Dead lithium in metal lithium batteries and solution strategies (II): Solution strategies for dead lithium

At present, the commercial application of lithium metal batteries still faces many challenges: ① During the battery cycling process, it is easy to form lithium dendrites, which brings the risk of battery short circuit; ② The lithium negative electrode reacts with the electrolyte to form an unstable solid electrolyte interface (SEI) film; ③ The serious volume effect during the charging and discharging process causes damage to the integrity of the electrode structure; ④ Continuously accumulating dead lithium due to electrochemical deactivation leads to rapid battery failure.
Researchers have conducted extensive research on these issues and made a series of progress. Through the design of three-dimensional lithium friendly frameworks, preparation of alloy negative electrodes, development of solid electrolytes, and regulation of SEI film structure and composition, the generation of dendrites has been effectively suppressed, and stable SEI films have been constructed to reduce the production and accumulation of dead lithium to a certain extent. However, in-depth research has found that the generation and accumulation of dead lithium on the surface are difficult to completely eliminate, and dead lithium remains a key challenge that limits the performance and lifespan of metal lithium batteries.
To effectively cure the problem of dead lithium, one must first have a certain understanding of the structural composition, physicochemical properties, and formation mechanism of dead lithium. However, dead lithium is an unstable lithium containing substance that is difficult to deeply analyze through general characterization techniques. Without basic understanding, it is also difficult to propose targeted strategies to solve the problem of dead lithium. Based on the recent achievements of the author and collaborators in the research of dead lithium, this article will first clarify the structural composition of dead lithium and its micro mechanism of formation; Subsequently, the application of some key new characterization techniques in the study of dead lithium will be introduced and evaluated, with a focus on exploring effective strategies to solve the problem of dead lithium, including multifunctional skeleton stabilized phases and interfaces, artificial interface protection layers, dead lithium activation, and solid-state electrolyte engineering; Finally, prospects are proposed for solving the problem of dynamic dead lithium and promoting the practical application of metal lithium batteries.

3 Dead Lithium Solution Strategies
With the assistance of advanced characterization techniques, researchers have gained a basic understanding of dead lithium. The formation of dead lithium is closely related to the deposition and dissolution behavior of lithium, as well as the properties of the SEI film. Therefore, there are two different considerations for solving the problem of dead lithium: ① strategies for suppressing dead lithium. By regulating the deposition morphology of lithium through lithium friendly frameworks and pressure fields, the growth of high curvature lithium dendrites can be inhibited, or a stable surface passivation layer can be constructed to suppress the corrosion of lithium by the electrolyte and promote uniform lithium removal Strategy for the conversion and utilization of dead lithium. By using specific substances to convert, re store, and reuse dead lithium, or by building an exogenous conductive network in situ within the dead lithium, the electronic pathway between the dead lithium and the electrode can be re established, allowing the dead lithium to be reused. Based on this, researchers have developed a series of strategies to attempt to solve the problem of dead lithium.
3.1 Functional skeleton stabilizes negative electrode phase structure and interface
Lithium still grows dendrites in conductive frameworks (such as pure carbon), forming dead lithium. Functionalized framework design can further enhance its stabilizing effect on lithium negative electrodes, such as lithium friendly frameworks. The lithium friendly skeleton is composed of a skeleton and a lithium friendly material. The former can reduce local current density, delay the formation of dendrites, and alleviate the volume effect during battery cycling. The latter can regulate the deposition behavior of lithium, further induce uniform deposition of lithium, and reduce the generation of dendrites and dead lithium.
According to the different types and functions of lithium friendly materials, they can be further divided into: ① Lithium storage active sites [Figure 6 (a)], which are materials that are easy to form alloys with lithium (such as zinc, silver, silicon, tin and their compounds). The formed alloys act as "buffer layers" to reduce the lattice mismatch between heterogeneous materials, thereby reducing the nucleation overpotential of lithium on lithium friendly seeds. Lithium friendly seeds evenly distributed in the skeleton can ensure the orderly deposition of lithium. In addition, when magnesium, aluminum, calcium, lanthanum and their compounds are used as lithium friendly seeds, concave "steps" will be formed between them and the skeleton. The critical nucleation volume and nucleation work of lithium in these areas are relatively small, which can effectively improve the formation of lithium. The nucleation ability of lithium [Figure 6 (b), (c)]. For example, by introducing magnesium oxide into carbonized natural wood, the prepared skeleton material can stably cycle at a super high current density of 15 mA/cm2, with an average Coulombic efficiency greater than 96% and no obvious dendrite formation [Figure 6 (d), (e)].

② Hybrid functional sites [Figure 6 (f)]. In addition to its lithium affinity, the skeleton can also be endowed with additional functions through physical and chemical treatments (doping, surface modification, composite), such as introducing fluorine elements into the carbon skeleton through fluorination treatment [Figure 6 (g)]. These fluorine elements can further participate in the formation of SEI films on the deposited lithium surface, constructing SEI films rich in lithium fluoride [77-79]. This type of functionalized skeleton design not only provides new ideas for the construction of fluorinated SEI films, but also further enriches the role of the skeleton in lithium negative electrodes. The use of these skeletons can effectively regulate the uniform deposition and extraction of lithium, thereby reducing the generation of dead lithium to a certain extent.
3.2 Artificial Interface Protection Layer
Recent studies have shown that the polymer components in the SEI film exhibit swelling behavior in the electrolyte. After swelling, the mechanical properties and ion conductivity of the SEI film will change, and as the electrolyte continues to solvate, the SEI film will gradually dissolve, causing exposure and corrosion of metallic lithium, and active lithium will transform into dead lithium. Therefore, compared to the native SEI film, introducing an artificial protective layer or constructing a highly stable SEI film through electrolyte optimization design is expected to suppress lithium corrosion and dead lithium formation, while ensuring the uniform transport of lithium ions.
By regulating the solvation structure of lithium ions, the structural composition of the SEI film can be effectively changed. For example, by introducing weakly solvated solvent molecules (such as DOL) to weaken their interaction with lithium ions, more anions can participate in the formation of the SEI film, which can construct a stable SEI film rich in inorganic substances, achieve uniform deposition of lithium, and suppress the occurrence of lithium corrosion [Figure 7 (a), (b)].
As for the artificial protective layer, the constructed protective layer should have low solubility, high ion conductivity, good adhesion and reduction stability, which can effectively isolate lithium and electrolyte. Polymer films synthesized through chemical processes (such as polyethylene oxide and polyvinylidene fluoride), inorganic passivation layers, natural polymer materials, and their composites are all potential protective layers. Natural biomass films with self-supporting structures, such as two-dimensional wood nanosheets, protein films, bamboo fiber films, etc., can promote the uniform deposition of lithium ions, construct stable SEI films, inhibit dendrite growth, and reduce the accumulation of dead lithium to a certain extent by utilizing the natural surface chemical properties and structure of these materials [Figure 7 (c) - (f)].

3.3 Dead Lithium Activation Strategy
The generation and accumulation of dead lithium are difficult to eliminate, so it is of great significance to develop effective strategies for processing the generated dead lithium. As mentioned earlier, dead lithium is composed of a failed SEI film shell and metal lithium fragments. Further research has also found that the SEI film contains a large amount of non-conductive nano lithium oxide, which blocks the electronic pathway between the metal lithium fragments and the electrode. To eliminate the ineffective SEI film shell, some reactive substances can be introduced. For example, using iodine mediator as a "dead lithium activator" can spontaneously react with metal lithium fragments in the "dead lithium" and Li2O in the SEI film during battery cycling, converting the dead lithium into soluble lithium iodide and lithium iodate [Figure 8 (a)]; As a "lithium carrier", lithium iodide can migrate to the positive electrode side under the action of concentration gradient and be oxidized by the charged positive electrode material, and the lithium ions are recovered and stored; The O18 isotope tracing experiment confirmed that lithium iodate, as an "oxygen carrier," can transport Li2O from dead lithium to the surface of active lithium and effectively passivate and protect it.

This strategy not only effectively solves the problem of "dead lithium" and achieves its recycling, but also reduces the gas production of the battery and suppresses dendrite growth, significantly improving the cycling stability of the battery. The preparation of iodine based sustained-release carbon capsules can be further applied in button full cell and 0.5Ah soft pack batteries [Figure 8 (b), (c)]. In addition to elemental iodine, metal iodide can also serve as a dead lithium activator and alleviate the corrosion of lithium by iodine mediators to some extent [Figure 8 (d), (e)]; In addition, the dead lithium activation strategy is also applicable in solid-state polymer metal lithium batteries, which can significantly improve battery life and cycle stability.
3.4 Dead lithium suppression in solid-state battery systems
In solid-state battery systems, there is also the problem of dead lithium. Whether it is a polymer electrolyte system or an inorganic solid electrolyte, due to the side reactions between lithium and the electrolyte, a large number of by-products with poor ion conductivity and good electronic conductivity will be generated at the interface, seriously deteriorating the stability and ion transport capacity of the interface, reducing the charging and discharging efficiency of the battery, and inducing the loss of active lithium. In addition, due to the abundance of grain boundaries and the inherent electronic conductivity of solid electrolytes, lithium ions will directly deposit at the grain boundaries, inducing dendrite growth, resulting in loss of active lithium and battery short circuits.
Optimizing electrolyte engineering can alleviate the generation and accumulation of dead lithium in solid-state battery systems: ① External conductive interface construction. Deposition of highly conductive and lithium conductive materials on the interface, such as platinum, gold, silver, etc., provides an exogenous conductive pathway for dead lithium [Figure 9 (a), (b)], allowing dead lithium to be partially reused Grain boundary modification. For example, introducing electronically insulating and ionically conductive lithium iodide into the grain boundaries of inorganic Li4SnS4 electrolyte, while stabilizing the electrolyte interface and bulk grain boundaries, effectively inhibits dendrite growth along the grain boundaries, reduces the generation of dead lithium, and can increase battery cycle life by 25 times [Figure 9 (c), (d)].