What Is a Lithium Battery?

Lithium batteries are cells that contain the cathode and anode materials. During discharge, an oxidation half-reaction at the anode produces positively charged lithium ions and negatively charged electrons. These reactants move through the electrolyte and external circuit to recombine at the cathode.

The separator keeps the anode and cathode apart, allowing only ions to pass while carefully avoiding direct flow of electrons. This allows for a high performance battery.

Battery Structure

A battery contains a positive electrode (cathode), a negative electrode (anode) and a liquid electrolyte that conducts electricity. When a lithium battery is being used, an external circuit applies an lithium battery over-voltage to the cell that forces charging currents to flow from the anode through the electrolyte and separator to the cathode. During discharge, the opposite happens: lithium ions move from the anode to the cathode and become embedded in the electrode materials through a process called intercalation.

When the battery is recharged, the positive and negative ions in the electrolyte are replaced back onto the respective electrodes. This isn’t perfect, though, because each charge cycle degrades the electrodes a little bit, and this is why even rechargeable batteries don’t last forever.

A battery’s performance depends on a number of factors, including its capacity, voltage and energy density. Its voltage is determined by the favorability of the oxidation and reduction reactions that take place in the anode and cathode, as well as the properties of the electrolyte and separator. It is also affected by the temperature at which the battery operates, as the higher the temperature, the more degradation of the battery pack occurs.

Electrolyte

The electrolyte is one of the four key components of lithium batteries and the main way they conduct electrons between the positive and negative electrodes. It is composed of high-purity organic solvents, electrolyte lithium salt (lithium hexafluorophosphate, LiF6), necessary additives and other raw materials, formulated in a certain proportion.

The battery’s energy comes from the fact that it can store lithium ions in both the anode and cathode. This process is called intercalation. It is what gives lithium batteries their high energy density, delivering 41.7 kJ per gram of lithium.

Unfortunately, repeated insertions of lithium ions into graphite—the standard material used in anodes—will eventually break it apart. To solve this problem, researchers have been looking at alternatives to graphite. One possibility is single-atom-thick sheets of carbon known as graphene. This would improve the battery’s performance and make it safer.

Cathode

The cathode is the electrode that attracts cations or positive charge and is the other side of the lithium battery battery to the anode. It is also the electrode that discharges and emits electrons in a process called electrochemical reduction.

During charging, the external power source applies an over voltage (which is higher than the cell produces and of the same polarity) which forces the charging current to flow from the positive terminal of the battery through the cathode to the negative terminal. This creates a potential difference between the cathode and anode that results in lithium ions moving from the anode to the cathode through a process called intercalation.

In lithium batteries, the cathode is often made of a combination of nickel, manganese and cobalt oxides (LiNiCoO2, or NMC) on a graphite anode. This material shows high specific energy, good load capabilities and low self-heating rates.

In the absence of an applied bias, thermally assisted diffusion of electrons into the p-doped layer of the cathode become what are termed minority carriers and tend to recombine on a timescale characteristic of the material. Similarly, holes diffuse into the n-doped layer of the anode and recombine on a similar timescale.

Anode

In a battery, the electrode that loses electrons during discharge is called the anode and the one that gains them during charge is called the cathode. The anode and cathode are separated by the electrolyte. This means that current flows between the two through ions rather than electrons.

Anodes are commonly made from metals like zinc or lithium. They need to be able to produce large amounts of electrical energy and have good conductivity, stability and a high coulombic output.

The anode is also known as the negative terminal of a battery because the direction of conventional current depends not only on the direction that charge carriers move but also on their electric charge, which is negative. However, the anode does not necessarily have to be negative, because batteries that are capable of recharging switch the roles of their electrodes during this process. This allows the anode to become positively charged and the cathode to be negatively charged again. This helps to prevent the formation of holes in the anode, which can cause a short circuit.

Separator

The separator keeps the positive and negative electrodes physically apart in order to prevent electrical current from passing between them. It also allows ions to move freely from the cathode to the anode on charge and back again on discharge. This process is referred to as self-discharge and it depletes batteries over time.

Early flooded nickel-cadmium and sealed lead acid batteries used glass fiber mats or cellulose soaked in sulfuric acid as separators. Modern sealed nickel-cadmium and maintenance free lead acid batteries use porous polyolefin films. These are usually made of a polyamide (nylon) or polyethylene, or a laminate of both.

Battery separators need to be thin, porous, mechanically durable and chemically stable. The pores in the separator must be wide enough to allow liquid electrolyte flow, yet small enough to keep ions from crossing the barrier. In addition, the pore size must be such that not all of it is filled with electrolyte, leaving some spaces to form gas channels. This is why LAKOS Standard Efficiency Separators are engineered with a pore size range of 30-50 nanometers (nm). This provides the right balance of pore size and porosity to achieve optimal performance.