How Do We Make a Mechanically Flexible Li-Ion Micro Battery for Wearable Electronics

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reversible capacities ranging from 200 mAhg−1 at 100 cycles for SWNTs to approximately 500 mAhg−1 for MWNTs24,25. By contrast, graphite has a capacity of ∼ 372 mAhg−1 25. As shown in figure 4, SWNTs and DWNTs have higher specific surface areas than MWNTs which, with defects, causes the decom- position of a higher quantity of solvent and hence the formation of a larger solid-electrolyte inter- phases (SEI) layer. The SEI layer increases the resistance to Li ions diffusion and consequently decreases the electrodes capacity. The reversible capacity of CNT papers is also affected by the composition of the electrolyte which alters the formation of the SEI layer27.

Figure 4: Schematic of a SWNT and a MWNT26.

Free-standing electrodes made of CNTs with integrated carbon layers (CLs) offer a high re- versible capacity of 572 mAhg−1 and long term- stability28. Moreover, as the electrochemical performance of CNTs is also affected by their surface conditions such as the degree of sur- face oxidation, their properties can be modified by changing the surface functional groups. For example, in addition to the storage space for

Li+ ions provided by the carbon layers, oxygen functional groups can store Li+ ions as well by chemically binding them through the following mechanism: C−O + Li+ + e– −−→ CO−Li.

Graphene-based electrodes

Graphene paper as a self-standing electrode, which can be designed by vacuum filtration of a reduced graphene oxide dispersion, delivers a reversible capacity of ∼ 250 mAhg−1 30.

Its electrochemical properties can be tuned dur- ing its formation by controlling the reduction process and the number of oxidizing agents present31. The electrochemical performance of graphene paper electrodes can also be enhanced by using additives such as polymeric materi- als or by using functional groups such as alkyl chains, metal oxide particles and carbon black nanoparticles32,33 .

Furthermore, the rearrangement of the graphene sheets and the introduction of defects such as mi- croscale pores, cracks and voids in the structure can improve not only the accessibility and ki- netics of the system but also its Li+ ion storage sites and hence its specific capacity. Liu et al. developed a porous graphene network electrode from a graphene aerogel by mechanical pres- sure that had a reversible specific capacity of 557 mAhg−1 at a 200 mAg−1 current density34. Nitrogen-doped graphene also displays a high number of defects and in result can deliver a reversible capacity of ∼ 1000 mAhg−1 at a 50 mAg−1 current density 35.

2.1.2 Composite electrode materials with flexible foundations

Flexible composite electrodes consist of flexible foundations made from polymers, CNTs-based or graphene-based papers with compatible active materials of a high energy density such as Si, Ge, Sn and other transitional metal oxides25. Some of the various fabrication methods for these elec- trodes are described below.

One of the easiest way to build flexible composite electrodes is by printing and coating active ma- terials onto flexible free-standing structures36 . Hu et al. made an electrode with a CNT-paper based conducting flexible substrate and a coat

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of Li4 Ti5 O12 and LiMn2 O4 active materials that had average initial discharge capacities of 149 mAhg−1 and 110 mAhg−1 for Li4 Ti5 O12 and LiMn2O4 respectively after 50 cycles at a C/3 rate. Another combination of active materials such as LiCoO2 and Li4 Ti5 O12 on a CNT-based paper that acts as a substrate as well as a sep- arator is illustrated in figure 5. This cell’s high energy density of 108 mWhg−1 for 200 cycles can be enhanced by stacking multiple cells to- gether in parallel for applications such as radio frequency sensing37.

Figure 5: Schematic of a paper LIB cell structure37.

One common method of making flexible com- posite graphene-based paper electrodes is by co-filtering and then by thermal reduction of a solution of graphene oxide dispersion with some high capacity active materials such as Si and SnO2 38 . Polymer-based flexible elec- trodes, on the other hand, are generally built by the intrusion and solidification of monomers inside ordered nanotubes of active materials. An aligned CNT/poly(3,4 thylenedioxythio- phene)/PVDF composite electrode fabricated using this method exhibited a reversible capac- ity of 265 mAhg−1 at 0.1mAcm−2 39.

Spray techniques can be used to make high- quality thin flexible LIBs films with great adhe- sion between the different components. Singh et al. made one with a capacity of ∼120 mAhg−1 at rate of C/8 by spraying on top of one an- other thin layers of CNTs, LiCoO2, a composite, Li4Ti5O12 and Cu to form respectively the cath- ode current collector, the cathode, the separator, the anode and the anode current collector of the battery40 .

Weaving graphene or CNT fabrics with electro- chemically active materials can create robust composite electrodes. It was demonstrated that a composite electrode made by weaving MWNTs fabric with LiFePO4 active materials delivered

a reversible capacity of 115 mAhg−1 at a C/3 rate41 .

Chemical (CVD) and physical vapour deposi- tion (PVD) use various deposition methods and chemical treatments to form flexible LIBs by providing stable binding between the active elec- trode materials and their substrates42. As can be observed in figure 6, the good adhesion and contact between the components confer the elec- trodes tenacity and robustness in addition to great electrochemical properties. Numerous ac- tive materials can be bonded to their carbon- based support with this technique. A few exam- ples are given below for CVD and PVD.

An electrode made from aligned CNTs deposited onto a graphene paper substrate by CVD de- livers a specific capacity of 290 mAhg−1 at a 30 mAg−1 current density while one with Si bonded to CNT fabrics has a stable capacity of 500 mAhg−1 43,44. A Fe2O3/SWNTs electrode, made by CVD and oxidization of Fe nanoparti- cles onto the SWNTs substrate, has a reversible capacity of above 1200 mAhg−1 at 50 mAg−1 45. An electrode made from amorphous V2O5 films deposited onto graphene paper exhibits a capacity of 414.4 mAhg−1 at 100 mAg−1 9.

Figure 6: Schematic of the synthesis of an inter- penetrating and robust CNTs/V2O5 nanocomposite network.46 .

2.2 Materials selection for the elec- trolyte

The electrolyte transports ions from one elec- trode to the other. Solid electrolytes are pre- ferred to liquid ones for flexible devices as they present much better safety. Indeed, even though