Ceramic Polymer Composite Electrolytes (CPCE) for a Solid-State, Conformal, Wearable Battery

Wearable and conformable electronics are being developed with increasing capabilities for man portable, light weight, low volume systems; however increased capability often comes with increased power demands. For a battery used in a volume and weight constrained system such as a wearable electronic, the volumetric packaging cost is significant and most of the packaging structure is dedicated to encapsulating sensitive liquid electrolytes, aqueous or aprotic. Solid electrolytes are a path forward that provides battery performance durability, and safety while facilitating the use of low cost – light weight – mechanically flexibile battery packaging materials that translate to low cost, high-density, conformal, wearable batteries. The full potential of solid electrolytes can only be realized if improvements in processing cost and stability with ultra high energy systems  results in disruptive energy density beyond the Li-ion technology. In particular, research and development of solid-state flexible electrolytes capable of coping with massive structural reconfigurations required in next-generation electrodes such as Si, SnO₂, sulfur is needed[1].

Lithium superionic conductor (LISICON)-type solid electrolytes are the most promising solid electrolytes for lithium batteries. The basic structure for LISICON (LiA₂^IV(PO₄)₃ (A^IV = Ge, Ti, Si, Zr, etc.) structure can be described as a covalent skeleton [A₂P₃O₁₂]^- constituted of AO₆ octahedra and PO₄ tetrahedra, which form 3D interconnected channels and two types of interstitial positions (M’ and M’’) where conductor cations (Li⁺) are distributed. The structural and electrical properties of LISICON-type compounds vary with the composition of the framework. The smallest unit cell has been obtained for LiGe₂(PO₄)₃. By substitution of trivalent cations (Al, Cr, Ga, Fe, Sc, etc) for tetravalent cations (Ge⁴⁺ or Ti⁴⁺) in the octahedral sites, a high bulk conductivity of 3 x 10^-3 S/cm at 25°C with (Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) [2] and 5 x 10^-3 S/cm with (Li_1.3Al_0.3Ge_1.7(PO₄)₃ (LAGP) [3] was achieved. In addition to high Li⁺ ion conductivity, LAGP or LATP combines many favorable properties such as solid-state nature, broad electrochemical window (>5 V), negligible porosity and single ion conduction (high transference number, no dendrite formation, no cross-over of electrode materials to opposite side of electrodes compartment, etc.)  enabling high-energy battery chemistries and circumventing safety and packaging issues of conventional lithium batteries [4]. While the pursuit of high ionic conductivity through innovative chemical structures remains as the momentum for novel lithium-ion conductors, novel material processing techniques are needed. Processing techniques have the potential to address the fact that these ceramics are brittle in nature and their thin film processing is elusive. Processing methods which can create thin (~30 micron), flexible, scalable, solid electrolytes are highly desirable for both high energy battery cells and batteries that need full conformability and flexibility.

We will present novel methods such as extrusion, solution processing, electrospinning, and polymer grafting on NASICON ceramics, to develop Ceramic Polymer Composite Electrolytes (CPCE) films for conformal and wearable lithium batteries. Structural, thermal, electrical, electrochemical characterization of the CPCE will be presented. Battery performance of wearable battery cells based on free-standing and flexible electrolyte (CPCE), anode (Silicon (Si) or Germanium (Ge) nanoparticles imbedded in graphene matrix) and cathode (LiNi_0.5Mn_1.5O₄ (LNMO) or lithium cobalt oxide (LCO) coated on carbon fiber mat) will be presented and discussed.

References:

[1] D Steingart, Ch: Micro Energy Storage: Consideration, page-401 in “Micro Energy Harvesting” edited by D. Briand, et. al. Wiley-VCH, 2015].

[2] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G-Y. Adachi, J Electrochem. Soc. 136 (1989) 590.

[3] B. Kumar, D. Thomas, J. Kumar, J. Electrochem. Soc. 156 (2009) A506.

[4] B. Kumar, J. Kumar, R. Leese, J.P. fellner, S. Rodrigues, K.M. Abraham, J. Electrochem. Soc. 157 (2010) A50.