Heuristics for Molten-Salt Synthesis of Single-Crystalline Ultrahigh-Nickel Layered Oxide Cathodes

Abstract

In pursuit of Li-ion batteries with higher energy density, ultrahigh-nickel layered oxides are a leading candidate for next-generation cathode materials. Single-crystalline morphology offers a neat solution to the poor stability of ultrahigh-Ni cathodes; a lower active surface area mitigates electrolyte decomposition at high voltages, and the elimination of grain boundaries improves mechanical resilience and increases volumetric energy density. However, single-crystal cathodes possess their own challenges, several of which originate from synthesis at elevated temperatures meant to induce grain growth. Molten-salt synthesis is an alternative method for obtaining single crystals, accelerating grain growth through the presence of a molten flux without the need for increased temperature. Herein, we offer heuristic guidelines for molten-salt synthesis, discussing key factors for designing reaction mixtures and the necessary exploratory research for novel molten salt/cathode systems. The influence of different salts and synthesis conditions on the morphology and properties of single-crystal LiNiO2 is presented. It is found that oxidative salts, such as Li2O2 and LiNO3, are crucial to supplementing dissolution of gaseous oxygen into the molten phase. Through these discussions, this work aims to provide a set of overarching principles for obtaining higher-quality single-crystal layered oxide cathodes and engender more rigorous and impactful investigation into their fundamental nature and applications.

Perspective on the Preparation Methods of Single Crystalline High Nickel Oxide Cathode Materials

“…Similar to sodium salts, lithium salts have also been widely utilized as flux agents for lowering the melting point of reactants, such as Li 2 MoO 4 , [62] LiCl, [63][64][65] LiNO 3 , [66][67][68] Li 2 O 2 [69] and Li 2 SO 4 . [70][71][72][73] For instance, Li 2 MoO 4 , possessing a melting point of 704 °C, has been utilized as a flux agent in the preparation of SC-NCM111.…”

“…study, in combination with Li 2 O 2 and LiOH, LiNO 3 effectively facilitated the oxidization of Ni, benefiting the formation of a well-structured SC-LNO with suppressed Li/Ni mixing. [69] Li 2 SO 4 possesses a suitable melting point of 859 °C and has been employed as a molten salt for the synthesis of SC-NCM622 [70] andSC-NCM811. [71] A plate-like polyhedral SC-NCM622 with the size of 2 μm was prepared by sintering at 900 °C for 10 h with 9.1mol% Li 2 SO 4 .…”

“…In another study, in combination with Li 2 O 2 and LiOH, LiNO 3 effectively facilitated the oxidization of Ni, benefiting the formation of a well‐structured SC‐LNO with suppressed Li/Ni mixing. [ 69 ]…”

Single Crystal Layered Oxide Cathodes: The Relationship between Particle Size, Rate Capability, and Stability

“…This will likely remain a significant challenge for the foreseeable future, as something simple such as targeting the same particle size with different dopants would require different synthetic conditions to compensate for changes in crystallization kinetics with inclusion of said dopants. In addition, something such as size distribution, which is observed to broaden with greater particle size, [33,40] can lead to aforementioned complications in material performance. In general, there exists three popular methods for SC preparation:…”

Crossover Effects in Lithium‐Metal Batteries with a Localized High Concentration Electrolyte and High‐Nickel Cathodes

“…[ 40 ] This is to be expected, as high‐nickel cathodes generally see minimal cation mixing over the course of only a few hundred cycles. [ 27,41 ] While high‐nickel cathodes are known to crack with extended cycling, this phenomenon is also generally observed at greater than 200 cycles. Furthermore, it has recently been shown that high‐stability electrolytes such as LHCE further mitigate cracking and microcracking by suppression of the stress‐corrosion cracking mechanism, and cracking is not expected to meaningfully contribute to capacity fade in this system.…”