The physicochemical properties at the metal/electrolyte interface, such as ion transport, interfacial energy, and mechanical factors determine the shape and structure of electrodeposited metals. This is particularly true for the electrochemical deposition of metallic lithium (Li), which is considered the ultimate anode for next-generation batteries with high energy densities due to its highest specific capacity (3860 mAh g⁻¹) and the lowest electrode potential among all possible alternative anodes [1], [2]. The electroplating of Li is especially challenging because of the immediate formation of a resistive interfacial passivation layer, known as the SEI [3], [4]. This layer forms due to the parasitic reduction of electrolyte components by the highly reactive Li [5]. The chemically heterogeneous SEI induces uneven Li+ flux and dendritic growth, risking internal short circuits and thermal hazards [6]. Additionally, the repeated breakdown and repair of the SEI result in ongoing loss of active materials, thus limiting the cycle life of the battery.
Extensive research has been dedicated to managing the surface reactivity of lithium metal [7], [8], [9]. Among the strategies explored—such as protective coatings, current collector engineering, separator modification, and electrolyte modulation, adjusting the electrolyte composition is particularly crucial and effective [10], [11], [12], [13], [14]. This approach directly affects the physicochemical properties of the SEI layer, thereby modifying the interfacial environment and influencing lithium deposition behavior. Functional metallic salt additives like InCl3 and Mg(NO3)2 in electrolytes offer significant advantages in suppressing dendrites [15], [16], [17]. They can form a rough protective layer by rapid reduction/alloying. However, the cycling performance of the anode can still be compromised under harsh conditions (e.g., lean electrolyte/high depth of discharge) due to the insufficient durability of the necessary concentration of additives and the rough interface (Fig. 1a).
Inspired by sustained drug-delivery systems that maintain stable concentrations for curing diseases (Fig. 1b), several carrier systems, such as gel capsules and metal-organic framework encapsulation, have been developed and incorporated into electrolytes to release nitrate additives gradually [18], [19], [20], [21]. This approach effectively extends the durability of lithium metal. However, nanocapsules in LMBs encounter challenges related to solubility, stability, weight, cost, and long-term structural integrity. Moreover, it should be noted that the release efficiency in these systems was mainly controlled by the pore size and tortuosity, typically enabling rapid release but making it difficult to ensure controlled release. These issues impact their efficiency and practicality for sustained SEI maintenance in high-energy applications. Additionally, it is crucial to consider that the safety of high-energy batteries using such electrolytes may be significantly compromised due to the potential reactivity of oxidative anions like nitrate.
In this study, we present a method for the controllable release of SbOCl nano-additives from ultrathin cellulosic paper separators (5 μm) to achieve both smooth lithium electroplating and enhanced safety. Cellulose was selected due to its excellent electrolyte affinity, thermal stability, chemical stability, and sustainability [22]. Due to the swelling of cellulose fibers, trace amounts of SbOCl are initially rapidly released into the electrolyte based on Ritger-Peppas kinetics, reacting with lithium metal to form stable Li3Sb clusters. Subsequently, SbOCl within the cellulose fiber exhibits sustained first-order controlled release, forming long-lasting Li3Sb replenishment, thereby enhancing battery life and stability. (Fig. 1c) [23], [24]. This suppresses dendrite growth and enhances the durability of thin lithium metal anodes under harsh conditions. The flame-retardant property of SbOCl, which improves the limiting oxygen index (LOI) from 18.4% to 39.27%, combined with the thermal stability of cellulose fiber, significantly enhance the safety of LMBs. In the meanwhile, this solution-based manufacturing process resembles the industrial preparation of commercial separators, showing a high availability for commercial production. As proof of concept, symmetric cells with thin lithium metal foil (50 μm) using this separator demonstrate stable operation for over 6000 h. Full cells with NCM22 cathodes, limited lithium metal, and lean electrolyte achieved stable cycling for over 250 cycles, exhibiting an impressive energy density of 368.65 Wh kg−1 (904.90 Wh L−1), surpassing that of the batteries in the previous reports and commercialized prototypes (Table S1). Considering the excellent fire retardancy and low thermal conductivity, Ah-level pouch cells with our separator maintain safety even under nail penetration tests.