Current Lithium Batteries (Li – Ion)

Lithium–ion batteries are the most commonly used type of lithium batteries. They were invented in response to safety problems with lithium–metal batteries and its instability in the liquid electrolyte. The negative electrode in lithium–ion batteries contains a structure that provides mechanical support and also enables bonding of lithium in the charged state. This new concept physically mobilizes and chemically bonds lithium, making it less active in the electrolyte than in a metallic state. Under ideal conditions, lithium is never reduced to its metallic state and that makes the battery safer by preventing the formation of metallic dendrites. In practice, however, under some conditions of charge, lithium metal formation might still be possible.

While many different materials can be used as negative electrodes, the most important is graphite. It has a layered structure and allows lithium insertion between its layers. It is also very light and enables good energy density and specific energy. A lithium–ion cell based on a graphite anode has similar principle of operation as a lithium–metal cell except that lithium is not present in its metallic form (Fig.6.4).

During discharge, lithium detaches from graphite, converts to Li⁺, and travels through the electrolyte from anode to cathode. Once it reaches the cathode, Li⁺ion intercalates between the layers of a cathode material, such as CoO₂. The equations for anodic, cathodic, and overall reaction are shown below for the case of general metal oxide cathode. The anodic reaction is fairly simple, whereby elemental lithium

Li⁺

Metal Li Intercalation

compound Liquid organic electrolyte e-

Fig. 6.3 Schematic of LOAD lithium metal battery

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is oxidized to Li⁺and an electron. The cathodic reaction is little bit more difficult to discern, Li⁺reacts with the metal oxide to get partially reduced since fractions are involved.

Negative electrode:

LixC6 ! xLi^þþxeþC6 ð6:2Þ Positive electrode:

LixMO2þxLi^þþxe ! LiM1‐xO2 ð6:3Þ Cell:

LixC6þLi1‐xMO2 ! LiM1‐xO2 ð6:4Þ Reactions in Li–ion battery are reversed during charge, LiM_1-xO₂is oxidized on the positive electrode, releasing Li⁺-ions, which are conducted through the electro- lyte to the negative electrode, to be reduced there and bonded to graphite.

The theoretical specific capacity of a lithium–ion graphite negative electrode is several times lower than for lithium–metal simply because one Li atom bonds with up to six carbon atoms, which present additional weight.

All lithium–ion batteries share a common Li anode, but there are several versions based on the cathode material. Each of these different battery types is characterized by distinct performance characteristics and is therefore used for different applications (Table6.1).

Lithium cobalt oxide (LiCoO2) is the most commonly used cathode material in lithium–ion batteries. This type of material (or chemistry, colloquially) has a layered structure (Fig.6.5, left) and is used as power supply for all cell phones and laptop computers. It has high specific energy and moderate cycle life, but limitations such as low discharge currents (<1 C), high cost and concerns regarding safety and cobalt

Li⁺

Li-ion in graphite Intercalation compound Liquid organic electrolyte e-

Fig. 6.4 Schematic of Li–ion LOAD battery

6.3 Current Lithium Batteries (Li–Ion) 97

availability. The importance of LICoO₂battery is slowly diminishing in favor of mixed oxide materials with better safety and less concerns about raw material availability.

Lithium manganese oxide (LiMn₂O₄) has been chronologically the second most popular version of lithium–ion batteries. Its layered, open structure enables high ion migrations rates, which lower internal resistance (Fig. 6.5, right). The battery characteristics include high power density, but at least 20% lower energy density compared with CoO₂. This type of lithium–ion battery has lower cost and performs better at higher temperatures. LiMn₂O₄is safer, more environmentally friendly, and abundant. For this reason, lithium manganese oxide batteries have the capability for high current rates (up to 30 C), both on discharge and on charge. Even at those high currents, this cathode material, also called spinel, has high thermal stability and needs less safety circuitry than the LCO system.

Lithium batteries based on nickel have interesting properties and higher energy density than LCO. By combining nickel, cobalt, and manganese (NMC battery), the expectation is that properties of the resultant cathode and the whole battery can be tuned. A simple approach is that safety can be improved compared to cobalt oxide and power density can be higher because of manganese presence but a tradeoff is that the energy density is lower.

Table 6.1 Main lithium battery chemistries and intended applications

Cathode General battery characteristics Applications

LCO (LiCoO₂) Very high energy, limited power, expensive Mobile phones, laptops, cameras

NMC (LiNiMnCoO₂)

High capacity and high power, thermally less stable

Evs, E-bikes, industrial NCA

(LiNiCoAlO₂)

Energy cell, high energy density, thermally less stable

Medical, industrial, EVs (Tesla)

LMO (LiMn₂O₄) High power, less capacity, relatively safe Power tools, medical devices, powertrains LFP (LiFePO4) High cycle number, low energy density,

very safe

Portable, stationary

MnO₂ Li

c Li

CoO₂

Fig. 6.5 Layered structure of LiCoO₂(left) and LiMn₂O₄(right). Small spheres represent lithium– ions intercalated between the layers of the host structure

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Another three-element cathode material is a combination of nickel, cobalt, and aluminum oxides (NCA) and it demonstrates improved safety, nearly unaffected energy density, but lower cell voltage.

A lithium–ion chemistry that has established an excellent record and interesting properties is lithium iron phosphate technology. These batteries have an excellent safety record, are virtually incombustible, and can better tolerate overcharge and overdischarge conditions as well as high temperatures. Because of this inherent stability of phosphate, these batteries have exceptionally long cycle lives, typically around 3000. This is many times higher than other lithium–ion chemistries and most other secondary battery systems. These batteries have reduced energy density and cell voltage compared to other Li–ion batteries such as LCO.