Traction battery

• 1970-80 Technology Development (Goodenough, Whittingham, Yazami, Mizushima)
• 1985 First prototype Li-ion battery (Yoshino)
• 1991 First commercial article Li-ion, fa. Sony (Nishi, Tozawa)
• 2019 Nobel Prize in Chemistry: Yoshino, Goodenough, Wittingham, for the development of Li-ion batteries
  

• Lithium belongs to the group of alkali metals.
• Alkali metals are soft, silvery-white substances with a low melting point and low density.
• Li is the lightest metal, lighter than water, softer than talc.
• Metal from a chemical point of view - an element that easily removes electrons and forms positively charged ions.

• Alkali metals have only one electron in the valence sphere.
• They are extremely reactive => in nature only as compound.
• Na and K are among the most common elements, but Li is very rare.
• Lithium burns in air (stored in kerosene):

• Production: ore mining (spodumene, Li2O.Al2O3.4SiO2) → lithium chloride production → electrolysis
• Beketov series of metals - according to the standard electrode potential (potential related to the hydrogen electrode, H has a potential of 0 V).
• Lithium is at the top in the standard electrode potentials table, i.e. it has the highest electrode potential: -3.04 V

Pure lithium	Lithium reacting in water	Red flame of lithium
Standard electrode potencials table

• Cathode - metal oxide (LiMO₂, M… Co, Mn, Fe, Ni), collector Al
• LiMO₂ - LiCoO₂, LiMnO₂, LiNiO₂
• Anode - graphite, Cu collector
• Separator - thin porous foils (polyethylene, polypropylene)
• Electrolyte - lithium salt and organic solvent

• Li + ions travel through the separator from the anode to the cathode.
• Electrons through an electrical circuit.
• The ions penetrate the LiMO2 metal oxide lattice.
• Discharged cell = steady state
  

• Li+ ions travel through the separator from the cathode to the anode.
• Layered graphite structures allow Li+ intercalation.
• Graphite does not play a role in the chemical reaction, it is only the "storage medium" for Li+.
• Charged cell = unstable state
  

• Temperature rise to critical values → electrolyte evaporation → internal short circuit → fire / explosion
• Separator - prevents internal short circuit, porous, through for Li+.
• Electrolyte applied to a thin separator.
• Cylindrical cell - all these layers are rolled into a roll.
• Electrons must not come into contact with the electrolyte → degradation of the electrolyte.
• SEI (solid electrolyte interphase) - a layer preventing the contact of electrons with the electrolyte and its degradation
• SEI is formed on the very first charge.
• Li+ → electrolyte → Li+ + solvent molecules → reactions with graphite → SEI
• About 5% of lithium is used to create SEI.
• Current battery life: 3000 cycles, 10 years
  

• R&D direction - replace graphite with Si
  
  • Increasing energy density
  • However, Si increases its volume during charging → possibly a combination of graphite and Si.

• The energy density of modern lithium batteries reaches a maximum of 200 Wh/kg.
• Compared to traditional lead batteries, it is 5-7 times more.
• The energy density of fossil fuels is many times higher.
  

• Oxygen from the external environment
• Electrochemical cell with theoretically highest energy density - 11.7 kWh/kg (petrol 13 kWh/kg)

• Efficiency of a car with an internal combustion engine: 20%
• Electric car efficiency: 85% (without charging)

• Oxygen consumption: a car with a fuel consumption of 7 l/100 km consumes about 122 liters of O₂ per 100 km.

Prismatic cells

• The electrodes are wound into a coil, the coil compressed into an elliptical shape and inserted into a housing.
  

• When connecting cells in parallel, the currents of individual cells are added.
• The voltage of the parallel connection is equal to the voltage of one cell.
• The performance of a parallel connection is the sum of the performance of individual cells.
• The capacity (Wh) of a parallel connection is the sum of the capacities of the cells.

• To assemble a battery with a nominal voltage of 400 V and a continuous power of 80 kW, 864 of these cells are required (108 in series, 8 in parallel).
• Total capacity: 108 * 3,7 * 2,6 * 8= 8,3 kWh (driving range approx. 50 km?)
• Weight (cells only): 864 * 0,088 = 76 kg
• Voltage range:
• Single cell approx. 3,2 – 4,2 V, i.e. in case of 108 cells connected in series: 345 – 454 V.
• For a range of 200 km: 3,500 cells, 308 kg, approx. 350 thousand CZK (cells only).
  

• Charging current lower than discharging (e.g. 10x or 20x lower).
• Lithium ions Li+ must intercalate into the graphite layers → it limits the charging speed.
• By charging, we force the cell to be in unstable state.
• The size of charging current I_nab is given by
  
• difference between the charging voltage U_nab and the internal voltage U₀,
• the size of the internal resistance R₀.

• Charging increases the battery voltage.
• By simultaneously increasing the charging voltage, we maintain a constant charging current = constant current charging (1^st charging step).
• However, the maximum cell voltage must not be exceeded.
• When the maximum voltage is reached (U_nab = U_max), the voltage must not increase further and must stay constant = charging at constant voltage (2^nd charging step, so-called saturation part).
• The charging current decreases exponentially => the charging at constant voltage is time consuming.
• Manufacturers often state charging time to reach 70-80% capacity.
• Maximum cell voltage depending on technology:
  
  •      usually 4.2 V for Co, Ni, Mn, Al.

• The cell charging current is usually recommended to be 0.5-1C.
• To increase the service life, low charging currents are recommended and charging below full SoC (e.g. do not start the saturation part) → reduced voltage stress.
• A large charging current speeds up the first charging step, but it prolongs the saturation part.

• The aging of the battery results in a shortening of the 1^st part (charging with constant current) and a prolongation of the saturation part.
• SoC is practically impossible to determine according to the current voltage.
• It is possible according to the OCV characteristic for the given temperature → in steady state, i.e. sufficient time from last loading.
• Usually, the calculated charge is counted:
  

• Overcharging is not possible - Li-ion will not take over another charge, charging must be stopped, otherwise safety is endangered.

• Charging Li-ion at temperatures below 5°C with limited current only, in frost is not allowed.
• Fast charging in the range of 5 - 45 ° C, but preferably 10 - 30 ° C.
• Charging in the cold damages the battery ("plating" the anode with lithium, permanent, irreversible changes).
  

• High temperatures are harmful for any type of battery.
• Usually, they cannot be charged at T> 50 ° C, it accelerates aging.
• Current limitation is necessary, charging with high current at high temperature reduces the capacity of the battery.
• May cause gas production and inflation.
  

• Internal resistance is a critical parameter of the battery.
• It causes a voltage drop.
• It affects the efficiency of charging and discharging → losses, consumption and range.
• It changes significantly, especially with temperature; it also depends on SoC.
  

Identification of internal resistance of Sanyo NCR20700A cell

Power

Cell discharging by constant current of 30 A at 0° C

Discharge characteristics during pulse discharging for different currents at a temperature of -5° C

• The installed capacity is not equal to the usable energy.
• The usable window ranges from approximately 80 - 90%.
• Sometimes the usable window decreases with a new vehicle and it is expanded as the battery ages.
  

• The electric traction battery is a relatively sensitive system.
• To maintain durability, it must be supplemented by a comprehensive electronic system.
• BMS in particular:
• Monitors the voltage and temperatures of individual cells / groups of cells
• Monitors the amount of energy received and consumed
• Balances articles
• Controls charging - communicates with the charging system
• Limits / enables max. Current
• Models cells, battery
• Estimates SoC, SoH
• It communicates with the control units in the car
  

Frequent terms

Thermal management of Tesla Model 3