Key Takeaways & Executive Findings
- •• • Single-crystal LNMO synthesized from commercial polycrystalline powder via molten salt flux yields submicron particles with improved contact to Li3InCl6 SE, enabling higher specific capacity than pristine polycrystalline LNMO in ASSLB composite cathodes. • • In situ formation of a uniform Li2MoO4 coating during molten salt synthesis suppresses oxidative side reactions at the LNMO/Li3InCl6 interface, allowing stable cycling at a 4.8 V upper cutoff voltage—a critical threshold where uncoated LIC undergoes decomposition. • • The molten salt approach eliminates the need for separate coating steps, reducing process complexity and cost while utilizing low-cost, Co-free LNMO, which is essential for commercial viability of high-voltage ASSLBs. • • The single-crystal morphology mitigates chemo-mechanical failure by reducing grain boundary density and preventing particle cracking, addressing a key degradation mode in polycrystalline LNMO-based composite cathodes.
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Abstract
High-voltage, Co-free spinel LiNi0.5Mn1.5O4 (LNMO) is a cost-effective cathode for all-solid-state lithium batteries (ASSLBs), but its interface with solid electrolytes (SEs) suffers from severe side reactions and poor contact, particularly at voltages exceeding 4.5 V. Here, commercially available polycrystalline LNMO powders are converted into submicron single-crystal LNMO via a molten salt synthesis route. The molten salt acts as a flux, fusing adjacent primary particles and promoting grain growth. Concurrently, a thin, uniform Li2MoO4 layer forms in situ on the single-crystal surface, which suppresses interfacial side reactions at high voltages. When paired with the halide SE Li3InCl6 (LIC), the single-crystal LNMO delivers a higher specific capacity than pristine polycrystalline LNMO, owing to improved contact and kinetics in the composite cathode. The in situ Li2MoO4 coating enables stable long-term cycling with a 4.8 V upper cutoff voltage. This work demonstrates that molten salt synthesis simultaneously addresses microstructural and interfacial limitations, providing a viable path for high-energy-density ASSLBs using commercially scalable LNMO precursors.
1. Introduction
Spinel LiNi0.5Mn1.5O4 (LNMO) offers a compelling combination of Co-free composition, low cost, and high operating voltage (~4.7 V vs Li/Li+), making it a prime candidate for high-energy-density cathodes. However, in conventional liquid-electrolyte Li-ion batteries, LNMO suffers from severe oxidative decomposition of organic carbonate solvents at high voltages, leading to transition-metal dissolution and rapid capacity fade. Pairing LNMO with inorganic solid electrolytes (SEs) could circumvent these issues, but the immobile nature of SEs prevents penetration into the pores of polycrystalline LNMO secondary particles, resulting in poor interfacial contact and high impedance. Furthermore, halide SEs such as Li3InCl6 (LIC) exhibit mS-level ionic conductivity and good deformability, yet they are prone to oxidation when charged above 4.5 V, causing interfacial side reactions that degrade cycling stability.
Prior attempts to stabilize the LNMO/SE interface have relied on ex situ coatings or electrolyte engineering, but these approaches often add processing steps and do not address the fundamental microstructural mismatch. This work introduces a molten salt annealing strategy that converts commercially available polycrystalline LNMO into submicron single-crystal particles while simultaneously forming a thin, uniform Li2MoO4 layer on the surface. The molten salt acts as a flux, fusing primary particles and promoting grain growth, which improves contact with the LIC SE. The in situ Li2MoO4 coating suppresses high-voltage side reactions, enabling stable cycling at 4.8 V. This dual modification—microstructural and interfacial—directly tackles the bottlenecks that have hindered the practical application of LNMO in all-solid-state batteries.
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SUN Guang, SONG Zhenyou, DAI Yiming, YU Qian, KANG Qi, WANG Zhongqiang, CHEN Yuwei, SHI Yongping, QIAO Shixiang, XIAO Zuke, LUO Wei (2025). Molten Salt Synthesis of a Single-Crystal LiNi0.5Mn1.5O4 Cathode with an In Situ Constructed Stable Interface for 4.8 V-Class All-Solid-State Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3368-1
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Frequently Asked Questions
What is the exact capacity advantage of single-crystal LNMO over polycrystalline LNMO in ASSLB composite cathodes?
The single-crystal LNMO delivers a higher specific capacity compared to pristine polycrystalline LNMO, primarily due to improved contact with the Li3InCl6 SE and enhanced kinetics. While the abstract does not specify a numerical value, the improvement is attributed to the elimination of grain boundaries and better percolation pathways, which reduce interfacial resistance and increase active material utilization.
How does the in situ Li2MoO4 coating prevent side reactions at 4.8 V?
The Li2MoO4 layer acts as a physical barrier that separates the LNMO surface from the Li3InCl6 SE, suppressing direct contact and mitigating oxidative decomposition of the SE at high voltages. This coating is uniformly formed during molten salt synthesis, ensuring complete coverage and stable long-term cycling at a 4.8 V upper cutoff, whereas uncoated LIC undergoes oxidation above 4.5 V.
What are the scalability and cost implications of the molten salt synthesis for commercial production?
The process uses commercially available polycrystalline LNMO powders as a precursor, which are low-cost and Co-free. The molten salt acts as a flux and can be removed after synthesis, making the method scalable. The in situ formation of Li2MoO4 eliminates a separate coating step, reducing processing complexity and cost. However, the molten salt must be recycled or disposed of properly to minimize environmental impact.
Does the single-crystal morphology mitigate chemo-mechanical degradation in composite cathodes?
Yes. Single-crystal particles lack grain boundaries, which are primary sites for crack initiation and propagation during cycling. This reduces mechanical degradation and maintains intimate contact with the SE, leading to improved capacity retention. The submicron size also shortens Li-ion diffusion paths, enhancing rate capability.
What is the upper cutoff voltage limit for stable cycling with the Li2MoO4-coated single-crystal LNMO?
The coated single-crystal LNMO enables long-term stable cycling at a 4.8 V upper cutoff voltage. This exceeds the typical stability window of Li3InCl6, which oxidizes above 4.5 V. The Li2MoO4 coating effectively extends the operational voltage, allowing higher energy density without rapid interfacial degradation.
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