Polymer design for solid-state batteries and wearable electronics

Abstract

Solid-state batteries are increasingly centre-stage for delivering more energy-dense, safer batteries to follow current lithium-ion rechargeable technologies. At the same time, wearable electronics powered by flexible batteries have experienced rapid technological growth. This perspective discusses the role that polymer design plays in their use as solid polymer electrolytes (SPEs) and as binders, coatings and interlayers to address issues in solid-state batteries with inorganic solid electrolytes (ISEs). We also consider the value of tunable polymer flexibility, added capacity, skin compatibility and end-of-use degradability of polymeric materials in wearable technologies such as smartwatches and health monitoring devices. While many years have been spent on SPE development for batteries, delivering competitive performances to liquid and ISEs requires a deeper understanding of the fundamentals of ion transport in solid polymers. Advanced polymer design, including controlled (de)polymerisation strategies, precision dynamic chemistry and digital learning tools, might help identify these missing fundamental gaps towards faster, more selective ion transport. Regardless of the intended use as an electrolyte, composite electrode binder or bulk component in flexible electrodes, many parallels can be drawn between the various intrinsic polymer properties. These include mechanical performances, namely elasticity and flexibility; electrochemical stability, particularly against higher-voltage electrode materials; durable adhesive/cohesive properties; ionic and/or electronic conductivity; and ultimately, processability and fabrication into the battery. With this, we assess the latest developments, providing our views on the prospects of polymers in batteries and wearables, the challenges they might address, and emerging polymer chemistries that are still relatively under-utilised in this area.

Fig. 1. Overview of polymer design for batteries and wearable technologies. Top: The synthetic polymer chemists' domain controlling polymer molar mass (M_n) and molar mass distribution (dispersity, Ð) through ‘living’ chain growth polymerisations that can allow access to different architectures, monomer sequences and chemical structures including backbone and pendant functionalities plus chain end-groups. Middle: Measurable properties important for polymers in batteries. Conductivity shown by a Nyquist curve from electrochemical impedance spectroscopy, mechanical properties measured by rheology (also dynamic mechanical analysis and stress–strain behaviour), processability window related to the polymer melt (dry) or electrode slurry (wet) viscosity and the electrochemical stability window measured by linear or cyclic voltammetry and thermal stability (not shown) by thermogravimetric analysis. Bottom: Challenges in solid-state and flexible batteries require (1) polymers as solid-electrolyte separators, (2) functional polymer components in composite electrodes, and (3) polymer active materials for flexible electrodes in bendable devices for wearable electronics.

Fig. 2. Ion transport in Solid Polymer Electrolytes (SPEs). (a) Relationships between chain segmental dynamics (τ₁) and ion mobility (μ₊); Vogel–Tammann–Fulcher (VTF) temperature-dependence of ion conductivity (σ_ionic) with terms for polymer glass transition temperature (T_g), pre-exponent A related to salt dissociation/free charge carrier concentrations and the activation energy for ion transport (E_a). (b) Coordination environment provided by polymer backbones for controlling ion transport. (c) Phase-separated block copolymers for optimised ionic-mechanical properties. Depicted morphologies: BCC = body-centred cubic spheres, HPL = hexagonally packed cylinders, Gyr = gyroid, Lam = lamellae. (d) Dual-ion (mobile anion and cation) vs. single-ion (immobilised anion) conductors. Cation transference number, t₊, is the ratio of cation mobility to total ion mobility. (e) Examples of common salts in SPEs and anions anchored to polymer backbones. (f) SPEs with high σ_ionic, t₊ ∼ 1 and are theorised to suppress detrimental Li dendrite growth, which currently limits Li-batteries.

Fig. 3. Polymer electrolyte chemistries. Properties of different families: (a) total ionic conductivities (σ_ionic) measured by electrochemical impedance spectroscopy at 25–30 °C. (b) Mechanical shear-storage-moduli (G′) from rheological measurements. (c) Li-transference number (t_Li⁺) or effective Li-ion selectivity with values sometimes differing depending on whether the Bruce–Vincent or NMR methods were used. (d) Electrochemical stability window (ESW) where operating voltages for various electrodes are indicated. (e) Electrolytes in (a–d) are categorized according to type: polyethers (grey), polycarbonates (red), fluorinated (orange), polynitriles (dark blue), polyesters (green) and triblock copolymers (blue); single Li-ion conductors SLIC (purple) include Li borates and sulfonyl imides; inorganic solid electrolytes (ISEs) in yellow include sulfide Li₆PS₅Cl and oxides Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). (f) Controlled polymerisation strategies to synthesise polycarbonates. (g) Theoretical specific capacities for today's leading cathode materials (vs. graphite anodes) and anodes (vs. LiFePO₄ cathode). NB. Polymer chain ends are specified as –OH terminated but for PEO and PEC different methoxy- and acetate groups have been shown to influence (a)–(d). Ranges represented in (a) to (d) reflect dependence on salt, M_n, etc. See also Table 1.

Fig. 4. Advanced polymer electrolyte design approaches. (a) Examples of covalent bonds that can be broken and reformed under different heat (Δ) or light (hν) stimuli. Toughened SPEs from covalently linked networks can be rendered processable again via these dynamic chemistries. (b) Enthalpically-driven ring-opening polymerisation of cyclic monomers conducted in situ in the battery cell. (c) Machine learning models applied to SPE datasets to assist in materials discovery and the identification of missing structure–property correlations.

Fig. 5. Roles of polymers in solid-state batteries with inorganic solid electrolytes (ISEs). From the left: solid-state cell with thick composite cathode comprised of cathode active material (CAM), ISE, carbon additive and polymer binder in ∼70 : 23 : 2 : 5 wt% ratio (1), thin-film ISE layer (2) and Li or Li-rich metal foil anode (3). Polymers play roles as functional binders and coatings, protective interlayers and interphases and active electrode material. SEI, solid electrolyte interphase. Elastic polymers are required to buffer electrode volume changes and intrinsic ionic and/or electron polymer conductivities to facilitate ion/electron pathways. Adhesive/cohesive polymer properties are necessary to prevent delamination failure mechanisms that shorten battery lifetime and polymer processability, which inform cell fabrication methods and, in turn, performances.

Fig. 6. Examples of multifunctional polymer components in battery electrodes. (a) Ionic, adhesion and elastomeric binders employed in solid-state composite cathodes combining NMC cathode active material, inorganic Li₆PS₅Cl solid electrolyte and C. Adhesive handles are highlighted in green. (b) Ion (σ_ionic) and/or electron (σ_e) conductivities of conjugated polymer binders with their merit in battery performances. Blue, cell cycling with solid electrolyte (SPE or ISE); red, cell cycling with commercial liquid electrolytes; all cells use Li metal anodes. Thiophene repeat units are shaded in purple and EO in grey – these are combined with LiTFSI for ion conductivity.

Fig. 7. Polymer considerations for wearable technologies. (a) Bendable, conformable batteries to power smartwatches; (b) soft, skin-interfaced devices. (c) End-of-life of battery components; depolymerisation and/or (bio)degradation of polymeric materials.