http://pubs.acs.org/journal/aelccp
Ameliorating Interfacial Ionic Transportation in All-Solid-State Li-Ion Batteries with Interlayer Modifications Anirudha Jena,†,‡ Yedukondalu Meesala,† Shu-Fen Hu,*,∥ Ho Chang,*,‡ and Ru-Shi Liu*,†,‡ †
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 106, Taiwan ∥ Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan
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‡
ABSTRACT: Li-ion batteries (LIBs) are a class of electrochemical energy storage devices widely adapted for their versatile use. Commercialized liquid electrolyte-based batteries are developing various issues like explosions, limited energy density, and leakage. All-solid-state batteries (ASSBs) with Li-ion-containing solid electrolytes (SEs) can be a solution to these shortcomings. However, assembling ASSBs is a challenge due to the high interfacial resistance between the electrodes and SEs. In the current Review, we addressed the rising concern over the interfacial deterioration leading to high charge-transfer resistance. A comprehensive discussion on the addition of buffer layers between the SE and electrodes is presented to improve interfacial stability. From polymer layers containing Li-salts with and without supporting fillers to amorphous oxides and metal coating, the interlayers ameliorate the ionic transport. Mutual compression and cosintering of SEs and electrodes can make a compact interface. Finally, the influence of morphology at the contacting surfaces is discussed.
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separator containing Li-ions. During the battery cycle, the Li-ions shuttle between the positive and negative electrodes through the separator, as shown in Figure 1a. In general, the positive electrode, i.e.,the cathode, lies above 2 V vs Li+/Li, and the negative electrode, i.e., the anode, lies below 4 V vs Li+/Li.7 Li-ions from the cathode are extracted during charging and inserted into the anode; in subsequent discharge, the Li-ions revert back into the cathode to complete the cycles. Such a movement of Li-ions between both electrodes is driven by the energy barriers within the electrodes and electrolyte. Schematic representation of the energy levels of battery components, i.e., electrodes and electrolytes for a thermodynamically stable cell, is given in Figure 1b. The gap existing between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the electrolyte, Eg, is the “working potential window” of the electrolyte. If the electrochemical potentials μA of the anode lies at higher levels than the LUMO of the electrolyte, it will be decomposed unless a buffer layer is created between them. This interphase layer acts as a barrier for electron transfer between the anode and electrolyte to suppress electrolyte reduction. In a similar way, a passivation layer between the
echargeable Li-ion batteries (LIBs) have brought revolution to the field of energy storage with its stable cycle performance and compatibility from small-scale devices to large-scale energy storage systems. The source of ignition, long run of future electric vehicles (EVs), hybrid vehicles (HVs), and plug-in electric vehicles (PEVs) rely on the energy density of the rechargeable batteries.1,2 In an attempt for a healthier planet, developing ecofriendly transportation to minimize CO2 emission cannot be approached without LIBs in EVs and PEVs.3 Energy density has been the issue in the case of LIBs for its full implementation in EVs and smart grids. With the rise in energy density of 3 Wh/kg per year until the year 1990 for secondary batteries, the target of 500 Wh/kg still can be achieved as the rate has been improved to 5.5 Wh/kg in the last 2 decades.4 This accelerated growth in energy density is credited to recent developments in LIBs. From the presently developed LIBs using Li1−xCoO2 (LCO) vs graphite (LixC6) as an electrode, technological advancements are still underway to develop a new class of batteries, e.g., Li−sulfur and Li−air (LIA). However, the latter two systems are yet to be commercialized. Hence, LIBs are still the most efficient energy storage devices that are sustainable and clean. Several global ventures are competing in the bid for incorporation of LIBs in their next-generation breeds of small-scale devices and HVs.5,6 In a typical LIB, the positive (a Li-ion-containing material) and negative (material that can accommodate the Li-ions using different mechanisms) electrodes are sandwiched by the © XXXX American Chemical Society
Received: August 23, 2018 Accepted: October 5, 2018 Published: October 5, 2018 2775
DOI: 10.1021/acsenergylett.8b01564 ACS Energy Lett. 2018, 3, 2775−2795
Review
Cite This: ACS Energy Lett. 2018, 3, 2775−2795
ACS Energy Letters
Review
Figure 1. Schematic representation of (a) Li-ion shuttling through the separator between the cathode and anode during battery cycles. (b) Relative energy levels of the electrodes and electrolytes in a thermodynamically stable battery cell. Adapted with permission from ref 8. Copyright 2010, American Chemical Society.
Interfacial Phenomena at the Electrode−Electrolyte Contact. The potential at the electrode−electrolyte interface is strongly influenced by the surface characteristics of the electrode material and its response to the electrolyte.11,12 Understanding the interfacial phenomena at the electrode−electrolyte contact is crucial.13 When two distinct conductors are placed in contact, there occurs a charge separation at the interface. Regardless of their medium, whether solid−solid, solid−liquid, or solid−gas, the interface remains dynamic as long as the conductors are in contact. In the case of ASSBs, for the solid
electrolyte HOMO and cathode can suppress oxidation of the cathode. Therefore, in an ideal situation, the electrochemical potentials μC of the cathode should not lie below the HOMO of the electrolyte. Hence, the completeness of the cell and its thermodynamic stability need positioning of the electrode parameters like μA and μC between Eg, giving a definition to the opencircuit potential Voc of the cell, Voc = (μA − μC)/e, where e is the electronic charge.8 Such potential barriers are inescapable irrespective of the nature of the electrolyte, whether solid or liquid. Zhu et al. have performed first-principles calculations to understand the thermodynamic and kinetic factors influencing the interface between solid electrolytes (SEs) and electrode materials by considering the chemical potential values of the contacting surface and the degradation products.9 In the case of rechargeable secondary LIBs, overall cell resistance can be curbed by reducing the thickness between the cathode and anode with a highly ionically conducting separator. The contact surface on both sides of the separator experiences either charge accumulation or depletion due to a mismatch between the μA/μC of electrodes and the LUMO/HOMO of the electrolyte. The passivation layer so-called solid electrolyte interphase (SEI) is formed due to side reactions of the liquid electrolyte with electrodes. The SEI is ionically conducting with a low electrical conduction pathway.10 The battery operating circuit gets completed by ionic transfer through the SEI and electrode. Large-scale implementation of batteries in HVs calls for strategic improvements in the battery components, e.g., electrolytes, stable electrodes, highperformance conductive additives/binders/current collectors, and efficient packaging are most important. Among these approaches, the electrolyte holds the key to the operating potential window of batteries. The state-of-the-art electrolytes mainly consist of lithium salts and organic solvents. Therefore, they cause irreversible capacity losses resulting from the formation of a stable SEI, hindering the increase in cycle life, limiting the temperature window, and posing severe safety concerns for batteries. Converting the cell components to the solid state is an alternative to address these safety issues. Allsolid-state batteries (ASSBs) have no issues like electrolyte leakage and flammability and can be accommodated in largescale to small-scale electronics. However, large interfacial resistance is still a concern. In this Review, we have addressed several aspects to minimize the interfacial problems and to solve the issue.
Regardless of their medium, whether solid−solid, solid−liquid, or solid−gas, the interface remains dynamic as long as the conductors are in contact. Li-ionic conductors, upon assembling with Li-ion-containing electrode materials, such a dynamic interface plays a vital role in transferring charge between the electrode and electrolyte. In such an instance, the imbalance of Li-ion distribution at the electrode−electrolyte interface is due to uneven diffusion of the Li-ion between the electrode and electrolyte. Also, during battery cycles, the formation of discharge products and loss of contact at the interface due to volume change further deteriorated at the interface. A schematic of such a charge imbalance is shown in Figure 2a. At the electrode−electrolyte interface, the mobile charge carriers establish an equilibrium. The chemical potential of the components in the bulk of both the electrode and electrolyte varies to that of the interface. Depending on the potential of the contact surfaces, the charge may either get accumulated or may be depleted. As in the case of SEs, with the Li-ion being the sole charge carrier, the stoichiometry at the interface can be altered. In the case of mixed conduction, i.e., both electronic and ionic, the interfacial phenomena are not straightforward and become further complicated. The presence of pores, grain boundaries, and defect states within the SEs makes the determination of the local ionic distribution further cumbersome. Furthermore, there occur structural adjustments at the interface due to uneven charge distribution that change the elastic properties of the SEs in the bulk. The operation of batteries using 2776
DOI: 10.1021/acsenergylett.8b01564 ACS Energy Lett. 2018, 3, 2775−2795
ACS Energy Letters
Review
Figure 2. (a) Distribution of charges at the electrified interface and defect states in SE. (b) Interaction of ions with the electrode , resulting in uninterrupted formation of dendritic structures.
carriers tending to equilibrate with the bulk and the blocking interface without any equilibrium of charges.24 However, the nonblocking nature of the interface appears like a blocking type in the case of a very low exchange current (1000 °C), such space-charge relaxes out due to enhancement of the ionic mobility. Crystalline materials, e.g., SEs, create space-charge regions along with the adjoining crystals, influencing the ionic mobility with the induction of structural and chemical disorder. Such a depletion in charge carriers results in low grain boundary conductivity in SEs.23 In-depth exploration of the charge-transfer dynamics at the electrified interface brings up two distinct situations: the nonblocking interface with the contacting surface charge
SEs create space-charge regions along with the adjoining crystals, influencing the ionic mobility with the induction of structural and chemical disorder. 2777
DOI: 10.1021/acsenergylett.8b01564 ACS Energy Lett. 2018, 3, 2775−2795
ACS Energy Letters
Review
Figure 3. (a) Road map and challenges in the development of all-solid-state batteries. (b) Current and future targets of solid-state batteries and their cell characteristics. Reprinted from ref 29. Copyright 2014, with permission from Elsevier.
of Li0, as a result of which the Li-dendrite propagates within the garnet SE. Dark spots in the electron-backscattered images as shown in Figure 4b,c further confirm the metallic Li. Such a contrast by the backscattering electrons generally formed due to the presence of lighter elements on the surface. The mechanism of dendrite formation and their growth are shown in Figure 4d. The Li-dendrites pass through the SE from the anode side to the cathode side and lower the cell resistance. The abrupt spikes in the voltammogram suggest that the Li-dendrites are not continuously growing; rather, a “dendritic arc” produces cracks in the SE and hence results in battery failure. Cheng et al. have directly visualized the growth of Li-dendrites in gel electrolyte using stimulated Raman scattering microscopy (SRS).34 With the help of SRS imaging, the charge accumulation or depletion as well as the growth of Li-dendrite can be seen. A positive feedback loop of dendrite formation and Li heterogeneity are also discussed. Richards et al. have developed computational methodology to understand the interfacial products between the Li metal and a combination of electrode materials.35 Utilizing the concept of anodic and cathodic stability due to Li-ion transfer at the interface, the chemical potential values, Li-ion concentration, and phase diagram have been constructed and related to the crystal structure. Not only does contact with Li metal deteriorate the SE−electrode interface, but the presence of moisture also has an adverse effect on the surface of SEs. Brugge et al. have studied the impact of water on Ga-doped LLZO and observed the change in transport properties of SEs influenced by the change in the chemical properties at the interface.36 Sharafi et al. have studied the contaminated layer on the LLZO surface due to air exposure.37 The LLZO surface in contact with moist air leads to the formation of protonated and carbonated species on the surface. The carbonated species concentration goes down to 40 nm into the bulk. Design of electrode materials at both cathode and anode is a key challenge for batteries with high energy density.38 However, the role of the electrolyte in this regard is large. Conventional LIBs with nonaqueous liquid electrolytes have reached their limit of ∼4 V, compared to that of aqueous electrolyte, still limiting its operation to ∼2 V. Considering the currently available electrodes and electrolytes, the commercial LIBs in the present scenario have delivered gravimetric energy densities of
