4.1 Introduction

In Chapter 1, we discussed the issue of side reactions and solid–electrolyte interface (SEI) formation at the lithium anode in a lithium sulfur cell. In addition to electroplating and electrodissolution of lithium in the core redox chemistry of the cell to release cations into the electrolyte, we note that in a typical construction the anode provides a source of reductant species, whilst excess lithium acts as a lightweight current collector and helps combat poor coulombic efficiency. Thus, the degradation of the anode is a significant contributor to reduced cycle life and limits application. If the energy density of a lithium–sulfur cell is set at 400 Wh kg⁻¹, the thickness of lithium metal is estimated at 25–50 μm (5–10 mAh cm⁻¹). Commercial foils are 70–130 μm in most cases.

Lithium is highly reactive and lightweight, making it an ideal candidate for a battery technology designed for high gravimetric energy density. Unfortunately, lithium reacts with many species it comes into contact with, forming unwanted side products. These unwanted side reactions do not add value and may lead to irreversible loss of lithium and other electrolyte components. Consumption of electrolyte or drying of the cell and or loss of lithium results in accelerated capacity fade.

Lithium has an extremely high theoretical specific capacity of 3860 mAh g⁻¹ and a low density of 0.59 g cm⁻³ and the lowest electrochemical potential −3.040 vs. the standard hydrogen electrode.

4.2 SEI Formation

The reactions of lithium and electrolyte components form an SEI on the surface of the lithium that slows reaction of electrolyte components with the anode and can reduce degradation and thus improve cycle life. The SEI covers the surface of the anode, and primary electrochemical reactions occur through the SEI layer. The nature of the SEI layer affects reaction kinetics and can lower cell voltage due to increased internal resistance. Despite this, the SEI layer and its properties are critical to the performance of the anode and the focus of materials research related to the anode in a lithium sulfur cell. While materials research in the electrolyte arena focuses on choosing stable solvent systems or reactive additives that promote a favorable SEI composition, solvents are the main source of organic lithium salts in SEI films [1].

Disordered structure promotes ionic conductivity, while the thickness of the SEI layer increases internal resistance. The film stops growing when electron transfer is blocked, typically tens of angstroms. The compact stratified layer model is commonly used to describe the SEI on a lithium anode. It is considered that the surface film on the anode consists of a porous interphase and a compact interphase consisting of sublayers. The porous outer layer close to the solution is nonuniform because the reduction of solution species cannot take place over the entire film–solution interface, but rather at defects or holes where electrons can tunnel to the surface. The composition of the SEI changes gradually as you move from solution/SEI to SEI/Li. Close to the lithium surface, lower oxidation states are found and the SEI becomes more compact.

The formation of an SEI is a double‐edged sword [2] depending on chemical and physical properties. A coarse and inhomogeneous SEI such as the disordered mosaic type from soaking promotes preferential growth through cracks and where the SEI is thinner. An intact and smooth SEI where localized defects are largely eliminated suppresses both intrinsic and induced dendrite growth. Ideally, an SEI should be chemically stable, lithium‐ion conductive, compact, uniform, and possess mechanical rigidity and elasticity to accommodate volume change.

4.3 Anode Morphology

In addition to SEI formation during charge and discharge, lithium stripping and plating leads to morphological changes over time. Natural imperfections in the soft metal anodes act as nucleation points, the uneven stripping and plating of lithium over time increases the surface area of the anode and introduces porosity; this is known as 3D mossy growth. While this process increases the anode reactive surface area for electrochemistry, it also promotes continual breaking and reforming of the SEI. This cyclic process depletes reactive SEI forming electrolyte components in the cell over time. While irreversible side reactions nibble away at the active material and perhaps more significantly the anode as current collector. The process accelerates as degradation increases.

Mossy growth [3] is a 3D omnidirectional moss or bushlike growth. 1D growth forms 3D growth by broadening and branching during filament growth. It can be explained by the raisin bread expansion model; there is no preferred direction and the distance between each raisin increases as the loaf expands. It has no growth center, but the movement can be restricted due to its support, where the metal base fixes the moss. Lithium atoms are inserted over the whole structure, and growth does not necessarily occur at tips but at distributed growth points. Growth and dissolution of Li mossy structures is a nonlinear dynamic process, motion appears random and is not dominated by any direction of the electric field in the bulk electrolyte. During dissolution, large parts may become electrically disconnected, which can happen even if the lithium remains attached to its original position at the substrate because the electrical contact sites are substituted by an insulating and passivating SEI layer.

The lithium surface must be smooth to ensure a uniform SEI layer. Stan and coworkers [4] studied the effect of controlling the starting lithium surface roughness and the nature of the native SEI. This was achieved by a simple roll press with a controlled surface finish. This had the effect of reducing overpotentials for plating and deplating in a symmetrical cell.

4.4 Polysulfide Shuttle

Discussed elsewhere, it is worth repeating here that the anode has a role in the polysulfide shuttle mechanism unique to lithium sulfur cells. The anode is able to reduce any high‐order polysulfide species, which find their way to the anode surface from the cathode and permeate through the separator dissolved in the electrolyte. Toward the end of charge, high‐order polysulfide species accumulate prior to sulfur precipitation at the cathode and diffuse back to the anode. At the anode, these high‐order polysulfides are reduced before diffusing back to the cathode to be reoxidized.

The whole process generates an internal parasitic current, in which the polysulfide species transport electrons from the anode to the cathode inside the cell. The effect is to prevent overcharge where the charge current is countered by the shuttle current at the top of charge. This in turn causes internal heating of the cell. The effects must be managed at the level of the battery management system to detect shuttle and manage self‐discharge or by materials research to eliminate the parasitic reactions through design of the SEI in the anode case. This chapter focuses on the latter approach.

4.5 Electrolyte Additives for Stable SEI Formation

Electrolyte additives are an effective method to modify electrode/electrolyte interfaces in lithium batteries. Lithium nitrate is a common additive in lithium–sulfur cells to manage the shuttle effect and to form a protective SEI layer. Xiong et al. [5,6] studied the properties of the surface film on a lithium anode with lithium nitrate salt in the electrolyte. They showed that the effect of adding lithium nitrate is to reduce or eliminate the polysulfide shuttle mechanism until it is depleted, and the effect can be most observed at the end of charge when the cell voltage readily reaches a cutoff voltage (2.4 V) compared to a cell subject to shuttle that will fail to reach its cutoff voltage due to the internal current leveling the voltage. It was proposed that a lithium anode exposed to an electrolyte containing lithium nitrate prevents parasitic reactions between polysulfides and lithium anode, demonstrated by Liang et al. [7].

Lithium was supplied with a native SEI consisting of LiOH, Li₂O, and Li₂CO₃. All are inorganic crystalline salts that provide a rough surface. When immersed in LiN(CF₃SO₂)₂/1,3‐dioxolane/dimethoxyethane, the lithium reacts to form LiF, Li₂S₂O₄, and ROLi creating a new smooth surface. If Li₂S₆ is added, then Li₂S is also detected in the SEI.

Analysis of the SEI formed from an electrolyte composition containing LiNO₃/1,3‐dioxolane/dimethoxy ethane showed that the top layer consists of both inorganic species such as LiN _x O _y and organic species such as ROLi and ROCO₂Li. With increasing depth, the order of the nitrogen in reduction products becomes lower, from Li₂N₂O₂ to Li₃N. Under the top layer, the film mainly consists of ROCO₂Li, Li₂O, LiN _x O _y , and Li₃N; these species prevent the continuous electron transfer from lithium metal to polysulfide solutions and electrolyte solutions.

A homogeneous surface film was obtained ensuring a uniform ionic conductivity on the surface of the anode, leading to uniform plating and stripping. Minimizing morphological changes reduces SEI damage and repair, thus reducing electrolyte depletion. Reduction of 1,3‐dioxolane synthesizes oligomers of poly‐dioxolane in situ that are insoluble and adhere to the lithium surface by its –OLi edge groups. These elastomers provide flexibility in the SEI that can better accommodate volume changes compared to many other solvents' SEI, making 1,3‐dioxolane a common solvent of choice.

By comparison, the SEI formed from the same electrolyte without lithium nitrate gave a top layer consisting of Li₂CO₃, Li₂O, LiOH, RCH₂Li, and RCH₂OLi with RCH₂OLi and Li₂CO₃ as the main components, suggesting the SEI is composed of lithium metal and mixed solvents. Under the top layer, Li₂CO₃, Li, and Li₂O were the main components similar to the native film found on metallic lithium. The surface of the lithium was very smooth due to ROLi and a polymer film of polymerized 1,3‐dioxolane, and the salts formed a regular meshlike pattern on the homogeneous polymer layer, suggesting that the polymer layer formed first.

Impedance spectroscopy determined that the native film on the lithium is disrupted following immersion of the anode in electrolyte. A new surface film forms from reaction of the anode and electrolyte components, a reaction that slows and stops with time as the SEI grows thicker and electron tunneling ceases. If the reaction rate is slow, then the SEI film can form uniformly. However, if the rate of SEI formation is too slow, then when it is damaged during cycling (due to volume changes) inefficient repair may form an uneven surface. The smoother and more compact the SEI, the more low viscosity solvents and lithium polysulfides are separated from reaction at the anode, thus reducing degradation.

This is a valid approach to anode protection in combination with other methodologies; by itself, this technique has been unsuccessful in significantly delaying degradation to meaningfully extend cycle life. Lithium nitrate is ultimately depleted due to constant repair of the SEI as the anode changes volume during cycling. Ultimately, the effect of suppressing the shuttle mechanism is defeated by constant repair of the SEI [8].

Others have used lithium salts in a similar way, such as Li bis(perfluoroethylsulfonyl)imide (LiBETI), which can form stable, thin, uniform, and compact surface films on lithium containing mainly LiF that can also improve cycling efficiency.

Nazar and coworkers [9] used Li₂S₆ and P₂S₅ as electrolyte additives to form a chemically stable thin amorphous layer on the anode consisting of Li₃PS₄ in situ. The film reduces the heterogeneity of the SEI, thus allowing a nonimpeding lithium ion flux. The layer has a theoretical lithium transference number of 1, which effectively eliminates ion depletion and strong electric field buildup at the lithium surface, both mechanisms linked to dendrite growth. Stable long‐term plating and stripping of symmetrical cells over 2500 hours without short circuit was demonstrated.

Mo and coworkers [10] modeled candidates for stable SEI components from a large materials database from first principles. They found most oxides, sulfides, and halides are reduced by lithium metal due to the reduction of metal cations and that the nitride anion has unique stability against lithium metal either intrinsically or through passivation.

4.6 Barrier Layers on the Anode

An excess of lithium is used as current collector in a lithium sulfur cell and to combat low coulombic efficiency. In lithium ion cells using lithium metal, the formation of lithium dendrites has caused safety concerns due to the potential for internal short circuit. The term dendrite covers a range of structures including needle‐like, snowflake‐like, treelike, bushlike, whisker‐like, and mosslike. In most lithium sulfur systems, only mossy growth is observed in practice and internal short circuits due to dendritic growth have not been a reported practical issue to date. Barrier layer approaches have been advanced to deal with dendrite growth that has the potential to pierce the separator for lithium ion technology, some of which can be applied to lithium sulfur technology to combat the shuttle effect and other degradation processes.

Most approaches to dendrite prevention in rechargeable lithium cells [1] have focused on the stability and uniformity of the SEI through the use of electrolyte additives. Because lithium metal is thermodynamically unstable in organic solvents, such methods are often short‐lived, as discussed earlier. Despite this, their simplicity for scale‐up and commercialization make them attractive.

An alternative approach is to form an ex situ mechanical barrier on the lithium foil. Examples include polymer coatings or ceramics with a high shear modulus to reduce damage and repair to the protective layer that would otherwise deplete reactive components in the electrolyte. Reel‐to‐reel coating techniques can be developed for lithium coatings; such techniques are used, for example, in the semiconductor industry.

Barriers rely on forming a strong mechanical layer while attempting to reduce the impact upon the primary electrochemistry taking place. The approach can easily lead to high internal resistance within the cell if the barrier layer blocks electrochemical activity.

Polymer layers can be cast onto lithium and dried; the advantages are that flexibility of the polymer makes them robust to volume changes during cycling. The issue is to find a conductive polymer or to achieve a thin coating that does not significantly increase internal resistance in the cell. Polymer layers are required to be insoluble in the electrolyte and stable in the presence of polysulfides, nucleophiles, and radicals. Wu and coworkers [11] developed a cation‐selective polymer blend of Nafion and polyvinylidene difluoride (PVDF), a hierarchically nanostructured composite that provides the flexibility and strength to cope with breathing of the lithium during charge and discharge. The polymer improved rate performance, coulombic efficiency, and cycle life.

Guo and coworkers [12] note that the SEI formed from organic solvents is brittle and cannot withstand mechanical deformation, leading to the formation of cracks. Cracks enhance lithium ion flux and result in dendrite formation and new SEI formation. The recurring breakage and repair of the SEI consumes lithium and electrolyte causing battery failure. Volume change is the main issue that defeats most approaches to forming a stable SEI. The team thus fabricated a smart SEI layer with elasticity using an in situ reaction between lithium and polyacrylic acid (PAA). Lithium PAA has good uniform binding properties and is flexible enough to accommodate lithium deformation.

An alternative to flexible polymer layers are ceramic coatings applied via a range of techniques used in the photovoltaic industry, for example, chemical vapor deposition (CVD) or atomic layer deposition (ALD). Ceramic layers may act as thin conductive barrier layers to address internal resistance issues of thicker polymer layers but tend to be brittle and crack during cycling.

Elam and coworkers [13] employed ALD to prepare conformal, ultrathin aluminum oxide coatings on lithium. They found it improved wetting of the surface toward both carbonate and ether electrolytes, leading to dense and uniform SEI formation with reduced electrolyte consumption and a practical 98% coulombic efficiency. During cycling, the surface was smooth and uniform compared to lithium without the coating. The coating allowed lithium ion diffusion and remained intact, resulting in an increase in cycle life with a reduced electrolyte volume in coin cells.

Zhang et al. [14] offer a review of techniques for lithium protection and point to the example of ex situ CVD of 2D boron nitride as a flexible, chemically stable, barrier layer with sufficient mechanical strength despite ultrathin thickness, yet noted its contribution to interfacial resistance which must be overcome.

4.7 A Systemic Approach

Hu and coworkers [15] conclude in their review of protected lithium metal anodes that a systemic approach is required to address anode stability. A systemic approach combines modification of the lithium electrode, the organic electrolyte and its additives, the SEI, the separator properties, and the battery configuration and management system itself. The systems approach is a theme that runs throughout this book.

In another review by Zhang and coworkers [3], they conclude that many issues need to be considered for the improvement of the lithium metal anode. Nonaqueous organic electrolyte is the most adopted system for lithium metal anodes, resulting in an unstable interface. The fundamental understanding of the SEI today is inadequate. Solid‐state electrolytes offer advantages over liquid in terms of safety and chemical stability, but low ionic conductivity and instability vs. lithium metal reduces their technology readiness level. Control of volume changes could be achieved through a structured matrix in place of a metal foil.

There is a lack of consistency in anode research, making it difficult to directly compare results; the performance of a material depends on the design of the cell and current density. To better understand the underlying mechanism, more smartly designed in situ or operando analytical techniques are required. There is a need for thinner commercial foils of 25–50 μm with a tailored SEI layer and lithium plating matrix that can be processed on a roll‐to‐roll commercial scale. These challenges make anode research a hot topic for next‐generation battery research.

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