Hidden structural order controls Li-ion transport in cation-disordered oxides for rechargeable lithium batteries (1806.06096v1)

Abstract: Crystal structures play a vital role in determining materials properties. In Li-ion cathodes, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining Li-ion diffusion kinetics. While a perfect crystal has infinite structural coherence, a class of recently discovered high-capacity cathodes, Li-excess cation-disordered rocksalts, falls on the other end of the spectrum: Their cation sublattices are assumed to be randomly populated by Li and transition metal ions with zero configurational coherence based on conventional X-ray diffraction, such that the Li transport is purely determined by statistical effects. In contrast to this prevailing view, we reveal that cation short-range order, hidden in diffraction, is ubiquitous in these long-range disordered materials and controls the local and macroscopic environments for Li-ion transport. Our work not only discovers a crucial property that has previously been overlooked, but also provides new guidelines for designing and engineering disordered rocksalts cathode materials.

Analysis of Hidden Structural Order in Cation-Disordered Oxides

The research paper investigates the pivotal role of hidden structural order (short-range order, SRO) in dictating lithium-ion (Li-ion) transport in cation-disordered oxides, specifically Li-excess cation-disordered rocksalt (DRX) cathodes. Historically, the lack of discernible long-range order in these materials, as inferred from conventional X-ray diffraction (XRD), led to the assumption that their cationic arrangement was entirely random. Contrary to this longstanding assumption, this paper reveals the existence of SRO that influences Li-ion conductivity.

Key Findings

1. Cation Short-Range Order (SRO): The authors demonstrate that SRO is prevalent in DRX cathodes and crucially impacts the population and connectivity of Li-migration channels. This hidden order, undetectable by typical XRD techniques, is revealed through electron diffraction (ED) and neutron pair distribution function (NPDF) measurements.
2. Comparison of LMTO and LMZO: The paper compares Li1.2Ti0.4O2 (LMTO) and Li1.2Mn0.4Zr0.4O2 (LMZO), two similar Li-excess DRX compositions. Despite expectations of comparable electrochemical performance based on their chemical composition and structural similarities, LMTO exhibits superior Li transport properties. The underlying reason is attributed to different SRO patterns in LMTO and LMZO, resulting in varying Li-ion diffusion pathways.
3. Li-ion Transport Pathways: Analysis reveals that in LMTO, Li4 tetrahedron configurations, which are optimal for Li-ion transport, occur more frequently compared to LMZO. The connectivity of these clusters significantly determines macroscopic Li-ion transport efficiency. LMTO supports extensive 0-TM (transition metal) Li networks, while such networks are sparse in LMZO, explaining its inferior Li-ion mobility.
4. Chemical Composition and SRO: The paper extends its findings to assert a general relationship between SRO features and chemical composition. The charge and ionic size of the constituent elements significantly affect SRO, thereby influencing Li diffusion pathways.

Implications

This research provides critical insights into the design and optimization of DRX materials for high-capacity Li-ion batteries. By uncovering the impact of SRO on Li-ion transport, it proposes a new variable for tuning electrochemical performance. The findings imply that tailoring SRO in DRX materials via careful selection of transition metal combinations can enhance Li-ion conductivity, thereby improving the efficiency and capacity of Li-ion batteries.

Theoretical and Practical Implications

Theoretically, the paper challenges traditional views on the role of disorder in functional materials, highlighting the necessity of understanding local structural discrepancies within otherwise disordered matrices. Practically, it offers a new paradigm in DRX cathode design, emphasizing the utilization of non-precious, earth-abundant elements to achieve high-performance electrical storage solutions.

Future Directions

Future research should focus on further characterizing the atomic-scale mechanisms by which SRO affects Li-ion dynamics, potentially employing advanced simulation techniques to predict and verify SRO patterns in a wider array of DRX compositions. Additionally, scalable synthesis methods to engineer specific SRO types could effectively lead to commercially viable high-capacity DRX-based batteries.

In conclusion, this paper underlines the importance of recognizing non-random local ordering in designing next-generation Li-ion battery materials. By identifying and adjusting subtle structural components, researchers can better predict and enhance the electrochemical properties necessary for efficient energy storage solutions.