Conjugated polymers are attractive for energy storage but typically require significant amounts of conductive additives to successfully operate with thin electrodes. Here, side-chain engineering is used to improve the electrochemical performance of conjugated polymer electrodes. Naphthalene dicarboximide (NDI)-based conjugated polymers with ion-conducting ethylene glycol (EG) side chains (PNDI-T2EG) and non-ion-conducting 2-octyldodecyl side chains (PNDI-T2) are synthesized, tested, and compared. For thick (20 µm, 1.28 mg cm −2) electrodes with a 60 wt% polymer, the PNDI-T2EG electrodes exhibit 66% of the theoretical capacity at an ultrafast charge-discharge rate of 100C (72 s per cycle), while the PNDI-T2 electrodes exhibit only 23% of the theoretical capacity. Electrochemical impedance spectroscopy measurements on thin (5 µm, 0.32 mg cm −2), high-polymer-content (80 wt%) electrodes reveal that PNDI-T2EG exhibits much higher lithium-ion diffusivity (D Li+ = 7.01 × 10 −12 cm 2 s −1) than PNDI-T2 (D Li+ = 3.96 × 10 −12 cm 2 s −1). PNDI-T2EG outperforms most previously reported materials in thick, high-polymer-content electrodes in terms of rate performance. The results demonstrate that the rate performance and capacity are significantly improved through the incorporation of EG side chains, and this work demonstrates a route for increasing the rate of ion transport in conjugated polymers and improving the performance and capacity of conjugated-polymer-based electrodes.

We review molecular design principles for polymer binders for silicon anodes. Their impact on performance is complex and includes mechanical properties, adhesion, electrolyte uptake, ionic and electronic conductivity, and electrochemical stability.

Silicon anodes have a high theoretical capacity for lithium storage, but current composite electrode formulations are not sufficiently stable under long-term electrochemical cycling. The choice of polymeric binder has been shown to impact stability and capacity of silicon anodes for electrochemical energy storage. While several promising polymeric binders have been identified, there is a knowledge gap in how various physicochemical propertiesincluding adhesion, mechanical integrity, and ion diffusionimpact electrochemical stability and performance. In this work, we comprehensively investigate the physical properties and performance of a molecular-weight series (3−20 × 10 6 g/ mol) of partially hydrolyzed polyacrylamide (HPAM) in silicon anodes. We quantify the mechanical strength, electrolyte uptake, adhesion to silicon, copper, and carbon, as well as electrochemical performance and stability and find that HPAM satisfies many of the properties generally believed to be favorable, including good adhesion, high strength, and electrochemical stability. HPAM does not show any electrolyte uptake regardless of any molecular weight studied, and thin films of mid-and high-molecular-weight HPAM on silicon surfaces suppress lithiation of silicon. The resulting composite electrodes exhibit an electrochemical storage capacity greater than 3000 mAh/g initially and 1639 mAh/g after 100 cycles. We attribute capacity fade to failure of mechanical properties of the binder or an excess of the solid electrolyte interphase layer being formed at the Si surface. While the highest-molecular-weight sample was expected to perform the best given its stronger adhesion and bulk mechanical properties, we found that HPAM of moderate molecular weight performed the best. We attribute this to a trade-off in mechanical strength and uniformity of the resulting electrode. This work demonstrates promising performance of a low-cost polymer as a binder for Si anodes and provides insight into the physical and chemical properties that influence binder performance.

MXenes, 2D nanomaterials derived from ceramic MAX phases, have drawn considerable interest in a wide variety of fields including energy storage, catalysis, and sensing. There are many possible MXene compositions due to the chemical and structural diversity of parent MAX phases, which can bear different possible metal atoms “M”, number of layers, and carbon or nitrogen “X” constituents. Despite the potential variety in MXene types, the bulk of MXene research focuses upon the first MXene discovered, Ti3C2T. With the recent discovery of polymer/MXene multilayer assemblies as thin films and coatings, there is a need to broaden the accessible types of multilayers by including MXenes other than Ti3C2T z ; however, it is not clear how altering the MXene type influences the resulting multilayer growth and properties. Here, we report on the first use of MXenes other than Ti3C2T z , specifically Ti2CT z and Nb2CT z , for the layer-by-layer (LbL) assembly of polycation/MXene multilayers. By comparing these MXenes, we evaluate both how changing M (Ti vs Nb) and “n” (Ti3C2T zvs Ti2CT z ) affect the growth and properties of the resulting multilayer. Specifically, the aqueous LbL assembly of each MXene with poly­(diallyldimethylammonium) into films and coatings is examined. Further, we compare the oxidative stability, optoelectronic properties (refractive index, absorption coefficient, optical conductivity, and direct and indirect optical band gaps), and the radio frequency heating response of each multilayer. We observe that MXene multilayers with higher “n” are more electrically conductive and oxidatively stable. We also demonstrate that Nb2CT z containing films have lower optical band gaps and refractive indices at the cost of lower electrical conductivities as compared to their Ti2CT z counterparts. Our work demonstrates that the properties of MXene/polycation multilayers are highly dependent on the choice of constituent MXene and that the MXene type can be altered to suit specific applications.

Silicon anodes are promising for high energy batteries because of their excellent theoretical gravimetric capacity (3579 mAh/g). However, silicon’s large volume expansion and poor conductivity hinder its practical application; thus,...