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# LiCoO2

Topic reserved by: Fasiha Israr

# Introduction

LiCoO2 was discovered in 1980 as a lithium-ion intercalation material by Professor John B. Goodenough. it is considered as one of the important compound in battery chemistry due to its high energy density and outstanding cycle stability, it is commonly known as lithium cobalt oxide. The Cobalt atoms has +3 oxidation state in the compound. It is a dark bluish gray crystalline material.it is a frequently utilized as cathode material in rechargeable lithium-ion batteries (LIBs).LIB is a particular kind of rechargeable battery in which lithium ions travel from the positive electrode (cathode) to the negative electrode (anode) during discharge and return during charging.

LixCoO2 crystallizes in α-NaFeO2 layered structure with trigonal R-3m symmetry. LixCoO2 is the general formula for cathode material for lithium-ion batteries which indicates different amounts of lithium content as Li is deintercalated from LiCoO2. LixCoO2 is a lithium-doped variant of the chemical LiCoO2. The formula's 'x' stands for the variable number of lithium ions that is inserted into the substance's crystal lattice.

LiCoO2 has a layered structure, where CoO2 layers are separated by Li layers. The CoO2 layers consist of a hexagonal close-packed arrangement of oxygen atoms with Co ions occupying the octahedral sites, while the Li ions are located in the interstitial sites between the CoO2 layers

High-temperature phase layered structure (HTLCO) shows outstanding electrochemical performance.

# Working Principle

Figure 1. Charge–discharge process of a lithium-ion cell using graphite as anode and LiCoO2 as cathode (Figure by Fasiha Israr)

The cathode, anode, electrolyte, and separator are the four main parts of a standard Lithium ion batteries. LiCoO2 act as negative electrode materials, whereas graphite is the most typical positive electrode material. The conductive media for ion migration is provided by the electrolyte in a form of liquid, solid, or semi-solid and separator helps to prevent short circuit in battery.The traditional LIB used electrolyte consists of lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate and linear organic carbonates mixture. These traditional electrolytes are considered as dangerous due to its flammability and volatility. Nowadays, less flammable electrolyte material such as Organosilicon compounds, Adiponitrile, Gel-polymer etc, are in use to minimize the hazard of explosion of batteries. While the device is being discharged, an electric current is created as the lithium ions move through the electrolyte from the anode to the cathode. Similarly, when the device is being charged, the cathode releases lithium ions that subsequently go back to the anode as illustrated in Figure1.

$LiC6 + CoO2 ➜ LiCoO2 + C6$

# Synthesis

LiCoO2 can be synthesized using two procedures, sol-gel preparation method and Solid-solution method are commonly used. In sol-gel method, Lithium acetate dihydrate  and cobalt acetate tetrahydrate were used as reactants in the sol-gel production process to synthesized LiCoO2. Deionized water was used to dissolve the reactants in stoichiometric proportions, and citric acid was added to the mixture as a chelating agent for the gel. The solutions were heated at 80 °C. while being vigorously agitated until a thick gel was produced. The heating times used were 6-36 hours. Before being calcined in air using a tube furnace at 700 °C for 7 hours, the gel was dried in an oven at 80 °C.

# Crystal Structure

Table 1. Properties and Crystallographic data for LiCoOusing single crystal XRD Data

 Melting Point about 1100℃ Color Dark bluish gray Structural Formula LiCoO2 Temperature (K) 300 Crystal System Rhombohedral Space Group R-3m Molar mass 97.87 g mol−1 a / Å ~2.82(10)diffraction peak occurs at 2θ = ~2.82 degrees with a FWHM (Full Width at Half Maximum) of 10 units of 2θ c / Å ~14.05(76)diffraction peak occurs at 2θ = ~14.05 degrees with a FWHM (Full Width at Half Maximum) of 76 units of 2θ

LiCoO2 has rhombohedral symmetry and  belongs to  the space group  R-3m, This  crystal  structure  of  closed-packed  oxygen  layers stacked in an ABC arrangement with Co and Li ions residing in octahedral sites in alternating layers between the oxygen planes. Layers of intercalated lithium and cobalt(III) ions are present in Figure 2a. In order to create the Co-O layers, the cobalt(III) ion forms a stable connection with the nearby oxygen anion at the octahedral sites Figure 2b. At the end, the CoO2 plans are intercalated between the lithium layers.  Lithium and cobalt(III) ions alternately occupy the octahedral positions of these layers, forming an ABCABC-type sequential stacking with oxygen ion layers as shown in Figure 2c. It has an α-NaFeO2 type layered structure in which the oxygen atoms are combined in a cubic close-packed  system and  with  the  cell  parameters  of  a=  ~2.82(2)  Å  and c= ~14.05(1) Å, the Lithium and Cobalt ions are organized in alternating  (111)  planes  of  R-3m  space  group. .

Figure 2.  (a) Layered rhombohedral crystalline structure of the LiCoO2 (b) The octahedral CoO6 structure; (c) layered arrangement of
the layers (CC by 4.0)

Figure 3. polyhedral crystal structure of LiCoO2; purple color represents the Li ions, blue represents Co ions and Pink shows the Oxygen ions (Figure created by Fasiha Israr on VESTA).

# Characterization:

Figure 4. Shows the XRD pattern of LiCoO2 (Figure by Fasiha Israr using VESTA)

The Figure 4 shows the diffraction peaks that correlate to the LiCoO2 layered hexagonal structure with lattice parameters c  =  14.05Å . The c-axis is the distance between the layers of CoO6 octahedra in the crystal lattice is specifically shown by the  strongest (003) peak at 19 degree angle.

# Application Modification

## Porous LiCoO2 - High-Rate Cathode

The LiCoO2 is widely used in LIBs as cathode material due to its excellent cycling performance, great working potential, and high specific capacity.  Porous material is considered as most important in almost every field and it has several uses like, including energy storage, catalysts, adsorption, etc. Porous LiCoO2 prepared by cobalt base MOF (Metal-organic frameworksmaterial are very stable and shows the highest rate capacity with 96.4% capacity retention up to the 100 cycles and reversible capacity of 106.5 mAh/g. The capacity of commercial LiCoO2 rapidly declined due to increase in the density of current and its capacity decreased to almost zero when the current density reached 6C as seen in Figure 5.

Figure 5. The capacity of commercial(dense) and porous LiCoO2 at different rates. (CC BY-NC 3.0)

The Figure 6 shows the difference between the rate of mechanism of LIB using porous and non-porous LiCoO2 material. The Porous LiCoO2 exhibits the highest rate mechanism by increase in charge transportation due to availability of large number of contact sites between electrodes and electrolyte. The pores could facilitate the dispersion of Lithium ions owing to shorter pathways for ion transmission. It also helps to retain the crystal structure of LiCoO2 in the course of lithium ion intercalation/de-intercalation process.

Figure 6. Comparison between mechanism rate of porous and non-porous LiCoO2 material. (CC BY-NC 3.0)

# References

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