Cathode

 

Cathode (LIB) is one of the two electrodes used in the cell which has lithium ions in it and these ions can be removed under the application of voltage or charging condition. In discharging condition, when circuit is closed the lithium ions move back to the cathode from anode.

By the word cathode it means positive, while cathode material is reduced by taking an electron, thus acting as a oxidizing electrode. Lithium ion cells are reversible and therefore oxidation and reduction takes place at the same electrode when we charge and discharge.

 There are different material chemistry for cathode and the simplest can be lithium metal, oxide materials (like lithium cobaltate - LCO), lithium manganese oxide - LMO), NCA, NCM, etc) and posphates (like lithium phosphate-LPO)). Oxides like NCA and NMC are more popular today which has structure similar to LCO but more cheaper along with more energy density. NCA and NMC are coined from the transition metals from which they are made of, i.e N for Nickel, C for Cobalt, A for Aluminium and M for manganese. They are often described together with numbers like for NMC (111), (622), (811) and the number indicates the ratio of the respective elements.

There are three main characteristics which decide the performance of any cathode material viz Voltage, Capacity, and Fast charging capability. Among the three voltage is inherent property of the elements from which the material is made up of, while the later two can be engineered by different methods which will be discussed elsewhere.

The voltage of the cathode is generally above 3V, LTO which has a nominal voltage of 2.4V and all properties similar to cathode is generally used as anode.

Increasing capacity and voltage is the game in R&D of lithium ion batteries. Research for increasing capacity in cathode is not leading to promising result, while the present chemistry is limited to around 200 mAh/gm and voltage around 4.2 V. Only a new chemistry with good voltage as well as capacity can solve the problem, but the present research is more focused on decreasing the amount of cobalt used in cathode and thereby decreasing the cost of battery.
Due to many chemistries in cathode available with different properties (performance, energy density, safety and cost), they are used in different application depending upon the need.

Let us study about different cathodes in brief

LCO has a good performance higher discharge voltage, low self-discharge, and around 1000 cycle life. This material has a high theoretical specific capacity and volumetric capacity but in reality it cannot give due to the strucutral stability of the layered structure after delithiation. The drawbacks are its low thermal stability, rapid capacity loss with cycling, and higher amounts of toxic and expensive cobalt.

LNO has very high structural instability especially when completely charged (highly delithaited state) it leads to structural disorientation and local structure collapse due to Li/Ni cation-mixing.

NCA chemistry has been evolved far from 33.3% cobalt to around 10% or atleast what present technology wants to get with high nickel content and low cobalt while alluminium is just a stability factor not contributing to capacity. Most of them are using NCA with 8:1.5:0.5 which still has more amount of expensive and toxic cobalt. It has high usable discharge capacity and long cycle life. Li/Ni cation-mixing is the major problem and it is minimized by by cationic doping with other metals.

LMP: Olivine-structured metal phosphates LiMPO4 (M=Fe,Mn, Co, Ni) are an interesting material family to replace the layered metal oxides LiMO2 as the positive electrode.Their safety owing to their intrinsic structural stability is very tempting for large-scale batteries. The olivine structure is composed of LiO6 and MO6 octahedra and PO4 tetrahedra, such that Li+-ion diffusion during lithiation/delithiation takes place only in one dimension, along the b axis. This material has high safety, cycle stability, cheap and environmentally friendly.