*3.1. Previous Technology Developments Associated with Lithium Batteries*

According to the cathode material utilized, Li-ion batteries are frequently categorized [80,81]. The common iron and phosphate are used to create LFP (lithium iron phosphate) batteries, on the other hand. Due to the material's hard olivine structure, these batteries have a very long lifespan and are capable of producing very high power. Unfortunately, this technology's intrinsic low potential compared to Li+ and particular capacitance make it less suitable for high-energy applications. LFP is still a good option for power applications (hybrid cars, power equipment, etc.) or situations requiring a lot of cycles, whereas both lithium nickel cobalt aluminum oxide (NCA) and lithium nickel manganese cobalt oxide (NMC) are energy-dense technologies, and electric cars frequently employ them. The amount of cobalt is being decreased in favor of the amount of nickel in both technologies, which is a definite trend. This guarantees a better energy density and lessens reliance on pricey cobalt. Studying different stoichiometric proportions enables us to see several types of NMC which are now available for commercial usage. Considering NMC111, NMC532, and NMC622, each of the three components are present in the same quantity. NMC111 is better suited for higher power applications, since it has less nickel and more manganese, but NCA, NMC-532, and NMC-622 are considered to be cutting-edge cathode materials, as seen in Figure 3.

**Figure 3.** Illustration of Energy density vs. specific energy.

There are many constraints of negative electrodes in terms of their availability for commercial usage. Because of their low potential when compared with Li+, it is observed that high specific capacitance and carbon anodes have outclassed them since 1991. In the year 2016, almost 90% of commercial batteries were based on graphite, whereas only 7% of them had amorphous carbon and only 2% had lithium titanate oxide. These materials have the ability to charge the batteries quickly but at the same time the raw material seems quite expensive, along with the low energy density [82]. There is a plethora of perks of the electrode material available today in the market, and this is because of the recent research and developments occurring in the field of Li-ion batteries, as illustrated in Figure 3. In the near future, silicium's contribution to this will be crucial. It has been observed while going through the literature that silicon is one of the alternate solutions for the next-generation materials for anodes [83], and this leads toward the reduction in the low prices: almost eight to ten percent less than graphite. In addition to this, the life cycle of silicon-based batteries is still short, even with the amalgamation of graphite electrodes in lower amounts. A few examples of this amalgamation have occurred already, for example, the 5 percent incorporation in Panasonic cells which were later utilized in Tesla X. We all expect that the technology that we have today will surely expand in upcoming years, and with this expansion there will surely be a boost in nickel-based cathodes that may lead to lessen the silicon's content and immediately will result in an increment in energy density. This is something that is expected until 2025, along with the anticipation of technologies based on lithium-sulfur-oxygen and solid-state batteries. In the coming eight to ten years, the current market is predicted to head toward the next generation of technology based on the lithium-ion battery. It is concluded that with an inclusion of cobalt or nickel the energy densities can be increased at the cell level as well as at the pack level, as shown in [84,85]. In order to fulfil this prediction, extensive research is required on these solid-state electrolytes. These are the thicker electrolytes with higher energy densities. They are not flammable and do not have any impact on concentration polarization voltage losses like the other liquid electrolytes. These are found to have brilliant dendritic growth resistance that allow us to use Li-based anodes easily [86].

Specifically, for a solid electrolyte to be used for the applications associated with an electric vehicle, it must have a fast charging capability. In addition to this, the maximum current density over which the battery becomes short circuited due to the Li-dendrite penetration phenomenon is one of the key things to be kept in mind. The modern parametric reading for critical current density values should be below the value of 5 mA/cm2 [87,88], but in actuality they are even below 0.12 mA/cm2. Furthermore, variable current densities are needed for charging and discharging. Recently, it was discovered that critical current densities are higher during charging than during discharging [86].

Investigating the interfaces such as electrode to electrolyte in solid-state batteries is one of the significant aspects in acquiring high specific energy with a longer life cycle. Moreover, one of the reasons due to which cells usually fail is the electro-chemical interfacial instability. For example, solid-state electrolytes presently have a stability window up to 6 V as compared to Li+/Li batteries. The cell impedance may eventually rise as a result of the breakdown of the solid electrolyte–solid electrode contact. The purpose of some techniques, such as liquid–solid hybrid electrolytes, is to explain the interface instabilities [87]. It is seen that the polymers and their composite electrolytes are majorly centered on solid-state batteries; this is because of their well-oriented nature towards the field of energy storage. In addition to providing improved mechanical flexibility, processability, and scaling up, they are less flammable than liquid electrolytes. Due to its broad ionic conductivity ranges, poly (ethylene oxide) (PEO) and its derivatives have fascinating solid-state battery possibilities. Compared to the conventional organic liquid electrolytes, ion conduction is still less efficient and more challenging [89–91]. Solid-state battery production comprises distinct lines for anode, cathode, and electrolyte sheets, much like standard Li-ion battery assembly. The production of battery parts and the methods by which they are assembled, however, differ. In contrast to conventional Li-ion batteries, electrodes must be added after the electrolyte has been created. Additionally, the creation of extremely poisonous H2S (in the case of sulphides such as Li6PS5Cl) and somewhat high temperatures (over 1000 C in the case of Li7La3Zr2O12) are required for the production of solid electrolytes [92].
