1.4.2 - Improving battery energy density and power

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    Despite remarkable performance from back in the 19th century, electric vehicles have not taken off in the same way as internal combustion engine vehicles. The limited autonomy, price of batteries and charging time of several hours, compared with filling a tank in a few minutes are all obstacles mentioned by potential buyers.

    How can autonomy be increased without increasing the weight of the battery, which would lead to overconsumption of energy, with an increase in the price and volume? The only response to this question is to increase the energy density (or specific energy) of the batteries.

     

    Batteries for electric vehicles

     

    While combustion engine vehicle tanks only contain fuel without the air required for combustion (12 kg of air per liter of fuel), a battery has to contain all the reactants. The energy densities of batteries are therefore much lower than those of liquid fuel. The weight of the thermal stabilization system required for certain batteries (Li-ion for example) reduces the specific energy of the battery pack accordingly. For example, while a Li-ion element can reach 300 Wh/kg, a battery pack integrating the assembly conditioning and voltage and temperature regulation (BMS - Battery management system) supplies an average of 100 Wh/kg.    

     

    Batteries

    Lead-acid

    Nickel-metal hydride
    (Ni-MH)

    Lithium-ion (Li-ion)

    Lithium-metal-polymer

    Zebra
    (Na-NiCl2)

    Energy density (Wh/kg)

    20 - 30

    50 - 70

    75 - 120

    100 - 120

    100 - 120

    Power density (W/kg)

    200 - 500

    1000 - 1500

    1000 - 3000

    320

    150

    Cost (€/kWh)

    100 - 200

    1000

    < 1000

    < 1000

    670

    Service life (cycles)

    300 - 800

    2000

    1000 - 3000

    800 - 1000

    1000 - 1500

    Recharge time (hours)

    8

    3 - 6

    3 - 6

    6 - 10

    10 - 12

    Operating temperature (°C)

    -20 to +70

    -10 to +50

    -20 to +60

    No limits

    No limits

    Limits

    Environment

    Temperatures

    Voltage / temperature regulation

    Currently being marketed

    Only one manufacturer


    Source: Sylvain Vitet – EDF 2007

     

    The power required by the vehicle, supplied without any problem by the electric motor should not be limited by the battery. Thus, power density (or specific power) is also an important characteristic of batteries.


    It is possible to favor power density or energy density for a given type of battery, playing on the morphology of the active materials when designed (shape and size of grains, use of nanomaterials) and on the quantity of additives that collect the electrons, etc. However, some types of battery are better suited to delivering a high peak of power and others to providing greater autonomy. The first will tend to be used in hybrid vehicles and the second in purely electric vehicles.



    Source: Michelin Challenge Bibendum Berlin 2011 Workshops Page 10


    The positive electrode approximately represents 20 % of the total cost of a battery. Research and development is endeavoring to substantially reduce these production costs. Most batteries have a mean capacity of 16 to 30 kWh. Their service life is 2,000 to 4,000 cycles and many manufacturers guarantee them for 8 to 10 years or 180 to 240,000 km (150,000 miles).

     

    Lithium batteries

     

    There are various types of lithium battery technology on the market or currently being developed; the major objectives being the best possible longevity and safety notably when overloads or intensive discharges occur.


    Example: Diagram of a Lithium L-ion type of exchange battery. (Extract from the Shanghai 2007 report p 97 by Bruno Scrosati - University of Rome.)

     

    Lithium-ion batteries are by far the most commonly used. The thermal instability appearing in the event of an overload has made implementation of a regulation system necessary. The energy density given is that of the battery pack, including regulation and the control system.

     

    Lithium-metal-polymer batteries have been created to improve thermal stability which is the case. However, the power density is divided by six and the charging time doubled versus lithium-ion batteries.

     

    A lithium-air battery with an energy density of 500 Wh/kg is being developed in France by EDF. This is expected to increase the distance per cycle and reduce the cost.

     

    Despite the advantage offered by lithium, it would undoubtedly be inadvisable to base the future of electric vehicles on one basic material, already much used elsewhere in an increasing number of devices such as mobile phones, laptop and tablet computers, portable electric tools, etc.


    Metal nickel-hydride batteries (Ni-MH) have replaced the old nickel-cadmium batteries due to their toxicity and charge memory requiring them to be fully run down before they are charged up. The energy density of Ni-MH batteries is twice as low as that of Li-ion batteries for an equivalent power density. As a counterpart to their extensive reliability, the entire charge capacity must not be used which limits the actual energy density. They are therefore not really adapted to purely electric vehicles but are widely used in most hybrid vehicles.

     

    The Zebra battery uses the electrochemical Nickel-sodium chloride couple (Ni-NaCl) and offers an energy density of 100 Wh/kg. It functions at a temperature of 270 to 350°C which calls for a thermal control device. Currently produced by only one manufacturer, this technology has demonstrated its viability, for fleets of buses in particular. These captive professional fleets undoubtedly constitute the best scope for application of these batteries. 

     

    Other electrochemical couples are also being studied, with the aim of increasing autonomy and, at the same time, reducing the weight and price of batteries, as well as retaining a sufficient service life, little affected by frequent quick recharging.