Revealing the importance of suppressing formation of lithium hydride and hydrogen in Li anode protection

The reviving of the “Holy Grail” lithium metal batteries (LMBs) is greatly hindered by severe parasitic reactions between Li anode and electrolytes. Herein, first, we comprehensively summarize the failure mechanisms and protection principles of the Li anode. Wherein, despite being in dispute, the formation of lithium hydride (LiH) is demonstrated to be one of the most critical factors for Li anode pulverization. Secondly, we trace the research history of LiH at electrodes of lithium batteries. In LMBs, LiH formation is suggested to be greatly associated with the generation of H2 from Li/electrolyte intrinsic parasitic reactions, and these intrinsic reactions are still not fully understood. Finally, density functional theory calculations reveal that H2 adsorption ability of representative Li anode protective species (such as LiF, Li3N, BN, Li2O, and graphene) is much higher than that of Li and LiH. Therefore, as an important supplement of well‐known lithiophilicity theory/high interfacial energy theory and three key principles (mechanical stability, uniform ion transport, and chemical passivation), we propose that constructing an artificial solid electrolyte interphase layer enriched of components with much higher H2 adsorption ability than Li will serve as an effective principle for Li anode protection. In summary, suppressing formation of LiH and H2 will be very important for cycle life enhancement of practical LMBs.


| INTRODUCTION OF LITHIUM METAL BATTERIES (LMBS)
The increasing fossil energy crisis stimulates demands for using renewable energy, such as solar, wind, and tidal power, and so on. Because of the intermittent nature of renewable energy, development of stationary energy storage systems has become more and more important. [1] As one of the most attractive electrochemical energy storage systems, rechargeable lithium-ion batteries (LIBs) have enabled revolutionary advancements in portable electronics, electric vehicles, and smart grids. [2,3] However, the energy density of conventional LIBs is nearly approaching their limits. To fulfill the ever-increasing demands for high energy density battery devices, conventional graphite anode (372 mAh g −1 ) of LIBs is proposed to be replaced by "old" anode of lithium (Li) metal (3860 mAh g −1 ) to rejuvenate "Holy Grail" lithium metal batteries (LMBs) with theoretical energy density exceeding 500 Wh kg −1 . [4][5][6][7][8] Actually, before the birth of LIBs in 1990s, the "Holy Grail" LMBs had already experienced numerous upheavals and downfalls since their birth in the 1970s. [9] Since 2010, despite great efforts having been devoted again ( Figure 1A), large-scale commercial applications of LMBs are still full of difficulties. [10,[12][13][14] The fact is that despite Li anode provides higher energy density compared to graphite anode, severe failure of Li anode becomes the "bottleneck" in LMBs. Specifically, in practical LMBs coexisting of lean electrolyte and thin Li anode, full depletion of either Li ( Figure 1B) or electrolyte ( Figure 1C), whichever comes first, quickly terminates cell operation. [10] As a result, most of practical large-format high areal capacity LMBs (cathode areal capacity ≥2 mAh cm −2 , Li metal anode ≤50 μm, and lean electrolyte ≤5 g Ah −1 ) still always suffer from sudden capacity failure within 100 cycles. [10,12] When disassembled from 300 Wh kg −1 NMC/Li pouch cells after 50 cycles, the cycled Li anode becomes already dark rough, severely expanded and pulverized ( Figure 1D-E). [11,15] Therefore, understanding and suppressing the intrinsic parasitic reactions between the Li anode and electrolyte is crucial to achieve long-cycle life LMBs under practical conditions.

| FAILURE MECHANISMS AND PROTECTION PRINCIPLES OF LI ANODE
As an alkali metal with low electronegativity, Li anode readily reacts with electrolyte chemically and electrochemically, leading to continuous excessive formation of solid electrolyte interphase (SEI) layer, Li dendrites and "dead Li" (Figure 2A). [3,13,18] Presently, as for Li anode protection, it is widely accepted that constructing an effective SEI layer is effective in suppressing Li dendrites growth and alleviating expansion/pulverization/cracking of Li anode ( Figure 2B,C). The representative strategies include liquid electrolyte engineering, [7,13,19] Li metal hosts fabricating [14] and artificial SEI layer constructing, [3] and so on. In general, there are three commonly used key principles of designing both natural SEI layers and artificial SEI layers for Li anode protection [3] : first, SEI layers must be mechanically stable and robust to prevent Li anode expansion/pulverization/cracking; second, SEI layers with fast/uniform Li + transport and high Li + transference number is highly required to suppress Li dendrites growth; third, favorable SEI layers should be able to passivate Li anode surface, minimizing parasitic reactions between Li anode and electrolytes. Successful SEI layers on the Li anode surface are always composed of rigid inorganic species and flexible organic polymer species. Presently, the representative identified favorable species for Li anode protection includes LiF, [20,21] Li 3 N, [22][23][24] BN, [25,26] carbon nanomaterials, [27][28][29][30][31] carbon nitride, [32][33][34] Li 2 O, [35] and so on. There are mainly two classic theories to effectively guide the selection of Li protective species and explain their working mechanisms in suppressing Li dendrites growth and protecting Li anode: lithiophilicity chemistry theory [16,28,[30][31][32][33][34][35][36][37][38] and high interfacial-energy theory. [17,29,39] The lithiophilicity of protective species is usually evaluated by theoretically calculating their binding energy ( Figure 2D) [16,28,[30][31][32][33][34][35]37,38] or adsorption energy [37] to Li atoms. The SEI layer enriched by lithiophilic species with high binding energy and adsorption energy to Li atoms demonstrates a strong interaction with Li metal, promoting homogeneous Li + distribution and Li metal nucleation. [16,23,30,31,35,38] On the other hand, high interfacial energy theory (also called lithiophobic chemistry theory) is also used ( Figure 2E). [17,29,39] A SEI layer (especially an artificial coating layer) with a large interfacial energy (to Li metal) and a high mechanical strength can improve the mobility of Li atoms along the interphase and suppress Li dendrites growth into SEI layer, consequently promoting the lateral growth of deposited Li metal. [17,29] Nonetheless, these two theories lack universality and possess limitations: the protected Li anode by lithiophilic species or high interfacial energy species still always suffer from severe pulverization in practical LMBs with limited Li anode and electrolyte. Therefore, both lithiophilicity theory and high interfacial energy theory exhibit limited guidance in suppressing intrinsic parasitic reactions between Li anode and electrolyte in practical LMBs.

| WHY LITHIUM HYDRIDE (LiH) AND ITS RESEARCH HISTORY
The underpinning mechanisms on intrinsic parasitic reactions (pathways, rate, area, potential, etc.) between the Li anode and electrolytes leading to the Li anode Reproduced with permission. [11] Copyright 2019, Springer Nature Limited. failure and electrolyte depletion are still undiscovered. In lithium batteries, large amounts of H 2 evolution have been reported, which can be very important for unveiling intrinsic parasitic reactions between Li anode and electrolytes. However, its generation paths and roles are seldom concerned. [40][41][42][43] One source of H 2 evolution is H 2 O reduction, however, the contribution of H 2 O to H 2 evolution is limited because H 2 O content in commercial electrolytes is always extremely controlled below 20 ppm. Metzger et al. proposed that migration of protic electrolyte oxidation species (R-H + ) from cathode to anode and their subsequent reduction is the origin of enhanced H 2 gassing. [40] Early in 1999, the pioneering work conducted by Aurbach et al. showed preliminary evidence that fresh Li can react with H 2 to form LiH in wet electrolyte solutions at room temperature. [44] Inspired by this conclusion, recently, we have demonstrated that, in a practical LMB (50 μm Li anode and 2.805 mAh cm −2 LiCoO 2 cathode), the regular, continuous and large amounts of H 2 evolution and in subsequent gradual LiH accumulation serve as main origins for volume expansion and pulverization of Li anode. [41] Thus, it is deduced that the external driving reason for Li anode failure is severe Li dendrites growth, while the intrinsic law of Li anode expansion and pulverization is formation of H 2 and LiH from intrinsic parasitic reactions between Li anode and electrolytes. However, the presence of LiH at Li anode and its crucial role in deteriorating Li anode are embroiled in controversy. Hence, it is necessary to trace the research history of LiH and its unique properties at electrodes, especially at Li anode ( Figure 3).
The research history of LiH in lithium batteries still begins in 1999. [44] By FT-IR spectroscopy, Aurbach et al. preliminarily identified that freshly Li reacts with H 2 to form surface LiH at room temperature in wet electrolytes. This conclusion challenges the conventional understanding of how LiH is formed by reacting liquid Li (melting point~180°C) and H 2 at elevated F I G U R E 2 Failure mechanisms and protection principles of Li anode. Li anode with different SEI layers before and after initial and long-term cycling: (A) with a typical SEI layer, (B) with an in situ formed high-quality SEI layer, and (C) with both artificial and in situ formed SEI layers. Reproduced with permission. [13] Copyright 2020, American Chemical Society. Selection of SEI components according to (D) Li binding energy [16] and (E) γE (γ and E represents interfacial energy and Young's modulus). [17] Reproduced with permission. [16] Copyright 2019, Elsevier B.V. Reproduced with permission. [17] Copyright 2020, American Chemical Society. SEI, solid electrolyte interphase.
In the surging revival of LMBs, until 2018, by using cryo-transmission electron microscopy (cryo-TEM), Kourkoutis et al. identified that LiH is a main component in a type of Li dendrites (in Li/Li cells) and the spatial distribution of LiH within the Li dendrites is mapped. [48] The lithium particle on the tip and the uniformity of the LiH within the type II dendrites, as well as their aspect ratio, suggest a root or tip growth mode of LiH. It is preliminarily pointed out that LiH possesses poor electrical conductivity and LiH is much more brittle than Li metal. In other words, LiH can disconnect deposited Li from bulk Li, leading formation of "dead Li" and subsequently contributing to capacity loss of LMBs. Yet, the direct relationship between LiH and "dead Li" has not been resolved at present. Moreover, adopting hydrogenless fluorinated electrolyte suppresses the formation of LiH substantially. Nevertheless, in other many cases (in Li/Cu cells), LiH is not detected, [49,56,57] even by combing methods of cryo-TEM and deuterium-oxide (D 2 O) titration gas chromatography (TGC). The feasibility of identifying LiH by cryo-TEM method needs further validation. It is noted here that the proposed D 2 O titration method (LiH + D 2 O = LiOD + HD↑, 2Li + 2D 2 O = 2LiOD + D 2 ↑) by Meng et al. is accurate, feasible, and economic, laying the foundation for rapid LiH and Li differentiation. [49] In retrospect, [49] LiH is not detected because Cu foil without Li plating/stripping processes is titrated by D 2 O, while LiH formation is related to H 2 evolution during Li plating/ stripping processes. Later, Armand et al. mentioned that coexistence of LiF and LiH in the SEI layer can inhibit Li dendrites' growth, [50,58] but experimental support for this conclusion is not given. The study of LiH at the Li anode is embroiled in huge controversy.
Encouragingly, since 2021, the research of LiH at Li anode has entered a flourishing stage. First (January 19, 2021), through delicately designed online D 2 O titration mass spectrometry (MS) system, Cui et al. (our group) found that LiH accumulation is negatively correlated with cyclability of practical LiCoO 2 /Li LMBs. [41] Surprisingly, the LiH content (mg LiH per mg Li) in the final failed and pulverized Li anode is at the same order of magnitude for different electrolytes (with a high average value of 0.1705 mg LiH per mg Li). This is the first case of identifying the existence of LiH at pulverized Li anode in LMBs under practical conditions. More importantly, it is revealed that a temperature sensitive equilibrium (Li + 1/2 H 2 ⇌ LiH) governs the formation and decomposition process of LiH at Li anode. On the one hand, at room temperature, large amounts of generated H 2 can react with deposited Li to form LiH, which induces Li anode expansion/pulverization/cracking. On the other hand, the equilibrium can be partially pushed back at high temperatures. Then, by online D 2 O titration MS and solid-state magic-angle spinning NMR of 7 Li, P. G. Bruce et al. quantifiably showed that less LiH is formed when fluoroethylene carbonate (FEC) additive is employed (in Li/Cu cells), suggesting an additional benefit of a fluoride-rich (LiF) interphase for promoting high cycling efficiency of Li anode. [51] Subsequently, Xiao et al. used synchrotron-based X-ray diffraction (XRD) and pair distribution function (PDF) analysis to identify and differentiate LiH and LiF in SEI layer of Li anodes (in Li/Cu cells). [52] Two possible reasons are given to explain why LiH is not being focused or identified in previous literature reports: One is that both LiH and LiF possess face-centered-cubic (FCC) phase, exhibiting a very similar lattice parameter (4.084 Å for LiH and 4.026 Å for LiF) and general XRD pattern appearance; On the other hand, LiH is extremely sensitive to moisture (LiH decomposes after 1 s exposure to moisture). Additionally, because of the same FCC structure and similar lattice parameters, there is a possibility of forming a solidsolution phase of LiH x F 1−x , which can substantially increase ionic conductivity because Li-H bond is much weaker than Li-F bond. Then, Yang et al. adopted mass spectrometry titration (D 2 O) and solid state nuclear magnetic resonance (NMR) to confirm that LiH accumulates during the cycling of anode-free LiFePO 4 /Cu and LiCoO 2 /Cu cells. [53,59] Differently, Yu et al. identified the positive role of LiH in promoting fast diffusion of Li + by building a unique three-dimensional (3D) Li anode composed of LiMg alloys uniformly confined into graphene-supported LiH nanoparticles. [54] Very recently, Hu et al. used synchrotron-based XRD, PDF and isotope labeling to confirm that LiOH on pristine Li foil is another important contributor to LiH generation (LiOH + 2Li → Li 2 O + LiH, this reaction has already been proposed in Hu et al. [46] ). Otherwise, when lithium accessibility is very limited, as in the case of anode-free cells, LiOH tends to grow into plate-shaped large crystals, dramatically limiting the cycle life of anode-free cells. Obviously, as aforementioned, the formation mechanisms of LiH and its roles in affecting Li anode are also embroiled in huge controversy. Based on the aforementioned research history of LiH, to effectively protect Li anode, we believe more focus must be given on generating pathways of H 2 and LiH from Li/ electrolyte intrinsic parasitic reactions and their regulating or suppressing strategies.

| A NEW PRINCIPLE FOR LI ANODE PROTECTION
After a thorough analysis of failure mechanisms and protection principles of the Li anode, and combing research history of LiH, we propose a novel insightful perspective based on high H 2 adsorption ability to seek protective species for effective artificial SEI layer engineering. Why? It is surprising to find that not only Li and LiH, but also the most frequently reported species, such as LiF, Li 3 N, BN, Li 2 O, carbon nanomaterials, and carbon nitride are excellent H 2 storage materials. [60][61][62][63] In other words, serving as lithiophilic/lithiophobic materials and H 2 storage materials, these Li protective species are interdisciplinary materials. Then, density functional theory (DFT) calculations are conducted to clarify the H 2 adsorption energy differences among these species (See details in Supporting Information). Encouragingly to see that,  Figure 4A). In other words, the H 2 adsorption ability of these protective species (LiF, Li 3 N, BN, Li 2 O, and graphene) is much higher than that of Li and LiH. Our DFT calculations indicate that a slow and complicated "H 2 adsorption competing" process exists at the interface of Li anode. Obviously, the winning or losing of H 2 by Li determines the failure or protection of Li anode ( Figure 4B,C). It is reported that LiH is a hydrogen storage promoter, [62] well explaining why composites with trace LiH (such as LiF-LiH composites [50,58] and LiMg-graphene-LiH composites [54] ) are reported to be favorable for Li anode protection. Therefore, constructing an artificial SEI layer enriched with components with much higher H 2 adsorption ability than Li will be a potential supplemented principle for LiH suppression and Li anode protection. Moreover, from the perspective of catalytic chemistry, an effective artificial SEI layer must also possess the ability of deactivating (passivating) catalytic sites for H 2 production from electrolyte decomposition.

| PERSPECTIVES AND CONCLUSIONS
Here, it is suggested that suppressing the formation of LiH and H 2 will serve as a supplemented principle for Li anode protection. While the formation of LiH is inferred to be greatly associated with generation of H 2 from intrinsic parasitic reactions between Li anode and electrolytes. As a result, it is necessary to explore exact and objective intrinsic parasitic reactions (pathways, rate, area, potential, byproducts, etc.) between Li anode and electrolytes leading to Li anode failure and electrolyte depletion. Except for cathode passivating, electrolyte formulating (formulating electrolytes with less or no hydrogen), separator engineering, and radical scavenger searching, [41] our DFT calculations definitely indicate that constructing an artificial SEI layer enriched of components with much higher H 2 adsorption ability than Li will be a potential way for LiH suppression and Li anode protection. This novel perspective based on H 2 adsorption ability will enlighten us to seek more effective artificial SEI layer components from the database of H 2 storage materials. Of course, theoretically selected H 2 storage materials are better to be lithiophilic/lithiophobic and must also satisfy the aforementioned three key principles of designing SEI layers for Li anode protection: mechanical stability; uniform ion transport; and chemical passivation. In other words, components with much higher H 2 adsorption ability than Li will not necessarily protect Li anode effectively. In experiments, LiH content determination is required to evaluate the effects of theoretically selected H 2 storage materials in protecting the Li anode. Moreover, to validate the supplemented principle, it is also necessary to experimentally confirm whether conventional representative Li protective species (such as LiF, Li 3 N, BN, Li 2 O, and graphene) will obviously decrease formation of LiH and H 2 or not. However, due to its unique properties like high moisture sensitivity, LiH can only be reliably detected by several few methods, such as aforementioned qualitative and quantitative online D 2 O titration, cryo-TEM, synchrotron XRD, and solid-state NMR. Thus, developing advanced and generally acceptable tools for LiH determination is urgently needed. Analogously, it is noted here that efforts are also required to clarify the role and formation mechanism of sodium hydride (NaH) in the SEI layer of Na metal anode. [64,65] Finally, we hope our unusual concept of suppressing the formation of LiH and H 2 will serve as a modest spur to draw forth more insightful efforts to realize excellent Li anode protection and the ultimate practical applications of "holy grail" LMBs. the relation of the charge, orbital, and spin degrees of freedom to the magnetic and transport properties of the strongly correlated materials and renewable energy materials using X-ray absorption spectroscopies.
Guanglei Cui is currently a professor at QIBEBT, CAS. He is the leading principal investigator of the Solid Energy System Technology Center. His research topics relate to next-generation energy storage materials and devices.

SUPPORTING INFORMATION
Additional supporting information can be found online in the Supporting Information section at the end of this article.