Lithium ion battery - Materials & Components

In general we may state lithium ion (Li-ion) battery as one (or best) among many portable batteries packed with lot of power. It has these common traits which makes it special

• Small and light (Energy Dense)
• Long power backup when not used (Low self-discharge)
• Rechargeable for years (Long cycle life)
• Reliable and cheap

Alternative technologies, (more info) even though looking promising on paper but cannot match Li-ion technology in the foreseeable future. Till these alternative technologies mature the standard of Li-ion is going above with increase in power (energy density) with newer chemistries. Though the cost is seen to increase but dollar/kW will be less. It means we can have the same capacity of battery what we have now at less price and probably even more smaller and lighter.

At present there is no replacement for portable rechargeable batteries other than lithium ion and the need for its performance in every area makes it challenging for manufacturers. The need to create Li-ion batteries to perform in real world operating conditions led to different chemistries.

Working : A Li-ion battery is a battery in which the electric current comes when lithium ions move between electrodes (anode/cathode). Lithium ions move from the negative electrode to the positive electrode during discharge and back when charging. The movement of ion leads to oxidation/reduction with intake/uptake of electrons in the process. While the electrons move from the cathode to anode outside the cell while charging and vice-versa while discharging. Note: Both ions as well as electrons move from cathode to anode while charging but the ions move inside the cell and electrons outside the cell.

The main components of a cell are its electrode(anode/cathode),binder, conductive agent, electrolyte and separator. Thou there are other components current collectors (copper/aluminium foil), cell cases, connectors, etc also which are necessary for making a cell.

Electrodes - Anode and Cathode : It has two electrodes one is negative electrode (anode) and other is positive electrode (cathode). These are the most important parts of the Li-ion cell in terms of chemistries others are separator and electrolyte. It should be noted that the coated electrode for anode or cathode contains a binder and conductive agent. Basically anode is a material which absorb more lithium and cathode which is a source of movable lithium. Therefore, to get good batteries with high energy density, we need a cathode which contains more lithium and anodes which can absorb more lithium. Previously, lithium metal batteries where we use lithium metal as one of the electrode was used since they have very high energy density (theoretically infinite). But while charging, lithium deposits at lithium metal as dendrites which grow into the separator and other electrode thereby short circuiting between anode and cathode. It causes the battery to fail and can result in a fire or explosion. Therefore in industries it not used at present.

Cathode : There are different material chemistry for cathode which basically has a source of lithium ions and can be moved under the application of voltage. The simplest can be lithium metal and other oxides like lithium cobaltate (LCO), lithium manganese oxide (LMO), lithium phosphate (LPO). Oxides like NCA and NMC are more popular today which has structure similar to LCO but more with more performance. NCA and NMC are coined from the transition metals which they posses, 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) they are the ratio of the respective metals.
Research for increasing capacity in cathode is not promising and the present chemistry is limited to around 200 mAh/gm and voltage around 4.2 V. Only a new chemistry can solve the problem with good voltage as well as capacity, 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(and their performance, energy density, safety and cost), different cathodes are used in different application depending upon the need.

Anode : Relatively anode has simple chemistry limited to carbon, silicon, blends of carbon-silicon and Lithium titanate. In the research area, there are numerous materials for anode but the advantage of high capacity and cheap sources like carbon is difficult to replace. Presently the trend is to incorporate silicon wither in composites or alloys of silicon or nano-structured form. Carbon has itself very high capacity which hasn't been exploited properly while silicon has even more capacity, but the main disadvantage of silicon over carbon is that while charging/discharging carbon expands only 4% but silicon expands over 300% making the silicon particles to crack. These broken particles are coated on the active particles thereby hindering the charging/discharging process (This process is called as SEI layer formation) The main advantage over the well adopted carbon is that carbon is not effective at high charging/discharging rate while silicon is good. (Note: Lithium titanate is the only material with lithium ion acting as anode)

Separator : It is used to separate anode and cathode electrically but ironically it is in contact, i.e., it means that electrons cant flow though the separator while lithium ions can flow through it. It is a micro-porous film made of polymers like polyethylene or polypropylene. Therefore it prevent from direct short circuits between the electrodes and since it is a polymer with low melting point, it acts as a safety device in such a way that if  the  cell overheats then  the polymer melts and thereby sealing porous structure which seals the electrodes from each other irreversibly. Separators  for  either  the  high-energy  or  high-power  lithium  ion.

Electrolyte : It is composed of solvent with lithium salt. The solvents can be any of the following, either one or the combination of all, i.e., EC, DC, PC.. and lithium salts are LIPF6, LiClO4... The selection of electrolyte is crucial to the formation of stable SEI layer, unstable growth of this layer leads to decrease in the capacity of battery and thereby lower cycle life. The SEI layer is formed of various oxides present in the battery components and it is non conductive.A stable SEI layer is less porous, increase in pore size lead to more growth of SEI layer. Electrolytes are added with additives to ensure the SEI formed have been engineered for its stability and/or conductivity. Recent research indicate nitrile and amide type of electrolytes are promising for high rate and stable batteries. The liquid electrolyte are the main concern for safety and research is moving towards solid electrolytes but the ionic conductivity is too low at room temperature for replacing liquid electrolyte.

Conductive Agent :  It is used to transfer electrons from the oxidation/reduction site of active material to the current collector. The active material is not enough conducting to transfer the electrons to the current collectors and added with two more points (i) the non conducting binder over the active material (ii) maximum thickness possible for the electrode. Since,the active electrode in the porous electrode is connected with the binder and conductive agent, while the pores are flooded with the electrolyte. For fast charging there is a need of proper conducting network and this is been fulfilled by incorporating the conductive agent. These material are of different morphology like nano-particles, nano-fibers, nano-tubes, nano-sheets, etc the more specific area in these forms gives a proper contact and hence more conductive.

Binder : For the active material and current collector to be together on the current collector we need a proper binder. An important property for a good binder is that it shouldn't be reactive with electrolyte. The binders should have a very high melting point and generally they are the  polymers with high molecular weight, they can be classified as homopolymer and crosslinked polymer.


Main Challenges : 

• Formation of unstable SEI layer and thereby decreasing efficiency in batteries
• Stability at high and low temperature

Future Technologies

• The newer lithium–sulfur batteries promise the higher performance-to-weight ratio and considered as the future batteries.
• Solid state batteries with no liquid electrolyte possess one of the best characteristics in terms of safety but low energy density and cost remains a challenge for the industry.
• Cost of lithium and availability of its ores is a debatable issues with few estimating that by 2025 the lithium would be very expensive and limited, alternate sodium ion batteries are of importance in this perspective. Till then there is no competition for smaller lithium ion which is more mobile than sodium ion.
• Other like lithium air batteries and hydrogen fuel cells are in competition to replace lithium ion batteries in some applications.