## 1 Introduction  
Printed flexible electrodes have attracted great attention in a variety of newly emerging areas such as printable transistors, flexible displays, electrodes, sensors, antennas, radio-frequency identification tags, solar cells, etc. [1]. Currently, inkjet [2], screen [3], gravure [4] and aerosol printing [5] techniques can be used to print desired patterns on flexible substrates, such as polyimide, polyethylene terephthalate and polycarbonate. During printing, conductive inks are essential, associating with the feasibility of electrode deposition, bonding at electrode-substrate interface, offering conductivity, etc., which usually contain highly conductive materials, such as silver, gold, carbon nanotubes or graphene, binders, solvents and other fillers [6], to adjust their physicochemical properties. Among them, the used binders are typically polymers, such as acrylics [7], alkylds [8], cellulose [9] and rubber resins [10]. Although these binders can connect conductive filler elements to each other and offer good formability during printing, they also bring insufficient electrical conductivity due to their non-conductivity. Further, the presence of such binders complicates the curing and sintering processes because longer time or higher temperature is needed to remove or decompose them [11]. In traditional thermal sintering, a high temperature is needed to decompose this organic content and promote atomic interdiffusion to form a metallic bond between metallic fillers [12]. However, flexible substrates usually cannot withstand such high temperature [13]. Therefore, sintering temperature is particularly important when heat sensitive substrates are involved, making low-temperature sintering essential to avoid mechanical or thermal degradation of the substrate [14]. This requires reduced organic content in conductive inks. Besides that, the two prevalent droplet jetting mechanisms for inkjet printing, continuous inkjet and drop-on-demand inkjet, require low viscosity inks, which are beneficial to stable jetting and suppression of generation of satellite droplets [15]. The addition of binders will significantly increase the viscosity of inks [11]. In recent years, there has been increasing interests given to binder-free conductive inks. [PERSON] et al. [16] demonstrated a binder-free, stable black phosphorus ink formulation using a mixture of IPA/2-butanol solvents. The low surface tension ensured wetting of a variety of untreated substrates, enabling good consistency and spatially uniform. Printable radio frequency identification antenna with ultra-low resistance was achieved by binder-free graphene laminate [17]. The conductivity of graphene laminate is improved by more than 50 times with rolling compression, reaching 4.3 \(\times\) 10\({}^{4}\) S/m, almost double the 2.5 \(\times\) 10\({}^{4}\) S/m of previously reported graphene laminate with binder. Referring to electronic industry, silver inks with heavy binder content are widely used. To increase the electrical performance and mechanical reliability of printed electrodes, adding the second phase of filler materials for inkjet printing was reported. For example, [PERSON] et al. [18] proposed that adding cellulose nanocrystals into silver ink can improve electrical conductivity and mechanical performance of printed films, providing a new solution to obtain conductive patterns on a porous substrate with a low sheet resistance of 50 \(\Omega\)/sq. [PERSON] et al. [19] demonstrated a composite copper nanoparticle based ink with 5 wt.% copper nanowires filler can reach a resistivity of 2.3 \(\times\) 10\({}^{-7}\)\(\Omega\)\(\cdot\)m for electrodes compared with 9.4 \(\times\) 10\({}^{-7}\)\(\Omega\)\(\cdot\)m for those printed by pure copper nanoparticle inks. However, if binders are removed, the silver particles have difficulty forming a continuous track without linkages by binders, leading to cracking of printed structure [20], large interface resistance [17] and less mechanical reliability [21].