New bendable and thin sulfide solid electrolyte films enable higher performance solid-state Li-ion batteries
|Schematic diagram of new cell. Credit: ACS, Nam et al. Click to enlarge.|
Researchers in South Korea have developed free-standing and stackable all-solid-state lithium batteries (ASLBs) with high energy density and high rate capabilities. A paper on their work is published in the ACS journal Nano Letters.
To make the batteries, the team developed bendable and thin sulfide solid electrolyte (SE) films reinforced with a mechanically compliant poly(paraphenylene terephthalamide) non-woven (PPTA NW) scaffold. With thin (∼70 μm) NW-reinforced SE film, the new solid state batteries show up to a 3-fold increase of the cell-energy-density to 44 Wh kgcell−1), compared to that of a conventional all-solid-state cell without the NW scaffold.
Spurred by desperate demands for safe LIBs, all-solid-state lithium batteries (ASLBs) using noninflammable inorganic solid electrolytes (SEs) have attracted significant attention as ultimately safe energy storage device. LiPON (Li3.3PO3.9N0.17) is a well-known commercialized SE material and is used to fabricate thin-film-type ASLBs. However, owing to its low room-temperature ionic conductivity in the range of ∼10−6 S cm−1 and high preparation cost via vacuum deposition the application of thin-film-type ASLBs is limited to low energy devices such as smart cards and microelectronics.
In contrast, bulk-type ASLBs in which composite electrodes comprise a mixture of electrode materials, SE particles, and conductive carbon are considered to be fabricated by a scalable process and are especially promising for outperforming the conventional LIBs. … However, sintering at a high temperature of at least ∼800 °C is necessary to form two-dimensional contacts between active materials and oxide SEs. Unfortunately, high-temperature sintering deteriorates the interfaces between active materials and oxide SEs, leading to extremely poor electrochemical performances.
In sharp contrast, promising performances for bulk-type ASLBs have been reported using sulfide SE materials such as glass-ceramic Li2S−P2S5 … Even though sulfide SE materials suffer from instability in air (due to generation of toxic H2S gas when reacting with moisture in air), the investigation of ASLBs using sulfide SEs has been accelerated because sulfide SEs are far superior to their oxide counterparts in terms of the following properties: First, sulfide SEs exhibit higher ionic conductivities than oxide SEs. … Second, the sulfide SE is ductile, exhibiting Young’s moduli in between those of organic polymers and oxide ceramics, which enables intimate contacts with active materials by means of a simple cold pressing procedure.—Nam et al.
|Comparison of the energy densities of the all-solid-state battery as a function of the overall weight fraction of SEs varied by electrode chemistry, the presence of SE coating, and the bendable NW-SE film. Credit: ACS, Nam et al. Click to enlarge.|
In the study, the team used either glass-ceramic Li3PS4 or tetragonal Li10GeP2S12. PPTA NW—a high-performance polymer with good thermal, chemical, and electrochemical stability, served as a mechanically compliant scaffold that provided flexibility and toughness to the NW-SE film.
The bendable NW-SE films have lower conductivity values than conventional SE counterparts—the NW scaffold is not Li+-ion conductive. However, the use of NW allows the fabrication of very thin (∼70 μm) bendable composite films which appear to have higher conductance values than those of the conventional thick SE film.
The team constructed all-solid-state cells incorporating LiTiS2 (LTS) and Li4Ti5O12 (LTO) as the cathode and anode, respectively.
In addition to the improved energy density, the LTS/ LTO all-solid-state cells that contained the NW-SE films also showed improved rate capabilities.
We believe that the NW-SE films proposed herein hold significant promise as a compelling building unit and their combination with the elaborately designed cell architecture provides a new route for the development of high-performance ASLBs.—Nam et al.
Young Jin Nam, Sung-Ju Cho, Dae Yang Oh, Jun-Muk Lim, Sung Youb Kim, Jun Ho Song, Young-Gi Lee, Sang-Young Lee, and Yoon Seok Jung (2015) “Bendable and Thin Sulfide Solid Electrolyte Film: A New Electrolyte Opportunity for Free-Standing and Stackable High-Energy All-Solid-State Lithium-Ion Batteries” Nano Letters doi: 10.1021/acs.nanolett.5b00538