Visualization of the Interfacial Decomposition of Composite Cathodes in Argyrodite-Based All-Solid-State Batteries Using Time-of-Flight Secondary-Ion Mass Spectrometry

160

104

While Ni‐rich cathode materials combined with highly conductive and mechanically sinterable sulfide solid electrolytes are imperative for practical all‐solid‐state Li batteries (ASLBs), they suffer from poor performance. Moreover, the prevailing wisdom regarding the use of Li[Ni,Co,Mn]O2 in conventional liquid electrolyte cells, that is, increased capacity upon increased Ni content, at the expense of degraded cycling stability, has not been applied in ASLBs. In this work, the effect of overlooked but dominant electrochemo‐mechanical on the performance of Ni‐rich cathodes in ASLBs are elucidated by complementary analysis. While conventional Li[Ni0.80Co0.16Al0.04]O2 (NCA80) with randomly oriented grains is prone to severe particle disintegration even at the initial cycle, the radially oriented rod‐shaped grains in full‐concentration gradient Li[Ni0.75Co0.10Mn0.15]O2 (FCG75) accommodate volume changes, maintaining mechanical integrity. This accounts for their different performance in terms of reversible capacity (156 vs 196 mA h g−1), initial Coulombic efficiency (71.2 vs 84.9%), and capacity retention (46.9 vs 79.1% after 200 cycles) at 30 °C. The superior interfacial stability for FCG75/Li6PS5Cl to for NCA80/Li6PS5Cl is also probed. Finally, the reversible operation of FCG75/Li ASLBs is demonstrated. The excellent performance of FCG75 ranks at the highest level in the ASLB field.

Section: Resultssupporting

confidence: 89%

Ni‐Rich Layered Cathode Materials with Electrochemo‐Mechanically Compliant Microstructures for All‐Solid‐State Li Batteries

Jung

Kim

et al. 2019

160

104

“…During the TOF‐SIMS measurement, the initiated secondary ions have a strong correlation with chemical bonding, and thus, the detected fragments could suggest the chemical environment of the composite. [ 41 ] As shown in Figure 3h, the observation of different CS fragments (CS − at m / z = 44, C 2 S − at m / z = 56, and C 4 S − at m / z = 80) verify the assumption that sulfur is covalently bonded to carbon. In addition, the intense signal of CNS − ( m / z = 58) indicate that a large amount of sulfur is bonded to the carbon atoms beside nitrogen.…”

Section: Resultsmentioning

confidence: 60%

Covalent Encapsulation of Sulfur in a MOF‐Derived S, N‐Doped Porous Carbon Host Realized via the Vapor‐Infiltration Method Results in Enhanced Sodium–Sulfur Battery Performance

Xiao

Yang

Wang

et al. 2020

129

Practical applications of room temperature sodium–sulfur batteries are still inhibited by the poor conductivity and slow reaction kinetics of sulfur, and dissolution of intermediate polysulfides in the commonly used electrolytes. To address these issues, starting from a novel 3D Zn‐based metal–organic framework with 2,5‐thiophenedicarboxylic acid and 1,4‐bis(pyrid‐4‐yl) benzene as ligands, a S, N‐doped porous carbon host with 3D tubular holes for sulfur storage is fabricated. In contrast to the commonly used melt‐diffusion method to confine sulfur physically, a vapor‐infiltration method is utilized to achieve sulfur/carbon composite with covalent bonds, which can join electrochemical reaction without low voltage activation. A polydopamine derived N‐doped carbon layer is further coated on the composite to confine the high‐temperature‐induced gas‐phase sulfur inside the host. S and N dopants increase the polarity of the carbon host to restrict diffusion of sulfur, and its 3D porous structure provides a large storage area for sulfur. As a result, the obtained composite shows outstanding electrochemical performance with 467 mAh g−1 (1262 mAh g−1(sulfur)) at 0.1 A g−1, 270 mAh g−1 (730 mAh g−1(sulfur)) after 1000 cycles at 1 A g−1 and 201 mAh g−1 (543 mAh g−1(sulfur)) at 5.0 A g−1.

“…The oxidation of S 2− in thiophosphate electrolytes has also been observed in previous X-ray photoelectron spectroscopy studies. [31,32] It should be pointed out that while the key findings presented in this study are based on an SSB system, the electrochemical stability of a coating material is relevant in liquid electrolytes as well. LZrO will be prone to Li extraction under high voltage in both liquid and solid electrolyte.…”

Section: Discussionmentioning

confidence: 97%

Direct Visualization of the Interfacial Degradation of Cathode Coatings in Solid State Batteries: A Combined Experimental and Computational Study

Zhang

Tian

Xiao

et al. 2020

known inorganic solid electrolytes (SEs), thiophosphates offer the advantage of combining high ionic conductivity with low cost and ease of processing, making them strong candidates for large-scale applications. [3,4] However, both ab initio computations and experimental studies indicate that thiophosphate SEs and commonly used oxide cathode materials are not stable in contact with each other. [5-9] This interfacial instability leads to increased resistance during cycling, [7,10-12] resulting in capacity fade and poor rate performance, indicating the need for interface engineering in SSBs. To suppress the reaction between the cathode and thiophosphate SEs, various cathode coating materials have been developed to serve as a buffer layer and prevent direct contact between the cathode and SE. [13,14] Most investigations on cathode coatings in thiophosphate-based SSBs have focused on exploring ternary metal oxides [13] such as LiNbO 3 , [12,15,16] LiTaO 3 , [17] and Li 2 ZrO 3 (LZrO). [18,19] However, such coating materials do not completely solve the interfacial issue between the cathode and SE. For example, despite the reduced interfacial resistance and enhanced capacity retention achieved in SSBs using LiNb x Ta 1−x O 3 coatings, Co diffusion from the LiCoO 2 cathode to the thin film coating is still observed. [12,20] First-principles computation also indicates that the high binding energy of a PO 4 group creates a driving force for S/O exchange between LiNbO 3 , LiTaO 3 , and The interfacial instability between a thiophosphate solid electrolyte and oxide cathodes results in rapid capacity fade and has driven the need for cathode coatings. In this work, the stability, evolution, and performance of uncoated, Li 2 ZrO 3-coated, and Li 3 B 11 O 18-coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathodes are compared using first-principles computations and electron microscopy characterization. Li 3 B 11 O 18 is identified as a superior coating that exhibits excellent oxidation/chemical stability, leading to substantially improved performance over cells with Li 2 ZrO 3-coated or uncoated cathodes. The chemical and structural origin of the different performance is interpreted using different microscopy techniques which enable the direct observation of the phase decomposition of the Li 2 ZrO 3 coating. It is observed that Li is already extracted from the Li 2 ZrO 3 in the first charge, leading to the formation of ZrO 2 nanocrystallites with loss of protection of the cathode. After 50 cycles separated (Co, Ni)-sulfides and Mn-sulfides can be observed within the Li 2 ZrO 3-coated material. This work illustrates the severity of the interfacial reactions between a thiophosphate electrolyte and oxide cathode and shows the importance of using coating materials that are absolutely stable at high voltage.