Basic Lithium-Ion Batteries

Basic Lithium-Ion Batteries – After the electric motor, the main component in an electric vehicle is the battery. Batteries play an important role in the journey of electric vehicles, even now the research process is ongoing to get the best battery. Let’s say the latter is research on solid-state batteries. However, all of this requires basic knowledge. And in this article and several other articles – in order – we present the following;

Easy Battery Fundamentals

Table of Contents

Since the vast majority of electric vehicles on the market today use lithium-ion batteries, we will focus on this particular chemistry. We will first take a look at the components that make up the battery, followed by the processes that make it run.

Cathode (+)

The cathode of a lithium-ion battery is made of a lithium-containing compound: most commonly, lithium cobalt oxide ( $LiCoC_2$ ). Other cathode materials include lithium manganese oxide ( $LiMn_2O_4$ ), lithium iron phosphate ( $LiFePO_4$ ), lithium nickel cobalt aluminum oxide ( $LiNiCoAlO_2$ ), or lithium nickel manganese cobalt oxide ( $LiNi_xMn_yCo_zO_2$ ). Given that the basic processes are essentially the same, we will use lithium cobalt oxide as our example.

The cathode is the natural home of the $Li^+$ ions (and corresponding electrons) that make the battery run. When the battery is fully discharged (and when the battery is being manufactured), the cathode is where the lithium lives. On charging, the $Li^+$ ions take a vacation to the anode. On discharging, the $Li^+$ ions return home to the cathode.

Anode (-)

The anode of a lithium-ion battery is made of a compound that can temporarily accept $Li^+$ . Most commonly, it is made of graphite ( $C_6$ ). Other anode materials include lithium titanate ( $Li_4Ti_5O_{12}$ ), hard carbon, an alloy of tin and cobalt, or a combination of silicon and carbon. Given that the basic processes are essentially the same, we will use graphite as our example.

The anode is like a vacation home for the Li+ ions. On charging, the anode temporarily accepts them in, only to send them home to the cathode on discharging.

Electrolyte

The electrolyte is what allows the $Li^+$ ions to move between the cathode and the anode. The key characteristic of the electrolyte is that it is an ionic conductor and an electrical insulator. This means that lithium ions can easily pass through it, but electrons can’t.

Circuit

Though technically not part of the battery itself, the circuit is an integral part of its operation. Similar to (but the exact opposite of) the electrolyte, the circuit is an electrical conductor and an ionic insulator— that is, electrons can easily pass through it, but lithium ions can’t. This means that while lithium ions travel from one electrode to the other through the electrolyte, a corresponding amount of electrons travel from one electrode to the other through the circuit. It’s like the electrolyte and the circuit are a boat and a train. When traveling from home (cathode) to vacation home (anode), or vice versa, the $Li^+$ ions always take the boat (electrolyte), while the electrons always take the train (circuit). They travel in parallel on different routes, meeting up at their destination.

Separator

The separator is what keeps the anode and cathode from touching and prevents a short circuit. This separation of charge is what allows the battery to store energy. In lithium-ion batteries, the separator is generally a thin, porous material such as cloth made from fiberglass or film made from nylon, polyethylene, or polypropylene. The separator allows $Li^+$ ions to travel through the electrolyte while ensuring electrons travel through the circuit instead.

Charging

When the battery is being charged, the charger uses power from the grid to transfer electrons from the cathode to the anode. At the same time, a corresponding number of $Li^+$ ions move from the cathode through the electrolyte and separator to the anode.

In our example, $Li^+$ is extracted from lithium cobalt oxide at the cathode and inserted into graphite at the anode. The technical terms for this extraction and insertion are deintercalation and intercalation. When we say something is intercalated, this simply means it is stored within the lattice structure of a compound. Figure 1 shows lithium intercalated in $LiCoO_2$ . It shows three layers of $CoO_2$ with many individual lithium atoms intercalated between the layers. $LiC_6$ has a similar structure, except the $C_6$ layers are made of carbon rings.

Equations (1) and (2) – sequentially – show what is happening at the cathode and the anode during charging. At the cathode, $LiCoO_2$ gives up an $Li^+$ and an $e^-$ , and at the anode, they combine with $_6$ to form $LiC_6$ .

$Cathode: LiCoO_2 \longrightarrow CoO_2+Li^++e^-.$

$Anode: Li^++e^-+C_6\longrightarrow LiC_6.$

Discharging

When the battery is being used (discharged), electrons move from the anode to the cathode, powering the load. At the same time, a corresponding number of $Li^+$ ions move from the anode through the electrolyte and separator to the cathode.

Equations (3) and (4) – sequentially – show what is happening at the anode and the cathode during discharging. At the anode, $LiC_6$ gives up an $Li^+$ and an $e^-$ , and at the cathode, they recombine to form $LiCoO_2$ .

$Anode: LiC_6\longrightarrow Li^++e^-+C_6$

$Cathode: CoO_2+Li^++e^-\longrightarrow LiCoO_2.$

The chemical processes involved in other lithium-ion chemistries are similar, simply replacing $CoO_2$ and $C_6$ with the corresponding compounds that serve as the lattice structures at the cathode and anode.

Another way to think about charging and discharging is to imagine that, when charging, we’re pushing the lithium ions and electrons up a hill, so to speak, to a place of higher potential energy. Discharging, in this analogy, is equivalent to letting them fall back down the hill in a controlled manner, converting their stored potential energy into usable work.

Basic Lithium-Ion Batteries