Lithium-Ion Batteries

Currently the lithium ion battery is the most rapidly developing energy storage device. Soon after its invention in 1991, the CoO2-Li ion battery became the predominant power source for small portable electronics such as mobile phones, digital cameras, and laptop computers. It is widely believed to be the best candidate for powering automobiles because it has the highest specific energy and the longest lifetime; see Table 12.4.

In some sense, the electrochemistry of Li ion batteries is the simplest. The only ion involved is the lithium cation, Li+. It has the smallest radius and the highest standard potential, -3.01 eV. The negative electrode is made of graphite, where the small Li ion intercalates into the space between adjacent sheets of grapheme. The positive electrode is a transition metal oxide, where the base metal can have different valence states to allow the lithium atom to join in or leave out.

Figure 12.8 shows the electrochemical processes in a Li ion cell. When fully charged, most of the lithium ions are buried in the planes of graphite. During the discharging process, as shown in Fig. 12.8(a), the lithium ions leave the negative electrode, drift through the electrolyte, pass the microporous separation film, and combine with the metal oxide in the positive electrode. At the end of the discharging process, most of the lithium ions are combined with the metal oxide in the positive electrode. During the charging process, as shown in Fig. 12.8(b), the lithium ions are forced by the external voltage to leave the metal oxide, drift through the electrolyte, pass the microporous separation film, and are intercalated into graphite.

Most Li ion batteries for small electronic devices, such as cell phones and digital cameras, use CoO2 as the basis of the positive electrode. During discharging, at the positive electrode, the lithium ion is combined with CoO2,

Li1-xCoO2 + xLi+ + xe – —> LiCoO2. (12.26)

At the negative electrode, lithium ions are extracted,

LixC6 —> C6 + xLi+ + xe-. (12.27)

During charging, at the positive electrode, lithium ions are extracted,

LiCoO2 —> Lii-xCoO2 + xLi+ + xe-. (12.28)

At the negative electrode, lithium ions are intercalated into graphite

C6 + xLi+ + xe – —> LixC6. (12.29)

In the above reactions, 0 < x < 1 is the fraction of lithium ion reacted.

The CoO2-based Li ion battery has very high specific energy. For applications where weight is an important factor, it is preferred. However, cobalt is expensive. In addition, it has been recorded that for large-size CoO2-based Li ion batteries, explosion has occurred. For power applications, Li ion batteries based on manganese oxide and iron phosphate are more preferred.

image814

Figure 12.8 Electrochemical processes in a Li ion cell. (a) during discharging, lithium ions leave the negative electrode, drift through the electrolyte, pass the microporous separation film, and combine with the metal oxide in the positive electrode. (b) during charging, lithium ions are forced by the external voltage to leave the metal oxide and intercalated into graphite.

LiFePO4 was discovered by John Goodenough’s research group at the University of Texas in 1996 as a material for the positive electrode of Li ion batteries. Because of its low cost, nontoxicity, high abundance of iron, excellent thermal stability, safety char­acteristics, good electrochemical performance, and high specific capacity (170 mA-h/g, or 610 C/g), it gained acceptance in the marketplace.

While LiFePO4 cells have lower voltage and energy density than LiCoO2 Li ion cells, this disadvantage is offset over time by the slower rate of capacity loss (aka greater calendar life) of LiFePO4 when compared with other lithium ion battery chemistries. For example, after one year on the shelf, a LiFePO4 cell typically has approximately

image815

Figure 12.9 Power Li ion batteries. (a) Single Li ion battery with nominal voltage of 3.7 V. (b) With 10 Li ion batteries connected in series, the nominal voltage is 37 V. Photo taken by the author. Courtesy of Phylion Battery Co., Suzhou, China.

the same energy density as a LiCoO2 Li ion cell. Beyond one year on the shelf, a LiFeP04 cell is likely to have higher energy density than a LiCoO2 Li ion cell due to the differences in their respective calendar lives.

The basic electrochemistry of the LiFePO4 battery is as follows. Iron has two oxidation states, Fe(II) and Fe(III). The Fe(III) compounds are often strong oxidizers. For example, the standard method of etching copper to make printed circuit boards is to use FeCl3,

2FeCl3 + Cu – t 2FeCl2 + CuCl2. (12.30)

Therefore, both FePO4 and LiFePO4 are stable compounds. Because the lithium ion is very small, these two compounds has negligible differences in volume per mole and share the same crystallographic structure.

The charging and discharging processes are as follows. In a fully charged LiFePO4 battery, the positive electrode is mostly FePO4, and graphite in the negative electrode is filled with lithium atoms. During discharging, at the positive electrode, the lithium ions are squeezed into FePO4 ,

FePO4 + Li+ + e~ —t LiFePO4. (12.31)

At the negative electrode, lithium ions are extracted from graphite,

LiC6 —t C6 + Li+ + e“. (12.32)

During charging, at the positive electrode, lithium ions are extracted from iron phos­phate,

LiFePO4 —t FePO4 + Li+ +e~. (12.33)

At the negative electrode, lithium ions are intercalated into graphite,

C6 + Li+ + e“ —t LiC6. (12.34)

Updated: August 23, 2015 — 11:38 pm