Characteristics of the Polymer Electrolyte

The following are the characteristics necessary to act as a successful polymer host for polymer electrolytes.

  • • A polymer repeat unit should have a donor group (an atom with at least one lone pair of electrons) to form coordinate bonds with cations.
  • • Low barriers to bond relations should be exhibited, so that segmental motion of the polymer chain can take place readily
  • • A suitable distance should exist between coordinating centers, because the formation of multiple intra-polymer ion bonds appears to be important.

Almost all of the polymer electrolytes have been optimized with respect to the ionic conductivity of the electrolytes. The ionic conductivity of the polymer electrolyte depends mainly on the structure and nature of the base polymer, the nature of the ionic salt, the concentration of the salt, the dissolution, ionic radius, etc. In the subsequent sections of this chapter (Sections 12.4-12.7), the role of biopolymers on the ionic conductivity of the polymer electrolytes will be discussed in detail.

Biopolymer-Based Polymer Electrolytes and Their Properties

Biopolymers are biodegradable polymers which are found in nature or can be synthesized from non-biodegradable monomers. Carbohydrates and proteins are examples of natural biopolymers, whereas polylactic acid (PLA) and polycaprolactone (PCL) belong to synthetic biopolymers. Derivatives of biopolymers are being produced commercially nowadays and find applications in biomedical uses, tissue engineering, and the packaging industry. Some of the biopolymers are extensively used as electrolytes in energy storage devices. Biopolymer-based electrolytes have overcome the main drawbacks of synthetic electrolytes, such as high cost and environmentally unfriendly nature. These biopolymer-based electrolytes exhibit high ionic conductivity, low cost, and good dimensional and mechanical stability. Chitosan-based polymers, starch-based derivatives, carrageenan-based biopolymers, cellulose-based compounds, polyethylene glycol (PEG), polycaprolactone (PCL), etc. are biopolymer electrolytes widely used in energy storage devices.

Chitosan-Based Polymer Electrolytes

Chitosan is a linear polysaccharide composed of randomly distributed p-(l->4)- linked D-glucosamine (deacetylated unit) and (V-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin in shells of shrimps and other crustaceans with an alkaline substance, like sodium hydroxide. Table 12.1 lists some of the lithium ion and proton-conducting polymer electrolytes based on chitosan.

TABLE 12.1

Room-Temperature (RT) Ionic Conductivity of Various Chitosan-Based Polymer Electrolytes

Materials

Optimized Composition

Conductivity (S cm-') (RT)

References

Chitosan (CH), methylcellulose (MC), lithium tetrafluoroborate (LiBF4)

60 wt.% CH-MC (75:25) and LiBF4 (40 wt.%)

3.747X10-6

[3]

Chitosan (CH), polyethylene oxide (PEO), lithium

bis(trifluoromethane sulfonyl) imide (LiTFSI)

1 wt.% CH-1 wt.% PEO-30 wt.% LiTFSI

1.40x10-6

[4]

Chitosan (CH), lithium triflate (LiCF,SO,)

40 wt.% LiCFjSO,

-10-'

[5]

Chitosan (CH), lithium triflate (UCF3SO3), ethylene carbonate (EC)

70 wt.% CH-30 wt.% EC + LiCFjSO,

2.75x10-'

[6]

Chitosan (CH), dextran, ammonium thiocyanate (NH4SCN)

40 wt.% dextran-60 wt.% chitosan-40 wt.% NH4SCN

1.28x10-*

[7]

Chitosan (CH), oxalic acid

60 wt.% chitosan-40 wt.% oxalic acid

4.95x10-7

[8]

Chitosan, ammonium nitrate, acetic acid

1.9 wt.% CH-0.17 wt.% ammonium nitrate- 96.3 wt.% acetic acid

1.46x10-'

[9]

Chitosan (CH), acetic acid, glycerol

-

2.9x10-'

[10]

Chitosan-LiOAc-oleic acid

-

10-'

[11]

Chitosan (CH), ammonium acetate

lg (CH) + 40 wt.% ammonium acetate

2.87x10-*

[12]

Chitosan (CH), ammonium chloride

lg (CH) + 20 wt.% ammonium chloride

5.35x10-'

[13]

Chitosan (CH), ammonium iodide (NH4I), Ethylene carbonate (EC)

55 wt.% chitosan-45 wt.% NH4I-40 wt.% EC

7.60x10-**

[14]

Chitosan (CH), ammonium thiocyanate (NH4SCN), aluminum titanate (Al,Ti05)

57 wt.% chitosan -38 wt.% NH4SCN -5 wt.% ALTiO,

2.10x10-'

[15]

Chitosan (CH), ammonium thiocyanate (NH4SCN), alumina

(A1A)

  • 60 wt.% chitosan-40 wt.% NH4SCN
  • 60 wt.% chitosan-40 wt.% NH4SCN-6 wt.%. ALO,
  • 1.29x10-*
  • 5.86x10-*

[16]

Poly(vinyl alcohol) (PVA), chitosan (CH), phosphoric acid (H,P04), niobium oxide (Nb,0,)

PVA:CS (80:20)-40% phosphoric acid-6% of Nb,Os

3.00x10-'

[17]

Chitosan (CH), poly(viny) alcohol), ammonium nitrate (NH4N03), ethylene carbonate (EC)

7 wt.% chitosan-l 1 wt.% PVA-12 wt.% NH4NO,-70 wt.% EC

1.60x10-'

[18]

TABLE 12.1 (CONTINUED)

Room-Temperature (RT) Ionic Conductivity of Various Chitosan-Based Polymer Electrolytes

Materials

Optimized Composition

Conductivity (S cm-') (RT)

References

Chitosan (CH), iota-carrageenan, phosphoric acid (H,P04), polyethylene glycol)

37.50 wt.% chitosan-37.50 wt.% iota- carrageenan-18.75 wt.%, H;PO.,-6.25 wt.% PEG.

6.29x10-4

[19]

Chitosan (CH), kappa- carrageenan, ammonium nitrate (NH4NO,)

42 wt.% CH-42 wt.% k- carrageenan-16 wt.% NH4NO,

2.39x10-*

[20]

Chitosan (CH), ammonium iodide (NH4I), l-butyl-3- methylimidazolium-iodide (BMII)

27.5 wt.% (CH>-22.5 wt.% NH4I-50 wt.% BMII

3.43x10-5

[21]

Chitosan (CH), poly(ethylene oxide) (PEO), ammonium iodide (NH4I)

27 wt.% CH-27 wt.% PEO-44 wt.% NH4I

4.32x10-"

[22]

Chitosan (CH), poly(ethylene oxide) (PEO), ammonium iodide (NH4I), l-butyl-3- methylimidazolium-iodide (BMII)

30 wt.% C -70 wt.% PEO-9 wt.% NH4I-80 wt.% BMII

5.52x10-*

Chitosan (CH), praseodymium (III) trifluoromethanesulfonate (PrTriO

20 wt.% CH - 30 wt.% PrTrif

2.38x10-" (at 90 »C)

[23]

Chitosan shows biocompatibility and biodegradability characteristics, which are suitable to create an environmentally friendly, inert, and flexible polymer for sensing and manipulating macromolecules and microorganisms in devices. In particular, the chitosan matrices provide a better environment for doping, blending, and grafting of acids, oxides, and salts to improve the conductance level to one comparable with synthetic ion-conducting polymers. In recent years, much research has been carried out on chitosan and its derivatives as components in batteries, supercapacitors, etc.

Chitosan is an odorless powder and its color varies from yellow to white. On the other hand, spray-dried chitosan salts exhibit a smooth texture and a pale color. The physico-chemical properties of chitosan are influenced by the degree of deacetylation and molecular weight. The process of deacetylation involves the removal of acetyl groups from the molecular chain of chitin, leaving behind a compound (chitosan) with a high degree of chemically reactive amino groups (-NH,). This makes the degree of deacetylation (DD) an important property in chitosan production, as it affects the physicochemical properties, and, hence, determines its appropriate practical applications [24]. The degree of deacetylation of chitosan ranges from 50% to 99%, with an average of 80%, depending on the crustacean species and the preparation methods. Chitin with a degree of deacetylation of 75% or above is generally known as chitosan.

Chitosan is hydrophilic in nature, finding utility in high-temperature and low- relative-humidity environments. Chitosan is soluble in most organic and alkaline solvents, but is not soluble in water, which limits its applications. In order to improve its properties, various chemical modifications, like sulfonation [25], phosphorylation [26], quaternization [27], chemical crosslinking [28], phthaloylation [29] and acylation [30], have been employed. Linear sweep voltammetry and variation in the discharge capacity of chitosan-based iota-carrageenan is shown in Figure 12.1.

 
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