Challenge Bibendum - Lithium-based Batteries for Electric and Hybrid-Electric Vehicles

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Home DISCOVERING THE SOLUTIONS Energies & vehicles Lithium-based Batteries for Electric and Hybrid-Electric Vehicles

Lithium-based Batteries for Electric and Hybrid-Electric Vehicles

05-20-2010 by Yves Chabre

Resume:
The principles of lithium secondary batteries have been established on the end of the 1970's, both for lithium-ion and lithium-metal polymer systems. The LiCoO2/Graphite system that has been commercialized from early 90'ths cannot be considered for traction applications for cost reason, and mainly lack of intrinsic safety. But several new systems have been developed in the 90'ths.

A Large Variety of Li-ion batteries for EVs and HEVs

Lithium-ion batteries are presently produced with two main types of positive electrode materials, oxides of transition metals (nickel, cobalt, manganese) that can form mixed compounds with lithium, and phosphates, mainly lithium iron phosphate, LiFeP04. Most of them are still using graphite (or other forms of carbon) as negative electrode material. The most used systems for traction applications are Li(Ni1/3Co1/3Mn1/3)O2,/C, Li(NixCoyAl1-x-y)O2,/C, LiMn204/C, and LiFeP04/C (frequently referred as NCM, NCA, LMO, and LFP).
Two classes of electrolyte exist for these Li-ion batteries: liquid electrolytes, that are solution of a lithium salt in various mixtures of organic liquids, wetting the porous membrane separating the electrodes; "polymer" electrolytes that are gels where the liquid-like part of the gel solvates Li salts and give the ionic conduction. The polymer stiffening the gel is chosen for acting as separator and adhesion to the electrodes. This latter type is usually referred to as "polymer lithium batteries".
There are two main types of casing : metallic, usually prismatic, used for cells with liquid electrolytes, and "soft bag" casing, a bag formed with resin films sandwiching an Al foil, in which the electrodes-electrolyte assembly is under vacuum. The term "polymer lithium batteries" is however misleading and supposed to refer to cells with gel-polymer electrolytes only and not to soft-bag types that can contains an electrode assembly with liquid electrolyte.

Safety Concerns of Lithium Batteries

One must keep in mind that the thank of a thermal engine, is a reservoir of fuel; and getting energy calls for an oxidizing agent taken outside (oxygen from air) for combustion. Occurrence of fire in an accident can be stopped by putting a physical barrier (cover, dry ice,..) to oxygen. But, when charged, batteries are reservoirs of energy, containing both oxidizing and reducing agents, close to each other. Thus it would be very difficult to stop an accidental occurrence of an internal reaction. As safety is the first requirement, great care should be taken in the choice of the chemical system, the quality of the design of the cells and of their assembly, and the electrical and thermal control of the batteries (BMS-Battery Management System and BTM- Battery Thermal Management).
From the chemical point of view, there is an intrinsic safety level related to the thermal stability of the positive electrode materials, the energy liberated when transforming and the fact that in case of a thermal runaway it can/cannot release oxygen. Going from NCM to LMO and LFP, the onset temperatures of structural transformations and energies liberated are about 220°C-500J/g, 250°C-300J/g and 270°C/a few 10J/g); and the onsets of oxygen release for NCM and LMO are at about 250 and 400°C respectively. Due to the presence of organic flammable compounds in the electrolyte an oxygen release can leads to severe accidents. Thus a significant advantage to LMO compared to NCM, and a very large one to LFP, thanks to the stability of the PO4 group. Unfortunately these benefits are at the expense of the energy density that in practice decreases from about 150-180 Wh/kg for NCM based cells to 110-120 Wh/g for LMO and 100 Wh/kg for LFP ones.

Needs for Electric and Thermal Management

Lead-acid Lead-acid and alkaline batteries have an intrinsic limit of potential that is the electrolysis of the water-based electrolyte (close to 1.40V in Ni-MH cells). Thus over-charge leads to oxygen and hydrogen evolution at the positive and negative electrodes respectively. Addition of catalyst in sealed cells make recombination possible, with moderate heating only. In lithium-ion batteries there is no such intrinsic physical limit for the organic solvents based electrolytes. Those used presently decompose irreversibly more or less rapidly above about 4.5 volts. Thus a lithium battery must have an associated electronic system that controls independently the potential of each individual cell and would stop charge of a cell as soon as its potential has reached the upper-limit potential; but continuing to charge the other cells in order that every cell reaches its maximum potential for storing the maximum of energy in the battery-pack (so-called "balancing"). These functions are the basic functions of the BMS (Battery Management System). As to temperature effects, alkaline batteries suffers from severe self-discharge when above 45°C. But specific properties are recovered when the temperature is lowered. On the contrary, reaching temperature above 60 °C inside a lithium-ion battery irreversibly deteriorate the electrode materials and the electrolyte. The reversible capacity is decreased and in the same time there is an increase of internal resistance, thus a lower capability to deliver power.

Fast Charge Lithium Batteries

Tentative users are worried about EVs when considering the duration of a standard charge , 3 to 5 hours usually. This duration comes from the fact that lithium plating is about 100 mV only below the potential of complete lithium insertion in the graphite electrode. Thus one cannot apply large overvoltage for accelerating the charge. "Hard carbons" that accommodates lithium at higher potential make faster charge possible but they still have limitation due to "SEI" a solid electrolyte interphase that develops below 1V vs. Li metal. The present solution is the use of lithium titanate (Li4Ti5012) that can accommodate reversibly up to 3 extra lithiums at 1.5 V vs Li metal. That makes possible using large over voltage for lithium insertion in the negative electrode and very fast charge (up to 80% of the full capacity in a few minutes). They also have very high cycle life (several thousand cycles) due to the absene of SEI, but these properties are at the expense of the specific energy that, at complete cell level is in the 50 to 60 Wh/kg only, due to the lowering of the unit cell potential (2 to 3,5V depending on the positive electrode material). Nevertheless these specifications make this type of cells very interesting for high power low capacity batteries, i.e. for HEVs.

Lithium Metal - Polymer batteries

The specificities of this type of battery, referred as LMP, are : i) the negative electrode is a thin film of lithium metal; ii) a thin film of a dry polymer [Poly(Ethylene Oxide) – PEO] that acts both as electrolyte – it dissolves lithium salts – and as mechanical non electron conducting separator between the electrodes; and iii) it should be operated in the 60_80°C range, 60°C being the onset temperature of a fast enough Li-ion conduction of the P(EO) based polymer electrolyte. This latter condition is not a problem and may be considered as an advantage: it is easier to control temperature in this range than to prevent a Li-ion battery to exceed 55°C in its core. This type of battery, initially developed by Hydro-Quebec, and presently produced on small scale by BatScap, has a high level of intrinsic safety (due to the large heat capacity of polymers that can go over melting) but suffers of limited specific power that, for EV applications, makes necessary to have in addition a pack of super-capacitors in front of the battery for delivering/absorbing the power peaks for acceleration/upon deceleration (regenerative braking).

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Categories: Battery - Energy transformer or storage - Electricity - Energy - Vehicles - Electric vehicles - Hybrid vehicles

Keywords: battery - electric - hybrid - - Start and stop - Transportation

THE AUTHOR

Yves Chabre