Interfaces in Solid-State Lithium Batteries

Lithium-ion batteries (LIBs) are the promising power sources for portable electronics, electric vehicles, and smart grids. The recent LIBs with organic liquid electrolytes still suffer from safety issues and insufficient lifetime. Solid-state batteries (SSBs) are expected to address these issues. In principle, the nonflammable solid electrolytes could prevent battery combustion and explosion, and only Li ions are mobile in solid electrolytes, which could suppress side reactions. Some solid electrolytes, such as sulfides, have sufficiently high ionic conductivity, which is comparable to that of with organic liquid electrolytes. Thus, solid-solid interfaces appear to be the key to push SSBs toward practical applications. In this review, we start by introducing the challenges in solid-solid interfaces versus liquid-solid interfaces. We then discuss different interfaces in SSBs, including cathode-electrolyte interface, anode-electrolyte interface, and interparticle interface. Lastly, we present the advanced characterization techniques to help deepen understanding of the composition and structure evolution at the interfaces during battery cycling. The on-chip single-nanowire electrochemical devices developed by our group are highlighted as a unique platform for in situ characterization. We suggest and emphasize some future directions for SSBs. First, different in situ or operando characterization techniques should be developed and combined to track the real-time composition and structure changes at the interfaces in SSBs. Second, in addition to metal ions, metal-air and metal-sulfur systems with much higher energy density should also receive sufficient attention for SSBs. Lastly, a unique advantage of SSBs over liquid-electrolyte batteries is that SSBs could be flexible, stretchable, and shrunk on a chip. Thus, SSBs are promising for integration with microelectronic circuits to fabricate self-powered wearable or implantable micro-/nanoscale devices. The influence of interfaces represents a critical factor affecting the use of solid-state batteries (SSBs) in a wide range of practical industrial applications. However, our current understanding of this key issue remains somewhat limited. Therefore, this review presents the mechanisms and advanced characterization techniques associated with interfaces in SSBs. First, we compare liquid- and solid-state batteries and emphasize the challenges in solid-solid interfaces. Second, we discuss different aspects of interfaces including interphase formation, cathode-electrolyte interface, anode-electrolyte interface, and interparticle interface, which contain a detailed description of the formation mechanisms and current research. Third, the characterization strategies for effective interfacial observation and analysis are summarized and discussed. In particular, two unique characterization techniques, vibrational sum-frequency generation spectroscopy and on-chip single-nanowire battery characterization, are highlighted. Lastly, we summarize the scientific issues associated with solid-solid interfaces and provide our perspectives to better understand the fundamental issues and improve the performance of SSBs. The influence of interfaces represents a critical factor affecting the use of solid-state batteries (SSBs) in a wide range of practical industrial applications. However, our current understanding of this key issue remains somewhat limited. Therefore, this review presents the mechanisms and advanced characterization techniques associated with interfaces in SSBs. First, we compare liquid- and solid-state batteries and emphasize the challenges in solid-solid interfaces. Second, we discuss different aspects of interfaces including interphase formation, cathode-electrolyte interface, anode-electrolyte interface, and interparticle interface, which contain a detailed description of the formation mechanisms and current research. Third, the characterization strategies for effective interfacial observation and analysis are summarized and discussed. In particular, two unique characterization techniques, vibrational sum-frequency generation spectroscopy and on-chip single-nanowire battery characterization, are highlighted. Lastly, we summarize the scientific issues associated with solid-solid interfaces and provide our perspectives to better understand the fundamental issues and improve the performance of SSBs. As a key element in today's information-rich world and the devices that power it, rechargeable lithium-ion batteries (LIBs) are considered to be essential devices for a cleaner and more sustainable distributed energy supply.1Goodenough J.B. Park K.-S. 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For example, sharp, sticky Li dendrites that grow on the Li-metal anode during cycling, which can then penetrate through the separator between two electrodes, will induce sudden failure of the battery and the unwanted release of heat, the result of which is an increased fire hazard due to short-circuiting.6Cheng X.-B. Zhang R. Zhao C.-Z. Zhang Q. Toward safe lithium metal anode in rechargeable batteries: a review.Chem. Rev. 2017; 117: 10403-10473Crossref PubMed Scopus (299) Google Scholar, 11Zheng G. Lee S.W. Liang Z. Lee H.-W. Yan K. Yao H. Wang H. Li W. Chu S. Cui Y. Interconnected hollow carbon nanospheres for stable lithium metal anodes.Nat. Nanotechnol. 2014; 9: 618-623Crossref PubMed Scopus (579) Google Scholar For an Li-S battery, in addition to safety concerns associated with dendrites, the polysulfides that form during cycling can dissolve into the liquid electrolytes, thus rendering the cell life too short for most applications without specified electrode modifications.12Li B. Li S. Xu J. Yang S. A new configured lithiated silicon-sulfur battery built on 3D graphene with superior electrochemical performances.Energy Environ. Sci. 2016; 9: 2025-2030Crossref Google Scholar, 13Chung S.-H. Chang C.-H. Manthiram A. A core-shell electrode for dynamically and statically stable Li-S battery chemistry.Energy Environ. Sci. 2016; 9: 3188-3200Crossref Google Scholar, 14Liu J. Galpaya D.G.D. Yan L. Sun M. Lin Z. Yan C. Liang C. Zhang S. Exploiting a robust biopolymer network binder for an ultrahigh-areal-capacity Li-S battery.Energy Environ. 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Nanotechnol. 2008; 3: 31-35Crossref PubMed Scopus (0) Google Scholar Considering both safety and performance, and the urgent challenges of meeting 21st-century power demands, the most promising direction for battery development is to use solid electrolytes, and preferably all solid-state batteries (SSBs).3Armand M. Tarascon J.-M. Building better batteries.Nature. 2008; 451: 652-657Crossref PubMed Scopus (8629) Google Scholar A solid-state system yields many intriguing advantages.16Wang Y. Richards W.D. Ong S.P. Miara L.J. Kim J.C. Mo Y. Ceder G. Design principles for solid-state lithium superionic conductors.Nat. Mater. 2015; 14: 1026-1031Crossref PubMed Scopus (277) Google Scholar First, a solid-state system alleviates safety concerns by abandoning the use of flammable organic liquid electrolytes. Second, it can potentially prevent short-circuits by blocking Li dendrites on one side. Third, some SSBs can be bent, punched, or even pierced without risking unwanted safety hazards. Fourth, the stable solid electrolyte potentially facilitates a wider electrochemical window, thereby enabling the use of certain cathode materials with higher voltage capability. It is worth noting that a recent study shows the electrochemical stability window of most inorganic solid electrolytes to be overestimated by the traditional Li/solid electrolyte/inert metal cell test (see detailed discussion in the subsection “Electrochemical Characterization”).17Han F. Zhu Y. He X. Mo Y. Wang C. Electrochemical stability of Li10GeP2S12 and Li7La3Zr2O12 solid electrolytes.Adv. Energy Mater. 2016; 6: 1501590Crossref Scopus (108) Google Scholar All told, it is expected that SSBs will exhibit excellent cycle life, high reversibility, wider working temperatures, and other highly advantageous features. However, four specific standards must be met if the use of solid electrolytes is to be expanded: (1) high ionic conductivity, σLi+ > 10−4 S cm−1; (2) sufficient strength and minimum defects to inhibit Li dendrite penetration18Takeda Y. Yamamoto O. Imanishi N. Lithium dendrite formation on a lithium metal anode from liquid, polymer and solid electrolytes.Electrochemistry. 2016; 84: 210-218Crossref Scopus (20) Google Scholar, 19Yang C. Fu K. Zhang Y. Hitz E. Hu L. Protected lithium-metal anodes in batteries: from liquid to solid.Adv. Mater. 2017; 29: 1701169Crossref Scopus (99) Google Scholar, 20Monroe C. Newman J. The impact of elastic deformation on deposition kinetics at lithium/polymer interfaces.J. Polym. Sci. 2005; 152: A396-A404Google Scholar; (3) affordable raw materials and facile fabrication methods; and (4) low activation energy for Li+ diffusion. In general, solid electrolytes can be divided into two major groups: organic solid polymers and inorganic solids, including oxides and sulfides, etc.21Janek J. Zeier W.G. A solid future for battery development.Nat. Energy. 2016; 1: 16141Crossref Scopus (231) Google Scholar At present, a number of solid electrolytes with superb ionic conductivity have shown great promise to replace current commercial organic electrolyte batteries, especially for Li10GeP2S12 and Li2S-P2S5 with their high ionic conductivities of 1.2 × 10−2 S cm−1 and 1.7 × 10−2 S cm−1, respectively. Their conductivity is comparable with that of organic liquid electrolyte (usually on the order of 10−2 S cm−1, 25°C), indicating that solid electrolyte materials show great promise for next-generation batteries.22Kamaya N. Homma K. Yamakawa Y. Hirayama M. Kanno R. Yonemura M. Kamiyama T. Kato Y. Hama S. Kawamoto K. et al.A lithium superionic conductor.Nat. Mater. 2011; 10: 682-686Crossref PubMed Scopus (1179) Google Scholar, 23Seino Y. Ota T. Takada K. Hayashi A. Tatsumisago M. A sulphide lithium super ion conductor is superior to liquid ion conductors for use in rechargeable batteries.Energy Environ. Sci. 2014; 7: 627-631Crossref Google Scholar However, a key scientific issue that will hinder the practical application of SSBs concerns solid-solid interfaces. In this review, we assess solid-state interfaces with respect to a range of important factors: interphase formation, interface between cathode and inorganic electrolyte, interface between anode and inorganic electrolyte, interface between polymer electrolyte and Li metal, and interface of interparticles. This review also summarizes existing characterization strategies, including microscopy observation, chemical composition analysis, and electrochemical characterization. Finally, we introduce several advanced methods that are used to comprehensively study interfaces in SSBs. For a liquid-electrolyte system, wetting enables the liquid electrolyte to be fully infiltrated in the electrode, thereby facilitating simultaneous Li+ intercalation/conversion/alloying reactions for all active materials in the electrode, assuming that the time gap for electron transport can be overcome. Charge carriers in the electrolyte reduce with the consumption of Li+ during solid-electrolyte interphase (SEI) formation, further increasing the resistance for ion transfer and therefore hindering the ability for high-rate charging/discharging. Although electrolyte decomposition can be partially recovered through the use of a stable SEI and additional electrolytes in the battery wetting the electrode, this phenomenon is less obvious for anodes such as Li4Ti5O12 (LTO, ca. 1.5 V versus Li+/Li), Li-In alloy (0.6 V versus Li+/Li), and others.24Takada K. Aotani N. Iwamoto K. Kondo S. Solid state lithium battery with oxysulfide glass.Solid State Ionics. 1996; 86: 877-882Crossref Scopus (162) Google Scholar Briefly there are four key challenges in SSBs versus liquid batteries. First, the ionic conductivity of most solid-state electrolytes is lower than that of liquid electrolytes. Although some sulfide electrolytes can achieve high ionic conductivity, a number of instability issues at the interface must be addressed. Second, in contrast to liquid electrolytes, the interfacial compatibility between solid electrolytes and electrodes is usually poor and needs significant improvement for application in SSBs. The interfaces in an inorganic solid-electrolyte battery can feature several basic structures: the cathode-electrolyte interface, the anode-electrolyte interface, and the interparticle interface, as illustrated in Figure 1. In addition to the ionic conductivity of the electrolyte, ionic transport of Li+ at the interfaces also determines the overall ionic conductance that ultimately defines the rate performance of these batteries. This relationship explains why overall electrochemical impedance can be high, even in cases when the ionic conductivity of a solid electrolyte is excellent, as well as why achieving high Li+ conductance in the solid state comparable with existing liquid electrolytes is a challenging task. In industrial settings, for example, the best electrolyte-electrode interaction is achieved by enabling the wetting of the electrodes with electrolytes for a week or even two (and even in a laboratory setting, a day's wetting is required). Adopting a solid bulk that has literally no self-diffusion—in other words, a “wetting effect”—can lead to severe interface impedance. In addition to the conventionally conceived electrolyte-electrode interface, an electrode's interparticle interface also plays a prominent role in a composite electrode material. For example, the relatively low electronic and ionic conductivities of cathodes that occur as a result of poor contact at the phase-phase or particle-particle interface could limit efforts to improve the power density of SSBs.25Yan K. Lu Z. Lee H.-W. Xiong F. Hsu P.-C. Li Y. Zhao J. Chu S. Cui Y. Selective deposition and stable encapsulation of lithium through heterogeneous seeded growth.Nat. Energy. 2016; 1: 16010Crossref Google Scholar Third, preventing Li dendrite penetration is still challenging in SSBs, even for ceramic electrolytes with high mechanical strength. Fourth, despite growing research efforts to elucidate the formation of interphases between solid electrolytes and electrodes, we still do not have a complete understanding of the microstructure, dynamic behavior, and chemical reactivity of SEI during cycling. Thus, advanced characterization techniques that provide in situ and real-time information about batteries in a working environment are essential for the further development of SSBs. We define the term “interface” as a combination of interphase formation, contact condition, energy status, and defects. Physical and chemical processes that take place at an interface during cycling should be taken into consideration. These essential processes include SEI formation, dendrite growth, Li-depleted space-charge layer, and interfacial adhesion variation due to structural and volume change, among others. This section features a detailed discussion of interphase formation and different categories of interfaces and their particular characteristics. Understanding the interfacial reaction is important, as it provides critical information on the interfacial products (interphase) and their various effects on the stability and electronic performance of a SSB. The chemical stability of an electrolyte can be predicted using the energy diagram shown in Figure 2. The “window” for both liquid and solid electrolytes can be determined by the energy separation Eg between the lowest unoccupied molecular orbital (LUMO) or conducting band (CB) and the highest occupied molecular orbital (HOMO) or valence band (VB)1Goodenough J.B. Park K.-S. The Li-ion rechargeable battery: a perspective.J. Am. Chem. Soc. 2013; 135: 1167-1176Crossref PubMed Scopus (2493) Google Scholar, 26Goodenough J.B. Kim Y. Challenges for rechargeable Li batteries.Chem. Mater. 2010; 22: 587-603Crossref Scopus (4278) Google Scholar of the electrolyte material, which is thermodynamically stable when the chemical potential (μa for anode and μc for cathode) of the electrode materials is within the LUMO-HOMO range. In other words, the interface is not stable if μa > LUMO (or CB) or μc < HOMO (or VB), unless an SEI forms at the interface as the passivation layer. The formation of SEI has been widely studied in liquid-electrolyte batteries, and the concept can potentially be extended to SSBs upon careful comparison of these two very different battery systems.27Verma P. Maire P. Novak P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries.Electrochim. Acta. 2010; 55: 6332-6341Crossref Scopus (1092) Google Scholar, 28An S.J. Li J. Daniel C. Mohanty D. Nagpure S. Wood D.L. The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling.Carbon. 2016; 105: 52-76Crossref Scopus (172) Google Scholar, 29Wang A. Kadam S. Li H. Shi S. Qi Y. Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries.npj Comput. Mater. 2018; 4: 15Crossref Scopus (2) Google Scholar The interfacial reaction and the interphase products play important roles in reaching the full potential of SSBs, as Li+ ions diffuse from the bulk electrolyte mostly via Schottky vacancy and grain boundaries.30Cheng X.-B. Zhang R. Zhao C.-Z. Wei F. Zhang J.-G. Zhang Q. A review of solid electrolyte interphases on lithium metal anode.Adv. Sci. 2016; 3: 1500213Crossref Scopus (263) Google Scholar, 31Porz L. Swamy T. Sheldon B.W. Rettenwander D. Frömling T. Thaman H.L. Berendts S. Uecker R. Carter W.C. Chiang Y.-M. Mechanism of lithium metal penetration through inorganic solid electrolytes.Adv. 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First principles study on electrochemical and chemical stability of solid electrolyte-electrode interfaces in all-solid-state Li-ion batteries.J. Mater. Chem. A. 2016; 4: 3253-3266Crossref Google Scholar Importantly, their theoretical results agree with a series of recent experiments. For example, X-ray diffraction (XRD) was used to characterize Li2S at the Li/LGPS interface (LGPS stands for Li10GeP2S12) and unknown products at the acetylene black/LGPS interface.36Oh G. Hirayama M. Kwon O. Suzuki K. Kanno R. Bulk-type all solid-state batteries with 5 V class LiNi0.5Mn1.5O4 cathode and Li10GeP2S12 solid electrolyte.Chem. Mater. 2016; 28: 2634-2640Crossref Scopus (35) Google Scholar Additional studies have characterized Li3P, Li2S, and Li15Ge4 at the Li/LGPS interface, as well as the reduction product of Li3xLa2/3−xTiO3 (LLTO) and Li1+xAlxTi2−x(PO4)3 (LATP).37Wenzel S. Randau S. Leichtweiss T. Weber D.A. Sann J. Zeier W.G. Janek J. 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Interface reactions between LiPON and lithium studied by in-situ X-ray photoemission.Solid State Ionics. 2015; 273: 51-54Crossref Scopus (46) Google Scholar Quite recently, XPS was also used to confirm that the once-believed extremely stable Li7La3Zr2O12 (LLZO) was reactive with Li at 300°C–350°C.40Wolfenstine J. Allen J.L. Read J. Sakamoto J. Chemical stability of cubic Li7La3Zr2O12 with molten lithium at elevated temperature.J. Mater. Sci. 2013; 48: 5846-5851Crossref Scopus (32) Google Scholar Additional studies conducted by Wenzel et al. using in situ XPS determined that (1) the decomposed Li10GeP2S12 interphase consists of Li3P, Li2S, and Li-Ge alloy,37Wenzel S. Randau S. Leichtweiss T. Weber D.A. Sann J. Zeier W.G. Janek J. Direct observation of the interfacial instability of the fast ionic conductor Li10GeP2S12 at the lithium metal anode.Chem. Mater. 2016; 28: 2400-2407Crossref Scopus (110) Google Scholar and (2) decomposed LLTO interphase consists of Ti3+, Ti2+, and Ti metal.41Wenzel S. Leichtweiss T. Krüger D. Sann J. Janek J. Interphase formation on lithium solid electrolytes—an in situ approach to study interfacial reactions by photoelectron spectroscopy.Solid State Ionics. 2015; 278: 98-105Crossref Scopus (0) Google Scholar A detailed discussion of several characterization techniques is included in the section “Advanced Characterization Techniques”. It is worth noting that some sulfide materials have been found to exhibit both excellent Li+ ion conductivity and electrochemical stability—principally Li9.54Si1.74P1.44S11.77Cl0.332Hu Y.-S. Batteries: getting solid.Nat. Energy. 2016; 1: 16042Crossref Google Scholar, 42Kato Y. Hori S. Saito T. Suzuki K. Hirayama M. Mitsui A. Yonemura M. Iba H. Kanno R. High-power all-solid-state batteries using sulfide superionic conductors.Nat. Energy. 2016; 1: 16030Crossref Google Scholar and Li9.6P3S12.42Kato Y. Hori S. Saito T. Suzuki K. Hirayama M. Mitsui A. Yonemura M. Iba H. Kanno R. High-power all-solid-state batteries using sulfide superionic conductors.Nat. Energy. 2016; 1: 16030Crossref Google Scholar A further understanding of the nature and behavior of interphase materials, including their formation mechanisms, conductivity for electrons/ions, mechanical strength, and crystal structure is absolutely essential for advancing the science of battery chemistry. Unlike metal anodes, cathode materials are composed of particles of active materials such as LiCoO2 (LCO), LiMn2O4 (LMO), LiNiMnCoO2, LiNiCoAlO2, and LiFePO4 that are mixed with appropriate amounts of solid electrolytes (ionic conductors), carbon (electronic conductors) and/or binders to enhance the ionic and electronic transport. In liquid-electrolyte batteries, the liquid electrolyte wets the gaps between the cathode particles as well as the gaps between the electrode and electrolyte; in contrast, physical contact is expected for solid-solid interfaces in SSBs. To clarify the effects of each interface, Luntz et al.43Luntz A.C. Voss J. Reuter K. Interfacial challenges in solid-state Li ion batteries.J. Phys. Chem. Lett. 2015; 6: 4599-4604Crossref PubMed Scopus (111) Google Scholar employed a symmetric cell to evaluate the internal resistance (IR) drop at the cathode-electrolyte interface and anode-electrolyte interface. Their assessment scheme and results are shown in Figures 3A–3D . In their experiment, an Au/SSE/Au cell (SSE refers to solid-state electrolyte) served as the baseline for their IR drop measurements. The SSE featured LiPON with low reduction voltage and Li+ conductivity at a magnitude of 10−5–10−6 S cm−1. For the Li/SSE/Li cell, a slight IR drop was evidenced, confirming an interface resistance between Li and SSE. However, when they combined SSE with carbon cathodes (Figures 3C and 3D), the IR drop increased much more significantly in comparison with analogous results for Li, indicating a much higher interfacial resistance. The IR drop on the Li/SSE interface could be attributed to the formation of SEI and an incomplete contact; for the cathode-electrolyte interface, however, the electrochemical processes can be much more complicated. Although their results do shed light on the different contributions of interface resistance at the cathode and anode, they only considered carbon cathodes in reporting that the highest interface resistance occurs at the cathode-electrolyte interface. Several kinds of interfacial resistance can coexist at the same time. One type of resistance is contact resistance, which results from poor interfacial contact or contact loss during cycling44Ohta S. 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