What is a polymer battery

  Li-polymer system
(Secondary system)

LiCx, Conductive polymer or gel LiMOx

Other names  Li-polymer battery, solid-state lithium battery

application  The originally planned development as a traction battery has so far failed due to the low electrolyte conductivity. Li-polymer batteries have been used in portable computers and cell phones since 2000. Since the cells are generally manufactured as foil batteries, the more favorable form factor compared to round cells gives them an advantage in the field of portable devices. The development focuses on improving the conductivity and stability of the polymer electrolyte.

Simplified electrochemical reaction equations:
(Help formulas)

Lithium forms so-called intercalation compounds with certain types of graphite, i.e., deposits of ions in the layer lattice of the graphite crystal, in which the ion is formally discharged but does not form a chemical bond. In the positive electrode, too, there are intercalation compounds of lithium in certain oxides with layer structures, for example manganese oxides. The positive electrode consists of a metal oxide (MOx ‘), in which a certain number of Li-ions can be reversibly stored between the levels of the crystal lattice (LiyMOx). In practice, mainly LiCoO is used2, partly LiNixCoyO2 and to a lesser extent LiMn2O4 used. LiNiO2 was also used on a trial basis, but was not found to be sufficiently stable, especially at elevated temperatures. The substitution of cobalt by nickel results in a lower price (cobalt is approx. 10 times more expensive than nickel), and on the other hand, the realizable capacity per gram of cathode material is higher: LiCoO2 = 140 to 170 mAh / g; LiNi0,8Co0,2O2 = 180 to 200 mAh / g.

The charge / discharge equation is:

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Negative electrode:LiyC.n¬ C.n + Li+ + e-
Positive electrode:Li1-yMOx + yLi+ + ye-¬ LiMOx
Total: Li1-yMOx + LiyC.n¬LiMOx + Cn

The system is identical to the Li-Ion battery, with the exception of the electrolyte. Gel electrolytes or polymer solid electrolytes are used. In addition to ion transport, the polymer electrolyte also serves as a separator (electrical separation of positive and negative electrodes) and as a flexible composite material that holds the system together without external pressure. Plastic-laminated aluminum foils are sufficient as a housing to protect against environmental influences. In these foil batteries, the weight proportion of the housing material is extremely low.

Technical specifications 
Electrolyte solution: Polyethylene oxide (PEO), poly-phenylene-plastic (PPP), polyvinylidene difluoride-hexafluoropropylene copolymer (PVDF-HFP) and polymers with molecularly bound solid ions are used as electrolyte carriers. The ionic conductors mentioned include: LiCF3SO3, Li1,3Al0,3Ti1,7(PO4)3, LiTaO3, SrTiO3, LiTi2(PO4)3· Li3PO4, LiCl, LiBr, LiJ.
Examples:
Polymer electrolyte PEO9 LiCF3SO3
 Layer thickness: 80 - 200 m m (foils)
 Conductivity: approx. 5 x10-4 S / cm at 120 ° C
 Polymer electrolyte PVDF-HFP Li1,3Al0,3Ti1,7(PO4)3
 Layer thickness a few 100 m m
 Conductivity: ³ 10-4 S / cm, residual conductivity ³ 10-10 S / cm
Operating temperature range: - 20 to 40 ° C; at PEO up to 120 ° C, preferably 80 ° C (here the conductivity is good and temperature-related parasitic reactions do not yet take place).
Theoretical specific energy: 450 - 900 Wh / kg
Practical spec. Energy: 130 - 144 Wh / kg
Energy density: 230 - 410 Wh / l
Service life: after 300 cycles still 95% of the initial capacity,
Self-discharge: low at low temperatures
Open circuit voltage: 4 V, nominal voltage: 3.7 V.
spec. Charge: 1157 Ah / kg (theoretical), 680 Ah / kg laboratory cells, 372 Ah / kg in use
Energy efficiency: no information
Types: consumer-oriented dimensions
Sizes: 500 mAh to 60 Ah (PEO Li batteries)


Particularities:  One tries to compensate for the poor conductivity with thin layers and large areas.

Advantages disadvantages  Advantages:High energy density. Easy production of any dimensions. Safer than the Li-Ion system. Absolutely leak-proof. Comparatively high cell voltage.

Disadvantage: Poor conductivity at low temperatures. No reversible overcharge reaction. At present (2001) the cycle stability compared to Li-ion cells is worse.

address Contact Person:
Dr. Jens Tübke
Tel .: (0721) 4640-343
Fax: (0721) 4640-111
 

Fraunhofer Institute for Chemical Technology

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Telephone (0721) 46 40-0
Fax (0721) 46 40 - 111



  

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