with more electrons than protons).  But other cathode compositions do exist and all have their positive benefits and negative side effects.

Some other cathode material compositions include: LiCoO2  , LiMn2O4  ,LiNiO2 , Li2FePO4F. There are others but all variations include oxygen.

We are coming to the end of our article on the manufacturing of lithium battery cells. Earlier we introduced the battery cell and looked at it a macro level, the lithium metal and saw how it was formed, we looked at the cathode and its material composition, and now we are going to look at the battery’s electrolyte.

Every battery has an anode, a cathode, and an electrolyte solution. There are mnay variations of an electrolyte solution. One common solution is sulfuric acid. Another common solution that is in use today is a Lithium hexaflourophosphate (LiPF6) in a mixture of organic solvents including: [Ethylene Carbonate (EC) + DiEthyl Carbonate (DMC) + DiEthyl Carbonate (DEC) + Ethyl Acetate (EA). This electrolyte solution like others is used to facilitate the transport of ions between the anode and the cathode. In fact that is the purpose of the electrolyte in a battery is to conduct or transport ions from the negative and positive terminals.

One other newly developed electrolyte solution is a high-purity lithium hexafluorophosphate (LiPF6), a conductive salt that was developed by Honeywell International. In fact this electrolyte research and development was paid for through the American Recovery and Reinvestment Act of 2009. The U.S. Department of Energy awarded Honeywell a $27.3 million grant that is designed to accelerate the market introduction and penetration of advanced electric drive vehicles, reducing fuel consumption and vehicle emissions of greenhouse gases. This electrolyte is one of the primary components that is intended to be used in these upcoming electric cars.