Fully Integrated Design of a Stretchable Solid‐State Lithium‐Ion Full Battery

Abstract

a conventional battery, such as good electrochemical performance, affordable price, and high safety.A conventional full cell LIB is a complicated device, composed of a current collector, a cathode, an anode, a separator, an electrolyte, and a proper packaging. To fabricate a stretchable full cell, we need to replace all these components with materials that can provide simultaneously the appropriate electrochemical functionalities as well as the intrinsic mechanical stretchability. Many components that could potentially be used in a stretchable battery have been presented in the literature. For example, several approaches have been reported for fabricating stretchable electrodes by incorporating rigid electrode materials within a porous framework, with a wavy design, or with helically coiled spring configuration. [6] However, these structures are usually complicated and costly to produce and scale-up, and, due to the use of polydimethylsiloxane (PDMS) or other substrates, the cells are thick and heavy. [7] Also, stretchable composite conductors for applications in electronics have been widely investigated. [8] However, there are only very few reports targeting battery current collectors. [7,9] The sheet resistances of the reported materials range from around 150 to 200 Ω □ −1 in the unstretched states and get much higher when stretched, producing significant internal resistance. Polymer gel electrolytes (PGEs) composed of poly(vinylidene fluoride) (PVDF), [10] poly(ethylene glycol) (PEG), [11] poly(ethylene oxide) (PEO), [12] or poly(ionic liquid) [13] are widely accepted as promising solution for flexible LIBs due to their advantageous flexibility, safety, and packaging behavior compared to the liquid electrolytes with a porous separator. Unfortunately, PGEs usually show quite a low room temperature ionic conductivity that ranges from 10 −4 to 10 −3 S cm −1 . Since 2015, "water-in-salt" (WiS) aqueous electrolytes containing extremely concentrated lithium salt have been investigated. [14] Some of these WiS-based gel electrolytes have also been reported to show better safety and robustness compared with the conventional PGE. [15] However, none of these materials exhibited mechanical stretchability.It is interesting to note that although so many stretchable battery components have been reported, their processing into full cells is rarely described, and even in these cases, usually not all the LIB components were made stretchable. [7,16] The reason for this is the diffculty in obtaining robust conductive interfaces between the battery components enabling efficient ion and A solid-state lithium-ion battery, in which all components (current collector, anode and cathode, electrolyte, and packaging) are stretchable, is introduced, giving rise to a battery design with mechanical properties that are compliant with flexible electronic devices and elastic wearable systems. By depositing Ag microflakes as a conductive layer on a stretchable carbon-polymer composite, a current collector with a low sheet resistance of ≈2.7 Ω ...

Stretchable Polymerized High Internal Phase Emulsion Separators for High Performance Soft Batteries

“…With our polyHIPEs we achieve a MacMullin number of less than 2, unmatched by any other stretchable separator technology. [ 8–10,40–48 ] Introducing potassium hydroxide (KOH) as electrolyte boosts the separator conductivity to record high 0.39 ± 0.05 S cm −1 while keeping the MacMullin number below 2, corroborating the performance increase achievable with polyHIPE separators.…”

“…Recent developments exploit high‐energy‐density materials, such as Zn and Li‐ion based batteries, [ 7–10 ] to enhance battery capacity and close the gap to rigid‐island designs. [ 11 ] Despite the progress in capacity and rechargeability, the principle design of planar intrinsically stretchable batteries has largely remained unchanged.…”

“…Hydrogels are often used in water‐based systems, serving as gel electrolyte and separator at the same time. [ 8–10 ] Although hydrogels contain large amounts of electrolyte (>80%), they have an intrinsically lower conductivity due to interactions of ions with the hydrophilic polymer network. [ 20 ] They are also limited to water‐based electrolytes, impeding the use of ionic liquids.…”

“…Fabricated membranes with 75%, 80%, and 85% porosity have pore‐sizes in the range of ≈5 –30 µm (Figure 2b), similar to other reported battery separators. [ 10,46,49 ] We characterize the conductivity of our separators with impedance spectroscopy utilizing a custom‐made measuring cell (Figure S2, Supporting Information). The specific resistance of the electrolyte‐filled separator and of the electrolyte itself is determined from the high‐frequency plateau of the real part of the impedance (Figure 2c).…”

“…[ 52 ] Higher energy densities and more charge cycles are possible with lithium ion based chemistries. PolyHIPE separators are compatible with state‐of‐the art Li‐based electrolytes containing Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), rendering them applicable for chemistries that utilize LiMn 2 O 4 ‐cathode and V 2 O 5 ‐anode compositions, [ 10 ] or NMC/graphite‐cells using graphite anodes and Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 ‐cathodes. [ 53 ] In combination with 11 m LiTFSI we find a conductivity of 9.8 ± 1.2 mS cm −1 , resulting in a MacMullin number as low as 3.0 ± 0.7 (Figure S8, Supporting Information).…”