REVIEWS Designing polymers for advanced battery chemistries Jeffrey Lopez 1, David G. Mackanic 1, Yi Cui 2,3 and Zhenan Bao 1* Abstract | Electrochemical energy storage devices are becoming increasingly important to our global society , and polymer materials are key components of these devices. As the demand for high- energy density devices increases, innovative new materials that build on the fundamental understanding of physical phenomena and structure–property relationships will be required to enable high-capacity next-generation battery chemistries. In this Review , we discuss core polymer science principles that are used to facilitate progress in battery materials development. Specifically , we discuss the design of polymeric materials for desired mechanical properties, increased ionic and electronic conductivity and specific chemical interactions. We also discuss how polymer materials have been designed to create stable artificial interfaces and improve battery safety. The focus is on these design principles applied to advanced silicon, lithium-metal and sulfur battery chemistries. The development of new electrochemical energy storage of existing materials or to enable the application of new technologies has been instrumental to the proliferation battery chemistries. In this Review, we summarize the of portable electronic devices and the increasing adop- fundamental polymer science and engineering con- tion of electric vehicles. Intermittent electricity genera- cepts related to the development of polymers for next- tion (for example, from wind and solar power sources) generation battery applications (Table 1). We specifically has further intensified the demand for high-energy den- discuss how these core concepts drive the design of new sity, high- power and low-cost energy storage devices1,2. polymer materials for advanced battery chemistries, Polymer materials are ubiquitous in these energy stor- including Si, Li-metal and S electrodes. age devices and are commonly used as binders, elec- trolytes, separators and package coatings to provide Advanced battery chemistries structural support, adhesion and mechanical stability Si anodes. Si has a high theoretical specific capacity of (Fig. 1; Table 1) −1 to the devices . Separators with pore sizes 3,579 mAh g for Li3.6Si and has the potential to replace of ~30–100 nm are commonly made from uniaxially graphite (372 mAh g−1) as the negative- electrode active stretched polyethylene or polypropylene and serve to material in Li- ion batteries5,6. Although Panasonic has electrically isolate the two electrodes while providing already begun to incorporate Si into their graphite ionic conduction pathways through the liquid electro- anodes7,8, the large volume expansion (~300%) of Si dur- lyte that fills the pores3 (Fig. 1c). Polymers are also used ing lithiation has prevented its utilization as a majority as binders to increase the cohesion of particles in the component of the active materials6,9,10. As Si expands, composite electrodes and their adhesion to the metal large anisotropic stresses build up that cause the particles 1Department of Chemical current collector4 (Fig. 1b). The binder also increases the to fracture and the capacity to quickly fade9,11,12 (Fig. 2a). Engineering, Stanford University, Stanford, CA, USA. viscosity of the electrode slurry to ensure high- quality The failure of Si electrodes results from two mechanisms. 2Department of Materials (that is, uniform and smooth) coating of the slurry onto First, cracks in the Si particles provide fresh surfaces for Science and Engineering, the current collector. The scale at which batteries are electrolyte decomposition reactions and solid electrolyte Stanford University, Stanford, being manufactured is growing quickly, and thus the interphase (SEI) formation13,14, and second, portions of CA, USA. cost and processability of devices are key considerations. Si can also become electrically disconnected from the 3Stanford Institute for For commercial application in energy storage devices, rest of the electrode, which prevents further cycling15. Si Materials and Energy new polymer materials should ideally be easy to syn- nanoparticles circumvent these issues because fracture Sciences, SLAC National Accelerator Laboratory, thesize from inexpensive reagents and processable in does not occur below a critical particle size of 150 nm 12 Menlo Park, CA, USA. environmentally friendly and non-toxic solvents. (REF. ), and many successful materials have been devel- 16–18 *e- mail: [email protected] As the demand for higher performance batteries oped based on this approach . However, nanosized https://doi.org/10.1038/ grows, researchers are turning to new polymers with Si can be 1–2 orders of magnitude more expensive s41578-019-0103-6 advanced functionalities to help improve the operation than larger micrometre- sized particles. Unfortunately, 312 | MAY 2019 | VOLUME 4 www.nature.com/natrevmats REVIEWS abCylindrical Pouch Composite electrode S cathodes. For the positive electrode, Li- metal oxides (LiMxO, M = Co, Ni or Mn) or phosphates (for example, LiFePO4) are currently used, but the capacity of these materials is limited7. Sulfur offers a promising alternative −1 with a high capacity of ~1,700 mAh g for Li2S, which is approximately seven times higher than the capacity Anode of the metal oxides (~250 mAh g−1)41. Unfortunately, ··· Separator Repeated stacks the intermediate lithium polysulfide species that form Cathode Active Polymer as S is reduced are soluble in the liquid organic electro- material binder lytes42 (Fig. 2c). These soluble species eventually migrate c Separator Conductive to and deposit on the anode. This self-discharge reduces carbon the capacity of the cell but also contributes to the build- up of an insulating layer on top of the anode that limits cycling stability. Moreover, the S electrodes have poor electronic conductivity and thus require large amounts of conductive carbon to function, which reduces the 43 1 µm energy density . Efforts to prevent polysulfide disso- lution have focused on the development of functional cathodes that can chemically bind the polysulfide spe- | | Fig. 1 Polymers in commercial Li- ion batteries. a A cylindrical cell and pouch cell with cies. Nitrogen- doped carbon materials have been some rolled and stacked configurations, respectively. b | Illustration of a composite electrode | of the most successful in this regard because of their high containing a polymer binder, active material and conductive additive. c Scanning 42 electron microscopy image of a porous polymer separator. Panel c is adapted with electronic conductivity and polysulfide-binding ability . permission from REF.3, American Chemical Society. Additionally, nanostructuring of the cathode can help to prevent polysulfide dissolution by physically trapping the species44. We discuss below polymer materials that serious particle fracture and rapid failure prevent the are used to increase the conductivity of the composite effective use of micrometre- sized Si (REFS12,19,20). To suc- electrode or to trap polysulfide species. cessfully enable the use of larger Si particles or to further Overall, new battery chemistries offer promising improve the performance of nanosized Si, researchers paths towards high-performance energy storage (Fig. 2d) have turned to encapsulating the Si particles in protec- for improved sustainability, and there is a significant tive carbon shells21 or modifying the polymer binder opportunity for innovation in polymer science and used in the composite electrode22–24. The polymer-based engineering to help solve longstanding problems and approach is discussed further below from the perspec- enable the development of these devices. This Review tive of mechanical properties, electronic conductivity serves as an introduction to the fundamental materi- and binding interactions. als requirements for advanced battery chemistries and the central concepts related to polymer design for these Li- metal anodes. Beyond Si, Li metal offers the high- applications. Owing to the breadth of topics covered, we est theoretical energy density owing to its low potential focus only on central works that are directly related to (−3.04 V versus the standard hydrogen electron) and the design of polymers for advanced battery chemistries. high capacity (3,860 mAh g−1)25, but successful applica- tion has eluded researchers for more than four decades Designing binder and separator mechanics owing to poor Coulombic efficiency and safety issues7. Polymer mechanics. Perhaps some of the most familiar A key challenge for ensuring safe and efficient Li- metal aspects of polymer materials are their mechanical prop- electrodes is the formation of a stable, uniform SEI layer erties. The main method used to analyse the mechanical on the Li surface that can withstand the large volumet- properties of a polymer is the stress–strain curve (Fig. 3a). ric change during cycling26–28. Local variations in the As strain is applied to a polymer, the stress increases line- composition of the SEI layer can lead to non- uniform arly at first, but as the strain continues to increase, the mate- deposition of Li owing to changes in Li- ion conductiv- rial may continue to stretch elastically (no slope change), ity across the electrode or breakage of the SEI. These begin to yield (a reduction in slope) or break. Materials SEI defects facilitate the growth of high- surface-area that yield undergo irreversible plastic deformation, Li dendrites that increase electrolyte