Introduction

LiCoO₂ was discovered in 1980 as a lithium-ion intercalation material by Professor John B. Goodenough.^[1] 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.^[2][3]it is a frequently utilized as cathode material in rechargeable lithium-ion batteries (LIBs).^[4]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.^[5]

Li_xCoO₂ crystallizes in α-NaFeO₂ layered structure with trigonal R-3m symmetry. Li_xCoO₂ is the general formula for cathode material for lithium-ion batteries which indicates different amounts of lithium content as Li is deintercalated from LiCoO₂. Li_xCoO₂ is a lithium-deficient 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.^[6][7]

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 ^[8]

High-temperature phase layered structure (HTLCO) shows outstanding electrochemical performance.^[9]

Working Principle

The cathode, anode, electrolyte, and separator are the four main parts of a standard Lithium ion batteries. LiCoO₂ acts as positive electrode material, whereas graphite is the most typical negative 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^[10].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 negative electrode to the positive electrode. Similarly, when the device is being charged, the positive electrode releases lithium ions that subsequently go back to the negative electrode.^[11]

Synthesis

LiCoO₂ can be synthesized using two procedures, sol-gel preparation method and Solid-state 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. ^[12]

Crystal Structure

Table 1. Properties and Crystallographic data for LiCoO₂  using single crystal XRD Data^[13]

LiCoO₂ crystallizes in the trigonal crystal system and belongs to the space group  R-3m. The crystal  structure consists 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 coordinates with nearby oxygen anion at the octahedral sites Figure 2b. Lithium layers are intercalated between the CoO₂ planes.  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^[13]. 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 Li and Co ions are organized in alternating  (111)  planes  of  R-3m  space  group. ^[1][15].

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) ^[13]

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. Powder XRD pattern of LiCoO₂ (Figure by Fasiha Israr using VESTA)^[16]

The Figure 4 shows the diffraction peaks that correlate to the layered LiCoO₂ structure with 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.^[16]

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 frameworks) material 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)^[17]

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.^[17]

Figure 6. Comparison between mechanism rate of porous and non-porous LiCoO₂ material. (CC BY-NC 3.0) ^[17]

References