Understanding Lithium-ion batteries | A long read • EVreporter

Lithium and its Applications 

With an atomic number of 3 & a density of 0.534 g/cm3, Lithium is the first & lightest metallic element in the periodic table^[1] and falls in the category of “alkali metals”, and like all alkali metals, it is highly reactive, rapidly reacting to its surroundings and releasing heat and hydrogen gas by forming lithium hydroxide. It also burns in the air, so pure Lithium must be stored in a vacuum, inert atmosphere or inert liquid such as purified kerosene or mineral oil. Under ordinary conditions, like Lithium, only two other metallic elements are light enough to float on water: potassium (density: 0.862 g/cm3) and sodium (density: 0.971 g/cm3), which are also part of alkali metals, falling under the same column of the periodic table. Hydrogen is the first & lightest element of the periodic table, also from the same alkali metal group, but it behaves as a nonmetallic gas.

Because of its highly reactive nature, Lithium does not appear in its pure metallic form & is mainly found as its oxides or chlorides, either in ores as minerals or in salt solutions such as brines. Based on the types of deposits & their geological formation, extraction technologies differ, and so is the associated costs and time. Furthermore, Lithium is also dissolved in ocean water as an almost “unlimited” resource; however, since the extraction techniques have not yet matured, seawater extraction of Lithium is not expected to be viable in the near future. (https://www.nature.com/articles/s41467-020-18402-y).

If the 2020/2021 estimates of the US Geological Survey (USGS) are to be believed, the Worldwide identified reserves of Lithium are estimated to be between 17 MMT to 21 MMT but this quantity could vary drastically due to the fact that most of the Lithium classification schemes have evolved based on information of solid ore deposits, whereas the quantity of Lithium in seawater [Brine (ClH2NaO)] is difficult to quantify under the same classification due to varying concentrations but still a large part of the lithium is extracted from the seawater which has a high concentration of Lithium Carbonate (Li2CO3). The brines are also found in the earth’s crust, known as continental brines/subsurface brines, which are the main source for the production of Lithium. For record sake, with 40,000 MT annual production, Australia is the global leader for solid ore mining of Lithium, while Chile (20,600 MT), China (14,000 MT) and, Argentina (6,200 MT) are the other major contributors.

Apart from its usage in rechargeable LICs (Lithium Ion Cells), Lithium is also used in some non-rechargeable cells used in pacemakers, toys, and clocks. Lithium is also used as an additive to form alloys with aluminium and magnesium, making them lighter yet stronger. While both Al-Li & Mn-Li alloys find use in aerospace and performance needing applications requiring high strength, low density, high stiffness, and superior damage tolerance, excellent corrosion resistance, superior fatigue cracking resistance, and weldability, Mn-Li alloys also find application in armor plating. The oxides of Lithium are also used in special glasses and glass ceramics. The compounds like Lithium Chloride (LiCl), are one of the most hygroscopic materials known and are extensively used in air conditioning and industrial drying systems [as is Lithium Bromide (LiBr)]. Lithium Stearate [LiO₂C(CH₂)₁₆CH₃] is an all-purpose high-temperature lubricant/ grease and is widely used in many industrial applications, e.g. automobile, steel, aircrafts, heavy machinery, and railcars & represent > 70 % of the worldwide grease market. Lithium hydroxide is a reagent used to produce lithium stearate (mainly lithium 12-hydroxy-stearate), which acts as thickener in the final grease formulation. Lithium greases are water resistant and additionally very resistant to high and low temperatures. They are also characterized by a very wide working temperature range. Lithium Carbonate (Li₂CO₃) is used in drugs to treat depression (although its action on the brain is still not fully understood) whereas Lithium Hydride (LiH) is used as a means of storing hydrogen for use as a fuel.

Evolution Of Li-Ion Cell (LIC)

By definition, cells are devices which directly convert the chemical energy stored within the constituent electrode materials into electrical energy by electrochemical redox reactions. In the case of primary cells, the reactions are unidirectional or irreversible, while in the case of rechargeable or secondary cells, the electrochemical reactions responsible for the generation of electricity are reversible. Starting from Baghdad cell (common name is Baghdad battery), to the breakthrough works of Galvani and Volta, the evolution of cells/ batteries have be continually happening.

Around 1899, the first rechargeable Nickel Cadmium (NiCd) Cells were created by Waldemar Jungner of Sweden, which were the direct competitor to the then most commonly used Lead Acid Cells (LAC). However, they were more costly than that of the lead–acid battery, and were also having high self-discharge rates. With times the energy densities of NiCd cells rapidly increased than LAC. By 1946, the first sealed NiCd cells were produced in the USA. By the middle of the 20th century, sintered-plate NiCd cells became increasingly popular, touching about 1.5 billion production volumes by the year 2000. Up until the late 1990s, NiCd cells had an overwhelming majority of the market share for rechargeable cells in home electronics.

Unfortunately, many times NiCd cells were found to be suffering from the “memory effect”, meaning that these NiCd cells could remember “how much energy was drawn on previous discharges and then would not deliver more than that in the next cycle & vice versa in charging cycle i.e. how much energy was pumped in the cells during previous charges and then would not allow accepting more than that in next cycle” & at restricted the cell voltage to go up during charging or caused a sudden drop of voltage during discharging. Apart from this factor, there were two other reasons for the demise of NiCd cells: first, it was the concern over the environmental issues since NiCd cells contain Cadmium, a particularly toxic material, and hence all NiCd cells needed to be sufficiently sealed so as to not expose anyone to the cadmium inside, during its usage and also at their EOL. Finally, in 2006, the EU Battery Directive restricted the sales of NiCd cells to consumers for portable devices. Secondly, the advancement in battery technology which brought NiMH (Nickel Metal Halide) cells in 1986/1989, having 2~3 times greater energy densities and with up to 40% longer service life than standard NiCd cells. This was soon followed by Li-ion technologies, bringing more improved energy densities, high discharge voltage (~3.7 V), and much longer cycle life and made which made the LICs an ideal choice for energy storage.

The beginning of LICs goes back to 1912 when the first step toward creating lithium cells was initiated by G.N. Lewis, but it had to wait for another 60 years when, in 1970, the first non-rechargeable lithium cells (primary cells) could be commercially made. Then, in 1980, came the development of Lithium Cobalt Oxide (LixCoO2) as the cathode material by John Bannister Goodenough, and which is considered as the single most important component leading to the present day rechargeable LICs. Unfortunately, the early rechargeable LICs were plagued with many safety problems due to the formation of dendrites on recharging lithium-metal anodes (-), leading to their unsafe operation. Understanding the safety issues of the highly active pure lithium metal as well as dendrites formation, the focus shifted to the use of a lithium-intercalation^[2] material as an anode (-). In 1985, I. Kuribayashi and A. Yoshino developed the new cell design using an intercalation carbon anode (-) and a LiCoO2 cathode (+) and filed patents worldwide. Then, in 1991, Sony Energytec Inc. began producing commercial Li-MnO2cells (or the first LIC) based on the Asahi patents. They also introduced “electronic safety circuitry” to control the charge-discharge cycle of LICs (with an interrupter to break current flow on a buildup of excessive internal cell pressure) as well as also used a “shut-down” polymer separator. Despite the fact that now the “Li-ion” cell/ battery is a globally accepted name, the fact is that there is no “Lithium Metal” in the LIC, which have both the electrode as the intercalation form of Li-ions into the structure of their active materials [While the cathodes (+) are made of ionic lithium compounds for which multiple chemistries exist, such as lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), lithium manganese oxide (LiMn2O4), and lithium nickel manganese cobalt oxide (LiNiMnCoO2), the anodes (-) consist of a porous material like graphite & silicone that sequesters Li-ions and releases them to the electrolyte during battery discharge]. For electrolytes, LICs use lithium salts dissolved in organic compounds and being flammable by nature, are the most critical factor in the LIC chemistries. This chemistry also governs the operating conditions for the LICs, making it very specific and, if not followed properly, may lead to the failures of LICs by explosion or fire. Though, with proper design chemistry, testing, protection & usage, this limitation is being addressed, but still these critical design intricacies still make the LICs, fragile. Another detrimental factor in the context of LICs is the high cost of lithium mining, a process which is also not environmental friendly. However, alternatively, some manufacturers tried using other metals like cobalt in LIC chemistries, but then it drives up the prices of the cells even more.

However, despite these concerns, the numbers of LICs being manufactured, have only been increasing, from their introduction in 1991. The top 15 manufacturers of LICs have already added a total of about 200 GWh of production in 2021, taking total cumulative production capacities to about 600 GWh. Meanwhile, 3,000 GWh of capacity is at the planning or construction stage, a growth which is led by CATL (China). It has been projected that the global cumulative LIC capacity could rise over 5,500 GWh by 2030^[3]. The Fortune Business Insights^[4], in its report, titled, “Li-ion Battery Market, 2021-2028”, states that the global LIC market size, which was about US$ 36.90 billion in 2020, is expected to touch US$ 44.49 billion in 2021 and is projected to reach about US$ 193.13 billion by 2028, exhibiting a CAGR of 23.3% during 2021-2028.

LIC Manufacturng Capacities(GWh) & %age By 2025

With the exponential growth in the volumes of LICs, the cost of LICs has also been dropping over time, from over $1,000/kWh in early 2000 to $200/kWh in 2021 (Tesla’s Giga factories are expected to slash these costs to below $100/kWh soon), while their energy density has increased from 150 Wh/kg to 300 Wh/kg during the same period (With Tesla’s new 4680 cell form factor energy density may increase to 5 times that of currently available). These improvements have made LICs a most favoured choice for energy storage.

Difference Between Cell & Battery

The cell is a single unit device which converts the chemical energy into electrical energy, whereas the battery is the group of cells connected together.

Understanding Basic Cell/ Battery Terminologies

There are many cell/ battery terminologies which are commonly known & understood. Still, with the rise of EVs as well as the need to store electrical energy, these terminologies need to be clarified & understood. However, they must be understood in relation to the application of cell/ battery as some of these definitions are applicable only to the specific application e.g. EVs and thus may differ for other applications such as electronics, portable and stationary energy storage systems etc. 

• EV: The abbreviation EV generally refers to a 100% Battery Operated Electric Vehicle (BEV), i.e. a vehicle that is fully electrified with no internal combustion engine (ICE) whatsoever. 

• Voltage (Volts) & Current (Amp): These are the most commonly known & understood terms.

• Capacity (Ah) Rating: This is the capacity of the cell/ battery, which is measured in Ampere hours (Ah) & indicates how much current can be drawn from the cell/ battery for how long. (i.e. from a 100Ah cell/ battery, one can draw 100 Amp current for 1 hour or 50 Amp current for 2 hours. Normally this time is said to be 1 hr i.e. 1C rating, if not specifically defined)

• Energy (Wh) Rating: When the Ah of the cell/ battery is multiplied by cell/ battery voltage, it gives the energy stored in the cell/ battery (VAh or Wh).

• Power (W): Power of cell/ battery is the rate of flow of energy stored in cell/battery (Wh), meaning how quickly the energy stored in cell/ battery can be flowed out/ flowed in when needed. Here, it must be kept in mind that even if two batteries have similar energy (Wh) ratings, the cell/ battery which can discharge that energy in a shorter time, would have a higher power rating. 

• C Rate, E Rate & P Rate: These factors are a measure of the rate at which a cell/ battery can be charged/ discharged w.r.t. Current (C), Energy (E) & Power (P). For example, a 3.7V, 1200 mAh Li-ion cell/ battery, with a 5C rating can be made to discharge 6000 mA in 12 mins. (since 1200 mAh cell/ battery means 1200 mA current can be drawn in 1 hour), BUT for this discharge rate of 5C, the cell/ battery must be designed to sustain the flow of 6000mA current both electrically & mechanically. However, the same cell/ battery with a 0.1C rating would mean that only 250 mA (1/10th of the 1200 mA) can be drawn from it, at which rate the cell/ battery will last for 10 hrs. & can be designed for lower electrical/ mechanic strength. For E Rating, instead of current (mA), energy discharge (mWh) rate is used, whereas, for P Rate, Power Flow (mW) is used. C/ E / P Rates are applicable to both discharging & charging. Higher E Rate would be needed for fast charging of EVs, while a higher P rate is needed for quick acceleration of EVs.

• Beginning of Life (BOL)/ End of Life (EOL) – The term BOL refers to the energy, capacity, and power of LICs when they are first built, while as a general rule, the EOL for batteries used in EVs has been decided to be about 80% of their BOL measurements. However, these cells/ batteries with lower EOL (from EVs) could be used in other applications like UPS etc.

• Depth of Discharge (DOD) – This is the energy range between which the cell/ battery should be made to operate for their efficient & effective utilization. Typically, a LIC is expected to be used somewhere between 20% and 80% of the total amount of energy which prevents overcharge as well as low-end voltage.

• State of Charge (SOC) – This is the measurement of the amount of energy left in a cell/ battery, at any given time. SOC is usually measured from 0% to 100%. It reduces with time as the battery performance deteriorates over time.

• Energy Density – Energy density is the measurement of how much energy a cell/ battery contains in relation to either its mass (Wh/Kg) or its volume (Wh/L).

• Internal Resistance of the Battery: The internal resistance of the cell/ battery is the mathematical sum of electronic resistance (of metal covers, internal components and how the metals are connected with each other) and ionic resistance (the resistance offered by the electrolyte to the flow of charged particles). As & when this sum exceeds the threshold limits specified by the manufacturers at BOL, the cell/ battery can be seen as deteriorating.

• Charge-Discharge Cycles: In an ideal situation, a cycle of 100% charge & 100% discharge is considered to be one “Charge-Discharge Cycle” of the cell/ battery. However, for improved cell/ battery life, it is often advised to charge-discharge the cell/ battery in the range of 20% to 80% of its full capacity. As the cells/ battery undergoes certain %age of Charge-Discharge Cycles, the cell’s health starts deteriorating.

• Battery Management System (BMS) – The BMS is the electronic control system within the battery pack that manages the charging and discharging, monitors the temperature and voltage, communicates with the EV’s system, balances the cells, controls charging (during regenerative breaking) & discharging & power flow (during regular running, acceleration & de-acceleration) and manages the safety functionality of the battery pack.

Series & Parallel Connection of Cells/ Battery^[5]

Since there is a limitation of cell voltage & cell energy (in the case of LIC, it is 3.7 V, 3200 mAh), multiple cells need to be arranged in a specific combination to increase the Voltage/ Energy in order to meet the load requirement. This combination of cells is called a battery. The batteries can achieve the desired operating voltage when several cell are connected in series; each cell adds battery’s voltage which is the sum of the voltages of all the cells in series (by Kirchhoff’s voltage law). The Ah of the battery in this series connection remains the same as that of one cell. Alternatively, in a parallel connection of cells, the battery can attain higher Ah capacity by adding up the Ah of all the cells (by Kirchhoff’s current law), whereas the Voltage of this battery remains the same as that of one cell. For Example, when three cells of 2V & 20Ah are connected in series to form a battery, the battery voltage would be 6V  while the capacity of the battery would only be 20 Ah resulting in a battery pack of 120Wh (6V*20Ah) energy capacity. On the other hand, when the same three cells are connected in parallel, the battery capacity would 60Ah while the voltage of this battery pack would only be 2V & thus, this combination would also give a battery pack with an energy capacity of 120 Wh (2V*60 Ah) however the voltages of two pack will be different.

Working Of LICs

LICs work completely differently from electrochemical cells. Whereas traditional electrochemical cells depend on redox reactions (a chemical reaction in which electrons are transferred between two reactants participating in it) to generate voltage & create energy, changing the chemical properties of electrodes surfaces, in the LICs, it is only the flow of Lithium Ions which do not react with the electrode materials, per se. Instead, they keep shuttling between both electrodes, through the electrolyte & separator and get inserted into (a process called intercalation) or expelled from (de intercalation) from the structure of the electrode materials, depending on the direction of current flow (Charging/ discharging). Typically, in a LIC, anode (-) is of graphite materials while the cathode (+) is a layered LiCoO2, and the electrolyte is usually LiPF6 (Lithium hexafluorophosphate) dissolved in a non-aqueous organic solvent. The separators in the battery provide a barrier between the anode (-) and the cathode (+) while enabling the smooth exchange of Li-ions from one side to the other. During the charging processes, the Li-ions from the host anode (-) material (LiCoO2) “de intercalate” and move through the electrolyte and “intercalate” within the graphite layers of the cathode (+); during the discharge process, the reverse happens. This results in the continuous movement of Li-ions between two zones during the charging and discharging processes causing the current to flow- because of this shuttling movement of Lithium Ions, LICs are also referred to as Rocking Chair Cells. However, the electrolyte in LIC does undergo a limited reaction with lithium to form a multi-layered film at the interface of both the electrodes, which is referred to as the “Solid Electrolyte Interphase (SEI)”. This SEI plays a vital role in preventing the electrolyte from undergoing side reactions with the electrode surface. Thus, in the subsequent cycling process, the movement of Li-Ions occurs through the surface films, in a reversible electrochemical reaction.

A Typical Li-ion Battery Construction, Its Current Flows & Reactions

Cathode Material (+) [or LIC Chemistries]

The Li-ion is a very generic term, and as these ions can be generated from Cathode using a variety of chemistries, bringing a range of performance characteristics. The most common chemistry combinations used in LICs are: LFP, LMO, LTO, LCO, NCA, and NMC (see below), and cell manufacturers also use their combination to get improved performance. Some of the LIC chemistries are indicated below:

Li-ion Chemistries

Anode (-): The negative side of the electrode is known as the anode (-). Today, most anodes (-) are made of a mixture of one of two materials, either graphite or soft or hard carbons. The quality of cell majorly depends upon the quality and this material & it plays a critical role in the performance of the LICs.

Separators: The separator is the single most important part of any LIC design and separates anode (-) & cathode (+) by creating a physical partition. It is often made of plastic or ceramic. While the separator must maintain the isolation of two electrodes, it must also be able to withstand the corrosive hydrocarbon (HC) based electrolytes used in LICs. However, if this separator fails due to any reason, the two electrodes will come into direct contact, leading to internal short-circuiting, causing cell failure & in some cases, a fire. While some manufacturers also use multilayer PP/ PE allowing it to sustain higher temperatures (∼135 °C), new technologies are also allowing to integrate ceramics in these separators as they allow higher temperatures at the separator surface, increasing the safety of LIC.

Electrolytes: This is usually a liquid or gel-based solution in which anode (-) and cathode (+) are submerged & act as a conductor for the Li-ions to move between the anode (-) and the cathode (+). This electrolyte is typically an HC-based mixture that includes multiple additives to provide different functionality within the LIC. These additives are the “secret sauce” of any cell manufacturer and are one of the key research areas of cell makers. Typical electrolytes may include a mixture of alkyl carbonates such as ethylene carbonate, dimethyl, diethyl, and ethyl methyl carbonates and lithium salts (LiPF6). The main function of the additives is to facilitate the formation of the solid electrolyte interphase (SEI) layer, to reduce the irreversible capacity and gas generation to enhance the thermal stability of the cell.

LIC Shape, Types and Sizes

There are essentially three main types of LIC form factors: small cylindrical, large prismatic, and pouch (or polymer) cells.

Cylindrical Cells: They are made by rolling long strips of cathode foil, separator, and anode foil together and inserting them into a rigid tubular stainless steel or aluminium cell housing or “can”. The “can” is filled with liquid electrolyte, safety disks are inserted into the top, and the electrodes are welded to the outer battery terminals (in this case, the top and bottom of the cell). The cell is hermetically sealed by crimping the top disk assembly closed. These cells are identified by their nomenclature such as 18650 (18 cm is its diameter while 65 cm is its length) & come in different nomenclatures such as 10440 (AAA), 14500 (AA), 16340 (CR123A), 18650, 21700, 26650 & 32650 & 32330 depending upon their application. 

Dimensions Of Various Cylindrical Lithium-Ion Rechargeable Cells

Prismatic Cells: These are similar in construction to cylindrical cells but use a flat rectangular housing to lower the overall thickness of the cell. The electrode/separator assembly can be rolled, as with cylindrical cells, or it can be a rectangular stack of individual electrodes (similar to a deck of cards). The battery terminals can be placed as contact pads on the top or side of the housing. The prismatic cell thin form factor is well suited to use in consumer electronics, particularly when ease of battery replacement is desirable.

Pouch Cell: They also have a thin rectangular form factor & are composed of rectangular stacks of individual electrode/separator layers, but instead of a rigid metal case, they use a laminated flexible polymer/aluminium “bag”. The electrodes have tabs along one side; these are welded together with battery terminal tabs that stick out of the top of the bag. The assembly is saturated with a liquid electrolyte and the bag is heat-sealed. By eliminating the rigid housing, pouch cells save on cost, weight, and thickness. The flexible pouch is, however, prone to swelling, and this can pose problems with lifetime, capacity loss, and safety.

However, by far the highest volume LIC format in production today is the 18650 cylindrical cell, with nearly 660 million cells produced annually. When it comes to EV applications, nearly all of the major auto manufacturers have identified NiMH for HEVs, while BEVs manufacturers have preferred to use 18650 cells or pouch type LICs e.g. Tesla uses about 7000 no. of 18650 cells in their Model 3 (85 kWh battery pack) whereas the General Motors uses 288 pouch type cells in their Chevrolet Volt (16.5 kWh battery pack) and Nissan uses 192 pouch-type cells in their LEAF (with 24 kWh battery pack). Yet again, in order to improve its EV performance, Tesla, recently, introduced a 4680 form factor for LIC, which is expected to have six times power and about five times the energy capacity of 21700.

Safety in LICs

 Like all other energy storage devices, LICs also have some inherent safety challenges. Additionally, there is always the potential for contamination creeping in or errors occurring during the cell manufacturing process, causing cell failure or sudden energy release. In order to help mitigate these risks, a few safety features are in-built into these cells during manufacturing. The LICs have a hard can type container, in which the first and most common in-built safety is a cell vent which is basically designed as an engineered failure point within the cell in the event of a buildup of pressure inside the cell. However, in the pouch cell there is a notch or other weak point to fail in case of pressure build up in the cell. Secondly, a type of non-resettable fuse (generally it is pressure based) or the “Current Interrupting Device (CID)” is often integrated within the LIC. In essence, the CID is a two-part mechanism that is designed to separate and break the flow of current to the terminals if the pressure builds up beyond a certain point. The third, type of safety that is included in some cell designs is a “Positive Temperature Coefficient (PTC)” device which is essentially a resettable thermal fuse, which is designed to break the current flow if the temperature of the cell rises above a predetermined point and when the temperature of the cell falls back to normal operating temperatures, the PTC will reset and the cell will again be usable. 

The biggest benefit of the cylindrical cell is that it uses either nickel-coated steel or aluminium “can” offering a high-strength packaging requiring a lot of energy to damage it, and it provides “stack” pressure on the jellyroll inside the can. However, one of the challenges involved in using such tubular format cells, apart from their sizes, is in their lid assembly. Manufacturers are slowly transitioning to laser welding of the lids onto the cells, as the crimping process to attach the lid has been found to fail under certain operating conditions. [In 2006, Sony recalled its $250 million dollar worth of cells which had started to fail as some metallic particles began to make their way inside the cells (Mook, 2006)].

Electrical Safety: Since EVs’ eco-system needs electricity, electrical safety becomes the most critical factor to avoid any mishap. While the EV connectors must be polarized and configured so that it is non-interchangeable with other electrical devices such as electric dryers, the method by which the charging equipment couples to the EV (which can be either conductive or inductive), must also be designed so as to prevent against unintentional disconnection. Additionally, the new electrical codes require that EV charging loads be considered continuous; therefore, the premises wiring for the EV charging equipment must be rated at 125% of the charging equipment’s maximum load. All EV charging equipment must have ground-fault circuit interrupter devices for personnel protection. To avoid any physical damage & water ingress in the EV-mounted battery packs, it is generally expected that these battery packs must be rated for IP67 ratings. Also, an interlock to de-energize the equipment in the event of connector or cable damage must be incorporated. Furthermore, a connection interlock is required to ensure that there is a non-energized interface between the EV charging equipment and the EV until the connector has been fastened to the vehicle. A ventilation interlock is also required in the EV charging equipment; this interlock enables the EV charging equipment to determine whether a vehicle requires ventilation and whether ventilation is available. If ventilation is included in the system, the ventilation interlock will allow any vehicle to charge. However, if ventilation is not included in the system, the mechanical ventilation interlock will allow vehicles equipped with non-gassing batteries to charge, but not vehicles equipped with gassing batteries.

Cell, Battery & Battery Pack

Cells, as shared above, are the smallest individual electrochemical unit and deliver a voltage & energy that depends on the combination of chemicals and compounds chosen to make the cell. Single-use cells are called primary cells, while rechargeable cells are called secondary cells. Batteries or battery packs are made up of groups of cells. Technically, a cell is different from a battery since the battery refers to an electrically connected group of cells. On other hand, a “battery pack” is a set of identical batteries or individual battery cells, connected in series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density. This also includes BMS to manage charging & discharging of all the cells so as to keep the voltages & energy levels of each individual cell below its maximum value during charging, allowing the weaker batteries to become fully charged with active energy balancing, shuttling energy from strong cells to weaker ones in real-time for better balance. Battery regulators are also part of BMS, for balancing the whole battery pack back so as to give longer life and deliver better performance.

Possible Battery Pack Combination In Few EVs On Indian Roads

Just to clarify for general reader once more, the Battery Pack voltage of EV decides the number of cells needed to be connected in series while the KWh rating of battery decides the total number of cells needed for the desired battery pack.

Discharge Characteristics Of LIC Battery Packs: 

On a Battery Pack level, the LICs are connected in a combination of series & parallel connections to form the Battery Packs, including many protection as well as a thermal management system based on air or liquid cooling. This system is designed to keep the temperature of the battery pack within the optimal temperature range, either by cooling during heavy-duty driving/high-temperature conditions or by heating when the battery operating temperature is low. Also, based on the speed of EVs, the current drawn by the driving motor varies; the higher the speed, the more the current would be. Similarly, when there is a sudden increase in load on the motor e.g. while acceleration, the power drawn would be more. Under all these conditions, as the current goes up, so would the heating of the battery pack leading to a fall in operating efficiencies. This will also result in a loss in the driving range of the vehicle. Also, a higher current generates higher mechanical force within the battery, and the complete battery system must be designed to sustain that force.

More importantly, till the current drawn remain below the designed C rating (which can vary from 4C to 6C for an EV, depending upon the make, design & fast charging capabilities of the battery pack & associated circuitry of the EV), the driving range of EV can be predictably estimated as heating losses & related ventilated system is designed to take care of the associated heat generated in the battery modules, however, if the C rating of discharge exceeds the deign value, the battery performance becomes unpredictable and can cause even the battery failure.

Impact of Operating Temperature Of LICs

Unfortunately, the extreme ambient/ working temperatures of India (which are very critical for both cylindrical and prismatic cells), have a great impact[6] on the electrochemical kinetic reaction of LICs and the transfer of substances within. The LiFePO4 cells show good performance at 20~30℃, which is the best operating temperature, while beyond this range, the LICs would show poor performance. It is worth mentioning that at a higher temperature of LIC, the chemical reactions within cells also quicken, which on the positive side, improves the performance as well as increases the storage capacity of the LIC (many studies state that this increase could be in the range of 20% if the operating temperature of LIC goes up from 25 DegC to 45 DegC). Unfortunately, this advantage comes with a much bigger detrimental side effect which is a decrease in the life of LIC over time, at such higher temperatures (few studies indicate that the LICs degrade by 6.7% when charged at 45 DegC, compared to 3.3% degradation when charged at 25 DegC), hence prolonged heat exposure of LICs & their charging at higher temperature must be avoided.

Similarly, when the temperatures of LICs drop, the internal resistance of the LICs increases (depending upon their chemistry), requiring higher current by the LICs to charge. This, in turn, lowers the capacity of LICs. (e.g. LAB may provide just half the nominal capacity at 0° C) and also increase internal heating within the LICs. 

However, it must be remembered that the operating temperatures of LICs also differ based on their types/ chemistries e.g. some of the LICs can be charged from 0 DegC to 45 DegC and discharged from –20 DegC to 60DegC. However, if one starts operating these LICs at such a wide range of temperatures, apart from running into the problems mentioned above, LICs may draw very high current, increasing internal temperature, causing a higher rate of gas release within LIC, which may build up internal pressure, leading to the explosions of LICs.

Hazards Associated With LICs

The fire hazards associated with LICs are primarily due to their high energy densities^[7], which gets intensities as the flammable organic electrolyte are used in them. Studies have shown that physical damage, electrical abuse (e.g. short circuits, overcurrent withdrawal or overcharging) and exposure to high temperatures can cause a thermal runaway^[8]. Apart from the poor quality of LIC due to imperfections and/or contaminants in the manufacturing process, it can also cause thermal runaway. During the thermal runaways, the organic electrolyte inside the cell vaporizes, causing the release of various gases, resulting in pressure build-up within the cell. If the pressure exceeds beyond a limit, the cell casing punctures/ explodes, releasing these flammable and toxic gases in the surroundings impacting other cells. The severity of the thermal runaway is also dependent on many other parameters, including battery size, chemistry, construction and the battery state of charge (SOC). In almost every significant battery reaction, the same hazardous components are produced, flammable by-products (e.g., aerosols, vapour and liquids), toxic gases and flying debris (some burning), and in most instances, sustained burning of the electrolyte and casing material. To address this issue present-day LICs are provided with the pressure release vents.

Best Practices in Handling LICs

Recycling Of Li-Ion Cells: 

Due to the complex structure and number of materials used in LICs, they need to be subjected to a set of complete electrical, mechanical & chemical processes to recover a good amount of materials from them. LICs must be first classified and need to be discharged or inactivation before disassembling them, after which only they can be subjected to different paths of recycling e.g. direct recycling (physical), pyro-metallurgy (smelting) recycling, hydro-metallurgy (leaching) recycling, or a combination which can be used to create a new integrated process, depending upon the quantity, conditions, chemistry and characteristics of the cells available for recycling.

The “Direct recycling” process separates the different components of the black mass (active material powder from the shredding of cells) by physical processes, like gravity separation, which recover separated materials without causing chemical changes, enabling the recovery of cathode material that is reusable with minimal treatment. While the “Pyro-metallurgy” process uses high temperature to facilitate the oxidation and reduction reactions in which transition metals like Co and Ni are reduced from oxides to metals, and recovered in a mixed metal alloy, the alternative process like “hydro-metallurgy” can also be used to recover the same materials, to make new cathode material. Other materials, including aluminium, anode, and electrolyte, are oxidized in the smelter, supplying much of the process energy. The aluminium and lithium oxides end up in the slag and are not generally recovered. The “hydro-metallurgy” process uses acids to dissolve the ions out of a solid like the cathode, producing a mixture from which recoveries can be made by using precipitation or solvent extraction or by reacting with other recovered materials to produce new cathode material. Sometimes, techniques like membrane separation are also used to recover the basic metals from the old LICs. Below given a pie chart indicating the possibility of recoverable components from an old LIC, which is about 87% however, since VOC are not recoverable, if Oxygen is also considered recoverable, then this %age goes up to 92%.

Historically, the main objective of Li-ion battery recycling has been the recovery of cathode materials (~9.0%) [cobalt (~3.10%)/ nickel (~3.1%)/ Manganese (~2.8%)] and not Lithium, because of their high values. Everything else has been secondary. However, it’s the present requirement of EU Regulation which demands that at least, 50% of a cell’s weight must be recycled. This requirement is going to be increased to 65% for LICs by 2025 and to 70% by 2030, when specific recycling requirements will have to be introduced for the lithium, cobalt, copper, nickel, and lead content of batteries.

The basic materials make up over half of the initial cost of cells, and the cathode material (cobalt, nickel and manganese) is the largest contributor. There is a financial incentive to recover cathode material. Further, the value of cathode material is greater than that of cells’ other constituents, so the recovery of reusable cathode provides more revenue. Recovery of cobalt from LCO cathode by smelting or leaching recovers about 70% of the cathode value, a percentage that falls drastically for other cathode chemistries containing less cobalt. This could be a considerable advantage for direct recycling, also called “Cathode-to-Cathode” recycling, because of the importance of cathode value recovery. The recovery process should ensure that the quality of these recovered cathode materials is at par with the quality of virgin materials and must not be contaminated or degraded.

Future of Li-Ion Cells

Unfortunately, Moore’s Law^[9] of Semiconductors does not apply to cells (LICs included)! If we go back to the introduction of the mass market of LIC in 1991, the cell capacities have only improved by about 5%~6% per year. Cell technology has also not advanced so as to double the energy densities every year. In fact, it has only improved about eightfold since the first commercial batteries were introduced in 1854 i.e. over about 170 years.

So the question is, “What would the Cells/ Batteries of the future look like?” Is there really a “game-changing” or “disruptive” technology out there in this field of energy storage (read cells/ batteries)? Unfortunately, the answer is clear “No”, since by definition & application, a disruptive technology is the one which entirely changes the way things are done in present times. In the early 1900, the evolution of ICEV was a disruptive technology (as it completely changed the travel behaviours of society and the way people used to travel, till then). The rise of PCs in the 1980s was a disruptive technology (as it completely changed the way offices, schools & businesses functioned back then). In the 1990s, touch phone technology was a disruptive technology (it changed the social interaction of people & the way they communicate & interact). However, the evolution of the present day’s LIC from the first Lead Acid Cell was patented in 1859 by Gaston Planté of France, does not show the same pattern as all the technologies have been coexisting and complementing each other. To become a true disruption technology, LICs technology needs to bring out innovation with much-improved energy storage, and higher power densities, both at much lower cost apart from being safer.

On the other side, the oil, being liquid, has the advantage of its handling & transporting. It also is a very convenient energy source. It also has a very high sp. energy density @ 46 MJ/kg, whereas present-day LICs only offer sp. energy densities between 0.36 MJ/Kg to 0.90 MJ/Kg, which is about 1/128th ~ 1/85th of the energy of oil. This, in turn, means no present-day batteries will be able to completely replace the oil as fuel (which also means that EVs can never completely replace ICEVs). In order to displace ICEVs, the battery technology must get smaller (volumetrically) & safer while at the same time increasing energy and power densities to a point where it is on par with the oil. Even if any battery technology starts offering the same energy content as a liquid fuel, but the cost is exorbitantly high, it will not become a feasible solution to be acceptable for mass market adoption. From an automotive standpoint, once any technology is engineered into a vehicle, it is likely to last somewhere between 5 to 10 years, as that is about the life of a standard vehicle architecture. This means that the batteries of EVs which are being introduced today were actually designed, using battery technologies that are a minimum of three to five years old and any breakthrough in battery technology, happening today, would only be available on EVs, a few years down the line. This cycle will continue for many years to come. However, the silver lining for EVs is that while oil needs to be pumped out from wells impacting nature, and since the oil is a non-renewable energy source, they become scarce with time, costing higher with the passage of time. Electricity can be generated from multiple renewable energy sources without impacting Mother Nature (e.g. solar, wind, geo thermal, tidal, etc.)

Epilogue

While the theoretical energy density improvement, that can be gained using silicon or thinner anodes in LIC, is in the range of 300% or more, the practical energy density increase is often about one-third of this theoretical number. However, if the theoretical number of energy densities is achieved, it will almost bring LICs at par with liquid fuels. So what could happen to the battery technologies that could make LIC truly disruptive? That is difficult to say, as Lithium is already one of the lightest materials in the periodic table, and so there is no visible opportunity for improvement in the lithium-based cell technology, and hence perhaps the disruption may not come from the lithium base cell technologies, but from any other new material chemistries. We are already beginning to see some of these chemistries emerging in the portable power industry, as we see personal electronics getting smaller and thinner and perhaps even wearable. However, since these appliances tend to have a much shorter life as compared to the EVs or stationary energy storage applications, the new improvement in cell technologies needs to prove their worth with EVs/ large energy storage applications. 

Yet, there is a lot of work is going on with nano-materials, coating the silicon or tin with graphite, graphene or other materials, organic electrolytes, as well as new methods for manufacturing, etc. that may improve the performance of the present-day cell chemistries to make them at par with present-day liquid fuel oil-based energy sources. Perhaps, one of the most interesting new technology that is in development is the solid-state battery (SSB). Another battery technology that is also receiving a lot of research attention today is the lithium-air battery & offers extremely high energy density, very flat discharge curves, essentially unlimited shelf life as long as it is not exposed to air, has low cost, and no environmental issues. Some other technologies include fuel cells (micro fuel cells, automotive fuel cells, and very large fuel cells) and supercapacitors and ultra-capacitors. However, there are many challenges with them also, the biggest of which is that it is dependent on the environment for oxygen and it has limited power output capacity. However, there are many challenges with them also which needs to be tackled for their long-term acceptability as a replacement for liquid fuel (i.e. Oil) and to emerge as a disruptive technology.

About the Author

Prabhat Khare | Mob: +91-9910490088 | BE (Electrical), Gold Medalist, IIT Roorkee

Automotive & Engineering Consultant, Energy & Safety Auditor, Trainer | Technology Article Writer

Auto Sector Expert (Tata Motors, Honda Cars & Ashok Leyland) | Energy Sector Expert (Cement & Fertilizers) | Energy Manager (Bureau of Energy Efficiency)

Life Member of National Safety Council of India | Lead Assessor for ISO 9K, 14K, 45K & 50K (BSI)

Email: prabhat.pkmail@gmail.com/ prabhatkhare22659@gmail.com | LinkedIn: https://www.linkedin.com/in/prabhatkhare2/

References

^[1] It is the table of the elements, arranged in the order of their atomic numbers, usually in rows, so that elements with similar atomic structures (and hence similar chemical properties) appear in vertical columns. The idea of such a table was put forward in 1869 by a Russian chemist called Dmitri Mendeleev.

^[2] In chemistry, intercalation is the reversible inclusion or insertion of a molecule into layered materials with layered structures. One famous intercalation host is graphite.

^[3] https://www.woodmac.com/press-releases/global-Li-ion-battery-capacity-to-rise-five-fold-by-2030/

^[4] https://www.fortunebusinessinsights.com/industry-reports/Li-ion-battery-market-100123

^[5] In both cases, precaution would be taken that cells of similar rating and condition should be combined to avoid any mishap.

^[6] https://www.cedgreentech.com/article/how-does-temperature-affect-battery-performance

^[7] The progress is tremendous. In 2008, the number was only 55 Wh/Liter, while in 2020, it was 450 Wh/l, according to the study. That’s an 8-fold increase in 12 years. With Tesla’s new 4680 Cells, these numbers will go further up.

^[8] Thermal runaway is an uncontrolled chain reaction which begins when the amount of heat generated within a system exceeds the amount of heat that is getting dissipated to its surroundings.

^[9]Moore’s Law was introduced by Gordon Moore, one of the founders of Intel, back in 1965 when he stated that the amount of transistors on an integrated circuit will double every two years (Intel, 2014). This has proven to be very accurate for integrated circuits.