ABSTRACT

Materials with the reversible intercalation properties, such as LiCoO2, LiMn2O4, LiFePO4, and LiNi1/3Mn1/3Co1/3O2, have been explored as viable cathode materials for the lithium-ion batteries in the past two decades along with the extensive research on Li2MnO3and its derivatives, also known as lithium rich solid solution. Intensive structural and electrochemical research has led to the lithium-ion technology to a stage where it is commercialized in almost every crucial electronic device of modern society. In order to enhance the energy density of the material and make it work for the longer time span, requirements such as high specific capacity, high operating voltage, and structural stability should be met. Researchers across the world have also made an effort to reduce the amount of costly and toxic materials ingredients so as to minimize the environmental impact without compromising

the performance. This chapter reviews various experimental and computational approaches that have been utilized for obtaining the materials’ best electrochemical properties. We have tried to cover all such crucial breakthroughs in the cathode materials’ research related to layered, spinel, and olivine structures. 6.1 IntroductionIn the past two decades, much emphasis has been made on the materials advancement for lithium-ion batteries. These batteries hold the future for the next-generation energy storage devices. Efforts have led to their successful application in most of the portable devices of today’s life, such as camera, laptops, and next-generation mobile phones. In the vision of using these batteries as an alternative to petrol or diesel in cars, these batteries have been improved on at both materials as well as engineering level. Some of the major drawbacks of using these batteries in cars include their cost and the use of organic electrolytes. These factors make lithium-ion batteries less safe than other battery technologies. In the early development of these batteries, graphite in the form of MCMB (Meso carbon micro beads) and LiCoO2 were used as anode and cathode material, respectively. Electrolytes composed of salts such as LiAsF6 or LiPF6, etc., dissolved in organic solvents such as DEC (di-ethyl carbonate), DMC (di-methyl carbonate), and EC (ethlyene carbonate), etc., have been used. It has been observed that the major contributor to the cost of these batteries is the cathode material, which comprise materials that can reversibly intercalate lithium ions in the structure. LiCoO2is a state-of-the-art cathode material in lithium-ion batteries due to its excellent performance from capacity, reversibility, and rate capability as proposed in the early 1980s [1]. However, this material is thermally unstable and also contains toxic cobalt, which is not an environment-friendly element. In an effort to identify an alternative to LiCoO2, several materials based on other transition metals have been proposed. However, the layered materials’ development without cobalt has not moved fast enough due to a number of requirements that battery materials should meet before their actual use in the market.LiMn2O4 is considered an alternative material to LiCoO2. The basic crystal structure of LiMn2O4, which is a spinel, is entirely

different from LiCoO2 [2]. LiMn2O4 meets almost all of the requirements for a battery cathode such as good thermal stability, reasonably good capacity, and capability to charge and discharge at high current rates. Mn is less toxic and inexpensive compared to Co. Therefore, this material has drawn much attention toward its use in the electric vehicles. Spinel-based LiMn2O4 possesses low charge/discharge capacity compared to LiCoO2 and have some disadvantages regarding the stability of the structure, which lead to failure of the cell after certain number of cycles. Mn oxidation state in case of LiMn2O4 is +3.5, which implies that Mn3+ and Mn4+are in present in 1:1 in ideal stoichiometry. Mn3+ is known as Jahn-Teller active ion, which undergoes distortion at low voltages. Therefore, the lower cutoff voltage in case of the spinel electrodes plays important role in the stability of the electrode material. It has also been pointed out that even though Jahn-Teller distortion is highly dependent on the voltage cutoff, but some structural fading also occur at high voltages because of the dissolution of the Mn ions in the electrolyte. Distortion in the structure leads to decrease in capacity with the increase in number of cycles. In order to improve Jahn-Teller distortion dopants such as Cr and Ni have been tried. An improved version of LiMn2O4 has been proposed by substituting some of Mn with Ni. Micron sized Ni doped LiMn2O4has shown very good performance at high C-rates. Capacity as high as 130 mAh/g has been achieved at 1C rate of charging/discharging [3]. Li2MnO3 have also reported another manganese-based composition, that belongs to C2/m space group symmetry and it is electrochemically inactive [4]. However, it has been shown that it can be activated by chemically or electrochemically removing Li2O from the host structure and leaving MnO2 cage structure. Many articles have been published to explain the reaction mecha-nisms that occur at high potentials however the cause behind slow transformation of the layered to the spinel structure is still under research and is thorny issue in the lithium ion battery cathode material development. Another interesting aspect of this material is that it can be structurally integrated into other well-known layered materials to have composite structure with enhanced properties. LiMnO2 is another compound that lies in the category of the manganese-based oxides for lithium-ion batteries. Therefore, cathode materials based on manganese have drawn much attention in the past and still a more advanced research is being carried

out in this area and has been discussed in this review. LiMnO2shows very high capacity, however, due to the presence of Mn in its +3 oxidation state it undergoes Jahn-Teller distortion and it transforms to spinel structure. This is a major issue that needs to overcome because such Jahn-Teller distortion can lead to the formation of the lithiated spinel cathode and performance of the cell deteriorates as per the following reaction: Li2Mn2O4  Li2MnO3 + MnOMn2+ is soluble in the electrolytic solution and hence gives rise to poor cycling stability. In order to avoid the Jahn-Teller distortion a part of Mn was replaced by Ni and Co. LiNi1/3Mn1/3Co1/3O2has been reported as a promising cathode compound with high energy density for its use in lithium-ion batteries [5].A new composition based on iron phosphates was reported in the form of LiFePO4, which is known to have olivine structure [6,7]. This material initially had very low electronic conductivity. However, this drawback has been overcome by its doping or coating with carbon. A breakthrough in research based on this material made by MIT researchers led to improvements in its rate capability [8]. It has been shown that the synthesis of off-stoichiometric phase LiFe1-2yP1-yO4-δ gives rise to a material that is capable of charge and discharge at very high C-rates, because such kind of off-stoichiometry synthesis leads to the formation and coating of glassy phases that are highly conductive in nature. This communications has been criticized by John Goodenough and coworkers on the technical grounds in one of the letter to the editor of Journal of Power Sources. 6.2 High Voltage Spinel Electrode MaterialsIn the recent past, much focus has been made on the high energy density and high power density materials for lithium-ion batteries, which can be utilized for EV and HEV. The materials need to be low cost and nontoxic in nature. The thermal stability of these materials is another factor that should be taken into account. One of the promising candidates in this category is LiMn2O4. This material is quite inexpensive, less toxic and environment friendly. It is thermally stable compared to the layered LiCoO2. However, its

practical capacity is limited to ~120 mAh/g. The working voltage lies in the 4.2 to 3.0 V range and the voltage profile is flat compared to the layered LiCoO2. Some of the reports related to thin-film batteries have also implied LiMn2O4 as a cathode material and checked its electrochemical properties as thin-film electrodes [9,10].