High energy-density lithium metal batteries (LMBs), which are expected as one of the most promising next-generation battery technologies, are designed with high-capacity lithium metal anode (3860 mAh g− 1) and cathode materials [1]. They also contain volatile and flammable organic electrolytes (such as carbonates and ethers) [2], which are not favorable to inhibit the lithium dendrites growth to prevent the potential safety hazards [3]. Therefore, developing solid-state electrolytes (SSEs) that are not only ionic conductive but also nonvolatile, nonflammable, and mechanically robust is crucial to fundamentally improve the safety characteristics of LMBs. Until now, several different types of SSEs have been developed, including inorganic solid electrolytes (ISEs) [4], solid polymer electrolytes (SPEs) [5] and organic-inorganic composite electrolytes [6–10]. Among the various SSEs, SPEs that are composed of polymer matrix and lithium salts exhibit high lithium ions (Li+) conductivity, high flexibility, facile processability, and excellent electrode/electrolyte interfacial compatibility. These advantageous properties are favorable for building high-performance and high-safety LMBs [11–13]. In this regard, SPEs are a research hot topic that is being intensively purposed in the battery community.
From the perspective of the ion conductive mechanisms, it has been generally acknowledged that it is the consecutive polar groups (e.g. -O-, =O, -S-, -N-, -P-, -C = O and -C = N, etc.) [14–16] of the SPE that coordinates with the Li+, resulting in the dissociation of the Li salt and generation for mobile charge carriers. Typically, the SPEs initiate the amorphous phase at temperatures above the glass transition temperature (Tg), resulting in the continuous movement of the chain segments to obtain a large free volume, which gives the ability of Li+ conductivity [17–19]. Hence, polymers with polar groups and low Tg are receiving increasing attention for building ideal SPEs. On the other hand, thermal stability and mechanical strength are also crucial properties of SPEs for building safety-improved SPE-based LMBs. Conventional strategies that employ plasticizers (such as ionic liquids, acetonitrile, succinonitrile, etc.) to increase the ionic conductivity of SPEs may also reduce their flame retardancy and mechanical strength [20–24]. Additionally, the addition of inorganic or halogen-free organic flame retardants in SPEs [25, 26] may lead to the deterioration of their mechanical property and the unwanted increase of the interfacial impedance of SPEs [27–29]. Despite the using of some flame retardant polymers containing P, N, and Si elements (such as triethyl phosphate, polyimide, siloxane, etc.) in SPEs can guarantee their mechanical and Li+ conductive properties, their preparation procedures are often complex, toxic and costly [19, 30–32]. Therefore, improving both the thermal stability and mechanical strength of SPEs, besides their ionic conductivities, is critical for their practical applications in LMBs.
Recently, the application of natural bio-based materials as flame retardants has received increasing attention in the battery field [33–36]. Among them, alginate fibers (AF) with high mechanical strength and intrinsic flame retardancy are being intensively studied [33, 37]. This unique feature of AF is ascribed to the abundant carboxyl groups and the chelation between the polysaccharide and polyvalent metal ions. The limit oxygen index (LOI) of AF is about 45%, which is superior to that of organic phosphate and halogenated flame retardants. Nevertheless, the lack of Li+ transport capability makes AF unsuitable to be directly used as a polymer matrix for SPEs. The previous report demonstrates that direct casting of poly(ethylene oxide)/lithium bis(trifluoromethane sulfonimide) (PEO/LiTFSI) onto the AF backbone to form a cross-linked structure can exhibit decent Li+ transport property, stills its intrinsic Li+ conductivity is not changed [38]. Considering that AF contains abundant functional groups (-OH, -COOH, C-O-C), which offers the possibility of introducing high Li+ conductive chain segments to improve the Li+ transport of AF [39]. That initiates us to judiciously design side chain modified AF-based SPEs with excellent flame retardancy and high Li+ conductivity to solve the thermal stability and improve the mechanical strength of traditional polymer-based SPEs in LMBs.
In this work, the AF grafted polyetheramines (PEA) [40, 41] with different molecular weights (AF-PEA) were synthesized as the high ion conductive and flame-retardant porous backbone to prepare the SPEs (called PEO@AF-PEA) via casting PEO/LiTFSI. The three-dimensional (3D) structure of the separators provided SPEs with excellent mechanical properties, flame retardancy and thermal stability, further enhancing the safety of the electrolyte. The grafted PEA acts as a Li+ transport chain segment providing more anchor points for Li+ rapid transport. The excellent ionic conductivity (σ = 6.7×10− 4 S cm− 1) and high ion mobility (tLi⁺ = 0.58) of PEO@AF-PEA SPEs enable the assembled LMBs (the lithium iron phosphate as the cathode) to achieve high energy storage performance, which the specific capacity of the cathode can maintain 103.5 mAh g− 1 after 1500 cycles at 2 C current density (at 80°C) with less than 0.016% capacity loss for each cycle. Through the simulation of Li+ distribution in AF-PEA and migration rate at constant voltage, the high adsorption ability brought by grafted PEA chain segments is beneficial to the dispersion of Li+ between fibres. The PEA chain length affects the adsorption role of the alginate backbone and the grafted PEA side chains, in which the medium length PEA chains are more uniformly distributed in the alginate space, allowing Li+ transport between alginate to show a faster trend. This study opens new opportunities for the development of bio-based solid polymer electrolytes.