Lithium garnet Li 7 La 3 Zr 2 O 12 (LLZO) is being investigated as a potential solid electrolyte for next-generation solidstate batteries owing to its high ionic conductivity and electrochemical stability against metallic lithium and high potential cathodes. While the LLZO/Li metal anode interface has been thoroughly investigated to achieve almost negligible interface resistances, the LLZO/cathode interface still suffers from high interfacial resistances mainly due to the high-temperature sintering required for proper ceramic bonding. In this work, the LLZO solid electrolyte/LiCoO 2 (LCO) cathode interface is investigated in an all-thin-film model system. This architecture provides an easy access to the interface for in situ and ex situ characterization, allowing one to identify the degradation processes taking place under high-temperature cosintering and to test solutions such as interface modifications. Introducing an in situ-lithiated Nb 2 O 5 diffusion barrier at the interface, we were able to lower the LLZO/LCO charge transfer resistance to about 50 Ω cm 2 , a 3-fold reduction with respect to previously reported values. The low interfacial resistance combined with the high conductance through the LLZO thin-film electrolyte allows one to investigate the charge transfer at high charge−discharge rates, unlike in bulk systems. At 1C, discharge capacities of about 140 mA h g −1 were measured, and at 10C, 60% of the theoretical capacity was retained with a cycle life over 100 cycles. Besides the role of this architecture in the interface investigation, this work also constitutes a milestone in the development of thin-film solid-state batteries with higher power densities.

reversible capacity of current cathodes is lower than that of conventional graphite anodes (372 mAh g -1 ) and far behind that of high-capacity anodes such as lithium metal (3860 mAh g -1 ) and silicon (4200 mAh g -1 ). [2,3] Simultaneously, the cathode is the main contributor to the material cost at the cell level. [4] Currently, LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) is gaining commercial adoption due to its capacity of ≈200 mAh g -1 assuming one Li-ion per formula unit and low content of expensive cobalt. [5] Two intriguing research questions stand out: i) Is it possible to increase the cathodic discharge capacity by introducing additional lithium into the host NMC811 structure? ii) Can we control the reactivity of Ni-rich cathode material with the electrolyte and prevent the formation of a solid electrolyte interphase (SEI) layer and consequent low capacity retention? [6,7] NMC cathodes can actually store more than one lithium ion per formula unit. This phenomenon is by no means a recent discovery, and the first reports of Li-rich NMC-type layered oxide cathodes date back to 2003. [8] When discussing Li-rich layered cathode materials, it is important to differentiate between Mn-based and Ni-based compositions. Most of the investigated Li-rich NMC compounds are based on Mn as main redox-active transition metal. [9,10] This allows for excess lithium to be stored in a Li 2 MnO 3 composite structure embedded in Li(Ni, Mn, Co)O 2 , where oxygen atoms are involved in the redox process above 4.5 V. [11] Such Among cathode materials, LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) is the most discussed for high performance Li-ion batteries, thanks to its capacity of ≈200 mAh g -1 and low Co content. Here, it is demonstrated that NMC811 can reversibly accommodate more than one Li-ion per formula unit when coupled with a solid-state electrolyte, thus significantly increasing its capacity. Sputtered Li-rich NMC811 cathodes are tested with lithium-phosphorus-oxynitride as a solidstate electrolyte in a thin-film architecture, which is a simplified 2D model with direct access to the cathode-electrolyte interface. The solid-state electrolyte helps to stabilize the interface and prevents capacity fading, voltage decay, and interface resistance growth, thus allowing cycling at extended voltage ranges of 1.5-4.7 V. While the liquid electrolyte cells suffer from rapid capacity decay, the Li-rich NMC811 cells with the solid-state electrolyte can cycle at a fast rate and an initial capacity of 149 mAh g -1 from 1.5 to 4.3 V for 1000 cycles. The all-solidstate thin-film cells with a lithium metal anode yield a discharge capacity of up to 350 mAh g -1 at C/10 because of multi-electron cycling with a coulombic efficiency of 90.1%. The results demonstrate how solid-state electrolytes that are stable against NMC811 cathodes can unlock the full potential of this Li-rich and Ni-rich cathode class.

Protective coatings applied to cathodes help to overcome interface stability issues and extend the cycle life of Li-ion batteries. However, within 3D cathode composites it is difficult to isolate the effect of the coating because of additives and non-ideal interfaces. In this study we investigate niobium oxide (NbO x ) as cathode coating in a thin-film model system, which provides simple access to the cathode-coating-electrolyte interface. The conformal NbO x coating was applied by atomic layer deposition (ALD) onto thin-film LiCoO 2 cathodes. The cathode/coating stacks were annealed to lithiate the NbO x and ensure sufficient ionic conductivity. A range of different coating thicknesses were investigated to improve the electrochemical cycling with respect to the uncoated cathode. At a NbO x thickness of 30 nm, the cells retained 80% of the initial capacity after 493 cycles at 10 C, more than doubling the cycle life of the uncoated cathode film. Elemental analysis using TOF-SIMS and XPS revealed a bulk and surface contribution of the NbO x coating. These results show that in situ lithiated ALD NbO x can significantly improve the performance of layered oxide cathodes by enhancing interfacial charge transfer and inhibiting surface degradation of the cathode, resulting in better rate performance and cycle life.

Crystallization of cathode films in solid-state microbatteries requires thermal annealing at high temperatures, restricting the choice of substrate and current collector materials. Here, flash lamp annealing (FLA) is explored to crystallize LiCoO 2 (LCO) cathodes on aluminum foils. Millisecond pulses of visible light induce rapid heating of the LCO films up to 900 °C, whereas the aluminum never exceeds the melting point. Microbatteries consisting of an FLA-processed LCO cathode, a LiPON electrolyte, and a Li metal anode are fabricated on flexible aluminum foil, with performance comparable to those on rigid silicon. This method can enable new microbattery designs at lower production costs.

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The sequential production process of thin-film solid-state batteries (TF-SSB) requires high temperatures up to 700 °C in order to achieve crystallized cathode films with high capacities and ceramic electrolytes with...

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