Rechargeable Lithium Batteries with Electrodes of Small Organic

Subscriber access provided by UOW Library

Review

Rechargeable Lithium Batteries with Electrodes of Small Organic Carbonyl Salts and Advanced Electrolytes Qing Zhao, Chunyang Guo, Yong Lu, Luojia Liu, Jing Liang, and Jun Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01462 • Publication Date (Web): 06 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Rechargeable Lithium Batteries with Electrodes of Small Organic Carbonyl Salts and Advanced Electrolytes Qing Zhao, Chunyang Guo, Yong Lu, Luojia Liu, Jing Liang, and Jun Chen* Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China.

ABSTRACT: Rechargeable lithium batteries with organic electrode materials are promising energy storage systems with advantages of structural designability, low cost, renewability, and environmental friendliness. Among the reported organic electrode materials, small organic carbonyl compounds are powerful candidates with high theoretical capacities and fast kinetics. However, these compounds are plagued with high solubility in aprotic electrolyte, which is considered as the main issue leading to capacity decay and short cycling life. Herein, we review two major methods to solve such problems, including the preparation of small organic carbonyl salts and the optimization of the electrolyte. The polarities of organic electrode materials can be enhanced by forming salt. Thus, the dissolution of organic compounds in aprotic electrolyte with low polarity is depressed. Meanwhile, the optimization of electrolyte with increasing viscosity can also reduce the dissolution. These two strategies provide guidance for future study on rechargeable lithium batteries with organic electrode materials.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

1. Introduction: Li-ion batteries (LIBs) have achieved a great success on portable electronics such as smart phones and laptops in the past two decades.1-3 Large-scale application of LIBs has also been recently demonstrated in electric vehicles and smart grids.4 However, further development of LIBs with inorganic transition-metal oxides as the cathode materials faces the bottleneck of limited capacity and resources (Figure 1a).5-8 A feasible solution towards such troubles is developing organic electrode materials with high capacity and wide abundance (Figure 1b).9-11 Organic electrode materials, which mainly consist of C, H, O, N and S, possess the advantages of low cost, structure designability, and green sustainability.12-14 There are four major types of organic electrode materials: conductive polymers,15 organsulfurs,16 organic free radical compounds17 and organic carbonyl compounds.9, 10 In particular, organic carbonyl compounds have been widely studied for their high theoretical capacity and fast reaction kinetics.10,

18

According to the molecular weight, organic carbonyl compounds can be divided into two major categories: small organic carbonyl compounds and organic carbonyl polymers.19-22 Small organic carbonyl compounds have shown the merits of facile synthetic process, high capacity and sufficient space for Li-ion insertion, but suffered from low electronic conductivity and high solubility in aprotic electrolyte. This leads to serious capacity decay and short cycling life of the electrodes. To improve the conductivity of organic carbonyl compounds, carbon materials such as carbon nanotubes23 (CNTs), graphene24 and porous carbon25, 26 with high conductivity have been applied. With the methods of mixing in selected solvent,24 thermal treating,26 impregnation25 and in situ grafting,27 organic materials can be in-situ loaded on highly conductive carbon materials to form organic carbonyl compounds/carbon composites. In addition, the organic materials can be


2

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


immobilized on specific substrates through π-π stacking interaction23 or covalent interaction,27 which can simultaneously decrease the dissolution in aprotic electrolyte. Further research on carbon composites should focus on optimizing the ratios of organic active materials in the composites to achieve high electrochemical performance with high energy/power density, high stability and long cycling life.28

Figure 1. a) Content of transition metal elements (Fe, Mn, Ni, Co) in the crust,8 which are presently used in the cathodes of LIBs. b) A proposed recyclable diagram with sustainable and green organic electrode materials.11-13

The high solubility of small organic carbonyl compounds in aprotic electrolyte can be derived from the similar polarity because both organic carbonyl compounds and electrolyte show low polarity.9,10 Efforts have been tried on solving this problem, mainly including the formation of small organic carbonyl salts and the optimization of the electrolyte. On one hand, compared with pure carbonyl compounds, carbonyl salts with O−-Li+···O− chelate bonds show increased polarity, therefore inhibiting the dissolution in aprotic electrolyte.9,10 On the other hand, optimizing electrolytes such as the preparation of high-concentration,29 quasi-solid-state30 and all-solid-


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

state31 electrolytes is also an efficient way to solve the dissolution problem. The increased viscosity and restrained solvent in optimized electrolyte will certainly inhibit the dissolution. Herein, we review recent development on electrodes with small organic carbonyl salts and advanced electrolytes in LIBs. Moreover, the design strategies of organic salt electrodes have been concluded such as the tunability of operation potential by introducing hetero-atom, the enhancement of theoretic capacity by adding more carbonyls and the increase of conductivity by extending the conjugated structures. Meanwhile, the challenge and prospect of advanced electrolytes have been also discussed, including the solution towards the low conductivity of solid-state electrolyte and the universality of high-concentration electrolyte. As results, this review should not only provide basic acquaintance on small organic carbonyl salts and advanced electrolytes but also point out the directions towards further R&D of rechargeable batteries with organic electrode materials.

2. Results and discussions 2.1 Electrodes of small organic carbonyl salts Up to now, there are three broad categories of small organic carbonyl salts, namely carboxylates, quinone (or oxocarbon) salts and amide salts. Generally, carboxylates with decreased lowest unoccupied molecular orbital (LUMO) energy are applied as anodes with one Li-ion insertion per -COOLi group (Type1, Scheme 1 and Figure 2).32 In comparison, most quinone or oxocarbon salts with –OLi group are applied as cathode materials with remained -OLi group for inhibiting dissolution (Type 2, Scheme 1 and Figure 2).11 The diimide dilithium salts are expected to occur tautomerisation reaction with half carbonyls utilization (type 3, Figure 2).33 Compared with most inorganic electrode materials prepared under high temperature,


4

Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


organic carbonyl salts are usually prepared under room temperature or moderate temperature with methods of proton-replacement reaction or dehydration reaction.32 Thus, the preparation of organic electrode materials has the advantages of lower energy consumption and shorter time production.

Scheme 1. The electrochemical process with three types of small organic carbonyl salts.

Figure 2. LUMO energy comparison of selected salts with carboxylates (a, b), oxocarbon (c), and quinone (d).

2.1.1 Carboxylates


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

Two kinds of aliphatic and aromatic carboxylates have been reported as anodes in LIBs. All aliphatic carboxylate with reversibility are trans versions (Compounds 1-3, Figure 3).32,

34

Increasing the conjugation numbers will decrease the capacity due to enhanced molecule weight and lower the polarization due to the increased electron delocalization. Meanwhile, the aliphatic carboxylates can actually uptake about one Li-ion per unit formula, which is caused by the rapid increasing energy for inserting the second Li according to result of the density functional theory (DFT) calculation. As comparison, the aromatic carboxylates with π-conjugated structures are able to uptake 2 Li-ions per unit. For instance, Li2C8H4O4 (Li2TP, Compound 4) can insert two Li-ions with a reversible capacity of 300 mAh/g and a capacity retention of 78 % after 50 cycles. Moreover, Li2TP have two advantages exceeding traditional graphene anode, the better thermal stability and faster Li-ion diffusion process.35 The DFT results reveal that Li-ion diffusion barrier of Li2-Li2TP is 270 meV lower than LiC6. In addition to Li2TP, CaTP and Al2/3TP (Compound 5, 6) can also be obtained with the displacement reaction

36

Among the three TP salts, CaTP

displays the best capacity retention. The lowest dissolution in electrolyte (1.8 ppm) and the maintained crystal/morphological structures contribute to this result. The simple conjugated carboxylates usually exhibit low conductivity. With in-situ carbon coating and smaller particle size, the high rate performance can be rationally enhanced (Compound 7). 37-39


6

Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Structures and theoretical capacities (Cth) of typical aliphatic (group 1) and aromatic (group 2) carboxylates. The Cth is calculated from the formula: Cth = nF/3.6 Mw, where n is the transferred electron number and equals to 2 (Compound 1-7), F is the Faraday constant, and Mw is the molecular weight.

Through introducing hetero-atom and extending the conjugated structures, the working potential of carboxylates can be rationally tuned. A heterocyclic carboxylate (Compound 8, Li2TDC, Figure 4) displays a discharge potential of 1.0 V (first cycle), which is about 0.2 V higher than that of Li2TP.40 Meanwhile, the N-hetero compound also shows higher discharge potential (Compound 9 and 10).41 Although the enhanced potential will lower the total energy density coupled with cathode, the high potential can effectively inhibit the formation of solid electrolyte interphase (SEI) film and thus increase the initial coulombic efficiency.41 In addition, the relatively high discharge cut-off potential makes cheap Al current collector appropriate for these type anodes. Interestingly, some carboxylate compounds show an excess discharge capacity at low discharge voltage (Li2TP and Li2TDC) 40 This manifests that the double bond in the heterocyclic and aromatic core can also reversibly insert excess Li-ions, which has been well


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

characterized with high resolution solid-state nuclear magnetic resonance (NMR) spectra with 13

C-isotopelabeled Li2TP and Li2TDC.

To improve the high-rate stability of organic salts in molecule design level, a few compounds with π-extended conjugated structures have been designed (group 4).42-47 Simultaneously, the extended conjugations are beneficial for lowering the polarization between discharge and charge (Compound 12).42 Among them, dilithium 2,6-naphthalene di-carboxylate (Li2NDC, compound 11) has been comprehensively investigated.43-45 In addition to the better high-rate performance, Li2NDC has also been demonstrated as a favorable anode for safe Li-ion batteries due to the maintained framework structure with very small volume change after about two Li-ions intercalation.44 Moreover, further study has shown that the capacity of Li2NDC can be improved by narrowing the distance of naphthalene layers with annealing process, which is helpful for the formation of efficient ion and electron pathways of the framework.45 As an application example, the full cells with Li2NDC anode and LiNi0.5Mn1.5O4 cathode display excellent cycling performance, including a capacity retention of 96 % after 100 cycles, a high operation voltage near 4 V with specific and power density of 300 Wh/kg and 5 kW/kg, respectively. As a general comparison with the commercial anode materials such as graphite in LIBs, the carboxylates usually display a higher operation voltage with a bit lower theoretical capacity. The main dominant of carboxylate anodes is considered to be high safety. For instance, some reported organic anodes such as Li2C6H4O4 and Li2C8H4O4 have been proved to remain more stable electrochemical reactivity than that with graphite at 80 °C.34 Meanwhile, DFT calculation has demonstrated the smaller Li-diffusion barriers in some organic electrodes in comparison with that of graphite. Finally, the relative high operation voltage potential enhances the energy utilization efficiency at the first cycle owing to the suppressive formation of SEI film.


8

Page 9 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Structures and Cth of typical heterocyclic (group 3) and extended conjugated (group 4) carboxylates. The transferred electron number is 2 in Compound 8-12 and 4 in Compound 13.

Organic electrode materials take the advantages of structure designability. Rationally managing active functional group on carboxylates will tune the reactions of electrodes. For instance, adding carboxylate group on the body of quinone can not only reduce the dissolution but also enhance the theoretic capacity. Yoshida’s group introduced two lithiooxycarbonyl groups into the molecule of 9,10-anthraquinone (AQ), 9,10-phenanthrenequinone (PQ) and pyrene-4,5,9,10tetraone (PYT) and prepared three organic carboxylate electrodes (Compound 14-16, Figure 5).48 The capacity retention of carboxylates were 80~100 % after 20 cycles, while the normal quinone only retained 20~40% after the same cycles. Connecting the –OLi group on the benzene ring of Li2TP would construct an organic electrode with dual-function. The obtained Li4C8H2O6 (Compound 17) can also be applied as both anode and cathode materials (Figure 5).49,

50

Meanwhile, Li4C8H2O6 has been initially reported with a low-polluting and original synthesis scheme, which makes it possible as a green electrode in rechargeable lithium batteries.49 At anode side, the reversible reaction of Li4C8H2O6/Li6C8H2O6 takes place at a voltage of 0.8 V. At


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 33

cathode side, the reversible reaction of Li4C8H2O6/Li2C8H2O6 is observed at voltage of 2.6 V. Morphologies of nanosheets, nanoparticles and bulk Li4C8H2O6 were prepared. In particular, Li4C8H2O6 nanosheet electrode exhibited best capacity retention (95%) after 50 cycles at 0.1 C charge/discharge rate. More importantly, the as-prepared Li4C8H2O6 can be assembled as symmetrical cells with Li4C8H2O6 as both cathode and anode. As expected, the open-circuit voltage of this symmetrical cell is ~ 0 V. However, after charging, this organic battery displays a discharge voltage of 1.8 V and an energy density of nearly 130 Wh/kg.50 In addition, As a structural isomer, another Li4C8H2O6 (Compound 18) with two hydroxyl-Li in the ortho-position has also been studied.51 Compared with the prior structure of two hydroxyl-Li in para-position, this Li4C8H2O6 molecule exhibits a higher operation voltage of 2.85 V as cathode materials, which is supposed as the lower LOMO energy of ortho-Li4C8H2O6 than that of para-Li4C8H2O6.

Figure 5. Structures and Cth of typical quinone body (group 5) and lithiated enolate-based carboxylates (group 6). The reaction mechanism of Compound 17 is also presented.50 The transferred electron number is equal to 4 (Compound 14 and 15), 6 (Compound 16) and 2 (Compound 17 and 18), respectively.


10

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.1.2 Quinone and oxocarbon salts Quinone electrode materials have been investigated for over 40 years. The high solubility is regarded as the major issue for capacity fading. Fabricating oxocarbon and quinone electrodes with salt substituents are proved available for solving this problem.52 An oxocarbon Li2C6O6 (Compound 19, Figure 6) electrode with proposed four Li-ions insertion and high theoretic capacity of 589 mAh/g has been studied, which displays a high initial discharge capacity of 580 mAh/g.11 However, this electrode undergoes fast decays for the multiple two-phase reactions. Particularly, the reaction of initial two Li-ion insertion process from Li2C6O6 to Li4C6O6 suffers a major morphological change, which leads to the exfoliation and isolation of the active material and then the decrease of the capacity.53 The cyclability can be improved with limited operation region between 1.5-2.5 V with expected reaction of Li4C6O6/Li6C6O6 (Compound 22). Another oxocarbon salt of Na2C5O5 has encountered similar problem, while the prepared nanowire structured Na2C5O5 (Compound 20) reveals improved cycle life for released structural stress. This demonstrates the significance of preparing nanostructured organic electrodes.54 Inspired from the multi-step reactions of Li2C6O6, an all-organic Li-ion battery has also been constructed with Li6C6O6 as anode and Li2C6O6 as cathode.55 This “greener and sustainable” all-organic Liion battery exhibits a working voltage of about 1.0 V with a reversible capacity of 200 mAh/g. Another derivative of Li2DHDMQ (Compound 23) can reversibly de-intercalate about one Li at an average potential of 3 V and the γ-Al2O3 additive has been proved beneficial for absorbing the dissolved active materials.

56

Similar with the carboxylates, a bigger π-extended conjugated

structure is helpful for increasing the conductivity and maintaining long cycle stability (Compound 24).57 In addition to the lithiated enolate functional group, the –SO3Na group can also solve the dissolution problem of the quinone (Compound 25, 26).58 Moreover, –SO3Na is an


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

electrode withdrawing group, which is able to decrease the LUMO energy of organic compound and results in an enhanced working potential.

Figure 6: Structures and Cth of typical oxocarbon (group 7) and quinone (group 8 and 9) salts. The reaction mechanism of Compound 20 is also displayed.54 The transferred electron number is equal to 4 (Compound 19) and 2 (Compound 20-26).

2.1.3 Aromatic amide and other salts


12

Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7: Structures and Cth of aromatic imide, indigo carmine and ester salts. The transferred electron number is equal to 2 (Compound 27-30).

The substitution of H by Li in diimide compound with obtained diimide dilithium salts (Compound 27 & 28, Figure 7) can also effectively resolve the solubility issues.33,

59

For

instance, comparing the electrochemical properties of three N contained naphthalenediimides (NDs), NDLi (Compound 28), NDH (H occupy Li site) and NDMe (methyl occupy Li site), NDLi, NDH, and NDMe showed 87.6%, 45% and ignorable capacity retention after 100 cycles, respectively. The aromatization of NDLi with lithiated state contributes the superior performance. Other carbonyl salts such as a commercial blue dye of indigo carmine with the inherent sodium sulfonate (-SO3Na, compound 29) group60 and an organic ester salt with lithiated enolate (-OLi) group61 have also been applied as cathode for rechargeable lithium batteries. Here we only review the small organic salts for rechargeable lithium batteries (the detailed electrochemical properties are listed in Table S1). Recently, a few salts (Na2C6H4O4,62,

63

Na2C6O6,64 Na2C8H2O6,65 Na2C5O5,66 MgC6H4O4 •xH2O(x=0, 2)67) have been prepared for rechargeable sodium-ion batteries, the summed strategies are also feasible. Future development of electrodes with small organic salts should depend on the following aspects. 1) Taking the operation voltage and capacity into account, the anode design with organic carbonyl salts should focus on aromatic compounds with multi –COOLi groups. It is because aromatic compounds usually exhibit lower operation voltage. Meanwhile, multi –COOLi groups provide more electrons with high theoretical capacity. 2) As cathode materials, increasing the operation voltage with reserved high capacity still remains a challenge. Our group proved that using molecule design strategy with the introduction of heteroatoms can reduce the lowest unoccupied molecular


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

orbital (LUMO) energy level, then leading to the increase of reduction potential.24 Meantime, connecting the benzene ring with the strong electron withdrawing groups such as -CN, -F, and Cl are also available to enhance the operation voltage.68, 69 3) Insights into the crystal structure change with Li-ion intercalation and de-intercalation reaction process are still limited for electrodes with small organic salts, which also provide broad development space for further study. 2.2 Advanced electrolytes Small organic carbonyl electrodes suffer from high solubility in traditional electrolytes. In addition to the preparation of organic carbonyl salts, the optimization of electrolytes such as allsolid-state, quasi-solid-state and high-concentration electrolytes is another efficient way. A typical solid-state electrolyte usually contains a polymer host such as (polyethylene oxide) (PEO), poly(methacrylate) (PMA)/poly(ethylene glycol) (PEG)) and a Li-salt such as LiClO4, LiTFSA (Figure 8a).31,

70-74

Meanwhile, there are also studies on solid-state electrolyte based on

conductive ionic-liquid composites (lithium bis (trifluoromethanesulfonyl)amide (Li-TFSA) and 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) amide (EMI-TFSA)).30, 75, 76 Specially, the LiTFSA shows high solubility in ether-based electrolyte, which makes it possible to fabricate high-concentration electrolyte. The high-concentration electrolyte can trap the motion of solvent, thus inhibiting the dissolution of organic electrodes.77, 78 In general, the ionic conductivity of different electrolytes at room-temperature is in the order of all-solid-state < quasi-solid-state < high-concentration liquid electrolyte (Figure 8b).


14

Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8: a) Typical structures of polymer and Li-salt for advanced electrolytes. b) Ionic conductivity comparison of different type electrolytes at room-temperature (the PEO/LiTFSI electrolyte is at 100 ○C ).

Figure 9: Structures and Cth of selected small organic carbonyls for rechargeable lithium batteries with advanced electrolyte. The transferred electron number is equal to 10 (Compound 31), 8 (Compound 33) and 2 (Compound 32, 34-37).

2.2.1 All-solid-state electrolyte


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

The all-solid-state electrolyte is endowed with advantage of high-safety. An all-solid-state lithium battery was fabricated based on organic pillar[5] quinone (Compound 31, Figure 9) cathode and composite polymer electrolyte (CPE).31 The selected poly(methacrylate) (PMA)/poly(ethylene glycol) (PEG)-LiClO4-3 wt % SiO2 CPE has a highest ionic conductivity of 0.26 mS cm−1 at room-temperature. The addition of SiO2 is an effective method to improve the ionic conductivity of solid-state electrolyte.79 With the selected CPE, an initial capacity of 418 mAh g−1 and 94.7% capacity retention after 50 cycles at 0.2 C rate is obtained. As a comparison, the electrode performance of P5Q tested in traditional liquid electrolytes with 1 M LiPF6 in ethylene carbonate:diethyl carbonate solutions (1:1 in volume) only remain a capacity of ~230 mAh g-1 after 3 cycles), which ascribes to the seriously dissolution of active materials into the liquid electrolyte. Afterwards, tetramethoxy-p-benzoquinone (Compound 32) with commercial all-solid-state PEO/LiTFSI-based electrolyte tested at 100 ○C also displays better cycling stability than liquid electrolyte.74 The increased operation temperature contributes to high conductivity (0.1 S cm-1) of electrolyte and further improves the high-rate performance. At a high discharge rate of 1 Li+ per 12 min, a 70% of the theoretical capacity can still be retained. 2.2.2 Quasi-solid-state electrolytes Quasi-solid electrolyte is also an alternative candidate for organic batteries. Our group has developed a poly(methacrylate) (PMA)/PEG-based GPE as the electrolyte with satisfactory ionic conductivity for dye-sensitized solar cells.80 Since this electrolyte shows the ability of entrapping large numbers of liquid electrolyte, (PMA)/PEG-based GPE has been further adopted to inhibit the dissolution of calix[4]quinone (C4Q) cathode material (Compound 33). In addition to traditional quasi-solid electrolyte, the ionic liquid based quasi-solid-state electrolytes have also been proved feasible for trapping the dissolution.30,

75, 76

Particularly, silica-RTIL composite


16

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


quasi-solid electrolyte has been applied to assemble a solid-state cell with small organic carbonyl compounds, showing high theoretical capacities (Compound 34-36).30 Electrochemical tests manifests that the liquid cell exhibits serious fading within limited capacity retention (

Rechargeable Lithium Batteries with Electrodes of Small Organic

Recommend Documents