Cell, Battery & Battery Pack
Cells as shared before, are the smallest individual electrochemical units 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 the rechargeable cells are called secondary cells. Batteries or batterypacks are made up from 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 delivers better performance.
Just to clarify for general readers once again, 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 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, higher the speed, more would be the current. Similarly, when there is sudden increase in load on motor e.g., while accelerating, the power drawn would be more. Under all these conditions, as the current goes up, so would be the heating of battery pack leading to fall in operating efficiencies. This will also result in the loss of 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 remains below the designed C rating (which can vary from 4C to 6C for an EV, depending upon the make, design & fast charging capabilities of 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 at the extreme ambient/ working temperatures of India (which are very critical for both cylindrical and prismatic cells), have a great impact onthe electrochemical kinetic reaction of LICs and the transfer of substances within. The LiFePO4 cells show a good performance at 20~300C, which is the best operating temperature, while beyond this range, the LICs would show poor performance. It is worth mentioning that at higher temperature of LIC, the chemical reactions within cells also quicken, which on 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 decreased in the life of LIC over the 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 increase (depending upon their chemistry), requiring higher current by the LICs to charge. This in turn, lowers the capacity of LICs, (e.g., a 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 wide range of temperatures, apart from running into the problems mentioned above, LICs may draw very high current, increasing internal temperature, causing higher rate of gas release within LIC, which may build up internal pressure, leading to the explosions of LICs.
The fire hazards associated with LICs are primarily due to their high energy densities, which gets intensities as the flammable organic electrolytes are used in them. Studies have shown that a physical damage, electrical abuse (e.g., short circuits, over current withdrawal or overcharging) and exposures to high temperatures can cause a thermal runaway. Apart from the poor quality of LIC due to imperfections and/or contaminants in the manufacturing process, can also cause thermal runaway.
During the thermal runaways, the organic electrolyte inside the cell vaporizes, causing release of various gases, resulting in the 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 surrounding 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, vapours 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.
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 for recovering good amount of materials from them. LICs must be first classified and need to be discharged or inactivated 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 that 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 shredding of cells) by physical processes, like gravity separation, which recover separated materials without causing chemical changes, enabling 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 the 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 materials. Sometimes, techniques like membrane separation are also used to recover the basic metals from the old LICs. Above given a pie chart indicating the possibility of recoverable components from an old LIC, which is about 87%, however since VOCs are not recoverable, but if Oxygen is also considered as recoverable then this percentage goes up to 92%.
Historically, the main objective of Li-ion battery recycling has been 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.
Since the basic materials make up over half of the initial cost of cell in which the cathode material (cobalt, nickel and manganese) is the largest contributor, so 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 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 of Semiconductors does not apply on cells (LICs included)! If we go back to the introduction of the mass market of LIC in 1991 the cell capacities have only improved about 5%~6% per year. The 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 “how would the Cells/ Batteries of 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 clearly “No”, since by definition & application a disruptive technology is the one which entirely changes the way things are done in present times. In early 1900, 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 the 1980s was a disruptive technology (as it completely changed the way offices, school & businesses functioned back then). In 1990s, the touch phone technology was a disruptive technology (it changed the social interaction of people & the way they communicate & interact). However, the evolution of 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 becoming a true disruption technology, LICs technology need to bring out an innovation with much improved energy storage, higher power densities, both at much lower cost apart from being safer.
On the other side, while, the oil, being liquid, has the advantage of its handling & transporting, but it also, is a very convenient energy source, it also as 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 that 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 in 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 batteries technologies that are minimum three to five years old and any breakthrough in battery technology, happening today, would only be available on EVs, 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 passage of time, electricity can be generated from multiple renewable energy sources without impacting the Mother Nature (e.g., solar, wind, geo thermal, tidal, etc.).
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 are 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 of improvement in the lithium-based cell technology, and hence perhaps the disruption may not come by the lithium based cell technologies, but from the 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 much shorter life as compared to the EVs or stationary energy storage applications, the new improvement in cell technologies need to prove their worth with EVs/ large energy storage applications.
Yet, there is a lot of work 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 technologies 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 the oxygen and it has limited power output capacity. The challenges need to be tackled for their long-term acceptability as replacement of liquid fuel (i.e., oil) and emerge as a disruptive technology.
Prabhat Khare holds BE (Electrical) & a Gold Medalist from IIT, Roorkee. He is an Automotive (EV) & Engineering Consultant, as well as a Technology Article Writer. He is a Certified Energy Manager (BEE) & Lead Assessor for ISO 9K, 14K, 45K & 50K. He can be reached at LinkedIn: https://www.linkedin.com/in/prabhatkhare2/.