The origin of high electrolyte–electrode interfacial resistances in lithium cells containing garnet type solid electrolytes

Volatility of lithium during preparation of lithium-stuffed garnet-type metal oxide solid Li ion electrolytes is a common problem, which affects phase formation, ionic conductivity, mechanical strength and density. Synthesis of Li-stuffed garnets has been performed generally using the conventional solid-state reactions at elevated temperature in air. The present study describes the effect of excess LiNO 3 (2.5 to 15 wt.%) addition during the ceramic synthesis on the structural and electrical properties of garnet-type Li 6 La 3 Ta 1.5 Y 0.5 O 12 . Powder X-ray diffraction (PXRD) confirmed that cubic phase was formed in all tested cases, and there is no significant variation in lattice parameter with amount of excess LiNO 3 used. However, increasing amounts of excess lithium decreased inter-particle contact and increased grain growth during sintering, producing sharply varied microstructures. PXRD showed no secondary phase and scanning electron microscopy (SEM) analysis showed rather uniform morphology and absence of "glassy" materials at the grain-boundaries. The bulk Li ion conductivity was found to increase with amount of excess lithium, reaching a maximum room temperature conductivity of 1.62 × 10 −4 Scm −1 for the sample prepared using 10 wt.% excess LiNO 3 . Raman microscopy study indicated the presence of Li 2 CO 3 in all aged Li 6 La 3 Ta Li-stuffed garnet-type metal oxides have been considered as a potential candidate solid electrolyte material to replace conventional organic liquid based Li ion conducting electrolytes in Li-ion batteries, since members of garnet solid electrolytes exhibit high bulk ion conductivity of 10 −4 -10 −3 Scm −1 at room temperature. Furthermore, some of the garnet-type electrolytes show excellent chemical stability against reaction with elemental Li, Li-ion insertion or intercalation cathodes and high electrochemical stability window.1,2 However, the synthesis of high bulk Li ion conducting garnet-type metal oxidebased lithium ion conductors is a challenging process, very sensitive to preparation conditions used, that are not yet fully characterized. For example, Li 7 La 3 Zr 2 O 12 calcined at 1230• C results in highly conducting (7 × 10 −4 Scm −1 at room temperature) cubic phase with a space group Ia-3d whereas a calcining temperature of 980• C leads to tetragonal phase with a space group I4 1 /acd, which showed 2 orders of magnitude less conductive than cubic phase.3,4 Traditional solid-state techniques are the most commonly used to synthesize garnet-type metal oxides, but usually require the highest sintering temperature resulting in the production of large particles. [5][6][7][8] It is known that volatilization of lithium occurs in the furnace, therefore, several researchers commonly compensate with a 10 wt.% excess lithium precursors.3,9,10 Al-doped Li 7 La 3 Zr 2 O 12 prepared without adding any excess lithium salts during synthesis, but varying the processing using powder cover condition for sintering, greatly affects the morphology, grain size and density. 11Attempts h...

Section: Resultsmentioning

confidence: 99%

Effect of Excess Li on the Structural and Electrical Properties of Garnet-Type Li₆La₃Ta_1.5Y_0.5O₁₂

Narayanan

Hitz

Wachsman

et al. 2015

“…In addition, it has been reported that the Li 2 CO 3 layer results in the large chargetransfer resistance at the interface between the surface of oxide solid electrolyte such as LLZO and the lithium metal anode. 33 Therefore, it is common to remove Li 2 CO 3 from the surface of cathode and solid electrolyte material by post treatments such as heating, washing and polishing. [32][33][34] However, in this study, it is observed in the ASS-LIB system using sulfide solid electrolytes that the Li 2 CO 3 acts as an effective coating material rather than an impurity phase, which results in suppression of the interfacial reaction by formation of a physical barrier.…”

Section: Resultsmentioning

confidence: 99%

Electrochemical Properties of Li_1+xCoO₂Synthesized for All-Solid-State Lithium Ion Batteries with Li₂S-P₂S₅Glass-Ceramics Electrolyte

Kim

Park

et al. 2015

The over-stoichiometric Li 1+x CoO 2 (x = 0.1, 0.2, and 0.3) cathode materials were synthesized from an aqueous solution of lithium nitrate (LiNO 3 ) as an excess lithium precursor and their effects on the electrochemical properties of all-solid-state lithium batteries employing Li 2 S-P 2 S 5 glass-ceramics solid electrolytes are investigated. A combination of X-ray diffraction, Fourier transform infrared spectroscopy, and inductively coupled plasma atomic emission spectroscopy reveals that the excess lithium forms a residual Li 2 CO 3 layer on the surface of as-prepared Li 1+x CoO 2 particles during the synthesis process. While regarded as an impurity phase in lithium battery systems using liquid electrolytes due to its detrimental effects on electrochemical performance, the Li 2 CO 3 formed on the surface of the over-stoichiometric Li 1+x CoO 2 powders is identified in all-solid-state lithium battery systems using sulfide solid electrolytes to act as an effective coating material to suppress the interfacial side reactions despite its low ionic and electronic conductivity. All-solid-state lithium ion batteries (ASS-LIBs) including inorganic solid electrolytes are consistently at the center of attention in the discussion on safety issues arising from the flammability of liquid electrolytes in conventional LIBs. ASS-LIB technology has come a long way in the last few decades through the development of oxide and sulfide solid electrolytes with practical levels of high ionic conductivities comparable to organic liquid electrolytes.1-4 Therefore, the rate determining step is now no longer Li + ion migration in the solid electrolyte and overall cell resistance is determined mainly at the interface between the cathode active material and solid electrolyte. However, ASS-LIBs employing oxide and sulfide solid electrolytes have some crucial interfacial problems resulting in low reversible capacity and capacity fade for practical applications, due to limited contact area and undesirable interfacial side reactions between the cathode active material and the oxide/sulfide solid electrolyte. 5,6 Many studies are in progress to understand and resolve the interfacial side reaction problem responsible for this capacity loss in order to develop ASS-LIBs with high energy density and long cycle life required for electric vehicles. Several researchers have suggested that the origin of the interfacial degradation is mutual diffusion of chemical species between solid electrolyte and cathode, 5,7 or the formation of a space-charge layer. 8,9 When garnet-structured Li 7 La 3 Zr 2 O 12 (LLZ) is contacted with LiCoO 2 , mutual diffusion of Co and La, Zr occurs at the interface during a thermal treatment to improve adhesion between the cathode material and the oxide solid electrolyte.10 Problems regarding this mutual diffusion of Co and S have been also reported in ASS-LIBs with Li 2 S-P 2 S 5 solid electrolytes, especially during initial charging process. At present, the fundamental mechanism is not clear, but interfacial control via sur...

“…X-ray photoelectron spectroscopy studies show that a post-sintering process to create a pristine mating surface to the Li metal, such as polishing and processing of sintered bodies in a controlled Ar atmosphere, can be significant in reducing Li ion interfacial resistance due to surface carbonate formation. 51 Meeting thickness requirements with oxide SSEs.- Figure 3 illustrates potential paths to forming LLZO having thickness <100 μm. PVD growth techniques such as sputtering 58 or PLD 59,60 have been demonstrated at research scale, but pose concerns from a scalability and cost perspective.…”

mentioning

confidence: 99%

“…48,49 Similarly, reaction with the solvent may produce ionically insulating surface layers, such as carbonates, on particles in solution, complicating subsequent sintering. [50][51][52] The successful sintering of metal oxides to high density requires control of multiple process parameters, and processes must typically be tuned to the specific compound being sintered. 53 Here, factors of importance based on LLZO are highlighted.…”

mentioning

confidence: 99%

Review—Practical Challenges Hindering the Development of Solid State Li Ion Batteries

Kerman

Luntz

Viswanathan

et al. 2017

552

464

Solid state electrolyte systems boasting Li + conductivity of >10 mS cm −1 at room temperature have opened the potential for developing a solid state battery with power and energy densities that are competitive with conventional liquid electrolyte systems. The primary focus of this review is twofold. First, differences in Li penetration resistance in solid state systems are discussed, and kinetic limitations of the solid state interface are highlighted. Second, technological challenges associated with processing such systems in relevant form factors are elucidated, and architectures needed for cell level devices in the context of product development are reviewed. Specific research vectors that provide high value to advancing solid state batteries are outlined and discussed. Solid state battery systems are of great interest because of potential benefits in gravimetric and volumetric energy density, operable temperature range, and safety in comparison to traditional liquid electrolyte based systems. However, unresolved fundamental issues remain in the quest to fully understand the behavior of all-solid batteries, especially in the area of electrochemical interfaces.1 There are also a number of significant engineering challenges that require methodical effort to enable a tangible product. Some transitions from academic laboratories to entrepreneurial efforts attempting to overcome these challenges remain unsuccessful in efforts to bring a product to the market.2,3 Vital parameters that require robust understanding from a product development standpoint are material cost, cell lifetime and shelf life, cell energy density on a volumetric and gravimetric basis, operable capabilities for given temperature conditions, and safety. The advantage of energy density remains to be realized in solid state electrolytes (SSEs) since most studies to date utilize thick SSEs or cathodes with low active loading compared to liquid counterparts. 4,5 Furthermore, the desire to use SSEs in conjunction with Li metal anodes requires understanding and managing the morphology of Li metal plating, which can impact volumetric energy density. Operation at both higher and lower temperature compared to conventional technologies is a significant potential advantage of SSE systems. However, reports of solid state cells achieving parity with traditional systems at room temperature or any other temperature do not currently exist. The safety, specifically decreased flammability, of SSE systems is another potential advantage but requires ongoing validation and study.6 Unlike current liquid electrolyte systems, 7 the manufacturability and material component costs of SSEs have not been well characterized, and thus the value of these features will need to be weighed accordingly with any added cost. Operating lifetime of SSEs capturing intrinsic materials parameters such as voltage stability, 8 as well as catastrophic failure modes such as shorting, 9 have been briefly investigated, but in the absence of high energy density electrode formulations and appli...