Solid-state lithium metal batteries are considered the ultimate choice for future energy storage systems due to their high theoretical energy density and safety.
However, the practical application of solid-state batteries is hindered by severe interfacial problems, such as high interfacial resistance, poor electrochemical/chemical compatibility, and poor stability. In addition, Li dendrite growth and mechanical performance degradation caused by interfacial stress during cycling are the main reasons for the failure of solid-state batteries.
Professor Yuan Hong from the Special Research Institute of Beijing Institute of Technology and Professor Zhang Qiang from Tsinghua University introduced the current basic understanding of the influence of metal lithium/solid electrolyte interface on solid state ions and interface chemistry. The electrical, chemical, electrochemical, and mechanical failure mechanisms of solid-state lithium batteries are reviewed, as well as emerging perspectives on future research directions.
Research Background
Solid electrolytes can be divided into two categories: solid polymer electrolytes (SPE) and solid inorganic electrolytes (SIE). SIEs generally have excellent mechanical modulus, wide electrochemical window, and good ionic conductivity, but poor chemical stability and poor interfacial compatibility, while SPEs are the opposite. Unfortunately, both have open issues.
Driven by interface science and nanotechnology, efforts have been devoted to improving the physicochemical properties of SSE (solid-state electrolytes), such as interfacial wetting, lithiophilic engineering, alloying, and artificial interface modification. But compared with liquid batteries, SSE-based SSLMBs (solid-state lithium metal batteries) still exhibit much lower electrochemical performance, which largely limits their practical industrial applications.
At present, it is generally believed that the main reasons for the failure of SSLMBs are large interface impedance, severe dendrite growth, unfavorable interface reaction, interface evolution deterioration and mechanical deformation, etc., but the in-depth analysis and comprehensive summary of the failure mechanism of SSEs are still lacking.
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Solid State Ions in SSEs
Fast ion transport kinetics in SSE is a key factor for high electrochemical performance. Among them, the ionic conductivity of SPE is generally lower than 10-4 S cm-1, and the volume ionic conductivity of perovskite type, garnet type, LiSICON type and arginite at room temperature is in the range of 10-4–10- 3 S cm-1, and sulfides can reach 10-2 S cm-1.
For crystalline ceramic electrolytes, the ionic conductivity of SSE can be effectively enhanced by increasing the ratio of vacancies and interconnected interstitial sites by doping, substitution, and non-stoichiometry.
In addition to charge carriers, ion transport paths related to ion mobility within the solid crystal lattice also contribute to the ion transport behavior. In general, anisotropic three-dimensional ion diffusion has become prevalent in fast Li-ion conductors, such as garnet-type, NASICON-type electrolytes.
Commonly used polymers include polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and polyvinylidene fluoride-hexafluoropropylene (PVDF) -HFP), of which PEO is the most attractive. The prevailing view is that the conduction of lithium ions is achieved by segmental relaxation of the amorphous region. The lithium ions are coordinated with polar groups on the segmented polymer chains , under the action of an electric field, lithium ions migrate from one coordination site to another through intra-chain or inter-chain transitions and continuous chain segment rearrangements, thereby realizing long-distance transport of ions. Reducing crystallinity can effectively significantly improve the ionic conductivity of SPE.
Solid electrolyte interface
High interfacial stability between electrodes and SSE is crucial for the efficient operation of batteries. However, the Li/SSEs interface is chemically unstable due to the lowest electrochemical potential and high reactivity of metallic Li anodes. Most SSEs spontaneously reduce upon encountering the Li anode and form a passivated interfacial layer at the interface, which greatly affects Li-ion transport kinetics and battery performance.
According to the characteristics of the interface layer, it can be divided into three types of Li-SSE interfaces: 1. Thermodynamically stable interface without the formation of interfacial reaction phase, this interface is very ideal for SSLMB, it can not only achieve uniform Li-ion 2. Thermodynamically unstable interface with mixed ion-electron conducting (MIEC) interface, this MIEC interphase allows continuous electrochemical reduction of SSE and eventually leads to battery failure; 3. Thermodynamically unstable interfaces with ionically conducting but electronically insulating interfaces, also known as "stable SEIs", can suppress the transfer of electrons between SSEs and thus maintain stable interfaces during charging cycles, which usually exist in typical In SSE, including LLZO, LiPON and Li7P3S11.
Space charge layer theory
Since the interface between electrodes and SSEs is always heterogeneous, there is a chemical potential gradient when they contact, which provides the driving force for Li ion redistribution and spontaneously generates a space charge layer at the electrode/SSE interface.
The inter-charge region is usually highly resistive and deteriorates the transfer of lithium ions through the interface, resulting in high interfacial resistance and poor cycling capability.
More deadly, the existence of the space charge layer may also lead to the gradual depletion of lithium ions from the electrode and accumulation in the electrolyte during battery cycling, thereby aggravating charge segregation and ultimately reducing the reversible capacity.
Most of the research results mainly focus on the interface between the high voltage cathode and SSE, and there is a lack of information on the space charge layer at the Li anode/SSE interface.
Electric Failure
Dendrites easily penetrate most SPEs because their relatively low elastic modulus cannot withstand the growth of dendrites, leading to cell failure.
In addition, pre-existing local surface inhomogeneities at the Li/SPE interface, such as impurity particles or defects, are considered to be a critical point for Li dendrite growth in polymer batteries.
The nucleation and growth of Li can preferentially focus on the edges of these impurities due to the increase in local conductivity or electric field strength, resulting in the formation of spherical or dendritic structures. In addition to this, irregular Li deposition also creates voids on top of the impurities.
Studies have shown that increasing the elastic modulus of SPE will generate high compressive stress around the dendritic protrusions, resulting in a lower exchange current density at the peaks of the protrusions than at the valleys, thus effectively preventing dendrites under higher current conditions. grow.
As for SIE, it is more controversial. In general, dendrite infiltration is prominent in garnet-type or some sulfide electrolytes. The microstructural features of these SIEs, such as grain boundaries (GBs), voids, pores, cracks, and protrusions, contribute to dendrite-induced short-circuit behavior.
GBs are widely considered to be the preferred sites for Li dendrite growth. Li metal nucleates initially at the Li anode/SSEs interface during cycling and, given their low elasticity and low ionic conductivity, propagates along the GBs, eventually leading to battery failure.
It has been found that the relatively high electronic conductivity of GBs contributes to the reduction of Li ions in SSEs. The high electronic conductivity of SSE (which can be caused by impurities, dopants, GB or electrochemical reduction) is the origin of dendrite nucleation and growth within SSE.
Besides the intrinsic properties of SIE, Li metal also plays an important role as a double-edged sword in regulating the dendrite growth of SSLMB.
On the one hand, the rigid interfacial contact between Li anode and SSE can be improved by the plastic deformation of metallic Li. On the other hand, severe deformation of lithium (also known as creep) causes lithium to propagate along voids, defects, cracks, and GBs within the SSE, eventually leading to short-circuiting of the battery.
Chemical Failure
Due to the high reactivity of Li metal anode, it can easily react with most SSEs and spontaneously form an interfacial layer on the surface of Li anode. The nature of the phases directly determines the overall performance of SSLMB.
For those spontaneously formed, electronically insulating but poorly ionically conductive interfacial phases, the ion transport kinetics of the entire battery system are weakened, thereby significantly reducing the cycling capability (such as the lithium-sulfide SSE interface).
SSEs containing high-valent metal ions with high ionic conductivity, such as NASICON-type LAGP, LATP, fast ion conductor LGPS, perovskite-type LLTO, etc., are more inclined to form MIEC interfaces when in contact with Li. The mixed conductive properties of the interface will accelerate the transfer of electrons across the interface, leading to rapid electrolyte degradation and eventual battery failure.
The chemical failure is governed by the thermodynamic interfacial reaction between the lithium anode and the SSE. If the formed interfacial features have uniform composition and high ionic conductivity, the unfavorable interfacial evolution during cycling will be largely alleviated. Rational design of the structure and composition of SSEs is effective for tuning the physicochemical properties of the interface.
Electrochemical Failure (Mechanical Failure)
It has been shown that the severe redox reaction of Li7P3S11 (LPS) occurs in a wide electrochemical window, and the amount of decomposition products (Li2S and S) increases with the depth of the redox reaction. More importantly, the redox reaction of the electrolyte is a continuous degradation process, resulting in the continuous generation and accumulation of by-products during cycling. Such a result enlarges the interfacial polarization and increases the cell resistance, ultimately leading to a rapid capacity drop.
In addition, the increased inhomogeneity of lithium distribution during electrochemical cycling also affects the electrochemical performance. For example, the Li-deficient region exacerbates the Li concentration polarization in LGPS electrolytes, increasing the interfacial resistance, leading to capacity fading.
The evolution of the interface during cycling and its impact on the electrochemical kinetic behaviors such as lithium ion diffusion and transport, interface morphology and chemical evolution, and potential changes remain to be further investigated. More importantly, unlike interfaces in liquid electrolyte systems, solid-solid Li/SSEs interfaces are difficult to operate and observe in situ. Advanced characterization techniques need to be developed to obtain
more detailed information about the interface behavior in SSLMB.
Mechanical Failure
The mechanical stability of the Li/SSEs interface also contributes to the battery performance. During the Li deposition/stripping process, the huge volume expansion of the anode can cause severe fluctuations at the Li/SSEs interface due to the rigid nature of the solid-state electrode and solid-state electrolyte. Such interfacial fluctuations can lead to impaired contacts or even delamination at the electrode/electrolyte interface.
Unlike the case of conventional liquid electrolytes, the interfacial volume change due to Li deposition/stripping cannot be buffered or absorbed by the SSE, but is limited by the space of the interfacial contact between the anode and the SSE. Therefore, this naturally creates large stresses that mechanically damage the interface.
More fatally, some generated or pre-existing surface defects can in turn serve as preferential sites for lithium dendrite penetration. The localized strain accumulates throughout the cycling process, resulting in high stress concentration at the tip of the Li filament (original Li filament), which further promotes crack propagation and leads to accelerated infiltration of the Li filament (original Li filament), ultimately leading to battery failure.
Relatively speaking, SSE with higher fracture toughness can significantly increase the overpotential and fracture stress required for cracks at the same size, thereby reducing the risk of decay. The improved fracture toughness of SSEs will help resist crack propagation and mitigate the risk of mechanical failure of the battery.
On the other hand, considering the high reactivity of Li anode towards SSEs, the formation and evolution of interfacial phases also have an impact on the mechanical degradation of SSLMBs. Li intercalation and interfacial transition during interphase growth lead to volume expansion within the SSE and large internal stress, which mechanically destroys the bulk SSE and leads to high resistance.
At high current densities, the promotion of short ion transport paths may be amplified due to the higher overall overpotential, leading to severe inhomogeneities.
The intrinsic properties of the (electro)chemically formed interface also influence the mechanical properties. Those SSEs that can chemically react with lithium metal to form the MIEC interfacial phase tend to fail mechanically, and they fail the battery during repeated charge/discharge processes.
Attached:
references
Liu J, Yuan H, Liu H, et al. Unlocking the failure mechanism of solid state lithium metal batteries[J]. Advanced Energy Materials, 2022, 12(4): 2100748.
Literature link
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