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​Strategies for improving rechargeable lithium-ion batteries

Lithium-ion batteries are the kind of batteries in which lithium ions are present which are responsible for the carrying of charge from the negative electrode to the positive electrode and vice versa. There are certain types and ways in which these Li-ion batteries process and one of the most worked and authenticated ways is the utilization of rechargeable lithium-ion batteries.

They are being used at a great level owing to the potential that they have and the characteristics that these bring forth. In recent times their use in the market has been accelerated and for that certain strategies have been made to improve the working of these rechargeable lithium-ion batteries. All these strategies when either used separately or together make a huge difference and add up to the credibility of these batteries.

Introduction

Typically a lithium-ion battery is also known as a Li battery which is one of the types of rechargeable batteries found in the market. In rechargeable Li batteries, the Li-ions carry the charge from the negative end towards the positive end. This process is carried out with the help of an electrolyte and is done during the discharge process. Two different materials are used at both ends of the battery. Intercalated lithium is used at the positive electrode end whereas graphite is used at the negative electrode end. These batteries are most certainly used for electric vehicles and electronics that are portable. They have a great reach in military and aerospace areas as well.

 

Characteristics of li-ion batteries

The three basic characteristics that are interpreted by the Li-ion batteries are as follows:

  • High energy density.
  • No memory effect at all except for the LFP cells.
  • Low self-discharge.

There are different purposes for the manufacture of cells which can be either done to give prioritization to energy or power up the density. Like everything comes in the market with a lot of perks, there can be a few safety hazards as well in regards to these as they consist of electrolytes which are highly flammable and once they are left untreated in terms of charging can cause serious damages and harms.

Prototype Li-ion battery

In 1985, Akira Yoshino first came up with the idea of a prototype Li-ion battery and developed it by keeping in view the research that was previously carried out by John Goodenough and his fellows in the early 1970s and 1980s. Later in 1991, another type of Li-ion battery was developed by Yoshino.

Li-ion battery

Lithium polymer batteries

Lithium polymer batteries are the ones that comprise a polymer gel as an electrolyte, a cobalt oxide comprising of lithium as a cathode material, and graphite as an anode. All of these when combined bring forth a high energy density battery. These offer longer lives and their rate capability is much better than the rest. Their use is evident in mostly medical supplies and electric tools.

Research

According to research, lithium-ion batteries have a wide range of characteristics that add up to the goodwill of these batteries which include long-lasting life, high energy density, and enhanced safety measurements, less cost, and increased charge speed. It highly supports the idea of rechargeable batteries as these are considered one of the best-known batteries for the characteristics that these bring forth.

Electrochemistry

It is known well that certain electrochemical reactions are going on in any cell. Likewise in a lithium-ion cell, the reactants which are present and responsible for carrying out the reaction are anode and cathode materials which are compounds comprising of lithium atoms.

Oxidation

In lithium-ion cells, an oxidation half-reaction is carried out at an anode which is responsible for producing lithium ions as the positive charge and electrons as the negatively charged. This reaction may as well be responsible for the production of an uncharged material that stays at the anode. The li-ions which work as the positive charge move via an electrolyte whereas the electrons which work as a negative charge move via an external circuit and eventually they combine at the cathode in addition to the cathode materials which then becomes a reduction half-reaction.

Reduction

In this reaction, the electrolyte and external circuit are responsible for providing the charge to both, lithium ions and electrons. However, these are not involved in any electrochemical reaction. In the process of discharge, the electrons present at the anode move towards the cathode via an external circuit. In the process of discharge, reactions are capable of lowering the chemical potential of the cell and the energy in return is shifted from the cell to wherever the electric current is being transferred which is mostly in the external circuit.

The direction of movement

In the case of charging reaction and transports both moves in opposite directions. Electrons travel from the positive electrode following to the negative electrode via an external circuit. This way the external circuit is also responsible for providing the electric energy to charge the cell. The energy which is produced is now stored and named as chemical energy in the cell.

Liquid electrolytes

Lithium-ion batteries also consist of liquid electrolytes which comprise lithium salts such as LiPF6, LiBF4, or LiClO4.

Liquid electrolyte serves as a pathway of conduction for the process of discharge while cations move from the negative electrodes towards the positive electrodes. There is a combination that goes around consisting of linear and cyclic carbonates. These consist of ethylene carbonate formally known as EC and dimethyl carbonate formally known as DMC. High conductivity and SEI which is known as solid electrolyte interphase, forming ability is offered via this combination.

Organic solvents

During charge, organic solvents are capable of decomposing on negative electrodes. Usage of appropriate organic solvents is very important and when it is carried out then the solvent becomes capable of decomposing on initial charge as a result of which a solid layer is formed known as the solid electrolyte interphase. Solid electrolyte interphase is usually electrically insulating yet is capable of providing iconic conductivity. However, the best thing about interphase is that it stops any additional decomposition of the electrolyte after the second charge.

Composite electrolytes

Composite electrolytes are usually based on polyoxyethylene also known as POE and are responsible for providing a stable interface. It is present in two forms, liquid and solid. When present in solid form it comprises of higher molecular weight and is suitable for use in dry Li-polymer cells. While when present in liquid form it comprises of lower molecular weight and is suitable for use in regular lithium-ion cells. One of the approaches to decreasing the flammability and volatility of organic solvents is the RTIL approach known as room temperature ionic liquids.

Charging and discharging

There are two types of processes which are carried out, one is charging and the other one is discharging.

Discharge

In the process of discharge, lithium ions are responsible for carrying out the current within the battery from negative towards positive and no external force or circuit is used. However, it is done via a non-aqueous electrolyte and a separate diaphragm.

Charging

In the process of charging, the opposite of discharging is done for which an external power source is required for the application of overvoltage to make it possible for the current to follow within the battery from positive towards the negative electrode. In this way, the lithium ions become capable of moving from the positive to the negative electrode, and this process is then known as intercalation as the ions embedded in the electrode material in this process.

Energy loss

A certain amount of energy loss arises due to the electrical contact resistance at the interfaces which is calculated to be about 20 percent of the entire energy present in the batteries.

Charging procedures

There is a complete set of charging procedures for a li-ion cell and a complete li-ion battery separately and both of these slightly vary from each other.

A single Li-ion cell is categorized in two stages following which it can charge:

  • Constant current (CC).
  • Constant voltage (CV).

A Li-ion battery (the one which comprises a series of Li-ion cells) is categorized in three stages following which it can be charged:

  • Constant current.
  • Balance (not required once a battery is balanced).
  • Constant voltage.

There is a phase known as the constant current phase in which constant current to the battery is applied by the charger while increasing the voltage gradually till the time when the voltage reaches the limit of the cell.

Charging temperature limits

There are certain charging temperature limits for lithium-ion batteries but because these batteries are rechargeable so their charging limits are stricter as compared to the operating limits. It is observed that the chemistry of lithium ions performs better when the temperatures are increased but it is important to keep in mind that excessive exposure to heat is destructive for the battery health and life. They perform excellently when charged at lower temperatures and also activate the ‘fast charging’ system of the battery. The temperature range for this should be 5 to 45°C as it is considered the best temperature range for charging.

Low temperature

When the temperature is dropped down and is brought in the range of 0 to 5 °C then the current of charge should be massively reduced yet charging is possible. When the low-temperature charge is going on then a slight increase in temperature is proved advantageous as it happens because of the internal resistance of a cell.

However, keeping the required temperature limits is very much important for the proper charging of lithium-ion batteries because it will directly affect the working mechanics of the battery and will add up to the battery’s life and health.

Strategies for improving rechargeable lithium-ion batteries

Intercalation cathodes

The use of intercalation cathodes has been introduced as one of the greatest strategies to improve the working mechanics of rechargeable lithium-ion batteries. These intercalation cathodes have four different crystalline structures named layered, spinel, olivine, and tavorite. It has been proven that a lot of different materials can work up as intercalation cathodes but the transition materials and polyanion have been emerged as the best choice owing to their excellent operating voltages. The maximum specific capacity range for these oxide cathodes is said to be ∼150–200 mA h/g at the cathode level. However, researches are being carried out in this regard to achieve a high specific capacity range for these intercalation cathodes because it will be regarded as another strategy to improve the working of rechargeable lithium-ion batteries.

Layered intercalation cathodes

The first type of intercalation cathodes is the layered intercalation cathodes but Whittingham and Gamble were the first ones to develop these intercalation cathodes in 1975. Although, it was due to the low voltage value that the first-ever intercalation cathodes were changed with the layered ones, the formula for which is LiMO2. The reason behind shifting to these cathodes was that they possess high values of operating voltages and despite that high theoretical specific capacity as well.

LCO

Then came Goodenough who introduced LCO as a cathode that can be alternatively used which was later on commercially launched by SONY too. This material comparatively possessed more capacity but its cost was a little too high. Other than that it has some weird structural defects as well which enable it to suffer from fast capacity fade.

Other oxides

Additionally, a lot of other isostructural oxides have also been considered for use as a cathode for lithium-ion batteries keeping in mind the budget and statistics. The alternative options included LNO and LiMnO2, both of which are cheaper but thermally unstable too which excludes them from the working categories.

Spinel intercalation cathodes

It was proposed by Thackeray et al., in 1983 that the product LiMn2O4 is one of the best products to be used as an intercalating cathode as its specific capacity range is ∼150 mA h/g. This product is also known as LMO. The structure of LMO is spinel and in its structure, Li is located at the tetrahedral sites whereas Mn is located at the octahedral sites. The reason for using LMO as an intercalating cathode is that comparatively, it is cheap and environment friendly though it does have a little drawback which is the cycling stability. It is poor in the case of Mn as it has the property of dissolving in two electrolytes known to be LiPF6 and LiAsF6.

Result

As a result, it has been reported that the spinel intercalation cathodes are effective when used appropriately to serve the purpose of improving rechargeable lithium-ion batteries. It is important to know the characteristics of materials being used and then incorporate them in the industrial uses to be sure of what they can bring to the table. The researches and experiments have proved that ever since the spinel intercalation cathodes have been introduced in the market, the efficacy and working of rechargeable lithium-ion batteries has increased which makes them sustainable and an important material as they add up to the value of products that they become a part of.

Olivine intercalation cathodes

Olivine phosphates known as LiMPO4 as well as olivine silicates known as Li2MSiO4 are the materials in which M = Fe, Mn, Mg, and Co are considered as the cathode materials. There are some layered cathode categories as well which comparatively possess more stability and power capability. These are known as the LCO and LNO cathodes. LFP has been previously used as a conversion type cathode in lithium-ion batteries but it has been observed that the circuit potential and electrical conductivity are relatively less portrayed and to increase that several dopants have been brought to use too which aid in increasing the conductivity for LFP.

New approach

However, a new approach has been introduced which is extremely beneficial when it comes to incorporating the materials that can work great as conversion type cathodes in lithium-ion batteries. All of these techniques are responsible for taking care of the charging current and the rate at which it flows and for that the use of vertically aligned carbon nanotubes also known as VACNTs has been introduced in this market.

Result

As a result, the technique of using olivine intercalation cathodes for the improvement of lithium-ion batteries is considered very beneficial as they notably add up to the betterment and bring forth better opportunities for these batteries.

Conversion-type cathodes

The first commercial launch of lithium-ion batteries was conducted by SONY in 1991 and from that time, constant improvement strategies have been emerging for the better working of these batteries. In recent times the alternate materials that are used as a conversion type cathode have been the mode of attraction due to the practicality that they bring forth. There are a lot of conversion type AMs but out of all those, sulfur is considered as the best one for all the portable devices that are next generation as it possesses 1,675 mA h/g as its high theoretical gravimetric capacity and 2,500 W h/kg as its gravimetric energy density yet its best feature is the low cost.

Efficacy of sulfur

Though it does bring along certain challenges too which can be overcome through proper strategy making and following that strategic planning. This will not only increase the battery life but will also add up to the better working of the entire battery as well as the product that this battery will be a part of. Sulfur is an excellent product and when it is used as a conversion type cathode it promotes better battery health which is the factor for the proper functioning of the battery. However, it is important to stay updated with researches and continue to add and subtract from the advancements as per the researches.

Intercalating anodes

The graphite anodes are the ones that contain ∼372 mA h/g as their theoretical capacity. Their initial purpose is to allow the li-ion batteries to have a commercially viable approach maintaining their low cost yet great stability as this is the key factor due to which these batteries are widely being used. The principle on which these graphitic anodes work is that the Li-ions start intercalating in between the graphene planes as it is capable of giving out great 2D stability mechanically, excellent electric conductivity, and overall ionic transport. Carbon has various forms which can be used as anode graphite being the greatest of all and the rest include carbon fibers, rGO, CNTS, and exfoliated graphene. Recently, due to the wide use of rechargeable lithium-ion batteries, the demand for new and updated anode materials has excessively increased as to promote commercially good and stable anodes to maintain the excellent working of these batteries.

Efficacy of rechargeable lithium-ion batteries:

Rechargeable lithium-ion batteries are extremely beneficial and hold great importance in the market as these bring along a lot of ease and mastery to the products and processes that they are a part of. A lot of strategies have been mentioned in the article which can help improve the working of rechargeable lithium-ion batteries but it is important to follow them step by step and avoid all those which can drastically harm the battery or the product that they become a part of. In this way, industries can ensure that these batteries will be up to the mark because initially as well the characteristics and the properties that these batteries are built on are great and ensure the smooth running of the products.

Conclusion

A huge number of strategies are introduced now and then to improve the efficacy of ongoing materials or products. The same is the case with lithium-ion batteries. All these strategies have been formulated under strict observance and after all the experimental works, they have been chosen to put forward for the industries to start introducing these strategies. In this way, the efficacy of rechargeable lithium-ion batteries can be improved and enhanced massively. It is very important to stay updated with all the new strategies and advancements in the previous ones to incorporate those changes in the products and industries.

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